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

As an emerging class of multifunctional optical materials, chiral-nematic p oly(p-phenylene)s with cyanobiphenyl and (S)-1-phenylethyl pendants were sy nthesized and characterized. Nematic homopolymer was also synthesized for p reparation into a uniaxially aligned film. Linearly polarized photoluminesc ence (LPPL) and FTIR dichroism revealed that both the conjugated backbone a nd the nematic pendant are oriented along the director with an orientationa l order parameter, S = 0.62 and 0.73, respectively. Films prepared with chi ral-nematic copolymers were found to selectively reflect left-handed circul arly polarized light in the ultraviolet, visible, and infrared region at a decreasing chiral content. The helically stacked poly(p-phenylene)s were fu rther characterized in terms of circularly polarized photoluminescence (CPP L). On the basis of a recent theory, S = 0.67 was found for the orientation al order of conjugated backbones within quasinematic layers comprising the chiral-nematic film. Within experimental uncertainty, the S values determin ed with the LPPL and CPPL techniques are in good agreement, thereby validat ing the proposed supramolecular structure on the basis of which theories go verning light absorption, emission, and propagation in structured media wer e constructed.