Influence of different environments on the excited-state proton transfer and dual fluorescence of fisetin

The influence of different protic and aprotic solvent environments on the excited-state intramolecular proton transfer (ESIPT) leading to a dual fluorescence behaviour of a biologically important, naturally occurring, polyhydroxyflavone, fisetin (3,3',4',7-tetrahydroxyflavone), has been investigated. The normal fluorescence band, in particular, is extremely sensitive to solvent polarity with nu(max) shifting from 24 510 cm(-1) in dioxane (E-T(30) = 36.0) to 20 790 cm(-1) in methanol (E-T(30) = 55.5). This is rationalized in terms of solvent dipolar relaxation process, which also cm accounts for the red edge excitation shifts (REES) observed in viscous environments such as glycerol at low temperatures. Significant solvent dependence of the tautomer fluorescence properties (nu(max), yield and decay kinetics) reveals the influence of external hydrogen bonding perturbation on the internal hydrogen bond of the molecule. These excited-state relaxation phenomena and their relevant parameters have been used to probe the microenvironment of fisetin in a membrane mimetic system, namely AOT reverse micelles in n-heptane at different water/surfactant molar ratio (w(o)). (C) 1999 Elsevier Science B.V. All rights reserved.