Fluorescence of 7-Substituted Coumarins

Abstract

Substituent effects on the absorption and fluorescence spectra of coumarin derivatives have been investigated in various solvents. No absorption band attributed to nπ* transition is observed. However, its existence is suggested from the fact that coumarin is non-fluorescent. As the electron donating tendency of the substituent at 7-position increases, the wave number of the first absorption band decreases and the amount of solvent shifts increases. These observations indicate that the first excited singlet state or the fluorescent state has a character of the intramolecular charge transfer (CT) state arising from an electron transfer from the substituent to the coumarin skeleton. Coumarin itself is non-fluorescent. However, coumarin derivatives having an electron donating group at 7-position are fluorescent. The relative fluorescence yield becomes higher with the increase in the electron donating tendency of the substituent. Since nπ* state is not so much affected by the substituent, the above results lead to the conclusion that an increase in the energy gap between nπ* and ππ* state enhances the fluorescence process. Effects of the solvent and temperature on the relative fluorescence yield are also discussed.