Fluorescence Intensity Modulation in Photochromic Conjugated Polymer Systems

DOI: 10.1002/ijch.201300016

Fluorescence Intensity Modulation in PhotochromicConjugated Polymer SystemsElizabeth J. Harbron*[a]

1 Introduction

Fluorescence photomodulation, the reversible switchingof emission intensity in response to a light signal, has at-tracted increased interest in recent years due to applica-tions in optical data storage and ultrahigh-resolutionimaging. Photochromic molecules are the ideal basis forsystems designed to undergo fluorescence photomodula-tion. These molecules, also called photochromes, havetwo different molecular forms that interconvert upon irra-diation with specific wavelengths of light.[1] By definition,the two forms of a photochrome have different absorp-tion spectra, and they also differ in terms of other molec-ular properties including shape and dipole moment.

Many photochromes are non-fluorescent in both forms;in photochromes for which fluorescence is observed, typi-cally only one form emits. Such fluorescent photochromesoffer a convenient means of photomodulation, as light in-duces switching between the non-fluorescent and fluores-cent forms of the photochrome. This approach is exempli-fied by the work of Li and coworkers, who have devel-oped a series of core-shell polymeric nanoparticles witha photochromic spiropyran in the hydrophobic core.[2] Intypical solution-phase environments, neither the UV-ab-sorbing spiropyran nor its visible-absorbing merocyanineform is fluorescent. In the rigid, hydrophobic interior ofthe nanoparticle, however, the merocyanine becomesstrongly fluorescent, and photoswitching from spiropyranto merocyanine forms activates the fluorescence. Thefluorescence photomodulation of these nanoparticles has

been used as the basis for the high-resolution imaging ofcells.[3]

An alternative but popular strategy for fluorescencephotomodulation involves using non-fluorescent photo-chromes in combination with an additional fluorophore.In this method, depicted in Scheme 1, the photochromeand fluorophore are selected such that they have no inter-action when the photochrome is in its initial form. Photo-switching to the alternate form changes the properties ofthe photochrome such that it now acts as a fluorescencequencher. The quenching mechanism is typically eitherphotoinduced electron transfer or fluorescence resonanceenergy transfer (FRET), and the fluorescence remainsquenched by this mechanism until the photochrome un-dergoes thermal or photochemical reversion to its non-quenching form. The photochromic FRET-based ap-proach has been used to modulate the emission of a varie-ty of fluorophores, including quantum dots,[4] metal nano-crystals,[5] conjugated polymers,[6] and small-moleculedyes.[7] Fluorescence photomodulation via the reversibleactivation of photoinduced electron transfer or FRET hasalso been reviewed.[8]

Abstract : The ability to modulate fluorescence intensity witha light signal enables a variety of applications based on fluo-rescence imaging. One approach to fluorescence photomo-dulation involves using a photochromic moiety that re-sponds to a light signal in conjunction with a nearby fluoro-phore. We employ conjugated polymers based on poly(p-phenylene vinylene) (PPV) as the fluorophore in photo-chrome-fluorophore systems for fluorescence modulation.Advantages of using conjugated polymers for this purposeinclude their intrinsic energy migration processes that

enable amplified fluorescence quenching as well as theirprocessability. Here we present examples of PPV-based pho-tomodulation systems that employ photochromic dyes fromthree common photochromic families: azobenzenes, spiro-naphthoxazines, and diarylethenes. In all cases we observereversible fluorescence quenching due to fluorescence reso-nance energy transfer to the photogenerated form of thephotochrome. Examples of the photomodulation of photo-chromic PPV systems in organic solution, polymer films,and conjugated polymer nanoparticles are presented.

Keywords: fluorescence · FRET · photochromism · polymers

[a] E. J. HarbronDepartment of ChemistryThe College of William and MaryWilliamsburg, Virginia 23187-8795 (USA)e-mail: [email protected]

256 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Isr. J. Chem. 2013, 53, 256 – 266

Review E. J. Harbron

Our work utilizes this photochrome-fluorophore strat-egy for the FRET-based fluorescence modulation of con-jugated polymers with a poly(p-phenylene vinylene)(PPV) backbone.[9] Other conjugated polymer structureshave also been employed as the fluorophore in photo-chrome-fluorophore systems.[6] The properties of conju-gated polymers such as the well-studied MEH-PPV(Scheme 2) make them particularly well suited to actingas the fluorophore in systems designed for photomodula-tion. Conjugated polymers are multichromophoric withextremely high extinction coefficients and decent quan-tum yields. The brightness of some conjugated polymersin nanoparticle form is high enough to be competitivewith quantum dots and the brightest organic fluoro-phores.[10] In addition to enhancing their brightness, themultichromophoric nature of conjugated polymers ena-bles amplified quenching. Upon excitation, higher-energychromophores can transfer their energy via FRET tolower-energy chromophores in close proximity. Suchenergy migration processes can effectively transfer the ex-citation energy of multiple high-energy chromophores toa single low-energy chromophore. A single quencher res-onant with that low-energy chromophore will quench theexcitation energy of multiple chromophores, boosting thequenching efficiency beyond what is possible witha single, isolated chromophore. These photophysical fea-tures are complemented by the physical properties of thepolymers, namely their processability, and they are ableto undergo fluorescence modulation in solutions, films, or

even nanoparticles suspended in water. This feature con-fers a degree of versatility not always possible with small-molecule fluorophores.

