Photochromism of spiropyran in ionic liquids: Enhanced fluorescence and delayed thermal reversion

Photochromism of spiropyran and thermal reversion of MC in a series of imidazolium-based ionic liquids (ILs) were investigated via fluorescence and UV-vis absorption spec. The fluorescence emission maxima (λ_em) of the colored merocyanine (MC) form is polarity dependent in all ILs and fluorescence intensity enhancement was first observed in ILs with respect to that observed in organic solvents (e.g., in [HOMIm][PF₆], ne32-fold enhancement than ethanol). Moreover, the thermal decay of MC in all ILs is uniformly first order, and in comparison with polar solvents possessing comparable polarity (such as ethanol and acetonitrile), a significant retardation of the decay rand increase of half-lifetime of MC form was observed, in particular at elevated temperature (e.g., at 313 K, MC decay with k of 5.19 × 10^-4 and 34.7 × 10^-4 s^-1 in [BMMIm][BF₄] and ethanol, respectively). Abnormal thermodynamics of tthermal decay of MC in ILs were observed, activation energies in less polar ILs (88-97 kJ mol^-1) have a larger value than in polar ILs (70-82 kJ mol^-1), opposite that in organic solvents (70-80 kJ mol^-1 in nonpolar solvents and around 100 mol^-1 in polar solvents), and all ILs exclusively exhibit minus activation entropies (-85.4 to -24.5 J K ^-1 mol^-1), similar to that of nonpolar solvents. Desolvation due to the conformational change during the thermal reversion between MC and the transition state may be the key to interpret this experimental result.