Excited state intramolecular proton transfer in 2-(2 '-N-palmitoyl-aminophenyl)benzimidazole: effect of carbonyl group

Excited state intramolecular proton transfer in 2-(2'-N-palmitoyl-aminophen yl)benzimidazole has been studied in different solvents. Except in water, d ual fluorescence is observed in each solvent. The fluorescence excitation s pectra and effect of excitation wavelength clearly indicate that the normal Stokes shifted fluorescence is observed from the rotamer I and the large S tokes shifted fluorescence from the tautomer III, formed from rotamer II in the S-I state. Semi-empirical quantum mechanical calculations and the time resolved studies support the presence of at least two isomers of each rota mer, The normal Stokes shifted emission band is very sensitive, whereas the large Stokes shifted emission is nearly insensitive to the solvent polarit y. The presence of the electron withdrawing carbonyl group increases the po larisability of -NH bond and also the fluorescence quantum yield of the tau tomer emission when compared to that in 2-(2'-aminophenyl)benzimidazole, Th e excited state pK(a), value for the deprotonation of amino group in the fo rmer is 12.0, whereas in the latter case it is 12.7. (C) 1999 Elsevier Scie nce B.V. All rights reserved.