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Doctoral Thesis
Light-emitting polymer systems for display applications
Author(s): Montali, Andrea
Publication Date: 2000
Permanent Link: https://doi.org/10.3929/ethz-a-003876621
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ETH Library
Diss. ETHNr. 13444
Light-Emitting Polymer Systems for
Display Applications
A dissertation submitted to the
EIDGENÖSSISCHE TECHNISCHE HOCHSCHULE ZÜRICH
for the degree of
Doctor of Technical Sciences
presented by
Andrea Montait
Dipl. Werkstoff Ing. ETH
born November 13th. 1970
citizen of Basel
accepted on the recommendation of
Prof. Dr. Paul Smith, examiner
PD Dr. Christoph Weder, co-examiner
Prof. Dr. Plans-Werner Schmidt, co-examiner
Zürich 2000
Table of Contents
Summary 1
Zusammenfassung 5
1. Introduction 9
1.1 Preamble 9
1.2 Photoluminescent Liquid-Crystal Displays 10
1.3 Polymeric Light-Emitting Diodes 17
1.4 Poly(/?-phenylene ethynylenc)s 20
1.5 Objectives and Scope of this Thesis 22
1.6 References 25
2. Polarizing Energy Transfer in Photoluminescent Materials for
Display Applications 31
3. Polarizing Energy Transfer in Photoluminescent PolymerBlends 41
4. Time-Resolved Fluorescence Study on the Mechanism of
Polarizing Energy Transfer in Uniaxially Oriented Polymer
Blends 57
5. Deformation Induced Formation of Molecular Polymer Blends
Probed by Single-Molecule Microscopy 73
6. Phase Behavior and Anisotropic Optical Properties of
Photoluminescent Polarizers 85
7. Ultra-High Performance Photoluminescent Polarizers Based on
Melt-Processed Polymer Blends 99
8. Poly(p-phenylene ethynyleneVBased Light-Emitting Devices 117
9. Polymerie Light-Emitting Diodes Based on Poly(p-phenylene
""
ethynylene), Poly(triphenyldiamine) and Spiro-quinoxaline 127
10. Conclusions and Outlook 143
Acknowledgements 147
Curriculum Vitae 149
1
Summary
In recent years there has been a considerable interest in the photophysical properties
of conjugated polymers, since these materials may combine the processibility and
outstanding mechanical properties of polymers with the exceptional, readily tailored
electronic and photophysical properties of organic semiconductors. Much research has
been focused on the photoluminescence (PL) and electroluminescence (EL) characteristics
of conjugated polymers. The focus of the present thesis are these two latter properties of the
conjugated polymer class of pofy(p-phenylene ethynylene) (PPE) derivatives and their
oligomers in view of applications in display devices in order to improve, among others, the
energy efficiency of liquid-crystal displays (LCDs).
Dichroic sheet polarizers together with color filters are used in numerous products
that make use of polarized, chromatic light, including color LCDs as the most important
application. However, this combination converts a major fraction of incident light into
thermal energy, which limits brightness and energy efficiency of these devices. One
concept to partially overcome these drawbacks was proposed earlier, in which the polarized
absorption and emission of light by oriented conjugated polymers is exploited. So-called
PL polarizers were presented as components in LCDs to replace a dichroic sheet polarizer
and the color filter. However, these PL polarizers, in certain configurations, exploit
maximally only 50% of the incident light. Thus, a substantial amount of energy passes the
polarizer unused and LCDs based on such PL polarizers are intrinsically limited in their
energy efficiency. In this thesis, we report a new concept for polymer-based PL polarizers
which overcome the limitations described above and can be used in PL LCDs with, in
principle, an ultimate efficiency of unity. These PL polarizers comprise a nearly randomly
oriented sensitizer that maximally harvests light by isotropic absorption, efficiently
transfers the energy to a uniaxially oriented PL polymer which, subsequently, emits highly
linearly polarized light. Key step is a polarizing energy transfer which, to a certain extent,
mimics the concept used by nature in photosynthesis to optimally use optical energy.
It is further shown that this polarizing energy transfer, which was demonstrated here
for the first time, is a more general phenomenon, provided that appropriate materials are
adequately combined, and we quantify the efficiency of the polarizing energy transfer,
demonstrating that it can be highly efficient, with tranfer efficiencies as high as 85%
determined in optimized blend films. Some of the required physico-chemical properties of
the materials to be combined for the polarizing energy transfer to occur efficiently, are
elucidated, in order to optimize the materials performance. The key elements in effectively
preparing such PL polarizers are the photophysical and the chemical compatibility between
the donor and the acceptor molecules, the melting temperature of the donor molecules and
the form of the latter which must be isotropic in order to avoid orientation in the matrix
polymer, here ultra-high molecular weight polyethylene (UHMW-PE)
A time-resolved study of polarizing energy transfer in oriented blends of a
conjugated polymer (EHO-OPPE, a dialkoxy substituted PPE derivative) and an organic
laser dye (7-diethylamino-4-methylcoumarin, DMC) in UHMW-PE is presented. The
transfer is described in terms of a Förster mechanism, based on long-range dipole-dipole
interactions. Förster radii were determined in oriented blend films as well as in solutions. It
was found that the transfer process is critically influenced by the phase behavior of the
system. A depolarizing homoIransfer between the donor molecules was found to be a key
step in the polarizing nature of the transfer which, ultimately, allows excitation light
polarized perpendicularly to the film orientation direction to be emitted with the
polarization direction parallel to its orientation.
The phase behavior of dilute blend films of EHO-OPPE in UHMW-PE was studied
on a molecular level with scanning confocal optical microscopy, revealing a phase
separated system in unoriented films, which upon tensile deformation undergoes a phase
"transformation", finally yielding a near-molecular blend in the highly oriented PL
polarizers. These investigations also demonstrate that solid-state tensile deformation of
initially phase separated mixtures allows to produce stable molecular blends of intrinsically
immiscible polymers. It was shown that the phase behavior of polymer blends used for the
preparation of PL polarizers can influence the anisotropy of their optical properties. The
orientational behavior of the guest PL molecules and of the matrix were investigated; it was
found that the extent of phase separation in the unoriented blend films, i.e. the domain-size
of the PL dye guest in the polyethylene host, inhibits an efficient orientation of the PL guest
at low draw ratios (<20). Consequently, PL polarizers with a controlled phase behavior
were produced, which exhibit high optical anisotropy (DR>40) at draw ratios as low as 10.
These PL polarizers consist of a linear low-density polyethylene matrix and a low-
molecular PL dye, which exhibits an improved solubility in the matrix material.
Finally, the electroluminescence properties of certain PPE-derivatives were
investigated. Light-emittmg diodes (LEDs) were produced witli PPE as the emitting layer
and aluminum as the cathode, and a peak brightness of around 80 cd/m2 was obtained.
From the determination of the band edges of the highest occupied molecular orbital
(HOMO) and of the lowest unoccupied MO (LUMO) with ultraviolet photoelectron
spectroscopy as well as with cyclo voltammetry, it was established that the energy barrier
for hole injection and hole transport are the limiting factors in PPE-based LEDs. Therefore,
LEDs were prepared, in which the EHO-OPPE was combined with a hole conducting
poly(triphenyldiamine) derivative (poly-TPD). Increased efficiencies as well as a peak
3
brightness of around 150 cd/m were obtained. In LEDs with an additional hole-blocking
layer vapor deposited on the emitting layer the efficiency was further increased to above
0.14 cd/A with a brightness of around 260 cd/m".
3
Zusammenfassung
Die photophysikalischen Eigenschaften von konjugierten Polymeren haben in den
letzten Jahren grosse Beachtung gefunden, denn solche Werkstoffe vereinen die
Verarbeitbarkeit und die ausgezeichneten mechanischen Eigenschaften von Polymeren mit
den hervorragenden und auf die spezifischen Anwendungen zugeschneiderten
elektronischen und photophysikalischen Eigenschaften organischer Halbleiter. Die
Photolumineszenz (PL) sowie die Elektrolumineszenz (EL) von konjugierten Polymeren
standen im Mittelpunkt der bisherigen Forschung. Diese beiden Eigenschaften der Familie
von Poly(p-phenylen ethinylen) (PPE)-Derivaten und -Oligomeren, bilden den
Schwerpunkt der vorliegenden Dissertation und werden im Hinblick auf eine mögliche
Anwendung in Flüssigkristallbildschirmen (LCDs) untersucht, wo unter anderem die
Energieeffizienz verbessert werden könnte.
Die Kombination von Folienpolarisatoren mit Farbfiltern wird in vielen Produkten
verwendet, in denen farbiges, polarisiertes Licht benötigt wird, insbesondere und
hauptsächlich in LCDs. Diese Bauteile wandeln allerdings einen grossen Teil des
einfallenden Lichtes in Wärme um, was die Helligkeit und die Energieeffizienz dieser
Bildschirme beeinträchtigt. Eine Möglichkeit, um diese Nachteile zu überwinden wurde
bereits in einer früheren Arbeit vorgeschlagen. Dabei werden die polarisierte Absorption
und Emission von Licht durch orientierte konjugierte Polymere ausgenutzt. Sogenannte PL
Polarisatoren wurden als Bauteile in LCDs eingesetzt, wo sie einen Folienpolarisator sowie
den Farbfilter ersetzen. Allerdings werden von diesen PL Polarisatoren in bestimmten
Anordnungen maximal nur 50% des einfallenden Lichtes ausgenutzt. Ein grosser Teil der
Hintergmndbeleuchtung geht also verloren, und die Energieeffizienz von LCDs, die auf
solchen PL Polarisatoren basieren, ist intrinsisch begrenzt. In dieser Dissertation wird ein
neues Konzept für polymère PL Polansatorcn vorgestellt, das die oben erwähnten
Nachteile überwindet und in PL LCDs eingesetzt werden kann, die prinzipiell eine
Effizienz von 100% besitzen. Diese PL Polarisatoren bestehen aus einem nahezu isotrop
ausgerichteten Sensibilisator, der isotropes Licht absorbiert und anschliessend die
absorbierte Energie an das orientierte PL PoKmer transferiert, das sie als polarisiertes Licht
emittiert. Der zentrale Prozess ist ein polarisierender Energietransfer, der teilweise die in
der Natur bei der Photosynthese ablaufenden Prozesse wiederspiegelt.
Es wird ausserdem gezeigt, dass dieser Energietransferprozess, der in dieser Arbeit
zum ersten Mal überhaupt beschrieben wurde, ein allgemeines Phänomen ist, sofern
geeignete Substanzen miteinander kombiniert werden und dessen Effizienz wird
quantitativ bestimmt, dabei konnten Effizienzen von bis zu 85% nachgewiesen werden.
6
Zusätzlich werden die erforderlichen physikalisch-chemischen Eigenschaften der
Werkstoffe erläutert, die kombiniert werden, um einen effizienten Energietransfer zu
ermöglichen. Die wichtigsten Punkte sind dabei die photophysikalische sowie die
chemische Kompatibilität zwischen den Substanzen, die Schmelztemperatur des Donors
sowie seine geometrische Form, die isotrop sein muss, damit der Donor beim
Verstreckungsprozess die Orientierung der Matrix, in diesem Fall ultra-hochmolekulares
Polyäthylen (UHMW-PE), nicht übernimmt.
Der Mechanismus des Energietransfers in orientierten Blends zwischen einem
konjugierten Polymer (EHO-OPPE, ein mit zwei Alkoxy-Gruppen substituiertes PPE) und
einem organischen Laserfarbstoff (7-diethylamino-4-methylcoumarin, DMC) in UHMW-
PE wurde mit zeitaufgelöster Fluoreszenzspektroskopie untersucht. Der Mechanismus
kann mit dem von Förster vorgeschlagenen Modell, das auf Wechselwirkungen zwischen
Dipolen basiert, beschrieben werden. Die kritischen Radien für das untersuchte System
wurden in orientierten Filmen wie auch in Lösung bestimmt. Es konnte gezeigt werden,
dass der Energietransfer durch das Phasenverhalten der involvierten Farbstoffe beeinflusst
wird. Eine Depolarisation der Energie durch Homotransfer zwischen Donor Molekülen
wurde als entscheidender Schritt für die polarisierende Eigenschaft des gesamten Prozesses
bestimmt, die es erlaubt, Licht mit einer Polarisation zu absorbieren, die senkrecht zur
Orientierungsrichtung der Filme liegt, und es mit paralleler Polarisation zu emittieren.
Mit konfokaler optischer Rastermikroskopie wurde das Phasenverhalten in stark
verdünnten, orientierten Blends von EHO-OPPE und UHMW-PE untersucht. Es konnte
gezeigt werden, dass während des Verstreckvorganges eine Phasenumwandlung erfolgt, die
von einem phasenseparierten System bei unverstreckten Filmen zu annähernd molekularen
Blends in verstreckten Filmen führt. Diese Untersuchungen haben auch gezeigt, dass
stabile molekulare Blends aus Polymeren hergestellt werden können, die sonst intrinsisch
inkompatibel sind. Ferner wurde gezeigt, class das Phasenverhalten in PL Polarisatoren
einen grossen Einfluss auf die Anisotropie der optischen Eigenschaften hat.
Untersuchungen des Orientierungsverbaltens der PL Gastmoleküle und der polymeren
Matrix haben gezeigt, dass das Ausmass der Phasenseparation, d.h. die Grösse der
Domänen des Gastmoleküls in der UHMW-PE Matrix, eine effiziente Ausrichtung der PL
Gastmoleküle bei niedrigen Verstreckimgverhältnissen (<20) verhindern. Daraufhin
wurden PL Polarisatoren mit einem kontrollierten Phasenverhalten hergestellt, die bei
einem Verstreckungsverhältnis von nur 10 schon eine sehr hohe optische Anisotropie (DR
>40) aufweisen. Diese PL Polarisatoren bestehen aus einer Matrix aus linearem
Polyäthylen niederer Dichte und einem niedermolekularen photolumineszierenden
Farbstoff, der eine erhöhte Löslichkeit im Matrixmaterial besitzt.
7
Schliesslich wurden Licht emittierende Dioden (LEDs) auf der Basis von einigen
PPE Derivaten hergestellt, Aluminium erwies sich als optimales Kathodenmaterial, es
wurde eine maximale Leuchtstärke von 80 cd/m erreicht. Die mit Ultraviolett
Photoelektronenspektroskpie und zyklischer Voltammetrie bestimmten Werte für die
Bandkanten des höchsten besetzten Molekülorbitals (HOMO) und des niedrigsten
unbesetzten MO (LUMO) deuten auf eine höhere Energiebarriere für die Lochinjektion an
der Anode als für die Elektroneninjektion an der Kathode. Folglich wurden LEDs
hergestellt, in denen EHO-OPPE mit einem lochleitenden poly(triphenylendiamin) Derivat
(poly-TPD) kombiniert wird. In diesen LEDs wurden eine erhöhte Effizienz und eine
maximale Leuchtstärke von 150 cd/m" erreicht. Als letzter Optimierungsschritt wurde eine
zusätzliche Schicht eines Lochblockers auf die emittierende Schicht aufgedampft. Die
Effizienz konnte bis auf 0.14 cd/A und die Helligkeit auf 260 cd/m2 gesteigert werden.
9
1. Introduction
1.1 Preamble
Various forms of "conducting polymers" have been known, in one form or another,
for almost a century. In 1977, the first report of high electrical conductivity in Iodine-
doped free-standing films of polyacetylene triggered extensive research in the area of %-
conjugated polymers. These materials can exhibit a series of attractive properties such as
electrical conductivity,2 photoconductivity.4 photoluminescence (PL)5 or
electroluminescence (EL). The possibility to tailor these properties through chemical
modification, as well as the possibility of simple processing through conventional methods
make conjugated polymers interesting from a fundamental scientific point of view as well
as for technological applications.7
Among others, potential applications for conjugated polymers exist in flat-panel
display technologies, where use can be made of their luminescence properties. The
principal interest in the use of polymers is based on their promise for low-cost
manufacturing, using e.g. solution-processing of film-forming polymers. The prospective
of light-weight, thin and flexible displays has triggered significant research and
development efforts for polymeric light-emitting diodes (LED) which exploit the EL of
conjugated polymers.9 In the years after the first demonstration of EL from poly(/?~
phenylene vmylene) (PPV). virtually all research focused on the EL properties of PPV and
PPV-dcrivatives.1043 With no apparent reason, the EL properties of other conjugated
polymers, with few exceptions, were only marginally explored until about 1996.
The PL properties on the other hand can be used in liquid-crystal displays (LCDs),
where in particular the emission of polarized light from oriented structures17 can be
exploited. This was recently proposed as a possibility to improve the energy efficiency and
I o
the viewing angle of LCDs. c which are two important drawbacks of such displays, as will
be elaborated on below.
Thus, with the above described technological potential in mind, and in view of the
general scientific interest, we set about to investigate applications of PL polymers in flat-
panel displays. In particular, we have focused on alkoxy substituted derivatives of poly(}>
phenylene ethynylene) (PPE). which will be discussed in more detail in Paragraph 1.4.
10
In the present chapter, the technologies which are addressed in this thesis are
presented and their relevant scientific and technological aspects will be treated, in order to
provide a background and a framework to the successive chapters.
1.2 Photoluminescent Liquid-Crystal Displays
99 OALCDs are the dominant technology in the field of flat-panel displays. With the
enormously increasing importance of information technologies, the visualization of
information through displays, screens and projection systems detains a role of extreme
importance, in which visualization devices represent the interface between the user and the
information system. Therefore, the need for smaller, lighter and more energy-efficient
screens continues to spur the research in the field of flat-panel displays. Despite many new
inventions and improvements,25"27 the majority of LCDs produced worldwide is still based
on the original invention of 1971 by M. Schadt and W. Helfrich, and includes a twisted
nematic (TN) cell. The basic principle of this electro-optical light-shutter is briefly
discussed below; its structure is shown in Figure 1.
A nematic, liquid-crystalline material is sandwiched between two Indium Tin Oxide
(ITO)-coated glass plates and two crossed polarizers. The glass plates are both covered
with an orientation layer consisting of rubbed polyimide and the rubbing direction
coincides with the polar axis of the respective polarizer (thus, the two orientation layers are
arranged perpendicular to one another). The liquid-crystalline molecules close to the glass
walls align parallel to the orientation layers and, clue to the crossed position of the latter and
directioned by a small concentration of a chiral dopant, the molecules arc forced to perform
a 90° twist across the cell gap.22-28-29 Jf no electric field is applied to the ITO electrodes,
the electromagnetic field vector of the incident light will also be twisted over 90° when
passing the cell. Light will therefore pass the second polarizer and the LCD appears
transparent. However, if an electric field is applied between the ITO-electrodes, the
nematic molecules tend to align parallel to the electric field. Incident light in this case will
encounter the second polarizer in a crossed position and, thus, will be blocked. Images
are produced on the display by selectively addressing defined sectors (segments or pixels)
Il
of the TN-cell through the patterned ITO electrodes. While reflective LCDs rely on
ambient illumination, backlit flat-panel displays usually comprise a white, diffuse light
source that provides unpolarized illumination from the back of the display, in order to
improve the visibility under poor lighting conditions22
Lisht is transmitted Lislit is blocked
5
3
3
5
4
3
2
1
Analyzer
Nematic liquid crystals
Glass-supported ITO electrodes with alignment layer
Linear absorbing polarizer
Isotropic backlight
Fig. 1: Structure of a backlit liquid-crystal display
To date, the polarizers employed in the majority of flat-panel displays are still based
on the original invention of Land.'0,31 These polarizers were the first large-area, flexible,
thin film polarizers produced. They consist of sheets of polyvinyl alcohol) which are
oriented by tensile deformation at elevated temperatures and are used as the carrier
material; iodine complexes formed by the absorption of iodine by these sheets absorb
virtually exclusively one polarization of incident light. Originally, such sheet polarizers
were used in applications such as anti-glare sunglasses, anti-glare desk lamps, photography
filters, optical mstiuments (microscopes), and stereoscopic projection devices. The
invention of the fN LCD broadened the applications of polarizers significantly.
12
Nevertheless, such absorption polarizers exhibit a series of important drawbacks. Most
importantly, conventional polarizers absorb at least fifty percent of the light and the
absorbed energy is transformed into thermal energy. Consequently, to obtain sufficiently
bright displays, which retain high contrast ratios and clear images when observed in
daylight, the absorbance losses need to be compensated for by increasing the illumination
intensity. The extra power consumption and bulkiness of the backlight system reduces the
lifetime of batteries, and in addition adversely affects the size and weight of, for example,
lap-top computers. Furthermore, conventional sheet polarizers exhibit limited stability at
high temperatures and humidity, and their performance, thus, deteriorates in time thereby
reducing the lifetime of displays.'4
In colored LCDs, colors are generated by absorbing color filters, which are often
placed between the LC-cell and the analyzer- and, clue to their working principle,
represent another source for significant energy loss. To obtain the full color spectrum, the
three primary colors, red, green, and blue, are needed."'' Three color filters, each absorbing
the complete spectrum with the exception of one primary color, are used in full-color
LCDs; the filters are patterned to spots of a size of the order of 100 um (so called pixels) in
OOaccordance with the patterned ITO electrodes.-"" Typically one such filter absorbs 70-80%
of the incident white light. It is, therefore, evident that these color filters severely limit
the efficiency of LCDs.
These limitations have triggered the development of polarizers based on selective
reflection or scattering of light which can replace the dichroic polarizer in a conventional
LCD configuration. Such polarizers selectively reflect or scatter light of one
polarization and allow recycling of the reflected or scattered light. The ultimate efficiency
of these polarizers is, in principle, unity. However color applications based on these
elements still rely on absorbing color filters which, as discussed above, are extremely
inefficient.
A different approach to increase the efficiency of color LCDs is the use of isotropic
photoluminescent materials, which act as ''active" color filters41 Emissive layers have
been used in so-called photoluminescent liquid-crystal displays (PL LCD). In one possible
design of PL LCDs, light from a narrow-bandwidth near-UV lamp44 is modulated by a LC
cell and is then incident on a photoluminescent screen on the front of the display41 The
photoluminescent screen converts the near UV "image" into a visible image using
13
photoluminescent phenomena. If the PL screen has red, green and blue pixels aligned in
registration with the pixels of the LCD, a color image is generated directly without the need
for absorbing color filters. Such PL LCDs exhibit the viewing angle characteristics of
cathode ray tabes or of LEDs in a flat-panel display thai in principle can be manufactured
in existing LCD production facilities. These displays promise a significant improvement in
energy efficiency because the rather inefficient absorbing color filters are replaced by
actively emitting substances. The limit of the efficiency of the light-conversion is the
intrinsic quantum efficiency of the luminescent species employed, which in principle can
approach 100%.45
Another possibility is presented by the use of photoluminescent polarizers, which
arc based on uniaxially oriented, form-anisotropic PL dyes. PL polarizers exhibit highly
anisotropic absorption and emission properties and may emit polarized light of one
particular color.17"18 They can be used to replace one absorption polarizer and the color
filter in conventional LCDs. Several configurations can be envisioned to incorporate PL
polarizers in an LCD, as was shown m the work of Weder et al.18
Analyzer
TN-cell
PL, polarizer
(UV) lisht source
PI, polarizer
(Analyzer)
TN-cell
Polarizer
(UV) light source
Fig. 2: Simplified schematic representation of a photoluminescent LCD. where, respectively, the
polarized emission (a) or absorption (b) properties of the PL polarizer are exploited.
fffff^l^ ^
14
Two particular device configurations are shown in Figures 2a and b respectively, in
which the PL polarizer is placed between the LC cell and the backlight (a), which of course
must be adapted to emit at a wavelength that is suitable for the excitation of the PL
polymer, or, alternatively, between the LC cell and the viewer (b). In the former
configuration, the anisotropic emission properties of the PL polarizer are exploited.
Alternatively, the PL polarizer functions as an analyzer and its anisotropic absorption
properties are relevant to the application (Figure 2b). In the latter arrangement, the viewing
angle is substantially improved, since it is determined by the lambertian emission
properties of the PL polarizer, and the usually strongly angle-dependent LC effects are
essentially eliminated.
Uniaxially oriented photoluminescent materials, as they are used for the above
described PL polarizers, usually exhibit anisotropic, i.e. linearly polarized, absorption and
emission. Polarized photoluminescence has been known for a long time, it was observed
and documented as early as the last century and m the 1930s it was observed from
47
organic substances embedded in an oriented polymeric matrix. However, despite the
availability of polarized photoluminescence, this source of highly polarized light was not
technologically exploited.
In order to efficiently orient photoluminescent molecules, and, thus, to create
materials that are characterized by highly polarized absorption and emission properties,
two important requirements must be met:
• The shape of the molecules must be such that they can be oriented efficiently, of
course, a high aspect ratio, i.e. ratio between length and diameter of the molecule,
ARfavors orientation. This makes linear polymers with a rigid, conjugated backbone
ideal candidates for applications in which anisotropic electronic properties are
required.
• Depending on whether the anisotropic absorption or emission properties of the ori¬
ented PL molecules are relevant for the envisioned application, the main angular
offset between the geometrical long axis of the molecule and the absorption or the
emission dipole moments, respectively, must be zero or close to zero, in order to
guarantee an efficient alignment of all transition dipole moments upon orientation
of the molecules.
15
Several possibilities to orient PL dyes are known and have been extensively studied
in the past, such as the Langmuir-Blodgett technique, ' mechanical deformation '
or
rubbing of a conjugated-polymer layer,' and the deposition of PL dyes onto a highly
ordered orientation layer, such as poly(tetrafluoroethylene) '
or rubbed poly(imide).
Other possible methods include the orientation of luminescent species in electric, magnetic
or flow fields or so-called optical alignment (i.e. with polarized light).3 However, the
degree of orientation and, hence, the dichroic ratios obtained with these methods are
usually only modest, typically well below 10 and, therefore, limit the device characteristics
with respect to contrast and resolution.
A different, efficient technique is tensile deformation of host-guest systems, in
which the guest molecules adopt the orientation of the host Polarized absorption
and emission studies of fluorescent molecules, which were embedded in a polymeric
matrix, revealed that the extent to which the orientation of the matrix is adopted by the
guest molecules is strongly related to their aspect ratio.
Films of ultra-high molecular weight polyethylene (UHMW-PE) produced through
the gel phase can reach extremely high degrees of order when elongated to draw ratios of
up to 130 times their initial length. The orientation is obtained through tensile
deformation at elevated temperatures of around 130°C. i.e. at a temperature close to. but
below the melting point of the polymer. The macromolecular chains in such oriented films
exhibit a single-crystal like order and are nearly completely extended along the drawing
direction of the film.64'65 To prepare such films, UHMW-PE typically is dissolved at low
concentrations in the order of 1-2% wAv in a hot solvent. Upon dissolution of the UHMW-
PE, a highly viscous solution is obtained, where the UHMW-PE molecules form a
continuous network with a very low entanglement density. This solution is subsequently
cast into a mold; upon cooling a gel is formed which is left to dry until all the solvent is
evaporated. The remaining film retains the very low entanglement density of the gel. Since
the maximal obtainable draw ratio is directly related to the molecular weight of the
polymer and the entanglement density, very high draw ratios can be obtained by tensile
deformation at elevated temperatures of samples prepared through the gel phase.
Previous studies have demonstrated that co-dissolving a fluorescent conjugated
polymer with UHMW-PE and subsequent tensile deformation of the blend film will yield
oriented films containing well-aligned fluorescent molecules.17'66 The photoluminescence
J 6
from undrawn polymer blend films is impolarized due to the disordered structure of the
latter, whereas the oriented blend films emit highly polarized light. Figure 3 schematically
illustrates these processes. These oriented polymer blend films, or PL polarizers, are
characterized by highly polarized absorption as well as emission of light, as can clearly be
seen in Figure 4, which shows polarized absorption and emission spectra of a film
consisting of 2% w/w EHO-OPPF (a poly(2.5-dialkoxy-/>phenylene ethynylene)
derivative) (cf. Figure 6b)19 embedded in UHMW-PE and drawn to 80 times its original
length.17 The spectra show the absorption and emission (under isotropic excitation) of light
polarized parallel and perpendicular to the drawing direction of the film.
>
iûb
Fig. 3: Polarized photoluminescence from photoluminescent molecules embedded in a polymeric
matrix in the unoriented and the oriented state.
These investigations demonstrate that tensile orientation of polymer blends based
on UHMW-PE is a powerful tool for the production of highly luminescent films which
absorb and emit light in a highly polarized fashion. As will be shown in this thesis, further
development of the process and an adequate choice of the materials hold the promise to
extend the concepts to more common industrial processes, such as melt-extrusion, to
produce films of oriented photoluminescent polymer blends.
17
I L_ , . I , i I i I , i_ 1 1
300 400 500 600 700
A. [nm]
Fig. 4: Polarized absorption and emission spectra of an oriented 2% w/w EHO-OPPE / ULIMW-PE
blend, draw ratio 80. Absorption and emission parallel (solid lines) and perpendicular
(dotted lines, insets) to the film drawing direction are shown. Emission spectra were
recorded under isotropic excitation at 440 nm.
1.3 Polymeric Light-Emitting Diodes
The phenomenon of electroluminescence was first observed in organic materials in
1963 by Pope et al. who demonstrated EL on an anthracene single crystal. Tang et al.
made a significant breakthrough in 1987, by optimizing the charge injection in a sandwich-
structured LED based on organic emitters and demonstrating a brightness of more than
1000 cd/m2 at driving voltages of 10 V.68 The first polymeric LED was reported in 1990 by
Burroughes et al. based on PPV as the emissive polymer.0 Since then, research in the field
of polymeric LEDs has been the focus of many academic and industrial groups and the
performance of LEDs has experienced a dramatic improvement. Polymeric LEDs now are
being considered for applications in flat-panel displays.69"71 The relatively simple
processing techniques such as spin-coating or spread-casting allow the simple preparation
of large-area devices,9and conjugated polymers can be chemically tailored in order to
emit virtually every color of the visible spectrum.1'"14 Furthermore, flexible and
18
transparent displays can be envisioned on the basis of polymers. Electroluminescent
displays also have the significant advantage of a wide viewing angle, since the emissive
layer, which is a lambertian emitter, is located on the very top of the display, as is the case
in cathode ray tubes."^ Finally, the potential application in LCDs of polymeric LEDs as
backlight sources which directly emit polarized light, promises a substantial increase in the
energy efficiency of the latter, since the polarizer, which absorbs at least half of the
backlight would become unnecessary.72 As in the case of photoluminescence, polarized EL
can be achieved by the use of oriented conjugated polymers as the emitting layer.
Understanding of the physical processes occurring in an LED and of device engineering, as
well as the knowledge of the chemistry of EL polymers have been considerably advanced,
these topics were recently extensively reviewed. Polymeric LEDs have now reached the
point of introduction on the market.71 The field of polymeric LEDs now offers all colors
of the visible spectrum, including white,^
external quantum efficiencies of up to 20
lm/W, peak brightness of several ten thousand cd/nry and lifetimes in the order of
lO'OOO hours and more. Transparent and flexible LEDs and polarized EL were
demonstrated as well. Nevertheless, it must be noted that these truly astonishing
improvements in performance which were achieved m only about ten years, are all limited
to single examples, and no devices have yet been demonstrated, that combine all the above
mentioned features.
The simplest structure of a polymeric LED is shown in Figure 5. A thin layer of the
emitting conjugated polymer is deposited, usually through spin-casting, on an ITO-coated
glass substrate. ITO is a good hole transportei and, thus, is usually employed to serve as the
anode. On top of the organic layer, a low-work-function metal (e.g. calcium, aluminum,
magnesium alloys or indium) is vapoi-deposited as the electron-injecting cathode. The
emitting layer, which typically has a thickness of around 100 nm, is sandwiched between
these electrodes. When a DC voltage is applied to the LED and a sufficiently high field is
generated, electrons are injected from the metal cathode into the conduction band of the
semiconducting polymer, holes from the ITO anode into the valence band, and excitons can
be formed which may recombme under the emission of light.
