FLUORESCENCE BEHAVIOR OF LOW MOLAR-MASS AND POLYMER LIQUID-CRYSTALS IN ORDERED SOLID FILMS

Fluorescent low-molar-mass and polymeric materials capable of freezing nematic and chiral-nematic liquid-crystalline mesomorphism in a vitre ous state were synthesized for the investigation of light absorption, steady-state fluorescence, and decay dynamics via single photon counti ng, all at room temperature. Monomer(i.e., isolated chromophore) emiss ion was established as the sole decay pathway for singly dispersed chr omophores in dilute solution. For chromophores constrained on a cycloh exane ring or a polymethacrylate chain, monomer emission was identifie d as the predominant decay pathway in dilute solutions. Furthermore, d eviation from a single-exponential decay is accompanied by some loss o f quantum yield to intramolecular processes. Emission from vitrified f ilms 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 aris en from a microenvironmental effect. The decay dynamics was found to b e similar to that in dilute solutions, with a greater loss in quantum yield to both intra- and intermolecular processes existing in solid fi lms. At modest values of order parameter, 0.4-0.6, vitrified nematic a nd chiral nematic films produced a fluorescence anisotropy of 0.42 and a dissymmetry factor of 0.25, respectively. The observed degrees of p olarization were found to agree fairly well with the theories governin g linearly and circularly polarized fluorescence from uniaxially and h elically arranged chromophores.