Captodative radicals enable the coexistence of monomer and dimer single-molecule junctions with 100-fold difference in conductance

Atomic-scale manipulation of chemical bond cleavage and formation offers significant advantages of extending the reaction controllability to the single-molecule regime and enabling the elucidation of fundamental reaction mechanisms. Herein, we drive reactions between two chemical species, open-shell monomeric radicals and closed-shell dimers in single-molecule junctions. Both monomers and dimers form a C-Au covalently-linked highly conducting junction in situ, with the conductance about 100 ́ higher than that of the dimer junction. First- principles calculations suggest that the substitution groups on the carbon in the C(sp3)-Au linkage bond effectively tunes the molecular junction conductance. Notably, captodative radicals enable the formation of weakly bonded spin-spin interactions in solution, yielding diamagnetic dimers, and we can reversibly switch between the dimer and the weakly- bonded radical pair by mechanically controlling the gap between the two electrodes.