[acs symposium series] polymers for second-order nonlinear optics volume 601 || development of...

Chapter 13

Development of Functionalized Polyimides for Second-Order Nonlinear Optics

Dong Yu, Zhonghua Peng, Ali Gharavi, and Luping Yu1

Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637

Several functionalized polyimides (both aromatic and aliphatic) with pendent nonlinear optical chromophores have been synthesized. The detailed characterization indicated very promising features for these polymers. The polyamic acids were soluble in several aprotic polar solvents, permitting thin film processing. The polyamic acid can be imidized at ca 200°C without damaging the NLO chromophore. Long term stability was observed at 170°C. The most interesting point is that the reaction approaches are versatile and allow for further developments of better materials.

The development towards practical applications of second order nonlinear optical polymers is gaining momentum.1-3 However, in spite of great deal of research effort focused on these oriented polymer systems, hurdles towards that goal still remain, i.e. higher thermal stability and larger optical nonlinearity. It is known that in order to be useful in optical device applications, second-order NLO polymers should be chemically and physically stable. They should also maintain a significant bulk nonlinearity at possible device processing temperatures of 200 °C-250 °C and at elevated working temperatures that can reach 80 °C or higher. These practical considerations have led investigators to search for new polymeric systems which exhibit better temporal and thermal NLO stabilities.

Polyimides have been extensively studied as high performance materials for applications in integrated electronic circuits and aerospace devices due to their exceptional thermal, mechanical, optical and dielectric properties.4*5 Recently, the unique properties of polyimides have aroused much attention from the optical community for their photonic applications. 6 - 8 The high glass transition temperature of these materials have been utilized to prepare second order nonlinear optical (NLO) composite materials which exhibit exceptionally high temperature stability in the dipole orientation.6 While these systems enjoyed a certain degree of success, several problems also occurred. For example, NLO chromophore bleaching was observed in the polyimide composite systems and the glass transition temperature decreased because of the plasticizing effect of the small chromophore at prolonged high

1Corresponding Author

0097-6156/95/0601-0172$12.00A) © 1995 American Chemical Society

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In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

13. YU ET AL. Functionalized Polyimides for Second-Order Nonlinear Optics 173

working temperatures, and so on. These problems severely hindered the further development of these materials for practical applications.

Recently, we synthesized a series of functionalized, nonlinear optical polyimides exhibiting large and exceptionally stable second harmonic generation (SHG) coefficients.9-11 The rationale to synthesize these polymers was that the motion of the free volume in the polymer matrix can be frozen due to a high glass transition temperature. Also, the covalent attachments of the nonlinear optical chromophore can eliminate several problems associated with the composite systems, such as chromophore sublimation, limited effective chromophore number density due to the incompatibility between the polymer host and the chromophore guest etc.. This paper summarizes our synthetic efforts and reports detailed results on physical characterizations.

Experimental Section

THF was purified by distillation over sodium chips and benzophenone. DMF was purified by distillation over phosphorous pentoxide. 1,2,4,5-Benzenetetracarboxylic dianhydride (PMDA) and 3, 3\ 4, 4'-benzophenone tetracarboxylic dianhydride (BTDA) were purified by recrystallization from acetic anhydride and then dried under a vacuum at 150 °C overnight before use. All of the other chemicals were purchased from Aldrich Chemical Company and used as received unless otherwise stated. The synthetic procedure for the intermediates used to produce the monomers can be found in references 9-11.

