Solvent and substituent effects on intramolecular charge transfer of selected derivatives of 4-trifluoromethyl-7-aminocoumarin

Abstract

A comparative study of the fluorescence of selected 4-trifluoromethyl-7-aminocoumarin derivatives has been made in a variety of protic and aprotic solvents. It was found that the fluorescence-decay kinetics were dependent on both the substitution pattern of the aminocoumarins and properties of the solvents used. These dependences are, in general, reminiscent of the corresponding 4-methyl homologues reported previously, but an intriguing difference is found for compounds possessing an unrestricted amine group in polar aprotic solvents. Furthermore, there is a ‘dielectric threshold’ for the solvent-dependence of fluorescence lifetime of non-rigid 4-trifluoromethyl-7-dialkylaminocoumarins, above which their solvent response is markedly reduced. These findings are discussed in connection with the factors affecting formation of a twisted intramolecular charge-transfer (TICT) state and lead to an hypothesis that hydrogen bonding between solvent and excited solute molecule seems not to be a determining factor for TICT state formation in the case of aminocoumarins.