Makromol. Chem. 194,859-868 (1993) 859

Synthesis and non-linear optical characteristics of crosslinked and linear epoxy polymers with pendant tolane chromophores

Rudolf ZenteP"), Dietmar Jungbauerb), Robert .l mieg, Do I: Yoon, C. Grant Willson

IBM Research Division, IBM-Almaden Research Center, 650 Harry Road, San JosC, CA 95120-6099, USA

(Received: February 26, 1992; revised manuscript of May 8, 1992)

SUMMARY Linear and crosslinked epoxy polymers containing high concentrations (ca. 86 wt.-Yo) of

pendant 4-(4-nitrophenylethynyl)aniline [4-amino-4'-nitrotolane] chromophores were syn- thesized. They exhibit a smectic phase in the bulk once heated above the glass transition tempera- ture (ca. 70°C). The appearance of the highly scattering smectic phase could be avoided by starting the crosslinking process of thin polymer films in the presence of solvent. Application of corona poling during crosslinking resulted in a highly polar polymer, exhibiting a very large birefringence of ca. 0,17 at 633 nm. Moreover, the birefringence remained stable during annealing at 80-90 "C for ca. 30 days. The electro-optic coefficient, determined from the Pockels-effect, was roughly 8 pm/V for r,13 at 633 nm.

Introduction

A variety of applications of nonlinear optical (NLO) polymers for frequency doubling and electro-optical modulation (Pockels-effect), for example, require second- order @)) nonlinear coefficients which are both large and stable. To date, numerous combinations of NLO chromophores and polymer structures have been investigated, to prepare poled amorphous polymers *) in which the required noncentrosymmetry is achieved by electric field-induced orientation above the glass transition temperature. Among them the concept of covalent attachment of nonlinear chromophores into a crosslinked matrix has been shown to be most promising for stability2). Very recently, the 4-amin0-4-nitrotolane chromophore (ANT) 3, was shown to lead to a large nonlinearity (d33 = 89 pm/V at 1,06 pm fundamental) when incorporated into a linear epoxy polymer4). This is due to its large molecular hyperpolarizabilities (p) and the colinearity of the dipolar vector with the principal axis of the hyperpolarizability tensor. In order to increase the concentration of NLO-chromophores in the polymer, and in order to incorporate them into a crosslinking matrix, new functionalized tolane derivatives had to be synthesized. An epoxytolane was desirable to increase the weight percent of NLO-chromophores, and a diaminotolane for crosslinking. In this paper we present the synthesis of ANT-based monomers and polymers, and their NLO characte- rization.

a) IMB-visiting scientist, present address: Institut fur Organische Chemie, Universitiit Mainz,

b, IBM-visiting scientist, present address: Hoechst-AG, GFP, D-6000 Frankfurt am Main 80, Becher-Weg 18 - 20, D-6500 Mainz, Germany.

Germany.

0 1993, Huthig & Wepf Verlag, Base1 CCC 0025-1 16X/93/$05.00

860 R. Zentel, D. Jungbauer, R. J. Twieg, D. Y. Yoon, C. G. Willson

Results and discussion

For this purpose we chose 4-[bis(2,3-epoxypropyl)amino]-4'-nitrotolane (l), 3,4-di- amino4'-nitrotolane (2) and 4-amino-4'-nitrotolane *) (3) (Scheme I ), which were prepared by a palladium-catalyzed ethynylation 5, of the corresponding 4-iodoanilines and 4-nitrophenylacetylene, as described in Schemes 2-4 . The polyaddition of two molecules of 1 to one molecule of 2 under the condition of a melt polycondensation led to the branched oligomer 4, which could be densely crosslinked during the poling procedure and which has an extremely high degree of loading (86 wt.-Yo) of the NLO- chromophores. For comparison, the corresponding linear polymer (polymer 5 , Scheme 1 ) was also prepared from 1 and 4-amino-4'-nitrotolane (3).

Scheme I :

1 2 3

1 a n d 2 - crosslinked polymer 1

OH

L

1 a n d 3 - linear polymer 5

*) Tolane = diphenylacetylene; the systematic names are given in the Exptl. part.