Compound 3: Diethyl azodicarboxylate (DEAD, 1.20 g, 6.9 mmol) in THF (5 ml) was added dropwise to the mixture of 4-(N-ethyl-N-P-hydroxyethyl)-4'-(methylsulfonyl)-stilbene (Compound 1, 1.83 g, 5.3 mmol), 2, 5-dinitrophenol (Compound 2,0.78 g, 5.3mmol) and triphenyl phosphine (1.81 g, 0.69 mmol) in 35 ml THF at room temperature. After the reaction mixture was stirred for 24 hr, water (50 ml) was added. The resulting solid was collected by filtration and recrystallized from acetone/methanol to yield dark black crystals (2.30 g, 85 %, m.p. 176-177 °C). lH NMR (CDC13, ppm): 6 1.22 (t, J=6.9 Hz, -CH2CH3, 3 H), 3.04 (s, -SO2CH3, 3 H), 3.52 (q, J=7.0 Hz, -CH2CH3, 2 H), 3.86 (t, J=5.2 Hz, -OCH7CH9N-. 2 H), 4.36 (t, J=5.2 Hz, -OCH2CH2N-, 2 H), 6.68 (d, J=8.6 Hz, ArH, 2 H), 6.87 (d, J=16.2 Hz, =CH, 1 H), 7.13 (d, J=16.2 Hz, =CH, 1 H), 7.39 (d, J=8.5 Hz, ArH, 2 H), 7.56 (d, J=8.1 Hz, ArH, 2 H), 7.82 (d, J=8.3 Hz, ArH, 2 H), 7.87 (m, ArH, 3 H). Anal. Calcd. for C25H25N3O7S: C, 58.71; H, 4.89; N, 8.22; Found: C, 58.63; H, 4.96; N, 8.18.

Monomer A: Stanneous chloride (3.71 g, 19.6 mmol) was added to compound 3 (1.0 g, 2.0 mmol) in 50 ml hydrochloric acid (36.5 % - 38 %). The mixture was stirred at room temperature for 24 hr and then warmed to 40 to 50 °C for 2 hr until the solution became completely transparent. Deoxygenated sodium hydroxide solution (40 %) was then added to the mixture at an ice bath until the pH value of the solution reached 12 - 13. Yellow solid was collected by filtration under nitrogen and then recrystalized from MeOH/H20 to yield bright yellow crystals (0.5 g, 57 %, m.p. 166 - 168 °C). lH NMR (CDCI3, ppm): 6 1.22 (t, J=6.9 Hz, -CH2CH3, 3 H), 3.03 (s, -SO2CH3, 3 H), 3.30 (brs, Ar-NH2, 4 H), 3.48 (q, J=7.0 Hz, -CH2CH3, 2 H), 3.77 (t, J=5.8 Hz, -OCH?CH?N-. 2 H), 4.09 (t, J=5.8 Hz, -OCH2CH2N-, 2 H), 6.16 (d, J=8.2 Hz, ArH, 1 H), 6.19 (s, ArH, 1 H), 6.51 (d, J=8.1 Hz, ArH, 1 H), 6.72 (d, J=8.6 Hz, ArH, 2 H), 6.86 (d, J=16.2 Hz, =CH, 1 H), 7.12 (d, J=16.2 Hz, =CH, 1 H), 7.37 (d, J=8.3 Hz, ArH, 2 H), 7.56 (d, J=8.1 Hz, ArH, 2 H), 7.81 (d, J=8.0 Hz, ArH, 2 H). Anal. Calcd for C25H29N3O3S: C, 66.52; H, 6.43; N, 9.31. Found: C, 66.67; H, 6.49; N, 9.16.

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174 POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

Monomer B: To a two-neck flask containing compound 4a (0.800 g, 1.36 mmol) and THF (20ml) was added the hydrazine monohydrate (0.389g, 6.80 mmol). The mixture was refluxed for 5 hours under nitrogen and then cooled down to room temperature. The precipitate was filtered out and the filtrate was dried under a vacuum to yield a red solid. Ethanol (20ml) was then added to this solid and the resulting mixture was stirred for 15 minutes at ca. 50°C. The insoluble part was removed by filtration. Deoxygenated water was then added dropwise to the filtrate and a dark red product crystallized out The crude product was recrystallized from MeOH/water to yield monomer B (dark-red plate crystals, 0.2g, 46.2%, mp, 160.2 °C ( DSC result)). *H NMR (DMSO-d6, ppm): 6, 1.50 (d, J = 4.07 Hz, -NH2 4 H), 2.67 (t, J = 6.90 Hz, -CH2NH2 4 H), 3.31 (t, J=6.98 Hz, -CH2N-, 4 H), 6.71 (d, J = 8.37 Hz, ArH, 2 H), 7.03 ( d, J = 16.31 Hz, =CH, 1 H), 7.36 (d,J = 16.32 Hz, =CH, 1 H), 7.41 (d, J = 8.33 z, ArH, 2 H), 7.70 (d, J = 8.36 Hz, ArH, 2 H), 8.12 (d, J = 8.40 Hz, ArH, 2 H). Anal. Calcd for C18H22N4O2: C, 66.23; H, 6.80; N, 17.17. Found: C, 66.04; H, 6.75; N, 17.06.

