Influence of the solvent and excited state prototropic equilibria on photochemical intramolecular acylatio of trans-3-(2-hydroxybenzylidene)-4,5-dihydrofuran-2(3H)-one

The solvent dependence of the kinetics of the geometrical isomerization of trans-3-(2-hydroxybenzylidene)-4,5-dihydrofuran-2(3H)-one and the subsequent intramolecular acylation of the cis substrate was studied. Even if the quantum yields Φ1 and Φ2 of the two processes increase on increasing the polarity of the methanol solutions, addition of water is found to quench dramatically Φ1 and Φ2. This behaviour is interpreted in terms of the proton exchange equilibria of the excited singlet state. A detailed analysis of the dependence of the fluorescence on the pH was carried out and a mechanistic model which is able to describe satisfactorily the experimental results is proposed.