Excited state intramolecular proton transfer in 2-(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine: effect of solvents

Excited state intramolecular proton transfer in 2-(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (2-HPIP) in different solvents and mixtures of binary solvents has been studied by means of UV–Visible absorption, fluorescence, fluorescence excitation and time-correlated fluorescence spectroscopy. Dual fluorescence is observed in all the solvents except in cyclohexane where only large Stokes-shifted emission is noticed. The fluorescence quantum yield of the long-wavelength emission decreases, whereas that of the short wavelength emission increases, with increase in the polarity and hydrogen bonding nature of the solvent. Steady-state fluorescence and fluorescence excitation spectra and the time-correlated study have clearly established the presence of only two distinctly different rotamers in the ground state. Semi-empirical quantum mechanical calculations (AM1) have been carried on different isomers, rotamers and tautomers of 2-HPIP to help establish the presence of true species.