Stimuli‐Induced Fluorescence Switching in Azine‐Containing Fluorophores Displaying Resonance‐Stabilized ESIPT Emission

Abstract This article reports the synthesis, along with structural and photophysical characterization of 2‐(2’‐hydroxyphenyl)benzazole derivatives functionalized with various azaheterocycles (pyridine, pyrimidine, terpyridine). These compounds show dual‐state emission properties, that is intense fluorescence both in solution and in the solid‐state with a range of fluorescent color going from blue to orange. Moreover, the nature of their excited state can be tuned by the presence of external stimuli such as protons or metal cations. In the absence of stimuli, these dyes show emission stemming from anionic species obtained after deprotonation (D * transition), whereas upon protonation or metal chelation, ESIPT process occurs leading to a stabilized and highly emissive K * transition. With the help of extensive ab initio calculations, we confirm that external stimuli can switch the nature of the transitions, making this series of dyes attractive candidates for the development of stimuli‐responsive fluorescent ratiometric probes.