NLO Behavior of Polymers Containing Y-Shaped Chromophores

Full Paper

1900

NLO Behavior of Polymers ContainingY-Shaped Chromophores

Antonio Carella,* Fabio Borbone, Ugo Caruso, Roberto Centore,Antonio Roviello, Alberto Barsella, Alessia Quatela

New polyurethanes containing Y-shaped chromophores, symmetrical derivatives of4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran, have been prepared.The polymers show high glass transition temperatures (Tg) and a good thermal stability.SHGmeasurements on poled polymer films of the synthesized polymers have been carried outand a maximum d33 of 15 pm �V�1

has been found at 1 368 nm funda-mental wavelength. Time stabilitymeasurements on the most activepolymer have shown that after theinitial fast relaxation, the d33 valueremains constant at 80 8C for 60 d.

Introduction

Organic polymers are currently under investigation for

their possible application in the field of nonlinear optics

(NLO). Features that make them interesting as compared

with inorganic materials traditionally used in the field are

their potentially higher activity, their easier processability

and compatibility with integrated circuit technology and

the possibility of modulating the properties through

proper modification of their chemical structure.[1–3]

Second order NLO polymers generally consist of NLO

active chromophores covalently linked to the polymeric

backbone and oriented along a preferential direction by

the application of a strong electric field (electric poling);

A. Carella, F. Borbone, U. Caruso, R. Centore, A. RovielloDipartimento di Chimica, Universita di Napoli ‘‘Federico II’’, 80126,Via Cintia, Napoli, ItalyE-mail: [email protected]. BarsellaIPCMS-CNRS, Groupe d’Optique Nonlineare et Optoelectronique,23 Rue du Loess, 67037 Strasbourg Cedex, FranceA. QuatelaDipartimento di Fisica Universita di Roma ‘‘Tor Vergata’’, Via dellaRicerca Scientifica 1, 00133 Roma, Italy

Macromol. Chem. Phys. 2007, 208, 1900–1907

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the chromophores typically used in these systems are

push-pull molecules, characterized by one dimensional

(1D) charge transfer of p-electrons.

Recently, organic chromophores with multidimensional

charge transfer emerged as interesting candidates for NLO

quadratic applications, offering potential advantages on

linear push-pull molecules such as increased b without

undesirable loss of visible transparency.[4–7] It has been

shown that off-diagonal tensor components of b can

become significant in dipolar molecules with C2v symme-

try that display electronic transitions for which the

direction of the transition dipole moment is perpendicular

to the C2 axis.[8–10] Because of their off-diagonal tensor

components, compounds of this type, characterized by a

2D charge transfer, could offer new possibilities for

achieving phase-matched second harmonic generation.[11]

Among those chromophores, symmetrical derivatives

of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)-

styryl]-4H-pyran (DCM), a chromophore typically used

as a very efficient red emitting dye, have received

particular interest.[12] Both theoretical and experimental

works have proved the good NLO activity of Y-shaped

chromophores based on DCM with two donor segments

and one (dicyanomethylene)pyran acceptor.[5,7,9,13] How-

ever, to date few reports on materials containing this kind

DOI: 10.1002/macp.200700275

NLO Behavior of Polymers Containing Y-Shaped Chromophores

Scheme 1. Molecular structure of the synthesized chromophores.

of chromophore have been published.[14–17] In one

report[14] DCM derivatives were used in the synthesis of

high glass transition temperature (Tg) poly(phenylquinox-

alines). Anyway those systems contain the NLO chromo-

phore only in lowweight amount leading to amodest NLO

activity.

In this paper we report on the synthesis and character-

ization of three new 2DNLODCM symmetrical derivatives;

these chromophores were functionalized to be used as

monomers in polycondensation reaction and new NLO

polyurethanes were prepared. The molecular structures of

the chromophores, Y1, Y2 and Y3, are sketched in Scheme 1.

