Complexation of spiropyrans with cyclodextrins: Effects of β- and γ-cyclodextrins on the thermal isomerization of 6-SO-spiropyran
Article first published online: 12 OCT 2004
Copyright © 1995 John Wiley & Sons Ltd.
Journal of Physical Organic Chemistry
Volume 8, Issue 5, pages 335–340, May 1995
How to Cite
Sueishi, Y. and Nishimura, T. (1995), Complexation of spiropyrans with cyclodextrins: Effects of β- and γ-cyclodextrins on the thermal isomerization of 6-SO-spiropyran. J. Phys. Org. Chem., 8: 335–340. doi: 10.1002/poc.610080503
- Issue published online: 12 OCT 2004
- Article first published online: 12 OCT 2004
- Manuscript Revised: 15 JAN 1995
- Manuscript Received: 28 SEP 1994
The thermal isomerizations of 1′,3′,3′-trimethyl-spiro[2H-1-benzopyran-2,2′-indoline]-6-sulphonic acid (6-SO-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-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-SP were determined at various pressures. It was found that the stability of the 1:1 inclusion complex between 6-SO-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-SP–β-CD system and 4·0 cm3 mol−1 for the 6-SO-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.