CORRELATIONS BETWEEN SOLVATOCHROMISM, LEWIS ACID-BASE-EQUILIBRIUM ANDPHOTOCHROMISM 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 empi rical parameter to measure the polarity of the solvents and as a refer ence process to predict the shift of the Lewis acid-base equilibrium b etween PC and the colourless form (SP) and the thermal decolouration r ates of the PC form in neat and mixed organic solvents. From the linea rity of the plot of ln K-c or ln k(dc) vs. E(T), the Lewis acid-base e quilibrium, photochromism and solvatochromism of the spiropyran were f ound 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 supp orting the view that the rate-determining ring-opening process is a co ncerted electrocyclic reaction.