Once the fluorophore for a photochrome-fluorophoresystem has been selected, a photochrome with propertiesthat are an appropriate match for the designated fluoro-phore and desired application must be chosen. Propertiesof concern include the photochrome absorption spectrum,extinction coefficient, redox potentials, and thermal sta-bility. In general, efficient photomodulation requiresa photochrome with one form that does not quench fluo-rescence and another that is an efficient quencher. Addi-tionally, a FRET-based system requires that the fluores-cence of the donor overlaps with the absorbance of theacceptor. This overlap is quantified by the spectral over-lap integral, J(l), which is given by Equation 1:

JðlÞ ¼Z1

0

FDðlÞeaðlÞl4dl ð1Þ

where FD is the corrected fluorescence intensity of thedonor and ea is the extinction coefficient of the accept-or.[11] Photochromes with forms that have distinctly sepa-rate absorptions such that only the spectrum of one formoverlaps with the donor fluorescence are desirable. Thequenching form of the photochrome should also havea high extinction coefficient if possible. For photochrome-fluorophore systems based on a photoinduced electrontransfer quenching mechanism, the redox potentials ofthe photochrome must undergo a distinct change upon

Elizabeth J. Harbron received her B. A.in Chemistry and Gender and Women’sStudies from Grinnell College (1995)and a Ph.D. in organic chemistry fromthe University of North Carolina atChapel Hill (1999), where she studiedorganic photochemistry with MalcolmD. E. Forbes as an NSF graduate re-search fellow. She was an NIH post-doctoral fellow with Paul F. Barbara atthe University of Texas at Austin, whereshe studied single-molecule spectros-copy. Since 2002, she has been on thefaculty of the College of William and Mary, where she is currentlyAssociate Professor of Chemistry and University Professor forTeaching Excellence.

Scheme 1.

Scheme 2.

Isr. J. Chem. 2013, 53, 256 – 266 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.ijc.wiley-vch.de 257

Fluorescence Intensity Modulation in Photochromic Conjugated Polymers

http://www.ijc.wiley-vch.de

photoswitching to generate a molecule capable of elec-tron transfer with the donor fluorophore. Regardless ofquenching mechanism, the mechanism(s) of return of thephotochrome from the photogenerated form must be con-sidered. Some photochromic families, including azoben-zenes[12] (1, Scheme 3) and spironaphthoxazines[13] (2) arenot thermally stable in their photogenerated form andcan return to their initial state thermally or photochemi-cally. Others, exemplified by the diarylethene[14] family(3), have thermally stable photogenerated forms and canonly return upon irradiation. Whether thermal stability inboth forms is desirable depends on the intended applica-tion, and the many photochromic families available offerchoice in this property.

This review provides an overview of our work on pho-tochromic PPV systems for fluorescence photomodula-tion. We have employed all three photochromic familiesdepicted in Scheme 3 as FRET acceptors for PPV-baseddonors. Our azobenzene-based PPVs undergo FRET witha relatively low efficiency, which enables study of thecompetition between FRET to the photochromes and in-trinsic PPV energy migration processes. In contrast, spiro-naphthoxazine- and diarylethene-based PPV systemswere designed for high FRET efficiency and exhibit muchmore dramatic photomodulation.

2 Azobenzene-PPV Systems

Azobenzene derivatives such as 1 (Scheme 3) exhibitphotochromism by isomerization from a thermally stable

trans form (1a) to a photogenerated cis form (1b). Asshown in Figure 1A, the absorption spectrum of 1a is do-minated by the p!p* transition at ca. 350 nm witha small contribution from the much weaker n!p* transi-tion above 400 nm. Ultraviolet (UV) irradiation near themaximum of the p!p* transition yields 1b, which fea-tures p!p* and n!p* transitions at 307 nm and 445 nm,respectively. The spectral relationship of azobenzene ab-sorbance to solution-phase PPV fluorescence is shown inthe Figure 1A inset. The photochrome-fluorophore spec-tral overlap is quite poor and on the blue, high energyside of the PPV fluorescence, which has a lmax of 550 nm.

Scheme 3.

Figure 1. A) Absorbance of model compound decyloxyazobenzenein dilute toluene solution in trans (solid) and cis (dashed) forms.Inset: overlap of trans- and cis-azobenzene absorbance with PPVfluorescence in toluene (dotted). B) Absorbance of 2a (solid) and2b (dashed) in dilute methanol solution. Inset: overlap of 2a and2b absorbance with fluorescence of MEH-PPV nanoparticles inaqueous suspension (dotted). C) Absorbance of 3a (solid) and 3b(dashed) in dilute THF solution. Inset: overlap of 3a and 3b absorb-ance with fluorescence of MEH-PPV nanoparticles in aqueous sus-pension (dotted). Adapted with permission from references[9c,9d,9a]. Copyright 2006, 2009, 2011, American Chemical Society.

