The thermal and photochemical processes associated with the acid-induced conversions of 6-nitroBIPS, SP-1, to form the protonated merocyanine (MC-OH+) were investigated via UV/vis spectrophotometric studies in acetone. It was found that the mechanism of trifluoroacetic acid (TFA)-induced ring-opening of the SP and the rate of MC-OH+ formation follows a general acid catalysis mechanism. In accord with this mechanism, the thermal growth of the acid-induced ring-opened form (MC-OH+) was retarded as the concentration of TFA in the medium was increased. The N-protonated SP, i.e., SP-NH+, is formed in a competing side-equilibrium process as an unreactive "sink", with the nitrogen lone-pair no longer available to drive the ring-opening process and resulting in the inverse rate dependence as a linear 1/kobs vs [HA] plot. Addition of a tertiary amine to MC-OH+ regenerated MC which underwent thermal ring closure to the SP, thus restoring its function as a molecular switch. NMR titration of SP samples showed a downfield shift of the N-substituent peak upon increasing the TFA concentration. However, a saturation behavior could not be observed with SP-1 up to 1 M acid, unlike the model compound, N,N-dimethylaniline (N,N-DMA), which indicates a base strength order of N,N-DMA > SP-1. Further, we have demonstrated that in solvent acetone, on acidification, the normal photo- and thermochromic behavior is reversed; now MC-OH+ is photochemically transformed into SP-H+, which undergoes thermal ring-opening to MC-OH+.

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 ) CH 3 , CH 2 CH 2 COOH, CH 2 CH 2 CH 2 SO 3 -, CH 2 CH 2 COO-Cholesteryl). The solvent effect was assessed by following the thermal decay of the photochemically ring-opened merocyanine to the spiropyran (MC H SP) via UV/vis spectroscopy at the λ 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 ∼2) leads to inhibition of the thermal reversion rate for the MCs in this study, independent of N-functionality. The MC H 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 (R, 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.