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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