Combined experimental and theoretical study of the photochemistry of 4- and 3-hydroxycoumarin

Steady-state and time-resolved optical spectroscopy techniques were employed to study the photophysics and photochemistry of 4- (4HC) and 3-hydroxycoumarin (3HC). We found that the p K_a of 4HC in aqueous solutions is about 7.5 and the p K_a* is only 4.5. We also found that the nonradiative rate of 4-hydroxycoumarin is high and the excited state proton transfer (ESPT) rate to water is rather slow and so an ESPT process is not observed. Quantum mechanical calculations confirm that the ESPT rate should be slow and the intersystem crossing rate S₁ → T is large. An ESPT process was also not observed for 3HC, but it might still take place and not being observed as the fluorescence quantum yield of the anion is two orders of magnitude smaller than that of the protonated form. The nonradiative decay of protonated 3HC is explained with an H-atom abstraction mechanism, leading to the formation of a radical species.