The synthesis of a rigid conjugated viologen and its cucurbituril pseudorotaxanes

A linear viologen (bis[4-(4-pyridinyl)phenyl] viologen, BPPV) characterized by a fully-conjugated structure was synthesized through Zinke reaction. BPPV consists of a unique linear sexiaryl structure and is apt to be encapsulated by cucurbit[n]uril (CB[n], n = 6 or 7) in aqueous solution, forming a [2]pseudorotaxane (BPPV⊂CB[6]) with CB[6] in a central-included mode and a final [3]pseudorotaxane (BPPV⊂2CB[7]) with CB[7] in a side-included manner. The reduction potentials became higher for BPPV⊂2CB[7], and the highest for BPPV⊂CB[6] when compared with BPPV. BPPV, BPPV⊂CB[6] and BPPV⊂2CB[7] all underwent reversible one-electron redox reaction in aqueous solution, accompanied by obvious color changes, showing a good prospect in the fabrication of electrochromic display devices.