Theoretical and spectroscopic studies of the photochemistry of 3-(4-dimethylaminophenyl)-7-methoxy-cyclohepta-1,3,5-triene

In this paper the photophysics and photochemistry of the title compound has been examined by laser, flash lamp and steady state methods. In protic solvents, photoheterolysis is able to compete with a rapid 1,7-hydrogen shift reaction to give the substituted tropylium methoxide, with the lifetime of the tropylium ion strongly depending on the nature of the alcohol. Hydrogen bonds between the solvent and the methoxy group of the cycloheptatriene derivative favor bond breakage and the solvation of the methoxide ion by protic solvents increases the lifetime of the free ions. Ab initio calculations suggest that the photoheterolysis in the gas phase takes places via a consecutive photohomolysis under formation of a radical pair, which then is undergoing an intra-pair electron transfer to form the tropylium and the methoxide ions. 7-hydrogen shift reaction leads exclusively to the production of 5-(4-dimethylaminophenyl)-1-methoxy-cyclohepta-1,3,5-triene. The fluorescence of the title compound exhibits a very large Stokes shift indicating considerable geometrical changes in the relaxed excited state and according to semi-empirical and ab initio calculations, the seven-membered ring is planar in the fluorescing state.