Lactim or lactam? A theoretical study on the possibility of excited-state intramolecular proton transfer in hydroxyphenyl-imidazo-isoindoles

The interest in organic molecules exhibiting Excited State Intramolecular Proton Transfer (ESIPT) has steadily increased in the last decades, as the peculiar properties of these compounds make them useful for various ap-plications. However, designing efficient ESIPT fluorophores may result a non-trivial matter. In this work we analyze the controversial case of a family of hydroxyphenyl-imidazo-isoindoles that has been recently proposed to experience ESIPT through an unusual four-membered hydrogen-bonding cycle. We demonstrate herein that such a process is unlikely to occur in excited state, and provide a simpler rationalization to the experimental observations.