Nitrostyrene-Modified 2-(2-Hydroxyphenyl)benzothiazole: Enol-Emission Solvatochromism by ESICT-ESIPT and Aggregation-Induced Emission Enhancement

Excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) are two competitive reactions that occur in the excited states of organic dyes that contain intramolecular hydrogen-bonds and electron acceptors and donors. Determining the mechanisms of these processes is key to understanding their multiple emission features, as the manner in which these processes interact can be modulated by modifying the dye structure. In addition, donor-π-acceptor (D-π-A) molecules often suffer from aggregation-induced quenching. Herein, we report the synthesis of three nitrophenyl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives, HBT-s-NO₂ , HBT-d-NO₂ , and HBT-t-NO₂ , which have C-C, C=C, and C≡C bonds between their HBT and nitrophenyl moieties, respectively. Compared with the enol emissions from HBT-s-NO₂ and HBT-t-NO₂ , that from HBT-d-NO₂ exhibits outstanding solvatochromism owing to consecutive ESICT-ESIPT. In addition, X-ray diffraction reveals that despite the highly planar and polar nature of HBT-d-NO₂ , which is strongly H-aggregated, it exhibits highly efficient fluorescence. Hence, this study provides new insight into the design of ESICT/ESIPT-coupled systems and for engendering planar dipolar molecules with excellent emission properties in the solid state.