A switching sensor of C[sbnd]H bond breakage/formation regulated by mediating copper (II)’s complexation

Copper (II)-catalyzed selective C[sbnd]H bond activation has been extensively investigated, but few examples are optically visible and reversible. We herein report a visible switching of copper (II)-catalyzed C[sbnd]H bond formation/cleavage regulated by the association/disassociation between copper (II) and calix[3]dimethoxycarbazole (2). Upon binding to copper (II), the C[sbnd]H bond of the bridging methylene of 2 is cleaved to produce a blue-color carbocationic intermediate 3; once the copper ion is removed by its stronger ligand triethylenetetramine (TETA), 3 is then converted back to 2, and its blue-color disappears correspondingly. The reversibility of the above optical behavior could be repeated up to 6 cycles by mediating the association/disassociation of 2-Cu²⁺ complexation. Thus, 2 could serve as a copper-selective switching sensor.