Role of nitrogen substitution in phenyl ring on excited state intramolecular proton transfer and rotamerism of 2-(2'-hydroxyphenyl)benzimidazole: a theoretical study

A comparative study of 2-(2'-hydroxy-3'-pyridyl)benzimidazole (2',3'-HPyBI), 2-(3'-hydroxy-4'-pyridyl)benzimidazole (3',4'-HPyBI), 2-(4'-hydroxy-3'-pyridyl)benzimidazole (4',3'-HPyBI), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (3',2'-HPyBI), and 2-(5'-hydroxy-4'-pyrimidinyl)benzimidazole (5',4'-HPymBI) with 2-(2'-hydroxyphenyl)benzimidazole (HPBI) was performed theoretically to evaluate the effect of nitrogen substitution in the phenolic ring on the photophysics and rotamerism of HPBI. Density functional theory (DFT) and configuration interaction singles (CIS) combined with time-dependent DFT were employed for ground and excited state studies, respectively. Different possible molecular forms were considered for each molecule viz., cis-enol, trans-enol, open-enol, and keto forms. The computational results revealed that cis-enol is the most stable form in the ground state for all the molecules except in 2',3'-HPyBI. In 2',3'-HPyBI, K-2 keto is the most stable form. Water molecule assisted interconversions between different forms of 2',3'-HPyBI were examined theoretically. Excitation and emission energies for all the forms have been calculated theoretically and the values are in good agreement with the available experimental data. The calculations show that intramolecular proton transfer (ESIPT) is endothermic in the ground state while it is exothermic in the first excited singlet state (except 5',4'-HPymBI). The barrier for the excited state ESIPT reaction increases with nitrogen substitution. Torsional rotation between the benzimidazole and the pyridinyl∕pyrimidinyl rings in the S(1) state depicts that twisted-keto structures involve charge transfer from the hydroxypyridinyl∕hydoxypyrimidinyl to the benzimidazole ring. However, the formation of twisted-keto is not energetically favored in these systems.