Insights into ESIPT-induced multicolor fluorescence emission in 2-(2′-hydroxy-5′-bromo)phenylbenzimidazole: a spectroscopic and TDDFT study

Although multicolor luminescent materials are widely used in information encryption and decryption based on the excited-state intramolecular proton transfer (ESIPT) reaction, there remains a significant gap in the mechanistic understanding of how solvent and pH conditions influence the ESIPT process. Owing to their ability to avoid self-absorption as well as provide large Stokes' shift and strong emission properties, ESIPT-generated molecules (ESIPT gens) have recently emerged as highly potential fluorophores. Herein, the ESIPT mechanism of bromine-based (2'-hydroxy-5'-bromo)phenylbenzimidazole (HBI-pBr) was investigated in solvents using spectroscopic measurements and time-dependent density functional theory (TD-DFT) calculations. The results indicated that multi-color fluorescence emissions were observed at 470, 458 and 416 nm in CH₃OH doped with a base and acid. The potential energy profile rationalized the fluorescence mechanistic insights into the ESIPT reaction and pH-dependent dual response. Notably, nucleus-independent chemical shift (NICS_ZZ) values were applied to reveal the ESIPT process. We leveraged the bromine atom as an electron withdrawing group to manipulate ground and excited-state proton transfer, thereby offering a strategic approach for designing and developing an ESIPT fluorescence sensor for the detection of H⁺ and OH^-. By studying the effect of solvent and pH conditions on HBI-pBr, the multicolor fluorescence mechanism of ESIPT was elucidated, thus laying a solid foundation for the design and synthesis of luminescent materials based on the ESIPT reaction.