Toward Directionally Controlled Molecular Motions and Kinetic Intra- and Intermolecular Self-Sorting: Threading Processes of Nonsymmetric Wheel and Axle Components

We have investigated the self-assembly of pseudorotaxanes composed of viologen-type axle and calix[6]arene wheel components. The distinctive feature of this system is that both components are structurally non-symmetric; hence, their self-assembly can follow four distinct pathways and eventually give rise to two different orientational pseudorotaxane isomers. We found that the alkyl side chains of the viologen recognition site on the molecular axle act as strict kinetic control elements in the self-assembly, thereby dictating which side of the axle pierces the calixarene cavity. Specifically, non-symmetric axles with alkyl side chains of different length thread the wheel with the shorter chain. Such a selectivity, in combination with the face-selective threading of viologen-type axles afforded by tris(N-phenylureido)calix[6]arenes, enables a strict directional control of the self-assembly process for both the face of the wheel and the side of the axle. This kinetic selectivity allows both intramolecular self-sorting between two different side chains in a non-symmetric axle, and intermolecolar self-sorting among symmetric axles with alkyl substituents of different length.