Main chain-containing azo-tetraphenyldiaminobiphenyl photorefractive polymers

The design of polymeric materials with optimized photoconductivity and nonlinear optical properties is of major interest for emerging optical applications using photorefractive properties. This paper reports the synthesis and preliminary optical characterization of a series of novel, fully functionalized, photorefractive polymers. They were synthesized by a post azo coupling reaction of a polyether carbonate polymer based on the charge transport N,N,N',N'-tetraphenyl-diaminobiphenyI group with 4-nitrobenzene-diazonium tetrafluoroborate by phase transfer catalysis. The amount of the inserted 4-nitro-phenyl-azo group was between 0.1 and 1.7 per structural unit. These polymers absorb in the UV-vis range, because of the presence of donor-acceptor substituted azobenzene chromophores. Light induced change of the refractive index was studied by birefringence measurements (lambda(probe) = 632.8 nm) as a function of azo content in thin films with thickness ranging from 200 to 500 nm. Surface relief grating inscription was also demonstrated in the same polymer thin films, with a weak surface relief amplitude and a low diffraction efficiency (<1%). These results indicate a relatively low mobility of the azobenzene chromophores. Photoconductivity was studied by measuring the photocurrent through 40 mum thick samples where a metal-free plthalocyanine layer deposited on electrodes served as charge generator, These samples have exceptionally high photoconductivity compared to other composite materials. In addition, preliminary two-beam coupling experiments demonstrated unambiguously the photorefractive effect under low poling field (2.5 V/mum) at room temperature, but no net gain was obtained because of a high absorption coefficient at 632.8 nm.