Primary Photophysical and Photochemical Processes in the Photolysis of 1,2-Dihydroquinolines in Different Solvents

The fluorescence lifetimes (τ_fl) of alkyl and alkoxy substituted dihydroquinolines (DHQ) were measured in hexane, isopropanol, methanol, and water. It was shown that τ_fl was determined by the solvent nature and weakly depended on substituents on the DHQ aromatic ring and heterocycle, with τ_fl being substantially lower in methanol than in the other solvents. The decrease in τ_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.