3-hexyl tetra-substituted sesquithienylene-phenylene polyazomethines with high molecular weight. Mechanistic considerations

The synthesis and characterization of copolymers constituted by the periodic alternation of six thienylenic and two phenylenic moieties linked by azomethine groups are reported. The azeotropic polycondensation used for this preparation yields higher molecular weight but sterically more disordered copolymers. By H-1 NMR experiments syn and anti isomers at imine linkages were evidenced. Mechanistic considerations on the different synthetic routes followed in polyazomethine preparation imply the lack of a polar control on the formation of the syn isomer. The capability of the substituents to rotate around the C-N bond of the intermediate is related to the overall bulkiness of the molecules. Due to their molecular weight, optical quality films were obtained either free-standing or spin-coated from tetrachloroethane solutions. The solid-state aggregation was investigated by XRD techniques, indicating for properly annealed samples some liquid crystal character, in agreement with microscopy observations; in fact a smectic or nematic arrangement was detected as a function of syn content. A proper control of anti/syn content in the copolymer allows a noticeable tuning in the photoluminescence spectra.