Radical‐Cation Dimerization Overwhelms Inclusion in [ n ]Pseudorotaxanes

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril ( CB7 ) in water is a well‐known phenomenon. Herein, two counter‐examples are presented. Two viologen‐containing thread molecules were designed, synthesized, and thoroughly characterized by 1 H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square‐wave voltammetry, and chronocoulometry: BV 4+ , which contains two viologen subunits, and HV 12+ , which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]‐ and [7]pseudorotaxanes that form on complexation with CB7 , that is, BV 4+ ⊂( CB 7) 2 and HV 12+ ⊂( CB 7) 6 , were also analyzed. The properties of two monomeric control threads, namely, methyl viologen ( MV 2+ ) and benzyl methyl viologen ( BMV 2+ ), as well as their [2]pseudorotaxane complexes with CB7 ( MV 2+ ⊂ CB7 and BMV 2+ ⊂ CB7 ) were also investigated. As expected, the control pseudorotaxanes remained intact after one‐electron reduction of their viologen‐recognition stations. In contrast, analogous reduction of BV 4+ ⊂( CB 7) 2 and HV 12+ ⊂( CB 7) 6 led to host–guest decomplexation and release of the free threads BV 2( . +) and HV 6( . +) , respectively. 1 H DOSY NMR spectrometric and chronocoulometric measurements showed that BV 2( . +) and HV 6( . +) have larger diffusion coefficients than the corresponding [3]‐ and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical‐cation dimerization. These results demonstrate that radical‐cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.