Fluorescence Behavior of Low Molar Mass and Polymer Liquid Crystals in Ordered Solid Films

Abstract

Fluorescent low-molar-mass and polymeric materials capable of freezing nematic and chiral-nematic liquid-crystalline mesomorphism in a vitreous state were synthesized for the investigation of light absorption, steady-state fluorescence, and decay dynamics via single photon counting, all at room temperature. Monomer (i.e., isolated chromophore) emission was established as the sole decay pathway for singly dispersed chromophores in dilute solution. For chromophores constrained on a cyclohexane ring or a polymethacrylate chain, monomer emission was identified as the predominant decay pathway in dilute solutions. Furthermore, deviation from a single-exponential decay is accompanied by some loss of quantum yield to intramolecular processes. Emission from vitrified films exhibited a red shift of no more than 30 nm and a peak broadening by less than 10 nm compared to dilute solutions of the same compounds. The limited extent of bathochromic shift was considered to have arisen from a microenvironmental effect. The decay dynamics was found to be similar to that in dilute solutions, with a greater loss in quantum yield to both intra- and intermolecular processes existing in solid films. At modest values of order parameter, 0.4−0.6, vitrified nematic and chiral-nematic films produced a fluorescence anisotropy of 0.42 and a dissymmetry factor of 0.25, respectively. The observed degrees of polarization were found to agree fairly well with the theories governing linearly and circularly polarized fluorescence from uniaxially and helically arranged chromophores.