Excited-state proton-transfer and dual fluorescence of robinetin in different environments

Excited-state intramolecular proton-transfer (ESIPT) and dual emission behaviour of robinetin: (3,7,3',4',5'-pentadydroxy flavone) a naturally occurring flavonoid, have been examined in different protic and aprotic solvents and in aerosol-OT (AOT) reverse micelles. The normal fluorescence band, in particular, shows dramatic sensitivity to solvent polarity, with lambda(max) shifting from 413 nm in dioxane (E-T(30) = 36.0) to 490 nm in methanol (E-T(30) = 55.5). This is interpreted in terms of a solvent dipolar relaxation mechanism, which also accounts for red edge excitation shifts (REES) noted in viscous environments, e.g. glycerol at low temperatures. The tautomer fluorescence properties (quantum yield and decay kinetics) also show significant solvent dependence. Relevant parameters of the dual emission bands have been used to infer the microenvironments of robinetin in AOT reverse micelles at different water/surfactant molar ratio (w(0)). (C) 1997 Published by Elsevier Science B.V.