Remarkable fluorescence enhancement of benzo[g]chromen-2-ones induced by hydrogen-bonding interactions with protic solvents

A coumarin analogue, 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (MMBC), is almost non-fluorescent in non-polar media, whereas it exhibits dramatically enhanced fluorescence in polar protic solvents. This study investigates the mechanistic features of the significant solvent effects on the fluorescence properties of MMBC and a related compound, 4-methyl-2H-benzo[g]chromen-2-one (MBC), by time-resolved fluorescence and photoacoustic measurements and by theoretical calculations. Time-resolved photoacoustic measurements reveal that the extremely fast non-radiative processes of MBC and MMBC in non-polar solvents can be attributed predominantly to internal conversion. The internal conversion rates of MBC and MMBC are remarkably reduced in a rigid matrix of 3-methylpentane at 77 K, suggesting that internal conversion is a thermally activated process. The photophysical properties of MBC and MMBC examined in selected solvents with different polarities and hydrogen-bond donating abilities show that the internal conversion rate is greatly reduced by hydrogen-bonding interactions with protic solvents. Furthermore, remarkable fluorescence enhancement is observed by adding a small amount of trifluoroethanol to n-hexane solutions of MBC and MMBC, indicating that internal conversion is suppressed by formation of hydrogen-bonded complexes with protic solvents. In light of theoretical considerations based on time-dependent density functional theory (TD-DFT) and INDO/S-CI calculations, the occurrence of fast internal conversion in MBC and MMBC can be explained in terms of the proximity effect, i.e., pseudo Jahn–Teller coupling between energetically close S₁ and S₂ states.