Phenoxy-Benzothiadiazole Donor–Acceptor Compounds Exhibit Solvatochromic Fluorescence and Are Precursors to Open-Shell Phenoxyl Radicals

In this work, we present the synthesis of two highly fluorescent phenoxy-benzothiadiazole donor-acceptor (DA) compounds, which exhibit quantum yields of up to 84% in solution under an atmosphere of air. These compounds display solvatochromic fluorescence within the visible region of the electromagnetic spectrum, characteristic of DA exciplex behavior, and exhibit reversible reactivity under acidic conditions as monitored by UV-Vis spectroscopy. We demonstrate the reversible oxidation of compound to the open-shell phenoxy radical 5·. In contrast, compound 6 undergoes irreversible oxidation to bis(quinone) 7, despite the loss of aromaticity. We also observe the comproportionation of 6 and 7 to compound 6· upon UV light irradiation in solution, as evidenced by EPR spectroscopy. The structures of compounds 5, 6, and 7 were confirmed through X-ray crystallography.