The Smallest “Carbon Nanoring”: [5]Cycloparaphenylene ([5]CPP)

A cycloparaphenylene (CPP), the so-called carbon nanoring, has a cyclic structure formed by linkages of p-substituted benzenes. The CPP attracted researchers in fundamental chemistry and material science, because it is a unit structure of a CNT. In fact, Itami et al. successfully synthesized a uniform CNT by a bottom-up procedure starting from CPP as a template.¹⁾ A CPP also receives attention as a molecular host material, because the CPP with a specified diameter forms an inclusion complex with fullerene.²⁾

Recently, this research area has been extended to synthesize a CPP of smaller diameter having large distortion. Yamago^3,4) and Jasti⁵⁾ groups independently reported synthesis of the [5]CPP (1), which has been the smallest CPP so far. It is expected that a bottom-up synthesis of the smallest diameter CNT may be possible by 1 as a template material. 1 is also a unit structure of C₆₀ with a similar diameter (0.7 nm) to the fullerene. 1 has narrow HOMO-LUMO gap that is comparable with C₆₀. 1 is nicely soluble in an organic solvent.