Materials for Nonlinear Optics - American Chemical Society

Chapter 14

Chromophore—Polymer Assemblies for Nonlinear Optical Materials Routes to New Thin-Film Frequency-Doubling Materials Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch014

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D.-R. Dai , M. A. Hubbard , D. Li , J. Park , M. A. Ratner , T. J. Marks , Jian Yang , and George K. Wong 1

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Department of Chemistry and the Materials Research Center and Department of Physics and the Materials Research Center, Northwestern University, Evanston, IL 60208

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The properties of polymer-based second harmonic generation materials are crucially dependent upon realizing high number densities of constituent chromophore moieties and upon achieving and preserving maximum microstructural acentricity. This article reviews recent progress toward these goals. Systems discussed include electric field poled, chromophore-functionalized polyphenylene ethers with second harmonic coefficients (d ) as high as 65 x 10 esu, T ~ 173°C, and with superior temporal stability of the poling-induced chromophore orientation. Also presented are successful strategies for simultaneously poling and diepoxide cross-linking chromophore-functionalized poly(p -hydroxystyrene). The result is a significant improvement in the temporal stability of chromophore orientation. Two approaches to chromophore immobilization are then discussed which involve highly cross-linkable epoxy matrices. In the first, chromophore molecules are embedded in a matrix which can be simultaneously poled and thermally cured. In the second, a functionalized high -β chromophore is synthesized for use asanepoxy matrix component. Finally, a strategy is discussed inwhichrobust, covalent, chromophore-containing self-assembled multilayers are built upon various surfaces. Very high chromophore layer second harmonic generation efficiencies are observed (d = 300 x 10 esu). 33

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The great current interest i n nonlinear o p t i c a l (NLO) materials based upon 7r-electron chromophores stems from the demonstrated p o s s i b i l i t i e s of large nonresonant s u s c e p t i b i l i t i e s , u l t r a f a s t response times, low d i e l e c t r i c constants, high o p t i c a l damage thresholds, and the great i n t r i n s i c t a i l o r a b i l i t y of the constituent structures (1-6). When such materials incorporate glassy polymeric architectures, the additional a t t r a c t i v e c h a r a c t e r i s t i c s of supermolecular organization, improved mechanical/dimensional s t a b i l i t y , improved o p t i c a l transpar ency, and p r o c e s s a b i l i t y into t h i n - f i l m waveguide structures can be envisioned. Nevertheless, the progression from the above ideas to

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In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch014

