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

Abstract

The pH effects on the 1:1 complexation behavior of β-cyclodextrin (β-CDx) with the title compounds have been investigated by measuring ultraviolet (UV) absorption, circular dichroism (CD), fluorescence and 1H nuclear magnetic resonance (NMR) spectra and fluorescence lifetimes of the β-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 β-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 1H 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 β-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 β-CDx inclusion in both the ground-state and the excited singlet-state. This finding strongly suggested that both the quinoid-type carbonyl and oxido groups in the tautomer anion guest are exposed to the bulk aqueous phase to interact with the surrounding water molecules.