Thermodynamic equilibrium of the transitions between the closed and open forms of a spiropyran-based molecular switch monitored

A benzothiazole spiropyran exhibited both photochromic and acid-chromic properties. Via variable-temperature 1H NMR spectroscopy, the equilibrium constants Kϕ, ΔHϕ and ΔSϕ of the ring-closed to ring-opened transition were calculated as ca. 10-7, 38.62 kJ mol-1 and -5.55 J K-1 mol-1, respectively. This is of importance for the application of new spiropyrans.