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The thermosolvatochromism of nitrospiropyran free andbound to cyclodextrin was studied in dimethylsulfoxide (DMSO)–waterbinary solvent systems. Spiropyran was interconverted to merocyanineby heating the sample to 55 °C. The merocyanine (MC) was convertedback to spiropyran (SP) either by cooling the sample to room temperatureor irradiating the sample with a visible light emitting diode. Steadystate absorption spectra of SP and MC samples in the free state andbound to cyclodextrin were obtained in several DMSO–water binarysolutions. Emission spectra of MC both free and cyclodextrin-boundwere also acquired. Blue-shifted absorption and emission spectra ofthe studied molecules with increasing solvent polarity suggest thatthe dipole moments of free and bound merocyanines are higher in theground state compared to the excited state. Merocyanine dipole momentsin the ground and excited states were determined using thermosolvatochromismmeasurements and the Lippert–Mataga, Bakhshiev, and Kawski–Chamma–Vialletpolarity functions. A large change in the dipole moment (ca. 16 D)of the merocyanine in aqueous DMSO was obtained upon electronic excitation,S1← S0. Analysis of the merocyanineStokes’ shifts as a function of solvent polarity indicatesthat both general solvent effects and specific solvent effects arepresent in all systems studied. These findings reveal that merocyaninecould be utilized as a polarity sensor for DMSO–water binarysolutions. [ABSTRACT FROM AUTHOR]