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 ph...