Chem. Mater. 1993,5, 1755-1761
1755
Poly[3-(alkoxyphenyl)thiophenes]: Synthesis and Linear and Nonlinear Optical Properties Lucie Robitaille and Mario Leclerc* Dbpartement de Chimie, Universite de Montrgal, Montrbal, Qubbec, H3C 3J7, Canada
Claire L. Callender Communications Research Center, P.O. Box 11490, Station H, Ottawa, Ontario, K2H 8S2, Canada Received July 19, 1993. Revised Manuscript Received September 17, 199P
Poly[3-(alkoxyphenyl)thiophene] derivatives were chemically and electrochemically synthesized. It has not been possible to obtain soluble, highly conjugated poly[3-(methoxyphenyl)thiophenes] while chemically prepared poly[3-( (octyloxy)phenyl)thiophenes] were found to be completely soluble in common organic solvents. Temperature-dependent UV-visible absorption measurements on these polymers have revealed that the chemical structure as well as the nature and the position of the alkoxy chain on the phenyl ring have a strong influence on the linear optical properties. Third harmonic generation measurements a t 1.053 pm were carried out on spin-coated films of poly[ 344'- (octy1oxy)phenyl)thiophene] and poly[3-(2'- (octy1oxy)phenyl)thiophene]. x(3)values of about 10-l2esu were obtained which are similar as those obtained with other polythiophene derivatives. Introduction Nonlinear optical materials have attracted great attention because of their potential applications in ultrafast and highly efficient devices for optical communication, information processing, storage,and computing.l-" During the past few years, it has been demonstrated that organic materials possess many advantages over existing inorganic materials, for example, ease of processing, good chemical, and thermal stability, versatility of synthesis, and fast optical nonlinear responses.6.6 Earlier studies by Hermann et al.7 and theoretical work by Rustagi and Ducuing8 have established the importance of an extended a-system in increasing the third-order nonlinear susceptibilities ( ~ ( 3 ) ) . Conjugated polymers, because of the a-electron delocalization along the polymer backbone, are expected to show large third-order nonlinear properties. Indeed, relatively large optical nonlinearities (10-1"10-9 esu) have been observed by several research groups in neutral a-systems such as polyacetylenes,%l2
* To whom correspondence should be addressed.
Abstract published in Aduance ACS Abstracts, October 15,1993. (1)Prasad, P. N.; Ulrich, D. R., Eds. Nonlinear Optical and Electroactiue Polymers; Plenum Press: New York, 1988. (2)Heeger, A. J., Orenstein, J., Ulrich, D. R., Eds. Nonlinear Optical Properties of Polymers; Materials Research Society Symposium Proceedings; MRS: Pittsburgh, 1987;Vol. 109. (3) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (4)Marder, S.R.; Sohn,J. E.; Stucky, G. D. Materials for Nonlinear Optics; ACS Symposium Series 455;American Chemical Society: Washington DC, 1991. (5)Robello, D. R.; Dao, P. T.; Phelan, J.; Revelli, J.; Schildkraut, J. S.;Scozzafava, M.; Ulman, A.; Willand, C. S. Chem. Mater. 1992,4,425. (6) Prasad, P. N.; Williams, D. J. Nonlinear Optical Materials; Wiley: New York, 1991. (7) Hermann, J. P.;Ricard, D.; Ducuing, J. Appl. Phys. Lett. 1973, 23, 178. (8) Rustagi, K. C.; Ducuing, J. Opt. Commun. 1974,10,258. (9) Le Moigne, J.; Hilberer, A.; Kajzar, F. Makromol. Chem. 1991, 192,515. (10) Wong,.K. S.;Vafdeny, Z . V. Synth. Met. 1992,49-50, 13. (11) Le Moigne, J.; Hilberer, A.; Strazielle, C. Macromolecules 1992, 25,6705.
