An Electrochromic Tristable Molecular Switch

A tristable [2]catenane, composed of a macrocyclic polyether incorporating 1,5-dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) units along with a 4,4′-bipyridinium (BIPY^•+) radical cation as three very different potential recognition sites, interlocked mechanically with the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺), was synthesized by donor-acceptor templation, employing a "threading-followed-by-cyclization" approach. In this catenane, movement of the CBPQT⁴⁺ ring in its different redox states among these three potential recognition sites, with corresponding color changes, is achieved by tuning external redox potentials. In the starting state, where no external potential is applied, the ring encircles the TTF unit and displays a green color. Upon oxidation of the TTF unit, the CBPQT⁴⁺ ring moves to the DNP unit, producing a red color. Finally, if all the BIPY²⁺ units are reduced to BIPY^•+ radical cations, the resulting CBPQT^2(•+) diradical dication will migrate to the BIPY^•+ unit, resulting in a purple color. These readily switchable electrochromic properties render the [2]catenane attractive for use in electro-optical devices.