A TDDFT exploration on the excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)-benzimidazole derivatives.

Excited-state intramolecular proton transfer (ESIPT) reactions are one of the fundamental energy transformation reactions in catalysis and biological process. The combining ESIPT with the twisted intramolecular charge transfer (TICT) brings the richness of optical, photoelectronic performances to certain functional compounds. Delineating the mechanism of ESIPT + TICT reactions and further understanding why a specific functional group dominates are fundamentally crucial for the design and application of the functionally photoelectric materials. In this paper, six 2-(2'-hydroxyphenyl) benzimidazole (HBIgens) derivatives involved in ESIPT + TICT were investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to have an insight into the photophysical and photochemical process in acetonitrile. The optimized geometries indicated that the intramolecular hydrogen bonds (-O-H···N-) were enhanced in the corresponding first singlet, which provided the fundamentally outstanding prerequisites of the ESIPT reactions. By further charge analysis, it is indicated that the introduction of substitutes to the different positions would determine the Stokes' shifts, and the electron-adopting p-cyanophenyl group mainly contributed to the TICT structure. Constraint scanning the potential energy curves of both ground and first singlet excited states, the electron-adopting N,N-diethylamino group on the meta position could enhance the barrier and inhibit the ESIPT reaction. Furthermore, the nucleus independent chemical shift (NICS(1)_ZZ) values of phenol groups indicate the relationship between the reversal aromaticity and the barrier of ESIPT, both of which were proved to be negatively correlated in the ESIPT reaction. It is concluded that not only both types and positions of substituents can tune the excited-state proton transfer behaviors in HBIgen derivatives, but also the aromatic rule can easily be applied to elaborate the ESIPT reaction.