Reactive pseudorotaxanes: Inclusion complexation of reduced viologens by the hosts β-cyclodextrin and heptakis(2,6-di-O-methyl)-β-cyclodextrin

The complexation of three guests containing 4,4'-bipyridinium redox-active residues by β-cyclodextrin (β-CD) and its heptakis-(2,6-O-dimethyl) analogue (DM-β-CD) was investigated by means of voltammetric techniques. The three 4,4'-bipyridinium (viologen) derivatives used as guests were designed to be water-soluble in all three accessible oxidation states. The N-substituents chosen to enhance aqueous solubility were: 2-(2-(2-ethoxy)ethoxy)ethanol (guest 1²⁺), 6-hexanoate (guest 2), and 3-propanesulfonate (guest 3). Detailed analysis of the voltammetric results by digital simulation techniques revealed that the oxidized forms of the guests did not interact appreciably with either CD host; the two-electron reduced guests formed extremely stable inclusion complexes, with association constants in the range 10³ - 10⁴ M^-1, while the cation radical forms exhibited intermediate binding affinities (≃10² M^-1). In all cases, DM-β-CD was found to form more stable complexes than unmodified β-CD.