A Double-walled Noncovalent Carbon Nanotube by Columnar Packing of Nanotube Fragments

Herein, we report the synthesis of double-walled noncovalent carbon nanotubes (CNTs) through host-guest complexation of nanotube fragments and tube-forming crystal engineering. As the smallest fragment of double-walled CNTs, a host-guest complex of perfluorocycloparaphenylene (PFCPP) and carbon nanobelt (CNB) was synthesized by mixing them in solvents. The immediate complexation of the PF[12]CPP⸧(6,6)CNB complex with a remarkably high association constant (K_a) of 2×10⁵ L/mol was observed. Time-dependent ¹H NMR and thermogravimetry measurements revealed that the stability of (6,6)CNB was significantly improved by encapsulation. X-ray crystallography confirmed the robust belt-in-ring structure of this complex. As indicated by the short distance between PF[12]CPP and (6,6)CNB (2.8 Å), intermolecular orbital interactions exist between the belt and the ring, which were further supported by theoretical calculation and phosphorescence quenching experiments. While the PF[12]CPP⸧(6,6)CNB complex adopts various crystal packing structures, chloroform was discovered to be a magic "glue" solvent inducing one-dimensional alignment of the PF[12]CPP⸧(6,6)CNB complex to build an unprecedented double-walled noncovalent CNT structure.