258 www.ijc.wiley-vch.de � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Isr. J. Chem. 2013, 53, 256 – 266



Because the rate of energy transfer depends on the over-lap integral,[11] poor spectral overlap limits the energytransfer rate and, ultimately, efficiency. The fact thatspectral overlap occurs predominantly on the blue edgeof the fluorescence spectrum also has negative conse-quences for energy transfer efficiency due to the donorconjugated polymer�s own intrinsic energy migration pro-cesses. Brunner has shown that overlap on the red edgeof a conjugated polymer�s fluorescence spectrum yieldsmore efficient energy transfer than on the blue edge dueto the migration of excitations to the lowest energy acces-sible chromophores.[15] Despite these limitations, the factremains that the 1a and 1b absorption spectra are differ-ent enough to yield a nearly fourfold increase in thevalue of the overlap integral for the azobenzene-PPVFRET pair upon trans!cis photoisomerization.[9b]

We have studied PPV-to-azobenzene energy transfer ina family of azobenzene-functionalized PPVs, shown inScheme 2 along with non-photochromic reference poly-mers.[9e] Originally synthesized by Yoshino and cowork-ers,[16] poly(2-methoxy-5-(4-phenylazophenyl-4’-(1,10-di-oxydecyl))-1,4-phenylenevinylene) (MPA-10-PPV) hasa standard methoxy-functionalized PPV backbone withpendant azobenzenes attached to each repeating unit bya solubilizing decyldioxy tether. The MPA-10-PPV ab-sorbance spectrum in dilute THF (Figure 2A) shows theexpected contributions from the trans-azobenzene sidechains (lmax =349 nm) and the PPV backbone (lmax =470 nm). UV irradiation induces substantial changes inthe spectrum, all due to the azobenzene isomerization, asthe PPV backbone absorbance is unaffected by the irradi-ation. The intense 1a p!p* transition at 349 nm decreas-es substantially, while the far weaker 1b p!p* and n!p* transitions on either side of the major 1a peak in-crease subtly. These absorbance changes reflect conver-sion to a photostationary state (pss) in which the sidechains are approximately 53 % 1a and 47% 1b. Figure 2Bshows the effect of the azobenzene isomerization on thefluorescence of MPA-10-PPV, which is due entirely to thePPV backbone as the 1a and 1b side chains are non-emis-sive. The fluorescence intensity decreases to 66% of itsinitial intensity upon conversion of the side chains to thepss. The quenching is reversed upon application of visiblelight, which both excites 1b directly and activates a sensi-tized isomerization pathway. Upon alternating applicationof UV and visible light, the fluorescence intensity can berepeatedly cycled between bright (side chains 100% 1a)and dim (side chains pss) states. The 1b isomers can alsoundergo thermal return to 1a, but this process is slow inMPA-10-PPV and related polymers (first order half-life>30 min).

We quantify the extent of photomodulation by themodulation efficiency (Emod, Equation 2):

Emod ¼ 1� Ipss

I0ð2Þ

where I0 and Ipss are the fluorescence intensities ob-served prior to irradiation, when the side chains are alltrans, and after UV irradiation, when the side chains havereached their pss concentration of cis isomers, respective-ly. MPA-10-PPV yields Emod values ranging from 0.34 to0.41 depending on the individual sample and solvent,which is typically THF or toluene. The modulation effi-ciency is limited by the fact that both I0 and Ipss representquenched intensities. Due to the similarity of the absorp-tion spectra of 1a and 1b, the PPV backbone is able totransfer energy to both isomers, albeit more efficiently to1b. The Figure 2B inset compares the relative intensitiesof reference polymer DM-10-PPV, MPA-10-PPV withtrans side chains, and MPA-10-PPV at the pss. Clearly, thereversible photomodulation of MPA-10-PPV occurs be-tween two dim states rather than bright and dark states.

In spite of the limited brightness and modulation effi-ciency of azobenzene-functionalized PPVs, they undergoemission intensity modulation in the solid state as well asin solution.[9f] Figure 3 shows fluorescence spectra forHPA-10-PPV (Scheme 2), an analogue of MPA-10-PPV

Figure 2. A) Absorbance and B) fluorescence spectra of MPA-10-PPV in dilute THF solution during 365 nm irradiation with Dt =1 min between spectra. Inset: relative fluorescence intensities ofcontrol polymer DM-10-PPV (black), MPA-10-PPV with trans sidechains (gray), and MPA-10-PPV at the pss concentration of cis sidechains (white). Adapted with permission from reference [9e]. Copy-right 2004, American Chemical Society.