19
cathode
polymeric emitting layer
ITO anode
glass substrate
Fig. 5: Schematic representation of a polymeric light-emitting diode
Due to the position of the energy levels of conjugated polymers, which mostly favor
hole-injection over electron-injection, methods were developed to obtain a more
balanced charge injection at the electrodes and an equilibrated charge transport in the
polymeric layer. These efforts resulted mainly in multilayer LEDs,12'68 in which layers of
different polymeric or low-molecular organic substances of particular properties and
functions are stacked in the LED.8 Typically, a hole-conducting / electron-blocking layer is
deposited on the ITO anode, the emitting layer is positioned on top of this layer and an
electron-conducting / hole-blocking layer is deposited at the interface to the cathode. This
structure leads to a charge confinement in the emitting layer and enhanced efficiencies can
be obtained. The above described multi-layer structure for optimization of charge injection
and transport, of course, poses several problems regarding manufacturing of polymeric
LEDs; during the preparation of multilayer devices with conjugated polymers, extreme
care has to be taken with respect to the solvents employed to produce the different layers of
materials. It is of crucial importance that the solvent employed for the deposition ol a
following substance will not dissolve the layers already deposited. Therefore, LEDs were
produced, which combine the properties of all layers in one single layer, by blending of
materials that contribute the various desired properties.1f ''
Despite being very attractive for the reasons mentioned above, organic LEDs still
suffer from a few significant drawbacks, most importantly the typically poor stability of the
devices. Since organic LEDs are extremely sensitive to oxygen and humidity.82"84 they can
not be operated in air and costly packaging technologies are required to protect and assure
long-time operation of these devices.(l-(
Adequate packaging requires sealing organic
LEDs in glass; unsolved problems remain in achieving adequate performance for flexible
substrates78 Even when packaged, the stability of LEDs remains an important issue, since
20
additional causes for degradation and failure exist, such as the presence of pinholes in the
cathode material or delammation of the latter from the polymeric semiconductor.
Further the oxygen present in ITO has been suggested to be involved in degradation, and
current-induced heating of the LEDs during operation was also shown to contribute to
degradation processes. Finally, the emission of highly polarized light from oriented
polymers in LEDs has proven to be difficult;1
only recently a dichroic ratio of 15 was
demonstrated in LEDs based on polymeric liquid crystals.79
1.4 Poly(p-phenylene ethynylene)s
Poly(/?-phenylene ethynylene) (PPE) (Figure 6a) belongs to the class of n-
conjugated polymers, which, as was mentioned above, have encountered interest in recent
years due to their electronic and optical properties.
Unsubstituted PPE is characterized by a rather rigid, linear conjugated backbone
and, as a result, is intractable.87,88 Substitution with alkyl- and alkoxy-sidechains leads to
an improved solubility of the polymer, which is mandatory for simple processing and
preparation of films, coatings or blends. '"
Control of the effective conjugation length or substitution with adequate side-
groups allow to tunc the emission color of PPE and its derivatives over a wide spectral
range. Unsubstituted PPE, oligomers. PPE-copolymers which comprise conjugated
segments of well-defined length and (aliphatic) spacers in a strictly alternating fashion, as
well as alkyl-substituted PPEs exhibit blue or blue-green emission with a maximum around
400-430 nm. The emission is shifted to longer wavelengths for 2,5-dialkoxy-
substituted PPEs, which exhibit an emission maximum around 480-500 nm.19'89"91 The
derivatization of the backbone with electron-withdrawing substituents leads to small shifts
of the absorption and emission spectra towards lower energies, leading to orange emission
centered around 550 nm.93
Poly[2,5-dioctyloxy-1.4-diethynyl-phenylene-z///-2,5-bis(2'-ethyhexyloxy)-l,4
phenylene] (EHO-OPPE), a PPE derivative consisting of alternating units substituted with
sterically hindered and linear alkyloxy side-chains, respectively (Figure 6b),19 features
21
several characteristics which make it of particular interest for applications involving
luminescence:
• It can be directly synthesized in high purity without the need for precursor poly-
mers.16'19-21
• It has a high PL quantum efficiency of around 85% in solution and of 35% in the
solid state.
• The rigidity and the linearity of the conjugated backbone results in an outstanding
orientability of the molecules, which is required for applications in which polarized
absorption and / or emission of light are desired. ''
a)
Fig. 6: Chemical structures of poly(y>phenylene ethynylene) (a), and ELIO-ÖPPE, a poly(2,5-
dialkoxy-/?-phenylene ethynylene) derivative (b).
Surprisingly, the EL properties of PPE and its derivatives have been explored only
sporadically to date, and PPE is generally not thought to be a promising material for LED
applications. '' This, in fact, is remarkable in view of the fact that alkoxy-substituted
PPEs exhibit high quantum efficiencies for photolumincscencc, good film-forming
properties, and a high glass-transition temperature: these are, among others, some of the
main prerequisites for applications in LEDs. Thus. EHO-OPPE was employed in the
majority of the studies presented here.
22
1.5 Objectives and Scope of this Thesis
Driven by the continued interest in luminescent polymer systems for the above
mentioned applications in flat-panel displays, the main objective of the present thesis is to
explore and exploit some relevant aspects of PPEs in this context.
The PL polarizers presented in previous studies exhibit highly polarized absorption
and emission properties. An application in PL LCDs was proposed, in which the PL
polarizer can replace either the polarizer or the analyzer and in both cases also the color
filter. However, when used in the former configuration (cf. Figure 2a), only at most 50% of
the unpolarized light incident from the light source is used. This represents an intrinsic
limit to the efficiency of PL LCDs. Therefore, one objective of the present thesis was to
develop more energy-efficient polarizers, which absorb light isotropically and reemit the
absorbed energy in a highly polarized fashion. A fundamentally new concept for polymer-
based PL polarizers is presented in Chapter 2, which overcomes the above described
efficiency limitations, and can be used in PL LCDs with, in principle, an ultimate
efficiency of 100%. These PL polarizers comprise a nearly randomly oriented sensitizer
that maximally harvests light by isotropic absorption, efficiently transfers the energy to a
uniaxially oriented PL polymer which, subsequently, emits highly linearly polarized light.
This unique property of the blends presented here is made possible by the specific
exploitation of the polarization dependence encountered in long-range energy transfer
based on dipole-clipole interactions. This phenomenon is shown here for the first time and
is termed as polarizing energy transfer.
In order to demonstrate that the polarizing energy transfer is a more general
phenomenon when appropriate materials are adequately combined, the results of
experiments made on tensile-oriented blend films of various donor-acceptor combinations
are presented in Chapter 3. The efficiency of the transfer as well as the optical absorption
and emission properties of all investigated blends are quantified by polarized absorption
and steady-state PL spectroscopy. The mechanism of the polarizing energy transfer is
analyzed in depth in Chapter 4. By means of time-resohed fluorescence spectroscopy in
solutions as well as on oriented blend films, it is shown that the mechanism can be
adequately described by a Förster-type model based on long-range dipole-dipole
interactions. The influence of the particular phase behavior encountered in PL polarizers, in
23
which the two mutually miscible chromophores tend to be present in clusters in the
polymer matrix, on the expected and measured values of the critical Förster radius is
shown.
The previously investigated PL polarizers17 were produced through solution-casting
and tensile deformation of blends of a conjugated polymer with UHMW-PE. With a large-
scale production of PL polarizers in mind, this process has a few significant drawbacks, in
spite of the outstanding anisotropic optical properties which can be obtained. A solvent-
based process is not convenient for obvious reasons and the high draw ratios required to
obtain high optical anisotropics in the PL polarizers (>40) lead to an unclesircd fibrillar
structure in the highly oriented UHMW-PE films, that tend to easily split along the
orientation direction. Therefore, it is one objective of the present dissertation to explore a
more efficient production process, which ideally is based on standard melt-processing
techniques. In Chapter 5, the morphology and phase behavior of tensile-oriented films of
EHO-OPPE / ULIMW-PE blends are presented. A deformation-induced phase transition
from a phase-separated state in pristine, unoriented blend films to a near-molecular blend in
the oriented films was observed on a molecular scale with scanning confocal optical
microscopy (SCOM). It was thereby also shown that tensile deformation of phase-
separated polymer blends at elevated temperatures allows to produce stable molecular
blends of otherwise immiscible polymers. Based on the insights acquired through SCOM
regarding the phase-behavior of oriented polymer blends, a connection between these
observations made on the extremely dilute films used for SCOM investigations and the
anisotropic optica] properties observed in more concentrated films17'18 is established in
Chapter 6. The relations between the phase behavior in the PL polarizers and their
anisotropic optical properties are elucidated. It is shown how. through control of the phase
behavior, it is possible to maximize the anisotropy of the optical properties of PL
polarizers. In Chapter 7 it is demonstrated indeed that conventional melt-processing can
yield PL polarizers with extremely high optical anisotropies. The results presented in this
chapter show the important possibility to produce state-of-the-art PL polarizers through
simple and industrially accessible processing routes.
A separate effort described in this thesis addresses the EL properties of PPE-
derivatives. which were not thoroughly studied before and are commonly assumed to be
poor.1 • Polymeric LEDs are treated in the Chapters 8 and 9. In Chapter 8, single-layer
24
LEDs based on PPE-derivatives are presented. It is shown that bright PPE-based LEDs can
be produced and operated with an aluminum cathode, and misconceptions with respect to
the use of PPE-derivatives, including EHO-OPPE. as useful emitting layer in polymeric
LEDs73'95 are corrected. The results presented indicate good electron transport properties
for PPE. Subsequently, as discussed in Chapter 9, LEDs were prepared and characterized,
in which PPE is combined with a hole-transport and an electron-transport material. In these
devices, a substantial increase in the EL properties was experimentally observed, which is
due to the more balanced charge injection and transport properties in these LEDs which
combine the employed materials.
The present dissertation is presented as a collection of papers which have already
been published or have been submitted for publication:
Chapter 2: A. Montait. C. Bastiaansen, P. Smith, Ch. Weder, Nature, 1998, 392,
261.
Chapter 3: A. Montail. P. Smith, Ch. Weder, J. Mater. Sei., in press.
Chapter 4: A. Montali, O.S. Harms. A. Renn, Ch. Weder, P. Smith, IIP. Wild, Phys.
Chem. Chew Phys.. 1999, 1, 5697.
Chapter 5: W. Trabesinger, A. Renn, B. Hecht, U.P. Wild. A. Montali, P. Smith, Ch.
Weder, Science, submitted.
Chapter 6: A. Montali. A.R.A. Palmans, M. Eglin, Ch. Weder, P. Smith, W.
Trabesinger. A. Renn, B. Hecht, U.P. Wild, Macromol. Symp., in press.
Chapter 7: M. Eglin, A. Montali, A.R.A. Palmans, T. Tervoort, P. Smith, Ch.
Weder. J. Mater. Chem., 1999.9,2221.
Chapter 8: A. Montali. P. Smith. Ch. Weder. Synth. Met.. 1998, 97, 123.
Chapter 9: C. Schmitz. P. Posch, M. Thclakkat. H.-W. Schmidt, A. Montali, K.
Feldman, P. Smith. Ch. Weder, Adv. Mater, submitted.
25
1.6 References
1 R. Wizinger, J. Fontaine, Per. Dtsch. Chem. Ges.1927, 60. 1377.
2 H. Shirakawa, E.l. Louis, A.G. MacDiarmid. CK. Chiang, A.J. Heeger, Chem.
Commun., 1977.578.
3 Various contributions in Conjugated Polymers and Related Materials: The Intercon¬
nection ofChemical and Electronic Structure, Proceedings of the 81st Nobel Sym¬
posium, eds. W.R. Salaneck, 1. Lundström. B. Ranby. Oxford University Press,
Oxford, 1993
4 N.S. Sariciftci, D. Braun. C. Zhang, V.l. Srdanov, A.J. Heeger. G. Stucky, F. Wudl,
Appl. Phys. Lett., 1993. 62, 585.
5 N.F. Colanen, D.D.C. Bradley, R.H. Friend, P.E. Burn, A.B. Holmes, C.W. Span-
glev, Phys. Rev., 1990, B42, 11670.
6 J.H. Burroughes. D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, PL. Burn,
A.B. Holmes. Nature, 1990. 347, 539.
7 A.J. Heeger, P. Smith. Solution Processing ofConducting Polymers: Opportunities
for Science and Technology, in Cojfugated Polymers, eds. EL. Brédas, R. Silbey,
Kluwer Academic Publishers, Dordrecht, 1991.
8 R.H. Friend, R.W. Gymer, A.B. Holmes. J.H. Burroughes, R.N. Marks, C. Taliani,
D.D.C. Bradley, D.A. Dos Santos. EL. Bredas, M. Löglund, W.R. Salaneck, Nature,
1999,397, 121.
9 CDT, Seiko-Epson, CDT and Seiko-Epson demonstrate World'sfirst Plastic Televi¬
sion Screen, http://www.ccltltd.co.uk7seiko.htm
10 D. Braun, A.J. Heeger, Appl. Phys Lett.. 1991. 58, 1982.
11 M. Fahlmann. M. Löglund. S. Stafström. W.R. Salaneck, R.LI. Friend, P.L. Burn,
A.B. Holmes, K. Kacriyama. Y. Sonoda. O. Lhost, F. Meyers, EL. Bredas, Macro-
molecules, 1995,28. 1959.
12 N.C. Greenham, S.C. Moratti. D.D.C. Bradley. R.H. Friend. A.B. Holmes, Nature,
1993,365,628.
13 D.D.C. Bradley, Curr Opin. in Solid State & Mater Sei, 1996, 1, 789.
14 M.R. Andersson, M. Berggren, O. Inganäs. G. Gustafsson, J.C. Gustafsson-Carl-
berg, D.Selse. T. Lljertberg. O. Wennerström, Macromolecules, 1995, 28.7525.
15 G Grem, G. Lcditzky. B. Ullrich, G Leismg,, tdv. Mater, 1992, 4, 37.
16 L.S. Swanson. F. Lu, J. Shinar. Y.W. Ding. T.J. Barton, Synth. Met., 1993, 55, 1.
17 Ch. Weder, C. Sarwa, Bastiaansen, P. Smith..h/r. Mater, 1997, 9, 1035.
26
18 Ch. Weder, C. Sarwa, A. Montali, C. Bastiaansen, P. Smith, Science, 1998, 279,
835.
19 Ch. Weder, M.S. Wrighton. Macromolecules. 1996, 29, 5157.
20 T.M. Swager, C.J. Gil, M.S. Wrighton, J. Phys. Chem., 1995, 99, 4886.
21 R. Giesa, J.M.S. Rev. Macromol. Chem Phys., 1996, C36, 631.
22 TJ. Nelson, J.R. Wullert II, in Electronic Information Display Technologies. World
Scientific Publishing, Singapore, 1997.
23 Stanford Resources Inc.. Fiat Panel Monitrak,Market Report, http://www.stanfor-
dresource.com/
24 LA. Castellano, Stanford Resources Inc.. Display Markets: Facts and Fantasies,
Presentation at the Electronic Information Displays Conference, Esher, 1998.
25 A. Takeda, S. Kataoka, T. Sasaki, H. Chida, H. Tsuda, K. Ohmuro, Y Koike, T.
Sasabayashi, K. Okamoto, SID International Symposium. Digest ofTechnical
Papers. 1998,29, 1077.
26 D.J. Broer, J.A.M.M. van Haaren, P. van de Witte, C Bastiaansen, ACS Polym.
Prepr., 1999,40, 1149.
27 A.G. Verludst, G Cnossen. J. Fiinfschilhng and M. Schadt, SID Journal 1995, 3,
133.
28 M. Schadt, W. Helfrich, Appl. Phys. Lett.. 1971, 18, 127.
29 V.G Chigrinov, in Liquid Crystal Devices. Physics and Applications, Artec House.
Boston, 1999.
30 E.H. Land, J Opt. Soc. Am., 1940. 30, 230.
31 E.H. Land. J Opt. Soc. Am., 1951. 41, 957.
32 M. Grabau, J Appl Phys., 1938. 9.215.
33 CD. West. R. Clark Jones, J. Opt. Soc. Am., 1951, 41, 976.
34 Y. Dirix, Polarizers Based on Anisotropic Absorbance or Scattering ofLight, Ph.D.
Thesis, Technische Universiteit Eindhoven. 1997.
35 J.-Ph. Perez, in Optik. Spektrum Akademischer Verlag, Heidelberg, 1996.
36 M. Schadt, J. Fünfschilling, Jpn J. Appl. Phys.. 1990, 29, 1974.
37 DJ. Broer, J. Lub. G.N. Mol. Nature, 1995. 378, 467.
38 D. Coates, M.J. Gouldmg. S. Greenfield, J.M.W. Hanmcr, E. Jolliffe, S.A. Marden,
OL. Parri, M. Verrai 1 SID International Symposium. Digest ofTechnical Papers,
1996 27,67.
39 D.L. Wortman, SID International Symposium, Digest of Technical Papers. 1997,
28, 98.
40 Y Dirix, H. Jagt, R. Hikmet, C. Bastiaansen, J. Appl. Phys., 1998, 83, 2927.
27
41 W.A. Crossland, I.D. Springle, A.B. Davey, SID International Symposium, Digest of
Technical Papers, 1997', 28, 837.
42 G Baur, W. Greubel. Appl. Phys. Lett., 1977. 31, 4.
43 H.J. Coles, Liq. Cryst., 1993. 14, 1039.
44 A. Vecht. A.C. Newport, P.A. Bayley. W.A. Crossland, J. Appl. Phys., 1998, 84,
3827.
45 A. Gilbert, J. Baggott, in Essentials of Molecular Photochemistry, Blackwell Sci¬
ence, Oxford, 1991.
46 E. Lommel, Annalen der Physik und Chemie, 1879, 8, 634.
47 A. Jablonski. Acta Phys Polon.1934, A14. 421.
48 E.W. Thulstrup, J. Michl, J. Am Chem. Soc. 1982, 104. 5594.
49 V. Cimrova, M. Remmers, D. Neher. G Wegner, Adv. Mater, 1996, 8, 146.
50 A. Bologncsi. G Bajo. J. Palohcimo, T. Ostergard, LI. Stubb, Adv. Mater., 1997. 9.
121.
51 U. Lemmer, D. Vacar, D. Moses, A.J. Heeger, T. Ohnishi, T. Noguchi, Appl. Phys.
Lett, 1996,68,3007.
52 P. Dyreklev, M. Bcrggren, O. Inganäs, M. R. Andersson. O. Wennerström. T. Hjert-
berg,yl<7v. Plater, 1995, 7, 43.
53 M. Hamaguchi, K. Yoshino. Appl. Phys. Lett., 1995. 67, 3381.
54 K. Pichler. R.H. Friend. PL. Burn, A.B. Holmes. Synth. Met., 1993, 55-57. 454.
55 D. Fenwick, K. Pakbaz. P. Smith. J. Mat. Sei., 1996, 31,915.
56 G. Liissem, G. Festag, A. Greiner, C. Schmidt, C. Unterlechner, W. Heitz, ELI. Wen-
dorff, M. Hopmeier, J. Feldmann, Adv. Mater, 1995, 7, 923.
57 M. Eich, J. Wendorff, B. Reck, H. Ringsdorf. Makromol. Chem. Rapid Commun.,
1987,8,59.
58 M. Schadt, Mol. Cryst Liq. Crvst. Sect. A, 1997. 58, 1997.
59 V Shibaev, in Polvmers as Electrooptieal and Photooptieal Active Media, Applied
Science Publishers, London. 1996.
60 P.J. Phillips, Chem. Rcv. 1990. 90. 425.
61 H. Springer. R. Neuert, ED. Müller, G. Hmrichscn, Coll. & Polym. Sei., 1984, 262,
46.
62 T. Damerau, M Hennecke, J Chem. Phys., 1995.103, 6232.
63 EW. Thulstrup. J. Michl. J.H. Eggers, J Phys. Chem., 1970, 74. 3868.
64 P. Smith, P.J. Lemstra, J.P.L. Pijpcrs, A.M. Kick Coll. & Polym. Sei, 1981, 259,
1070.
65 P. Smith, P.J. Lemstra../ Mater. Sei. 1980. 15, 505.
66 T.W. Hagler, K. Pakbaz. K.E Voss, A.J. Heeger. Phys. Rev B, 1991, 44, 8652.
28
67 M. Pope, H. Kallmann, P. Magnante, J. Chem. Phys., 1963, 38, 2042.
68 C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett.. 1987, 51, 913.
69 PODIA, Press Release, May 18th. 1999. http://www.cdtltd.co.uk/podia.htm70 Philips Research, Press Release Archive No. 9802e, March, 1998, http://
www.research.philips.com/pressmedia/releases/9802e.html
71 UMAX Corporation, Press Release. April 6th.1999, http://uniax.com/
Press%20April%2099.htm
72 M. Grell, D.D.C. Bradley, Adv. Mater.. 1999,11, 895.
73 A. Kraft, A.C. Grimsdale. A.B. Holmes. Angew Chem. Lnt. Ed., 1998, 37, 403.
74 J. Kido, H. Shionoya, K. Nagai, Appl. Phys Lett, 1995. 67, 2283.
75 N. Tcsslcr, N. T. Harrison, R.H. Friend, Adv Mater, 1998, 10, 68.
76 UNIAX Corporation, Typical Display Performance, http://uniax.com/
77 G Gu, G Parthasarathy. S.R. Forrest, Appl Phys. Lett., 1999, 74, 307.
78 G Gustafsson, Y Cao, GM. Treacy, E Klavetter. N. Colaneri, A.J. Lleeger, Nature,
1992,357,477.
79 M. Grell, W. Knoll. D. Lupo, A. Meisel. T. Miteva, D. Neher, H.-G Nothofer, U.
Scherf, A. Yasuda, Adv Mater, 1999, 11.675.
80 C.-C Wu, EC Sturm, R.A. Register. J.Tian. E P. Dana, M.E. Thompson. IEEE
Transactions on Electron Devices. 1997, 44. 1269.
81 C Zhang, H. von Seggern, K. Pakbaz. B. Kraabel, H.-W. Schmidt, Synth. Met,
1994,62,35.
82 R.D. Scurlock, B.J. Wang. P.R. Ogilby, .1 R. Sheats. R.L. Clough, J. Am. Chem. Soc.,
1995,117, 10194.
83 K.Z. Xing, M. Fahlman, M. Logdlund, D A. Dos Santos, V. Parente, R. L,azzaroni,
J.L. Bredas, R.W. Gymer. W.R. Salaneck. Adv Mater., 1996, 8, 971.
84 P.E. Burrows, V Bulovic, S.R. Forrest. L S. Sapochak, D.M. McCarty, and M.E.
Thompson,^/?/. Phys Lett., 1994, 65, 2922.
85 S. Karg. EC Scott, J.R. Salem. M. Angelopoulos, Synth. Met., 1996, 80, 111.
86 N. Tessler, N.T. Harrison, D.S. Thomas, R.H. Friend. Appl. Phys. Lett., 1998, 73,
732.
87 T.R. Trumbo, CS. Marvel, J Polym. Sei Part A, 1987, 25, 1027.
88 R. Giesa. R.C Schulz, Makromol Chem.1990,191. 857.
89 D. Ofer, T.M. Swager. M.S. Wrighton. Chem Mater., 1995. 7. 418.
90 M. Moroni, J. LeMoigne. S. Luzzati. Macromolccules, 1994, 27, 562.
91 D. Steiger, P. Smith, Ch. Weder. Macromol Rapid Commun., 1997,18, 643.
92 CE. Halkyard, M.E. Rampey, L. Kloppeiiburg, S.L. Studer-Martinez, U.LI.E Bunz,
Macromolccules. 1998, 31. 8655.
29
93 S. Dellsperger, F. Dötz, P. Smith, Ch. Weder, Macromol. Chem. Phys., in press.
94 A.R.A. Palmans, M. Eglin, A. Montali, Ch. Weder, P. Smith, Chem. Mater, in press.
95 W. Chen, S. Ijada-Maghsoodi, T.J. Barton, T. Cercvenik, J. Shinar, ACS Polym.
Prepr., 1995,36,495.
31
2. Polarizing Energy Transfer in Photoluminescent
*
Materials for Display Applications
Abstract
Dichroic sheet polarizers together with color filters are used in numerous products
that make use of polarized, chromatic light.'"4 including color liquid crystal displayso 1
(LCDs) as the most important application.""" However, this combination converts a major
fraction of incident light into thermal energy,1'4 which limits brightness and energy
efficiency of these devices. Here, we report a new concept for polymer-based
photoluminescent (PL) polarizers which overcome this limitation and can be used in PL
LCDs with, in principle, an ultimate efficiency of unity. These PL polarizers comprise a
nearly randomly oriented sensitizer that maximally harvests light by isotropic absorption,
efficiently transfers the energy to a uniaxially oriented PL polymer which, subsequently,
emits highly linearly polarized light. Key step is the polarizing energy transfer which, to a
certain extent, mimics the concept used by nature in photosynthesis to optimally use
optical energy\
This chapter is reproduced from: A Montait. C. Bastiaanscn. P. Smith. Ch. Weder, Nature, 1998,
392.261.
32
The current limitations of LCDs recently have triggered the development of
polarizers based on selective reflection or scattering of light6"10 which can replace the
dichroic polarizer in a conventional LCD configuration. Using appropriate supplementary
elements to recycle reflected or scattered energy, the ultimate efficiency of these systems
can, in principle, approach unity (thus, twice that of dichroic polarizers); but in practice is
up to 80 %.6" However, color-applications require filters, which absorb at least two-
thirds of light and, thus, reduce the ultimate overall-efficiency to below 30 %. In another
recent approach, PL polarizers were demonstrated to efficiently combine the polarization
of light and production of bright colors1112 concomitant with a substantial increase in
brightness and efficiency of PL LCDs based on these elements.lj'14 These polarizers
comprise uniaxially oriented PL polymers, which, after photoexcitation, emit linearly
polarized light. Their efficiency is chiefly limited by the luminophore's quantum yield
which, ultimately, can approach unity, but for PL polymers typically is up to 80 %.15'16
However, when used in a standard PL LCD configuration,b only -50 % of light incident
from the light source is used, since the absorption of these PL polarizers is also
anisotropic.11'12 We now demonstrate materials which exhibit nearly isotropic absorption,
but emit the absorbed energy in highly polarized fashion, and, consequently, allow the
production of PL polarizers of ultimate efficiency. Such polarizers can directly replace the
standard polarizer in conventional LCDs and, using an appropriate (UV) backlight and a
dichroic mirror (to direct all emitted light towards the viewer), result in ultra-efficient,
colored PL LCDs. The thermoplastic character of these materials also allows the
production of pixilated structures with standard technologies, which enable multicolor
devices. While the reverse effect, i.e. PL depolarization, is well known,1749 the polarizing
energy transfer exhibited by the new materials is not only of technological relevance but
also manifests a new photophysical phenomenon.
The PL films we report here are based on uniaxially oriented, ternary blends of
ultra-high molecular weight polyethylene (UHMW PE). a poly(2,5-dialkoxy-/?-phenylene
ethynylene) derivative (EHO-OPPE)16 (2 % w/w), and 7-diethylamino-4-methylcoumarin
(DMC) (2 % w/w) as the sensitizer (Figure 1). The respective binary blends (LJHMW PE /
EHO-OPPE and ULIMW PE / DMC) were used as reference systems. Blend films were
prepared by solution casting of EHO-OPPE (10 mg), and / or DMC (10 mg), and LÏFIMW
PE (500 mg) from xylene (50 g) according to standard procedures.12 The resulting films
33
were uniaxially drawn at 120 °C to draw ratios of -80, yielding oriented PL films of a
thickness of 1 - 2 urn and of low optical density.
Fig. 1: Molecular structures of the poly(2.5-dialkoxy-p-phenylene ethynylene) derivative used
(EHO-OPPE, number-average molecular weight ~ 1 104 gmof1) and 7-diethylamino-4-
methylcoumarin (DMC, Aldrich).
DMC was selected as the sensitizer because of its low form-anisotropy, suitable
photophysical prerequisites and particularly beneficial phase-behavior. The melting
temperature of 74°C makes DMC compatible with the orientation process which requires
mobility of the guest molecules during deformation; in addition, DMC and EHO-OPPE
are miscible at elevated temperatures, which enables a most favourable morphology of the
oriented blends (see below). The absorption of DMC around 364 nm optimally overlaps
with the emission of common UV lamps that may be used as excitation source in PL
LCDs.13'14 Importantly, DMC seems not to quench emission of EHO-OPPE and,
mandatory for energy transfer." exhibits an own emission that favourably overlaps with
the absoiption of EHO-OPPE.
We investigated photophysical characteristics of PL films based on the ternary and
the binary reference blends, employing polarized LTV-VIS absorption and steady-state PL
spectroscopy. Since the absorption at 440 nm is exclusively related to EHO-OPPE and at
365 nm principally due to DMC, experiments were performed at these two wavelengths to
separately address the conjugated polymer and the sensitizer. (Note that different phonon
bands are observed for ELIO-OPPE for p- and ^-polarized light, making the dichroic
behavior wavelength-dependent, with maximum distortion at absorption maxima; to better
reflect the 'average' dichroic behavior, expressed by the division of the integrated
spectra,"
experiments to address EHO-OPPE were conducted at the empirically
determined wavelength of 440 nm).
34
300 400 500 600
X[nm]
Fig. 2: Polarized absorption spectra of
oriented films obtained with p- (solid
line) and s- (dashed line) polarized
light; recorded on a Perkin Elmer X
900; films were sandwiched with
silicon-oil between quartz slides for
all experiments to minimize light-
scattering at film surfaces; (see text
for a definition of s and p) (A)
Binary UHMW PE / DMC blend; (B)
Binary UHMW PE / EHO-OPPE
blend; (C) Ternary UHMW PE /
EHO-OPPE / DMC blend. UHMW
PE (weight-average molecular weight
~ 4105 gmof1) was from Hoechst
AG,
300 400 500 600
X [nm]
300 400 500 600
X fnml
Polarized absorption spectra, acquired with incident light polarized parallel (p) and
perpendicular (s) to the orientation direction of the films (Figure 2) show that the
characteristics of the ternary blend are a combination of those of the two respective binary
blends. The ternary blend exhibits high absorption dichroic ratios DRA (ratio between
absorption foxp- and s-polarized light) of up to 13 at 440 nm, resulting from a high degree
of orientation of EHO-OPPE. By contrast, the absorption at 365 nm is essentially isotropic
35
(DRa = 1.5) and reflects the nearly random orientation of the sensitizer within the oriented
UHMW PE matrix.
350
350
/I
),_450 550
X.[nm]
650
- jl ;
450 550
X [nm]
650
Fig. 3; Emission spectra of oriented films
obtained under isotropic excitation at
365 nm and polarized detection in p-
(solid line) and s- (dashed line) mode;
(A) Binary UHMW PE / DMC blend;
(B) Binary UHMW PE / EHO-OPPE
blend; (C) Ternary UHMW PE / EHO-
OPPE / DMC blend. Emission spectra
were recorded on a SPEX Fluorolog
F212; corrected PL intensities are
given in arbitrary units, however,
spectra compared in one graph always
have identical scale.
Polarized emission spectra, obtained under isotropic excitation at 365 nm and
polarized detection in either/?- or „v-mode are shown in Figure 3. In binary UHMW PE /
DMC films, the emission from DMC, centred around 400 nm, exhibits only minor
polarization, expressed by an emission dichroic ratio, DRE (ratio between the integrated
36
PL spectra inp- and .y-mode), of 2.3, consistent with the low degree of orientation of the
sensitizer. By contrast, the binary UHMW PE / EHO-OPPE films display state-of-the-art
1 ^
emission anisotropy"
(DRE= 27). In the ternary blend, importantly, the DMC emission is
almost fully suppressed, while the emission from EHO-OPPE is highly polarized (DRe =
16). The fact that DRE is somewhat lower in the ternary than in the binary UHMW PE /
EHO-OPPE blend is explained with a plastisizing effect of DMC on ELIO-OPPE that
reduces the efficiency of the orientation process.
