Nonlinear conjugated polymers

J. L. Bredas and R. Silby, Conjugated Polymers The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials, Kluwer Academic, Boston, 1991. 

Optical Nonlinearities and Photoinduced Solitons in Conjugated Polymer Crystals... 

The linear and nonlinear optical properties of one-dimensional conjugated polymers contain a wealth of information closely related to the structure and dynamics of the ir-electron distribution and to their interaction with the lattice distorsions. The existing values of the nonlinear susceptibilities indicate that these materials are strong candidates for nonlinear optical devices in different applications. However their time response may be limited by the diffusion time of intrinsic conjugation defects and the electron-phonon coupling. Since these defects arise from competition of resonant chemical structures the possible remedy is to control this competition without affecting the delocalization. The understanding of the polymerisation process is consequently essential. 

Agrawal, G. P. Cojan, C. Flytzanis, C. "Nonlinear Optical Properties of One-Dimensional Semiconductors and Conjugated Polymers," Phys. Rev. 1978, B17, 776. 

Up to now, many conjugated polymers have been found to possess large and very rapid third-order nonlinear optical response, which originates from the one-dimensionally delocalized ji-conjugation system along the polymer chain. Their application to the all optical signal processing devices has been expected. 

Lee and co-workers reported an interesting example of a conjugated polymer obtained by polymerizing 5-phenyl-2-(propynylamino)-4(57/)-oxazolone in the presence of palladium or platinum chlorides. The authors predict this unique material may have applications for polymer electrolytes, semiconductors, and nonlinear optical (NLO) materials. 

Polysilanes are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

Another chemical approach to improve our microscopic understanding of optical nonlinearities is a study of nonlinear optical behavior of sequentially built and systematically derivatized structures. Most past work for third-order nonlinearities have focused on conjugated polymers. This ad hoc approach is not helpful in identifying functionalities necessary to enhance optical nonlinearities. A systematic study and correlation of Y values of systematically varied structure is an important approach for material development. 

Conjugated polymers, in the undoped state, present remarkable nonlinear optical properties23. The material which has been most thoroughly investigated ever since 197624 is polydiacetylene para-toluene sulfonate (PTS). The polydiacetylene backbone consists of a... 

S. Abe, Excitonic Effects in the Linear and Nonlinear Optical Properties of Conjugated Polymers, in N. S. Sariciftci (Ed.), Primary Photoexcitations in Conjugated Polymers, World Scientific, Singapore, 1997, p. 115. 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

In the mid 1980s, conjugated polymers were a hot topic in nonlinear optics. Many people promoted third-order nonlinear optics based on conjugated polymers and predicted their incorporation in future all-optical applications. However, the disillusionment came during the beginning of nineties when it was realized that it will not be so easy to have all required parameters under control. The initial hopes did not fulfill on the short time scale and quite some problems were left to be tackled. 

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