Complexation between a triptycene-derived oxacalixarene and π-extended viologens: linker-length-dependent orientation of the macrocycles in pseudo[3]rotaxanes

With an expanded electron-rich cavity and a fixed conformation, macrocycle H was found to encapsulate π-extended viologens G1-G4 to form the first case of pseudo[3]rotaxanes based on oxacalixarenes. The complexation was investigated in detail both in solution and in the solid state using NMR spectroscopy and X-ray crystallography. Due to the three-dimensional nonsymmetric structure of H, three orientational isomers of the pseudorotaxanes could be expected theoretically. However, as the crystal structure analysis revealed, only one of the three isomers was obtained with either G1 or G3. Moreover, with regard to the different lengths of the linkers in the guest molecules, completely opposite orientations of the macrocycles on the axles were observed, which could be explained by different complexation modes between the components in the pseudo[3]rotaxanes. Additionally, the complexation between the host and the guest could be reversibly switched on and off using a suitable acid and base. These results will provide us with the opportunity to design and elaborately regulate high-order molecular devices.

Unique binding behaviour of water-soluble polycationic oxacalix[4]arene tweezers towards the paraquat dication

The first water-soluble polycationic oxacalix[4]arene molecular tweezers able to recognize - under pH control - the paraquat dication as a result of a delicate balance between electrostatic repulsion, Coulombic shielding and attractive π-stacking interactions are reported.

Investigating structural alterations in pyrogallol[4]arene-pyrene nanotubular frameworks

Small-angle neutron scattering (SANS) and diffusion NMR studies are performed to investigate the stability and geometry of hydrogen-bonded pyrene-guest-containing C-hexylpyrogallol[4]arene (PgC(6) -pyrene) nanotubular frameworks in solution. In the solid state, hydrogen-bonded pyrogallol[4]arene tubes are formed; however, the scattering data for PgC(6) -pyrene assemblies in acetone are best modeled as dimeric spheres of PgC(6) with no pyrene guest. The result of diffusion NMR study also indicates the rearrangement of tubular entity into spherical framework in acetone. This is the first example of structural transformation of pyrogallol[4]arene nanotubes (guest-exo) in solution. Individual hydrogen-bonded spheres of PgC(6) exhibits a uniform radius of ca. 8.6 Å and a diffusion coefficient of 9.12 × 10(-10) m(2) s(-1) in acetone. The diffusion NMR measurements further gave, for the first time, insights into how the type of solvent (acetone vs. methanol vs. acetontitrile/D(2) O) governs the structural differences in these nanoassemblies. Solution-phase structural alteration observed for PgC(6) -pyrene gives evidence of enhanced stability of pyrogallol[4]arene nanocapsules over nanotubes.