Excited-state intramolecular proton transfer and rotamerism of 2-(2'-hydroxyphenyl) benzimidazole

Abstract

Excited-state intramolecular proton transfer (ESIPT) and rotamerism of 2-(2'-hydroxyphenyl benzimidazole (HPBI) have been studied using steady-state and time-resolved emission spectroscopy and by semi-empirical quantum-chemical methods. Two ground-state rotamers (I and II) with distinct excitation and emission spectra and lifetimes have been identified. Excitation of one of them (I) produces the normal emission while the other after excitation undergoes ultrafast ESIPT to form the tautomer (III) with Stokes-shifted emission. At 77 K the tautomer emission is markedly supressed as the rotamer, II, responsible for it, is less stable than I. CNDO/S-CI calculations were performed optimising the ground-state geometry by the AM1 method. These calculations give good estimates of the ground- and excited-state energies of the rotamers I and II and the tautomer, III.