In the effort to obtain highly stable systems without

depressing the activity, we polymerized our chromophores

with 2,4-tolylendiisocyanate obtaining in this way poly-

mers with a high chromophore concentration. Working on

the chemical structure of both the chromophores and the

other monomer we managed to increase the Tg of our

systems and finally obtained polyurethanes with Tg up

to 212 8C

Experimental Part

Materials

All reagents and solvents were purchased from Aldrich, Lancaster

or Carlo Erba and used without further purification except for

2,4-tolylendiisocyanate which was distilled before use.



Monomer Synthesis

The monomers Y1 and Y2 were synthesized by exploiting the

acidity of methyl protons of (2,6-dimethyl-4H-pyran-4-ylidene)

propanedinitrile (A) with a proper aldehyde in a Knoevenagel

condensation. The synthesis of Y3 was achieved through the

double Knoevenagel condensation of Awith 4-nitrobenzaldehyde

followed by reduction of the obtained dinitro-compound and a

final double diazo-coupling with N-methyl-N-(2-hydroxyethyl)-

aniline. The synthesis of monomers is outlined in Scheme 2 and 3.

(2,6-Dimethyl-4H-pyran-4-ylidene)propanedinitrile (A)

2,6-Dimethyl-4H-pyran-4-one (25 g, 0.20 mol) and malononitrile

(13.791 g, 0.21 mol) were refluxed in 50 mL of acetic anhydride for

6 h. The system was then slowly cooled down at room tem-

perature and the crystallization of a brown product occurred

which was recovered by filtration. Yield 60%.

m.p.: 193 8C1H NMR (CDCl3): d¼2.31 (s, 6H), 6.54 (s, 2H).

5-[N-(2-hydroxyethyl)-N-methylamino)]thiophene-2-carbaldehyde (D1)

2-Bromothiophene-5-carboxyaldheyde (15 g, 0.078 mol) and

2-methylaminoethanol (17.530 g, 0.234 mol) were refluxed for

9 h in 400mL ofwater. The systemwas then cooled down at 0 8C in

an ice bath and after few minutes the formation of a yellow solid

occurred (eventually, NaAcaq could help the coagulation of the

solid). The product was recovered by filtration. Yield 60%.

m.p.: 120 8C

www.mcp-journal.de 1901

A. Carella et al.

Scheme 2. Synthesis of Y1 and Y2.

1902

1H NMR (DMSO): d¼ 3.06 (s, 3H), 3.44 (t, 2H, J¼5.4 Hz), 3.59 (m,

2H), 4.85 (t, 1H, J¼5.4 Hz), 6.10 (d, 1H, J¼ 4.5 Hz), 7.63 (d, 1H, J¼4.5 Hz), 9.25 (s, 1H).

5-(4-Hydroxypiperidin-1-yl)thiophene-2-carbaldehyde (D2)

The synthesis was performed with the same procedure used for

the synthesis of (N-methyl-N-hydroxyethyl)-2-aminothiophene-

5-carboxyaldheyde except that in this case 4-hydroxypiperidine

was used instead of 2-methylaminoethanol. Yield 66%.

m.p.: 143 8C1H NMR (DMSO): d¼1.46 (m, 2H), 1.78 (m, 2H), 3.17 (m, 2H),

3.53–3.85 (m, 3H), 4.80 (d, 1H, J¼ 4 Hz), 6.30 (d, 1H, J¼4.4 Hz), 7.66

(d, 1H, J¼ 4.4 Hz), 9.43 (s, 1H).

2-{2,6-Bis((E)-2-{5-[N-methyl-N-(2-hidroxyethyl)-aminothiophen-2-yl]vinyl}-4H-pyran-4-ylidene)malononitrile (Y1)

D1 (8 g, 0.043 mol), A (3.489 g, 0.014 mol) and piperidine (3.088 g,

0.036 mol) were refluxed for 8 h in 100 mL of pyridine. Then



the solution was poured in 400 mL of water and the formation of

a solid occurred. The solid was recovered by filtration and

recrystallized from pyridine/H2O. The yield after recrystallization

was 78%.

