Mechanistic insights into substituent position effects on ESIPT and singlet–triplet interactions in BrA-HBI

Abstract

Positional substitution of functional groups plays a decisive role in modulating excited-state intramolecular proton transfer (ESIPT) and its coupling with singlet–triplet interactions, thereby directly impacting thermally activated delayed fluorescence (TADF) and the associated photophysical properties. Three novel derivatives (BrA-1-HBI, BrA-2-HBI, and BrA-3-HBI) were designed by systematically varying the positions of Br and –CHO substituents in the (2′-hydroxyphenyl)benzimidazole scaffold (BrA-HBI). Density functional theory (DFT) and time-dependent DFT (TD-DFT) methods were employed to investigate their ESIPT properties and photophysical responses. The results show that all positional isomers exhibit reduced ESIPT barriers and red-shifted emission spectra. Remarkably, BrA-3-HBI undergoes a two-step excited-state double proton transfer (ESDPT), yielding an exceptionally large Stokes shift of ∼1.24 × 10⁴ cm⁻¹. Moreover, positional substitution alters the transition character from weak n → π* to strong π → π*, thereby activating Enol-state emission, enabling dual fluorescence channels, and reprogramming ISC pathways from the Keto state toward the Enol state. These results show that the Br inductive effect and –CHO conjugation strengthen the intramolecular H-bond and promote ESIPT, offering a rational strategy for designing high-performance ESIPT-based optoelectronic materials.