Purple-to-Green Emission: Excited-State Intramolecular Proton Transfer in an Aromatic D-π-A System

Abstract

Excited-state intramolecular proton transfer (ESIPT) has been proven as a powerful mechanism for molecular photoswitches, and in the design of novel photoelectronic materials . In proton-donor-π-acceptor (D-π-A) aromatic ESIPT systems, the enol tautomer usually dominates the ground-state, while the keto tautomer is often difficult to stabilize. Herein, we show that single N-methylation of 2-(2′-hydroxyphenyl)-benzimidazole (HBI) enables the co-existence of enol and keto tautomers in the ground electronic state and thereby provides precise modulations of the emission by adjusting the excitation energy. In protic solvents, solvation inhibits ESIPT, significantly enhancing enol fluorescence while quenching keto emission. In addition, the anion formation from intermolecular proton transfer adds 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 applied to provide insights into the ESIPT mechanism, revealing the role of solvents, structural modifications and intermolecular proton transfer. This study establishes a general strategy for modulating the photochemical properties in molecular materials by the introduction of substituents into ESIPT systems, thereby unlocking substantial potential for the stimuli-responsive sensors and anti-counterfeiting materials.