Td¼283 8C1H NMR (DMSO): d¼ 3.05 (s, 6H), 3.41 (t, 4H, J¼4.8 Hz), 3.60 (m,

J¼ 4.8 Hz), 4.84 (t, 2H, J¼4.8 Hz), 5.98 (d, 2H, J¼4.2 Hz), 6.27 (d, 2H,

J¼ 15.6 Hz), 6.44 (s, 2H), 7.27 (d, 2H, J¼4.2 Hz), 7.68 (d, 2H, J¼15.6

Hz).

(C26H26N4O3S2) (506.65): Calcd. C 61.64, H 5.17, N 11.06, S 12.66;

Found C 61.54, H 5.24, N 10.61, S 12.57.

2-(2,6-Bis{(E)-2-[5-(4-hydroxypiperidin-1-yl)thiophen-2-yl]vinyl}-4H-pyran-4-ylidene)malononitrile (Y2)

The synthesis was performed in a similar way for the synthesis

of Y1 except that D2was used instead of D1. Thework-up procedure

is slightly different because the chromophore precipitated from

the reaction solution, on cooling, as a crystalline product without

the need of further purification. The yield was 88%.

Td¼316 8C

DOI: 10.1002/macp.200700275


Scheme 3. Synthesis of Y3.

1HNMR (DMSO): d¼1.51 (m, 4H), 1.80 (m, 4H), 3.16 (m, 4H), 3.45

(m, 4H), 3.69 (m, 2H), 4.77 (d, 2H, J¼ 3.8 Hz), 6.20 (d, 2H; J¼4.4 Hz)

6.39 (d, 2H, J¼ 15.6 Hz), 6.54 (s, 2H), 7.29 (d, 2H, J¼ 4.4 Hz), 7.72 (d,

2H, J¼ 15.4 Hz).

(C30H30N4O3S2) (558.72): Calcd. C 64.49, H 5.41, N 10.03, S 11.48;

Found C 64.42, H 5.53, N 10.06, S 11.35.

2-[2,6-Bis((E)-4-nitrostyryl)-4H-pyran-4-ylidene]malononitrile (DNP)

A (5 g, 0.029 mol), 4-nitrobenzaldehyde (9.654 g, 0.064 mol) and

piperidine (0.862 g, 0.01 mol) were refluxed in 30 mL of pyridine

under nitrogen atmosphere for 30 min. During the reaction the

formation of a solid occurred which was recovered by filtration

and washed with ethanol. It was not possible to perform 1H NMR



analysis because of the poor solubility of this product in common

deuterated solvents. The yield was 42%.

2-[2,6-Bis((E)-4-aminostyryl)-4H-pyran-4-ylidene]malononitrile (DAP)

DNP (1.8 g, 0.004 mol) was suspended in 70 mL of hot ethanol.

Awater solution of Na2S (5.58 g of Na2S � 9H2O, 0.023mol, in 18mL

of H2O and 1.8 mL of 37% HCl) was poured in the DNP sus-

pension and the system was taken under boiling. The color of

the suspension turned from yellow to dark red. After 30 min the

system was cooled down to room temperature. The solid was

recovered by filtration and washed with water. A microcrystalline

product was obtained with 45% yield.


A. Carella et al.

Scheme 4. Polymer structures.

1904

1H NMR (DMSO): d¼5.88 (s, 4H), 6.59–6.63 (m, 6H), 6.95 (d, 2H,

J¼ 15.8 Hz), 7.49–7.62 (m, 6H).

m.p.: 276 8C.

2-{(E)-2,6-Bis-[4-((E)-{4-[N-methyl,N-(2-hydroxyethyl)aminophenyl]diazenyl}phenyl)vinyl]-4H-pyran-4-ylidene}malononitrile (Y3)

DAP (1.5 g, 3.96 mmol), was placed in a flask containing 20 mL

of H2O and 2 mL of 37% HCl; the suspension was cooled down to

0–5 8C in an ice-water bath. A solution obtained by dissolving

NaNO2 (0.596 g, 8.63 mmol) in 5 mL of water was added dropwise

to the suspension under stirring. Stirring at low temperature was

continued for 1 h after the addition of nitrite solution.

