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

Inclusion complex formation between pyrylium salts 1-6 and beta-cyclodextri n (beta-CD) was investigated. Because of the positive charge present in the molecule, simple pyrylium salts are hydrophilic and do not show any tenden cy for encapsulation in beta-CD. However, by tuning the hydrophobicity of t he substituents present in the 4-position, one can prepare pyrylium salts, which are sufficiently hydrophobic to undergo encapsulation in beta-CD cavi ties. Thus, 2-5 undergo encapsulation as is evident from their NMR and fluo rescence spectra in the presence of beta-CD Fluorescence quantum yields of 2-5 were enhanced in the presence of beta-CD and these enhancements were us ed 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 beta-CD cavity as a result of electron transfer was found to be long-lived and this is attributed to the reduced r ate of back electron transfer reaction achieved due to the protection affor ded to the radical by the hydrophobic beta-CD cavity.