Synthesis and non-linear optical characteristics 861

Scheme 2:

6 7

PdCl F- 7H3

O,N--@ + HCZC-SI-CH, 2 ----c 0,N I CH3

a

a 7

I 1

Scheme 3:

Scheme 4 :

a 3

In order to establish the ideal conditions for the electric field poling of the polymers 4 and 5, their thermal characteristics had to be determined. Thereby it turned out that

862 R. Zentel, D. Jungbauer, R. J. Twieg, D. Y. Yoon, C. G. Willson

the T, of polymer 4 increases during annealing at 147 "C from 73 "C for the freshly precipitated sample to 120°C after 15 min. This is due to further crosslinking, as described for other crosslinkable epoxy polymers in ref.2). Above 155- 170 "C both polymers decompose. The results for both polymers are compiled in Tab. 1.

Tab. 1 . Thermal characterization of the crosslinkable polymer 4 and the linear polymer 5

No. Transition temperatures in "Ca) Layer thickness in A

4 g 73b) s g 120') s 170 dec

g 64 s 155 dec 5

18,7

17,2 8,65 (17,3)d)

a) g: glass transition (onset), s: smectic A phase, dec: decomposition. b, Fresh sample. ') After curing at 147°C for 15 min. d, Second order.

Polarizing microscopy revealed that both polymers can form a liquid-crystalline (LC) phase above the glass transition temperature T. However, if they are precipitated from hexane, or if they are spincasted from solution, completely amorphous samples are obtained below T g . For the linear polymer 5, the LC phase forms as soon as the polymer is heated above T, (thereby the sample turns turbid). For the crosslinking polymer 4 however, an LC phase forms only if the carefully dried sample is heated quickly above T,. If the crosslinking is started in the presence of solvent, or if the sample is heated very slowly above T, , the crosslinking interferes with the formation of the LC phase. As a consequence, no birefringence is observed and the sample stays isotropic (non-scattering).

The LC phases of both polymers were investigated by polarizing microscopy and X- ray scattering. The linear polymer 5 shows textures in the polarizing microscope which resemble textures of smectic phases6). For polymer 4, an identification is not possible because of the quick hardening of the crosslinking material, which prevents the development of a characteristic texture. However, X-ray measurements show nearly the same structure for both polymers. The X-ray diagrams consist of one (polymer 4) or two (polymer 5) sharp small-angle reflections and a broadened wide-angle reflection which corresponds to a Bragg-spacing of 4,3 A. This structure is typical of smectic A or C phases7). In this case the small-angle reflection corresponds to the thickness of the smectic layers ('Itib. 1 and Scheme 5) . The length of the molecules, as determined with the help of "Dreiding models" (17 A), is nearly identical to the layer thickness. This suggests a smectic A phase in which the layer structure is stabilized by hydrogen bonds between the hydroxyl groups along the polymer chains and the oxygen atoms of the nitro-groups (Scheme 5) . Therefore, there should be two driving forces for the formation of the LC phase. One is the high concentration (about 86 wt.30) of formanisotropic chromophores; the other is a microphase separation into a sublayer which is rich in hydrogen bonds and a sublayer which contains the nonpolar cores of the chromophores.

Synthesis and non-linear optical characteristics . . . 863

Scheme 5:

OH OH

17A

Scheme 5: Proposed struc- ture of the liquid-crystalline phase of polymer 5

For the preparation of poled films, an 18 wt.-Vo solution of polymer 4 in a mixture of 61 vol-Vo ethylene glycol dimethyl ether, 25 vol-Vo cyclohexanone and 14 vol-Vo 3-methoxypropyl acetate was spincoated onto glass slides coated with indium tin oxide. The polymer films obtained in this way (thickness about 1 -2 pm) are completely clear and amorphous (not LC). UV spectra show absorption bands that are broadened due to the high concentration of chromophores (see Fig. 1). Since the polymers show a slight absorbance above 500 nm, only the linear electrooptic effect (the Pockels-effect) was investigated in detail.

