Complexation of spiropyrans with cyclodextrins: Effects of β‐ and γ‐cyclodextrins on the thermal isomerization of 6‐SO 3−‐spiropyran

The thermal isomerizations of 1′,3′,3′‐trimethyl‐spiro[2H‐1‐benzopyran‐2,2′‐indoline]‐6‐sulphonic acid (6‐SO 3−‐SP) between its spiro (SP) and merocyanine (MC) forms were followed spectrophotometrically in the presence of cyclodextrins (β‐ and γ‐CDs) at various pressures. From the pressure dependence of the isomerization rate, the activation volumes for the thermal isomerization of 6‐SO 3−‐SP were estimated to be 17·1 cm³ mol⁻¹ for MC → SP and 8·5 cm³ mol⁻¹ for SP → MC. The equilibrium constants (K) for the inclusion complex formation of CDs with 6‐SO 3−‐SP were determined at various pressures. It was found that the stability of the 1:1 inclusion complex between 6‐SO 3−‐SP and CDs decreases in the order β– > γ– > α‐CD. From the pressure dependence of K, the reaction volumes were estimated to be −0·9 cm³ mol⁻¹ for the 6‐SO 3−‐SP–β‐CD system and 4·0 cm³ mol⁻¹ for the 6‐SO 3−‐SP–γ‐CD system. Based on the activation volumes, the structure of the transition state for the thermal isomerization is discussed and an explanation for the effect of CDs on the reaction rate is given. The difference in the reaction volumes for the inclusion complex formation of β‐ and γ‐CDs is explained in terms of the difference in the number of water molecules excluded from the CD cavity.