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e f f i c i e n t NLO materials has presented great challenges, and numerous obstacles remain to be surmounted. o For polymer-based second harmonic generation (SHG, x ) materials, the c r u c i a l synthetic problem i s to maximize the number density of component high-/S chromophore molecules while achieving and preserving maximum a c e n t r i c i t y of the microstructure. One early approach to such materials was to "dope" NLO chromophores into glassy polymer matrices and then to a l i g n the dipolar chromophore molecules with a strong e l e c t r i c f i e l d (poling) (7,8). The performance of such materials i s l i m i t e d by the low chromophore number densities which can be achieved before phase separation occurs and the physical aging/structural relaxation c h a r a c t e r i s t i c s of a l l glassy polymers (9-14), which lead to randomization of the poling-induced p r e f e r e n t i a l chromophore orientation. Hence, the SHG e f f i c i e n c y of such "guest-host" materials i s generally short-lived. In addition, we have observed that the chromophore constituents are not strongly bound i n such matrices and that these materials r e a d i l y undergo d i e l e c t r i c breakdown during poling. A second approach to the construction of e f f i c i e n t filmbased SHG materials has been to incorporate NLO chromophores into Langmuir-Blodgett (LB) films (15-17). A p r i o r i . such an approach offers f a r greater net chromophore alignment than i s possible i n a poling f i e l d (where net alignment i s s t a t i s t i c a l l y determined), temporal s t a b i l i t y of the chromophore alignment, and controlled f i l m thickness. While preliminary results with LB film-based NLO films have been encouraging (15-17), s i g n i f i c a n t problems arise from the f r a g i l i t y of the films, the temporal i n s t a b i l i t y of chromophore alignment, the problem of scattering microdomains, and the s t r u c t u r a l r e g u l a r i t y of layer deposition that i s possible (18-22). With these results as a background, the goal of the present a r t i c l e i s to b r i e f l y summarize recent research i n this Laboratory aimed at the r a t i o n a l design, construction, and characterization of new types of polymer-based NLO substances. We discuss three classes of materials: i ) chromophore-functionalized glassy polymers, i i ) t o t a l l y cross-linked matrices, and i i i ) chromophore-containing s e l f assembled organic superlattices. In each case, the goal has been to develop approaches to enhanced a c e n t r i c i t y , chromophore number densities, and SHG temporal s t a b i l i t y . In each case, chemical synthesis i s also employed to test fundamental ideas about polymer s t r u c t u r a l dynamics, cross-linking processes , and monol aye r /multilayer synthesis. Poled Chromophore-Functionalized Polymers A f i r s t step i n ameliorating many of the d e f i c i e n c i e s of the aforementioned guest-host materials has been to covalently bind NLO chromophores to selected polymer c a r r i e r s (23-30). I n i t i a l work focussed upon functionalized polystyrene and poly(p-hydroxystyrene) systems (23-27). These materials provide greatly enhanced chromophore number densities, greater SHG temporal s t a b i l i t y (tethering of chromophore molecules to massive polymer chains greatly r e s t r i c t s r e o r i e n t a t i o n a l mobility), improved s t a b i l i t y with respect to contact poling-induced d i e l e c t r i c breakdown (presumably a consequence of the r e s t r i c t e d microstructural mobility), and enhanced chemical s t a b i l i t y (chromophore molecules are more strongly bound within the matrix). I t was found that contact poling f i e l d s as large as 1.8 MV/cm and d33 values


