Spectroscopic investigation of the three prototropic forms of a β-cyclodextrin-indolizine derivative from its inclusion-cum-charge-transfer complexes

By absorption and fluorescence (steady state and time-resolved) studies, three prototropic forms of (N-6-deoxy-β-cyclodextrin-6-yl-1-aminocarbonyl)-3-(4-fluorobenzoyl)-7-pyridine-4-yl indolizine, (1) have been established and shown to be a potential fluorescent pH sensor with switching action. Inclusion complexes of (1) with a number of quinones have been shown to exhibit charge transfer (CT) spectra in aqueous medium and variation of such spectra with change in pH yield the proton affinity of the biologically important indolizine moiety. Förster cycle reveals that in the excited state indolizine moiety has a greater proton affinity. The nature of the CT transitions (whether n-donor–π-acceptor or π-donor–π-acceptor) has been experimentally ascertained.