Wavelength- and Isomer-Dependent Photochemistry of Styryl- and (Phenylethynyl)fluorenones

Abstract

Fluorenones substituted with cis- and trans-styryl and phenylethynyl groups at C-1, C-2, and C-4 were synthesized, as were the trans-2- and trans-4-(3,4-dimethoxystyryl) and β-methyl-2-styryl derivatives. The static absorption and fluorescence spectra of these compounds were measured, as were their redox potentials in acetonitrile. Transient spectra in a range of solvents were obtained on a microsecond time scale. The compounds showed marked solvent-dependent behavior. In solvents of low polarity, the principal photochemical process for the styryl derivatives is cis−trans isomerization via triplet intermediates. In polar solvents, singlet processes become more important, including photoionization from upper excited states. There are also significant regiochemical differences. The triplets from the 2-substituted fluorenones have microsecond lifetimes and λmax values in the 400−500-nm range, indicating that they have a trans configuration. By contrast, 1- and 4-substituted fluorenones give rise to triplets ...