Thermal reversion mechanism of N-functionalized merocyanines to spiropyrans: A solvatochromic, solvatokinetic, and semiempirical study

In continuing studies of the effect of solvent and molecular structure on the behavior of photochromic and thermochromic dye molecules, especially spiropyran (SP)-merocyanine (MC) interconversions, we have examined a series of 6'-nitrobenzoindolinospiropyrans (6-nitro-BIPS) with varying N-functionalities (R = CH3, CH2CH2COOH, CH2CH2CH2SO3-, CH2CH2COO-Cholesteryl). The solvent effect was assessed by following the thermal decay of the photochemically ring-opened merocyanine to the spiropyran (MC reversible arrow SP) via UV/vis spectroscopy at the lambda(max) of the MC form. It was found that while modification of the N-moiety produced no perturbations in the solvatochromic behavior of these dyes, there was a marked effect on the solvatokinetic behavior. In nonpolar solvents, where the MCs possess predominantly quinoid character (unit central bond order), a constant thermal reversion rate was observed for the MCs with electron-rich N-ligands. This was attributed to electronic and steric interactions between the Ligands and the phenoxide moiety. However, in polar solvents the increased zwitterionic character of the MCs (central bond order similar to 2) leads to inhibition of the thermal reversion rate for the MCs in this study, independent of N-functionality. The MC reversible arrow SP interconversion has also been examined by means of semiempirical calculations. These reveal the lowest energy pathway for conversion of the trans-MC to a cis-MC form via sequential bond rotation of the three central dihedral angles (alpha, beta, and gamma). The calculations support the observed solvatokinetic behavior, leading to the assignment of the trans/cis thermal isomerization as the rate-determining step in the overall process.