with a longer alkoxy side chain for enhanced solubility.When irradiated alternately with UV and visible light inorganic solution, HPA-10-PPV undergoes fluorescence in-tensity quenching and recovery analogous to MPA-10-PPV. Nearly identical levels of quenching and recoveryare observed when the same irradiations are applied toneat polymer films, although the fluorescence spectrashift ca. 30 nm to the red upon going from solution tofilm. This spectral shift is typical of PPV-based polymersand reflects an increase in conjugation length and aggre-gated interchain species in the polymer films.[17] HPA-10-PPV can be used to demonstrate spatial control of fluo-rescence intensity, as shown in the Figure 3 inset. Aquenched state was first produced by irradiating the filmwith UV light. Next, lines in the shape of the letter Hwere drawn with a 488 nm laser, which converted 1b iso-mers back to their less-quenching 1a forms. The endresult is a bright-on-dim image, which demonstrates theapplicability of fluorescence modulation in azobenzene-PPVs to spatially resolved films in addition to bulk filmsand solutions.

These solution- and film-based examples of fluores-cence photomodulation are limited by the fact that both1a and 1b act as PPV quenchers via FRET. However, thislimitation enables study of the photophysical details ofa system undergoing FRET to two different covalently at-tached side chains with different quenching efficiencies,1a and 1b. We measured fluorescent lifetimes of azoben-zene-functionalized PPVs and non-photochromic refer-ence polymer DM-10-PPV by time-correlated single-photon counting (TCSPC).[9c] The decays of all azoben-zene-containing PPVs were fit to a sum of three exponen-

tials, from which an integrated lifetime, t̄, was derived ac-cording to Equation 3:

�t ¼Z1

0

FðtÞdt ¼X

i

aiti ð3Þ

The integrated lifetime of each photochromic PPV wascompared to that of control polymer DM-10-PPV, t0. Asshown in Table 1, the t̄/t0 value for MPA-10-PPV with 1a

side chains indicates that the azobenzenes quench thePPV emission significantly relative to the control poly-mer, as expected based on the steady-state results. How-ever, comparison of quenching levels in the time domain(t̄/t0 =0.41) and steady state (I/I0 =0.35) reveals thata smaller amount of quenching is observed in the timedomain. This discrepancy is a classic indicator of a subpo-pulation that is undergoing static quenching, in which thefluorophore and quencher form a non-fluorescent com-plex. Such a population is not observed after photoisome-rization to the pss concentration of 1b isomers, whichyields time-resolved and steady-state quenching levelsthat are the same within experimental error (t̄/t0 =0.19, I/I0 =0.21).

The time-domain data indicate that the 1a trans iso-mers are both better and worse quenchers than the 1b cisisomers: the FRET quenching to 1a is less efficient thanto 1b, as noted in the steady-state data, but the staticquenching observed only for 1a creates a more quenchedemission. Static quenching can occur when a fluorophoreand quencher are able to form a stacked structure. Wehypothesized that 1a, which has a structure that is geo-metrically similar to the trans-vinyl linkages of the PPVbackbone, might be able to form such a complex whilethe 1b isomers would not. To investigate this hypothesis,we studied the emission of MtBuPA-10-PPV (Scheme 2),an MPA-10-PPV analogue with a bulky tert-butyl group inthe azobenzene side chain. In terms of their absorbanceand theoretical FRET-acceptor properties, the two poly-mers are essentially identical. However, their quenchingbehavior is quite different. The time-domain and steady-state quenching levels of MtBuPA-10-PPV with 1a side

Figure 3. Photomodulation of HPA-10-PPV in dilute THF solution(a traces) and in a neat film cast from THF solution (b traces): fluo-rescence spectra of samples with trans side chains (solid), at thepss concentration of cis side chains produced by 365 nm irradia-tion (dotted), and upon return to the trans state after 488 nm irra-diation (dashed). Inset: 30 mm2 image of HPA-10-PPV film after365 nm irradiation to produce dim state and inscription of thecharacter “H” by 488 nm irradiation. The final image was back-ground subtracted to correct for film heterogeneity. Adapted withpermission from reference [9f] . Copyright 2007, American ChemicalSociety.

Table 1. Photophysical parameters of PPVs.[a]

PPV derivative t̄/t0 I/I0 Emod

MPA-10-PPV – – 0.41trans 0.41 0.35 –pss 0.19 0.21 –

MtBuPA-10-PPV – – 0.52trans 0.51 0.49 –pss 0.25 0.24 –

[a] Adapted with permission from reference [9c]. Copyright 2006,American Chemical Society.




