Inclusion complexation of a few pyrylium salts by β-cyclodextrin studied by fluorescence, NMR and laser flash photolysis

Abstract

Inclusion complex formation between pyrylium salts 1-6 and β-cyclodextrin (β-CD) was investigated. Because of the positive charge present in the molecule, simple pyrylium salts are hydrophilic and do not show any tendency for encapsulation in β-CD. However, by tuning the hydrophobicity of the substituents present in the 4-position, one can prepare pyrylium salts, which are sufficiently hydrophobic to undergo encapsulation in β-CD cavities. Thus, 2-5 undergo encapsulation as is evident from their NMR and fluorescence spectra in the presence of β-CD. Fluorescence quantum yields of 2-5 were enhanced in the presence of β-CD and these enhancements were used to calculate the association constants by a Benesi-Hildebrand treatment. Electron transfer to the encapsulated pyrylium salts from a water-soluble donor was studied by fluorescence quenching and laser flash photolysis. The pyranyl radicals formed within the β-CD cavity as a result of electron transfer was found to be long-lived and this is attributed to the reduced rate of back electron transfer reaction achieved due to the protection afforded to the radical by the hydrophobic β-CD cavity.