Excited state proton-transfer reactions of coumarin 4 in protic solvents

Excited state 7-hydroxy-4-methyl coumarin (4CUOH) has dual photochemical properties. In water, its neutral form (ROH*) is a strong photoacid and transfers a proton to the solvent with pK* ∼ 0. It is also a strong photobase capable of accepting a proton from some protic solvents or reacting with excess protons in solution. In this study, we measured the rate and dynamics of the reaction of excited state 4CUOH with excess protons in neat monols and water-glycerol solutions. The reaction shows diffusion-controlled reactivity and fits the Smoluchowski model. The reaction can be formulated by the kinetic scheme ROH* + H⁺ → ROH₂⁺*, where ROH₂⁺* and ROH* are the protonated and neutral forms of the photobase, respectively. Nonexponential decay of the ROH* fluorescence was observed. We analyzed the experimental data within the framework of the diffusive model. The model predicts that the survival probability depends strongly on both the intrinsic rate constant and the mutual diffusion constant.