Comparative Theoretical Study of Rotamerism and Excited State Intramolecular Proton Transfer of 2-(2′-Hydroxyphenyl)benzimidazole, 2-(2′-Hydroxyphenyl)imidazo[4,5-b]pyridine, 2-(2′-Hydroxyphenyl)imidazo[4,5-c] pyridine and 8-(2′-Hydroxyphenyl)purine

The effect of nitrogen substitution in the benzene ring of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) on the photophysics and rotamerization were examined theoretically by a comparative study of HPBI with 2-(2′-hydroxyphenyl)imidazo[4,5-b]pyridine (HPIP-b), 2-(2′-hydroxyphenyl)imidazo[4,5-c]pyridine (HPIP-c), and 8-(2′-hydroxyphenyl)purine (HPP). Density functional theory (DFT) was used for ground state calculations. Restricted configuration interaction singles (RCIS) combined time dependent DFT (TDDFT) was used for excited state calculations. The calculations reveal in the ground state all of the molecules have two stable rotameric forms, but their relative population is strongly affected by nitrogen substitution. The excitation and emission bands have been calculated theoretically for the rotamers and tautomers. Fluorescence emission and excitation spectra were recorded for HPBI in dioxane and compared with the theoretical results. Theoretical excitation and emission data are in good agreement with the available experimental data. The potential energy surface simulated for the proton transfer processes reflect that it is not favorable in S0 state, but it is feasible in S1 state in all of the molecules. Except in HPIP-b, HPIP-b′, and HPP′, in all other nitrogen substituted molecules, the energy difference between the keto and enol form along the excited state proton transfer coordinates decreases compared to that in HPBI. The study also reveals that torsional relaxation of tautomer to twisted state competes with radiative transitions and leads to fluorescence quenching. Nitrogen substitution enhances this torsional induced nonradiative process and it follows the order HPBI < HPIP-b < HPIP-c < HPP.