chains are nearly the same (t̄/t0 =0.51, I/I0 =0.49), dem-onstrating that addition of the tert-butyl group dramati-cally reduces the extent of static quenching. The bulkytert-butyl moiety presumably blocks the stacking interac-tion responsible for the static quenching in MPA-10-PPV.As was the case with MPA-10-PPV, MtBuPA-10-PPVwith 1b side chains exhibits the same quenching levelswithin experimental error (t̄/t0 =0.25, I/I0 =0.24). Thesmall increase in both time-domain and steady-state in-tensities for MtBuPA-10-PPV relative to MPA-10-PPVreflects an apparent increase in donor-acceptor distancefor the tert-butyl polymer, which is to be expected giventhe increase in steric bulk. The key difference betweenthe two polymers remains the near absence of staticquenching in MtBuPA-10-PPV. By causing additionalquenching of the bright state in MPA-10-PPV with 1aside chains, static quenching limited the modulation effi-ciency to 0.41. With static quenching blocked by unfavor-able steric interactions, MtBuPA-10-PPV yields brighteremission with 1a side chains and an increased modulationefficiency of 0.52. These results demonstrate that maxi-mizing modulation efficiency requires consideration ofgeometric factors in addition to FRET donor-acceptorproperties.

Any further increase in modulation efficiency for azo-benzene-functionalized PPVs would require answeringadditional photophysical questions about this system.How does energy transfer to 1a and 1b compete withPPV�s intrinsic energy migration processes? Why is thequenching as efficient as it is, given that spectral overlapis poor and on the high-energy side of the PPV emission(Figure 1A)? We explored these questions by performingwavelength-resolved TCSPC measurements so that wecould separately study blue-edge and red-edge PPV emis-sion.[9b]

Figure 4A shows emission decays of non-photochromiccontrol polymer DM-10-PPV from the red and blueedges of the emission in comparison with the total emis-sion. The red-edge and total emissions appear extremelysimilar to the eye due to the fact that both are dominatedby a major component with a typical PPV lifetime of t~300 ps, and both have similar t̄ values (Table 2). Theblue-edge emission is noticeably quenched relative to thetotal with t̄=126 ps, and fitting reveals that a third of itspopulation remains unquenched while most of the re-maining two-thirds of the emission is strongly quenchedwith a t of 19 ps. The strongly quenched component is as-signed to energy migration from shorter, higher-energychromophores to longer, lower-energy chromophoreswithin the PPV chain and is expected based on the char-acteristic energy migration processes of a conjugatedpolymer. Energy migration is further reflected in the red-edge emission, which exhibits a small rising edge that rep-resents the growing-in of the migrating excitons.

The DM-10-PPV decays provide a baseline represent-ing typical PPV photophysics against which the behavior

of azobenzene-functionalized PPVs can be compared.Figure 4B shows the edge and total emissions of MPA-10-PPV with 1a side chains. All decays are quenched relativeto DM-10-PPV, reflecting the quenching effect of 1a asseen in our previous time-resolved measurements of thetotal emission. The blue-edge emission of MPA-10-PPVhas a t̄ value that is roughly half that of DM-10-PPV, rep-resenting the increased quenching. Its emission decay isdominated by a strongly quenched component with a t of25 ps that represents two-thirds of the population, essen-tially matching the lifetime and amplitude observed forthe population of migrating excitons in the control poly-mer. The presence of 1a side chains clearly did not affectthis component, indicating that energy transfer to 1a isnot competitive with energy migration from high-energychromophores. Most of the remaining blue-edge emissionof MPA-10-PPV is partially quenched (t=154 ps), reflect-ing energy transfer to 1a side chains, and it is this quench-

Figure 4. Fluorescence decays measured in dilute toluene solutionfor A) DM-10-PPV total emission (solid), red edge (dashed), andblue edge (dotted); B) MPA-10-PPV total emission (solid), red edge(dashed), and blue edge (dotted); C) MPA-10-PPV blue edge in thedark (solid) and after 365 nm irradiation to produce the pss(dotted); D) MPA-10-PPV red edge in the dark (solid) and after365 nm irradiation to produce the pss (dotted). Adapted with per-mission from reference [9b]. Copyright 2007, American ChemicalSociety.

Table 2. Integrated lifetimes before and after irradiation.

PPV derivative Emission t̄dark[a] (ps) t̄UV

[b] (ps)

DM-10-PPV total 312 302blue 126 148red 298 299

MPA-10-PPV total 124 57blue 66 49red 177 97

[a] t̄dark = integrated lifetime prior to irradiation. [b] t̄UV = integratedlifetime following UV irradiation. Table adapted with permissionfrom reference [9b]. Copyright 2007, American Chemical Society.




ing that is responsible for the overall decrease in the inte-grated lifetime. Thus, while 1a cannot quench high-energy, rapidly diffusing excitons, it is able to quench thepopulation of non-diffusing excitons observed in the con-trol polymer. On the red edge of the MPA-10-PPV emis-sion, nearly three-quarters of the population has a t of142 ps, which is quite similar to the population observedon the blue edge that was assigned to chromophoresquenched by 1a. The remaining population is completelyunquenched and, unlike the red-edge control polymerdata, no rising edge was needed to fit this decay. Togeth-er, the red-edge results indicate that 1a likely quenchessome of the existing low-energy chromophores as well asdiffusing excitons before they reach those lowest energy,red-edge chromophores. With poor 1a-PPV spectral over-lap on the red edge, it is not surprising that some chromo-phores remain completely unquenched.