Monomer C: The hydrazine monohydrate (3.0 g, 60 mmol) in THF (10 ml) was added slowly to a suspension of compound 4b (3.05 g, 5.0 mmol) in 40 ml of THF under stirring. The mixture was refluxed for 3 hr under nitrogen and then cooled down to room temperature. The solid was removed by filtration and the filtrate was concentrated under a vacuum to ca. 5 ml. Ethanol (10 ml) was added to the concentrated filtrate and the solution was placed in a refrigerator overnight. The yellow crystals were collected and recrystallized twice from ethanol, yielding monomer C (0.77g, 42.8 %, m.p. 105-106 °C). *H NMR (DMSO-d6): 6, 2.66 (t, J = 6.55 Hz, -NH2, 4 H), 3.18 (s, -SO2CH3 3 H), 3.31 (t, J = 6.64 Hz, -NCH2-, 4 H), 6.70, 7.38, 7.70, 7.79 (d, ArH, 2 H), 7.00, 7.27 (d, =CH, 1 H). Anal, calcd. for C19H25 N3O2S: C, 63.49; H, 7.01; N, 11.69. Found: C, 63.54; H, 6.97; N, 11.49.

Polymerization. The polymerization procedure is examplified as follows: to a 25-ml two-necked, round-bottom flask was added monomer A (0.2148 g, 0.476 mmol) and the anhydrous NMP (6 ml) at -2 to -5°C under nitrogen. The 3, 3', 4, 4'-benzophenone tetracarboxylic dianhydride (BTDA, 0.1533 g, 0.476 mmol) was added at once. The dianhydride was completely dissolved in ca. 20 min. The mixture was stirred at -2 - -5°C for 4 hr after the addition. The polyamic acid was precipitated into methanol and collected by filtration (almost quantitative yield). To further purify the polymer, it was ground in acetone to remove the residual solvent and then washed with chloroform in a soxhlet extractor for 2 days. The lH NMR of polymer A2 in DMSO is shown in Figure 1.

Characterization: The lH NMR spectra were collected on a varian 500 MHz FT NMR spectrometer. The FTIR spectra were recorded on a Nicolet 20 SXB FTIR spectrometer. A Perkin-Elmer Lambda 6 UV/VIS spectrophotometer was used to record the UV/VIS spectra. The thermal analyses were performed by using the DSC-10 and TGA-50 systems from TA instruments under a nitrogen atmosphere. The melting points were obtained with open capillary tubes using a Mel-Temp apparatus. Elemental analyses were performed by Atlantic Microlab, Inc.

The NMP solution of the polyamic acid was filtered through 0.2-/*m syringe filters and then spin cast onto indium-tin oxide (ITO) coated glass slides or normal glass slides. The films were dried in an oven at 40 °C for 48 hr under nitrogen, then vacuum dried at 60 °C overnight The films were poled/ cured at a corona-discharge setup with a tip-to-plane distance of 1.0 cm at elevated temperatures from 60 to 200 C within two hours. The second order NLO properties of the poled polymeric films

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Figure 1. *H NMR spectrum of polyamic acid A2 in DMSO-d6-

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were characterized by second harmonic generation (SHG) measurements. A mode-locked Nd:YAG laser (Continuum-PY61C-10, 25 ps, 10 Hz repetition rate ) was used as a fundamental light source (1,064 /mi). The second harmonic signal was detected and amplified by a photomultiplier tube (PMT) and then averaged in a boxcar integrator. The temporal stability of the SHG signal of the polyimide was monitored at the following temperatures: room temperature, 90 °C, 150 °C and 170 °C respectively. The temperature reading error was within ± 5 °C. Detailed descriptions of the measurements are given in reference 12.

