Differences between the Interactions of Linear and Tetrahedron-like Ditopic Guests with Cucurbit[8]uril: Steric Hindrance and Molecular Structure Play Dominant Roles

A tetrahedron-like guest molecule (THV) with four ditopic arms, consisting of viologen (V) and hexylene moieties, was synthesized, and its interaction with cucurbit[8]uril (CB[8]) was investigated. As a reference, the interaction between CB[8] and an analogue of the arm of the linear ditopic guest (LHV) was first explored by ¹ H NMR and UV/Vis spectroscopy. In this case, CB[8] was located on the hexylene moiety and resulted in the formation of a binary complex. However, when THV interacted with CB[8], despite having four arms with four hexylene moieties, only two CB[8] molecules were identified as located on the hexylene moieties; the other two CB[8] molecules resided on the V moieties. Great steric hindrance among the arms played a dominant role in the interaction between THV and CB[8]. Regardless of whether the guest molecule was THV or LHV, CB[8] moved to its V moiety (electron-deficient part) to form the charge-transfer complex upon the addition of a stopper containing a naphthol residue (electron-rich part). Guests LHV and THV were reduced to LHV^+(+.) and THV^+4(+.) upon the addition of sodium dithionite. Dimerization of LHV^+(+.) was enhanced when CB[8] was present in less than 0.5 equivalents. In contrast, dimerization was suppressed when CB[8] was present in more than 0.5 equivalents. However, the V^+. units of THV^+4(+.) stacked intramolecularly because of the tetrahedron-like molecular structure of THV. The influence of CB[8] on the dimerization of THV^+4(+.) was weaker than that on LHV^+(+.) .