The interaction between cucurbituril (Q) and a series of symmetric viologen derivatives having aliphatic substituents of variable length [N,N'-dialkyl-4,4'-bipyridinium dianions; alkyl = CH₃(CH₂)n–,n = 0 (MV²⁺), 1 (EV²⁺), 2 (PV²⁺), 3 (BV²⁺), 4 (FV²⁺), 5 (HV²⁺) or 6 (SV²⁺); BPY²⁺ = diprotonated 4,4-bipyridine], determined by ¹H NMR and electronic absorption spectroscopy methods, is described. Some different binding models were observed in this work when compared to the interactions between cucurbituril (Q) and these guests. The experimental results revealed that the binding site of the guests by Q depended strongly on the length of the aliphatic substituents on the 4,4'-bipyridinium nucleus. While a 1:2 complex was observed for Q-BPY²⁺ under acidic conditions, a 1:1 complex was formed for Q-viologen derivatives with chains shorter than four carbon atoms. However, multiple Q molecules could be threaded on the longer-chain FV²⁺, HV²⁺ or SV²⁺ molecules to form 2:1 and even possibly 3:1 complexes.
Journal of Inclusion Phenomena and Macrocyclic Chemistry Vol. 61, Issue 1-2, p. 131-138