Interaction between cucurbit[8]uril and viologen derivatives

The interaction between cucurbit8]uril (Q8]) 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 cucurbit7]uril (Q7]) and these guests. The experimental results revealed that the binding site of the guests by Q8] depended strongly on the length of the aliphatic substituents on the 4,4′-bipyridinium nucleus. While a 1:2 complex was observed for Q8]-BPY²⁺ under acidic conditions, a 1:1 complex was formed for Q8]-viologen derivatives with chains shorter than four carbon atoms. However, multiple Q8] molecules could be threaded on the longer-chain FV²⁺, HV²⁺ or SV²⁺ molecules to form 2:1 and even possibly 3:1 complexes.