Dynamic solvent effects on the fading of a merocyanine formed from a spironaphthoxazine. A case of a strong coupling between chemical and medium coordinate

Abstract

The pressure dependence of the rate constant for the fading of a merocyanine formed from a spironaphthoxazine was measured in three pairs of nonviscous/viscous solvents of similar polarity, such as ethanol/2-methylpentane-2,4-diol, methyl acetate/glycerol triacetate, and methylcyclohexane/2,4-dicyclohexyl-2-methylpentane. The results unequivocally demonstrated a necessity to treat the medium and the chemical coordinate as two independent coordinates. These coordinates were found strongly coupled in the present reaction, especially in protic solvents. The results also revealed that hydrogen-bond solvation to the oxygen atom in the initial state and its desolvation during the activation step play important roles in both reaction mechanisms and dynamics.