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 H-1 DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square-wave voltammetry, and chronocoulometry: BV4+, which contains two viologen subunits, and HV12+, 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, BV4+ subset of(CB7)(2) and HV12+ subset of(CB7)(6), were also analyzed. The properties of two monomeric control threads, namely, methyl viologen (MV2+) and benzyl methyl viologen (BMV2+), as well as their [2] pseudo-rotaxane complexes with CB7 (MV2+ subset of CB7 and BMV2+ subset of 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 BV4+ subset of(CB7)(2) and HV12+ subset of(CB7)(6) led to host-guest decomplexation and release of the free threads BV2(center dot+) and HV6(center dot+), respectively. 1H DOSY NMR spectrometric and chronocoulometric measurements showed that BV2(center dot+) and HV6(center dot+) have larger diffusion coefficients than the corresponding [3]- and [7] pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radicalcation dimerization. These results demonstrate that radicalcation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.