Purple-to-green emission: excited-state intramolecular proton transfer in an aromatic D-π-A system

Excited-state intramolecular proton transfer (ESIPT) has emerged as a pivotal mechanism in the realm of molecular photoswitches and the design of novel photoelectronic materials. In aromatic proton-donor-π-acceptor (D-π-A) ESIPT systems, the enol tautomer typically dominates in the ground-state, rendering the stabilization of the keto tautomer a challenging task. Herein, we indicate that single N-methylation of 2-(2'-hydroxyphenyl)-benzimidazole (HBI) facilitates the co-existence of enol and keto tautomers in the ground electronic state, thereby enabling precise modulations of emission through adjusting the excitation energy. In protic solvents, solvation inhibits ESIPT, significantly enhancing enol fluorescence while quenching keto emission. In addition, the formation of an anion introduces a third, blue-emitting species, enabling a three-color fluorescence system. Femtosecond transient absorption (fs-TA) spectroscopy and time-dependent density functional theory (TDDFT) calculations were employed to elucidate the ESIPT mechanism, revealing the role of solvents, structural modifications and intramolecular proton transfer. This study establishes a general strategy for modulating the photochemical properties of materials by incorporating substituents into ESIPT systems, thus unlocking substantial potential for stimuli-responsive sensors and anti-counterfeiting materials.