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

摘要

The effect of nitrogen substitution in the benzene ring of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) on thephotophysics 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), and8-(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 forexcited state calculations. The calculations reveal in the ground state all of the molecules have two stablerotameric forms, but their relative population is strongly affected by nitrogen substitution. The excitation andemission bands have been calculated theoretically for the rotamers and tautomers. Fluorescence emission andexcitation spectra were recorded for HPBI in dioxane and compared with the theoretical results. Theoreticalexcitation and emission data are in good agreement with the available experimental data. The potential energysurface simulated for the proton transfer processes reflect that it is not favorable in So state, but it is feasiblein S1 state in all of the molecules. Except in HPIP-b, and HPP', in all other nitrogen substitutedmolecules, the energy difference between the keto and enol form along the excited state proton transfercoordinates decreases compared to that in HPBI. The study also reveals that torsional relaxation of tautomerto twisted state competes with radiative transitions and leads to fluorescence quenching. Nitrogen substitutionenhances this torsional induced nonradiative process and it follows the order HPBI < HPIP-b < HPIP-c < HPP.