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as high as 19 x 10" esu (greater than the corresponding c o e f f i c i e n t for LiNb03) could be r e a l i z e d . Considerably enhanced SHG temporal s t a b i l i t i e s were also observed. Nevertheless, neither optimum chromophore number densities nor maximum chromophore immobilization could be achieved i n these f i r s t - g e n e r a t i o n systems. In the following subsections, we describe the synthesis and properties of a functionalized polymer with greater than one chromophore moiety per polymer repeat unit and with a very high glass t r a n s i t i o n temperature (Tg - one index of polymer chain mobility) (31) . We next describe an approach to chromophore immobilization i n which thermal c r o s s - l i n k i n g chemistry i s effected i n concert with e l e c t r i c f i e l d poling of a chromophore-functionalized polymer (32). We also compare contact to corona poling methodologies and r e s u l t s . The l a t t e r technique offers larger (but not p r e c i s e l y known) e l e c t r i c poling f i e l d s as well as f a r greater resistance to d i e l e c t r i c breakdown (33). An NLO Chromophore-Functionalized Polyether. Poly(2,6-dimethyl-l,4phenylene oxide) (PPO) i s a high-strength, amorphous engineering thermoplastic with Tg « 205-210°C and excellent film-forming charact e r i s t i c s (34). For the present work, PPO was prepared by oxidative coupling of 2,6-dimethylphenol and was p u r i f i e d as described i n the l i t e r a t u r e (35) (Scheme I; ^ - 27,000). Bromination (36) i n r e f l u x i n g tetrachloroethane y i e l d s PP0-Br materials with functionali z a t i o n levels on the order of 1.6-1.8 Br/repeat unit (predominantly benzylic bromination) as judged by elemental analysis and ^H NMR. N(4-nitrophenyl)-(S)-prolinoxy (NPPO-, Scheme I) was chosen as a model chromophore synthon since the o p t i c a l properties have been extensively studied (37,38) and since i t i s readily amenable to NLO experiments at A = 1.064 /zm. Reaction of PP0-Br with NPPO" (from NPPOH + NaH) i n dry N-methylpyrrolidone (NMP) y i e l d s the chromophore-functionalized material (PP0-NPP ; Scheme I; 1.4 - 1.6 NPPO/repeat unit; T « 173°C) a f t e r p r e c i p i t a t i o n with acetone, washing with H2O, Soxhlet extraction with MeOH, and vacuum drying. Polymer films were cast i n a class 100 laminar-flow clean hood onto ITO-coated conductive glass from t r i p l y f i l t e r e d NMP solutions. The solvent was then slowly evaporated at 80°C, and the films dried i n vacuo at 150-170°C f o r 24 h. These PPONPP films have excellent transparency c h a r a c t e r i s t i c s (vide i n f r a : A - 405 nm) , adhere tenaciously to glass, and are insoluble i n most organic solvents. Contact poling of the PPO-NPP films was c a r r i e d out at 160-170°C with 1.2 MV/cm f i e l d s using aluminum electrodes and techniques described elsewhere (23-27). After cooling the films to 30°C, the f i e l d was maintained f o r an additional 1.5 h. Corona poling was c a r r i e d out at 180-190°C using a needle-to-film distance of 1.0 cm and a +4-+5 kV p o t e n t i a l . After the f i l m had cooled to room temperature, the f i e l d was maintained f o r an additional 1.5 h. Second harmonic data were measured at 1.064 fim i n the p-polarized geometry using the instrumentation and c a l i b r a t i o n techniques described previously (2327). Second harmonic c o e f f i c i e n t s ^ 3 3 values) were calculated from the angular dependence of I ^ and the formalism of Jerphagnon and Kurtz for u n i a x i a l materials, assuming a d d i t i o n a l l y that d3^ - &2i± = ^15 ^33/3 (39) . We have previously v e r i f i e d t h i s l a t t e r assumption for other poled, chromophore-functionalized polymers (23). In Table I are presented f u n c t i o n a l i z a t i o n l e v e l and d33 data for representative PPO-NPP films. Assuming approximate a d d i t i v i t y of PPO x

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Table I. Second-Harmonic Coefficients ( d ) and Temporal Decay Data for NPP-Functionalized Poly(2,6-Dimethyl-1,4-Phenylene Oxide) 3 3

Functionalization Level a

NPPO Number Density 10 /cm 20

Poling Method

d33 10' esu 9

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zzz

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o

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u r e

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chromophore dipoles with respect to the substrate surface normal. Our results thus suggest that \j> i s i n the range 35°-39° for the present self-assembled chromophoric superlattices. By c a l i b r a t i n g the 1.064 /im SHG from the s u p e r l a t t i c e samples against that from a quartz plate we obtain d33 - 100 x 1 0 e s u for these multilayer structures and d33 = 300 x 10"^ esu for a ChCp monolayer of 22 A estimated (from the ellipsometric data) thickness. These values are rather large compared to those observed i n poled polymer films (vide supra). The p o s s i b i l i t y of obtaining such large n o n l i n e a r i t i e s i n these self-assembled films i s consistent with the higher chromophore number density and the high degree of noncentrosymmetric alignment of the chromophores. In regard to whether the above A — 1.064 /im d33 r e s u l t s may include a very large resonant enhancement ( A = 510 nm for the chromophore), supplementary SHG measurements at A = 1.90 /im y i e l d a nonresonant d value only 40% smaller. Such large d-^ values also suggest that the formation of aggregates with c e n t r i c structures, which commonly occurs i n LB films and lowers considerably the maximum possible value of d33 that can be achieved (18-22,64,65), i s not important i n these covalently connected, self-assembled films. m a x

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INCIDENT ANGLE (deg.) Figure 6. SHG intensity from a glass s l i d e having self-assembled CpCh monolayers on both sides as a function of fundamental beam incident angle. The interference pattern i s due to the phase difference between the SHG waves generated at either side of the substrate during propagation of the fundamental wave. The s o l i d envelope i s a t h e o r e t i c a l curve generated f o r Xzzz/*zyy ™ ^'