Scheme 1. Syntheses of diamino monomers.

|1

0 SnCl 2

HC1

S0 2CH 3 i - | (1) DEAD= EtOOCN=NCOOEt

S0 2CH 3

(3) O H 2N

S0 2CH 3

Monomer A ,NH2

CH

<3

O P h 3 P

THF

• HQ* Hydrazinolysis CH

X (4a)x = N02

x Monomer B X = N02

(4b)x = S02CH3 Monomer C X = SO2CH3

0

Results and Discussion

Our strategy in synthesizing the functionalized polyimides was to functionalize the diamine monomers. The key step in these syntheses was to utilize the Mitsunobu reaction to attach an NLO chromophore to 2,5-dinitrophenol and then to reduce the nitro groups (Monomer A) or to convert dihydroxy groups into diamino groups (Monomers B and C). The synthetic approaches for these monomers are shown in scheme 1. The monomers obtained were easy to purify. Both spectroscopic and microanalysis results supported the structures as proposed.

Polymerization can be easily effected in aprotic polar solvents (Such as DMF, NMP and DMAc, etc.) with different dianhydrides. The polyamic acids obtained usually possessed relatively high molecular weights (with intrinsic viscosities in the range of ca 0.5-1.0dl/g) and were soluble in aprotic polar solvents such as NMP, DMSO, DMAc and DMF. The polyamic acid solution can be directly used to prepare films or precipitated into acetone to obtain solid materials. The films can be further cured at high temperature (>200°C) to form polyimides. Scheme 2 shows the structures of different polyimides synthesized.

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Spectroscopic studies, including lH NMR, FTIR and UV/vis, were in full agreement with the expected structures of the polymers. All of the polyamic acids exhibited an endothermic process at around 200 °C as shown by the DSC studies. This process was clearly due to imidization as correlated with the FTIR, UV/vis spectroscopic and TGA studies. The polyimides obtained exhibited a high glass transition temperature (all larger than 200 °C) which resulted in the high stability of the dipole orientation at elevated temperatures.

Scheme 2. Structures of second order NLO polyimides.

X

The UV/vis spectra of the polyamic acids showed a typical absorption maxim due to the dialkylamino-sulfonyl (or -nitro) stilbene chromophore. After thermal curing, the absorbance did not change significantly except for a slight blue shift caused by the matrix change due to the imidization (See Figure 2). However, after the polyamic acid was corona-poled as it was imidized, a large decrease in absorbance was observed due to the birefringence induced by the dipole alignment. Table I summarizes some of the physical data of these polymer systems.

Second harmonic generation measurements were performed at a wavelength of 1064 nm. A wide range of 033 values were obtained by the careful design of the NLO chromophore (See Table I). Listed in Table I are the dispersionless d33 values derived from the two level model. Due to the insufficient poling (O = 0.12), the d33 value of polymer CI is significantly lower than polymers Al , A2 and C2. Latest study after we submitted the paper showed comparable d33 value (43 pm/V) after improve the poling. The d33 value for polymer Bl is strongly resonance enhanced at 532 nm.

To evaluate the high temperature stability of our polymers, we studied the temporal stability of the second harmonic generation (SHG) signals at various temperatures. It was found that the SHG signals of all of these polyimides exhibited no decay at room temperature, 90 °C or at 120°C for more than 1200 hours. For polyimide A2, long term stability at 150 °C was observed (see Figure 3). Decay was

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Table I. Physical data for different polyimides.3

Polyimides

Tg(°Q m̂ax (nm)

cj>b n (532 nm)

d33 (532nm, pm/V)

<*33 (°°. pm/V)c

Al 240 384 0.20 1.781 51 18 A2 230 383 0.18 1.815 46 16 Bl 230 440 0.30 1.833 115 27 CI 250 372 0.12 1.769 29 10 C2 209 372 0.21 1.781 41 14

a. See Scheme 2 for structures of the polyimides. b. Order parameters derived from the UV/vis spectra before and after electric

poling. c. The values estimated from the two level model.

