Complexation of phenols with β- and γ-cyclodextrins: Determination of the association constants by using the isomerization of spiropyran

As a chemical indicator, the isomerization rate of so-called 6-SO3^--spiropyran was used to estimate the equilibrium constants for the inclusion complex formation of β- and γ-cyclodextrins (β and γ-CDs). The association constants for inclusion complexes of β- and γ-CDs with various kinds of phenols were determined. It was found that β- and γ-CDs form 1:1 and 1:2 inclusion complexes with phenols, respectively. The substituent effects on the association constants for the 4-substituted phenols-β-CD system can be interpreted in terms of the geometry of the inclusion complex. The magnitude of the association constants for the inclusion complex is related to molecular polarizability of the guest molecule. Based on the results, the dominat factor for CD complexation with phenols is discussed.