Synthesis and characterization of constitutionally isomeric oriented calix[6]arene-based rotaxanes

Oriented rotaxanes composed of tris(N-phenylureido) calix[6]arene wheel 1 and N,N′-dialkyl viologen-based axles were synthesized in which the span between the diphenylacetyl stoppers at the wheel upper and lower rim and the bispyridinium cation portion of the axial component is different in length. A sequential synthetic pathway was followed to achieve the selective formation of orientational isomers, wherein the short (or long) chain of the axle is positioned either on the upper or lower rim of the wheel. The synthetic strategy adopted involves the upper rim threading of axles 2a–c to yield oriented pseudorotaxanes, which were then stoppered in situ. The structure of the oriented rotaxanes was unravelled by detailed NMR experiments, and their spectroscopic, electrochemical and dynamic properties were investigated by UV/Vis, cyclic voltammetry, and EPR measurements. The results were compared with those of their symmetric counterparts.