Role of Aggregates In the Wavelength-Dependent Photochemistry of a Main-Chain Liquid-Crystalline Polyaryl Cinnamate
Abstract
Several aspects of the thermal and photochemical behavior of a main-chain nematic liquid crystalline polyarylcinnamate are discussed. Hypochromic and other effects that are attributed to chromophore aggregation are observed in the UV-VIS spectra of heated polymer films. These effects are more pronounced upon thermal annealing. They can only be eliminated by recasting the film out of solution. Irradiation (313nm) of the polymer leads to cyclobutane dimer and photo-Fries rearrangement products. Aggregated chromophores seem to form cyclobutane products in preference to photo-Fries products. Dimer to photo-Fries product ratios are reported as a function of phase type. Preliminary indications are that long-wavelength (365nm) excitation of aggregates leads to preferential cyclobutane formation. Only weak fluorescence can be observed from polymer films and this is tentatively attributed to monomeric chromophores. There is no evidence of emission from excited aggregates.