Energy transfer from DMC to the conjugated polymer is evident when comparing
the emission intensities (related to EHO-OPPE) of the ternary and the binary ULÏMW PE /
EHO-OPPE blend (Figures 4A, 4B) for isotropic excitation at 440 and 365 nm,
respectively. The binary reference blend shows a significantly lower emission intensity
when excited at 365 nm compared to excitation at 440 nm, due to the much lower
absorption of EHO-OPPE at the shorter wavelength (Figure 2B). The ternary blend, by
contrast, shows similar emission intensities when excited at 365 and 440 nm, as a result of
the sensitizing effect of DMC: the effective, isotropic absorption of the sensitizer,
evidently followed by energy transfer to the conjugated polymer, is the obvious
rationalisation for the increased emission intensity. The polarizing characteristic of the
energy transfer is demonstrated by the results presented in Figure 4C. The intensity of p-
polarized emission from the ternary blend was found to be only weakly depending on the
polarization of the incident light (when excited at 365 nm). In fact, the ratio of the
emission intensities for excitation with s- and />polarized light (1.5) is in gratifying
agreement with the slightly dichroic absorption of the film at 365 nm (DRA = 1.5). Thus,
the ternary blend unambiguously exhibits the phenomenon of polarizing energy transfer:
optical energy is isotropically absorbed by DMC. with similar efficiency for both
absorption (excitation) polarizations transferred to EHO-OPPE, which subsequently emits
polarized light. In the most unfavourable limit (Figure 4 C) the new material converts fully
5-polarized into highly 77-polarized light.
37
Fig. 4: (A) and (B): Emission spectra of
oriented films obtained under isotropic
excitation at 440 (solid line) and 365
nm (dashed line), and polarized
detection in p-mode; (A) Binary blend
of UHMW PE / EHO-OPPE; (B)
Ternary blend of UHMW PE / EHO-
OPPE / DMC. (C): Emission spectra
of an oriented UHMW PE / EHO-
OPPE / DMC ternary blend film
obtained under polarized excitation at
365 nm mp- (solid line) and s- (dashed
line) mode and polarized detection in
_p-mode.
350 450 550 650
X fnm]
; 1 :
: jlv :
350 450 550
a. [nm]
650
The polarizing energy transfer process observed in the present PL materials is
schematically represented in Figure 5 A-C. Principally, the phenomenon may originate
from either radiative9' (trivial), long-range coulombic"' (Forster) or short-range electron-
exchange"" (Dexter) energy transfer between the DMC sensitizer as donor and the oriented
EHO-OPPE as acceptor. The fact that energy is transferred between donor molecules that
have been excited with A-polanzed light and acceptor molecules which subsequently emit
/»-polarized light implicates a depolarization of the donor excited state, unless Dexter-type
38
coupling is involved.20 This depolarization can derive from randomizing energy migration21
or orientational relaxation of the donor'7 and is indeed observed when exciting the binary
DMC reference blend with polari/ed light. The low optical density of the samples
essentially excludes a radiative energy transfer.
i = DMC = PPE = UHMWPE Polarizing Energy Transfer
A:®
§ m
§ f
90
B:,,'"
C:
-§ **
DMC*
————fS*"
DMC
PPE*
hv_
PPE
DMC*
•^tET-
hv'"mS*1
A ü---^^—-
hv,
DMC
+ hv. t
pp[T*
PPE
Fig.5: Schematic representation of the photophysical processes observed for uniaxially oriented
films of the binary reference blends of UHMW PL / DMC (A). UHMW PE / FHO-OPPE
(B), and the ternary UHMW PE / EHO-OPPE / DMC blend (C): Arrows indicate
polarizations of incident and emitted light
A nonradiative energy transfer might, on the other hand, point to a very particular
phase-behavior of the oriented blends. We have shown earlier for binary blends, that EHO-
OPPE forms an apparent molecular dispersion in the UHMW PE matrbc.1" Thus, the
incompatibility of DMC and UHMW PE. and the demonstrated affinity of DMC and EHO-
OPPE make the formation of DMC / EHO-OPPE aggregates very ükcly, in which a
nonradiative energy transfer is enabled by the close proximity of donor and acceptor
molecules. Further experiments, including time-resolved measurements and the
39
determination of quantum efficiencies, are in progress to develop of a full understanding
of the underlying mechanism.
As direct indication for the practical impact of DMC sensitization, we measured
the absolute brightness under isotropic excitation with a 365 nm LJV lamp, i.e., in a
configuration of relevance to actual PL LCDs.~"
The luminosity of a ternary blend film
is dramatically increased (82 cd/m2). compared to an unsensitized binary blend (22 cd/m")
of similar optical density in the EHO-regime. Of course, the absolute brightness can be
further enhanced by an increase in optical density.
In order to explore other compositions of this new class of multifunctional
materials, films were prepared containing an alternative sensitizer (7-[dimethylamino]-
2,3-dihydrocyclopenta{c}{l}benzopyran-4f lHjone) and an alternative acceptor (poly|2-
methoxy-5-[2'-ethyl-hexyloxy]-/>phcnylenevinylene] [MEH-PPV]).24 These systems
feature similar energy transfer characteristics as the present ternary blend and, thus,
demonstrate the versatility of the concepts outlined.
Acknowledgements
We thank E. J. Visjager for help in preparing the manuscript.
References
1 D.S. Kliger, J.W. Lewis, CE. Randall, in Polarized Light in Optics and
Spectroscopy, Academic Press, San Diego. 1990.
2 L.K.M Chan., in The Encyclopedia ofAdvanced Materials, Vol. 2, eds. D. Bloor,
R.J. Brook, M.C. Flemings, S. Mahajan. 1294-1304, Elsevier Science Ltd..
Oxford. 1994.
3 P.E Drzaic. in Liquid Crystal Dispersions, World Scientific Publishing,
Singapore, 1995.
4 T.J. Nelson. ER. Wullert 11, in Electronic Information Display Technologies,
World Scientific Publishing, Singapore. 1997.
40
5 R. Van Grondelle, Biochim. Biophysica Acta, 1985, 811, 147.
6 M. Schadt, J. Fünfschilling, Jpn. J. Appl Phys., 1990, 29, 1974.
7 D.J. Broer, J. Lub, G.N. Mol, Nature, 1995, 378, 467.
8 D. Coates, M.J. Goulding, S. Greenfield. J.M.W. Hanmer, E. Jolliffe, S.A.
Marden, OL. Parri, M. Verrall, SID International Symposium, Digest ofTechnical
Papers, 1996, 27, 67.
9 D.L. Wortman, SID International Symposium, Digest ofTechnical Papers, 1997,
28, in press.
10 Y. Dirix, Polarizers based on anisotropic absorbance or scattering oflight, Ph. D.
thesis, Technische Universiteit Eindhoven, Eindhoven, 1997.
11 T.W. Hagler, K. Pakbaz, J. Moulton, F. Wudl, P. Smith. A.J. Heeger, Polym.
Comm.. 1991,32,339.
12 Ch. Weder. C. Sarwa, C. Bastiaansen. P. Smith, Adv. Mater., 1997,9, 835.
13 Ch. Weder, C. Sarwa, A. Montali, C. Bastiaansen, P. Smith, Science, 1998, 279,
1035.
14 C.Weder, C. Sarwa, C. Bastiaansen, P. Smith, European Patent No. 97111229.7,
1997
15 N. Tessler, G.J. Denton, R. Friend. Nature, 1996. 382, 695.
16 Ch. Weder, M.S. Wrighton. Macromolecules, 1996, 29, 5157.
17 J. Guillet, in Polymer Photophysics and Photochemistry, Cambridge Univ. Press,
New York, 1985.
18 S.E. Webber, Chem. Rev., 1990. 90. 1496.
19 N.L. Vekshin, in Energy Transfer in Macromolecules, SPLE Optical Engineering
Press, Washington, 1997.
20 A. Gilbert, J. Baggott, in Essentials of Molecular Photochemistry. Blackwell
Science. Cambridge. 1997.
21 T. Förster. Ann. Phys .1948, 2. 55.
22 D.L. Dexter. J. Chem Phys., 1953, 21. 836.
23 R.S. Knox, in Theorv of Excitons, Academic Press, New York, 1963.
24 F. Wudl, G. Srdanov. US Patent No. 5189136, 1993.
41
3. Polarizing Energy Transfer in Photoluminescent
Polymer Blends
Abstract
The occurrence of polarizing energy transfer in uniaxially oriented polymer blend
films is investigated. A poly(2,5-dialkoxy-/?-phenylene ethynylene) derivative (EHO-
OPPE) and poly[2-methoxy-5-[2'-ethyl-hexyloxyl-p-phenylene vinylene] (MEH-PPV)
were used as the acceptors, and various sensitizers were used as donors. Some of the
properties of the chromophores required for polarizing energy transfer to occur efficiently
are elucidated, such as form-isotropy and thermal characteristics. The energy transfer
efficiency is quantified, and for the present, optimized systems, values as high as 85%
were demonstrated.
This chapter is reproduced from: A. Montali. P. Smith, Ch. Weder. J. Maler. Sa., in press.
42
Introduction
Recently, we introduced a new concept for photoluminescent (PL) colored liquid-
crystal displays (LCDs), in which one PL polarizer replaces the conventional polarizer and
color filter.1 These polarizers are based on oriented PL polymers, which upon
photoexeitation emit linearly polarized light. The color of the emitted light can be tuned
through chemical modification of the PL polymer.2"4 This approach can simplify device
design and substantially increase device brightness, contrast, efficiency, and (in specific
configurations) viewing angle. However, the anisotropic absorption of these polarizing
films allows the use of only about half the incident light, while the other half passes the
film.
To overcome this drawback, an isotropically absorbing dye was introduced into the
PL polarizer to act as a light-harvesting sensitizer.5 The sensitizer absorbs light
isotropically and subsequently transfers the energy to the oriented PL polymer, which
emits polarized light. In this polarizing energy transfer, the isotropic dye acts as a donor
and the PL polymer as an acceptor and emitter. These PL polarizer films are of a clearly
improved brightness and efficiency when compared to the reference (unsensitized) PL
polarizing films.
In our previous studies, a poly(2.5-dialkoxy-/?-phenylene ethynylene) derivative
(ELIO-OPPE)6 was used as the PL-polymer,1,7 and 7-diethylamino-4-methylcoumarin
(DMC) as the sensitizer.'1 The chromophores were dispersed in an ultra-high molecular
weight polyethylene (LJHMW-PE) matrix; orientation of these blend films was achieved
through tensile deformation at elevated temperatures.
We now demonstrate that the polarizing energy transfer is a more general
phenomenon, provided that appropriate materials are adequately combined, and we
quantify the efficiency of fhe polarizing energy transfer. In addition, some of the required
physico-chemical properties of the materials to be combined for the polarizing energy
transfer to occur efficiently, are elucidated.
Energy transfer in its general, unpolarized form is a well-known phenomenon.8"1' It
may principally originate from either radiative (trivial).8 long-range Coulombic (Förster),9
or short-range electron exchange (Dexter)10 energy transfer between a donor and an
acceptor. A resonance, or dipole-dipole energy transfer mechanism, in which energy is
43
transmitted radiationless by resonance interaction between chromophores, as was
described by Förster,9 is assumed to be relevant in the present case.12 The transfer rate kj
for a Förster energy transfer is usually described by:8,9.13
9000 In 10k„
128 7T3N(1)
Here, K is the orientation factor. <I>D the quantum efficiency of the donor, J the
overlap integral for a given donor-acceptor combination, Td is the radiative lifetime of the
donor, n is the refractive index of the solvent. R is the average distance between the
donor-acceptor pairs, and N is Avogadro's number. From equation I, it is evident that
efficient energy transfer requires optimal overlap of the donor emission- and the acceptor
absorption spectra, and a close proximity of the molecules involved.
Materials that should exhibit a polarizing energy transfer, in which light is
absorbed isotropically and emitted in a highly polarized fashion, thus, require a careful
choice of the chromophores, as well as an adequate preparation process that yields highly
oriented acceptor molecules and (ideally fully) disordered donor molecules in close
proximity.5
Of course, the acceptor molecules must be highly luminescent, and have a high
aspect ratio, in order to orient efficiently. The donor molecules on the other hand, should
be of isotropic geometry in order to avoid orientation and to assure maximal absorption of
isotropic excitation light. For the tensile-drawing orientation process applied in the present
work, both the donor and the acceptor are preferably in a highly mobile phase during
stretching. The latter, in the ease of UHMW-PE-based blend films, is carried out at about
120°C. Furthermore, the chromophores in the final oriented films evidently should not be
phase separated, to fulfill the above requirement of close proximity of the donor-acceptor
pairs.
44
Experimental method
Preparation and characterization of oriented photoluminescent films
EHO-OPPE and poly[2-methoxy-5-[2'-ethyl-hexyloxy]-p-phenylene vinylene]
MEH-PPV14 were synthesized, and oriented blend films were prepared according to the
procedures described previously.' The donors were purchased from Aldrich (DMC, C138,
CI52) and Fluka (BPO). BPEB was synthesized in our laboratories according to the
procedures described below. UHMW-PE (Hostalen GUR 412, ~MW~ 4T06 g/mol) was
obtained from Hoechst AG. Polarized LEV-VIS spectra were recorded with a Perkin-Elmer
Lambda 900 instrument fitted with motor driven Glan-Thompson polarizers. PL spectra
were recorded on a SPEX Fluorolog 3 (Model FL3-12), also fitted with motor driven
Glan-Thompson polarizers, and with a 450W Xe-lamp for excitation. The films were
sandwiched between two quartz slides, applying a minor amount of silicon oil in order to
minimize light scattering at the film surfaces. The remaining scattering effects were
compensated in the absorption measurements by subtracting the spectra of neat UHMW-
PE films of comparable draw ratio and thickness. The brightness of films was measured
using a Minolta LS100 luminance meter fitted with a close up lens 110; a commercially
available 4W UV-light source of 365 nm was used as excitation source.
Synthesis of l,4-Bis(phenylethynyl)-2,5-dioctyloxybenzene (BPEB)
Phenylacetylene (0.155 g. 1.52 mmol). l,4-dioctyloxy-2.5-diiodobenzene (0.434 g,
0.740 mmol, prepared according to Ref. [61), Pd(PPh3)4 (0.0425 g. 0.0368 mmol), and
Cul (0.0070 g, 0.037 mmol) were combined in a degassed mixture of absolute toluene (15
mL) and diisopropyl amine (7 mL). The reaction mixture was then stirred at 70°C under a
dry Ar atmosphere for 18 h. After the reaction mixture was cooled to room temperature, it
was passed through a 4 cm plug of silica gel using toluene as eluent. The evaporation of
the solvent led to an orange oil. which crystallized upon standing. Recrystallization from
ethanol (twice) yielded yellow crystals (0.247 g, 62 %); mp 75°C. !h NMR (250 MLIz.
45
CHC13) ô 7.50 (m, 4 H, ar), 7.33 (m, 6 H, ar), 7.00 (s, 2 H, ar), 4.01 (t, 4 H, OCH2), 1.83
(m, 4 H, CEL2), 1.53 (m, 4 H,CH2), 1.24 (m, 16 H, CH2). 0.85 (t, 6 H, C-CH3).
Results and Discussion
An overview of the chromophores used in this work, selected according to the
above requirements, is given in Table 1 and Figure 1, including their structural and
photophysical characteristics.
Name Function Tm [°Cf A-ahmaOnm]3 'Cm.max [nm]
EHO-OPPE acceptor Tg = 98;
325.485 495
MEH-PPV acceptor n.a. 505 570
DMC donor 74 365 400
C138 donor 152 365 407
BPO donor 139 306 368
BPEB donor 75 317,365 397
See Figure 1 for molecular structure.
2Tm= melting temperature; as indicated by the supplier; except BPEB which was
determined by differential scanning calorimetry (DSC).
Tg= glass transition temperature, determined by DSC.
3
Absorption maximum of blend films comprising 2% w/w of the chromophore in
UHMW-PE
4Emission maximum of blend films comprising 2% w/w of the chromophore in
UHMW-PE under isotropic excitation at the respective absorption maximum
Table 1: Physical properties of chromophores (donors and acceptors) used in this work in
uniaxially oriented blend films (draw ratio ~ 80; thickness ~ 2pm).
46
Both EHO-OPPE and MEH-PPV are highly luminescent polymers with a high
aspect ratio, have previously been shown to be compatible with gel-processing in UHMW-
PE, and to emit highly polarized light in the oriented blends.7'15
hi the series of selected
donors (cf. Table 1, Fig. 1), DMC has adequate spectral overlap with EHO-OPPE, a
processing-compatible melting temperature of 74°C and is nearly form-isotropic. C138
has the same favorable characteristics as DMC with the exception of a higher melting
temperature of 152°C. Thus, the absence of a melting point depression, and a less
homogeneous distribution of this donor in the blend films might be expected. BPO and
BPEB both show favorable spectral characteristics, but due to their slightly anisotropic
geometry some orientation may occur during tensile-orientation.
=/
EHO-OPPE MEH-PPV
o ^o
o^
0
DMC BPEB
0 ^0
C138
Fig. 1: Chemical structures of the chromophores used in this work.
47
Binary reference films of UHMW-PE / EHO-OPPE or MEH-PPV, and ternary
films of UHMW-PE / EHO-OPPE or UHMW-PE / MEH-PPV and the donors listed in
Table 1, were produced by gel-processing, and subsequent tensile deformation at 120°C to
draw ratios of 70-80 according to the procedures described in detail before. During
drawing, the macromolecules are chain-extended and uniaxially oriented to a degree of
structural order that approaches that of a single crystal.1 A similar degree of orientation is
induced on conjugated polymeric guests incorporated in the blend,7'15 but not necessarily
on the form-isotropic donor molecules employed in the present studies.
All films were characterized with polarized and unpolarized UV-VIS absorption
spectroscopy, as well as with polarized and unpolarized steady-state fluorescence
spectroscopy. In order to quantify the anisotropic optical characteristics of the drawn
films, we determined the dichroic ratios, defined for absorption (DRabs) and emission
(DRem) as the ratio between the respective spectra measured with polarization parallel (p-)
and perpendicular (s-) to the drawing direction. In our determination of the dichroic ratio
in emission, we integrated the spectra, because the integrals are directly related to the
energy of the relevant electronic transitions and, hence, reflect the underlying physical
processes best. The dichroic ratio in absorption was determined using the values measured
at a single relevant wavelength; usually at the wavelength which was used for PL
excitation in the emission experiments.
300 400 500
a) Xex [nm]
600
c
CD
Q.
300
b)
400 500
X [nm]fix L J
600
Fig. 2: Photoluminescence (PL) excitation scans (unpolarized) of all oriented blend films
investigated (draw ratio ~ 80; thickness ~ 2pm); comprising ULIMW-PE, 2% w/w of the
donors indicated, and 2% w/w EHO-OPPE (2a) or MEH-PPV (2b). respectively.
48
The concentration of each chromophore in the blend films was 2% by weight,
unless indicated otherwise; all concentrations in this work are given in weight %. The
optical density of all investigated samples was between 0.04 and 0.2; the thickness of the
films was about 2 |im.
To demonstrate the occurrence of energy transfer in the various oriented blend
films, PL excitation spectra were recorded under isotropic excitation and detection.17 The
detection wavelength was chosen in a range where the donors show no emission: 580 nm
in the case of DMC, C138. and BPO; and 620 nm for BPEB. Hence, the detected signal is
virtually exclusively generated by acceptor emission.
All investigated ternary blends show maxima of acceptor emission when excited at
wavelengths at which the donor has its prominent absorption band. This behavior clearly
contrasts that of the reference binary films (Fig. 2) and demonstrates the occurrence of an
energy transfer process for all ternary films in the present study.
The orientation of the chromophores in the drawn blend films was investigated
with UV-VIS absorption experiments conducted with light polarized parallel and
perpendicular to the drawing direction (Table 2). Oriented films comprising DMC or C138
show nearly isotropic absorption at their absorption maximum of 365 nm (DRabs =1.7 and
1.5. respectively), indicating near isotropic orientation of these sensitizer molecules.
Drawn blends comprising BPO and BPEB, which both have a higher molecular aspect
ratio, exhibit dichroic ratios of 3.8 and 3.3 at the respective absorption maxima of 306 and
365 nm, which is indicative of non-negligible uniaxial order of these sensitizers.
In contrast to the sensitizers, the uniaxially oriented acceptors exhibit absorption
dichroic ratios (DRabs) in excess of 25 at their maximal absorption wavelength, reflecting
state-of-the-art uniaxial order. These values demonstrate that the orientation process of the
UHMW-PE matrix and of the acceptor molecules only marginally affects the donors.
When isotropically excited at the wavelength where the donor exhibits an
absorption maximum, all films displayed highly polarized emission of the acceptor as
evidenced by the dichroic ratio in emission (DRcm) around 20. Clearly, the observed
emission is largely due to acceptor emission, while donor emission is highly reduced due
to the (polarizing) energy transfer to the acceptor.
The polarizing nature of the energy transfer was demonstrated by exciting a ternary
blend film with;?- and s-polarized light in the absorption range of the donor, and detecting
49
the /»-polarized emission from the acceptor.3 This is shown in Figures 3a and 3b for an
oriented ternary EHO-OPPE / C138 / UHMW-PE blend film excited at 365 nm (3a) and
440 nm (3b), respectively. It can be clearly seen that the intensity of/^-polarized emission
by the acceptor is only weakly dependent on the polarization of incident light, when
exciting at the absorption maximum of the donor (Fig. 3a). In fact, the ratio of the
emission intensities for excitation with/?- and .v-polarized light (1.6), is in good agreement
with the slightly dichroic absorption of the film at this wavelength (DRabs=l.5). In
contrast, in the case of directly exciting the acceptor, ^-polarized excitation leads to a
substantially lower/»-polarized emission, than excitation with/»-polarized light, because of
the anisotropic absorption of the acceptor (Fig. 3b). The fact that energy is transferred
between donor molecules that have been excited with s-polarized light and acceptor
molecules, which subsequently emit /»-polarized light, implies a depolarization of the
donor excited state, if the occurring transfer follows a Förster mechanism.8 This
depolarization could originate in randomizing energy migration between donor
molecules13 or orientational relaxation of the latter.18
I i ! , 1 i I 1 1 j I , L_
400 500 600 400 500 600
a) *.em[nm] b) Xm [nm]
Fig. 3: Emission scans under/»-polarized (solid lines) and s-polarized (dotted lines) excitation at
365 nm (3a) and 440 nm (3b) and p-polarized detection for an oriented UHMW-PE / 2%
w/w EHO-OPPE / 2% w/w C 138 blend film (draw ratio ~ 80; thickness ~ 2u.m).
50
The efficiency of the energy transfer, Ot, was determined for the present materials
according to the relationship derived by Dale and Eisinger:19
cpr =
abs„
absd
emat!!
em„
(2)
Here, absa and absd denote the absorption of the acceptor and donor, respectively, at the
excitation wavelength; ema(i and era,, are the emission intensities of the acceptor excited in
the presence (emad) and in the absence (ema) of the donor, respectively. The energy
transfer efficiencies for the investigated ternary blends are listed in Table 2. These data
clearly demonstrate that in all materials energy transfer occurred with high efficiency.
The observed energy transfer efficiency appears to be related to certain physico-
chemical properties of the chromophores used: DMC combines a set of desired
photophysical properties, with a melting temperature that is below the applied stretching
temperature of the films. In addition, DMC and EHO-OPPE are miscible at elevated
temperatures, which allows for close proximity of the donor and acceptor. This
combination of properties results in a very high energy transfer efficiency of
approximately 73%, and in the nearly complete quenching of DMC emission; the residual
donor emission accounts for only about 2% of the total emitted energy.
A reduced energy transfer efficiency (47%) was observed when C138 was
employed as the donor; this, despite its favorable photophysical properties. We attribute
this finding to the relatively high melting temperature of C138, which is well above the
drawing temperature. Under these conditions, the CI38 domains in the blend film are not
expected to be in the highly mobile phase that is required for ideal mixing of the donor
and acceptor molecules. As a result, the average distance between donor and acceptor
moieties is likely to be substantially larger than in the previous (DMC) case. A higher
amount of donor emission (~ 9%) is consistent with this view.
BPEB and BPO both exhibit suitable photophysical properties for energy transfer
in PPE-based PL polarizing films. Energy transfer does occur in these systems, but a
lower transfer efficiency, and a higher amount of donor emission was observed, i.e.
around 25%. This is due to a less favorable phase behavior of the donor / acceptor
combinations. For example. BPO is only poorly miscible at elevated temperatures with
51
EHO-OPPE, as opposed to DMC. Furthermore as mentioned above, due to the slightly
higher aspect ratio of these two sensitizers, some alignment during stretching of the blends
occurred, as is evidenced by DRabS of the respective films; which hinders the efficient
absorption of excitation light of all polarizations. Thus, in these two particular cases, the
polarizing nature of the transfer can not be optimal.
Blend'
DR,bi
@ 365 nm2
DRem Donor
Emission4
oV Brightness
[cd/m2]6
EHO-OPPE 5.2 20 - - 27
EHO-OPPE +
DMC
1.7 19 J 0.02 0.73 78
EHO-OPPE +
C138
1.5 19 0.09 0.47 40
EHO-OPPE +
BPO
3.87
207
0.287
n.a. 41
EHO-OPPR +
BPEB
3.3 16 0.22 0.49 41
MEH-PPV 3.5 28 - n.a.
MEH-PPV
+ DMC
1.7 17 0 16 0.49 n.a.
AH blends comprise ULIMW-PE and 2% vv/w of the indicated chromophores
2Dichroic ratio in absorption (DRabs); in all samples DRabs is >25 at the absorption
maximum of the acceptor (EHO-OPPE = 484 nm, MEH-PPV = 508 nm).
Dichroic ratio in emission; isotropic excitation at 365 nm
4Relative to the total emitted energy, isotropic excitation at 365 nm and isotropic
detection
1
Energy transfer efficiency calculated according to equation 2
6
Isotropic excitation at 365 nm
7At 306 nm
Table 2. Emission and absorption characteristics of uniaxially oriented blend films (draw ratio ~
80; thickness ~ 2pm).
52
For different acceptors, the same concepts described so far for maximal polarizing
energy transfer, apply. For example, MEH-PPV shows favorable phase behavior with
LJHMW-PE and DMC. Indeed, as can be seen from the data in Table 2, oriented UHMW-
PE / MEH-PPV / DMC films display the desired phenomenon. The lower transfer
efficiency, when compared to the DMC / EHO-OPPE system, is most likely related to the
bathochromie shift of the absorption spectrum of MEH-PPV, compared to EHO-OPPE,
which results in a reduced overlap integral with DMC.
100|
. , 1 1 . , 1
80 - ...-•*'*
-
.*•'
g 60-
40 -
0 i l 1 , 1 . . 1
400 500 600 0 12 3
a) A.[nm] b) EHO-OPPE [% w/w]
Fig. 4: a) Isotropic emission scans of tensile-oriented blend films (draw ratio ~ 80; thickness ~
2pm) with different acceptor and donor concentrations (in % w/w) as shown, recorded
under isotropic excitation at 365nm.
b) Transfer efficiency, Or, vs. EHO-OPPE concentration of oriented blend films of a
constant DMC concentration of 1% w/w (draw ratio ~ 80; thickness ~ 2|xm).
The absolute brightness, under isotropic illumination, is of significant importance
when considering applications of fhe investigated films in. e.g., LCDs. The brightness was
measured for selected films under isotropic excitation at 365 nm. All films analyzed
contained the same concentration of emitter and were of similar thickness. The results are
shown in Table 2. The films that contained sensitizer were significantly brighter than the
corresponding binary (reference) blend comprising the conjugated polymer only. The
observed brightness appears related to the values for the transfer efficiency; with films
displaying a less efficient transfer being less bright. It should be noted, however, that the
brightness naturally depends also on other factors, such as the extinction coefficient of the
sensitizer, etc.; and, therefore, the above suggested correlation with <Pt may be somewhat
53
fortuitous. Of course, the absolute brightness of the films can readily be further improved,
by increasing their optical density. For such thicker films (a sample of ~ 8 mil thickness,
draw ratio of ~ 80, 2% EHO-OPPE and 2% DMC was investigated) a brightness as high
as 275 cd/m2 was obtained.
DRa|„ DReiy <E0 Brightness [cd/m2]4
3% EHO-OPPE
+ 1%DMC
2.4 21 0.85 56
2% EHO-OPPE
+ 1 % DMC
1.8 18 0.82 43
1% EHO-OPPE
+ 1%DMC
1.5 21 0.68 38
0.25% EHO-OPPE
+ 1%DMC
1.6 19 0 23 n.a.3
0.05% EHO-OPPE
+ 1%DMC
1.8 n.a."*
0 15 n.a.
2% EHO-OPPE 5.2
>25 @ 484 nm
20 27
2% EHO-OPPE
+ 2% DMC
1.7 19 0.73 78
2% ELIO-OPPE
+ 4% DMC
1.3 10 0 6 120
Dichroic ratio in absorption at 365 nm.
Dichroic ratio in emission; isotropic excitation at 36^ nm.
Energy transfer efficiency calculated according to equation 2.
4
Isotropic excitation at 365 nm.
DRcm and brightness could not be determined with sufficient accuracy for these
samples, since the acceptor concentration is low, and emission spectra are
predominantly governed by donor emission.
Table 3: Emission and absorption characteristics of uniaxially oriented blend films (draw ratio ~
80; thickness ~ 2u.m) of different donor and acceptor concentrations.
54
Finally, we investigated the influence of the donor and acceptor concentrations in
the UHMW-PE / EHO-OPPE / DMC system. Blend films with a constant DMC
concentration of 1% w/w and EHO-OPPE concentrations of 0.05%, 0.25%, 1%, 2% and
3% w/w, respectively, were prepared and characterized. At increased EHO-OPPE
concentration, the emission gradually changed from DMC to EHO-OPPE emission (Fig.
4a). At intermediate concentrations the emission spectra appeared to be a linear
combination of both the DMC and the ELIO-OPPE spectra. Under the employed
experimental conditions, the transfer efficiency was found to increase at higher ELIO-
OPPE concentration, and to level off at a value of approximately 80% at acceptondonor
weight ratios exceeding about 1 (Table 3, Fig. 4b), which corresponds to about 3 DMC
molecules per polymer repeat unit (cf. Fig. 1).
Similarly, films with a constant concentration of EHO-OPPE of 2% and different
concentrations of DMC (1%, 2% and 4%) were also prepared and characterized. The
results of these experiments are shown in Table 3. These oriented blend films exhibited an
analogous emission behavior as the films described above; the EHO-OPPE emission was
found to increase with the DMC concentration, and the DMC emission was virtually
completely quenched in these latter samples. As expected, the brightness increased at
increasing DMC concentration in the blends from 27 cd/m2 for the reference UHMW-PE /
2% EHO-OPPE blend to 120 cd/m2 for the blend film that contained 4% DMC. However,
the presence of a high concentration of DMC in the films led to a reduced polarization of
the emitted light. We attribute this finding to a highly increased (DMC-induced) mobility
of EHO-OPPE, which reduced the orientation efficiency of the latter. Furthermore, the
efficiency of the energy transfer was reduced to 60 % in the case of the UHMW-PE / 2%
EHO-OPPE / 4% DMC blend film. Thus, it appears that for the present system, the
optimal composition is 2% ELIO-OPPE / 2% DMC m ULIMW-PE. This particular
material offers an optimized combination of high energy transfer efficiency, high
brightness, near-isotropic absorption and highly polarized emission.
55
Conclusions
In summary, we have shown that polarizing energy transfer occurs for several
combinations of donor and acceptor molecules, provided that they exhibit a number of
appropriate physico-chemical and photophysical characteristics. In this work, some of the
required properties for polarizing energy transfer to efficiently occur are elucidated. For
example, the importance is demonstrated of form-isotropy, thermal characteristics, and
concentration of the donors. For the present, optimized materials systems, polarizing
energy transfer efficiencies as high as 85% were demonstrated.
AcknowledgementsH*
The authors thank Dr. A. Palmans for many fruitful discussions. S. Dellsperger for
the synthesis of EHO-OPPE, and S. Amhof for assistance in photophysical measurements.
