Ground- and excited-state structural orientation of 2-(2'-hydroxyphenyl)benzazoles in cyclodextrins

The effects of alpha-, beta-, gamma-, and 2,6-di-O-methyl-beta-cyclodextrins (CDs) on the ground- and excited-state properties of 2-(2'-hydroxyphenyl)benzoxazole, 2-(2'-hydroxyphenyl)benzothiazole, and 2-(2'-hydroxyphenyl)benzimidazole in aqueous media are investigated. Steady-state fluorescence measurements are used to characterize the interaction of CDs with these azoles. Absorbance measurements indicate increased solubility of the azoles in aqueous solutions of CDs. Measurements of acidity constants (pK(a)) and data from induced circular dichroism indicate increased ground- and excited-state acidities of the phenolic protons of the molecules in the presence of CDs and axial orientation of the molecules within the CD cavity, respectively. The data further suggest a planar stucture for HBO and a twisted confirmation for both HBT and HBI. The association constants of the inclusion complexes have also been estimated. These studies are further supplemented by comparative spectroscopic studies of 3-(2'-methoxyphenyl)benzothiazole in aqueous solutions of CDs. On the basis of the spectral data acquired, it is believed that the HBA molecules exist as zwitterionic tautomers in the presence of CDs.