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Since the present multilayers are extremely t h i n i n comparison to the wavelength of l i g h t being employed and to the expected coherence length, the intensity of the SHG signal should scale quadratically with the number of chromophore layers (66). This i s an NLO q u a l i t y diagnostic commonly applied to LB films (15-22). As can be seen i n Figure 7, the adherence of the present multilayer structures to quadratic behavior i s good, indicating that i t i s possible to maintain the same degree of noncentrosymmetric chromophore ordering in the additions of successive layers.


Conclusions These results i l l u s t r a t e the d i v e r s i t y of synthetic and processing approaches that can be taken i n the synthesis of t h i n - f i l m frequency doubling materials. S p e c i f i c a l l y , we have demonstrated that i t i s possible to assemble chromophore-functionalized polymers with greater than one chromophore substitutent per monomer subunit, with d33 values as high as 65 x 10" esu, with T values as high as 173°C, with improved temporal s t a b i l i t y , and with good transparency characterist i c s at A — 0.633 /im. We have also shown that known chromophorefunctionalized polymers can be simultaneously poled and cross-linked 9

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Number of Layers Figure 7. Plot of the square root of SHG i n t e n s i t y versus the number of chromophore layers i n the multilayer s u p e r l a t t i c e . The straight l i n e s are the linear least-squares f i t to the experimental data. The number labels correspond to the interferogram maxima i n Figure 6.


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to significantly retard the rate of chromophore disorientation following electric field poling. In addition, NLO systems based upon highly cross-linkable epoxy matrices have been prepared and shown to be viable candidates for both improved SHG temporal stability and improved frequency doubling efficiency. Finally, we have shown that i t is possible to sequentially construct robust, covalently linked, chromophore-containing organic superlatices with good structural regularity and high optical nonlinearity ^33 ~ 300 x 10' esu at A = 1.064 /xm) . 9


Acknowle dgment s This research was supported by the NSF-MRL program through the Materials Research Center of Northwestern University (Grant DMR8821571) and by the Air Force Office of Scientific Research (Contracts 86-0105 and 90-0071). We thank Mr. T. G. Zhang for helpful discussions and Drs. D. Lam and J . Parker of Argonne National Laboratory for assistance with the ellipsometry measurements.

Literature Cited 1. Messier, J . ; Kajar, F.; Prasad, P.; Ulrich, D., Eds.; "Nonlinear Optical Effects in Organic Polymers," Kluwer Academic Publishers: Dordrecht, 1989. 2. Khanarian, G., Ed. "Nonlinear Optical Properties of Organic Materials," SPIE Proc. 1989, 971. 3. Heeger, A. J . ; Orenstein, J . ; Ulrich, D. R., Eds. "Nonlinear Optical Properties of Polymers,"Mats.Res. Soc. Symp. Proc., 1988, 109. 4. Chemla, D. S.; Zyss, J., Eds. "Nonlinear Optical Properties of Organic Molecules and Crystals," Vols. 1, 2; Academic Press: New York, NY, 1987. 5. Zyss, J. J. Mol. Electronics 1985, 1, 25-56. 6. Williams, D. J. Angew. Chem. Intl. Ed. Engl. 1984, 23, 690703. 7. Singer, K. D.; Sohn, J. E.; Lalama, S. J. Appl. Phys. Lett. 1986, 49, 248-250. 8. Singer, K. D.; Kuzyk, M. G.; Sohn, J. E. J. Opt. Soc. Am.B 1987, 4, 968-975. 9. Hampsch, H. L.; Torkelson, J. M.; Bethke, S. J . ; Grubb, S. G. J. Appl. Phys., 1990, 67, 1037-1041. 10. Hampsch, H. L.; Yang, J . ; Wong, G. K.; Torkelson, J. M. Macromolecules 1988, 21, 526-528. 11. Hampsch, H. L.; Yang, J . ; Wong, G. K.; Torkelson, J. M. Polymer Commun. 1989, 30, 40-43. 12. Yu, W-C., Sung, C. S. P.; Robertson, R. E. Macromolecules 1988, 21, 355-364, and references therein. 13. Victor, J. G.; Torkelson, J. M. Macromolecules 1987, 20, 22412250. 14. Struik, L. C. E. "Physical Aging in Amorphous Polymers and Other Materials"; Elsevier: Amsterdam, 1978. 15. Bosshard, Ch.; Küpfer, M.; Günter, P.; Pasquier, C.; Zahir, S.; Seifert, M. Appl. Phys. Lett. 1990, 56, 1204-1206, and references therein.



