Red-Shifting ESIPT Fluorescence by Site-Specific Functionalization in 2-(2’-hydroxyphenyl)benzazole Derivatives

We describe the synthesis, full photophysical study, and ab initio calculations of 2-(2’-hydroxyphenyl)benzazole (HBX) fluorophores substituted, at the meta position of the phenol group, by pyridine derivatives. HBX are commonly used as model dyes to study the stimuli-induced modulation of the Excited State Intramolecular Proton Transfer (ESIPT) process. The meta-substituted fluorophores reported herein, display a photophysical profile different from the previously reported ortho- and para-substituted HBO pyridine isomers. Indeed, while all dyes undergo spontaneous deprotonation in neutral conditions, leading to highly emissive anionic species; upon protonation, ortho- and para-pyridine substitution leads to resonance-stabilized keto isomers, formed after ESIPT. Protonated meta derivatives, unable to stabilize their excited structure by such electronic delocalization process, display sizable intramolecular charge transfer (ICT) processes, translating into significantly redshifted emission. In addition, all dyes present a strong emission intensity, not only in neutral and acidic solutions, but also in the solid-state. The nature of the emissive transitions was confirmed in each case by theoretical calculations combining Time-Dependent Density Functional Theory (TD-DFT) and second-order Coupled Cluster (CC2) methods.