Influence of the Solvent on the Thermal Back Reaction of One Spiropyran

The solvent influence on the absorption spectra and the kinetics of the back reaction of the 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro(2H-1-benzopyran-2,2'-2H-indole) (6-NO2-BIPS) has been investigated by means of temperature-controlled, UV-visible spectroscopic measurements. The back reaction process was proved to follow first-order kinetics for five different solvents (toluene, THF, acetonitrile, DMSO, and ethanol) at several temperatures. Kinetics parameters (E-a, A, tau, Delta H-#, Delta S-#, and Delta G(#)) were extracted from absorbance decay profiles for all solvents, and a simple model based on solvation mechanism was proposed in good agreement with experimental results. Such a photochromic compound also exhibits strong solvatochromism in its merocyanine form. All these experiments were performed to illustrate solvent effects on kinetic processes and optical properties for students with an undergraduate physical chemistry theoretical background. All the experiments described could be performed in a physical chemistry undergraduate laboratory.