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16. Popovitz-Biro, R.; Hill, K.; Landau, E. M.; Lahav, M.; Leiserowitz, L.; Sagio, J . ; Hsiung, H.; Meredith, G. R.; Vanherzeele, H. J. Am. Chem. Soc. 1988, 110, 2672-2674. 17. Cross, G. H.; Peterson, I. R.; Girling, I. R.; Cade, N. A.; Goodwin, M. J . ; Carr, N.; Sethi, R. S.; Marsden, R.; Gray, G. W.; Lacey, D.; McRoberts, A. M.; Scrowston, R. M.; Toyne, K. J. Thin Solid Films 1988, 156, 39-52. 18. Allen, S.; McLean, T. D.; Gordon, P. F.; Bothwell, B. D.; Robin, P.; Ledoux, I. SPIE 1988, 971, 206-215. 19. Schildkraut, J. S.; Penner, T. L.; Willand, C. S.; Ulman, A. Optics Lett. 1988, 13, 134-136. 20. Lupo, D.; Prass, W.; Schunemann, U.; Laschewsky, A.; Ringsdorf, H.; Ledoux, I. J. Opt. Soc. Am. B 1988, 5, 300-308. 21. Ledoux, I.; Josse, D.; Vidakovic, P.; Zyss, J . ; Hann, R. A.; Gordon, P. F.; Bothwell, B. D.; Gupta, S. K.; Allen, S.; Robin, P.; Chastaing, E.; Dubois, J. C. Europhysics Lett. 1987, 3, 803-809. 22. Hayden, L. M.; Kowel, S. T.; Srinivasan, M. P. Optics Commun. 1987, 61, 351-356. 23. Ye, C.; Minami, N.; Marks, T. J . ; Yang, J . ; Wong, G. K. in ref. 1, pp. 173-183. 24. Li, D.; Minami, N.; Ratner, M. A.; Ye, C.; Marks, T. J . ; Yang, J.; Wong, G. K. Synthetic Metals 1989, 28, D585-D593. 25. Ye, C.; Marks, T. J . ; Yang, Y.; Wong, G. K. Macromolecules 1987, 20, 2322-2324. 26. Ye, C.; Minami, N.; Marks, T. J . ; Yang, J . ; Wong, G. K. in ref. 3, pp. 263-269. 27. Ye, C.; Minami, N.; Marks, T. J . ; Yang, J . ; Wong, G. K. Macromolecules 1988, 21, 2901-2904. 28. Singer, K. D.; Kuzyk, M. G.; Holland, W. R.; Sohn, J. E.; Lalama, S. J . ; Commizzoli, R. B.; Katz, H. E.; Schilling, M. L. Appl. Phys. Lett. 1988, 53, 1800-1802. 29. Eich, M.; Sen, A.; Looser, H.; Yoon, D. Y.; Bjorklund, G. C.; Twieg, R.; Swalen, J. D. in ref. 2, pp. 128-135. 30. Eich, M.; Sen, A.; Looser, H.; Bjorklund, G. C.; Swalen, J. D.; Twieg, R.; Yoon, D. Y. J. Appl. Phys. 1989, 66, 25592567. 31. Dai, D.-R.; Marks, T. J . ; Yang, J . ; Lundquist, P. M.; Wong, G. K. Macromolecules 1990, 23, 1894-1896. 32. Park, J . ; Marks, T. J . ; Yang, J . ; Wong, G. K. Chemistry of Materials 1990, 2, 229-231. 33. Comizzoli, R. B. J. Electrochem. Soc. 1987, 134, 424-429. 34. Aycock, D.; Abolins, V.; White, D. M. in "Encyclopedia of Polymer Science and Technology," Wiley: New York, 1988, Vol. 13, pp. 1-30, and references therein. 35. White, D. M. J. Org. Chem. 1969, 34, 297-303. 36. Cabasso, I.; Jagur-Grodzinski, J . ; Vofsi, D. J. Appl. Polym. Sci. 1974, 18, 196. 37. Zyss, J . ; Nicoud, J. F.; Coquillay, M. J. Chem. Phys. 1984, 81, 4160-4167. 38. Barzoukas, M.; Josse, D.; Fremaux, P.; Zyss, J . ; Nicoud, J. F.; Morely, J. J. Opt. Soc. Am. B 1987, 4, 977-986.



