Complexation and proton dissociation behavior of 7-hydroxy-4-methylcoumarin and related compounds in the presence of β-cyclodextrin

The pH effects on the 1:1 complexation behavior of beta -cyclodextrin (beta -CDx) with the title compounds have been investigated by measuring ultraviolet (UV) absorption, circular dichroism (CD), fluorescence and H-1 nuclear magnetic resonance (NMR) spectra and fluorescence lifetimes of the beta -CDx inclusion complexes formed. Steric effects of substituent attached at the four position of a guest molecule on the stability of the inclusion complex as well as on the induced CD spectral intensity demonstrated that the guest is included into the beta -CDx cavity so as to locate this substituent around the rim of the cavity at low and even at high pH (where a tautomer anion is exclusively produced). The H-1 NMR data were consistent with the intracavity immersion of a given guest molecule from the secondary hydroxy group side and, additionally, substantiated the structure of the beta -CDx inclusion complex generated at low pH. Through an analysis of the pH-dependent UV absorption and fluorescence spectra of 7-hydroxycoumarine derivatives, it was found that the proton dissociation abilities of these guests undergo negligible effects of the beta -CDx inclusion in both the ground-state and the excited singlet-state. This finding strongly suggested that both the quinoid-type carbonyl and oxide groups in the tautomer anion guest are exposed to the bulk aqueous phase to interact with the surrounding water molecules. (C) 2001 Elsevier Science B.V. All rights reserved.