Correlations between solvatochromism, Lewis acid-base equilibrium and photochromism of an indoline spiropyran

The solvatochromism, Lewis acid-base equilibrium and photochromism of an indoline spiropyran in neat and mixed organic solvents were studied by UV-visible spectroscopy. The variation of the transition energy E(T) of the coloured form (PC) in different solvents was used as an empirical parameter to measure the polarity of the solvents and as a reference process to predict the shift of the Lewis acid-base equilibrium between PC and the colourless form (SP) and the thermal decolouration rates of the PC form in neat and mixed organic solvents. From the linearity of the plot of ln K-c or ln k(dc) vs. E(T), the Lewis acid-base equilibrium, photochromism and solvatochromism of the spiropyran were found to be similar solvent-dependent processes. The changes in K-c and k(dc) were interpreted in terms of the differences in solvation of PC in solvents of different polarity. The relatively small influence of solvent polarity on the colouration rate k(op) serves as evidence supporting the view that the rate-determining ring-opening process is a concerted electrocyclic reaction.