Radical-Cation Dimerization Overwhelms Inclusion in[n]Pseudorotaxanes

摘要

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril (CB7) in water is a wellknown phenomenon. Herein, two counter-examples are presented. Two viologen-containing thread molecules were designed, synthesized, and thoroughly characterized by ~1H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square-wave voltammetry, and chronocoulometry: BV4+, 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+)?(CB7)2 and HV~(12+) ?(CB7)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+)üCB7 and BMV~(2+)üCB7) 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+)?(CB7)2 and HV~(12+)?(CB7)6 led to host–guest decomplexation and release of the free threads BV2(C+) and HV6(C+), respectively. ~1H DOSY NMR spectrometric and chronocoulometric measurements showed that BV2(C+) and HV6(C+) 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.