A Method of Evaluating the Rate Constant of the Thermal Isomerization of 6-SO3−-spiropyran by Using β-Cyclodextrin

The thermal isomerizations of 1′,3′,3′-trimethyl-spiro[2H-1-benzopyran-2,2′-indoline]-6-sulfonic acid between its spiro (SP) and merocyanine (MC) forms were followed spectrophotometrically by using β-cyclodextrin as the control of the isomerization reaction. The rate constants of the thermal isomerizations were estimated to be 2.27 × 10−3 s−1 for MC → SP and 3.03 × 10−3 s−1 for SP → MC.