Rational Modulation of Excited‐State Intramolecular Proton Transfer, Aggregation‐Induced Emission, and Intramolecular Motion on Polysubstituted Imidazoles

In this study, it is investigated how the N ‐substitution pattern modulates the excited‐state intramolecular proton transfer (ESIPT) in polysubstituted imidazoles, a class of compounds with promising photophysical properties. The selective use of a single isomer, either enol or keto, has emerged as a valuable strategy in developing new functional molecules. Using a combination of experimental and theoretical methods, it is addressed how N ‐alkyl substituents in combination with a 2‐(methoxy)phenyl group significantly influence the conformation and stabilization of enol and keto tautomers involved in the ESIPT process. Single‐crystal X‐ray diffraction reveals a clear influence on the dihedral angle involved in the ESIPT pathway in the crystalline solid state, while (time‐dependent) density functional theory reveals the importance of conformational flexibility and substituent π ‐donation in shaping the excited‐state potential energy surface to access a regime of tunable ESIPT. Supported by steady‐state solvent‐polarity‐dependent emission, aggregation‐induced emission, and viscosity‐dependent emission, an enhanced emission correlated with the stabilization of a specific isomer is demonstrated.