Thermotropic chiral-nematic poly(p-phenylene)s as a paradigm of helically stacked π-conjugated systems

As an emerging class of multifunctional optical materials, chiral-nematic poly(p-phenylene)s with cyanobiphenyl and (S)-1-phenylethyl pendants were synthesized and characterized. Nematic homopolymer was also synthesized for preparation into a uniaxially aligned film. Linearly polarized photoluminescence (LPPL) and FTIR dichroism revealed that both the conjugated backbone and the nematic pendant are oriented along the director with an orientational order parameter, S = 0.62 and 0.73, respectively. Films prepared with chiral-nematic copolymers were found to selectively reflect left-handed circularly polarized light in the ultraviolet, visible, and infrared region at a decreasing chiral content. The helically stacked poly(p-phenylene)s were further characterized in terms of circularly polarized photoluminescence (CPPL). On the basis of a recent theory, S = 0.67 was found for the orientational order of conjugated backbones within quasinematic layers comprising the chiral-nematic film. Within experimental uncertainty, the S values determined with the LPPL and CPPL techniques are in good agreement, thereby validating the proposed supramolecular structure on the basis of which theories governing light absorption, emission, and propagation in structured media were constructed.