Excited state intramolecular proton transfer in julolidine derivatives: an ab initio study

Abstract

We have studied, using ab initio tools, a series of recently prepared fluorescent julolidine derivatives, undergoing Excited State Intramolecular Proton Transfer (ESIPT). We show that the computed free energy change in the excited state (ΔG^ES) can be used to predict the preference for enol, keto, or dual emission. Indeed, two julolidine molecules experimentally show dual emission, consistent with our finding of a small ΔG^ES. In agreement with experimental outcomes the complexation between the ESIPT centre and BF₂ increases the rigidity of the fluorophore and greatly facilitates emission at energies close to the original enol (E*) fluorescence band. The protonation of the imino group also suppresses ESIPT and sole E* emission is obtained. We disclose that chemical substitution can significantly tune the radiationless deactivation of the enol related to the CN bond rotation of the ESIPT centre. While there is a significant barrier for the experimentally studied compounds we have found a strong correlation between the barrier height and the electron donating strength of the phenyl substituent. Strong donors such as amines facilitate the barrierless non-radiative decay from E* back to the ground state, while weak electron donors make the barrier sufficiently high to allow ESIPT. Strong electron accepting groups such as –NO₂ further increase this barrier. This work therefore illustrates the fine interplay necessary to design dual emitters.