Excited state intramolecular proton transfer in 2-(2′-amino-3-pyridyl)benzimidazole:: effect of solvents

Excited state intramolecular proton transfer (ESIPT) has been studied in 2-(2'-amino-3-pyridyl)benzimidazole (2-A3PyBI) in different solvents with the help of absorption, fluorescence excitation and fluorescence spectroscopy and time resolved emission spectrofluorimetry. Electronic structure calculations on each species were carried out using semi-empirical AMI method and hybrid density functional theory (DFT) B3LYP using 6-31 G** basis set and Gaussian 98 program. Dual fluorescence is observed in all the solvents. Based on theoretical and experimental results, normal small Stokes shifted emission is assigned to rotamer a-2 in non-polar solvents and a-3 in polar protic solvents. Long wavelength (LW) and large Stokes shifted band is due to tautomer formed by ESIPT from rotamer a-1. Presence of electron withdrawing =N-atom ortho to -NH2 group increases its acidity. Due to this rate of ESIPT process will be enhanced and thus the fluorescence quantum yield (Phi(f)(T)) of the tautomer will also be increased. Both AM1 and DFT calculations have shown that a-1 is the most stable among all the rotamers and semi-empirical calculations have indicated that the activation barrier for interconvertion of a-1 to a-2 is only 2.6 kJmol(-1) in the ground state (S-0) and 44.3 kJmol(-1) in the first excited singlet (S-1). On the other hand, activation barrier height for the conversion of a-1 to a-T is 132 kJmol-1 in S-0 state and 72 kJmol(-1) in S-1 state, obtained by AM1 method. (C) 2004 Elsevier B.V. All rights reserved.