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

Abstract

Absorption and emission spectra of the merocyanine (MC) form of 6-nitrobenzospiropyran (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 acetonitrile–toluene. For all these mixtures, the Stokes shift of MC shows the same linear dependence with the solvent parameter E_T(30). The thermal decay kinetics is monoexponential in acetonitrile–toluene mixtures and biexponential in those containing ethanol. The activation parameters of the rate constants display very different behaviours with E_T(30): whereas ΔG is linear in this parameter, E_a shows abrupt variations with this polarity parameter. This is compensated by changes in ΔS, resulting in a linear correlation between E_a and ΔS.