References
1 Ch. Weder, C. Sarwa, A. Montali, C. Bastiaansen, P. Smith. Science, 1998, 279,
1035.
2 A. Kraft, A.C. Gnmsdale, A.B. Holmes, Angew. Chem. Int. Ed. 1998, 37, 402.
3 O. Inganäs. M. Berssren. M.R. Andersson, G. Gustafsson. T. Hiertberg, O.
Wennerström, P. Dyreklev, M. Granström, Synth. Met, 1995, 71, 2121.
4 EL. Brédas, Adv. Mat, 1995. 7. 263.
5 A. Montali. C. Bastiaansen, P. Smith. Ch. Weder, Nature, 1998, 392, 261.
6 Ch. Weder, M.S. Wrighton, Macromolecules, 1996. 29, 5157.
7 Ch. Weder. C. Sarvv a, C. Bastiaansen, P. Smith. Adv. Mater.. 1997, 9, 835.
8 A. Gilbert, J. Baggott. in Essentials of Molecular Photochemistry, Blackwell
Science, Cambridge. 1997.
9 T. Förster, Ann. Phvs. 1948. 2. 55.
10 D.L. Dexter, .7 Chem Phvs., 1953. 21, 836.
56
11 S.E. Webber, Chem. Rev., 1990, 90, 1496.
12 Experimental proof that the observed energy transfer is indeed a Förster-type
energy transfer was recently obtained through time resolved spectroscopy; these
results were published in: Phys. Chem. Chem. Phys.. 1999,1, 5697.
13 B.W. Van Der Meer, G. Coker III, S.-Y. Chen, in Resonance Energy Transfer,
VC1I, New York, 1994.
14 F. Wudl, G. Srdanov. US Patent No. 5189136. 1993.
15 T.W. Hagler, K. Pakbaz, J. Moulton, F. Wudl. P. Smith. A.J. Heeger, Polym.
Comm., 1991,32,339.
16 P. Smith. P. Lemstra, J.P.L. Pijpers, A.M. Kiel. Coll. Polym. Sei, 1981, 259, 1070.
17 N.L. Vekshin, in Energy Transfer in Macromolecules, SPLE Optical Engineering
Press, Washington, 1997.
18 J. Guillet, in Polymer Photophysics and Photochemistry, Cambridge Univ. Press,
New York, 1985.
19 R.E. Dale, .1. Eisinger, in Biochemical Fluorescence: Concepts, Vol. 1. eds. R.F.
Chen, H. Edelhoch, Marcel Dekker Inc., New York, 1975.
20 The values for absa and absci in the ternary blend films were determined by
subtracting the scaled absorption spectrum of a binary reference blend film from
the spectrum of the former. ema was determined on binary reference blend films
comprising only the conjugated polymer, and scaled to the respective absa of the
ternary blend films. This method of evaluation was applied after verifying that the
quantum efficiency of the acceptor (when exciting directly its absorption band at
440nm) is similar for the reference binary and the ternary blend films.
57
4. Time-Resolved Fluorescence Study on the Mecha¬
nism of Polarizing Energy Transfer in Uniaxially
Oriented Polymer Blends
Abstract
A time-resolved study of polarizing energy transfer in oriented blends of a conjugat¬
ed polymer (a dialkoxy substituted poly Op-phenylene ethynylene) derivative) and an organ¬
ic laser dye (7-diethylamino-4-methylcoumarin) in ultra-high molecular weight
polyethylene is presented. The transfer is described in terms of a Förster mechanism, based
on long-range dipole-dipole interactions. Förster radii were determined in oriented blend
films as well as in chloroform solutions. It was found that the transfer process is critically
influenced by the phase behavior of the system under investigation. A depolarizing ho-
motransfer between donor molecules was found to be a key step in the polarizing nature of
the transfer which, ultimately, allows excitation light polarized perpendicularly to the film
orientation direction to be emitted with the polarization direction parallel to its orientation.
* This chapter is reproduced from: À. Montali, G S. Harms. A. Renn. Ch. Weder. P. Smith,U.P. Wild. Pins. Chem Chem. Phys., 1999. 1. 5697.
58
Introduction
Recently, we introduced a new concept for photoluminescent (PL) colored liquid-
crystal displays (LCDs), in which one PL polarizer replaces the conventional polarizer and
color filter.1 These polarizers are based on oriented PL polymers, which upon photoexeita¬
tion emit linearly polarized light along their orientation axis. This approach can simplify de¬
vice design and substantially increase device brightness, contrast, efficiency, and (in
specific configurations) viewing angle. However, the anisotropic absorption of these polar¬
izing films allows the use of only about half the incident light, while the other half passes
through the film.l
To overcome this drawback, an isotropically absorbing dye was introduced into the
PL polarizer to act as a light-harvesting sensitizer." The sensitizer absorbs light isotropically
and was found to subsequently transfer the energy to the oriented PL polymer, which emits
light polarized parallel to its orientation; and, thus, we refer to this process as "polarizing"
energy transfer.2 The envisioned application of such oriented polymer blend films as PL po¬
larizers in LCDs is made possible by the specific exploitation of the polarization dependence
encountered in the energy transfer.
In previous studies, we investigated oriented blend films of a poly(2,5~dialkoxy-/>
phenylene ethynylene) derivative (EHO-OPPEE as the emitter,1, ' and 7-diethylamino-4-
methylcoumarin (DMC) as the sensitizer," dispersed in an ultra-high molecular weight poly¬
ethylene (UHMW-PE) matrix. Energy transfer efficiencies of up to 85% were observed in
oriented blend films comprising 3% w/w EHO-OPPE and 1% DMC.'*'
Here, we present a detailed investigation regarding the mechanism of energy transfer
in the EHO-OPPE / DMC system. From the determined fluorescence lifetimes and steady-
state fluorescence measurements at different emitter concentrations in LJHMW-PE blend
films and chloroform solutions we conclude that long-range dipole-dipole interaction is in¬
volved in the transfer mechanism in which the sensitizer functions as a donor and the emitter
as an acceptor. Furthermore, the polarizing nature of the transfer, where, in the limit, per¬
pendicularly polarized excitation light is re-emitted with parallel polarization, is explained
in terms of fluorescence depolarization through homotransfer between donor molecules.
59
Experimental
Samples were prepared using commercially available UHMW-PE (Hostalen GLJR
412, Mw~ 4T06 g/mol, Hoechst AG), and DMC (Aldrich). EHO-OPPE was synthesized as
previously described. Films of 1-2 pan thickness were prepared by gel-casting and subse¬
quent tensile deformation at 120-130°C to about 70 times their initial length, as previously
described.4 The DMC (donor) concentration was held constant at 1% w/w in all films, while
the EHO-OPPE (acceptor) concentration was varied from 0.01 to 1% w/w. Additionally, bi¬
nary blend films containing only DMC (1% w/w) or ELIO-OPPE (1% w/w) in UHMW-PE
were prepared as reference samples. The same chromophore combination was investigated
in chloroform solutions of identical weight concentrations. The chemical structure of the
dyes employed in this study is shown in Fig. 1.
Fig. 1: Chemical structure of the substances employed in the present study, 7-diethylamino-4-meth-
ylcoumarin (DMC) as donor (a), and a dialkoxy substituted poly (/>-phenylene ethynylene)
derivative (EHO-OPPE) as acceptor (b).
Polarized UV-VIS spectra were recorded on a Perkin-Elmer Lambda 900 instrument
fitted with motor-driven Glan-Thompson polarizers. Steady-state PL spectra were recorded
on a SPEX Fluorolog 3 (Model FL3-12), fitted with motor-driven Glan-Thompson polariz¬
ers, and employing a 450W Xe-lamp for excitation. Fluorescence spectra of solid samples
were detected in front-face mode, at an angle of 22.5° from the excitation beam; fluores¬
cence spectra of solutions were measured in a standard 90° cuvette geometry. Correction of
all spectra for instrumentation influences and scattering effects were performed according
to the procedures previously described.' ^
Steady-state PL spectra at cryogenic temperatures were recorded under excitation
from a 2500W Xe-lamp; the fluorimeter was fitted with SPEX 1402 double monochroma-
60
tors in both, the excitation and the detection beam path; the sample photoluminescence was
collected in the front-face mode and detected with a photomultiplier cooled to 243K (R2949,
Hamamatsu); the sample was held in a continuous flow cryostat (CF1204, Oxford Instru¬
ments) with dynamic gas exchange cooled to a temperature of 1 IK with liquid helium.
Time-resolved data were recorded by time-correlated single photon counting (TC-
SPC); the essential method and procedure has been published elsewhere.7 The excitation
source utilized was a dye laser (Coherent Cr-599-04, LJSA) synchronously pumped from the
second harmonic output of a pulsed Ncl:YAG laser (Coherent Antares 76-S, USA). The dye
utilized was DCM (Radiant Dyes Chemie. Germany) dissolved in ethylene glycol. The puls¬
es produced were 5 ps at 680 nm and were frequency doubled to 340 nm by a BBO crystal
(CASIX, China). The instrument response function (1RF) was also measured at 340 nm, us¬
ing a scattering sample to deconvolve the IRF from the actual fluorescence signal. The pro¬
cedure for the fitting is found in ref. 7 and was here modified for the stretched exponential
fitting described below.
Results and Discussion
Within the framework of a Förster-type energy transfer process,8 overlap is required
between donor emission and acceptor absorption spectra, the presence of which is demon¬
strated for the DMC / EHO-OPPE system in Fig. 2. The overlap integral, O, is given in
wavelength form as:
CO
Q = JfD(X)£A(Z7)X4cb\ (Eq. 1)
0
Here, X is the wavelength of light in [nm], eA(À) is the molar extinction coefficient
of the acceptor at that wavelength in [(M cm)"1!, and fD(A,) is the donor fluorescence spec¬
trum normalized on the wavelength scale according to:
CO
jfDa)dX = 1 (Eq.2)
0
61
The overlap integral was determined from isotropic fluorescence emission and ab¬
sorption spectra of DMC and EHO-OPPE, respectively; the chromophores were either dis¬
solved in chloroform or in the respective oriented UHMW-PE blend films at concentrations
of 1% w/w. eA(X) of solid film samples was determined assuming a homogeneous chro¬
mophore concentration and a constant film thickness over the entire sample. The overlap in¬
tegral in oriented UHMW-PE blends was determined to be LI 10~12 [cm3 M"1!. This value
is in well agreement with the overlap integral m chlorofonu solutions, which was found to
be 8.5 HT13 [cm3 NT1].
i r—i | 1
'''"""' A
zi-
CTS
H—'
CO
c
CD
c-
-
_]
Q_
-
, 1 ...1 1 I.. .
, I . .1 , 1 JL
>cro>
o
c3~
oCD
300 350 400 450 500
X [nm]
550 600
Fig. 2: PL emission spectrum of DMC (dashed line) and absorption spectrum of EHO-OPPE (solid
line); both spectra were measured in oriented blend films of UHMW-PE and 1% w/w of the
respective chromophore.
The actual occurrence of energy transfer is demonstrated by a fluorescence excita¬
tion scan of an oriented ternary EHO-OPPE / DMC / UHMW-PE blend film, with detection
at 580 nm (Fig. 3). Here the donor. DMC, shows no emission and, hence, the detected signal
is virtually exclusively generated by acceptor, EHO-OPPE, emission. The "sensitized"
blend shows a maximum of acceptor emission when excited at wavelengths at which the do¬
nor has its prominent absorption band. This behavior clearly contrasts that of the "unsensi¬
tized" film and reveals the occurrence of enersv transfer.
62
3
>^
'coc
CD
300 350 400 450
X [nm]
500 550
Fig. 3: PL excitation scans (unpolarized) of an oriented blend film (draw ratio ~ 70; thickness ~
2pm) comprising UHMW-PE, 1% w/w DMC and 1% w/w EHO-OPPE (solid line); and of
a reference film (same draw ratio and thickness) comprising UHMW-PE and 1% w/w EHO-
OPPE without donor. DMC (dashed line).
i ' i- ' —r-
a DMC
' 1
EHO-OPPE
I 0
1 0.02
1—p- 0.1
-J— ------ i
1
>>
"wc
3 • ,y J0 1
1- ;
\ F .''
'. ^-^ \ „
Q.
--
ß^^ '-'cij'
' -'-'.~ "- y^~-—~
> 1i
400 500
X [nm]
600 700
Fig. 4: Steady-state PL emission scans of oriented blend films (draw ratio ~ 70; thickness ~ 2pm)
comprising UHMW-PE, 1% w/w DMC and the shown concentrations of EHO-OPPE [%w/
wl; excitation at 340 nm.
A gradual transition from the emission spectrum of the neat donor to a spectrum
dominated by the emission of the acceptor can be observed in steady-state experiments (Fig.
4) for a series of oriented blend films in which the acceptor concentration was systematically
increased. We should point out that the shape of the emission as well as absorption spectra
63
of both, donor and acceptor, appear similar in all oriented blend films, suggesting the ab¬
sence of excimer formation. For the same samples, a sharp acceleration in the decay of donor
fluorescence can be observed concomitantly (Fig. 5). This observation is inconsistent with,
and, thus, rules out the trivial possibility that the energy transfer occurs through a mecha¬
nism in which the acceptor molecules are excited by the actual re-absorption of photons that
have been emitted by the primarily excited donor molecules.8'9 Such a process would not
affect the decay of donor excited states, thus causing donor decay times to be unchanged
upon doping with an acceptor. This trivial energy transfer can further be excluded because
of the low optical densities (<0.15) of all investigated samples. An overview of donor decay
times (weighted mean of multiexponential decay fits of the donor excited state decays, de¬
tected at 430 nm) for different film compositions is given in Table I.
i—.—i 1 i i _i i i . ,i i
0 2 4 6 8
time [ns]
Fig. 5: Fluorescence decay curves of DMC in oriented UHMW-PE blend films (draw ratio ~ 70;
thickness ~ 2pm) comprising 1% w/w DMC and the indicated concentrations of EHO-OPPE
[% w/w]; excitation at 340 nm, detection at 430 nm. The data were fitted (solid lines) to Eq.
8. The instrument response (ERE) is shown as a dotted line.
The efficiency of the energy transfer, <ET, can be determined from the decrease of do¬
nor excited state lifetime which is caused by quenching through acceptor molecules (Eq. 3):8
<DT] = 1 - } (Eq.3)
64
where T and x0 are the lifetimes of the donor excited states in presence and in absence of
acceptor molecules, respectively.
Alternatively, the transfer efficiency may be determined from the relative increase
of acceptor fluorescence detected by steady-state absorption and fluorescence measure-
Q
ments (Eq. 4).
ab s a
absD
•emT
emA(Eq.4)
Here, absA and absD denote the absorption of the acceptor and donor, respectively,
at the excitation wavelength; emT and emA are the emission intensities of the acceptor ex¬
cited in the presence (emT) and in the absence (emA) of the donor, respectively. The values
for absA and absD in ternary blend films were determined by subtracting the scaled absorp¬
tion spectrum of a binary reference blend film from the spectrum of the former. The value
of emA was determined on binary reference blend films comprising only the conjugated
polymer, and scaled to the respective absA of the ternary blend films. This method of eval¬
uation was applied after verifying that the PL quantum efficiency of the acceptor (when ex¬
citing directly its absorption band at 440nm) is similar for reference binary and ternary blend
films.
The results are shown in Table 1 and demonstrate an increasing transfer efficiency
at higher acceptor concentrations. Most importantly, the energy transfer process was found
to be very effective; energy transfer efficiencies of up to 0.75 were determined for the most
concentrated samples investigated in the present study. The energy transfer efficiencies de¬
termined from steady-state PL measurements (Table 1) were found to be slightly but sys¬
tematically lower than those determined from time-resolved experiments. This circumstance
may be explained by the fact that donor lifetimes reflect all quenching processes which con¬
tribute to acceleration of donor decay. Thus, measurements of the latter tend to overestimate
the transfer efficiency in contrast to steady-state PL measurements which only reflect the ef¬
fect of excited states generated by energy transfer.
65
Donor
Concentration
[% w/w]
Acceptorconcentration
[% w/w]
Acceptorconcentration
[mol/1]
Donor decaytime [ns]
Ot1! <PT22
l3 - - 3.6±0.l - -
1 0.01 I4 0~5 3.06±0.1 0.15 n.a.4
1 0.02 24 0"5 2.89+0.04 0.2 4n.a.
1 0.05 5-icr5 2.34±0.05 0.35 0.15
1 0.1 1-10"4 2.05±0.07 0.43 0.25
1 0.25 2.54 0"4 1.03+0.11 0.71 0.56
1 1 PIO"3 0.89±0.09 0.75 0.68
- 1 1T0"3 0.72±0.075 - -
1 Transfer efficiency determined from donor lifetime decay according to Eq. 3.
" Transfer efficiency determined from the relative increase of acceptor fluorescence ac¬
cording to Eq. 4.
3 13Corresponds to 4.34 0"- mol/1, assuming a density of 0.97g/ml for UHMW-PE
4Not available due to the low acceptor emission in these films.
5Acceptor decay time [ns].
Table FFilm composition and donor decay times of oriented blend films comprising UHMW-PE,
DMC (donor) and EHO-OPPE (acceptor) (draw ratio ~ 70; thickness ~ 2pm).
Decay data were analyzed assuming a Förster transfer mechanism, governed by
long-range dipole-dipole interactions, effective up to distances of several nanometers.8 The
transfer rate kT according to that mechanism is inversely proportional to the sixth power of
the distance R between involved molecules (Eq. 5).8'10,11
kT = xl R(Eq. 5)
The Förster radius R0 is defined as the distance at which the transfer rate equals the
decay rate of direct donor fluorescence emission. At this distance, half of the donor excited
states generated through the absorption of photons decay through direct fluorescence and
half through energy transfer to the acceptor. R0 is defined by Eq. 6 and depends on the mag-
66
nitude of spectral overlap between donor fluorescence and acceptor absorption (Ql), the do-
nor quantum efficiency (&N), the relative orientation between interacting molecules (k~)
and the refractive index (n) of the solvent or solid matrix surrounding the chromophores.8'11
1287t'n NA
For the refractive index of the solvent a value of 1.44 was used in the case of chlo¬
roform,12 and 1.51 was employed for the polyethylene-based blend films. R0 can also be
expressed in terms of a critical acceptor concentration C0A, which corresponds approximate¬
ly to the number of dye molecules present in a sphere with radius R0, divided by its volume
(Eq. 7).8
R03 = fi!M_L_') (Eq. 7)zjc iNA^0A
Here, NA is Avogadro's number. 6.0224 0_i [moF1]. C0A is in [mol/1], and R0 in [cm].
The acceptor-induced decay of donor fluorescence (Fig. 5) follows a stretched expo¬
nential function (Eq. 8), in which the first exponential describes the first order deactivation
and the second term accounts for the quenching efficiency by acceptor molecules.14'15 The
latter term is strongly concentration dependent,-l;>
and. consequently, the measured donor
fluorescence decay at different acceptor concentrations fCA) can be fitted to this function in
order to determine the critical acceptor concentration C0A. For this procedure, the first ex¬
ponential in Eq. 8 is replaced by the measured decay function of the neat donor.9 Determi¬
nation of the factor y for each acceptor concentration yields a close-to-linear relation
between y and the acceptor concentration as shown m Fig's. 6a and b, for oriented blend
films and chloroform solutions, respectively. The critical acceptor concentration C0A can be
determined from a linear least squares fit of these data (Eq. 9); and application of Eq. 7
yields the critical îadius R0.
r/tU/2i
(Eq. 8)I(t) = I0e\p(--Mexpv t0;
r
/ty/2-,
y = fk-C'0A
(Eq. 9)
67
Evaluation of fluorescence decay data in chloroform solutions according to the
above procedure yields a critical acceptor concentration of (1.2± 0.08)40 mol/1, which cor¬
responds to a critical radius, R0, of 7.1 ± 1.4 nm. Determination of R0 from steady-state ex¬
periments according to Eq. 6 yields the very similar value for R0 of 7.0 nm. The outstanding
agreement of these two values clearly shows that the concentration dependence of the donor
lifetimes follows the behavior expected for a Förster-type transfer, and also the absolute val¬
ue of the determined critical radius is fully consistent with this mechanism. An experimental
error of around 1.5 nm can be estimated for these results, due to errors in determination of
the several variables present in Eq. 6 (<hD, EL k~). The value used for k in the evaluation of
Eq. 6 was 2/3. The quantum efficiency of DMC in CHC13 was determined according to pro¬
cedures described by Demas et al., using quinine sulfate in sulfuric acid as a reference,
yielding a value of 0.94. This value was additionally confirmed using DMC in ethanol
(quantum efficiency 0.74 ) as another reference.
.'' . , . , . , ,
/' '
10°
.
A
yv"y
'S
10' - y
. 1 , . ...
10'
a)
10'
CA [mol/l]
10'
1 1 * «"">"«"' > 1 ""»
y
m
101-
r 1
101
: -/-10
b)
10*
C [mol/l]
10J
Fig. 6: Fitting parameter y vs. EHO-OPPE (acceptor concentration) for DMC / EHO-OPPE, m ori¬
ented UHMW-PE blend films (a) and in chloroform solutions (b); concentrations [mol/1]
were calculated assuming a density of 1.48 g/ml for chloroform12 and of 0.97 g/ml for
UHMW-PE;13 C0A was determined from a linear fit (solid line).
R0 for oriented blend films, determined from time-resolved data, was found to be 9.3
± 1.0 nm. For the determination of R0 from steady-state data, a quantum efficiency of DMC
of 0.42 was used (measured in the oriented binary DMC / UHMW-PE blend using dipheny-
.19anthracene in PMMA as a reference ), and a value of 4/3 for k~. This corresponds to the
68
highest possible value which k can assume for the combination of linear and isotropic tran¬
sition dipole moments of the acceptor and the donor, respectively (p. 64 in ref. 8). This rather
high value was chosen considering the highly anisotropic structure of the blends, which
show a preferential orientation of the molecules in the direction of tensile deformation, as
well as a substantial amount of depolarization of fluorescence through homotransfer (see be¬
low). Through the homotransfer between donor molecules, the energy transfer between do¬
nor-acceptor pairs with favorable mutual orientation is likely to be enhanced, thereby
leading to a higher value for k~. Thus, from steady-state data R0 was calculated to be 6.8 nm.
The value of R0 as determined by steady-state experiments perfectly matches with
data obtained in solution, and indicates a Förster-type mechanism also for the solid samples.
Time-resolved measurements clearly result in a substantially higher value of R0 (9.0 nm) for
the oriented blend films. This behavior is consistent with, and explained by the particular
phase behavior encountered in the blend films, in which the chromophores are likely to be
present in mixed clusters that phase-separate from the UHMW-PE matrix." The miscibility
of the two chromophores further favors the close proximity of donor and acceptor mole¬
cules, and leads to higher local concentrations of chromophores in these clusters where en¬
ergy transfer is likely to occur. Thus, when compared to solutions (in which the
chromophores are presumably statistically distributed), it appears obvious that this state of
matter results in a higher value for the critical radius. However, we should point out that also
these experiments reveal a concentration dependence of the donor fluorescence decay,
which is consistent with a Förster-type mechanism.
The polarizing nature of the energy transfer observed in the present oriented blend
films, which permits light even polarized perpendicular to the drawing direction of the films
to be absorbed by DMC and subsequently reemitted by EPIO-OPPE with polarization par¬
allel to the drawing direction, stands in apparent contradiction to the Förster theory. The
transfer rate is proportional to the orientation factor K2, which is zero for perpendicular ori¬
entation between donor and acceptor transition dipole moments. Thus, in order for a transfer
to occur for such a mutual orientation of transition dipole moments, a depolarization of flu¬
orescence must occur prior to the transfer. Such a fluorescence depolarization can be caused
by rotational motion of the donor molecules or by homotransfer among the latter.20 Rota¬
tional motion is likely to be non-relevant due to the extremely high viscosity of the surround¬
ing solid UHMW-PE. Nevertheless, in order examine the likelihood of the latter possible
69
origin, measurements of fluorescence anisotropy were performed according to the proce-
90dures described by Lakowicz on oriented blend films of 1% w/w DMC in UHMW-PE at
room temperature as well as in a He-cryostat at 11 K. The latter temperature is well below
the glass transition temperature of DMC of around 261 K as measured by differential scan¬
ning calorimetry and, thus, the rotational motion of DMC molecules is frozen. Fluorescence
anisotropy (r) was calculated from the ratio between integrated fluorescence emission inten¬
sities of polarizations parallel (Ip) and perpendicular (Is) to the polarization of the excitation
Of)
light, according to Eq. 10.
L/L- I
r = f-—^ (Eq. 10)p s
"
The polarization of excitation light was chosen to be perpendicular to the drawing
direction of the film. The fluorescence anisotropy was found to be -0.09 at room tempera¬
ture, demonstrating a very low degree of polarization for the emitted light, as a result of an
efficient depolarization process. A similar situation was observed at cryogenic temperatures,
where the anisotropy under identical excitation and detection conditions was determined to
be -0.1. Interestingly, in spite of selective photoexeitation of the molecules oriented perpen¬
dicular to the drawing direction of the film in this particular measurement, the observed pho¬
toluminescence was found to be slightly polarized parallel to the drawing direction of the
film. However, this finding is fully consistent with the not perfectly statistical orientation of
the DMC molecules in the oriented UHMW-PE matrix. As a matter of fact, the observed
slight polarization of the emitted light, and the direction of the latter, is in perfect agreement
with polarized absorption measurements. These experiments show a dichroic ratio (ratio of
the absorption recorded through a polarizer oriented parallel and perpendicular to the draw¬
ing direction) of 1.3 and clearly indicate a slight (average) orientation of the DMC transition
dipole moments along the orientation direction of the film.
The above experiments clearly demonstrate that rotational motion of the DMC mol¬
ecules can be excluded as the depolarization mechanism, suggesting that indeed a ho¬
motransfer between multiple DMC molecules is the relevant depolarization mechanism in
the present films.
In order to obtain further proof for the latter, fluorescence anisotropy was subse¬
quently measured at room temperature in oriented DMC / UHMW-PE blends with decreas¬
ing DMC concentrations. For these materials, an increase of fluorescence anisotropy from
70
-0.05 at 1% w/w DMC to 0.12 at 0.05% w/w DMC was observed. This finding indicates that
the rate of depolarizing homotransfer decreases with decreasing DMC concentration, i.e.
with increasing distance between the molecules. Thus, the polarizing nature of the energy
transfer is a consequence of the highly ordered morphology of the investigated oriented
blend films. DMC molecules appear to retain a nearly isotropic orientation, whereas EHO-
OPPE molecules adopt the unidimensional orientation of the UHMW-PE matrix, and after
depolarization of excitation light through homotransfer among donor molecules an efficient
energy transfer to the acceptor is observed to occur regardless of the polarization of excita¬
tion light.
Summary
We have shown that energy transfer in oriented blend films of EHO-OPPE and DMC
in UHMW-PE follows a Förster mechanism based on dipole-dipolc interactions. The critical
radius for this chromophore combination was determined to be around 7.0 nm. A favorable
proximity of donor and acceptor molecules was found to occur in the oriented blend films,
that are based on an UHMW-PE matrix. This clearly enhances energy transfer, as demon¬
strated by an apparently increased R0 of 9.0 nm. Furthermore, it was found that in the present
highly ordered system, in which the acceptor is uniaxially oriented, the orientation factor K2
may assume larger values than in randomly oriented solutions. The polarizing nature of the
observed energy transfer, i.e. the transfer between states which apparently have perpendic¬
ular dipole moments, is shown to be enabled through an initial homotransfer between donor
molecules.
Acknowledgments
The authors thank Prof. Thomas Schmidt, Dept. of Biophysics. Leiden University,
The Netherlands for fruitful discussions. GSH gratefully acknowledges support by the Eul-
bright Foundation and the Schweizerische Bundesstipendat Kommission.
71
References
1 Ch. Weder, C. Sarwa, A. Montali. C. Bastiaansen, P. Smith, Science, 1998, 279,
1035.
2 A. Montali, C. Bastiaansen, P. Smith. Ch. Weder. Nature, 1998, 392, 261.
3 Ch. Weder, M.S. Wrighton. Macromolecules, 1996, 29, 5157.
4 Ch. Weder, C. Sarwa, C. Bastiaansen, P. Smith. Adv. Mater., 1997. 9, 835.
5 A. Montali, P. Smith. Ch. Weder, J. Mater. Sei, in press.
6 P. Smith, P.J. Lemstra. Coll Polym. Sei, 1980. 258, 891.
7 U.P. Wild, A.R. Holzwarth. H.P. Good, Rev. Sei Inst, 1977, 48, 1621.
8 B.W. Van Der Meer, G. Coker III. S.-Y. Chen, in Resonance Energy Transfer. VCH,
New York, 1994.
9 U. Lemmer, A. Ochse. M. Deussen. R.F. Mahrt. E.O. Göbel, IE Bässler, P. Haring
Bolivar, G. Wegmann. H. Kurz. Synth. Met, 1996, 78. 289.
10 T. Förster, Ann Phvs., 1948. 2, 55.
11 N.L. Vekshin, Energy Transfer in Macromolecules, SPPE Optical Engineering Press,
Bellingham, 1997.
12 CRC Handbook of Chemistry and Physics, 73th Edition, p. 3-320, ed. D.R. Lide. CRC
Press, Boca Raton, 1992.
13 Polymer Handbook, 31C Edition, p. V/17, eds. J. Brandrup, E.H. Immergut, Wiley
and Sons, New York. 1989.
14 R.G. Bennett, ,7 Chem Phvs., 1964. 41, 3037.
15 W. Klöpffer, in Electronic Properties ofPolymers, ecls. J. Mort and G. Pfister, Wiley
and Sons, New York, 1982.
16 N. Demas, G.A. Crosby. J Phvs. Chem.. 1971, 75, 991.
17 T. Lopez Arbeola, E. Lopez Arbeola. M.J. Tapia, I. Lopez Arbeola, J. Phys. Chem,
1993, 97, 4704.
18 G. Jones II, M.A. Rahman,,/ Phvs Chem., 1994. 98. 13028.
19 B.M. Conger, EC. Mastrangelo, S.H. Chen, Macromolecules, 1997. 30. 4049.
20 J.R. Lakowicz. in Principles of Fluorescence Spectroscopy, Ch. 5, Plenum Press,
New York, 1983.
73
5. Deformation-Induced Formation of Molecular
Polymer Blends Probed by Single-Molecule
Microscopy
Abstract
Dispersions of macromolecules in a solid polymer matrix are of interest, because
they can synergistically combine the properties of their components.1 For example,
blends of tailored properties of semiconducting conjugated polymers2"5 and electrically
inactive polymers have recently been advantageously used for applications in light-
emitting devices,6 transistors,' lasers8 and other optical components.4 Blending on a
molecular scale is, however, usually prevented by the limited miscibility of the
components, and as a result, phase-separated systems are mostly obtained.1'9'10 At the
example of rigid-rod conjugated macromolecules in a flexible-coil matrix we
demonstrate by means of single-molecule photoluminescence imaging11 that solid-state
tensile deformation of initially phase-separated mixtures allows to produce stable
molecular blends of intrinsically" highly incompatible polymers.
This chapter is reproduced from: W. Trabesinger, A. Renn. B. Hecht. U.P. Wild, A. Montali,P. Smith, Ch. Weder. Science, submitted.
