Temperature Effect on Dual Fluorescence of 2‑(2′-Hydroxyphenyl)benzimidazole and Its Nitrogen Substituted Analogues

The effects of temperature on the dual fluorescence of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues, viz., 2-(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (HPIP-c), were investigated in solvents of different polarity and hydrogen bonding capability. Absorption, steady-state, and time-resolved emission spectroscopic techniques were employed for the experimental study. Density functional theoretical calculations were performed to find the relative population of the conformers. The calculations predict that, with increase in temperature, the population of trans-enol increases, while that of cis-enol decreases. At all temperatures, the population ratio of cis-enol to trans-enol increases in the order HPIP-c < HPIP-b < HPBI. Except for HPBI in methanol and ethylene glycol, the fluorescence of both emissions decreases with an increase in temperature and is more pronounced in the tautomer band than in the normal band. The data are analyzed using the Arrhenius and van’t Hoff equations. The change in the fluorescence with temperature is governed by (i) the change in the relative population of conformers and (ii) the increase in non-radiative decay from the excited states. The increase in non-radiative decay from the normal emission competes with the increase in the relative population of trans-enol with a rise in temperature.