Synthesis and optical properties of thermotropic polythiophene and poly(p-phenylene) derivatives

Thermotropic nematic and chiral-nematic conjugated polymers consisting of polythiophene and poly(p-phenylene) backbones carrying cyanobiphenyl and (-)-cholesterol as pendant groups were synthesized and characterized as a new class of optical polymers; Spontaneous assembly of the conjugated backbone was promoted by liquid crystalline mesomorphism on the part of the pendants, thereby allowing uniaxially and helically aligned glassy films to be prepared for polarized photoluminescence (PL) studies. In the thiophene series, the conjugated backbone was found to yield an absorption peak red-shifted from those of the monomers and pendant groups. This permitted selective photoexcitation of the polythiophene backbone to be accomplished. In the p-phenylene series, the conjugated backbone did not give rise to a unique absorption peak. However, with excitation at the long wavelength edge of the absorption peak, PL of the poly(p-phenylene) backbone was found to be 1 order of magnitude stronger than that of the pendant cyanobiphenyl group. These ordered solid films produced significant degrees of linearly and circularly polarized FL. Existing theories were;Found to be capable of representing the experimental results with independently measured absorption coefficient, average refractive index, and optical birefringence.