74
Molecular mixing of two polymers in general, and of rigid-rod and flexible-coil
macromolecules in particular, is usually inhibited by the low entropy of mixing.1'9'10
This limitation can be overcome by preparation of copolymers12'13 or by increasing the
enthalpy of mixing, for example by introducing specific interactions between the blend
components.14 Significant efforts have also been devoted to the investigation of the
influence of external fields on the compatibilization of polymers, including electric-
field15 and shear-induced mixing.16 However, previous work has been limited to
selected, in fact, rather compatible polymer systems.^i6
Moreover, the stabilizing effect
of the previously applied externa] fields was found to be comparatively weak, and, thus,
compatibilization has been restricted to molten blends close to the critical point.15'16
Finally, these blends are far away from thermodynamic equilibrium and upon
interrupting the field, the high mobility of the (molten) polymers usually causes the
system to spontaneously de-mix, preventing the formation of long-term-stable
homogeneous blends. In closely related work, the influence of mechanical deformation
on the drop deformation in (compared to the present system also rather compatible)
polymer blends has been investigated.1 Some of the previous studies suggest that
mechanical deformation might induce compatibilization. However, few reports actually
imply compatibilization clown to a molecular level and, unfortunately, unequivocal
experimental proof for the latter could not be obtained, because the analytical tools
applied in the past (such as X-ray or light-scattering experiments, luminescence
spectroscopy, or phase-contrast, fluorescence, and near-field scanning opticalISIS
microscopy"*
) could not resolve single molecules, but monitored the properties of
large multi-molecule ensembles.
Fig. 1: Chemical structure of EHO-OPPE the poly(2,5-dialkoxy-p-phenyleneethynylene)
derivative used.
75
In our recent studies regarding the preparation of photoluminescent polarizers,19
we surprisingly discovered that solid-state-deformation of initially phase-separated
blends of a poly(2,5-dialkoxy-Jp-phenylene ethynylene) derivative (EHO-OPPE,20 Fig. 1)
and ultra-high molecular weight polyethylene (UHMW PE) resulted in blends in which
the conjugated species exhibited photoluminescent properties that indicated an apparent
molecular dispersion of the rigid-rod conjugated polymer guest in the UHMW PE host;
although a true molecular dispersion was thought to be unlikely for thermodynamic
reasons.19 With the potential of molecular composites in mind, a systematic
investigation of this most unusual observation has been undertaken. We now
demonstrate that solid-state-deformation indeed is a widely applicable, kinetically
controlled method for the fabrication of stable, molecular blends of intrinsically
incompatible polymers. The investigation of phase behavior, morphology, and
orientation in these molecular polymer-polymer composites, to the best of our
knowledge the first on a molecular level, has been enabled by the application of single
molecule detection techniques.11 Recent developments in scanning confocal optical
microscopy have allowed for the routine observation of single molecules at ambient
conditions,11'21"23 including the investigation of conjugated polymers24'23 and single
molecule polarization phenomena." In contrast to ensemble measurements, single
molecule detection provides distributions of molecular parameters such as absorption
and emission dipole orientation,"6 diffusional trajectories,27 and positions of individual
absorbers.28
Samples were prepared by casting a solution of EHO-OPPE20 (number-average
molecular weight -IxlO4 gmol"1; 10, 5xl0"4, IxlO"1, and 0 mg, respectively), and
UHMW PE (Hostalen Gur 412, Hoechst AG, weight-average molecular weight ~4xl06
gmoP1; 500 mg) in/j-xylene (Fluka p.p.a., 50 g; dissolution at 130 °C after degassing
the mixture in vacuum at 25 °C for 15 min) into a petri dish 11 cm in diameter. The gels
were dried under ambient conditions for 24 h, leading to phase-separation of the two
polymers under near-equilibrium conditions. The resulting pristine films were uniaxially
drawn at 120 °C to well-defined draw ratios (X = final / initial length) of up to 80,
yielding transparent, oriented films of 1-2 pm thickness. Importantly, the orientation
process was conducted m the temperature window above the glass transition of the
conjugated polymer (around 100 °C). and below the melting temperature of the
polyethylene (-135 °C), before quenching the samples to room temperature. The films
76
were embedded in silicon oil to provide index matching, and sandwiched between a
microscopy slide and a covei glass
Fig. 2: (a) Wide-field fluoiescence image of
an oriented (diaw ratio, X=80, vertical
dnection) bmaiy EHO-
OPPE/UHMW-PE blend film
comprising 2% w/w of the EHO-
OPPE guest (b) Scanning confocal
optical image of an oriented (A.=80)
bmaiy EHO-OPPE/UHMW-PE blend
film comptismg 104 % w/w of EHO-
OPPE obtained by recording the
parallel polarization component
(vertical dnection as in (a)) at an
excitation intensity of 2 kW/cnC (c)
Scanning confocal optical image of an
unoriented film obtained by lecordmg
the paiallel polanzation component
The concentiation and excitation
intensity aie the same as m (b) Insets
m (b) and (c) images of the
perpendiculat polanzation component
tecoided simultaneously at the
îespective positions (same scale)
77
Binary blend films and neat UHMW PE reference samples of different draw
ratios were investigated using standard wide-field fluorescence and scanning confocal
optical microscopy.29'30 Fig. 2a shows a 100x100 urn2 wide-field fluorescence image of
an oriented (A,=80) binary blend film containing 2 % w/w EHO-OPPE. Highly
fluorescent stripe-like structures are observed with intermittent darker elongated
domains oriented parallel to the stretching direction which coincides with the
preferential polarization of the fluorescence emission.19 This observation is in
agreement with earlier photoluminescence. X-ray-, and electron diffraction experiments
which indicated that the macroscopic tensile deformation leads to a breaking up and
smearing out of the originally phase-separated EHO-OPPE clusters.19
Figs. 2b and 2c show typical polarized fluorescence images of an oriented
(A.=80) and an unstretched dilute (10~4 % w/w EHO-OPPE) binary blend film,
respectively, recorded by detecting the polarization component parallel to the
deformation direction. Most strikingly. Fig. 2b exhibits well separated diffraction-
limited fluorescent spots of variable intensity, organized in rows along the direction of
tensile deformation. Many of these spots can be attributed to single EHO-OPPE
molecules because of their characteristic blinking and stepwise photobleaching apparent
from fluorescence time traces31 (Fig. 3a). These observations were absent for the
unstretched film (Fig. 2c), consistent with the presence of EHO-OPPE clusters that are
phase-separated from the UHMW-PE matrix. The insets in Figs. 2b and 2c show the
fluorescence signal recorded simultaneously for the perpendicular polarization
component at the corresponding positions. The intensity contrast between the
polarization directions in Fig. 2b clearly demonstrates on a molecular level the high
degree of orientation of the conjugated molecules. This is in sharp contrast to the
corresponding confocal images of the unoriented film (Fig. 2c). Reference samples
without EHO-OPPE show only very weak fluorescence, well below the intensity of
typical single conjugated polymer fluorescence spots. The number of fluorescent spots
observed in the oriented blends was found to scale with the nominal concentration of
conjugated polymer, but typically accounted for only about 12 % of the nominal EHO-
OPPE concentration. This finding is m agreement with other single-molecule studies of
conjugated polymers,2 "~
and is consistent with the existence of non-luminescent
molecules, an uncertainty in molecular weight of EHO-OPPE, and actual losses of the
latter during sample preparation.
78
Additional fluorescence time-traces (Fig. 3b) of the oriented films show
bleaching in several steps of equal intensity, suggesting that the brighter spots observed
in Fig. 2b represent clusters of multiple chromophores. Interestingly, the degree of
deformation-induced orientation appeared to be a function of cluster size. Weaker spots,
in general, were found to be predominantly visible in the parallel polarization image
(Fig. 2b), consistent with a high degree of orientation of the respective chromophores.
By contrast, more intense spots show the tendency to be also visible in the perpendicular
polarization direction, indicating a reduced orientation of the EHO-OPPE molecules
when clustered.
Fig. 3: (a), (b) Timetraces of oriented binary
EHO-OPPE/UHMW-PE blend films
(draw ratio, X=80) comprising 10"4 %
w/w of the EHO-OPPE guest, recorded
at an excitation intensity of 2 kW/cm.
The behavior suggest the presence of
one (a) and two (b) molecules
respectively, (c) Intensity probability
density functions (pdfs) of oriented
binary EHO-OPPE/UHMW-PE blend
films comprising 10"4 % w/w of the
EHO-OPPE guest, as a function of the
draw ratio; the inset shows a plot of the
average integrated cluster intensity vs.
assigned stoichiometry (the pdf for the
X=20 film is not included because the
uni- and bichromophoric peaks are
formed by only one fluorescence spot
and the ter-chromophoric peak happens
to be completely absent).
c) integrated intensity [10' counts]
70
In order to obtain more quantitative information on the degree of clustering and
orientation of single EHO-OPPE molecules as a function of draw ratio, larger areas
(-25x25 urn2) of oriented films (10~4 % w/w EHO-OPPE) were imaged. A non-linear
least-square fit of fluorescence spots in the images to two-dimensional Gaussians was
performed using the Levenberg-Marquard algorithm"
which determines their center,
integrated intensity, the local background, and the respective errors. All subsequent
statistical evaluations were based on fluorescent spots with a peak height exceeding the
local background by a factor of at least three, and which met the additional requirement
that the error of the intensity parameter" did not exceed 20 % of the integrated peak
intensity. By applying these two empirical criteria, fits of poor quality in areas with large
background signal, or due to e.g. photobleaching are discarded without introducing a
bias for any particular cluster size. Each fitted fluorescence spot permits the construction
of a probability density function (pdf) that specifies the probability for possible results
of repeated integrated intensity measurements.33 Combining all individual pdfs for
accepted fluorescence spots (typically about 100 for a given X) results in pdfs that
characterize the probability distribution for measuring a certain integrated intensity on a
randomly selected fluorescent spot. Fig. 3c shows such combined pdfs for films of
various draw ratios. At smaller integrated intensities, the combined pdfs show,
independent of X, well-defined maxima occurring at equidistant values (inset Fig 3c),
evidencing the an integer number of chromophores in each cluster. The probability for
measuring a given stoichiometry (i.e. the number of chromophores comprised in a
cluster) can be deduced from such combined pdfs by fitting a sum of Gaussians to the
distributions to determine peak positions and relative weight of the first peaks.33
Clusters with stoichiometrics >4 cannot be unambiguously assigned to a certain
stoichiometry while, however, still contributing to the pdf with the appropriate statistical
weight. This situation is related to an increasing error in the intensity measurement, due
to increased photobleaching probability and energy transfer effects in larger clusters, and
an intrinsic broadening of individual pdfs due to shot noise.3"3 We monitored the relative
weight of the different stoichiometrics to study cluster dispersion as a result of tensile
deformation. The obvious increase m the occurrence of single chromophores and bi-
chromophoric clusters with increasing draw ratio (Fig. 3c) at the expense of the number
of larger clusters illustrates the increased degree of molecular dispersion of initially
phase-separated domains. This effect is nicely illustrated by comparing, for example, the
80
pdf for A,=30 with the pdf for À,=40. The probability to find single chromophores and bi-
chromophoric clusters is strongly increased for the latter. For films of X=70 a strong
decrease of larger clusters is observed in the pdf, while the weight of the first three
peaks is approximately equal. For single chromophores and bichromophoric clusters in
the latter film, the weight is actually slightly smaller than for the film of X-4Ö. We
attribute this finding to minor local inhomogeneities in the films that could not
completely be averaged out, although at least two images recorded at different locations
were evaluated. Plowever, the combination of additional wide-field fluorescence and
scanning confocal optical images suggests that the presently employed (unstretched as
well as stretched) dilute blend films arc, on a large scale, rather homogeneous with
respect to size and distribution of EPIO-OPPE clusters. Thus, the results obtained from
scanning confocal optical images seem to adequately reflect the "average structure" of
the films and Fig. 3c provides clear quantitative evidence for a deformation-induced
molecular dispersion of small clusters, ultimately leading to blends that comprise a
significant fraction of isolated single chromophores.
13
11
9
i
G 7
CE
Q5
3
1
20 30 40 50 60 70
draw ratio [-]
Fig. 4: Average dichroic ratio of individual EHO-OPPE clusters and/or single molecules in
oriented binary EHO-OPPE/UHMW-PE blend films comprising 10"4 % w/w of the
EHO-OPPE guest, as a function of draw ratio. The maximum dichroic ratio that can be
achieved on the level of single molecules is limited by the non-negligible contribution
of background luminescence and is therefore smaller than in macroscopic studies.19
1 1 i >'
-
1~
-
- Tà i 1
1
1
i
1
I -
--
_
1-
-
-
, i 1 , i
81
To address the issue of deformation-induced orientation of the molecules, the
average dichroic ratio of isolated fluorescence spots was determined as a function of X
by dividing the integrated intensities of fluorescence spots of both polarization
directions. The results, obtained for the same set of fluorescence spots as used for Fig. 3,
are plotted in Fig. 4. Gratifyingly, the dichroic ratio increases with increasing draw ratio
and reaches saturation around /O=40, in accord with results obtained in previous
macroscopic studies,19 and also the dispersion-behavior reflected by Fig. 3c.
Finally, it is important to comment on the thermodynamic aspects and long-term
stability of the present systems. Evidently, the applied deformation process, if conducted
under experimental conditions that allow for large-scale rearrangement, transforms the
originally phase-separated blend into molecular blends of often well-isolated rigid-rod
macromolecules in a flexible-coil matrix polymer, i.e. systems that are clearly far away
from thermodynamic equilibrium.' ' However, due to the extremely slow diffusion of
the present macromolecules in the resulting solid state, the compatibilization is assumed
to persist virtually indefinitely. Indeed, confocal images of an oriented (A,=80) binary
EHO-OPPE/UHMW-PE blend film comprising 10~4 % w/w of EHO-OPPE (such as
shown in Fig. 2b) remain essentially unchanged when storing the sample for more than
6 months under ambient conditions and. thus, clearly demonstrate the outstanding long-
term stability of the compatibilized blends. In agreement with a plethora of previous
studies, it is anticipated that, at least in case of the presently investigated, rather dilute
blends, the guest molecules are present predominantly in the amorphous phase and
possibly in adsorbed states on the surfaces ol the UHMW PE crystallites.'1
Acknowledgements
We thank Dr. ITieo Tervoort for very helpful discussions.
82
1 L.A. Utracki, in Polymer alloys and blends: Thermodynamics and rheology, Carl
Hanser Verlag, München. 1989.
2 H.Vestweber, IOberski, A. Greiner, W. Heitz. R.F. Mahrt, H. Bässler. Adv.
Mater, for Optics and Electronics, 1993.2, 197.
3 M. Granström, 0. Inganäs, Appl. Phys. Lett, 1996. 68, 147.
4 Ch. Weder, C. Sarwa, A. Montali. C. Bastiaansen, P. Smith, Science, 1998, 279,
1035.
5 A. Montali, C. Bastiaansen, P. Smith. Ch. Weder, Nature, 1998, 392, 261.
6 J.H. Burroughes, D.D.C. Bradley. A.R. Brown, R.N. Marks. K. Mackay, P.L.
Burn, A.B. Holmes, Nature, 1990. 347, 539.
7 J.H. Burroughes, C.A. Jones, R.H. Friend, Nature, 1988, 335, 137.
8 N. Tessler, G.J. Denton, R. Friend. Nature, 1996, 382, 695.
9 P.J. Flory, Macromolecules, 1978, 11, 1138.
10 R. Koningsveld, W.H. Stockmayer, E. Nies, in Polymer Phase Diagrams,
Oxford University Press, in press.
11 X. S. Xie, J.K. Trautman, Annu. Rev. Phys. Chem., 1998, 49, 441. Frontiers in
Chemistry: Single Molecules Science, 1999, 283. T. Basché, W.E. Moerner, M.
Orrit, U.P. Wild, in Single Molecule Optical Detection, Imaging and
Spectroscopy, VCH, Weinheim, 1996.
12 M.B. Gieselman, J.R. Reynolds. Macromolecules, 1990, 23, 3118.
13 U. Lauter, W.H. Meyer. G. Wegner, Macromolecules, 1997, 30, 2092.
14 CD. Eiscnbach, J. Hofmann. K. Fischer, Macromol. Rapid. Commun., 1994, 15,
117. CD. Eisenbach, J. Hofmann, W.J. MacKnight. Macromolecules, 1994, 27,
3162.
15 D. Wirtz. D.E. Werner, G.G. Fuller,./. Chem. Phys., 1994,101, 1679.
16 J. Van Egmond, G.G. Fuller. Macromolecules, 1993, 26. 7182. S. Kim, E.K.
Hobbie, J.-W. Yu. C.C. Han, Macromolecules, 1997, 30. 8245.
17 U. Sundararaj, C.W. Macosko, Macromolecules. 1995, 28, 2647. U. Levitt, C.W
Macosko. S.D. Pearson, Polym. Eng. Sei, 1996. 36. 1647.
18 J.H. Hsu, P.K. Wei. W.S. Fann. K.R. Chuang, S.A. Chen. Ultramicroscopy,
1998, 71, 263. S. Webster. D.A. Smith, D.N. Batchelder. D.G. Lidzey, D.D.C.
83
Bradley, Ultramicroscopy, 1998, 71. 275. J.A. DeAro, K.D. Weston, S.K.
Buratto, U. Lemmer, Chem. Phys. Lett, 1997, 277, 532.
19 Ch. Weder, C. Sarwa, C. Bastiaansen, P. Smith, Adv. Mater., 1997, 9, 835.
20 Ch. Weder, M.S. Wrighton, Macromolecules, 1996. 29, 5157.
21 J.J. Macklin, J.K. Trautman, T.D. Harris. L.E. Brus, Science, 1996, 272, 255.
22 H. Lu, X.S. Xie, Nature, 1997, 385, 143.
23 S. Nie, D.E. Chiu, R.N. Zare. Science. 1994. 266, 1018.
24 D.A. Vanden Bout, W.-T. Yip, D. Hu, D.-K. Fu, T. Swager, P.F. Barbara,
Science, 1996,277, 1074.
25 W.-T. Yip, D. Hu, J. Yu, D.A. Vanden Bout, P.F. Barbara, J. Phys. Chem. A,
1998,102,7564.
26 T. Pia, T. Enderle, D.S. Chemla, P.R. Selvin. S. Weiss, Phys. Rev. Lett, 1996,
77, 3979. T. IIa, J. Glass. T. Enderle, D.S. Chemla. S. Weiss, Phys. Rev. Lett,
1998, 80, 2093. R.M. Dickson, DJ. Noms, W. E. Mocrner, Phys. Rev. Lett,
1998, 81. 5322. S.A. Empedocles, R. Neuhauser, M.G. Bawendi, Nature, 1999,
399. 126.
27 T. Schmidt, G.J. Schütz, W. Baum satiner, HJ. Gruber, H. Schindler, Proc. Natl.
Acad. Sei USA, 1996, 93. 2926. R.M. Dickson, D.J. Noms, Y.-L. Tzeng. W.E.
Moerner, Science, 1996, 274, 966.
28 M.A. Bopp, A.E Meixner, G. Tarrach, I. Zschokke-Granacher, L. Novotny,
Chem. Phys Lett, 1996,263,721.
29 EHO-OPPE was excited with circular polarization at a wavelength of 488 nm via
an immersion oil microscope objective (1.4 NA). EHO-OPPE fluorescence was
collected by the same objective and directed via a dichroic mirror and a cutoff
filter towards a polarizing beamsplitter. The resulting two orthogonal
fluorescence components, one coinciding with the direction of tensile
deformation, are detected by a single photon counting avalanche diode,
respectively.
30 L. Fleury. B. Sick, G. Zumofen, B, Hecht. U.P. Wild, Mol Phys., 1998, 95,
1333.
31 Fluorescence time traces were recorded by positioning well-isolated fluorescence
spots in the confocal volume and directing the fluorescence counts to a
multichannel analyzer.
84
32 P.R. Bevington, D.K. Robinson, in Data reduction and error analysisfor the
physical sciences, McGraw-Hill, New York, 1994.
33 T. Schmidt, G.J. Schütz, H.J. Gruber, H. Schindler, Anal. Chem., 1996, 68,
4397.
34 P.J. Phillips, Chem. Rev., 1990. 90. 425.
85
6. Phase Behavior and Anisotropic Optical Properties
of Photoluminescent Polarizers
Abstract
The phase behavior and anisotropic optical properties of tensile deformed blends of
a photoluminescent polymer guest in an ultra-high molecular weight polyethylene matrix
were studied on the level of single molecules by means of scanning confocal optical
microscopy. It is shown that upon tensile deformation of the blends, the system transforms
from a phase-separated svstem into a quasi-molecular solid solution. The influence of this
phase transition on the anisotropic optical properties of oriented blend films was also
investigated with polarized steady-state photoluminescence spectroscopy. We show that
well-dissolved guest molecules tend to reach higher degrees of orientation at lower draw
ratios of the blend films compared to guests that phase-separate from the matrix polymer.
Dichroic ratios in emission in the range of 50 were observed in optimized blend films
based on photoluminescent oligomers and linear low density polyethylene.
This chapter is reproduced from: A. Montali. A.R.A. Palmans. M. Eglin. Ch. Weder. P. Smith, W.
Trabesinger. A. Renn, B. Hecht. U P. Wild. Macromol. Symp.,in press.
86
Introduction
Color liquid-crystal displays (LCDs) suffer from limited brightness and energy
efficiency, originating from the use of absorbing polarizers and color filters.1
Photoluminescent (PL) polarizers consisting of uniaxially oriented blends of a PL polymer
and ultra-high molecular weight polyethylene (UHMW-PE) which, after photoexeitation.
absorb and emit light in a highly linearly polarized fashion, have been presented as a
possibility to increase the efficiency, brightness and viewing angle of LCDs." In our
previous studies, a highly luminescent and form-anisotropic conjugated polymer (EHO-
OPPE, a poly(2.5-dialkoxy-jD-phenylene ethynylene) derivative) was typically used as the
PL emitter, embedded in UHMW-PE."° The PL polarizers were prepared by the earlier
described gel-casting process.3 and uniaxially oriented through tensile deformation to
draw ratios, A., in excess of 70. Due to the thermodynamically unfavorable mixing
behavior of polymers," a phase-separated system was observed in the pristine, i.e.
unstretched PL films. Based on the interpretation of properties of large multimolecular
ensembles (i.e.. luminescence and diffraction spectra) it was previously suggested that the
phase-separated PL polymer domains are deformed during tensile deformation in order to
yield an apparent molecular dispersion of EPIO-OPPE in UHMW-PE.1 We now present
investigations carried out on the level of single-molecules with scanning confocal optical
microscopy (SCOM),5 which confirm this view. The influence of the initial domain size
on the anisotropic emission properties of the PL polarizers was investigated by
characterizing UHMW-PE blends with EPIO-OPPE concentrations of 0.05, 0.2, I, 2, 5 and
25% w/w. The importance of using PL dyes which are "soluble" in the matrix polymer
was shown by comparing blends based on l,4-bis(4-dodecyloxy phenyl ethynyl)benzene
(BPBC12) and EHO-OPPE. respectively, of similar concentrations; in the former blend, the
optical anisotropy was found to be substantially higher compared to the latter. Finally, an
optimized, melt-processed blend based on BPBC12 and linear low-density polyethylene
(LLDPE) was developed that allows efficient manufacturing of PL polarizers which at
draw ratios of only 10 exhibit emission dichroic ratios exceeding 50.
87
Experimental
EHO-OPPE6 (M„ ~ lO'OOO gmol"1) and BPBCi2? (Fig. 1) were prepared according
to procedures described elsewhere. The polyethylenes employed are commerciallyavailableandwereobtainedfromHoechst
(U
(LLDPE: Dowlex NG5056E, p=0.919gcm~\
availableandwereobtainedfromHoechst
(UHMW-PE: GUR 412, Mw ~ 4 106) and Dow
EHO-OPPE: -f
Fig. 1: Chemical structure of the photoluminescent dyes employed in this woik.
Film Preparation
Blend films of EHO-OPPE and UHMW-PE with EHO-OPPE concentrations of
0.05, 0.2, 1, 2, 5 and 25% w/w were prepared according to procedures described
elsewhere. Similarly, a film with a nominal concentration of 10" % w/w EHO-OPPE in
UPIMW-PE was prepared through gel-casting for the SCOM measurements, and the same
procedures were also employed for the pieparation of a blend film of 0.2% w/w BPBC12 in
UHMW-PE. Films of BPBC,2 and LLDPE with BPBC12 concentrations of 0.2, 0.8 and 2%
w/w were prepared by feeding PE pellets, which had previously been coated with the PL
dye, into a recycling, co-rotating twin-screw mini-extruder (DACA instruments, Santa
Barbara, CA). The pellets were mixed for 10 min at 155°C and subsequently extruded.
The extrudate was compression-molded in a Carver press at 150°C to yield films of
around 100 |im thickness. All films were drawn at temperatures of 70°C (LLDPE) or
120°C (UHMW-PE) on a thermostatically controlled hot shoe. Draw ratios were
calculated from the displacement ot distance marks printed on the films prior to drawing.
88
Optical Characterization
PL spectra were recorded on a SPEX Fluorolog 3 (Model FL3-12), fitted with
motor driven Glan-Thompson polarizers, and with a 450W Xe-lamp for excitation. The
films were sandwiched between two quartz slides, applying a minor amount of silicon oil
in order to minimize light scattering at the film surfaces. We quantified the anisotropic
optical characteristics of the drawn films by the dichroic ratio, defined for emission
(DRcm) as the ratio between the respective spectra measured with polarization parallel (p-)
and perpendicular (s-) to the drawing direction, hi our determination of the dichroic ratio
in emission, we integrated the spectra, because the integrals are directly related to the
energy of the relevant electronic transitions and, hence, reflect the underlying physical
processes best. SCOM measurements were performed under excitation from an Ar-lascr at
488 nm; the setup for the measurements was described by Fleury et al.8
Results and Discussion
SCOM images of an EHO-OPPE / UHMW-PE blend film with a nominal EHO-
OPPE concentration of 10~4 % w/w which was drawn to a draw ratio of 80 (Fig. 2a),
recorded by detecting the polarization component parallel to the drawing direction
(vertical in the Fig.) appear to reveal single fluorescent spots of variable intensity aligned
along the drawing direction. Many of these spots can be attributed to single EHO-OPPE
molecules because of their characteristic blinking and stepwise photobleaching apparent
from fluorescence time traces.9 The insets in Figs. 2a and 2b show the fluorescence signal
recorded simultaneously for the perpendicular polarization component at the
corresponding positions. The high contrast in intensity between the polarization directions
in Fig. 2a clearly demonstrates on a molecular level the high degree of orientation of the
conjugated molecules. The above observations are in sharp contrast to the corresponding
confocal images of the unoriented film (Fig. 2b and inset). For the latter, the absence of
blinking and stepwise photobleaching are fully consistent with the presence of EPIO-PPE
clusters that are phase-separated from the UHMW-PE matrix; and it is obvious that the PL
molecules are, as expected, fully disordered. These images, although obtained on films of
89
a significantly lower dye concentration than employed in eaihei experiments, confum
previous findings, demonstrating that an efficient orientation of the EHO-OPPE
molecules is achieved only altei the clusteis ol the lattei aie deformed and successively
dispersed m the UHMW-PE matrix, finally yielding an appaient molecular dispersion 01
solid solution
Fig. 2: a) SCOM image of an EHO-OPPE / UHMW-PT blend film oriented to a diaw îatio of 80
and b) of the same film m the pnstme, undiawn state Insets images of the perpenchculat
polanzation component iecoided simultaneously at the lespective positions (same scale)
In oidei to obtain moie quantitatrve mloimation on the degree of clusteimg and
orientation of single EHO-OPPE molecules as a function ot draw îatio, -25x25 urn'
squares of onented dilute films (104% w/w EHO-OPPE) of different diaw latios (20, 30,
40, 70) weie imaged Foi each diaw latio at least two images weie taken at difteient
locations ol the sample in oidei to minimize effects of mhomogeneous distubution of
EHO-OPPE in the matiix A non hneai least-squaie lit of fluoiescence spots appealing in
the images to two-dimensional Gaussians was petfoimed using the Levenbeig-Maiquaid
algorithm In addition to acciuately deteimmmg the centei of the fluoiescence spots, the
fitting pioceduie yields the integiated intensity the local background, as well as the
lespective enois on all paiameteis All subsequent statistical evaluations were based on
fluoiescent spots only with a peak height exceeding the local backgiound by a factoi of at
least thiee. and which met the additional lequnement that the enoi of the intensity
paiameter does not exceed 20% ol the integiated intensity ot a peak By applying these
90
two empirical criteria, fits of poor quality in areas with large background signal or
erroneous fits due to e.g. photobleaching during the measurement are discarded without
introducing a bias for any particular size of clusters. Each individually fitted fluorescence
spot permits the construction of a probability density function (pdD that specifies the
probability for possible results of repeated integrated intensity measurements.
Combining all individual pdfs for accepted fluorescence spots (typically about
100 for a given draw ratio, X) results in pdfs that characterize the probability distribution
for measuring a certain integrated intensity on a randomly selected fluorescent spot. Fig. 3
shows such combined pdfs for films of various draw ratios. The combined pdfs show
well-defined maxima at smaller integrated intensities. The peaks occur at equidistant
integrated intensity values, independent of X, as is obvious from the inset of Fig 3. This
finding is clear evidence for the presence of an integer number of chromophores in each
cluster.
12 3 4
integrated intensity [10° counts]
Fig. 3: Intensity probability density functions (pdfs) of oriented binary EHO-OPPE / UHMW-PE
blend films comprising 10"4 % w/w of the EHO-OPPE guest, as a function of the draw
ratio X.
The probability for measuring a given stoichiometry (i.e. the number of
chromophores comprised in a cluster) can be deduced from such combined pdfs by fitting
a sum of Gaussians to the distributions to determine peak positions and relative weight of
91
the first peaks." Peaks with stoichiometrics larger than 4 can hardly be observed in our
experiments. This is probably due to the strongly increased probability of photobleaching
in larger clusters; an increasing influence of energy transfer effects and an intrinsic
increase in the width of individual pdfs due to shot noise.11 We monitored the relative
weight of the different stoichiometrics in order to study cluster dissolution as a result of
tensile deformation. The obvious increase in the occurrence of single chromophores and
bi-chromophoric clusters with increasing draw ratio (Fig. 3) at the expense of a decrease
in the number of ter-chromophoric and larger clusters illustrates the increased degree of
molecular dispersion of initially phase-separated domains. This effect is nicely illustrated
by comparing, for example, the pdf for A,=30 with the pdf for A,=40. The probability to find
single chromophores and bi-chromophoric clusters is strongly increased for the latter. For
films of relatively low draw ratio (A,=20), only single fluorescence spots contribute to the
uni- and bichromophoric peaks in the pdf, respectively, while the ter-chromophoric peak,
in fact, happens to be completely absent. The pdf for A,=20 is, therefore, not included in
the inset. For films of X=10, a strong decrease of larger clusters is observed in the pdf,
while the weight of the first three peaks is approximately equal. For single and
bichromophoric clusters in the latter film the weight actually is slightly smaller than for
the film of A,=40. We attribute this finding to inhomogcneities in the films that could not
completely be averaged out although images were recorded at different locations.
Alternatively, it might be argued that single conjugated molecules may actually represent
two or more indistinguishable chromophores.12'1' Nevertheless, Fig. 3 provides clear
quantitative evidence for a deformation-induced molecular dispersion of small clusters,
ultimately leading to isolated single chromophores. Finally, in order to address the issue of
deformation-induced orientation of the molecules, the average dichroic ratio of isolated
fluorescence spots was determined as a function of the draw ratio. To this end. the ratios
of the integrated intensities of fluorescence spots in both polarization directions were
calculated for the same set of fluorescence spots as used for the clustering analysis. For
each value of the draw ratio A,, the average of the dichroic ratios of individual spots was
calculated. The results are plotted in Fig. 4a. Gratifyingly, the dichroic ratio increases with
increasing draw ratio and reaches saturation around A.=40. This behavior is m well accord
with the results presented and discussed below (Fig. 4b) as well as with previous findings,3
both obtained in macroscopic studies.