Given its improved spectral overlap with PPV, 1b maybe able to quench populations that 1a cannot, namely thehigh-energy diffusing excitons on the blue edge and theunquenched low-energy chromophores on the red edge.To determine whether this is so, wavelength-resolveddecays were measured for both DM-10-PPV and MPA-10-PPV following UV irradiation. As can be seen fromthe t̄ values in Table 2, the irradiation has essentially noeffect on the emission decays of control polymer DM-10-PPV, as expected. MPA-10-PPV, in contrast, exhibitsfaster emission decays upon UV-induced 1a!1b conver-sion, as can be visualized in Figure 4C and 4D for theblue- and red-edge emissions, respectively. On the blueedge, the increase in quenching is visually subtle andyields an integrated lifetime that is 74% of its value whenthe side chains were in their 1a form. The emission-decayfit reveals a decrease in the lifetime of the component as-signed to azobenzene quenching that is consistent withthe increase in azobenzene-PPV spectral overlap upon1a!1b conversion. The red-edge emission decay showsa greater change upon UV irradiation, reflected by a de-crease in the integrated lifetime to 55% of its pre-irradia-tion value. The fit of the red-edge emission decay nolonger shows any unquenched component, and all of thelowest-energy chromophores are at least partiallyquenched by 1b. Thus, the major contribution of 1b tooverall quenching comes from the red edge of the emis-sion, and the dimmer emission of MPA-10-PPV with 1bside chains can be largely attributed to the increase inspectral overlap on the red edge of the PPV emission.

Collectively, the lifetime measurements show that theazobenzene side chains are not effective quenchers ofrapidly diffusing excitons but do quench non-diffusingand slowly diffusing excitons with efficiencies that reflecttheir degrees of spectral overlap with PPV. The observedphotomodulation is essentially enabled by the reversibleactivation and deactivation of the spectral overlap of 1bwith the red edge of the PPV emission. If a system witha fully unquenched bright state and fully quenched dark

state is desired, then a photochrome that cannot quenchPPV emission in one form and has strong red-edge over-lap of the PPV emission in its alternate form would be re-quired. For this purpose, we turned to photochromes 2and 3 as described below.

3 Spironaphthoxazine-PPV and Diarylethene-PPVSystems

An ideal photochrome for fluorescence modulation ofPPV emission would have widely spaced absorbance spec-tra such that one form has no spectral overlap with thePPV emission while the other has strong overlap. As de-picted in Scheme 1, such a scenario would involve noFRET to one form of the photochrome and highly effi-cient FRET to the other form. Photochromes of the spi-ronaphthoxazine and diarylethene families meet these cri-teria for a photochromic PPV system. The absorptionspectra of spironaphthoxazine 2 and diarylethene 3 areshown in Figure 1B and 1C. The initial photochromicforms of 2 and 3 absorb only in the UV, below 400 nm for2a and below 350 nm for 3a. Irradiation with an appropri-ate UV wavelength converts the photochromes to their2b and 3b forms, both of which absorb strongly in thevisible region of the spectrum between 500 and 700 nm.A key difference between 2b and 3b is their return totheir initial forms: 2b undergoes thermal reversion to 2afairly rapidly (t1/2 =2.8 s in THF), while thermally stable3b does not revert to 3a until it is irradiated with visiblelight. Whether thermal return of the photochrome is de-sirable or undesirable depends on the application, and wehave found both types of photochrome to be useful inour studies of PPV photomodulation.

Covalent attachment of the photochrome to the PPVbackbone, as in the azobenzene-functionalized PPVs, pro-vides a well-defined structure with the close photo-chrome-fluorophore proximity required for FRET. How-ever, synthesis of the photochromic PPVs can be timeconsuming, and synthetic procedures must often be al-tered as new functional groups are introduced. For ourwork with 2 and 3 we turned to a new photochrome-PPVarchitecture, the conjugated polymer nanoparticle(CPN).[18] CPNs are small (5 nm and up), spherical parti-cles prepared from conjugated polymers in a surfactant-free method popularized by McNeill and coworkers.[10,19]

Briefly, the conjugated polymer is dissolved in a water-miscible solvent such as THF. A small portion of this pre-cursor solution is injected into rapidly stirring or sonicat-ing water, forming CPNs that are stable in aqueous sus-pension. If hydrophobic dopants are added to the precur-sor solution, they will be incorporated into the CPNs, andsuch doping can be used to alter the fluorescence color orother properties of the CPNs.[20] In addition to the ease ofpreparation, advantages of CPNs include their excellentphotostability and brightness, especially when compared




to conjugated polymer films.[10] We[9a,d] and others[6a,d,g]

have prepared CPNs doped or functionalized with photo-chromes for photomodulation.

