Excited state intramolecular proton transfer in 2-(2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine: effects of solvents

Excited state intramolecular proton transfer (IPT) reaction in 2-(2′-hyrdoxyphenyl)-1H-imidazo[4,5-c]pyridine in different solvents and mixtures of binary solvents has been studied by means of UV-vis absorption, fluorescence, fluorescence excitation, time correlated single-photon counting fluorescence spectroscopy and using AM1 semi-empirical quantum mechanical calculations. Dual emission from this molecule in organic solvents under the excitation of 310 nm was observed and was ascribed to different species, rotamer a-1 and tautomer Ta, formed by the IPT in the excited S1 state in rotamer a-2. The emission is discussed in terms of a four-state diagram in which the tautomeric form, in the ground state, is only obtained by emission from the excited tautomer and this species quickly converted into more stable species rotamer a-2. The fluorescence quantum yield of the tautomer emission decreases with the increase of polarity and hydrogen bonding nature of the solvents, whereas that of normal band increases under the similar conditions but it starts decreasing after certain composition of the solvent mixtures. Polarity of the solvents plays the major role in decreasing the fluorescence quantum yield of the tautomeric band.