92
In order to more clearly unveil the influence of a transition from a phase-separated
to an apparently dissolved system on the macroscopic emission properties of the polymer
blends, we prepared and characterized UHMW-PE blends of different EHO-OPPE
concentration. The draw ratio required to obtain an optimal dissolution or dispersion of the
guest polymer domains in the host material, and an optimal orientation of the latter, should
strongly depend on the initial phase behavior of the pristine blends. In homogeneous
blends or blends with smaller PPE clusters highly polarized emission can be expected at
smaller draw ratios than in phase separated blends; and it is assumed that the phase
behavior of the present EHO-OPPE / UHMW-PE system -at least to a certain extent- can
be governed by the composition of the blend.
a:Q
13
11
9
7
5
3
1
a)
20 30 40 50 60
draw ratio [-1
: I 1 I ;
M II |
11
70
a:
a
% w/w EHO-OPPE
1 '
A 0.05
0.2
A
O
A
330
20
1
2
5
0
A5K
XX
10 -
9
Î -
1
I
, : ! i 1.. T 1
b)
20 40 60
draw ratio
100 120
Fig. 4: a) DRem of individual EHO-OPPE clusters and/or single molecules in oriented binary
EHO-OPPE / UHMW-PE blend films comprising 10'4 % w/w of the EHO-OPPE guest, as
a function of draw ratio b) Macroscopic DRem vs. draw ratio for EHO-OPPE / UHMW-PE
blends with different EHO-OPPE concentrations. The absolute values of the maximum
dichroic ratio determined by single molecule microscopy of about 9 are significantly lower
than the values determined m the macroscopic experiments due to the significant
contribution of background luminescence in the former.
The determination of DRcm of samples of a draw ratio of 70 confirms this expected
behavior, showing a decrease of DRcm with increasing EPIO-OPPE concentration from 38
in a 0.05% EHO-OPPE blend, to 13 in a blend comprising 5% EHO-OPPE (cf. Fig. 4b).
For a blend comprising 25% w/w EHO-OPPE, a DRem of only 5 at a maximal draw ratio
93
of 60 could be measured. The proposed deformation and orientation mechanism for the
embedded PPE molecules is further demonstrated by the fact that at even higher draw
ratios (X ~110) all films with concentrations up to 2% w/w of EHO-OPPE reach very high
values of around 30 and more. It appears from Fig. 4b that the dichroic ratios of these
samples level off at draw ratios of 70-80, and a further increase of the draw ratio does not
cause an increase of the optical anisotropy, consistent with the fact that in these samples
the EHO-OPPE molecules are optimally dispersed and oriented. On the other hand, when
considering films comprising 5% w/w EHO-OPPE. DRem does not seem to reach
saturation even at a draw ratio of 110, indicating that - in contrast to the above described
diluted samples - in the latter the dispersion and orientation of originally phase-separated
EHO-OPPE is significantly stifled.
composition31 DRem @ X =70 ^max DRem @ X
mx
0.05% EHO-OPPE UHMW-PE 38 110 37
0.2% EHO-OPPE UHMW-PEo ->
110 33
1 % EHO-OPPE UHMW-PE 25 110 32
2% EPIO-OPPE UHMW-PE 92 no 28
5% EHO-OPPE UHMW-PE 13 110 20
25% EHO-OPPE UHMW-PE n.a.b) 60 5
0.2% BPBC1? UHMW-PE 45 80 >50
0.2% BPBC12 LLDPE u.a. 10 >50
0.8% BPBCI2 LLDPE n.a. 9 37
2% BPBC12 LLDPE n.a. 9 24
All compositions in % w/w.
Not applicable.
Table 1: Influence of the composition on the dichroic ratio of PL polarizers
94
It can be assumed that a further deformation and orientation of the EHO-OPPE
clusters and, thus, a further increase in DRem. could be obtained at even higher draw ratios;
however, such draw ratios (À, > 110) could not be obtained in the present films. The same
discussion applies to samples with 25% EHO-OPPE, for which such high draw ratios
could not be obtained, and, therefore, the maximum value for DRem was found to be
limited to only 5 (Fig. 4b, Table 1 ).
We have shown earlier that PL emission spectra of undrawn films allow further
insights regarding the relations between the EHO-OPPE concentration and the extent of
phase separation. As can be seen from Fig. 5, in the case of a low EHO-OPPE
concentration of 0.05% w/w the spectra of unoriented blend films show the characteristics
of a (molecular) solution, i.e. a strong predominance of the vibronic band associated to the
transition from the lowest vibrational energy level of the excited state to the corresponding
ground state. With increasing concentration of the PL polymer, the fraction of well-
dispersed EPIO-OPPE molecules decreases in favor of aggregated molecules, which at
higher concentrations account for the major part of photoluminescence emission.
Therefore, with more and larger aggregates present in the films the form of the emission
spectrum gradually approaches that of a pure EHO-OPPE film (Fig. 5). A similar effect
was observed earlier, when comparing the emission spectra of an undrawn and of an
oriented film.
, ,_
1 ' 1 ' 1
A ;'. il p\
! ' I ' 1
% w/w EHO-OPPE -
ri i \'
" / \1 C\f\
|(C| I uu
>, -
5"55c
\ t '. j '\
0.05CD j 'i !
'// \
^i ' i i ii \
>
'
i' / A.solution
A! : U \
-aCD
' ' 1 ' \
,N' ' I \ '
^ \.' : ' / - ' ^i
15 \^
E / /// ''Xx \y.,
oc
-
1 1 1.
, 1. i . ....i i
450 500 550 600 650 700
X [nm]
Fig. 5: Steady-state emission spectra of unoriented EHO-OPPE / UHMW-PE blend films with
different EHO-OPPE concentrations, and of an EHO-OPPE solution in chloroform. All
spectra were recorded under isotropic excitation at 440 nm.
95
The finding that at a low EHO-OPPE concentration the emission spectrum has the
characteristics of a molecular solution apparently stands in contradiction to the SCOM
measurements, which indicate that in undrawn films even at extreme dilutions, such as
10"4 % w/w. the EHO-OPPE is present in clusters. However, these results suggest that in
dilute samples well-dispersed EHO-OPPE molecules dominate the emission, and,
therefore, the emission spectra exhibit the characteristics of an apparent solution.
Although some clustering can be encountered in films of these concentrations, the
emission from these seems to be of rather low intensity due to possible luminescence-
quenching in the aggregates.14'^
In addition, one might speculate that the emission
characteristics of clusters of only a few EHO-OPPE molecules are essentially similar to
those of a "true" molecular solution.
From the above results we conclude that, in order to obtain highly dichroic
emission at moderate draw ratios, a system is required in which the PL dyes are already
optimally dispersed prior to the orientation process. Based on these findings, a blend film
was prepared, in which EHO-OPPE was substituted by a phenylene ethynylene derivative
oligomer, BPBQ2, based on the assumption that a low-molecular dye might be more
compatible with the UHMW-PE matrix. It could, therefore, be expected, that higher
dichroic ratios would be obtained at lower draw ratios than in the case of EHO-OPPE-
based blends. In Fig. 6, the emission dichroic ratios are shown as a function of draw ratio
for dilute 0.2% w/w EHO-OPPE / UHMW-PE and 0.2% w/w BPBC12 / UHMW-PE blend
films. It is evident that the orientation of the guest occurs much more efficiently in the low
molecular weight BPBC12-based blend films compared to the EHO-OPPE-based films.
High values of DRem (>30) were obtained at draw ratios of only 35. This compares
favorably to PL polarizers containing EHO-OPPE as the PL, dye, for which draw ratios of
more than 70 were necessary to obtain a comparable DRcm (cf. Fig. 4 and Fig. 6).
96
T ! r— , , „
, , ! , _j,
,
4 BPBC1?» EHO-OPPE
A
AÀ
A
s
j L i.„ I i I i_ I i I
20 40 60 80 100 120
draw ratio [-]
Fig. 6: DRem vs. draw ratio of BPBC,2 / UHMW-PE and of EHO-OPPE / UHMW-PE blend films,
the PL dye concentration in both blends was 0.2% w/w. All values were determined under
isotropic excitation at 325 nm (BPBC1;) and 440 nm (EHO-OPPE).
Triggered by the above findings, we have further undertaken to investigate the
orientational behavior of melt-processed blends based on LLDPE and BPBC12.16 Films
based on blends of 0.2 %, 0.8 % and 2 % w/w of the photoluminescent guest in LLDPE
were prepared by melt-processing as described above and were subsequently drawn at 90
°C to draw ratios of around 10. These comparably low draw ratios were limited by the
nature of the matrix polymer.
Drawn films of these blends show unexpectedly highly polarized emission, as
demonstrated in Figure 7 for a 0.2 % w/w BPBCV / LLDPE blend film of a draw ratio of
10; the latter was characterized by an (integrated) DRC!11 of about 50. The dichroic ratio of
the BPBC12 / LLDPE systems was found to slightly decrease when the concentration of
the photoluminescent guest was increased (0.2 % w/w, À =9, DRem = 38; 0.8 % w/w, X-
9, DRera = 37; 2 % w/w, X = 9, DRera = 24). The latter results reflect an analogous behavior
as was found for the EPIO-OPPE / UHMW-PE blends, namely that also in the case of
BPBC12 / LLDPE blend films higher dye concentrations may lead to a phase separation of
the dye in the LLDPE matrix. A thorough investigation of the orientational- and of the
phase-behavior of melt-processible PL polarizers based on low molecular weight dyes has
been published elsewhere.
60
50
_ 401
e
rr 30a
20
10
1
97
ri
>.
'55c
Bc
I
_l
350 400 450 500 550 600
X[nm]
Fig. 7: Emission spectra of an oriented (X = 10) 0.2% w/w BPBC12 / LLDPE blend under
isotropic excitation at 322 nm.
Conclusions
In summary, we have demonstrated that control of the phase behavior of blends
used for the production of PL polarizers is essential for obtaining maximal orientation of
the PL dyes and, consequently, high polarization of the emitted light. In the case of the
presently investigated blends comprising polymeric PL dyes, the latter are present in
clusters in the pristine polyethylene matrix. These clusters are deformed and elongated
upon tensile deformation of the blend films, eventually leading to a quasi-molecular "solid
solution". The initial size of such clusters determines the draw ratio which is needed to
obtain high optical anisotropy. Phase-separation between the PL dye and the matrix
material is observed for large concentrations of the PL dye, and leads to comparably low
optica] anisotropics at low draw ratios as well as a limited maximal orientation of the dye
molecules at maximum draw ratios. Due to a high compatibility with the polymeric
matrix, low molecular weight dyes have been demonstrated to yield PL polarizers with
extremely high emission dichroic ratios. Furthermore, the latter allow the use of melt-
processible matrix materials, such as LLDPE.
p-detection
s-detection
98
References
1 TJ. Nelson, LR. Wullert II, in Electronic Information Display Technologies,
World Scientific Publishing, Singapore, 1997.
2 Ch. Weder, C. Sarwa, A. Montali. C. Bastiaansen, P. Smith, Science, 1998, 279,
1035.
3 Ch. Weder, C. Sarwa. C. Bastiaansen, P. Smith, Adv. Mater., 1997, 9, 835.
4 H.-G. Elias, m Makromoleküle, 4th edition. Hiithig & Wcpf Verlag, Basel, 1981.
5 T. Basché, W.E. Moerner, M. Orrit. U.P. Wild, in Single Molecule Optical
Detection, Imaging and Spectroscopy, VCH, Weinheim, 1996.
6 Ch. Weder, M.S. Wrighton. Macromolecules, 1996. 29, 5157.
7 A.R.A. Palmans, M. Eglin, A. Montali, P. Smith. Ch. Weder, Chem. Mater., in
press.
8 L. Fleury. B. Sick, G. Zumofen, B. Hecht, U.P. Wild, Mol Phys.. 1998, 95. 1333.
9 Fluorescence time traces were recorded by positioning well-isolated fluorescence
spots in the confocal volume and directing the fluorescence to a multichannel
analyzer. Detailed results will be published elsewhere.
10 P.R. Bevington, D.K. Robinson, in Data reduction and error analysisfor the
physical sciences, McGraw-Hill, New York, 1994.
11 T. Schmidt, G.J. Schütz, HJ. Gruber, H. Schindler, Anal Chem., 1996, 68, 4397.
12 D.A. Vanden Bout, W.-T. Yip. D. PIu, D.-K. Fu, T. Swager, P.E. Barbara, Science.
1996,277, 1074.
13 W.-T. Yip, D. PIu, J. Yu, D.A. Vanden Bout, P F. Barbara, J. Phys. Chem. A, 1998.
102, 7564.
14 T.Q. Nguyen. V. Doan. B.J. Schwartz.,/ Chem Phys., 1999,110. 4068.
15 CE. Halkyard, M.E. Rampey, L. Kloppenburg, S.L. Studer-Martinez, U.H.F.
Bunz, Macromolecules, 1998, 31, 8655.
16 M. Eglin. A. Montali. A.R.A. Palmans. T. Tervoort. P. Smith, Ch. Weder, J. Mat
Chem., 1999.9,2221.
99
7. Ultra-High Performance Photoluminescent
Polarizers Based on Melt-Processed Polymer
Blends
Abstract
Photoluminescent polarizers that comprise uniaxially oriented photoluminescent
species which absorb and emit light in highly linearly polarized fashion, can efficiently
combine the polarization of light and the generation of bright colors. We here report the
preparation and characterization of such polarizers by simple melt-processing and solid-
state deformation of blends of a photoluminescent guest and a thermoplastic matrix
polymer. The orientation behavior of a poly(2.5-dialkoxy-/>phenylene ethynylene)
derivative (EHO-OPPE), 1.4-bis(phenylethynyl)benzene, and l,4-bis(4-dodecyloxy
phenylethynyl)benzcne was systematically compared in different polyethylene grades.
Experiments suggest that if phase-separation between the photoluminescent guest and the
matrix polymer is reduced during the preparation of the pristine (i.e. unstretched) blend
films, photoluminescent polarizers can be produced which exhibit unusually high dichroic
properties at minimal draw ratios. In connection with this finding, an optimized, melt-
processed blend based on 1.4-bis(4-dodecyloxy phenylethynyDbenzene and linear low-
density polyethylene was developed that allows efficient manufacturing of
photoluminescent polarizers which at draw ratios of only 10 exhibit dichroic ratios
exceeding 50.
This chapter is reproduced from: M. Eglin. A. Montali. A R.A. Palmans, T. Tervoort, P. Smith,Ch Weder. J Mater Chem., 1999.9,2221.
too
Introduction
Color liquid-crystal displays (LCDs) suffer from limited brightness and energy
1 ?
efficiency, originating from the use of absorbing polarizers and color filters."
These
o o
limitations have recently triggered, among other advances,'" the development of
photoluminescent (PL) polarizers which can efficiently combine the polarization of light
and the generation of bright colors,9,10 concomitant with a substantial increase in
brightness and efficiency of PL LCDs based on these elements.11 These polarizers
comprise uniaxially oriented PL polymers, which absorb and emit light in highly linearly
polarized fashion. The introduction and use of a polarizing energy transfer effect has led to
a second generation of photoluminescent polarizers with further enhanced efficiency.10
The latter PL polarizers additionally comprise a randomly oriented sensitizer which
maximally harvests optical energy by isotropic absorption, efficiently transfers the energy
to the oriented, photoluminescent polymer which, subsequently, emits linearly polarized
EHO-OPPE
BPBC,2
BPB
Fig. 1: Molecular structures of the photoluminescent polymer (EHO-OPPE) and small-molecular
dyes (BPB, BPBCp) employed in tlm work.
101
In our previous studies,~13
a strongly luminescent, highly form-anisotropic
conjugated polymer (EHO-OPPE, a poly(2,5-dialkoxy-p-phenylene ethynylene) (PPE)
derivative, see Figure 1) typically was used as the PL emitter, and ultra-high molecular
weight polyethylene (UHMW PE) was employed as a highly ductile matrix polymer.
Uniaxially oriented blend films which comprised 1 to 2 % w/w of the conjugated polymer
were prepared by solution casting, drying, and subsequent tensile drawing in the solid
state. Uniaxial deformation of these films to draw-ratios of up to 80 resulted in a high
degree of orientation of the PL polymer and, consequently, state-of-the-art optical
anisotropy. However, the orientation of the PPE molecules in these "gel-processed"
UHMW PE blends9 did not follow the pseudo-affine deformation scheme,14"15 which is
often used to describe the orientation process of small-molecular dichroic (absorbing)
chromophores in tensile-deformed polymer matrices.16"19 Within this theoretical
framework, which is based on the aggregate model originally proposed by Ward et al.,14
the average orientation (cos2 o) of initially randomly oriented, statistical chain segments of
the macromolecules with respect to the direction of uniaxial deformation depends on a
sole parameter, the draw ratio X, and is given by:13'20
/ 2 \ $ U S i-\cos el =—z -—
TTT arctan((A" - 1) ")
Assuming that dichroic guest molecules adapt the orientation of the host polymer in which
is on 99
they are dispersed or dissolved, and applying Hermans' orientation function (//,):'"
"
"
3(cos2^"1 DRA-\ D*A0+2 (2);'
2 DR\+2 DRA0-l
the absorption dichroic ratio (DRa) of an oriented blend film can readily be calculated
from the draw ratio and the ultimate absorption dichroic ratio (DRao)- The latter accounts
for the parallel and perpendicular components of the extinction coefficient; the fact that
the transition dipole moment of the chromophore is not necessarily parallel to its
molecular axis; and for a possible misorientation of the guest molecule in the matrix.18'2'1
Substitution of DRa bv DRc (emission dichroic ratio) and DRao by DReo (ultimate
emission dichroic ratio) leads to an equivalent relation that allows to describe the
development of anisotropic photoluminescence.
102
As mentioned above, the generation of polarized photoluminescence by tensile
deformation of gel-processed EHO-OPPE / UHMW PE blends was found to deviate from
an affine orientation mode: only relatively low dichroic ratios were observed at low draw
ratios, and draw ratios of more than 50 were required to produce blends with highly
dichroic properties.9 This - from a technological point of view highly undesirable -
circumstance is a direct consequence of the particular phase behavior of these blends: due
to an initial phase-separation of the system (during gel-casting and drying), a subsequent
efficient orientation of the PPE molecules is achieved only after the agglomerates of the
latter "break up" or "smear out" which required elevated draw ratios. The transformation
of an originally phase-separated blend into a mixture that exhibits the properties of an
apparent molecular dispersion or "solid-solution" was indeed observed upon tensile
deformation of the PPE / UHMW PE blends reported before.9
Thus, with the above summarized potential relevance of PL polarizers and the
general technological advantages of melt-processing in mind; and the notion that phase-
separation between the photoluminescent guest and the matrix should be reduced during
formation of the pristine (i.e.. unstretched) blend films, we embarked on the preparation of
PL polarizers based on melt-processed blends. We systematically investigated the
orientational behavior of selected polymeric and small-molecular photoluminescent dyes
in a variety of polyethylenes of different molecular architecture (i.e., branch type and -
content, as well as molecular weight). We compared melt-processed materials with gel-
processed blends, and also with films in which the photoluminescent guest molecules were
incorporated by the frequently used diffusion method."4 ^ Most importantly, we introduce
an optimized melt-processable system which exhibits outstanding dichroic PL properties
at minimum draw-ratios.
Results and Discussion
EHO-OPPE / PE Blends
Our initial experiments were focused on melt-processed blends of EHO-OPPE26 of
a number-average molecular weight (AM of -lO'OOO gmol"1 (Figure 1) and different
polyethylenes (PE). In order to systematically investigate the molecular architecture of the
103
matrix polymer on the orientation behavior of the PPE guest, we employed low-density
polyethylene (LDPE) and several linear low-density polyethylenes (LLDPE) of similar
melt-flow index and, thus, approximately comparable molecular weights, but different
type and content of branches (reflected in part by the polymers' density p). In addition, we
utilized high-density polyethylene (HDPE) as a melt-proeessable model system for
UPIMW PE (Table 1). Films based on blends of 0.2 and I % w/w EHO-OPPE in PE were
prepared by melt-mixing the two components in a co-rotating twin-screw mini-extruder
and subsequent compression molding (see Experimental Section for details) at
temperatures between 155 and 170 °C. thus, well above the glass transition of the PPE
guest (about 100 °C). The films were subsequently drawn at temperatures of 70 - 120 °C,
i.e., around or slightly below the glass transition temperature of the neat PPE. It should be
noted, however, that a minute amount of crystallinity might be present in the PPE phase,
which, of course, would reduce the mobility of the latter under the drawing conditions
employed to some extent. The maximum draw ratios (A,max) and optimum drawing
temperatures are summarized in Table 1.
polymerbranches' melt flow index"
[g/10 mm]
density
[g/cnT]
drawing
temperature f°C]
maximum
draw ratio [-1
LDPE Long-chain (C4) 1 2 0 922 75 5
LLDPE Short-chain (Cs) 1 0.905 65 7
LLDPE Short-chain (C8) 1 05 0 919 90 to
LLDPE Short-chain (C8) 1 0.942 100 12
HDPE - 0.16 0.958 120 20
UHMW-PEi
n a. 0.93 120 80
Symbol in brackets indicates the co-monomer (Cs: octene; C4: butène).
bMeasured at 190 °C / 2 16 kg, except HDPE: 190 °C / 2.21 kg.
n.a.: not applicable
Table 1: Properties and processing parameters of the polv ethylenes used.
104
The anisotropic photophysical behavior of the drawn films was studied employing
polarized UV/VIS absorption and steady-state PL spectroscopy. In all PL experiments,
unpolarized light was used for excitation. In order to quantify the anisotropic optical
characteristics of the drawn films, we determined the dichroic ratios, defined for
absorption (DRA) and emission fDRf) as the ratio between the respective spectra measured
with polarization parallel and perpendicular to the drawing direction. In our evaluations,
we integrated the spectra, because the integrals are directly related to the energy of the
relevant electronic transitions and. hence, reflect the underlying physical processes best.
13
jB>,
'wC
BC
2j0_
450 550 650
x [nm]
Fig. 2: Polarized photoluminescence spectra of a melt-processed, oriented (X = 12) 0.2 % w/w
EHO-OPPE / LLDPE blend film, recorded under isotropic excitation and polarized
detection in/?- (solid line) and 5- (dashed line) mode
Figure 2 displays, as an example, the polarized PL spectra (recorded parallel and
perpendicular to the orientation direction) of a 0.2 w/w % EPIO-OPPE / LLDPE blend film
of a draw ratio of 12. which was characterized by a DRr of 11. The influences of the draw
ratio and the architecture of the matrix polymer on the dichroic ratio of the different EHO-
OPPE / PE blends are summarized in Figure 3, For purpose of comparison, previously
published data9 of gel-processed 2 ck w/w EHO-OPPE / UHMW PE blend films are also
included. Data of the 1 w/w % EHO-OPPE / LLDPE blend films were comparable to those
of the 0.2 % w/w series but, for the sake of clarity, we omitted these results in Figure 3.
Also, our experiments indicate that the orientational behavior of EHO-OPPE is similar in
all LLDPE blends under investigation which therefore have been summarized as one series
105
in Figure 3. The latter observation suggests that the content of hexyl side chains in these
matrix materials only has a minor influence on the development of the guest's orientation
behavior; except that it leads to different optimum drawing temperatures and maximum
draw ratios. Importantly, the data in Figure 3 reveal that the orientation behavior of the
PPE guest significantly differs when comparing blends with LLDPE, HDPE, and UHMW
PE: the slope of the initial linear increase of DRj. with draw ratio is dramatically higher in
matrices of LLDPE than UHMW PE. HDPE seems to represent an intermediate between
these systems. LDPE - not shown in the graph - yielded similar dichroic ratios as
comparable HDPE-based films, but exhibited a maximum draw ratio of only about 5.
Clearly, at low draw ratios, the EHO-OPPE / LLDPE blends are characterized by a
significantly higher orientation of the PPE guest than UHMW PE-based blends of
comparable draw ratios. However, due to the lower maximum draw ratio of the present
LLDPE-based blends (Xmax ~ 12, see Table 1). and the outstanding drawability of the gel-
processed UHMW PE blends (Xmax ~ 80), the maximally achievable DRe was found to be
higher in the latter (18.5 vs. 12). The data presented in Figure 3 suggest that HDPE and
LDPE are less suitable matrix materials for the preparation of highly oriented blends with
EHO-OPPE, since these polymers seem to combine a limited drawability (particularly
LDPE) with a rather unfavorably low orientation efficiency of the PPE guest. Figure 3 also
shows theoretical data for optical anisotropy of EHO-OPPE / PE blends calculated under
the assumption of an affine orientation behavior of the luminescent guest molecules (Eqs.
1-2), and employing an ultimate dichroic ratio, DRfo. of 19 for this system (DRco was
semiempirically determined by matching with the highest experimental DRE(Xmaf)).
It is evident that in none of the blends under investigation, the conjugated
macromolecules exhibit an orientational behavior, which precisely followed the pseudo¬
affine deformation scheme. Plowever. the results unequivocally demonstrate that among
the various systems studied EHO-OPPE maximally adopts the orientation of the matrix in
LLDPE-based blends. This result is consistent and explained with the different phase
behavior of the investigated blends. As discussed above, pristine, gel-processed EHO-
OPPE / UHMW PE blends suffer from an initial phase separation of the two polymers. We
have shown earlier that in this system a transition into a molecular dispersion or "solid-
solution" can very sensitively be monitored by the shape of the emission spectra of these
materials.
106
i——i 1 1—-i < 1—-i 1 . r
/ Htf>
• LLDPE
'/ "*o
°. ° HDPE
/ * °m ' UHMW-PE
*.° L_i I i_J I i L_j___J i 1 ,
1 10 20 30 40 50 60 70 80
draw ratio [-]
Fig. 3: Emission dichroic ratios of oriented films based on blends of 0.2 % w/w EHO-OPPE /
LLDPE, 0.2 % w/w EHO-OPPE / HDPE, and 2 % w/w EHO-OPPE / UHMW PE, as a
function of draw ratio. The solid line reflects the theoretical limit, assuming affine
deformation and employing an ultimate dichroic ratio of 19 for the EHO-OPPE / PE
system (Eqs. 1-2).
The emission spectrum of the undrawn gel-processed EHO-OPPE / UHMW PE
blend is relatively broad and only poorly resolved; it is comparable to the one of the neat,
amorphous or partially crystalline film of the conjugated polymer (Figure 4a) and indicates
that a phase-separation between the PPE and LTHMW-PE appears to have occurred. By
contrast, the emission spectrum of the drawn EHO-OPPE / UHMW PE blend is fairly
narrow, shows well-resolved vibronic features, and virtually matches the spectra of the
PPE in solution (Figure 4b). Thus, in the latter system, the PPE appears to behave as if it
was molecularly dispersed or "dissolved"" in the solid polyethylene matrix. Importantly,
and very much in contrast to the pristine, undrawn. EHO-OPPE / UPIMW PE blends,
similar well-defined molecular features were observed for the emission spectra of the
undrawn melt-processed EHO-OPPE / LLDPE blends. The latter indicates that phase-
separation between the luminescent guest and the matrix polymer was - at least to a certain
extent - avoided in this system, which explains its favorable orientation behavior, as
discussed above. It should be recognized, however, that a truly molecular dispersion or
solution in the polymer blends is. of course, highly unlikely for simple thermodynamic
C-\J
15
X
Q
? 10
107
reasons, although the photoluminescent characteristics of a true solution appear to be
present.
It should be noted that the present melt-processed EHO-OPPE blends show
somewhat lower dichroic ratios in absorption than in emission (when calculating DRa
from the ratio of the peak maxima, DRa was found to be up to a factor of two lower than
the respective DRe). This behavior is in contrast to the previous results obtained for the
EPIO-OPPE / UHMW PE blends,9 and also at variance with those of the below presented
blends of small-molecular PL dyes and LLDPE, for which similar absorption and emission
dichroic ratios were measured. It suggests that the absorption characteristics of the melt-
processed EHO-OPPE / LLDPE blends are partially governed by some remaining, hardly
oriented and poorly dispersed (and therefore also less luminescent, see Refs. 9 and 26),
perhaps crosslinked EHO-OPPE-elusters; while the emission characteristics are dominated
by highly oriented, well dispersed (and therefore highly luminescent) luminophores.
solution (CHCI3) .
undrawn LLDPE
drawn UHMW-PE"
. .
I S . 1 1__~J u
400 500 600 700 800 400 500 600 700 800
a) X [nm] b) X [nm]
Fig. 4: a) Photoluminescence spectra of a spin-cast film of neat EHO-OPPE and an unoriented
EHO-OPPE / UHMW PE blend film, b) Photoluminescence spectra of EHO-OPPE
solution in CHCL. an unoriented EHO-OPPE / LLDPE blend film prepared by melt-
extrusion and subsequent molding, and an oriented EHO-OPPE / UHMW PE blend film.
Undrawn UHMW-PE
Neat PPE
go
cCD
Q_
=5
CO
GO
C
CD
D_
108
BPB / PE and BPBCn / PE Blends
Triggered by the above findings, we have further undertaken to investigate the
orientational behavior of melt-processed blends based on LLDPE and two low-molecular
weight analogues of PPE (Figure 1). Very much like EHO-OPPE, these luminophores -
based on a l,4-bis(phenylethynyl)benzene (BPB) moiety - exhibit high aspect ratios and
large PL quantum efficiencies,29 and we surmised that their electronic transition dipole
moments could optimally coincide with their geometric long axis. In order to increase the
compatibility with the PE matrix, in one instance we derivatized the 1,4-
bis(phenylethynyl)benzene moiety with dodecyloxy groups in the para positions of the
two terminal phenyl rings, resulting in BPBCi2-J°
Films based on blends of 0.2 % and 2 %, and in case of BPBC12 also 0.8 % w/w of
the photoluminescent guest in LLDPE (p = 0.919 gem"1) were prepared by melt-processing
as described above and were subsequently drawn at 90 T. For the purpose of comparison,
we also have incorporated the BPB and BPBCn guests into unstretched LLDPE films by
"guest diffusion", i.e.. by swelling the latter with a solution containing the
photoluminescent dye, prior to tensile deformation. Comparative absorption experiments
with the melt-processed films indicate concentrations of -0.05 % w/w of the
photoluminescent guest in the latter.
Blends containing BPB were processed at 155 or 185 °C, i.e., below or above the
melting temperature of the BPB guest (176 - 178°C). All BPB / LLDPE blends that were
either processed at 155 °C, or comprised 2 % w/w BPB were found to exhibit a large-scale
phase-separation between BPB and the LLDPE matrix, as unequivocally visualized with
optical microscopy. By contrast, a 0.2 % w/w BPB / LLDPE blend film processed at
185 °C had a homogeneous appearance (as determined by polarized optical microscopy).
Drawn films of this material were found to exhibit rather strongly polarized absorption and
emission, as shown in Figure 5a for a blend film of a draw ratio of 9, which was
characterized by a DRe of 11 and a DRA of 14. Interestingly, significantly higher dichroic
ratios were measured for the drawn BPB / LLDPE blend films that were prepared by
diffusion of the PL guest into a film of the PE matrix. For example, a film of a draw ratio
of 10 was characterized by a DRe of -44 and an about equally high DRa (Figure 5b;
additional data for some lower draw ratios are given in Figure 7). Note that the accurate
determination of DRa in these highly oriented films is stifled by the extremely low
109
absoiption of ^-polarized light and the comparably large contribution from light scattering
of the matrix, resulting in a potentially large experimental error for DRa.
i5.
280 300 320 340 360
X [nm]
_ ...
-J 1 1 1 ! 1
350 400 450 500 350 400 450 500
a) \[nm] b) l[nm]
Fig. 5: Polarized photoluminescence spectra of oriented (2 = 9) BPB / LLDPE blend films,
recorded under isotropic excitation and polarized detection in p- (solid line) and s-
(dashed line) mode, a) Blend film comprising 0.2 9c w/w BPB, prepared by melt-extrusion
at 185 °C and subsequent molding, b) Blend film comprising approximately 0.05 % w/w
BPB, prepared by diffusing the PL dye into an unstretched film of pure LLDPE and
subsequent tensile deformation: the inset shows polarized absorption spectra of the same
film, recorded with/?- (solid line) and s-polarized (dashed line) light.
The liquid-crystalline BPBCn displayed a more complex thermal behavior than
BPB, with transitions at 112 (solid-solid), 155 (solid-solid). 174 (solid-smectic), 185
30
(smectic-nematic), and 197 °C (nematic-isotropic melt). Blends containing BPBQ2 were
processed at 155 or 180 °C; all BPBC12 / LLDPE blends had a homogeneous appearance
(as determined by polarized optical microscopy). Drawn films of this materia] show
unexpectedly highly polarized absorption and emission, as demonstrated in Figure 6 for a
0.2 % w/w BPBC12 / LLDPE blend film of a draw ratio of 10.
