Viologens are among the most studied guests for cucurbituril (CB) and their complexation is usually driven by bipyridyl core inclusion inside the cavity to maximize both hydrophobic and cation-dipole interactions. The presence of alkyl substituents on the guest alters this complexation mode, switching to aliphatic chain inclusion in U-folded conformation. Here we report a thorough study of the influence of the alkyl chain length on the binding mode of methyl alkyl viologens. The chain length of the studied guests was increased by two methylene groups starting from methyl dodecyl viologen (MVC12) to the octadecyl analogue (MVC18). Complexation in water, investigated by NMR spectroscopy and ITC, revealed a clear switch from 1:1 to 2:1 stoichiometry moving from 12 to 16 carbon atoms, as a consequence of the chain folding of the major portion of the longer alkyl chain in one CB cavity and the inclusion of the full viologen unit by another host molecule. The CB2•MVC18 complex crystal structure evidences the unprecedented 2:1 stoichiometry and quantified in 12 the number of carbon atoms necessary to fill the CB cavity in U-shaped conformation.
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