Excited state intramolecular proton transfer reaction and luminescent properties of theortho-hydroxy derivatives of 2,5-diphenyl-1,3,4-oxadiazole

Abstract

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.