Encapsulation of 6-Hydroxyquinoline in Heptakis(2,6-di-O-methyl)-β-cyclodextrin

Abstract

Cyclodextrins have attracted long-standing interests in the field of artificial catalysts. Their cone-shaped cages consist of α,D-glucose subunits and possess nonpolar cavities with polar rims containing primary hydroxyl and secondary methoxy groups. Cyclodextrins are soluble in water and their hydrophobic interiors are capable of sequestering a variety of polar and nonpolar compounds, controlling the chemistry of reactive molecules. Three different types of water are generally assumed to exist in aqueous cyclodextrin solutions: water inside the cavity, water near the rim, and bulk water. A number of studies have reported the effects of micelles and cyclodextrins on proton transfer to shed light on the influence of lipophilic environment. Proton transfers of hydroxyquinolines having two prototropic groups of enol and imine in a molecule have been extensively explored. The excited-state proton transfer of 7-hydroxyquinoline (7HQ) in the molecular cage of βcyclodextrin or heptakis(2,6-di-O-methyl)-β-cyclodextrin (CD) has also been studied. The anionic intermediate during the excited-state proton transfer of 7HQ forms slower but decays faster in β-cyclodextrin cages than in water. However, the fluorescence spectral overlaps of normal molecule, enol-deprotonated anion, and imine-protonated and enol-deprotonated tautomer at S1, as well as the low solubility in water and the low association constant of βcyclodextrin with 7HQ, make the excited-state proton transfer kinetics of 7HQ very complex to require a sophisticated analysis. This has led us to investigate the encapsulation of 6-hydroxyquinoline (6HQ) in the molecular cage of CD in this work. CD is a derivative of β-cyclodextrin with a greatly improved solubility in water, encapsulating a predominant fraction of 6HQ to form host-guest complexes. The observed H chemical shifts of CD in water decrease