Ion-Controlled On–Off Switch of Electron Transfer from Tetrathiafulvalene Calix[4]pyrroles to Li⁺@C₆₀

Binding of chloride anion to a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) donor results in ET to Li⁺@C₆₀ to produce the radical pair (TTF-C4P^•+/Li⁺@C₆₀^•–), the structure of which was characterized by X-ray crystallographic analysis. The addition of tetraethylammonium cation, which binds more effectively than Li⁺@C₆₀^•– as a guest within the TTF-C4P cavity, leads to electron back-transfer, restoring the initial oxidation states of the donor and acceptor pair.