The latter was characterized by an (integrated) DRe of about 50 and a similar DRa-
Experiments with blends processed at 155 or 180 °C revealed that for this particular
composition the orientation of the photoluminescent guest, as reflected by DRe, was
independent of the processing temperature. The dichroic ratio of the BPBQ2 / LLDPE
systems was found to slightly decrease when the concentration of the photoluminescent
guest was increased (0.2 % w/w. X = 9. DRr=l%; 0.8 % w/w. zl= 9, DRE= 37; 2 % w/w,
=3
c<D
D_
>,
'toc0)
Ç
JjCL
110
X = 9, DRe = 24). We attribute the latter phenomenon to a limited solubility of the dichroic
PL dye in the matrix polymer. Oriented BPBC12 / LLDPE blend films prepared by guest-
diffusion exhibited an essentially similar maximum orientation of the photoluminescent
guest as the melt-processed 0.2 % w/w BPBC12 / LLDPE blend.
CDo
C
COjai~
o00
<
ZS
cri
C/>
c
0
350
M
400 450
X[nm]
500
Fig. 6: Polarized absorption and photoluminescence spectra of an oriented (A = 10) 0.2 % w/w
BPBC12 / LLDPE blend film, a) Polarized absorption spectra recorded with/)- (solid line)
and 5-polarized (dashed line) light, b) Polarized photoluminescence spectra, recorded
under isotropic excitation and polarized detection in p- (solid line) and s- (dashed line)
mode.
In order to compare and rationalize the orientation behavior of the
photoluminescent guest molecules in the above described films, the emission dichroic
ratios of oriented BPB / LLDPE and BPBCo / LLDPE blend films prepared by melt-
processing and guest-diffusion are summarized in Figure 7; together with the theoretical
limit, assuming affine deformation, and employing an ultimate dichroic ratio of infinity
(Eqs. 1-2). It is evident that in films prepared by guest-diffusion, the experimentally
determined values for DRe approximately follow the theoretical predictions,
demonstrating that the transition dipole moments of the presently employed luminophores
indeed coincide with their geometric axis, and perfectly adapt the orientation of the
polyethylene matrix. This behavior is consistent with the supposition that fhe
photoluminescent guest molecules are moleculariy dispersed in the amoiphous fraction of
the polyethylene during the diffusion process/ By contrast, the optical anisotropy of melt-
processed BPB / LLDPE blends is significantly lower than the predicted values. We
Ill
attribute the latter observation to phase-separation of this system resulting in an
'immobilization' of the high-melting BPB under the deformation conditions applied. The
observed macroscopic phase separation in many of the investigated BPB / LLDPE blends
corroborates this view.
80
70
60
150
£
X
Q
40
30
20
10
1
• BPB melt-processedo BPB diffusion
» BPBC.? melt-processedn BPBC,„diffusion
5 10
draw ratio 1-1
15
Fig. 7: Emission dichroic ratios of oriented BPB / LLDPE and BPBC12 / LLDPE blend films
prepared by melt-processing (comprising 0.2 % w/w of the photoluminescent guest) and
diffusion. The solid line reflects the theoretical limit (Eqs. 1-2), assuming affine
deformation and employing an ultimate dichroic ratio, DRko, of infinity.
Most importantly, melt-processed BPBC12 / LLDPE films exhibited extraordinary
high optical anisotropics that exceed values of the blends prepared by diffusion, as well as
those calculated on the basis of affine deformation. This extremely favorable behavior
points to a molecular dispersion of the BPBC12 luminophores in the polyethylene matrix
after thermoplastic processing (at least in a concentration regime of between 0.2 - 0.8 %
w/w of the luminescent guest), which ensures an eminent orientabihty of the latter. A
slightly "accelerated" or more efficient orientation of the former, when compared to the
affine deformation mode, might be explained with the fact that this model only reliably
reflects an average orientation, but fails to adequately separate the components of
crystalline and amorphous phase. Thus, it can be speculated that in case of the present
melt-processed BPBC 12 / LLDPE blends the photoluminescent guest molecules not
necessarily adopt an average orientation. However, we should also clearly point out the
noticeable scattering of the data presented in Figure 7. The experimental error in the
112
determination of DRe(X) is, in our view, largely related to two factors: first, the value of
DRe is computed by dividing the emission intensity of ^-polarized by that of ^-polarized
light. For the present, highly oriented systems, the latter was an extremely small number
and, thus, even relatively small fluctuations in this intensity caused, for example, by
scattering from structural defects, reflections, mismatch of angle, quality of the analyzer,
etc., may result in a significant experimental error for DRe. Perhaps an even more
important uncertainty arises from the determination of the draw ratio, which in laboratory
samples is not necessarily completely homogeneous throughout the whole film, and,
particularly in case of films of low draw ratio, may suffer from some inaccuracy.
Conclusions
In summary, we have shown that melt-processing and subsequent tensile
deformation of blends of different photoluminescent guest molecules and polyethylene - in
particular LLDPE - can lead to an outstanding orientation of the conjugated polymer guest.
resulting in state-of-the-art polarized photoluminescence and absoiption of the prepared
films. Experiments suggest that maximum orientation and optical anisotropy are obtained,
if the photoluminescent guest is of high aspect ratio, exhibits electronic transition dipole
moments that optimally coincide with its geometric long axis, and if phase-separation
between the photoluminescent guest and the matrix polymer is reduced or avoided during
the preparation of the pristine blend films. As a result of these findings, an optimized,
melt-proeessable blend based on 1.4-bis(4-dodecyloxy phenylethynyl) benzene and linear
low-density polyethylene was developed that allowed production of photoluminescent
polarizers which at draw ratios of only 10 exhibited (integrated) dichroic ratios exceeding
50.
Finally, we would like to briefly comment on the implications of the above
discussed observations for investigation of orientation processes of polymers with
polarized spectroscopy in general. In the past, the orientation development in tensile-
deformed polymers has been extensively studied, by monitoring the optical anisotropy
caused by dichroic absorbing as well as photoluminescent dyes incorporated in the
polymer of interest (see for example Refs. 23-25, 31). Here, we clearly demonstrated that
the orientation of an incorporated dichroic molecule not necessarily follows the one of the
113
polymer matrix. Significant variation can be observed for a given material system,
depending on, for example, the method of preparation, the concentration of the dichroic
guest, the deformation temperature, etc. Thus, we urge that great caution should be taken,
when relating the anisotropic optical properties resulting from incorporated guest
molecules to the orientation of the matrix polymer.
Experimental
Materials
EHO-OPPE26 (Mn -lO'OOO gmoll) and BPBC1230 were prepared according to
procedures published elsewhere. BPB was purchased from GFS Chemicals. All
polyethylenes (Table 1) are commercially available and were obtained from Dow (LLDPE:
Dowlex NG5056E, p = 0.919 gem"3; Dowlex BG2340, p = 0.942 gem"3; Attane SL4102,
p = 0.905 gem"'. LDPE: LDPE 310R) and DSM (HDPE: Stamylan HD8621). All solvents
were of analytical grade quality and were purchased from Fluka.
Preparation of Blends
PE pellets (5 g) were first coated with the PL dye by casting a solution of the latter
(10 or 100 mg) in toluene (EHO-OPPE) or CLIC13 (BPB, BPBC12) over the preheated (-60
°C) PE pellets and evaporating the solvent. The coated PE pellets were fed into a
recycling, co-rotating twin-screw mini-extruder (Commercially available from DACA
Instruments, Santa Barbara, CA), mixed for 10 mm at 155 °C (all dyes in LLDPE and
LDPE), 170 °C (EHO-OPPE in HDPE). 180 °C (BPBCj, in LLDPE) or 185 °C (BPB in
LLDPE), and subsequently extruded.
Preparation of Films
Films were prepared by compression-molding the extruded blends between two
Mylar®-foils in a Carver press at 150 °C (all dyes m LLDPE and LDPE), 165 °C (EHO-
114
OPPE in HDPE), 180 °C (BPBC12 in LLDPE) or 185 °C (BPB in LLDPE). All resulting
blend films had a thickness of about 110 - 130 p.m. The films were drawn at temperatures
of 70 - 120 °C on a thermostatically controlled hot shoe. Draw ratios were calculated from
the displacement of distance marks printed on the films prior to drawing.
In addition, blend films were also produced by diffusing the small-molecular PL
dyes into unstretched films of pure LLDPE. The LLDPE films were prepared as described
above (without the addition of the PL dye) and, subsequently, immersed for 24 h in a
solution of the dye (0.5 - 2 % w/w) in CHCE. In the following, the films were rinsed with
CHCL, dried, and oriented as described above.
Optical Characterization
For the photophysical experiments, the polymer films were sandwiched between
two quartz slides, applying a minor amount of poly(methylphenylsiloxane) 550® fluid
(Aldrich, viscosity 125 centistokcs) m order to minimize light scattering at the film
surfaces. Polarized IJV-VIS spectra were recorded with a Perkin Elmer Lambda 900
instrument, fitted with motor driven Glan-Thomson polarizers. Scattering effects of the
matrix were compensated in the absorption measurements by subtracting the spectra of
pure PE films of comparable draw ratio and thickness. Absorption dichroic ratios, DRA,
were determined by the ratio of the integrals of the main absorption bands (EPIO-OPPE:
350 - 540 nm; BPB and BPBCn: 280 - 370 nm) measured through a polarizer with its
optical axis parallel and perpendicular, respectively, to the deformation direction of the
film. Corrected PL spectra were recorded m front-face mode on a SPEX Eluorolog 3
(Model FL3-12), using unpolarized light for excitation (excitation at 440 nm for EHO-
OPPE-based blends, and 322 nm for BPB and BPBCn-based blends) and a Glan-Thomson
polarizer on the detector side. In order to compensate for the polarization-sensitivity of the
instrument, a depolarizer was placed behind the latter. Emission dichroic ratios, DRE, were
determined by the ratio of the integrals of the emission bands (EHO-OPPE: 450 - 675 nm;
BPB and BPBCi2: 330 - 520 nm) measured through a polarizer with its optical axis
parallel and perpendicular, respectiveh. to the deformation direction of the film.
115
Acknowledgments
We wish to thank Dr. C. Bastiaansen for many helpful discussions and S.
Dellsperger for crucial assistance with the polymer synthesis.
References
1 L.K.M. Chan, in The Encyclopedia ofAdvanced Materials, vol. 2, eds. D. Bloor,
R.J. Brook. M.C. Flemings, S. Mahajan, Elsevier Science Ltd., Oxford, 1994, p.
1294.
2 T.J. Nelson, J.R. Wullert II, in Electronic Information Display Technologies,
World Scientific Publishing, Singapore, 1997.
3 M. Schadt, J. Fünfschilling, Jprt. J. Appl Phys.. 1990, 29, 1974.
4 D.J. Broer, J. Lub, G.N. Mol, Nature, 1995, 378, 467.
5 D. Coates, M.J. Goulding. S. Greenfield, J.M.W. Hanmer, E. Jolliffe, S.A. Marden,
O.L. Parri, M. Verrall, SID International Symposium, Digest of Technical Papers,
1996,27,67.
6 R.A.M. Hikmet, Mol Cryst Liq. Cryst, 1991, 198, 357.
7 R.A.M. Hikmet.,/. Appl. Phys., 1990, 68, 4406.
8 Y. Dirix, H. Jagt, R. Hikmet, C. Bastiaansen. J., tppl. Phys.. 1998. 83, 2927.
9 C. Weder, C. Sarwa. C. Bastiaansen, P. Smith, Adv. Mater., 1997. 9, 1035.
10 A. Montali, C. Bastiaansen, P. Smith, C. Weder, Nature. 1998, 392, 261.
11 C. Weder, C. Sarwa. A. Montali. C. Bastiaansen, P. Smith. Science, 1998, 279,
835.
12 A. Montali, P. Smith. C. Weder, J. Mater. Sei, in press.
13 A.R.A. Palmans, P. Smith, C. Weder, Macromolecules, 1999, 32, 4677.
14 I.M. Ward, Proc. Phys. Soc. 1962, 80, 1176.
15 Y. Dirix, T.A. Tervoort. C. Bastiaansen. P.J. Lemstra. J Text Inst, 1995, 86, 314.
16 Y.T. Tang, P.J. Phillips. E.W. Thulstrup, Chem. Phys. Lett., 1982, 93, 66.
17 P.J. Phillips, Chem. Rev., 1990, 90. 425.
18 Y. Dirix, T.A. Tervoort, C. Bastiaansen. Macromolecules, 1995, 28, 486.
116
19 Y. Dirix, T.A. Tervoort, C. Bastiaansen, Macromolecules, 1997, 30, 2175.
20 O. Kratky, KolloidZ„ 1933, 64, 213.
21 R.B.D. Fraser, d. Chem. Phys., 1953, 21,1511.
22 P.H. Hermans, D. Pleikens, Rec. Trox. Chem. Pays Bas, 1952, 71, 49.
23 J. Michl, E.W. Thulstrup, in Spectroscopy with Polarized Light, VCH
Publishers Inc., New York, 1986.
24 E.W. Thulstrup, J. Michl, J. Phys. Chem., 1980, 84, 81.
25 E.W. Thulstrup, J. Michl, J. Am. Chem, Soc,. 1982, 104, 5594.
26 C. Weder, M.S. Wrighton. Macromolecules. 1996. 29. 5157.
27 It should be noted that differential scanning calorimetry and dynamic mechanical
thermal analysis data of the pure EHO-OPPE indicate an onset of thermal
decomposition due to crosslinking at about 130 °C (D. Steiger, P. Smith and C.
Weder, Macromol. Rapid. Commun., 1997.18, 643). However, the blends prepared
here were fully soluble after melt-processing and the absorption and PL emission
spectra of these solutions were identical to an untreated reference, indicating that
no significant degradation of the PPE occurred.
28 P.J. Flory. Macromolecules, 1978, 11, 1138.
29 S. Nakatsuji, K. Matsuda, Y. Uesugi, K. Nakashima, S. Akiyama, W. Fabian, J.
Chem. Soc. Perkin Trans., 1992, 1, 755.
30 A.R.A. Palmans, M. Eglin, A. Montali, P. Smith, C. Weder, Chem. Plater., in
press.
31 E.W. Thulstrup, J. Michl, J.H. Eggers. J Phys Chem., 1970, 74, 3868.
117
8. Poly(/?-phenylene ethynylene)-Based Light-
Emitting Devices
Abstract
We here report polymer light-emitting diodes with substituted poly(p-phenylene
ethynylenes) as the emissive layer. Yellow-green electroluminescence was observed for
different poly(t>-phenylene ethynylene) derivatives. Surprisingly, and importantly in view
of stability issues, devices with an Aluminium cathode were found to have a higher
external quantum efficiency (0.035%) and a lower onset voltage for electroluminescence
(10.8 V) than those with a Calcium cathode. These results are explained in terms of a
lower energy barrier for electron injection than for hole injection, consistent with the
ionization potential of polyOy-phenylene ethynylenes) which was determined to be 6.3 eV
below vacuum level with ultraviolet photoelectron spectroscopy and 5.8 eV with
cyclovoltammetry.
This chapter is reproduced from: A Montali, P. Smith. Ch Weder. Synth Met. 1998. 97, 123.
118
Introduction
Since the discovery of electroluminescence (EL) from polymeric light-emitting
diodes (LEDs),1 the performance and availability of colors has significantly improved by
making multilayer devices and using a variety of light-emitting polymers.2"1 PolyOp-
phenylene vinylene) (PPV) and its derivatives have hereto been the material of choice for
EL applications."
Only few groups have studied the photophysical properties of ~po\y(p-
phenylene ethynylene)s (PPEs), which feature a triple rather than a double bond in the
conjugated backbone.4"'1 EL properties of PPE-based LEDs were studied by Shinar et
al./" and PPE is generally not considered to be a promising material for LED
applications.3'7 Here, we report the fabrication and characterization of single layer LEDs
based on two different PPE derivatives, confirming preliminary experiments carried out
1 0
in our group,~
and, thus, demonstrate the principal suitability, and correct
misconceptions'7with respect to the use of PPEs as useful emitting layer in polymer
LEDs. Surprisingly, and importantly in view of stability issues, devices with an Al
cathode were found to have a higher external quantum efficiency and a lower onset
voltage for EL than those with a Ca cathode. These results are explained in terms of a
lower energy barrier for electron injection than for hole injection, which is consistent
with the ionization potential of PPEs which was determined to be 6.3 eV below vacuum
level with ultraviolet photoelectron spectroscopy (UPS) and 5.8 eV with
cyclovoltammetry (CV).
Experimental
The PPE derivatives selected for this work are O-OPPE, substituted with only
linear alkyloxy side chains, and EHO-OPPE. derivatized with linear and sterically
hindered alkyloxy groups in an alternating pattern (Fig. 1). Both polymers were prepared
according to the procedures previously described10 and had number-average molecular
weight Mn of about 10'000 gmol"1.
119
Fig. 1: Chemical structures of the substituted poly(p-phenylene ethynylenejs used in this work.
Single-layer EL devices were produced by spincoating filtered solutions of the
PPEs (1% w/w in toluene) onto ITO-coated (20 O/sq.) glass substrates. The thickness of
the films was determined to be ~ 100 nm ±10nm with a Tencor Instruments a-step
profilometer. An Al (100 nm), Ca (30 nm) or Cr (100 nm) cathode, and in the case of Ca
an additional gold protection layer (70 nm), was deposited onto the PPE films with a
Baltec MED 020 coating system at pressures of ~ 2.0'10"5 mbar, to yield pixilated
structures with active areas of 3x3 mm".
Devices were operated and characterized in a glove-box under inert N>
atmosphere at room temperature. The current-voltage charcteristics and light intensities
were simultaneously measured with a Keithley 237 SMU and a calibrated Si-photodiode.
Quantum efficiencies were derived according to the method described by Greenham et
1 7al. Emission spectra were recorded using a SPEX-Fluorolog 3 spectrometer. The
absolute brightness was measured using a Minolta LS 100 Luminance Meter, fitted with
a close-up lens 110. The valence band edge was determined on thin films (d<30 nm)
spincoated from toluene onto ITO-coated glass slides by UPS using a monochromatized
He I radiation source (hv= 21.2 eV). CV measurements were carried out in
acetonitrile/O.lM TBAPF6 solutions at glassy carbon electrode vs. Ag/AgN03 as
reference electrode.
120
Results and Discussion
Typical values for relevant parameters of the investigated PPE-based devices are
given in Table 1. Essentially identical characteristics were observed for LEDs based on
O-OPPE and EHO-OPPE, in agreement with previous studies, which revealed similar
photophysical properties for amoiphous films of these polymers.
Emitting Layer EHO-OPPE EHO-OPPE EHO-OPPE O-OPPE O-OPPE
Cathode Material Al Ca Cr AI Ca
EL threshold
voltage [V]
10.8 14.5 19.7 11.0 14.2
Ext. quantum
efficiency [%]
0.035 0.023 0.015 0.032 0.020
max. brightness
[cd/m2]
80 38 ! 33
ii
.y
_. .
80 35
Table 1: Electroluminescence-characteristics of substituted polyOy-phenylene ethynylenes),
EHO-OPPE and O-OPPE. single layer LEDs.
A typical EV curve, showing current density and luminance vs. voltage, for an Al
/ O-OPPE / LIT) LED is shown in Figure 2, the inset illustrates the dependence of light
output on the injected current density for devices with different electron injecting
contacts. In contrast to results reported for PPV-based LEDs.2"1 the nature of the cathode
material - when comparing aluminium and calcium - was found to only marginally
influence the characteristics of the devices investigated here. The EL threshold voltage
(defined as the voltage at which the photodiode signal starts to increase monotonically)
of devices with an Al rather than a Ca cathode, in fact, was slightly lower (10.8 V vs.
14.5 V) and external quantum efficiencies of Al devices were higher than the latter
(0.035% vs. 0.02%). Devices comprising a Cr cathode, however, exhibit a clearly higher
EL threshold voltage (19.7 V) and lower quantum efficiencies (0.015 %). All devices
were found to emit yellow-green light, with an emission maximum at 535 nm. The
121
brightness measured on devices with an Al cathode at a driving voltage of 22.0 V was 80
cd/m2. Highest current densities and, thus, maximum brightness were obtained for
devices comprising Al cathodes. Absorbance, photoluminescence (PL), and EL emission
spectra are shown in Figure 3 for the O-OPPE single-layer LEDs. The EL spectra
essentially match the PL spectra but additionally show a distinctive red-shifted band.
This feature is in accordance with data of other groups and can be attributed to defect-
induced trap states, which can block charge carriers and, thus, yield lower energy EL
• 4,5emission.
-
1 1 ' 1 • I ' f 1 • i '
-
° Al0
'
t"
nCa
3, »Or •/^,
3*
-
c- ;
a> ö"
"H-
UJyC<yrrA*."> * •
;-
-
.««sM^*^" •I -
•70 0 10 bO SI 100 120 ,*/
-
Current density [mA -nf]
J-
-
I
^f
i , i . i . i ,
i
_I , 1 , 1 >. J .1
I
, 1 _j L_
-5 0 5 10 15 20 25 30
CM
F 100
a
F80
>,
'Tn 60
c
<D"O 40
C(13
t ?03
Ü
0
Voltage [V]
Fig. 2: Typical current density vs. voltage (solid line) and EL-intensity vs. voltage (dotted line)
curves for an Al / O-OPPE / ITO LED. Inset: Luminance-current density characteristics
of O-OPPE single-layer LEDs with different cathode materials; the three curves are
shown in the same scale.
The fact that the nature of the cathode - when comparing Al and Ca - barely
influences the characteristics of the PPE-based devices contrasts the situation observed
for many single-layer devices based on PPE-4 and PPV-derivatives. In the latter case, a
change from an Al to a Ca cathode typically leads to a lower EL threshold voltage and an
improved quantum efficiency.2 due to the enhanced electron injection from Ca, which
has a significantly lower work function than Al (2.9 eV and 4.3 eV respectively18). As
our internal reference, and to control the quality of our Ca-layers, single layer devices
with unsubstituted PPV as the emitting layer were produced as well.10 and. as expected, a
122
significant increase in quantum efficiency was observed when using Ca rather than Al as
the cathode.
CDO
c
CO
-Q
ow
<
300 400 500 600 700
X [nm]
m
CD
22,f—t-
I?
800
Fig. 3: Absorbance (solid line), PL (dotted line), and EL (dashed line) emission spectra for an Al
/O-OPPE/ITO LED.
The behavior of our PPE-based devices is. however, consistent with the potentials
of the conduction and valence band edges of these materials. The upper edge of the
valence band was determined with UPS to be 6.3 eV below the vacuum level. The
bandgap was determined to be 2.4 eV for both. EHO-OPPE and O-OPPE, from the onset
of emission in fluorescence excitation scans. From these data, the lower edge of the
conduction band was calculated to be 3.9 eV below vacuum level. Valence and
conduction band edges of EPIO-OPPE determined with CV measurements were found to
be 5.8 eV and 3.6 eV below vacuum level respectively, slightly higher than the values
calculated for unsubstituted PPEÉ0 5.6 and 3.4 eV respectively. The discrepancy of a few
tenths of eV between these values and the UPS results is due to intrinsic differences
between measurements in solution and in the solid state."' According to these data, a
significantly lower energy barrier has to be overcome by the electrons being injected
from the metal cathode into the conduction band of the investigated PPEs than by the
holes injected from the ITO anode, the valence band edge of which we determined with
UPS to be at 5.0 eV below vacuum. This is in contrast to PPV derivatives, where hole
injection and transport are facilitated and electron injection is the limiting factor.2 The EL
m
chaiactenstics, which also demonstiate election injection and light emission with Ci as
the cathode, and the results of the UPS and CV measurements, suggest good election-
tianspoitmg piopeities toi the investigated PPL s
ta :yv
vi t y\
Ci 4 11 V
Fig. 4 Schematic lepiesentation ot eneisv levels m a PPb based sins>le layei LbD as
detei mined bv ITS
The highei elfictency obseived when using M as a cathode îathet than Ca is due
to a nioie balanced chaige injection, since election inject ion horn Ca occuis vsithout
having to oveicome an eneigy bamei (Pig 4) The disuepancy between the lesults
published by Swanson el al6 foi PPP-bascd LEDs and the picsent woik could be related
to the use of diffeient chalko\v-PPF dei natives hi addition we omitted to bake the
devices at elevated temperatmes (150 C). m oidei to ictain the device peitoimance. as
we have demonstiated eaihei." that at least the dialkoxv PPE dei natives investigated
heic undeigo uieveisible chemical ciosshnkmg undei the conditions denoted
Fig. 5 HO/LHO-OPPb / Al I bD mopeiatton the size ot the device is 9 mm the pictuie was
taken in da> light
i
i
i
r
i
i
i Î MO r
1
L•> iH\ PPL |no i
i
o y v i
HOMO j
124
Conclusions
In summary, we have demonstrated that dialkoxy-substituted PPEs can
effectively be used as the emitting layer in polymeric LEDs, as can be clearly seen in
Figure 5. While no efforts were made to optimize the device characteristics with respect
to brightness and efficiency, yellow-green electroluminescence with a brightness of up to
80 cd/m2 at external quantum efficiencies of up to 0.035 % was observed for the PPE-
based single-layer devices. The positions of the valence and conduction band edges
suggest that in the case of PPEs the hole injection barrier is a limiting factor, while the
injection of electrons from the low work function electrode is facilitated. The results
obtained for PPE-bascd LEDs comprising different cathode materials confirm this
assumption. Consequently, these devices favorably comprise a cathode with moderate
work function (i.e. Al rather than Ca) and, thus, exhibit enhanced oxidation stability.
Acknowledgements
The authors are deeply indebted to Dr. Ye Tao. Institute for Quantum Electronics,
ETH Zürich, for UPS studies, to Dr. Mukundan Thelakkat, Makromolekulare Chemie I,
University of Bayreuth, for CV studies, and to Dr. Andreas Greiner and Michael Ishaque,
Institut für Physikalische Chemie-Polymere, Philipps Universität Marburg, for
preparation of the PPV layers.
References
1 EH. Burroughes, D.D.C. Bradley, A.R. Brown. R.N. Marks, K. Mackay. P.L.
Burn, A.B. Holmes. Nature, 1990, 347, 539.
2 D.D.C. Bradley, Curr. Opin. Solid State Mater. Sei, 1996, 1, 789.
3 A. Kraft, A.C. Grimsdale, A.B. Holmes. Angew Chem. Int. Ed. 1998, 37,403.
4 L.S. Swanson. F. Lu, J. Shinar. Y.W. Ding. T.J. Barton. Proc. SPIE, 1993. 1910,
101.
5 J. Shinar. L.S. Swanson. Proc. SPIE, 1993. 1910. 147.
125
6 L.S. Swanson, J. Shinar, Y.W. Ding, T.J. Barton, Synth. Met, 1993, 55, 1.
7 W. Chen, S. Ijada-Maghsoodi. T.J. Barton. T. Cerkvenik, J. Shinar. Polym. Prepr.
ACSDiv. Polym. Chem.. 1995. 36, 495.
8 T.M. Swager, CG. Gil, M.S. Wrighton,./. Phys. Chem, 1995. 99, 4886.
9 D. Ofer, T.M. Swager, M.S. Wrighton, Chem. Mater, 1995, 7, 418.
10 Ch. Weder, M.S. Wrighton, Macromolecules, 1996, 29. 5157.
11 D. Steiger, Ch. Weder, P. Smith, Macromol Rapid Commun., 1997,18, 643.
12 A. Montali, Ch. Weder, P. Smith. Proc. SPIE. 1997, 3148, 298.
13 Ch. Weder, C. Sarwa, C. Bastiaansen. P. Smith, Adv. Mater., 1997, 9, 1035.
14 Ch. Weder, C. Sarwa, A. Montali, C. Bastiaansen, P. Smith, Science, 1998, 279,
835.
15 A. Montali, C. Bastiaansen, P. Smith, Ch. Weder, Nature, 1998, 392, 261.
16 For a recent review see R. Giesa, .7.M.S.-Rev. Macromol. Chem. Phys. C, 1996,
36,631.
17 N.C. Greenham, R.H. Friend, D.D.C. Bradley, Adv. Mater.1994. 6, 491.
18 CRC Handbook of Chemistry and Physics. 76th Edition, ed. D.R. Lide, CRC
Press, Boca Raton, 1995.
19 PPV layers were produced according to the procedures described by:
O. Schäfer, J. Pommerehne. W. Guss, H. Vestweber, H.Y. Tak, H. Bässler, G.
Lüssem, B. Schartel, C. Schmidt, V. Stümpflen, EH. Wendorff. S. Spiegel, C.
Möller, A. Greiner. Synth. Met., 1996. 82, 1.
20 EL. Bredas, R.R. Chance, R.H Baughman. R. Silbey. J. Chem. Phys., 1982, 76,
3673.
21 M. Thelakkat, R. Fink, P. Posch. L Ring. H.-W. Schmidt, ACS Polym. Prepr,,
1997,38.394.
127
9. Polymerie Light-Emitting Diodes Based on Poly(p-
phenylene ethynylene), Poly(triphenyldiamine) and
Spiro-quinoxaline
Abstract
Light-emitting diodes (LEDs) based on EHO-OPPE, a dialkoxy substituted poly(p-
phenylene ethynylene) derivative, have previously been demonstrated and EHO-OPPE was
found to be a good electron transporter. Here we present a study performed on LEDs based on
EHO-OPPE as the emitter, in which the device performance was enhanced by optimizing the
device composition and structure. EHO-OPPE was combined with a hole-conducting
poly(tripehnyldiamine) derivative (poly-TPD). Devices were prepared in a bilayer
configuration as well as in a single-layer configuration, with both materials blended in one
film that exhibits bipolar carrier transport abilities. The composition of the blends was
systematically varied to optimize the device performance.
The influence of an additional electron-transport and hole-blocking layer (a tetrameric spiro-
quinoxaline ether based on a spirobisindane core, spiro Qux) was subsequently investigated,
which was vapor-deposited on top of the hole-transporting and emitting layer. With respect to
a reference LED consisting of neat EPIO-OPPE only, the maximal photometric efficiency was
enhanced by a factor of 35 while the onset voltage for electroluminescence dropped from 15
V to 5 V. This improvement in the device performance is explained by a combination of a
favored hole injection by the poly-TPD and an electron-injecting and hole-blocking effect
exerted by the spiro-Qux.
This chapter is reproduced from: C. Schmitz. P. Posch, M Thelakkat. H.-W. Schmidt, A. Montali,
K. Feldman. P. Smith. Ch Weder. Adv. Mater., submitted.
128
Organic light-emitting diodes (OLEDs) based on low-molecular weight and polymeric1 9
semiconducting materials are in an advanced stage of application in flat-panel displays. '
After the first reports of Tang and Van Slyke in 1987.3 and Burroughes et al. in 1990,4 rapid
progress was achieved in the enhancement of device performance such as efficiency,
brightness and durability.2
However, many electroluminescent polymers which are currently under investigation
have only unipolar injection and transport ability. Deficiencies in the charge-carrying
properties of the emitting material can be overcome by either incorporating additional layers
of charge-carrier or -blocking materials in the LED'^1
or by blending these materials with the
actual emitter.7"9 In this latter configuration, a single-layer structure of the LED can be
maintained. The latter offers clear advantages during the production of the devices, since the
preparation of polymeric multi-layer devices is usually a delicate matter because of the
limited choice of solvents that can be employed for casting of the different layers.9 Clearly, it
is of crucial importance that a solvent employed for the deposition of a certain substance will
not dissolve the layers already deposited.
