Thermal Reversion Mechanism of N-Functionalized Merocyanines to Spiropyrans:  A Solvatochromic, Solvatokinetic, and Semiempirical Study

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 104; no. 39; pp. 9046 - 9055
• Main Authors: Wojtyk, James T. C, Wasey, Adnaan, Kazmaier, Peter M, Hoz, Shmaryahu, Buncel, Erwin
• Format: Journal Article
• Language: English
• Published: American Chemical Society 05.10.2000
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp001533x

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 ⇋ SP) via UV/vis spectroscopy at the λmax 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 ∼2) leads to inhibition of the thermal reversion rate for the MCs in this study, independent of N-functionality. The MC ⇋ 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 (α, β, and γ). 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.