A comparative study is presented of competitive fluorescences of three flavonols, 3-hydroxyflavone, 3,3',4',7-tetrahydroxyflavone (fisetin), and 4'-diethylamino-3-hydroxyflavone (DEIF). The normal fluorescence Si -* So (400-nm region) is largely replaced by the proton-transfer tautomer fluorescence S' --S' in the 550-nm region for all three of the flavonols in aprotic solvents at room temperature. For DHF in polar solvents the normal fluorescence becomes a charge-transfer fluorescence (460-500 nm) which competes strongly with the still dominant proton-transfer fluorescence (at 570 nm). In protic solvents, and at 77 K, the interference with intramolecular hydrogen bonding gives rise to greatly enhanced normal fluorescence, lowering the quantum yield of proton-transfer fluorescence. The utility of DHF as a discriminating fluorescence probe for protein binding sites is suggested by the strong dependence of the charge-transfer fluorescence on polarity of the environment and by various static and dynamic parameters of the charge-transfer and protontransfer fluorescence which can be determined.The purpose of this paper is to explore the application of molecular fluorescence as probes for protein binding sites. The competition between proton-transfer (PT) fluorescence (1, 2) (i.e., excited-state intramolecular proton transfer, ESIPT) and charge-transfer (CT) fluorescence, and the application of solvent polarity studies, is presented for three hydroxyflavones selected as fluorescence probes. The model compounds chosen are 3-hydroxyflavone (3-HF) and its derivatives 3,3',4',7-tetrahydroxyflavone (fisetin) and 4'-diethylamino-3-hydroxyflavone (DHF). In the first two, PT tautomer fluorescence (Amax 529 nm and 540 nm, respectively) is predominant in aprotic solvents at 298 K, the normal tautomer fluorescence being observed only very weakly. The third molecule (DHF) emits both a PT fluorescence at Amos 570 nm and a prominent CT fluorescence at Amass 460 nm.3-HF has been the object of extensive photophysical studies (1-33). Fisetin has not been studied extensively (29) and attracted our attention again recently because of its extraordinary performance as a lasing material (unpublished work). DHF is a newly synthesized molecule (34-36) which we explore as a protein fluorescence probe.The spectroscopic behavior of these three hydroxyflavones offers some contrasting properties which, for these cases, permit a discrimination between normal fluorescence, strong CT fluorescence, and PT fluorescence and indicate the utility of the latter two as fluorescence probe phenomena. MATERIALS AND METHODSSpectroscopic Measurements. Absorption spectra were measured on a Shimadzu UV-2100 spectrophotometer. The fluorescence spectra were recorded with a SPEX Fluoromax spectrofluorimeter (Spex Industries, Edison, NJ).Chemicals. 3-HF and fisetin were from Aldrich. tDHF and 4'-diethylamino-3-methoxyflavone were synthesized in the laboratory of Pi-Tai Chou (University of South Carolina, Columbia). All solvents were of spectrograde quality and...

Fluorenone sensitization in solution interconverts all-trans-1, 3, with its trans,cis,trans, cis,trans,trans, and cis,cis,trans (trace) isomers, tct-, ctt-, and cct-DPH, respectively. Photoisomerization quantum yields are reported for the three major isomers in degassed and air-saturated benzene. In degassed solutions the quantum yields are strongly concentration dependent due to quantum chain processes. The presence of air eliminates the quantum chain processes, as all DPH triplets are deactivated by oxygen. Triplet-triplet absorption spectra observed in the microsecond time scale starting from these three DPH isomers are identical. The concentration dependence of the DPH triplet lifetime is consistent with the concentration dependence of the isomerization quantum yields. The results indicate that in benzene DPH triplets exist as an equilibrium mixture of ttt, tct, and ctt isomers whose composition at 20 °C, 94% ttt, 5% tct, and 1% ctt is revealed by isomerization quantum yields in the presence of air. Photoisomerization quantum yields in the presence of air show that the isomeric triplets are fully equilibrated within less than 100 ns. † J.S. dedicates this paper to his teacher, George S. Hammond, whose illuminating and inspiring ways made this issue possible.

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A phenomenological analysis is given of the three cases of lowest triplet state potentials relative to ground singlet and lowest excited singlet state potentials in the presence of intramolecular proton transfer. In each case, singular excitation phenomena are observed respectively: enhanced normal tautomer molecule phosphorescence (TI -SO), dual phosphorescence, and unique proton-transfer (FT) tautomer phosphorescence (T1' -SO'). The application of these model cases to proton-transfer examples involving radiation-detector scintillators, biomolecule fluorescence probes, and four-level lasers is discussed. The development of the potential curve interaction models permits the optimization of the desirable large wavelength shift and maximization of quantum yield of the PT fluorescence. The competition of normal tautomer and PT tautomer phosphorescences with the FT fluorescence is discussed as a limiting factor.Researches on the excitation behavior of molecules exhibiting excited state intramolecular proton transfer (ESIPT) have been greatly broadened in recent years1,* with many new molecular systems investigated, involving numerous novel and highly detailed spectroscopic and dynamics researches. A determining factor in the efficiency of the fluorescence which can be observed from the tautomer produced by the ESIPT excitation is the role played by the lowest triplet state potential between the normal tautomer and the proton-transfer tautomer species. Nevertheless, very few cases have yet been reported in which direct involvement of the lowest triplet state potential has been revealed.In the present paper, we analyze the cases representing the three spectroscopic schemes which are possible and discuss the conditions for and consequences of the occurrence of each. The four reported cases of tautomer triplet state emissions and their subtleties will be analyzed on the basis of the three cases. Case A. Enhancement of TI -SO Phosphorescence via ESIPTordering Case A is considered to correspond to the state energy diagrammed schematically in Figure 1. For this case we assume that the energy difference TI' -T1 is too great for significant Boltzmann excitation of the TI' state from TI to occur, even at 298 K. This is the case suggested to be present for the aminosalicylates studied by Gormin et permitting a T1 state excitation enhancement mechanism via an ESIPT pathway. The reported comparative spectroscopy showed an apparently intensified T1 -SO phosphorescence relative to the S1 -SO fluorescence, for the two species in which ESIPT was possible.f This paper is dedicated to the celebration of the magnificent scientific research career of Professor Mostafa El-Sayed, who has made keynote contributions to the study of triplet states of molecules and many other areas.SO phosphorescence enhancement via a back-proton transfer in the lowest triplet state potential from the proton-transfer tautomer to the normal molecule tautomer.The qualitative spectroscopic results (Figure 3, ref 3) indicated a strong enhancement of the T1 -SO emi...

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