Synthesis of covalent organic pillars as molecular nanotubes with precise length, diameter and chirality

The construction of nanotubes with well-defined structures, although synthetically challenging, offers the prospect of studying novel chemical reactions and transportation within confined spaces, as well as fabricating molecular devices and nanoporous materials. Here we report a discrete molecular nanotube, namely the covalent organic pillar COP-1, synthesized through a [2+5] imine condensation reaction involving two penta-aldehyde macrocycles and five phenylenediamine linkers. A pair of enantiomeric nanotubes, obtained in a quantitative and diastereoselective manner, were characterized and resolved readily. NMR spectroscopy, isothermal titration calorimetric and X-ray crystallographic studies revealed that the 2-nm-long and 4.7-Å-wide one-dimensional channel inside COP-1 can accommodate α,ω-disubstituted n-alkyl chains with complementary lengths and electron density distributions. Furthermore, in a length-mismatched host-guest pair, we found that the nonamethylene dibromide thread not only displays a diminished binding constant in solution, but adapts an energetically unfavoured gauche conformation inside COP-1 in the solid state.