Tunable enforced cavities inside self-assembled capsules

Controlling and tuning the molecular recognition properties is a crucial task in host-guest chemistry. The size and dimension of the guest-binding space inside self-assembled capsules 1a-c is successfully determined by installing the substituents at the 6'-position of 2,2'-bipyridyl arms. X-ray diffraction analysis and DFT calculations at the M06-2X/6-31G(d, p)+ LanL2DZ level of theory demonstrate that the pi-methoxyphenyl group expands the dihedral angle of the 2,2'-bipyridyl arms through its pi-stacking interaction to the 2,2'-bipyridyl arm, whereas the steric interaction of the isopropyl group to the neighboring bipyridyl arm slightly reduces the dihedral angle of the two bipyridyl groups. The Cu(I)-united 2,2'-bipyridyl arms function as a hinge; accordingly, installing the pi-methoxyphenyl group extends the cavity by ca. 2 A, whereas the isopropyl group shrinks the cavity more than that of the capsule 1a with methyl groups at the 6'-positions. These steric interactions influence the molecular recognition of the capsules 1a-c for rigid ditopic guest 2a as well as flexible ones 2b-f. The guest selectivity inside the enforced cavity is determined by varying the substituents.