In order to obtain the polar order necessary for x(') effects, spincoated amorphous films of polymer 4 were subjected to a special heat treatment (heating during 2 hours slowly to 120 "C and keeping the film at that temperature for an additional hour), while applying the conditions of corona poling'). Thereby, a polar non-centrosymmetric orientation of the NLO-chromophores is obtained. As a result a completely clear (non- scattering) and highly oriented film is obtained (see Fig. 2). At the same time the polymer is cured, which leads to a stabilization of the polar structure. The dispersion of the refractive index of this polymer before and after poling is displayed in Fig. 2.

864 R. Zentel, D. Jungbauer, R. J. Twieg, D. Y. Yoon, C. G. Willson

200 300 LOO 500 600 700 800 Wavelength in nm

Fig. 1. UV-VIS spectrum of a film of the crosslinked polymer 4 (thickness: 530 nm, spincasted on fused silica wafers)

s-polarized IightITE): unpoled poled

Fig. 2. Dispersion of the refractive index of an unpoled and poled film (1,l pm) of polymer 4

500 700 900 1100 Wavelength in nm

The change of the birefringence with time was monitored in order to determine the stability of the structure obtained. No change of birefringence was observed at room temperature, and even an experiment at higher temperatures (80 to 90 "C) led only to a decrease of about 13% during 30 days (see Fig. 3). This material is therefore extremely stable.

Synthesis and non-linear optical characteristics . 865

a, u C a,

C Is)

Z 0,105 E .-

.- Q

C - a

0,100

0.095

Fig. 3. Relaxation of a poled film of cured polymer 4 at

-8OOC- I 9 o o c I I 11 I I

0,090 80-90°C (1340 decrease during 30 days) 0 2 L 6 810 20 30

Time in days

As a measure of the non-linear optical x(*) properties, the Pockels-effect was determined with a Mach-Zehnder interferometer. A value of r l13 = 7 3 pm/V was found for polymer 4 at 632,8 nm (He-Ne laser) with a modulation frequency of 5 kHz. For a comparison with x(*) values determined by frequency doubling, a d,, value of 20 pm/V can be estimated using the two-states approximation4**). This value is very high, but only about as high as the value of the linear epoxytolane described in ref. 4, la - o n -- 1x1 a - ?c -... / I 1 ..-A - 0 -... /xl\ -I+l.-....h +ha I--A:-.. ... :+I. (u3, = 07 piii/ v, u , ~ - LJ piiw v aiiu r l I 3 - o piii/ v J , airiiuusii LUG iuauiiig w i i i i

NLQ-chromophores has been raised for this polymer (86 instead of 46 wt.-Yo) and the birefringence of polymer 4 is higher than that of the linear polymer of ref.4). For a possible explanation, it is important to consider at first that the internal electric field, which might be especially reduced by ionic impurities, is not known during the corona poling procedure. In addition, a preference for an antiparallel orientation of the NLO- chromophores should be considered. This would explain the combination of high birefringence (apolar order) and smaller Pockels coefficients (polar order). The preference for such an antiparallel orientation might result (i) because of a cancellation of the dipol moments of the densely packed NLQ-chromophores and/or (ii) because of the LC short-range order. It may already be preformed in the highly branched prepolymer which is prepared without an external electric field.

Experimental part

Synthesis of monomers

The synthesis of N,N-bis(2,3-epoxypropyl)-4-(4-nitrophenylethynyl)aniline (1) is summarized in Scheme 2. N,N-Bis(3-chloro-2-hydroxypropyl)-4-iodoaniline (6) was prepared from 4-iodoaniline and

epichlorohydrin in analogy to general procedures in ref.9). 100 g of 4-iodoaniline (0,46 mol), 200 mL of epichlorohydrin and 2 mL of water were heated under nitrogen for 24 h at 100°C. Evaporation of the excess of epichlorohydrin gave a viscous brown oil (purity according to 'H NMR > 80'70, rest epichlorohydrin), which was used without further purification for the next step.

