Photoinduced substitution reaction in halothiophenes. Quantum-mechanical and photochemical studies on nitro- and cyano-derivatives

Abstract

2-Iodo-5-nitrothiophene, 2-iodo-5-cyanothiophene, 2-bromo-5-cyanothiophene and 4-iodo-nitrobenzene were studied by steady state and pulsed techniques and by semi-empirical quantum-mechanical calculations. The quantum yields for the direct and sensitized photoarylation of these haloaromatics following the photocleavage of the carbon–halogen bond were measured in benzene. The transient species (triplet states and free radicals) originated by direct and sensitized laser excitation were investigated by nanosecond laser flash photolysis in benzene and methylcyclohexane. The transients were characterized in terms of absorption spectra and decay lifetimes. The triplet states were also characterized by phosphorescence measurements in rigid matrices at low temperature. The quenching rate constants of the triplet states and the quantum yields for singlet oxygen production were determined in methylcyclohexane. The results obtained with sensitizers of different triplet energy indicate that an upper triplet state is involved in the photocleavage of the carbon–halogen bond.