From Packed Sandwich to Russian Doll: Assembly by Charge-Transfer Interactions in Cucurbit[10]uril

As the host possessing the largest cavity in the cucurbit[n]uril (CB[n]) family, CB[10] has previously displayed unusual recognition and assembly properties with guests but much remains to be explored. Herein, we present the recognition properties of CB[10] toward a series of bipyridinium guests including the tetracationic cyclophane known as blue box along with electron-rich guests and detail the influence of encapsulation on the charge-transfer interactions between guests. For the mono-bipyridinium guest (methylviologen, MV2+), CB[10] not only forms 1:1 and 1:2 inclusion complexes, but also enhances the charge-transfer interac tions between methylviologen and dihydroxynaphthalene (HN) by mainly forming the 1:2:1 packed sandwich complex (CB[10]center dot 2MV(2+)center dot HN). For guest 1 with two bipyridinium units, an interesting conformational switching from linear to U shape is observed by adding catechol to the solution of CB[10] and the guest. For the tetracationic cyclophane-blue box, CB[10] forms a stable 1:1 inclusion complex; the two bi-pyridinium units tilt inside the cavity of CB[10] according to the X-ray crystal structure. Finally, a supramolecular Russian doll was built up by threading a guest through the cavities of both blue box and CB[10].