The derivatives of 3-hydroxyflavone exhibit excited-state intramolecular proton transfer (ESIPT) reaction with significant (60-80 nm) shifts of fluorescence spectra between normal and phototautomer forms. This fact makes these compounds attractive as fluorescence probes in analytical chemistry, biophysics, and molecular biology. Different flavonol derivatives, including 4′-(monoaza-15-crown-5)flavonol, were synthesized, and their absorption and fluorescent spectra were studied in acetonitrile in the presence of different concentrations of Mg 2+ and Ba 2+ ions. It was shown that the general feature of flavonols is the ability to form two types of complexes with alkaline-earth cations: the low-stability "external" and high-stability chelating complexes. On the formation of the complexes, parent flavonols and their 4′-dialkylamino derivatives undergo different perturbations of their electronic structures. 4′-(Monoaza-15-crown-5)flavonol forms two types of complexes with both Mg 2+ and Ba 2+ ions; the sequence of steps in formation of Ba 2+ and Mg 2+ complexes is different.

Correlation between the character of electronic density redistribution in the excited normal and phototautomer forms, which are involved in the excited state intramolecular proton transfer (ESIPT) reaction, and the rates of primary photoprocesses--proton phototransfer and radiationless deactivation of the phototautomer form--was analysed on the qualitative level. It was revealed that noticeable improvement of fluorescence properties of ESIPT molecules could be achieved by directed modification of their chemical structure, most importantly by the introduction of electron withdrawing substituents into their proton donor moieties. Several facts are presented in favour of the last statement.

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Luminescent properties of several ortho‐hydroxy derivatives of 2,5‐diphenyl‐1,3,4‐oxadiazole were studied in solvents of different polarity and protolytic ability and compared with the analogous derivatives of 1,3‐oxazole, studied by us earlier. Evaluations of rate constants for the excited state intramolecular proton transfer (ESIPT) reaction and radiationless degradation of the energy of electronic excitation in the ESIPT reaction product, phototautomer form, were made on the basis of fluorescence quantum yields and fluorescence kinetic measurements. The proton phototransfer reaction is the most efficient primary photoprocess in the examined series of compounds. With rate constants of 1011–1012 s−1 it exceeds the rates of other primary photophysical processes in these molecules by up to 2–3 orders of magnitude. A connection between the rate of the ESIPT reaction and the electron density redistribution on electronic excitation was revealed for the molecules under study. The electron donor substituents in 5‐phenyl decrease the proton phototransfer rate constant up to complete inhibition of the ESIPT process in the case of the dimethylamino derivative of the oxazole series, whereas the electron acceptor substituents must accelerate this reaction. The determining role of the excited state increase of the proton donor group acidity was assumed on the basis of experimental data and quantum chemical calculations. The temperature investigations showed that the ESIPT reaction in the series of ortho‐hydroxy derivatives of 2,5‐diphenyloxazole and 2,5‐diphenyl‐1,3,4‐oxadiazole has a very low activation barrier. However, the radiationless processes in the phototautomer form are characterized by the presence of a certain barrier in both examined series. A dependence of the radiationless dissipation rate in the phototautomer form on the excited state redistribution of the electron density in it was also found. The more efficient is the charge transfer from the ortho‐phenolate fragment to the protonated heterocycle and the rest of the molecule, the more efficient are the radiationless losses. It was revealed that structural changes, which prevent the mentioned movement of electron density, result in an increase in the fluorescence efficiency of the phototautomer form and, hence, also to an increase in the total fluorescence quantum yield of the ESIPT molecule. Copyright © 2000 John Wiley & Sons, Ltd.

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