Refolding foldamers: triazene-arylene oligomers that change shape with chemical stimuli

We describe the preparation of five triazene-arylene oligomers (3, 4, 7, 8, and 11) and investigations of their folding properties in aqueous solution. These oligomers contain four 2-fold rotors and populate a conformational ensemble comprising at least 10 states. Extensive 1D and 2D NMR studies as well as X-ray crystallography establish that the presence of three members of the cucurbitn]uril family (CBn]), CB10], CB7], and CB8], results in the selective population of the (a,a,a,a)-, (a,s,s,a)-, and (a,a,a,s)-conformers. As a result of the high affinity and highly selective binding properties of the CBn] family, it is possible to fold a single foldamer strand (3) into the CB8].(a,a,a,s)-3 conformer by the addition of CB8], then unfold and refold it into the CB7].(a,s,s,a)-3.CB7] conformer by addition of CB7] and 3,5-dimethylaminoadamantane (17), then unfold and refold it again into the CB10].(a,a,a,a)-3 conformer by addition of CB10].CB5] and aminoadamantane (18). The transformation of CB8].(a,a,a,s)-3 into CB7].(a,s,s,a)-3.CB7] proceeds through the intermediacy of CB 8].(a,a,s,a)-3.CB7], which enhances the rate of dissociation of strand 3 from CB8].