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

James T.C. Wojtyk, Adnaan Wasey, Peter M. Kazmaier, Shmaryahu Hoz, Erwin Buncel

Research output: Contribution to journalArticlepeer-review

134 Scopus citations

Abstract

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.

Original languageEnglish
Pages (from-to)9046-9055
Number of pages10
JournalJournal of Physical Chemistry A
Volume104
Issue number39
DOIs
StatePublished - 5 Oct 2000
Externally publishedYes

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