Abstract
The complexation of three guests containing 4,4′‐bipyridinium redoxactive 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 103–104M−1, while the cation radical forms exhibited intermediate binding affinities (≈︁102M−1). In all cases, DM‐β‐CD was found to form more stable complexes than unmodified β‐CD.
Redox switchable and highly stable inclusion complexes are formed between β‐cyclodextrin or its heptakis(2,6‐di‐O‐methyl) analogue and the two‐electron reduced forms of a series of viologen guests functionalized to increase their aqueous solubility in all three oxidation states (see Scheme). These complexes exhibit a pseudorotaxane structure and the unusual feature that the cyclodextrin includes an electrochemically generated subunit, which, therefore, is highly reactive.