Separately, a water/ethanol solution containing sodium

acetate (2.200 g, 0.027 mol) and N-methyl-N-(2-hydroxyethyl)

aniline (1.197 g, 7.92 mmol) was prepared. To this solution, cooled

in an ice bath and under stirring, the suspension containing the

diazonium salt was rapidly added. Immediately the color of

the solution turned to red and in few second, a red precipitate of

the azo compoundwas formed. Thiswas collected by filtration and

then crystallized from DMF and water. Yield 55%.1H NMR (DMSO): d¼ 2.94 (s, 6H), 3.40 (t, 4H, J¼4.8 Hz), 3.48 (m,

4H), 4.65 (t, 2H, J¼ 4.8 Hz), 6.68–6.74 (m, 6H), 7.29 (d, 2H, J¼16.2

Hz), 7.62–7.79 (m, 14H).

MS: m/z Calcd. C42H38N8O3 702.31; Found 703.16.

Synthesis of Polymers

Polymers were prepared by polycondensation reaction of

chromophores and 2,4-tolylendiisocyanate. The chemical struc-

ture of the polymers is reported in Scheme 4. Synthesis of PU Y1 is

reported as an example.



PU Y1

Y1 (0.608 g, 1.2 mmol) and 2,4-tolylendiisocyanate (0.209 g,

1.2 mmol) were dissolved in 3 mL of anhydrous NMP and kept at

120 8C under stirring in N2 atmosphere for 4 h. The solution was

then poured dropwise into 100 mL of a methanol/H2O solution

(9:1 by volume). The precipitated polymer was collected by

filtration, dissolved in 10mL of DMF and poured again into 100mL

of the methanol/H2O solution. The solid formed was collected by

filtration and dried in oven. The yield was 74%.

Physico-Chemical Characterization

The thermal behavior of the compounds was studied by DSC

(Perkin-Elmer DSC-7, nitrogen atmosphere, scanning rate 10

K �min�1), temperature-controlled polarizing microscopy (Zeiss

microscope, Mettler FP5 microfurnace) and thermogravimetric

analysis (Mettler TA80, nitrogen, 10 K �min�1). 1H NMR spectra

were recorded with a Varian XL 200-MHz apparatus. X-ray

diffraction (XRD) patterns of polymers were recorded on a flat film

camera, using Ni filtered Cu Ka radiation. Inherent viscosity (hinh)

of polymer solutions at 25 8C was measured with an Ubbelohde

viscometer in DMF solutions (concentration was 0.5 g �dL�1).

Matrix-assisted laser desorption-ionization (MALDI-TOF) mass

spectrometry analysis was performedwith an Applied Biosystems

MALDI DE PRO spectrometer.

Film Preparation

In a typical procedure, 5 wt.-% pyridine solution of the polymer

was spin coated on a glass substrate (2.5�2.5 cm2) at 1 000 rpm

for 60 s and subsequently at 2 000 rpm for 20 s. Prior to use, the

glasseswere cleaned by treatmentwith a basic detergent solution,

rinsed with distilled water and ethanol, washed with methanol

and finally with dichloromethane.

The typical thickness of the films was 200–300 nm.

Poling and NLO Measurements

The corona poling set-up used a 25 mm diameter gold wire (biased

with þ4.5 kV for PU-Y1 and PU-Y2 poling and with 6 kV for PU-Y3poling) placed at about 1 cm from the film surface. The HV

generatorwas grounded to heating stage and thewhole apparatus

was held in a controlled atmosphere (dry nitrogen) box. In the

typical poling procedure the poling temperature was set 10 8Cbelow the Tg of the polymers and the poling time was 30 min. A

Quantel Brilliant Q-switched Nd:YAG laser (frequency up to 10 Hz,

5 ns pulse duration, 400 mJ per pulse) provided the fundamental

beamoutput at 1 064 nm for SHGmeasurements of polymers. This

source fed a Solid State Raman Shifter (MolTech CRS-14) which

shifted the beam output to 1 368 nm in order to reduce the

resonance enhancement of the nonlinear signal. The nonlinear

coefficient d33 of the samples is obtained by measuring the

intensity of SHG pulse, carefully calibrated with a Maker fringes

reference experiment[18] of a quartz crystal slab (110) (d11¼0.335 pm �V�1[19]). SH signal is corrected taking into account the

DOI: 10.1002/macp.200700275


Table 1. Chromophores.

