Development of New Synthetic Methods for Carbon Nanohoops

Abstract

Carbon nanotubes have received much attention from the material chemists after the synthesis and structure analysis by Iijima in 1991. It was envisioned that cycloparaphenylenes (CPPs), which comprise a sequence of benzene units connected at para position to make a cyclic ring structure, are possible templates for growing carbon nanotubes of a single chirality and single diameter for nanotechnology application. Recently, chemists turned their attention to synthesizing aromatic carbon nanobelts proposed as a general term for carbon molecules composed of fused benzene rings in cylindrical structure shape. Cyclophenacenes (CPAs) as carbon nanobelts are expected to be more suitable candidates for growing carbon nanotubes because it is anticipated that CPAs would be thermally more robust. In this investigation, an alternative and efficient pathway of preparing 4,4'-dibromo-3,3'-dimethyl(1,1'-biphenyl)-2,2'-dicarboxylic acid was developed, leading to 2,7-dibromo-1,8-bis(bromomethyl)phenanthrene containing suitable functional groups as a potential building block for CPAs. The presence of the functional groups provides the opportunity to connect multiple phenanthrene units together to form macrocycles bearing the carbon frameworks of CPAs. A synthetic pathway was developed using 2,7-dibromo-1,8-bis(bromomethyl)phenanthrene as a building block to form a macrocycle bearing two 2,7-dibromophenanthrene units connected via two dibenzylamine linkages at C1 and C8 positions. The structure of the macrocycle having two dibenzylamine linkages is not planar and the two phenanthrene units are nonequivalent on the NMR time scale at 25 °C. The DFT calculations and temperature-dependent NMR studies indicate that the two phenanthrene units preferred a conformation with one phenanthrene unit oriented perpendicular to the other unit.