Spectral fluorescent properties of new 5-R-2-(quinolin-2-yl)- and 5-R-2-(quinolin-4-yl)-1,3-benzoxazoles

The spectral-fluorescent properties of 5-R-2-(quinolin-2-yl)- and 5-R-2-(quinolin-4-yl)-1,3-benzoxazoles (R = Me and Cl) have been investigated in solvents of different polarity and proton donating ability. The effect of the solvent on spectral characteristics has been estimated. It has been shown that all the studied compounds have low fluorescence quantum yields in non-polar solvent. The experimental data together with the results of ab initio calculations indicate that the main quenching channel of the fluorescent S 1 ?-?* excited state in non-polar solvents is intersystem crossing to close-lying n-?* triplet state. By growth of solvent polarity and solvent hydrogen-bonding ability, long-wavelength shifts in the fluorescence maxima and increase of fluorescence quantum yields are observed for the studied quinolinyl-5-methyl-benzoxazoles, whereas no considerable changes of fluorescence band positions and of quantum yields are observed for the quinolinyl-5-chlorobenzoxazoles in polar and protic solvents. This difference is suggested to be caused by the difference in solvent cage relaxation process, occurring in polar and protic solvents after the absorption, for quinolinyl-5-methyl-benzoxazoles and for quinolinyl-5-chlorobenzoxazoles.