Prototropism of the methylated derivatives of 2-(2 '-aminophenyl) benzimidazole

The ground state (S-0) and the excited singlet state (S-1) prototropic reac tions of a series of three different methyl derivatives of 2-(2'-aminopheny l)benzimidazole have been investigated by means of absorption, fluorescence emission, fluorescence excitation and excited singlet state lifetime measu rements. Both the absorption and the fluorescence spectra of the monocation s (MCs) get largely red shifted, whereas a large blue shift is observed for the dications (DCs). Double and single exponential decay processes are obs erved for the MCs and the DCs, respectively. The fluorescence excitation sp ectra and the semi-empirical quantum mechanical calculations confirm the pr esence of mainly one kind of monocation and dication in the S-0 state under both isolated and hydrated conditions, whereas two kinds of MCs are presen t in the S-1 state. Molecular electrostatic potential energy calculations c arried out on different neutral molecules provide a good correlation betwee n global minima and pK(a) values for the monocation-neutral equilibrium and also indicate the chances of excited state intramolecular proton transfer in the S-1 state. Fluorimetric titrations indicate that prototropic equilib rium is not established in the S-1 state and no proton-induced fluorescence quenching is observed prior to the formation of the MCs and DCs. (C) 1999 Elsevier Science B.V. All rights reserved.