866 R. Zentel, D. Jungbauer, R. J. Tn .cs. D. Y. Yoon, C. G. Willson

The treatment of bischlorohydrin 6 with concentrated KOH restored the epoxide structure in N,N-bis(2,3-epoxypropyl)-4-iodoaniline (7). 140 mL of 45% KOH were added over 1 h to a vigorously stirred solution of 153 g of the crude 6 (about 80%) dissolved in 1 500 mL of toluene (room temperature). During this period a white precipitate formed. After 12 more hours of stirring, the toluene layer was washed with water, the toluene removed and the product 7 purified by flash-chromatography on silica-gel with ethyl acetateholuene (1 : 3 v/v) as eluent. Yield: 63,4 g (ca. 60%) of yellow oil.

4-Nitrophenylacetylene (8) was prepared by reaction of 1-bromo-4-nitrobenzene with trimethyl- silylacetylene and subsequent deprotection with potassium fluoride in analogy to general procedures ').

Monomer 1 was prepared by a palladium-catalyzed ethynylation of 7 with 8 in analogy to general procedures (ref. 5 ) ) . 7,35 g (0.05 mol) of 4-nitrophenylacetylene (8), 16,55 g (0,05 mol) of iodobenzene derivative 7 and 330 mg of triphenylphosphine were dispersed under nitrogen in 250 mL of diisopropylamine in a two-neck flask equipped with a condenser and a stirring bar. While heating the mixture at 70 "C, everything dissolved; at this time 82 mg of PdCI, and 48 mg of Cu(OAc), * H,O were added and then the mixture heated to a gentle reflux. After 20 min, an orange precipitate started to form, and after 1 h the reaction was stopped. The precipitate was filtered off, washed well with water and dried. The product was purified by flash-chromatography on silica-gel with hexane/ethyl acetate (1 : 1 v/v) as eluent and recrystallized from isopropyl alcohol. Yield: 6,7 g (38%) of orange needles, m. p. 123 "C.

'H NMR (CDC13): 6 = 2,56 [m; 2H, -CH,-0-1, 2,79 [m; 2H, -CH,-O-], 3,16 [m; 2H, >CH-0-1, 3,44 [m; 2H, >N-CH,-1, 3,78 [m; 2H, >N-CH,-1, 6,74 [d; 2HarOmat, Ar-Nl, 7,39 [d; 2H,,,,, Ar-Nl, 7 3 Id; 2H,,,,,, Ar-NO,], 8,14 [d; 2HarOmat, Ar-NO,].

C2OH18N204 (350) Calc. C 68,6 H 5,l N 8,O Found C 68,O H 5,6 N 7,8

The synthesis of 2-amino-4-(4-nitrophenylethynyl)aniline (2) is summarized in Scheme 3. 1-Iodo-3,4-dinitrobenzene (9) was prepared from 1 -iodo-3-nitrobenzene according to ref. lo).

Yield: 42%. 2-Amino-Ciodoaniline (10) was prepared by reduction of 9 with hydrazine and Raney-nickel in

analogy to general procedures (ref. ' I ) ) . A solution prepared from 20 g (0,068 mol) of 1 -iodo-3,4-dinitrobenzene (9) and about 4 g of Raney-nickel in 300 mL of dry ethanol was flushed with argon. Then 40 mL of hydrazine monohydrate was added to this boiling suspension over 2,5 h. After refluxing overnight, the nickel was filtered off, the ethanol removed and the residue purified by flash-chromatography on silica-gel with hexane/ethyl acetate (1 : 1 v/v). It was further purified by recrystallization from ethanol. Due to its sensitivity to air oxidation, it turns rapidly black in solution and is best immediately converted to monomer 2. Yield: 1,l g (7%) of slightly brownish crystals.

