Spectroscopy and thermal decay of a photomerocyanine in mixtures of polar and nonpolar solvents

Absorption and emission spectra of the merocyanine (MC) form of 6-nitrobenz ospiropyran (SP), as well as the kinetics of the thermal decay of MC to SP, were measured in solvent mixtures of ethanol-hexane, ethanol-toluene and a cetonitrile-toluene. For all these mixtures, the Stokes shift of MC shows t he same linear dependence with the solvent parameter E-T(30). The thermal d ecay kinetics is monoexponential in acetonitrile-toluene mixtures and biexp onential in those containing ethanol. The activation parameters of the rate constants display very different behaviours with E-T(30): whereas Delta G( not equal) is linear in this parameter, E-a shows abrupt variations with th is polarity parameter. This is compensated by changes in Delta S-not equal, resulting in a linear correlation between E-a and Delta S-not equal.