Regulated single‐axis rotations of a carbonaceous guest in a van der Waals complex with an entropy cost

In a tight host-guest complex assembled solely by nondirectional van der Waals forces, unique motions of the guest, such as solid-state inertial rotations, emerge. The regulation of dynamic motions is an important element to be explored for novel functions of such complexes, which may be seemingly difficult to achieve because of the nondirectionality of the assembling forces. A regulated, single-axis rotation was made possible by choosing an appropriate shape of the guest in the tubular host. Specifically, an ellipsoidal guest was made to stand along a cylinder axis of the host, which consequently resulted in single-axis rotations of the guest in the solid. The rotational frequency was considerably high for solid-state rotations but was suppressed to 10 GHz, which was 1/20 of the isotropic rotation of a spherical guest. In-depth kinetic analyses quantitatively revealed that the entropy cost was a determining factor that regulated the dynamics.