noticeable at the initial stage when the samples (Polymer A1, Bl CI and C2) were kept at 150°C and the signal was then stabilized at ca. 85% of its initial value. Several very promising features of these polyimides can be extracted from these results: first, its nonlinearity can be stabilized at 90 °C and 120 °C and 150°C for a significant long term (more than 1000 hr), which is the possible working temperature anticipated by device engineers. Second, the films can withstand high processing temperatures for a short term (e.g. 220 °C). Third, the reaction approaches are versatile in synthesizing different materials. We can obviously vary several structural parameters to obtain new materials. For example, the NLO chromophore can be changed from the stilbene to other structures which will lead to an improved NLO activity and higher thermal stability. Different dianhydride monomers can be utilized to synthesize polyimides so that the glass transition temperature can be finely tuned. Cross-linking units can be introduced into the polymers to further enhance its stability at high temperatures. Finally, due to the good processibility of polyamic acids, it is possible to fabricate several device elements.

Conclusions

Functionalized polyimides with pendent nonlinear optical chromophores can be synthesized by the careful design of suitable NLO chromophores. The polyamic acids obtained are soluble in several aprotic polar solvents, permitting thin film processing. The polyimides exhibited a high glass transition temperatures (>200 °C) after the polyamic acid was imidized at ca 200°C. The high glass transition temperatures of the polyimides resulted in a high stability of the optical nonlinearity at elevated temperatures. Long term stability was observed at 150°C.

Acknowledgments This work was supported by the Office of Naval Research grants N00014-

93-1-0092. The support from the Arnold and Mabel Beckman Foundation is gratefully acknowledged.

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- B e f o r e P o l i n g . A f t e r P o l i n g :

a t 2 0 0 ° C / 2 h ^ '

— B e f o r e c u r i n g . A f t e r C u r i n g

a t 2 0 0 ° C / 2 h

7(10

i 1 1 i — " T * 1 i 1 1 r

300 400 500 600 700 800

(nm)

Figure 2. UV/vis spectra of the polyamic acid and polyimide under different conditions.

Q , 80

200 400 600 800

Time(h)

Figure 3. Temporal stability of the SHG signals at 150 °C for polyimide A2.

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180 P O L Y M E R S F O R S E C O N D - O R D E R N O N L I N E A R O P T I C S

Literature Cited 1. Prasad, P. N., Williams, D. J., Introduction to Nonlinear Optical Effects in

Molecules and Polymers, J. Wiley and Sons, New York, 1991. 2. Materials for Nonlinear Optics: Chemical Perspectives, ACS Symposium

Series No. 455, ed. by Marder, S. R., Sohn, J. E., Stucky, G. D., Washington, 1991.

3. Nonlinear Optical Properties of Organic Molecules and Crystals, ed. by Chemla, D. S., Zyss, J., Academic Press, New York, 1987.

4. Polyimides, Mittal, K. L., Ed.; Plenum Press, New York, 1984, Vols. 1, 2. 5. Scroog, C. E., J. Polym. Sci., Macromol. Rev. 1976, 11, 161. 6. Wu, J. W., Valley, J. F., Ermer, S., Binkley, E . S., Kenney, J. T.,

Lipscomb, G. F., Lytel, R. J., Appl. Phys. Lett., 1991,58, 225. 7. Lin, J. T., Hubbard, M . A. , Marks, T. J., Lin, W., Wong, G. K. , Chem.

Mater. 1992, 4, 1148. 8. Becker, M . W., Sapochak, L. S., Ghosen, R., Xu, C. Z . , Dalton, L. R., Shi,

Y. Q., Steier, W. H. , Chem. of Mater., 1994, 6, 104. 9. Peng, Z. H. , Yu, L. P., Macromolecules , 1994, 27, 2638. 10 Yang, S. Y. , Peng, Z. H. , Yu, L . P., Macromolecules. 1994, 27, 5858. 11. a). Yu, D., and Yu, L . P., Macromolecules. 1994, 27, 6718. b). Yu, D.,

Gharavi, A. , and Yu, L . P., Macromolecules. to Appear in Jan. 1995. 12. Yu, L . P., Chan, W. K., Bao, Z. N., Macromolecules, 1992, 25, 5609

RECEIVED January 30, 1995

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[acs symposium series] polymers for second-order nonlinear optics volume 601 || development of...

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