Should I Stay or Should I Go? Applying a Combined Experimental and Theoretical Approach to Predict the Excited-State Intramolecular Proton Transfer (ESIPT) in Imidazo[1,5-a]pyridine-3-yl Phenols

The fluorescent behavior of a series of imidazo[1,5-a]pyridine-3-yl phenols was investigated both experimentally (in solution, the solid state, and thin films) and theoretically. Among these compounds, only a subset exhibited excited-state intramolecular proton transfer (ESIPT) emission, identified as a thermodynamically driven process. To understand this behavior, a comprehensive theoretical study was carried out to elucidate the nature of the electronic transitions and to identify the molecular factors governing the occurrence of ESIPT, revealing that fine-tuning the electron-donating and-withdrawing properties is needed for the proton transfer to take place. A structure-property relationship approach was employed, resulting in the development of a numerical descriptor (Omega), dependent on the substituents on the core architecture, capable of predicting whether, upon excitation, the proton "stays" on the oxygen atom or "goes" to the nitrogen atom via the ESIPT mechanism. The same approach also proved to be effective in predicting the frontier orbital energies of the studied systems.