Single-layer polymeric LEDs consisting of a dialkoxy substituted polyO>y-phenylene
ethynylene) derivative (EHO-OPPE)10 (Fig. 1) as the emitting layer were earlier shown to
yield devices of good brightness, which could be operated with different cathode materials,
such as aluminum, calcium, and chromium, the former giving the best results.11
Determination of the band edges of the highest occupied molecular orbital (HOMO) and of
the lowest unoccupied MO (LlTMO) by cyclic voltammetry (CV) revealed energy values well
below the vacuum level, at -5.8 eV and -3.6 eV, respectively. These characteristics suggest
good electron-transport properties of EHO-OPPE. and a higher energy barrier to be overcome
for hole injection than for electron injection. Therefore, production of devices was envisioned,
in which EHO-OPPE is used in combination with a hole-conducting material, in order to
facilitate the hole injection from the anode. This approach is also expected to yield a more
balanced charge transport across the LED and, thus, an increased efficiency of the device.
129
EHO-OPPE
poly-TPD
spiro Qux
Fig. 1: Structures of EHO-OPPE. a poly(2,5-dialko\y-p-phenylene ethynylene) derivative,
polyOYA'-diphenylben/iditiediphenylether) (polv-PPDh and a tetrameric spiro-quinoxaline
(spiro-Qux).
130
A polymeric triphenyldiamine (poly(A,Ar,-diphenylbenzidine-diphenylether), poly-
TPD) (Figure 1) was selected as the hole-conducting material,12 which was previously shown
to be a good hole-transporter with HOMO and LLIMO band edges of -5.2 eV and >-2.4 eV
respectively.12 Devices were prepared in a bilayer configuration as well as in a single-layer
configuration, with both materials blended in one film that exhibits bipolar carrier transport
abilities. The composition of the blends was systematically varied to optimize the device
performance.
The influence of an additional electron-transport and hole-blocking layer (ETHBL)
was subsequently investigated, which was vapor-deposited on top of the hole-transporting and
emitting layer. For this latter device configuration a tetrameric spiro-quinoxaline ether (spiro-
Qux) (Figure 1) based on a spirobisindane core was used as the electron-transport material.
Spiro-Qux exhibits a glass transition temperature Tg of 155 °C (determined by differential
scanning calorimetry, DSC) and forms stable amorphous films upon vapor deposition.13 The
LUMO and HOMO band edges, as determined from CV measurements, were found to be at -
2.8 eV and <-6.5 eV. respectively. Ehese values suggest favorable electron-injection
properties from the aluminum cathode as well as a high barrier to hole transport, thereby
confining the charges to the emitting layer and favoring the formation of excitons.
Device
-3.6
4.2W
Oh Al4.8 o
ITOo
Device 2:
>-2.4
-3.6
f? W
4.8"o
Oh
O
ITOo
-5.1
-5.8
4.2
Al
Device 3:
>-2.4
ITO
4.2
Al
-5.8
Device 4:
>-2.4
4.8
ITO
-3.6
-2.8
È1-5.8
<-6.5
4.2
Al
Fig. 2: Schematic view of the device-structures investigated and of their corresponding energy-level
diagrams; the band-edge energy levels were determined by cyclic voltammetry (CV).
131
Schematic views of the four investigated device structures along with the
corresponding HOMO and LUMO values (without taking energy-level modification at the
interfaces into account) are shown in Figure 2. The HOMO and LUMO values were
determined from oxidation and reduction potential values measured against ferrocene (Fc) as
an internal standard14'15 from CV measurements.
The optimal thickness of the ETHBL was determined by means of a combinatorial
approach to device preparation, which allows the preparation of a number of devices
characterized by a different layer thickness in one deposition step. 11n this method, a linear
thickness gradient of spiro-Qux was deposited on top of the spin-coated EHO-OPPE / poly-
TPD blend layer. Application of combinatorial methods for the optimization of the layer
thickness in OLEDs was previously demonstrated to be a highly efficient tool for screening a
large quantity of devices in a reproducible and comparable manner.13'16
Device
Photometric
efficiency [cd/A]
@ 10 mA/cm2
Power
efficiency [cd/W]
@ lOmA/cm2
Onset \oltage
IV]
@ 0.001 cd/m2
Max
brightness
[cd/m2]
100% EHO-
OPPE
0.004 210~4 15 4
Bilayer
poly-TPD /
EHO-OPPE
0.016 0.001 10 19
75% EHO-OPPE 410~4 210' 9 33
50% EHO-OPPE 0.002 6 10"' 8.5 31
25% EHO-OPPEi 0.02 0.001 9.5 146
10% EHO-OPPE 0.007 ' 5 10"4 9.5 4
Table 1: EL characteristics of the investigated LEDs based on blends of EHO-OPPE as the emitter
and poly-TPD as a hole-transport material; data for a bilayer EHO-OPPE / poly-TPD device
are also included; concentrations of the emitter are given m % w/w the remaining fraction
consists of poly-TPD
132
The layer composition and the EL characteristics of the EHO-OPPE / poly-TPD
devices (device structures 2 and 3 in Figure 2) are summarized in Table 1 together with a
reference device consisting of neat EHO-OPPE (device structure 1). The photometric
efficiency vs. current density characteristics of the LEDs are shown in Figure 3a. The greatly
enhanced efficiency observed due to the presence of poly-TPD (from 0.005 to maximally 0.07
cd/A) demonstrates, as expected, a more balanced charge injection and transport in the
devices, which favors the formation of excitons and their recombination. The maximal light
output could be increased from 4 cd/m to 146 cd/m". The improved charge injection is
further evidenced by a substantial decrease in the field for onset of electroluminescence, from
around 1.2 MV/cm for the reference 100% EHO-OPPE device to as low as 0.6 MV/cm for a
(25% w/w EHO-OPPE + 75% w/w poly-TPD") blend device with bipolar charge carrying
abilities (Figure 3b). The brightness and efficiency were observed to reach maximum at a
poly-TPD content of 75%. A further increase in the poly-TPD concentration had no beneficial
effect on the LED characteristics, as is demonstrated by a device comprising 90% of poly-
TPD, for which the EL characteristics, in fact, decay to lower levels (not shown in Figure 3,
cf. Table 1). A LED with a bilayer structure (device 2 m Figure 2), which exhibits optimal
conditions for charge-injection at the electrodes, showed similar characteristics as the blend
comprising 75% of poly-TPD up to current flows of around 50 mA/cm" (Figure 3). However,
at higher current densities (i.e. > 50 mA/cm") the bilayer device failed, while the single-layer
blend device exhibited stable operation up to current densities of above 200 mA/cm2. This
behavior, which was observed for several devices, may be due to insufficient film qualify in
the bilayer device as a consequence of its preparation; deposition of the EHO-OPPE layer
may have led to a partial dissolution of the poly-TPD layer. This finding demonstrates on the
one hand that through an appropriate blend composition, charge injection and transport in a
single-layer device can be enhanced to the same level as in a bilayer device. On the other
hand these experiments illustrate the clear advantages of the preparation from solution of a
single-layer device with bipolar charge carrying abilities in comparison with a multi-layer
structure.
All concentrations in this work are given in wemht-'/r unless stated otherwise.
133
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Fig. 3: a) Photometrie efficiency vs. curient density curves and b) luminance vs. field of LEDs based
on blends of EHO-OPPE as the emitter and poly-TPD as a hole-transport material; data for
bilayer EHO-OPPE / poly-TPD devices are also included.
The favorable properties of the (25% EHO-OPPE- / 75% poly-TPD) single-layer blend
device, may be directly related to the phase behavior of the polymer blend. Due to the
intrinsically poor miscibility of polymers.17 phase separation of the two polymers used in the
present single-layer blend LED is to be expected. Unfortunately, DSC is inadequate to verify
this behavior due to the limited thermal stability of the EPIO-OPPE (Tdccomposmon ~ 160 °C),
which would degrade prior to reaching the Tg of the poly-TPD (206 °C12). The phase behavior
of the 25% EHO-OPPE + 75% poly-TPD blend, therefore, was imaged by atomic force
microscopy (AFM, Figure 4) and b> phase-contrast optical microscopy.
As can be seen in Figure 4, the 25% EHO-OPPE / 75% poly-TPD blend indeed was
phase-separated: EHO-OPPE. which in this particular blend represents the minority phase, is
finely dispersed in the continuous poly-TPD phase, yielding a large interface between the two
phases. The same phase structure was observed by phase-contrast optical microscopy. When
comparing the properties of the single-layer blend device and those of the bilayer device
(devices 3 and 2. respectively), similar efficiencies were observed for both devices, but the
former required a lower field strength for the onset of electroluminescence (0.6 MV/cm vs.
1.0 MV/cm, Fig. 2b). This favorable behavior of the blend-based structure is attributed to a
134
beneficial effect of the increased interface between hole-transporter and emitter in the blend,
which might facilitate the formation of excitons and their recombination through radiative
processes. An analogous high-stirface-area effect was previously observed for charge-
injection from the anode when a porous polyaniline film was employed as the anode.18
Fig. 4: AFM images of a 25% w/w EHO-OPPE + 75% w/w poly-TPD blend film spiocoated from a
cyclohexanone solution. The sample was imaged in topography mode deft) and in amplitude
modulation mode (right).
The comparison of the above discussed device characteristics of a single layer 100%
EHO-OPPE device with those of the bilayer device poly-TPD (80 nm)/EHO-OPPE (60 nm)
and of the single-layer (25% EHO-OPPE + 75% poly-TPD) blend device, allows the
conclusion that the efficiency for hole injection and charge-carrier recombination are
considerably improved by the introduction of poly-TPD as a hole-injection material in the
latter two devices. Due to its superior performance, as well as its manufacturing advantages, a
single-layer device structure consisting of a (25% EHO-OPPE + 75% poly-TPD) blend film
was chosen for further investigations which addressed the additional introduction of an
ETHBL. Thus, the influence of spiro-Qux as an additional ETHBL positioned between the
emitting layer and the cathode was examined in a bilayer device structure consisting of a 11.0
nm layer of (25% EHO-OPPE + 75% p-TPD) and a layer of spiro-Qux, the thickness of
135
which had a gradient of 0-48 nm (see the Experimental Section for details regarding the
device preparation).
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0.6 0.8 1.0 1.2
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Thickness of spiro-Qux: 48 nm A 37 nm D 27 nm
• 43 nm 32 nmO 0 nm
Fig. 5: Efficiency and onset plots for the device: ITO / (25% EHO-OPPE + 75% poly-TPD) / spiro-
Qux gradient 0-48 nm / Al. a) photometric efficiency vs. current density; b) luminance vs.
field; c) power efficiency vs. field; d) current density vs. field.
136
The characteristics of these devices are shown in Figure 5. Since the EL characteristics
of devices with an additional spiro-Qux layer of up to 20 nm thickness remained unchanged
when compared to those without ETHBL, only devices with 0 nm, 27 nm, 32 nm, 37 nm, 43
nm and 48 nm of spiro-Qux are included. The influence of the spiro-Qux layer-thickness on
the efficiency of the devices is shown in Figure 5a. As is commonly observed for OLEDs,13'19
the photometric efficiency reaches saturation at comparably low current-density values of
around 5 mA/cm2 in all devices with an additional spiro-Qux layer and remains constant for
the remaining current range up to device failure. The device with 48 nm of spiro-Qux shows
the maxima] photometric efficiency of 0.15 cd/A at 5 mA/cm". A proportional increase of the
luminance with the current flow up to a saturation of the recombination rate was observed
(not shown in Figure 5) and a maximal brightness of 257 cd/m2 at 260 mA/cm2 was measured
for a spiro-Qux layer of 48 nm. This observation is in accordance with other studies
performed on OLEDs.13'16'19"21 The trend observed in Figure 5 suggests that even thicket-
layers of spiro-Qux may lead to further improved efficiency values.
With increasing spiro-Qux layer thickness from 0 nm to 43 nm. the onset of
luminescence was found to decrease from 0.6 MV/cm (6.0 V) to 0.3 MV/cm (4.5 V) (Figure
5b). Thus, the beneficial effect of an additional spiro-Qux layer on the onset of luminescence
is clearly demonstrated. Figure 5c illustrates its effect on power efficiency. A gradual
improvement in the maximum power efficiency from 0.0016 cd/W at a field of 1.2 MV/cm
and a current density of 1 17 mA/cm" for a device without spiro-Qux, to 0.01 cd/W at 0.9
MV/cm and 1.2 mA/cm" for the device with 48 nm of spiro-Qux can be observed.
Interestingly, the power efficiency reaches a maximum at a field of around 0.9 MV/cm and,
subsequently, decreases again, nearly converging to the maximum power efficiency of the
device without spiro-Qux. Phis behavior was observed for all devices with an additional
spiro-Qux layer, regardless of its thickness. This decrease in the power efficiency clearly was
not caused by a degradation of the devices, since the luminance of the latter increased with
increasing field values, also above 0.9 MV/cm (Figure 5b). The increase in power efficiency
in devices with a spiro-Qux layer up to a field of 0.9 MV/cm may be explained by a
cumulative effect of the hole blocking property of spiro-Qux and an increased electron
injection at the Al / spiro-Qux interface. The decrease in power efficiency at higher field
137
strengths may be caused by an improved electron injection and -transport at the spiro-Qux /
EHO-OPPE interface, which induces higher current flows. This, in fact, is revealed in a plot
of the current density as a function of field strength (Figure 5d), where a steep increase in the
current flow can be observed at field strengths above 0.9-1 MV/cm. As a direct consequence
of the increased electron injection into the (EHO-OPPE / poly-TPD) blend layer, exciton
formation and radiative recombination are favored. This is demonstrated by a steep increase
in light output in concomitance with the current flow, as often detected in OLEDs.1'1'19"22
Thus, from the above experiments it can be concluded that, for maximal photometric
and power efficiency at an appreciable light output (~100 cd/m"), a spiro-Qux layer-thickness
of about 40-50 nm yields an optimal rate of electron injection and recombination for the
present systems.
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Bilavcr polv-TPD (80 nmVEHO-OPPE (60 nm)
Fig. 6: Comparison of characteristics of the different device structures investigated, a) Photometric
efficiency vs. current density; and b) power efficiency vs. field.
138
One, or a combination of the following reasons may cause the improved efficiency
and onset. First, there may be a better electron injection at the Al / spiro-Qux interface
compared to the Al / blend interface. Another reason could be the hole-blocking effect of
spiro-Qux which confines charges to the emitting layer. This may lead to higher
recombination rates in the blend. In addition, the charge accumulation at the blend/spiro-Qux
interface causes the formation of an internal field, which may again favor the electron
injection from the cathode. Finally, an explanation for the improved device performance may
be found in the improved electron transport through the spiro-Qux layer into the blend.
In earlier investigations on the effect of spiro-Qux in TPD / Alq3 devices,13 this
compound functioned as an efficient hole-blocking material, but no improvement in electron
injection and -transport could be detected in these devices. In contrast thereto, spiro-Qux
exhibits efficient electron-injection and -transport properties in addition to its hole-blocking
behavior when employed - as shown here - in combination with the poly-TPD/EPIO-OPPE
system. When comparing our present results with the earlier investigations mentioned above,
it could also be concluded that electron-injection at interfaces is favored in the order, Al/Alq^
interface > Al/spiro-Qux/Alq3 interface > Al/spiro-Qux/ EHO-OPPE + p-TPD blend interface
> Al/EHO-OPPE -f p-TPD blend interface.1'1 However, the electron-transport properties of
spiro-Qux demonstrated in this work are in agreement with an appreciably high electron
mobility observed in starburst quinoxaline molecules."1
Finally, the device performances of all four device-structures investigated, i.e. 100%
EHO-OPPE (device 1). 80 nm poly-TPD / 60 nm EHO-OPPE (device 2). 25% EHO-OPPE +
75% poly-TPD (device 3) and (25% EHO-OPPE + 75% poly-TPD) / 48nm spiro-Qux (device
4), are compared in Figures 6a and b, and a summary of the EL characteristics is given in
Table 2. In Figure 6a, the photometric efficiencies of the four devices are plotted against the
current density. When comparing the photometric efficiencies at 10 mA/cm2 an increase by a
factor of 35 in device 4 with respect to device 1 and of 7-8 with respect to the devices 2 and 3
can be observed. This observation can be readily understood by comparing the HOMO and
LUMO values and the band offsets at the interfaces in the various device configurations (cf.
Figure 2). The barrier to hole injection decreased from 1 eV in device 1 to 0.3 eV in structures
2 and 3. In device structure 4, an additional hole-blocking effect is introduced by the spiro-
139
Qux layer, which has a HOMO value of <-6.5 eV. In a similar way, both power efficiency and
onset voltage were also improved from device 1 to device 4 (Figure 6b). The onset voltage
decreased drastically from 15 V for device 1 to 10 V for devices 2 and 3 and, finally, to 5 V
for device 4. It is, thus, possible to increase the power efficiency by a factor of 40 by
combining an EHO-OPPE emitter with poly-TPD as a hole-transport material and an
additional layer of spiro-Qux as ETHBL.
Device
Photometric
efficiency fcd/A]
@ 10 mA/cm2
Power
efficiency [cd/Wl
@ 10 mA/cm2
Onset voltage
[VI
@ 0,001 cd/m2
Max
brightness
[cd/nrj
Device 1 0.004 2 10"4 15 4
Device 2 0.016 0.001 10 19
Device 3 0.02 0.001 ! 9.5 146
Device 4 0.145 0.008 j 5 257
Table 2: Comparison of typical EL characteristics of the four device configurations investigated.
In conclusion, we have demonstrated that polymeric LEDs with improved
performance can be obtained through careful design of the device structure and composition.
Here EHO-OPPE was used as the emitting material; blending with the hole-conducting poly-
TPD resulted in an increase of the photometric efficiency by a factor of 5. An additional layer
of the electron-conducting and hole-transporting material, spiro-Quinoxaline, yielded a
further increase by a factor of 7. The maximum brightness of the investigated devices
increased from 4 cd/m for a device of pure EHO-OPPE to 260 cd/m2 in a device with 25%
EHO-OPPE + 75% poly-TPD as the hole transporting / emitting layer and an ETHBL of 48
nm thickness. The optimal thickness of the latter layer was determined with a combinatorial
setup for vapor deposition, which allows sampling of a large quantity of devices with
different layer thickness in one single preparation step.
140
Experimental
Materials: The substances used in this work, EPIO-OPPE10 as the emitter, poly-
TPD12 as the hole-transport material, and spiro-Quxlj as the hole-blocking and electron-
transport material were synthesized according to known referenced procedures; their chemical
structures are shown in Figure 1.
Device preparation: All OLEDs were based on Indium Tin Oxide (ITO)-coated
glass substrates (ITO thickness 110 nm. sheet resistance 30 Q D"1) which had a size of 38.0 x
25.4 mm. The long side was etched 9 mm wide to avoid shorts between the ITO and the
aluminum during contacting. An aluminum cathode (200 nm) was vapor deposited (pressure
~ 10"6mbar) onto the organic layer through a shadow mask; 12 devices resulted on each
substrate (device dimensions: 1.77 mm x 18.0 mm. area 0.32 cm").
Single-layer LEDs (device structure 1): Reference LEDs based on neat EHO-OPPE
were prepared by spincoating filtered solutions of EPIO-OPPE (1% in cyclohexanone) onto
ITO-coated glass substrates, to yield 100-110 nm thick films.
Bilayer LEDs (device structure 2): 60-70 nm thick films of poly-TPD were
deposited onto ITO-coated glass substrates by spincoating from filtered solutions (0.2% in
TPIE). Subsequently, a filtered solution of EHO-OPPE (1% in cyclohexane) was spincoated
onto the poly-TPD film. The total thickness of both layers was 120-130 nm.
Single-layer LEDs with bipolar charge carrying blends (device structure 3): For
the preparation of blend films, filtered solutions of EHO-OPPE / poly-TPD with the ratios
indicated in Table 1 were spincoated from a hot solution (1.2% in cyclohexanone. 80°C) onto
heated substrates. The spincoating parameters were adjusted in order to obtain a film
thickness between 110 and 130 nm for all devices.
Bilayer LEDs with an ETHBL (device structure 4: Two ITO-coated glass
substrates (38 mm x 25,4 mm each) each coated with a blend of 25% EHO-OPPE / 75% poly-
TPD (110 nm) were aligned in a vapor deposition chamber resulting in a double-sized
substrate with the dimensions 76.0 mm x 25.4 mm. Spiro-Qux as the ETHBL was vapor
deposited on top of the EHO-OPPE / poly-TPD blend. The ETHBL was deposited as a linear
thickness gradient (0-48 nm) over the substrates by using a combinatorial set-up, as
141
previously described. '
Evaporation of the Al-cathode through a shadow-mask yielded 24
devices with a layer of the (25% EHO-OPPE + 75% poly-TPD) blend of constant thickness
and an ETHBL layer with a thickness gradient from 0-48 nm.
Instrumentation: All devices were characterized at room temperature and in air.
Current-voltage characteristics were measured using a computerized set-up consisting of an
LS 100 luminance meter (Minolta), a Keithley 2000 multimeter and a PN 300 programmable
power supply (Grundig). For layer thickness measurements, a surface profilometer (DEKTAK
3030 ST) was used.
Acknowledgements
We acknowledge the financial support from SFB 481 (TP A6 and B4) and Fonds der
Chemischen Industrie-BMBF (C. Schmitz).
References
1 R.F. Service, Science, 1996, 273. 878.
2 J. Shi, C.W. Tang, Appl Phy v Lett. 1997. 70, 1665.
3 C.W. Tang, S.A. VanSlyke, Appl Phys Lett, 1987. 51, 913.
4 EH. Burroughes, D.D.C Bradley, A R Brown, R N. Marks. K Mackay, R.H. Friend,
P.L. Burn, A.B. Holmes. Nature. 1990, 347, 539.
5 A.R. Brown, D.D.C. Bradley, J.H Burroughes, R.H. Friend, N C. Grecnham, P.E.
Burn, A.B. Holmes. A. Kraft, Appl Phys Lett, 1992, 61, 2793.
6 N.C. Grecnham, S.C. Moratti. D.D.C. Bradley, R.H. Enend. A.B. Plolmes, Nature,
1993,365.628.
7 J. Kido. M. Kohda, K. Okuyama. K. Nagai, Appl Phys. Lett, 1992, 61, 761.
8 C. Zhang, H. von Seggern, B. Kraabel, H.-W. Schmidt, A.J. Heeger, Synth, Met,
1994, 62, 35.
142
9 C.-C. Wu, EC. Sturm, R.A. Register, J.Tian. E.P. Dana, M.E. Thompson, IEEE
Transactions on Electron Devices, 1997, 44, 1269.
10 Ch. Weder, M.S. Wrighton, Macromolecules, 1996, 29. 5157.
11 A. Montali, P. Smith, Ch. Weder, Synth. Met, 1998, 97, 123.
12 M. Thelakkat, R. Fink. F. Haubner, H.-W. Schmidt, Macromol. Symp., 1997,125, 157.
13 C. Schmitz, P. Posch, M. Thelakkat, H.-W. Schmidt. Phys. Chem. Chem. Phys., 1999,
1, 1777.
14 A. J. Bard, L. R. Faulkner in Electrochemical methods - Fundamentals and
Applications, ch. 14, p. 634, Wiley, New York. 1980.
15 H. M. Koepp, H. Wendt, H. Strehlow, 7, Electrochem., 1960, 64, 483.
16 C. Schmitz, M. Thelakkat. H.-W. Schmidt, Ach- Mat. 1999,11, 821.
17 L.A. Utracki, in Polymer alloys and blends- Thermodynamics and rheology, Carl
Hanser Verlag, München. 1989.
18 Y. Yang. E. Westerweele. 0. Zhang, P. Smith. A. J. Heeger, Appl Phys. Lett, 1995,
77, 694.
19 AW. Grice. D.D.C. Bradley. M.T. Bernius. M. Inbasekaran, WW. Wu, E.P. Woo,
Appl. Phys Lett, 1998. 73. 629.
20 V. Gebhardt, A. Bacher. M. Thelakkat, U. Stalmach. H. Meier, H.-W. Schmidt, D.
Haarer,/Wv. Mater., 1999. 11, 119.
21 G. Gu, G. Parthasarathy, P. Turn, P.E. Burrows. S.R. Forrest, J. Appl Phys., 1999, 86.
4076.
22 H. Spreitzer, H. Becker. E. Kluge, W. Kreuder. PI. Schenk, R. Demandt, H. Schoo,
Adv. Mater, 1998,10,1340.
23 M. Redecker, D.D.C. Bradley, M. Jandke, P. Strohriegl. Appl. Phys. Lett, 1999, 75.
109.
143
10. Conclusions and Outlook
In conclusion, it was shown in this thesis that the tensile deformation of blends of
conjugated polymers or their oligomers and appropriate matrix materials, such as UHMW-
PE or LLDPE, leads to an outstanding orientation of the PL guest molecules, resulting in
state-of-the-art polarized photoluminescence and absorption of the prepared films.
Blending of form-isotropic sensitizers with conjugated PI, polymers and UPIMW-PE was
presented as a possibility to produce PL polarizers with, in principle, an efficiency of
100%. These new PL polarizers are based on the specific exploitation of the polarization
dependence encountered in energy transfer processes, which was shown here for the first
time. This polarizing energy transfer was demonstrated for a variety of combinations of
different chromophores, and it was shown that it is a general phenomenon, provided that
suitable substances are combined. A thorough investigation of the mechanism of the
polarizing energy transfer was carried out with time-resolved fluorescence spectroscopy;
the mechanism was determined to be ruled by long-range dipole-dipole interactions and
can be described by the Förster model. The polarizing nature of the transfer, which,
ultimately, allows excitation light polarized perpendicularly to the film orientation
direction to be emitted with polarization direction parallel to its orientation direction was
shown to be based on a depolarizing homotransfer between donor molecules.
By using scanning confocal optical microscopy, which allows to detect the
fluorescence signals of single molecules, the phase behavior of PI, blends was studied. It
was shown that upon tensile deformation of the blend films, the system transforms from a
phase-separated system into a near-molecular blend. Thus, a deformation-induced phase
transformation was observed, which results in stable molecular blends of intrinsically
immiscible polymers. The relations between the phase behavior and the anisotropical
optical properties of PL polarizers were unveiled by analyzing the orientational behavior of
the PL guest molecules and the matrix polymer in blends containing different
concentrations of the PL guest. It was found that a good solubility of the PL guest in the
matrix polymer is of paramount importance to obtain high order m the guest molecules at
low draw ratios. The latter is important from a technological point of view. Through control
of the phase behavior, it was, finally, demonstrated that PL polarizers with very high
optical anisotropics can be efficiently produced through standard melt-processing
techniques, given that appropriate chromophores and matrix materials are combined. PL
polarizers were produced that exhibit dichroic ratios of over 40 at draw ratios of only 10.
The concepts developed and presented m this thesis regarding the exploitation of
polarized PL in liquid-crystal displays are of potential technological relevance. In
144
particular the polarizing energy transfer presented in the first chapters is an interesting
concept, since the efficiency of a device which exploits this phenomenon is, in principle,
limited only by the efficiencies of absorption, transfer, and emission of the respective
chromophores and of the mechanism. In theory, the ultimate efficiency of all these
processes is 100%. The ultimate embodiment of this concept, as recently noted by Grell
and Bradley (ref. 72 in Chapter 1), would be the incorporation of polarizing energy transfer
into the liquid-crystal light-valve itself, thus arriving at fluorescent LCDs that no longer
require polarizers and which could make use of 100% of the backlight.
The possibility to produce PL polarizers through standard melt-processing methods
is a valid demonstration of the suitability of PL polarizers for large-scale production. The
stability of the employed dyes under UV-irradiation is another important aspect when
applications in the field of displays are envisioned: the required lifetimes in display
applications lie at about lO'OOO hours. Preliminary experiments carried out during the
present work indicate a luminescence half-time of a PL polarizer based on UHMW-PE and
2% EHO-OPPE of over 6000 hours under irradiation with UV-light at 365 nm and
temperatures of around 50-60°C. This remarkable stability seems to be partially caused by
the polyethylene matrix, which encapsulates the PL polymer. It appears from these
experiments that the key to photostability. in addition to an appropriate choice of the
materials, might lie in the encapsulation of the devices. This is not surprising, considering
that encapsulation is also one of the key issues in the production of organic LEDs.
However, when considering the specific use of PL polarizers in high information-
density displays, which exhibit a very high resolution and are usually fitted with full-color
capability, it appears that tensile deformation of polymer blend films does not allow to
apply the chromophores with a high spatial resolution. Thus, it seems difficult to pixillate
PL polarizers with the proposed production process. In order to achieve this goal, other
orientation processes such as vapor deposition onto orienting substrates, e.g. oriented PTFE
or rubbed polyimide, or the orientation of guest PL dyes in a liquid-crystalline host should
be employed. Some experiments regarding the former method have been carried out during
this thesis and indicate that this method is, in principle, applicable.
A further objective of the present thesis was to investigate the suitability of EHO-
OPPE as an emitting material in polymeric LEDs. The EL properties of EHO-OPPE were
investigated and it was shown that LEDs can be produced on the basis of PPEs. The
particular electron-conducting properties of EHO-OPPE were optimally combined with a
hole-conducting polymer. poly-TPD, to yield fairly bright and efficient LEDs. These
results suggest that through appropriate chemical modification of the molecules and
through optimization of device manufacturing, LEDs could be prepared on the basis of
PPEs, which might compete with LEDs based on other conjugated polymers. In particular
145
the orientabihty of the PPE derivatives studied here represents a promising property in
view of applications focused on the emission of polarized light. The possibility to orient the
PPE through liquid-crystalline phases or by blending it with a liquid-crstallme matrix
seems to be a feasible and promising possibility which is certainly worth investigating in
the future.
147
Acknowledgments
A large amount of people have contributed to this thesis and have made the past
years to a time of my life which I will always look back to with great pleasure. First of all I
should thank Prof. Paul Smith for the possibility to work in his group for the past three
years and for all the opportunities he gave me during this time. Dr. Christoph Weder intro¬
duced me to the subject of light-emitting polymers and always gave me all the support I
needed, this work would not have been possible without his coaching. I should also thank
all co-authors of the papers presented in this thesis for the interesting, fruitful and pleasant
hours spent together in preparing, carrying out and discussing our joint projects. I am espe¬
cially grateful to Prof. Hans-Werner Schmidt for accepting me as a part-time member of his
group and for being co-examiner of this thesis, and to Dr. Mukundan Thelakkat, Christoph
Schmitz and the other members of the group MC I of the University of Bayreuth for their
warm hospitality and friendship during the time I could spend with them in Bayreuth. The
collaboration with the Physical Chemistry group of Prof. Urs P. Wild of the ETI! Zürich
has been an extremely fruitful and pleasant one and 1 would not miss thanking Dr. Alois
Renn, Dr. Bert Hecht, Dr. Greg Harms, Werner Trabesinger and Prof. U.P. Wild himself
for their help in successfully concluding the joint projects.
Furthermore. I thank Simon Dellsperger who dedicated a large amount of his work
to the synthesis of the EHO-OPPE on which this thesis was based. Dr. Anja Palmans has
been of great help by always judging my work in a very critical way. by opening my eyes
to the chemistry behind behind it and by being an great friend. I also thank all other mem¬
bers of the Polymer Technology group, who contribute to making this a very special group,
and whom I leave behind as friends, not just as colleagues; if I do not mention everyone
personally, this shall not dimmish the importance of their support, friendship and the time
spent together.
Finally I would like to thank my parents who stood by me not only during this thesis
but for the last 29 years.
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149
Curriculum Vitae
Andrea Montali was born on November 13th 1970 in Gorla Minore (VA) in Italy.
He has both the Italian and the Swiss citizenship and currently lives in Basel, Switzerland.
He attended high school in Lugano, Switzerland and graduated in 1989. He started
his studies at the faculty for Materials Engineering of the ETH Zürich in 1990, where he
obtained a degree as a Materials Engineer in 1996. He graduated with a thesis on the
Chemical Modification of Wood, which he carried out in the group of Prof. Nicholas
Spencer, in the Laboratory for Surface Science and Technology, at the ETH.
After working on a project on metal injection-moulding with Prof. P. Uggowitzer, in
the Institute for Metallurgy of the ETH, he joined the group of Prof. Paul Smith, in the
Department of Materials, for his doctorate studies in 1996.
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