Tda) mb lmax1d) lmax2

d) elmax1d)

-C 10S48 esu nm nm L �molS1 � cmS1

Y1 283 2 320b) 524 590 1.11T 104

Y2 316 2 200c) 515 580 1.25T 104

Y3 281 2 500b) 505 – 4.40T 104

a)Decomposition temperature taken as the temperature corre-

sponding to 5% weight loss in the thermogravimetric run

(10 -C �minS1, nitrogen atmosphere); b)Performed on acetyl deriva-

tives in chloroform solution; c)Performed in DMF solution; d)UV-vis

analysis in DMF solution.

sample thickness and the refractive index measured by spectro-

scopic ellipsometer.

The optical nonlinearities of the chromophores were measured

using second harmonic generation set-up. The pump beam was

generated by an Nd:YAG laser emitting 10 ns pulses at 1.064 lm

with a 10 Hz repetition rate. Wavelength conversion to 1907 nm

was achieved using a high pressure H2 Raman cell. The beamwas

then focalized on the sample and the SHG signal at 950 nm was

measured using a photomultiplier equipped with a narrow-band

interferometric filter.

Results and Discussion

Some properties of the chromophores are reported in

Table 1.

The chromophores decompose before melting and

therefore it was not possible to measure their melting

temperature. The decomposition temperatures (Td) were

measured by means of thermogravimetric analysis in

nitrogen atmosphere and in all the cases they are higher

than 280 8C. UV-vis spectra of Y1 and Y2 show three main

absorption maxima (the two at higher wavelength are

reported in Table 1) while for Y3 the spectrum features just

one broad maximum.

Table 2. Polymers.

Tg Tda) hinhb) Densityc)

-C -C dL � gS1 g � cmS3

PU-Y1 183 276 0.20 1.29

PU-Y2 212 309 0.22 1.28

PU-Y3 196 301 0.21 1.27

a)Decomposition temperature taken as the temperature correspondin

nitrogen atmosphere); b)Measured at 25 -C in 0.5 g �dLS1 DMF solutio



The NLO activity of the acetyl-derivatives of the

chromophores Y1 and Y3 was measured by the electric-

field-induced second-harmonic (EFISH) technique (in

chloroform solution) and fairly high mb values of

2 320� 10�48 and 2 500� 10�48 esu, respectively, were

found. The EFISH experiment on Y2 in DMF solution gave

mb value of 2 200� 10�48 esu. The EFISH experiments were

performed at a working wavelength of 1.907 mm, so mb

values were free of resonance enhancement.

In Table 2 some properties of the polymers are reported.

Tgs range between 183 and 212 8C. The higher Tg of PU-Y2as compared with PU-Y1 is probably related to the more

conformationally rigid nature of piperidinoxy group with

respect to ethyloxyamino group. In PU-Y3 the chromophore

has a longer rigid moiety and it is linked to the polymer

matrix through the flexible ethyloxyamino group; the

Tg of the polymer is intermediate between PU-Y1 and

PU-Y2. In all the cases, anyway, the Tgs are fairly high and

this should result in a good time stability of polar order

achieved upon poling. The thermal stability of the

polymers was evaluated by TGA analysis in nitrogen

atmosphere: Td close to 300 8Cwasmeasured (Table 2). The

gap between Tg and Td is in each casewide enough to allow

a safe poling procedure at temperatures around Tg.

All the synthesized polymers are amorphous as shown

by X-ray diffraction analysis and as requested to this class

of materials for second order NLO applications. The lack of

structuration is, moreover, retained also after annealing at

10 8C above the Tg for 1 h.

The UV-vis spectra of PU-Y1 and PU-Y2 are characterized

by an absorption maximum at around 520 nm with a

shoulder slightly beyond 600 nm. The UV-vis spectrum of

PU-Y3 features just one broad maximum at 500 nm. The

lcutoff is at 750 nm for PU-Y1 and PU-Y2 and at 663 nm for

PU-Y3 (see Table 2); these values are consistent for an

adequate use of these materials at the telecommunica-

tions working l of 1 550 nm. d33 coefficients of the

synthesized polymers are reported in Table 2.

