Synthesis and characterization of main-chain NLO oligomers and polymer that contain 4-dialkylamino- 4?-(alkylsulfonyl)azobenzene chromophores

An ω-amino carboxylic acid monomer that contained a nonlinear optical (NLO) chromophore was prepared by a convergent synthesis. Strategies for selective protection/deprotection of the amino and carboxylic acid functionalities were developed. The protected monomer, 4-[N-(4-benzyloxycarbonyl)butyl-N-methylamino]-4′-[2″,5″-bis(decyloxy)-4″-(phthalimidomethyl)benzylsulfonyl]azobenzene, could be deprotected selectively or sequentially to give HOOC-monomer-N-phthaloyl, benzyl-OOC-monomer-NH2, or HOOC-monomer-NH2. Sequential synthesis was performed to yield main-chain NLO dimers and tetramers. This was accomplished by selective deprotection and dicyclohexylcarbodiimide coupling. The HOOC-monomer-NH2 was polymerized by treatment with diphenylphosphoryl azide to give a main-chain NLO polyamide. The monomer, dimer, tetramer, and polymer NLO materials were characterized by 1H, 13C, IR, and UV–visible spectroscopy as well as by gel permeation chromatography, differential scanning calorimetry, and elemental analysis. The NLO properties of these materials were measured. Thin films of the oligomers and polymer were prepared by spin casting on indium-tin oxide coated glass. The second-order NLO properties of the oligomers and polymer thin films were studied by in situ corona poling/second-harmonic generation and attenuated total reflection methods. The optimal poling temperatures were significantly lower than the melting temperatures or glass-transition temperatures of the oligomers and polymer. The poling efficiency increased in the following order: monomer, oligomers, and polymer. An electro-optic coefficient of 4 pm/V at 1.06 μm was obtained for the polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 546–559, 2000