Redox switchable daisy chain rotaxanes driven by radical-radical interactions

We report the one-pot synthesis and electrochemical switching mechanism of a family of electrochemically bistable 'daisy chain' rotaxane switches based on a derivative of the so-called 'blue box' (BB⁴⁺) tetracationic cyclophane cyclobis(paraquat-p-phenylene). These mechanically interlocked molecules are prepared by stoppering kinetically the solution-state assemblies of a self-complementary monomer comprising a BB⁴⁺ ring appended with viologen (V²⁺) and 1,5-dioxynaphthalene (DNP) recognition units using click chemistry. Six daisy chains are isolated from a single reaction: two monomers (which are not formally 'chains'), two dimers, and two trimers, each pair of which contains a cyclic and an acyclic isomer. The products have been characterized in detail by high-field ¹H NMR spectroscopy in CD ₃CN - made possible in large part by the high symmetry of the novel BB⁴⁺ functionality - and the energies associated with certain aspects of their dynamics in solution are quantified. Cyclic voltammetry and spectroelectrochemistry have been used to elucidate the electrochemical switching mechanism of the major cyclic daisy chain products, which relies on spin-pairing interactions between V^•+ and BB ^2(•+) radical cations under reductive conditions. These daisy chains are of particular interest as electrochemically addressable molecular switches because, in contrast with more conventional bistable catenanes and rotaxanes, the mechanical movement of the ring between recognition units is accompanied by significant changes in molecular dimensions. Whereas the self-complexed cyclic monomer - known as a [c1]daisy chain or molecular 'ouroboros' - conveys sphincter-like constriction and dilation of its ultramacrocyclic cavity, the cyclic dimer ([c2]daisy chain) expresses muscle-like contraction and expansion along its molecular length.