A Systematic Characterization of the Solvent‐Dependent Photoswitching Behavior of an N ‐Substituted Nitrospiropyran Derivative

This study presents a comprehensive investigation of the switching kinetics of a widely used spiropyran–merocyanine (SP–MC) photoswitch in different solvents. Under UV irradiation, the forward SP→MC conversion follows apparent first‐order kinetics, yielding a mono‐exponential increase in MC population toward a photostationary state (PSS). In contrast, green‐light irradiation and thermal relaxation drive the reverse MC→SP conversion via exponential decay kinetics. These processes must be treated independently, as they originate from distinct photophysical and thermodynamic mechanisms. The proposed rate laws are validated experimentally in different solvent environments. Temperature‐dependent studies reveal that thermal back‐isomerization increasingly shifts the equilibrium toward the SP‐enriched state and can substantially suppress MC formation at elevated temperatures. Solvent polarity strongly influences the forward switching kinetics: while MC formation exhibits ideal monoexponential behavior in a polar solvent such as acetonitrile, decreasing polarity induces pronounced deviations from first‐order behavior due to aggregation phenomena involving the highly polar MC species. In contrast, the reverse MC→SP conversion remains largely monoexponential and insensitive to solvent polarity, consistent with the greater thermodynamic stability of the less polar SP state in nonpolar environments.