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39. Jerphagnon, J; Kurtz, S. K. J. Appl. Phys. 1970, 41, 16671681. 40. Singer, K. D.; Sohn, J. E.; King, L. A.; Gordon, H. M.; Katz, H. E.; Dirk, C. W. J. Opt. Soc. Am. B 1989, 6, 1339-1351. 41. Tien, P. K. Appl. Opt. 1971, 10, 2395-2413. 42. McAdams, L. V.; Gannon, J. A., in "Encyclopedia of Polymer Science and Engineering," Wiley: New York, 1986, Vol. 6, pp. 322-382, and references therein. 43. Mertzel, E.; Koenig, J. L. Adv. Polym. Sci. 1985, 75, 74-112. 44. Hubbard, M. A.; Minami, N.; Ye, C.; Marks, T. J . ; Yang, J . ; Wong, G. K. in ref 1b, pp. 136-143. 45. Hubbard, M. A.; Marks, T. J . ; Yang, J . ; Wong, G. K. Chemistry of Materials 1989, 1, 167-169. 46. Kloosterboer, J. G. Advan. Polym. Sci. 1988, 84, 3-61. 47. Oleinik, E. G. Advan. Polym. Sci. 1986, 80, 49-99. 48. Dusek, K. Advan. Polym. Sci. 1986, 78, 1-59. 49. Rozenberg, B. A. Advan. Polym. Sci. 1986, 75, 73-114. 50. Morgan, R. J. Advan. Polym. Sci. 1985, 72, 1-43. 51. Marks, T. J., 1989 International Chemical Congress of Pacific Basin Societies, Honolulu, HI, Dec. 1989. 52. Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235-241. 53. Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67-76. 54. Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153-162. 55. Sagiv, J. Israel J. Chem. 1979, 18, 339-345. 56. From perturbation theory and the PPP SCF MECI formalism (5759) we estimate β = 382 x 10 cm esu at a frequency of 1.17 eV. 57. Li, D.; Marks, T. J . ; Ratner, M. A. Chem. Phys. Lett. 1986, 131, 370-375. 58. Li, D.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 1707-1715. 59. Li, D.; Marks, T. J . ; Ratner, M. A., to be published. 60. Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. 61. Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. 62. Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J . ; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 321-335. 63. Shen, Y. R. "The Principles of Nonlinear Optics," Wiley: New York, 1984, Chapt. 2. 64. Williams, D. J . ; Penner, T. L.; Schildkraut, J. J . ; Tillman, N.; Ulman, A. in ref. 1a, pp. 195-218. 65. Fang, S. B.; Zhang, C. H.; Lin, B. H.; Wong, G. K.; Ketterson, J. B.; Dutta, P., Chemistry of Materials, in press. 66. Bloembergen, N.; Pershan, P. S. Phys. Rev. 1962, 128, 606622. -30

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