An insight into the solvent effect on photo-, solvato-chromism of spiropyran through the perspective of intermolecular interactions

The solvent effect on photochromism and solvatochromism of the SP (1′-ethyl-3′, 3′-dimethyl-6-nitrospiro [chromene-2, 2′-indoline]) is investigated in details. The photochromic phenomena indicate that the change of solvents can reverse the thermodynamic equilibrium between the ring-close Spiropyran and ring-open Merocyanine. Therefore, a more appropriate definition is suggested to emphasize the thermodynamic nature of the positive and negative photochromism; furthermore, a generalized definition of chromism is raised as a reference. Hansen solubility parameter is originally employed to analyze the roles of different intermolecular interactions in the solvatochromism of SP in diverse solvents. The kinetics of spiropyran in nonpolar solvents exhibits stepwise first order of reaction that does not follow the regular situation.