Inclusion of 4-pyrrolidinopyridine derivatives in a symmetrical α,α′,δ,δ′-tetramethyl-cucurbituril and a Ba2+-driven pseudorotaxane with characteristic UV absorption changes†
The host–guest interactions between a symmetrical α,α′,δ,δ′-tetramethyl-cucurbituril and three 4-pyrrolidinopyridine derivatives, specifically 4-pyrrolidinopyridine, N-butyl-4-pyrrolidinopyridine, and N-amyl-4-pyrrolidinopyridine, have been investigated both in aqueous solution and in the solid state using NMR spectroscopic methods, electronic absorption spectroscopy, isothermal titration calorimetry (ITC), and single-crystal X-ray diffraction analysis. In aqueous solution, TMeQ forms 1 : 1 inclusion complexes with these guests. Generally, TMeQ preferentially accommodates the pyridyl moiety of these guests, which was confirmed by the crystal structures of its complexes with 4-pyrrolidinopyridine and N-butyl-4-pyrrolidinopyridine. Moreover, TMeQ can form a thermodynamic inclusion complex with 4-pyrrolidinopyridine at elevated temperatures. The unthreading and rethreading of the TMeQ ring can simulate a molecular-level plug/socket system, that can be reversibly driven by Ba2+/SO42−, and is accompanied by obvious changes in the UV absorption spectrum, both in wavelength and intensity.