Kinetics of thermal decoloration of a photomerocyanine in mixtures of protic and nonpolar solvents

The kinetics of thermal conversion of a photogenerated merocyanine into its more stable spiropyrane form was studied in mixtures of ethanol with nonpolar solvents, toluene, n-hexane or CO2. For solutions of ethanol concentration higher than ca. 1.5 M two decay rates are observable. Below that concentration, the relative amplitude of the slow mode becomes negligible, compared to the fast, and the observed kinetics is monoexponential. As reported before in toluene–acetonitrile mixtures, the rate constant for the thermal decay in toluene–ethanol passes through a shallow maximum at low-ethanol concentrations. The activation energy and entropy of both processes were determined; the data corresponding to the fast process have a complex alcohol concentration dependence. A three-species reaction scheme proposed previously for neat alcohols, supplemented with an approximation regarding the effect of medium polarity on the assumed species, is able to capture the essential features of all activation thermodynamic functions in the studied mixtures.