Binding modes of cucurbit[6]uril and cucurbit[7]uril with a series of bis-pyridinium compounds

Binding behaviors of cucurbit6]uril (CB6]) and cucurbit7]uril (CB7]) with a series of bis-pyridinium compounds N, N’-hexamethylenebis(1-alkyl-4-carbamoyl pyridinium bromide) (HBPB-n) (alkyl chain length, n = 6, 8 and 10) guests were investigated using ¹H-NMR, ESI–MS and single crystal X-ray diffraction methods. The results show that CB6] and CB7] can form 2]pseudorotaxanes with HBPB-n easily. When increasing the length of tail alkyl chain, the binding site of CB6] at guest molecules changed from the tail to the middle part, while CB7] remained located over the tail chain. As CB6] and CB7] were added in HBPB-8 aqueous solution, a 3]pseudorotaxane was formed by the inclusion of the internal middle site in CB6] and the tail chain in CB7].