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 cucurbit[n]uril family (CB[n]), CB[10], CB[7], and CB[8], 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 CB[n] family, it is possible to fold a single foldamer strand (3) into the CB[8]·(a,a,a,s)-3 conformer by the addition of CB[8], then unfold and refold it into the CB[7]·(a,s,s,a)-3·CB[7] conformer by addition of CB[7] and 3,5-dimethylaminoadamantane (17), then unfold and refold it again into the CB[10]·(a,a,a,a)-3 conformer by addition of CB[10]·CB[5] and aminoadamantane (18). The transformation of CB[8]·(a,a,a,s)-3 into CB[7]·(a,s,s,a)-3·CB[7] proceeds through the intermediacy of CB [8]·(a,a,s,a)-3·CB[7], which enhances the rate of dissociation of strand 3 from CB[8].