We prepared 2a-doped MEH-PPV CPNs for the dualpurposes of modulating emission intensity and using thephotochromic reaction as a probe of the nanoparticle en-vironment.[9d] Good photomodulation is expected for thissystem because of the strong overlap of the 2b absorb-ance with the CPN emission, which is red shifted com-pared to solution in a fashion similar to polymer films(Figure 1B inset). As shown in Figure 5A, the absorbanceof the doped CPNs is dominated by the polymer at ca.490 nm, with a minor contribution from the 2a photo-chromes at 335 nm. Upon UV irradiation the polymer ab-sorbance is unchanged, as expected, but a tiny increase inabsorbance at ca. 600 nm is observed (Figure 5A, rightinset). That this small change represents the formation of2b can be seen in the difference spectrum shown in theleft inset of Figure 5A, which is consistent with the 2b ab-

sorption spectrum from Figure 1B. The concentration of2b in the CPNs can be calculated from the absorbanceand represents approximately eight dyes per CPN. Thesedyes undergo thermal conversion back to 2a in a few sec-onds, restoring the original absorbance spectrum.

Fluorescence photomodulation of 2-doped CPNs isdemonstrated in Figure 5B. The small number of 2bquenchers produced by UV irradiation has a dramaticeffect on CPN fluorescence, quenching it to 7.5 % of itsinitial value. The quenching is completely reversible, withthermal recovery of the initial 2a population restoring thefluorescence intensity to its initial value. The modulationis extremely robust and can be cycled repeatedly betweenbright and dark states without significant change in theintensities of those states (Figure 5B inset). The modula-tion efficiency for the dataset shown in Figure 5 is 0.93,and values for the 2-doped CPNs reach as high as 0.95 insome samples. That these values are so much higher thanthose observed for the azobenzene-functionalized PPVs isexpected based on the much greater donor-acceptor spec-tral overlap of PPV with 2b than 1b. Although the 2-doped CPNs exhibit strong photomodulation, the fact re-mains that Emod values never reach 1, i.e., some residualemission is observed in all samples. Determining why thisis so requires a more detailed understanding of CPNstructure.

Photochromic reactions have long been used as probesof their environments, particularly in polymeric systems.The kinetics of the thermal 2b!2a conversion are knownto be sensitive to matrix rigidity and free volume,[21] andwe took advantage of this fact to characterize the envi-ronment experienced by the photochromic dyes in thenanoparticle. Figure 6 shows the decay of the 2b absorb-ance as a function of time for four environments: CPNs,THF solution, and MEH-PPV and polystyrene films. Thesolution-phase kinetics are first order and rapid, witha 2.8 s half-life. In contrast, 2b decay kinetics in the poly-mer films are much slower and non-first order, reflectinga heterogeneous environment with a distribution of freevolume. These samples provide points of reference forthe conclusion that the 2b kinetics in the CPNs are re-markably solution-like, with a first-order half-life of 3.5 s,nearly as fast as in THF. The 2b environment clearly pro-vides ample free volume for the photochromic conversionand bears no resemblance to a polymer film. These re-sults imply that those dyes that are able to undergo thephotochromic conversion are located on solution-exposedregions of the CPNs, i.e., the nanoparticle surface, whiledyes embedded within the CPN remain in the 2a form.This conclusion is also supported by a separate project inwhich we used dye-doped CPNs to sense mercury ions inthe aqueous nanoparticle suspensions. Mercury-sensitivedyes on the CPN surface were able to undergo a structuralconversion upon contact with the ions, verifying theirpresence on regions of the nanoparticle exposed to theaqueous environment of the nanoparticle suspension.[22]

Figure 5. A) Absorption spectra of 28 wt. % 2-doped MEH-PPVnanoparticles suspended in water before (solid) and after 365 nmirradiation (dotted) and after thermal recovery (dashed). Rightinset: red-edge expansion of the three spectra. Left inset: differ-ence spectrum created by subtracting the spectrum prior to irradi-ation from the one recorded immediately after UV irradiation. B)Fluorescence spectra of same sample before (solid) and after365 nm irradiation (dotted) and after thermal recovery (dashed).Inset: peak fluorescence intensity of the nanoparticles over manycycles of alternate UV irradiation (low-intensity points) and thermalrecovery (high-intensity points). Adapted with permission from ref-erence [9d]. Copyright 2009, American Chemical Society.




With the location of 2b quenchers now known to belimited to the CPN surface, the question of why the 2-doped CPNs do not experience complete modulation(Emod =1) can be revisited. AFM analysis of the CPNs in-dicated an average CPN size of 8 nm. The Fçrster radius,which is the donor-acceptor distance at which the FRETefficiency is 50 %, is 3.9 nm for the 2b/MEH-PPV FRETpair. From this information, the FRET efficiency fora polymer chromophore at the center of the particle anda 2b quencher on the particle surface can be calculated tobe 46%. Thus, if chromophores near the CPN centermake a substantial contribution to the total emission,then the CPN will always have some residual emissiondue to FRET that is less than 100% efficient. Given thedistribution of particle sizes, which range from 5–13 nmwith a few larger aggregates, it is also possible that somesmaller particles are completely quenched while otherlarger ones exhibit significant residual fluorescence.