The highest NLO activity is shown by PU-Y3, the

chromophore having the longer conjugation path. Since

the three chromophores have a very similar molecular

Chromophore number density lcutoff d33

1020 cmS3 nm pm �VS1

11 750 7

11 750 5

9 663 15

g to 5% weight loss in the thermogravimetric run (10 -C �minS1,

n; c)Measured by flotation at 25 -C in hexane/CCl4.


A. Carella et al.

Figure 1. Time behavior of NLO activity for PU-Y3.

1906

nonlinearity (cf. Table 1), any difference in d33 values of the

corresponding polymers should be reasonably related to

the loading of chromophore in the polymer and/or to the

efficiency of the poling process. PU-Y3 was poled biasing

the corona wire with 6 kVwhile for PU-Y1 and PU-Y2 it was

not possible to bias the wire beyond 4.5 kV without

observing some degradation phenomena. Moreover, as

reported in Table 2, PU-Y3 is the polymer having the lowest

chromophore density within the set of prepared polymers.

Both these aspects could account for the higher activity of

PU-Y3 as comparedwith PU-Y1 and PU-Y2. In particular, it is

widely reported in the literature[2,3] that high chromo-

phore loading can seriously hinder the efficiency of poling

because of the chromophore-chromophore centrosym-

metric interactions. Moreover, the lower NLO activity of

PU-Y2 as compared with PU-Y1 is worth noticing: this

seems to suggest that the rigid piperidonoxy linkage, while

increasing the Tg of the system, probably hinders the

mobility of chromophore under the applied electric field in

the poling process leading to a lower final chromophore

orientation.

For PU-Y3, the time stability of the NLO activity was

studied by means of an accelerated ageing test.[20] Poled

films of PU-Y3 were kept in an oven at three different

temperatures (80, 100 and 140 8C), and their d33 was

periodically measured (utilizing the same procedure

described above) over a period of 60 d. For each oriented

sample, the same thermal treatment was also performed

on a non-oriented film used as the reference, and its UV-vis

spectrum periodically recorded. These spectra showed no



changes in absorbance, which allowed us to exclude the

chemical degradation of the material due to thermal

treatment. In Figure 1 the decay of the normalized d33 at

the three different temperatures is shown. The curves are

compatible with a biexponential behavior with two

characteristic relaxation times. The initial fast decrease

in SHG signal, actually reported in many previous

works[20–23] has been attributed to the discharge of

trapped charge[21] carriers or to the rotational mobility

of the chromophores[20,23], whereas the slower relaxation

time is related to processes involving longer segments of

the chains. It is worth noticing that at 80 8C, after the fast

initial reduction, the signal remains unchanged. Generally,

the minimum temporal stability required for electro-

optical modulators is the invariance of performance after

1 000 h at 85 8C.[24] Therefore, the high time stability of the

induced polar order in PU-Y3 indicates this material as a

potential candidate for applications.

Conclusion

Three new NLO polyurethanes containing Y-shaped

chromophores were synthesized and characterized. The

polymers showed a high Tg and a good thermal stability.

The NLO activity of poled films of the three polymers was

measured by means of SHG experiment and a maximum

value of 15 pm �V�1 for PU-Y3, the polymer containing the

chromophore with the longer conjugation path, was

found. Time stability of NLO activity of this polymer

was studied. At 80 8C the d33 value, after the initial

relaxation, remains unchanged after 60 d. The very high

time stability together with the fair d33 value makes PU-Y3an interesting material for the realization of a prototype

modulator.

Acknowledgements: The work was supported by a FIRB 2001grant from MIUR (n. RBNEOIP4JF).

Received: May 15, 2007; Accepted: May 23, 2007; DOI: 10.1002/macp.200700275

Keywords: NLO; polyurethanes; SHG; synthesis; Y-shaped chro-mophores

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NLO Behavior of Polymers Containing Y-Shaped Chromophores