Theoretical Investigation on the Excited-State Proton Transfer Mechanism of 2-(1 H-benzo[d]imidazol-2-yl)-6-(benzo[d]thiazol-2-yl)-4-bromophenol.

Excited-state intramolecular proton transfer (ESIPT) mechanisms incorporating dual hydrogen-bond acceptors (HBAs) demonstrate considerable practical applicability due to their tunable characteristics. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods were employed to explore the structural, photophysical properties, and solvent-polarity-dependent ESIPT behavior of the symmetric dual HBA molecule 2-(1 H-benzo[d]imidazol-2-yl)-6-(benzo[d]thiazol-2-yl)-4-bromophenol (BIBTB). The intramolecular hydrogen bonds (IHBs) and electronic spectral properties under different solvent polarities were first investigated. Calculations revealed that IHB strength increases with solvent polarity, accompanied by a pronounced blue shift in fluorescence. Furthermore, the dipole moments of both ground and excited states were computed in various solvents. The results show that the dipole moments in the excited state are consistently lower than those in the ground state, indicating that the ground state undergoes greater energy stabilization in polar solvents. This trend, consistent with the potential energy curves, corroborates the observed spectral blue shift. Additionally, hole-electron analysis combined with potential energy curves elucidated the barrierless ESIPT characteristics of BIBTB. Constrained optimizations further identified the origin of the weak experimental fluorescence peak at ~ 400 nm. Collectively, this study presents a comprehensive conformational analysis of BIBTB via quantum chemical methods and examines the influence of solvent polarity on the ESIPT process. The theoretical insights into this dual HBA system pave the way for developing high-performance fluorescent probes and luminescent materials.