Shortest Double-Walled Carbon Nanotubes Composed of Cycloparaphenylenes

Construction of a double-walled carbon nanoring

The nanoring structure of cycloparaphenylenes (CPPs) can be considered as the shortest fragment of a carbon nanotube. Herein, we successfully prepared a double-walled carbon nanoring of [6]CPP⊂[12]CPP, which can be regarded as the shortest double-walled carbon nanotube. [6]CPP⊂[12]CPP was constructed through the supramolecular assembly, and its crystallographic structure was unambiguously determined by single-crystal X-ray diffraction. The host-guest interaction and charge transfer in [6]CPP⊂[12]CPP were disclosed by UV-Vis absorption, fluorescence, and electrochemical studies. Electron paramagnetic resonance (EPR) spectroscopy disclosed the stability of the [6]CPP⊂[12]CPP cation radical, whose unpaired spin was fully delocalized on the inner [6]CPP and well protected by outer [12]CPP. Moreover, [6]CPP⊂[12]CPP shows highly enhanced photoconductivity and photocurrent under light irradiation compared to those of pristine monomers. The self-assembly behavior of [6]CPP⊂[12]CPP was also studied, and it was found that [6]CPP⊂[12]CPP molecules tend to form a square rod structure in the DMF solution. Thus, these results demonstrate that this double-walled carbon nanoring material has a great potential application in photoelectronic devices and organic semiconductors.