Primary photophysical and photochemical processes in the photolysis of 1,2-dihydroquinolines in different solvents

The fluorescence lifetimes (tau(fl)) of alkyl and alkoxy substituted dihydroquinolines (DHQ) were measured in hexane, isopropanol, methanol, and water. It was shown that tau(fl) was determined by the solvent nature and weakly depended on substituents on the DHQ aromatic ring and heterocycle, with tau(fl) being substantially lower in methanol than in the other solvents. The decrease in tau(fl) in MeOH is caused by an increase in the rate constants of the photochemical reaction and nonradiative transitions in this solvent. The quantum yield of fluorescence in H2O and MeOH, in which the photoinduced addition of the solvent molecules occurs, decreases with a decrease in the excitation wavelength within the limits of long-wavelength absorption band, thus providing, strong evidence that the photochemical reaction proceeds from the unrelaxed excited state.