Thermosolvatochromism of Nitrospiropyran and Merocyanine Free and Bound to Cyclodextrin

The therrnosolvatochromism of nitrospiropyran free and bound to cyclodextrin was studied in dimethylsulfoxide (DMSO)-water binary solvent systems. Spiropyran was interconverted to merocyanine by heating the sample to 55 degrees C. The merocyanine (MC) was converted back to spiropyran (SP) either by cooling the sample to room temperature or irradiating the sample with a visible light emitting diode. Steady state absorption spectra of SP and MC samples in the free state and bound to cyclodextrin were obtained in several DMSO-water binary solutions. Emission spectra of MC both free and cyclodextrin-bound were also acquired. Blue-shifted absorption and emission spectra of the studied molecules with increasing solvent polarity suggest that the dipole moments of free and bound merocyanines are higher in the ground state compared to the excited state. Merocyanine dipole moments in the ground and excited states were determined using thermosolvatochromism measurements and the Lippert-Mataga, Bakhshiev, and Kawski-Chamma-Viallet polarity functions. A large change in the dipole moment (ca. 16 D) of the merocyanine in aqueous DMSO was obtained upon electronic excitation, S-1 -> S-0. Analysis of the merocyanine Stokes' shifts as a function of solvent polarity indicates that both general solvent effects and specific solvent effects are present in all systems studied. These findings reveal that merocyanine could be utilized as a polarity sensor for DMSO-water binary solutions.