Solvation effects on spirooxazine to merocyanine thermal reversion kinetics in acetonitrile-water binary mixtures

In solution, the photochromic molecule 5-methoxyspiro[indole-phenanthrolinoxazine] (SPO) is in equilibrium with its corresponding merocyanine open isomer. In pure acetonitrile, there is about 22.5% of the open form, whereas in acetonitrile-water binary solvents this proportion increases to more than 60% when the water molar fraction chi(H2O) is around 0.80. Irradiation in the visible range leads to a photobleached steady state in which the proportion of the merocyanine open isomer decreases significantly. When irradiation is switched off, a slow thermal reversion of the closed spircoxazine to the open merocyanine takes place until the initial equilibrium position is reached. Kinetic analysis of a whole set of relaxation curves recorded at various water contents was used to establish a model involving multiple equilibria between three merocyanines in different solvation states (solvated by acetonitrile, solvated by an acetonitrile-water complex and hydrated). Their relative proportions, which are dependent on the binary mixture composition, were determined, in addition to their corresponding visible spectra. Copyright (c) 2004 John Wiley & Sons, Ltd.