0897-475619312805-1755$04.00/0
polythiophenes,l3-le polyanilines,20p21poly@-phenylene~ i n y l e n e s and ) ~ ~other ~ ~ ~aromatic Recently, Prasad and Reinhardt2' have measured the third-order nonlinear optical susceptibilities of a large number of aromatic model compounds. They have found that (1)a thiophene ring in a conjugated structure is more efficient than any other heteroaromatic ring in increasing optical nonlinearity, (2) the presence of an aromatic group in the side chain produces a two-dimensionala-conjugation that also increases the optical nonlinearity, and (3) the presence of an alkoxy pendant group on an aromatic ring enhances x ( ~values. ) To test these observations, Callender et al.13 have performed third harmonic generation measurements on a series of alkyl- and alkoxy-substituted polythiophenes. They have observed only a small effect of these substituents ) on the ~ ( 3 values. (12) Neher, D.; Kaltbeitzel, A.; Wolf, A.; Bubeck, C.; Wegner, G. J. Phys. D: Appl. Phys. 1991,24,1193. (13)Callender, C. L.; Karnas, S.J.; Albert, J.; Roux, C.; Leclerc, M. Opt. Mater. 1992,1, 125. (14)Torruellas. W. E.:Neher. D.: Zanoni. R.: Steneman. G. I.:. Kaizar. F.; Leclerc, M. Chem. Phys. L e k . 1990,175,11. (15) Meth, J. S.;Vanherzeele, H.; Chen, W.-C.; Jenekhe, S.A. Synth. Met. 1992,49-50,59. (16) Lee, T.-M.; Mittler-Neher, S.; Neher, D.; Stegeman, G. I.; Roux, C.; Leclerc, M.; Martin, J.; Najdi, S. I. Opt. Mater. 1992,1, 65. (17)Houlding, V. H.; Nahata, A,; Yardley, J. T.; Elsenbaumer, R. L. Chem. Mater. 1990,2,169. (18) Singh, B. P.; Samoc, M.; Nalwa, H. S.; Prasad, P. N. J. Chem. Phys. 1990,92,2756. (19) Neher, D.;Wolf, A,; Leclerc, M.; Kaltbeitzel, A.; Bubeck, C.; Wegner, G. Synth. Met. 1990,37,249. (20) Osaheni, J. A.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S.; Sun, Y.; MacDarmid, A. G. J. Phys. Chem. 1992,96,2830. (21) Chen, W.-C.; Jenekhe, S. A. Macromolecules 1992,265919. (22) Swiatkiewick, J.; Prasad, P. N.; Karask, F. E.; Druy, M. A,; Glatkowski, P. Appl. Phys. Lett. 1990,56, 892. (23)Spangler, C. W.; Lui, P.-K.; Hall, T. J.; Polis, D. W.; Sapochak, L. S.;Dalton, L. R. Polymer 1992,33,3937. (24) Osaheni, J. A.;Jenekhe, S. A. Chem. Mater. 1992,4 , 1282. (25)Yu, L.; Dalton, L. R. J . Am. Chem. SOC.1989,111, 8699. (26)Jenekhe, S.A.;Yang, C.-J.; Vanherzeele, H.; Meth, J. S. Chem. Mater. 1991,3,985. (27)Prasad, P. N.; Reinhardt, B. A. Chem. Mater. 1990,2,660. 1
0 1993 American Chemical Society
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1756 Chem. Mater., Vol. 5, No. 12,1993 Following the concepts developed by Prasad and Reinhardt,27 another possibility to enhance the nonlinear optical effects could be the introduction of an aromatic side group on a polythiophene backbone. In this regard, Sat0 et al.2shave recently reported the synthesis of poly(3-phenylthiophene), but this polymer has been found to be insoluble and infusible. However, it is well-knownthat the addition of long flexible side chains on the conjugated backbone enhances the solubility of the p01ymer.W~For ~ ~ reported the synthesis and example, Pei et a l . ” ~have characterization of a processable poly[3-(4’-octylphenyl)thiophene]. Moreover, the linear optical properties of conjugated polymers can also exhibit very interesting features for applications in some optical devices. For example, some polythiophene derivatives undergo some thermally induced conformational transitions which give rise to thermochromic p r o p e r t i e ~ . ~ W Indeed, there is a strong coupling between the electronic structure and the backbone conformation of conjugated polymers,4l which explains why conformational changes are accompanied by spectral changes (chromism). Therefore, to develop novel well-defined processable polythiophenes with interesting linear and nonlinear optical properties, chemical and/or electrochemical synthesis of ortho, meta, and para derivatives of 3-(alkoxypheny1)thiophenes was carried out. The linear optical properties of poly [3-(0-,m-pmethoxypheny1)thiophenesl and poly 13-(0-,m-,p-(octy1oxy)phenyl)thiophenes] were analyzed as well as the nonlinear optical properties of spincoated f i i s of poly[3-(4’-(octyloxy)phenyl)thiopheneland poly[3-(2’-(octyloxy)phenyl)thiophenel.
Results and Discussion Polymer Characterization. The oxidation potential of the monomers are listed in Tables I and 11. Since the substituents are electron-donating groups, values of oxidation potentials lower than the value of the unsubstituted thiophene (E, = 2.06 V vs SCE42) were effectively expected. However, it is important to note that the strength of the electron-donating effect of the substituent is highly dependent upon the position of the alkoxy group on the phenyl ring.
Robitaille et al. Table I. Summary of Properties of Chemically and Electrochemically Prepared Poly[3-phenylthiophene]and Poly[ 3-(methoxypheny1)t hiophenes] A,
(nm)
in the Eo,: EOxd solid CHCh (V vs SCE) (V vs SCE) state solution
polymer poly [3-(4’-methoxyphenyl) thiophene]’ poly[3-(3’-methoxyphenyl) thiophene]’ poly[3-(2’-methoxyphenyl) thiophene10 polyC3-phenylthiopheneIb poly 13-(4’-methoxypheny1)thiophenel poly[3-(3’-methoxyphenyl) thiophene] b poly[3-(2’-methoxyphenyl)thiophenelb
1.30
1.06 2.06 1.09 1.95 0.93 1.93 1.38 0.95 1.83 1.23 1.98 0.97 1.95
1.70 1.80 1.66 1.30 1.70 1.80
412
404
368
358
424
418
520 520