To determine whether all CPNs exhibit residual emis-sion or a mixed ensemble with “off” and “on” CPNsexists, we studied the fluorescence of individual CPNs.[9a]

The 2-doped CPNs were not suitable for these experi-ments due to the fast 2b!2a thermal reversion, so weused diarylethene 3, which is thermally stable in both ofits photochromic forms. Like 2b, 3b has good spectraloverlap with MEH-PPV emission, although its Fçrsterradius is somewhat smaller at 2.2 nm. In the bulk aqueoussuspension of CPNs, Emod levels of 3-doped MEH-PPVCPNs are comparable to those measured with 2b as thequencher. Figure 7 shows 10 mm2 images of individual 3-doped MEH-PPV CPNs embedded in an optically trans-parent poly(vinyl alcohol) matrix before UV irradiation(A), after UV irradiation (C), and after visible irradiation(E). The CPNs clearly undergo substantial fluorescencequenching upon 3a!3b conversion and recover most of

their initial emission intensity upon photoinduced 3b!3areversion. Figures 7B, 7D, and 7F show 1.5 mm2 expan-sions with two representative CPNs that undergo fairlystrong modulation. Although these two molecules appearto be almost completely quenched, they retain enough re-sidual emission in the quenched state to yield Emod valuesof 0.84 (left) and 0.79 (right). Indeed, only 15% of CPNswere quenched to a dark level indistinguishable frombackground, with the vast majority retaining a smallamount of residual emission. This finding provides sup-port for the idea that average CPN size is too large toallow dyes on the particle surface to quench all emissionfrom within the particle. Smaller CPNs would likely ex-hibit more complete quenching. CPN size can be con-trolled during preparation by the polymer concentrationin the precursor solution, but the polymer chain lengthsets a lower limit on CPN size. The MEH-PPV used inthese experiments had an average molecular weight of260,000, and size analysis indicated that many CPNslikely were composed of a single polymer chain. A conju-gated polymer with a lower molecular weight wouldpermit creation of smaller CPNs with higher modulationefficiencies.

4 Summary and Outlook

Conjugated polymers possess a unique combination ofphotophysical and physical properties that make them in-triguing and useful as the fluorophore in photochromic

Figure 6. Decay of 2b peak absorbance as a function of timeduring thermal 2b!2a recovery for a) 30 wt. % 2-doped polystyr-ene film, b) 30 wt. % 2-doped MEH-PPV film, c) 30 wt. % 2-dopedMEH-PPV nanoparticles, and d) 2 in dilute THF solution. Inset: ex-pansion of the decay during the first 15 s. Adapted with permis-sion from reference [9d]. Copyright 2009, American Chemical Soci-ety.

Figure 7. Single-nanoparticle fluorescence images (10 mm2, left)and expansions (1.5 mm2, right) of 3-doped MEH-PPV nanoparticlesembedded in poly(vinyl alcohol) A/B) before irradiation, C/D) after254 nm irradiation, and E/F) after visible irradiation from a whitelight LED. Reprinted with permission from reference [9a]. Copyright2011, American Chemical Society.




systems designed for fluorescence photomodulation.Their brightness and intrinsic energy migration processesproduce an intense fluorophore whose multiple chromo-phores can be quenched by a very small number ofquenchers. Their polymeric nature means that they canundergo fluorescence modulation in a variety of forms, aswe have demonstrated with solution, films, and nanoparti-cles suspended in water and dispersed in films. We havefocused on PPV, which is compatible with a variety of dif-ferent photochromes. Covalently functionalized azoben-zene-PPVs undergo FRET from the polymer backbone toboth forms of the photochromic acceptor 1 but with dif-ferent efficiencies. Fluorescence intensity photomodula-tion is enabled by photoswitching from the less efficient1a quencher to the more efficient 1b form. Time-resolvedmeasurements support the FRET mechanism and alsoprovide photophysical details that can be used to improvephotomodulation in this system. Unlike 1, photochromes2 and 3 each have just one form that is resonant with thePPV backbone, enabling modulation from an unquenchedbright state to a dark state with greater modulation effi-ciency than was observed with 1. Conjugated polymernanoparticles doped with 2 or 3 provide an easily pre-pared system for fluorescence photomodulation, althoughonly dyes residing on the particle surface appear to beable to undergo the photochromic conversion. These ex-amples demonstrate the utility of photochromic polymersystems for fluorescence photomodulation.

Acknowledgements

E. J. H. thanks the undergraduate researchers of the Col-lege of William and Mary who performed the work re-viewed here and gratefully acknowledges support of thiswork by the NSF through CAREER award CHE-0642513.

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Received: February 13, 2013Accepted: February 27, 2013

Published online: May 13, 2013




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Fluorescence Intensity Modulation in Photochromic Conjugated Polymer Systems