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The excited-state intramolecular proton transfer (ESIPT) of 2-(2‘-hydroxyphenyl)benzimidazole (HBI) has been studied using absorbance, steady-state, and time-resolved emission spectroscopies in cyclodextrins (β-, γ-, and 2,6-di-O-methyl-β-cyclodextrins) and in binary aqueous solvent mixtures. The spectral characteristics of HBI in cyclodextrins and binary solvent mixtures (dioxane:water, MeOH:water, and solutions of cyclohexane containing MeOH and trifluoroethanol) are compared to further study the effects of various microenvironments on the ground and excited-state properties of the molecule. The intermolecular interactions of HBI with cyclodextrins and various solvents appear to weaken intramolecular hydrogen bonding in HBI and facilitate the formation of strong intermolecular hydrogen bonds with the various cyclodextrins and solvent molecules. The acquired data are used to suggest the formation of a zwitterionic structure of HBI in cyclodextrins.

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The effects of α-, β- , γ-, and 2,6-di-O-methyl-β-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 2-(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.

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The host-guest complexation of 2-(4′-aminophenyl)-and 2-(4′-(N,N-dimethylamino)phenyl)benzothiazole (APB and DAPB, respectively) with β-cyclodextrin (β-CDx) were studied in aqueous solution by use of fluorescence spectroscopy. The induced circular dichroic (ICD) spectra of the inclusion complexes were also measured to confirm complexation and to estimate the orientation of the guest molecule in the cyclodextrin cavity. The association constants for binding of the molecules with β-CDx were estimated in phosphate buffer containing varying concentrations of NaClO 4 . The association constant for binding of DAPB is almost 4 times as high as that of APB. In the case of DAPB, the association constant was found to decrease with an increase in salt concentration. However, the association constant of APB initially increases and then decreases with an increase in salt concentration. The optical rotation data of β-CDx in the absence and presence of NaClO 4 suggest complexation of ClO 4 -with β-cyclodextrin.

A dual fluorescence of trans-stilbene (TS) is reported which involves an apparent ternary complex of TSI y-cyclodextridcyclohexane. This dual fluorescence is not observed to an appreciable extent in the absence of cyclohexane. Only monomer fluorescence of TS is observed in the latter samples. These observations are interpreted in terms of spatial and screening restrictions on the TS photoisomerization and cycloaddition. The cyclohexane in the temary complex is believed to induce restrictions by co-inclusion into the cyclodextrin cavity. As a result, formation of the cis-isomer upon irradiation is reduced. Consequently, the excimer yield of TS is increased. The inclusion of cyclohexane as a partition between two included stilbene molecules in the cyclodextrin cavity is also proposed. This is the first reported excimer of TS in fluid or aqueous cyclodextrin solutions which does not involve covalently linked or severely constrained TS molecules. Alcohols and other hydrophobic compounds such as toluene, xylene, and hexane are also examined as ternary components.