Synthesis and Nonlinear Optical Properties of Side Chain Liquid Crystalline Polymers Containing Azobenzene Push-Pull Chromophores

摘要

Azobenzene monomeric precursors bearing piperazine as donor moiety with different withdrawing groups and derived side chain polymethacrylates have been prepared and characterized. Monomers having terminal cyano or nitro groups, and the corresponding polymers, exhibited smectic A phases. Linear and nonlinear optical properties of every monomer and thin films of the cyano polymer (pol-PZ-CN) have been also studied. UV-vis spectroscopy revealed out-of-plane orientation in the as prepared films, as confirmed by waveguide refractive index measurements. Moreover, absorption spectra indicated the presence of azo aggregates in these films. The initial molecular arrangement has been modified by applying thermal annealing within the mesophase range and UV-blue irradiation. Although thermal annealing resulted in a significant amplification of the out-of-plane optical anisotropy due to thermotropic self-organization of side chain azo moieties, irradiation with 440 nm light induced some disruption of aggregates. The nonlinear optical response of Corona poled films has been studied by second harmonic generation measurements, and the influence of the molecular arrangement on the nonlinear d(ij) coefficients has been analyzed. The more efficient poling corresponded to preirradiated films. In any case, a noticeable degree of polar order (70% of the initial d(33) value) remained for several months after the poling in films kept at FIT