A photochemical and photophysical investigation of polymers containing the trans-4,4'-stilbene bis-(carboxylate) chromophore

Alline Peeler Somlai

Abstract

The photochemistry and photophysics of main chain liquid crystalline polymers containing the trans -4,4' -stilbene bis-(carboxylate) chromophore have been investigated. A homopolyester donated by Eastman Kodak (Jackson and Morris, 1985) exhibits a narrow mesophase range at very high temperatures making photochemical and photophysical studies in various phases difficult. A class of new polyesters with low transition temperatures was designed. The copolyesters synthesized by solution condensation under dry conditions have room temperature mesophases and below room temperature glass transition temperatures making photochemical and photophysical studies much easier in the various phases. All of the polymers show ground state aggregation by UV-Vis spectroscopy when cast as films and give red shifted fluorescence emission spectra. The latter are somewhat more dependent on film thickness. The homopolymer in 1:1 chloroform: cyclohexane shows both the isolated chromophore and 'excimer-like' fluorescence emission. Fluorescence studies of films at variable temperature show a decrease in the 'excimer-like' emission with an increase in temperature. One copolyester gives evidence of enhanced isolated chromophore emission at higher temperature. Variable temperature UV-Vis studies of the copolyester films show that the mesogens/chromophores become increasingly less aggregated with increases in temperature. The isotropic state of the polymer film has a spectrum resembling that of an isolated chromophore with minor evidence of residual aggregates. The photochemistry of all polymers is similar. Irradiation at 313 nm in solution produces the 2+2 cyclobutane adduct as well as a small percentage of cis -stilbene. Subsequent irradiation at 254 nm causes cyclobutane cleavage producing both trans - and cis -stilbene in the polymers. Irradiation at 313 nm or 366 nm of polymer films also produces the 2+2 cycloadduct but with little evidence of production of cis -stilbene. Subsequent cyclobutane adduct cleavage occurs at 254 nm either below or above the T g in the polymers to regenerate stilbene. The homopolymer shows enhanced photochemical reactivity above the Tg . A copolymer shows enhanced photochemical reactivity at 50°C, a higher temperature in the mesophase. The highest quantum yield of reaction, 0.21, occurred in a copolymer film at 50°C.