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Complexation of spiropyrans with cyclodextrins: Effects of β‐ and γ‐cyclodextrins on the thermal isomerization of 6‐SO 3 − ‐spiropyran
Author(s) -
Sueishi Yoshimi,
Nishimura Tomonori
Publication year - 1995
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610080503
Subject(s) - isomerization , chemistry , merocyanine , spiropyran , reaction rate constant , photochemistry , cyclodextrin , kinetics , photochromism , organic chemistry , catalysis , physics , quantum mechanics
Abstract The thermal isomerizations of 1′,3′,3′‐trimethyl‐spiro[2 H ‐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 cm 3 mol −1 for MC → SP and 8·5 cm 3 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 cm 3 mol −1 for the 6‐SO 3 − ‐SP–β‐CD system and 4·0 cm 3 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.