Monomer 2 was prepared by a palladium-catalyzed ethynylation of 10 (1,l g; 433 mmol) with 4-nitrophenylacetylene (8) (0,735 g; 5,00 mol), as described for monomer 1. Yield: 0,73 g (64%) of dark red needles, m. p.: 198 "C (dec.). 'H NMR (acetone-d6): 6 = 2,86 [-NHz], 6,64 [d; 1 H,,,,, , Ar-NH,], 6,80 [d; 1 Ha,,,,

Ar-NHJ, 6,87 [s; 1 HarOma,, Ar-NH21, 7,66 [d; 2Har0,,,, Ar-NO,], 821 [d; 2HarOmaf. Ar-NO,].

Cf4H11N302 (253) Calc. C 66,4 H 4,3 N 16,6 Found C 65,8 H 4,8 N 16,O

4-(4-Nitrophenylethynyl)aniline (3) was prepared by coupling of 8 with 4-iodoaniline (Scheme 4 ) .

A solution of 6,39 g (0,044 mol) of 8, 8.76 g (0,044 mol) of 4-iodoaniline, 200 mL of diisopropylamine, 35 mg (0,2 mmol) of PdCl,, 20 mg (0,l mmol) of Cu(OAc),. HzO and 263 mg (1,0 mmol) of triphenylphosphine was maintained at reflux for 90 min. Thereafter the mixture was cooled, excess solvent stripped off by rotary evaporation and the residue slurried with

Synthesis and non-linear optical characteristics . . . 867

methanol (200 mL). The solid was isolated by suction filtration, washed well with methanol/water and air dried. It was taken up in boiling toluene with a small amount of silica gel, filtered and crystallized (recrystallization from isopropyl alcohoMoluene). Yield: 8,47 g (88%).

Synthesis of polymers

The polymers 4 and 5 were prepared by melt polycondensation (cf. Scheme I). An exact 1 to 2 (polymer 4) or 1 to 1 (polymer 5) mole ratio of the amine components 2 or 3 and the epoxy component 1 was melted and stirred at 130-140°C under argon in a flask. For polymer 4 (crosslinkable), the reaction was stopped after 1 h as the viscosity increased strongly, but the resulting branched prepolymer was still completely soluble. For the linear polymer 5, the reaction was run for 5 h. Polymer 4 is soluble in a mixture of 61 vol-’70 ethylene glycol dimethyl ether, 25 vol-To cyclohexanone and 14 vol-Vo 3-methoxy propyl acetate, which was also used for the spin- casting. Polymer 5 was dissolved in N-methyl-2-pyrrolidone for the optical measurements. In order to convert the polymers into a powder, they were precipitated from hexane. Yield: nearly quantitative.

Measurements

The molecular weights were determined by gel-permeation chromatography (GPC) in THF versus polystyrene standards. 4 weight-average mol. wt. Mw : 1 580; 5: Mw : 1 130. They are very low, presumably because of the formation of cyclic structures.

The thermal characterization of the polymers was done with a Perkin-Elmer DSC-2c differential scanning calorimeter. The onset of the glass transition was used to define the glass transition temperature ( T ) .

The films for the optical measurements were prepared by spin-casting. For optical absorption spectroscopy, the samples were prepared on fused silica wafers, and the spectra were recorded on an IBM 9430 UV-Vis spectrometer. For corona poling, they were prepared on glass plates coated with indium thin oxide. The set-up described in refs. 4, j2) was used (voltage 15 kV, distance needle to sample: 10 cm). During poling, the second harmonic signal (SHG) was monitored. The tem- perature programme was started at room temperature and raised stepwise (5 - 10 K), allowing the SHG signal to reach a constant level at each temperature. The end temperature of 120°C was reached within 1 h, and the sample was kept at this temperature for another hour to achieve a glass transition temperature close the the poling temperature. Subsequently, the sample was cooled to room temperature within 10 min with the field still on. Finally, the corona field was switched off and the remaining surface charges were removed with a wet swab.

The refractive index for s- and p-polarized light was determined using a Metricon 2000 prism coupler and investigating TE and TM modes of light traveling in a slab waveguide. The thickness of the sample was also obtained by these measurements.

The set-up for the measurement of the Pockels-effect is described in refs. ’, j3). A 70-nm thick PVA coating was spun as a blocking electrode on top of the poled polymer film before depositing a 50-nm thick gold electrode4).

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