Red-Hair-Inspired Chromogenic System Based on a Proton-Switched Dehydrogenative Free-Radical Coupling

In the presence of micromolar peroxides or biometals (Fe(III), Cu(II), V(V) salts), and following a strong acid input, the stable 3-phenyl-2H-1, 4-benzothiazine is efficiently converted to a green-blue Delta(2,2')-bi(2H-1,4-benzothiazine) chromophore via dehydrogenative coupling of a 1,4-benzothiazinyl radical. The new system is of potential practical interest for colorimetric peroxide and redox biometal detection.