The cucurbit[n]uril family: prime components for self-sorting systems

We determined the values of Ka for a wide range of host-guest complexes of cucurbitn]uril (CBn]), where n = 6-8, using 1H NMR competition experiments referenced to absolute binding constants measured by UV/vis titration. We find that the larger homologues--CB7] and CB8]--individually maintain the size, shape, and functional group selectivity that typifies the recognition behavior of CB6]. The cavity of CB7] is found to effectively host trimethylsilyl groups. Remarkably, the values of Ka for the interaction of CB7] with adamantane derivatives 22-24 exceeds 10(12) M(-1)! The high levels of selectivity observed for each CBn] individually is also observed for the CBn] family collectively. That is, the selectivities of CB6], CB7], and CB8] toward a common guest can be remarkably large. For example, guests 1, 3, and 11 prefer CB8] relative to CB7] by factors greater than 10(7), 10(6), and 3000, respectively. Conversely, guests 23 and 24 prefer CB7] relative to CB8] by factors greater than 5100 and 990, respectively. The high levels of selectivity observed individually and collectively for the CBn] family renders them prime components for the preparation of functional biomimetic self-sorting systems.