Between Aromatic and Quinoid Structure: A symmetric UV to Vis/NIR Benzothiadiazole Redox-Switch.

Reversibly switching the light absorption of organic molecules by redox processes is of interest for applications in sensors, light harvesting, smart materials, and medical diagnostics. Here, we present a symmetric benzothiadiazole (BTD) derivative with a high fluorescence quantum yield in solution and in the crystalline state and show by spectroelectrochemical analysis that reversible switching of UV-absorption in the neutral state, to broad-band Vis/NIR absorption in the 1st oxidized state, to sharp band Vis absorption in the 2nd oxidized state, is possible. For the one-electron oxidized species, formation of a delocalized radical is confirmed by electron paramagnetic resonance spectroelectrochemistry. Furthermore, our results reveal an increasing quinoidal distortion upon the 1st and 2nd oxidation, which can be used as the leitmotif for the development of BTD based redox switches.