Chemistry and Application of Methylene-bridged Cycloparaphenylenes

Cyclic molecular nanocarbons represented by carbon nanorings and nanobelts are recent emerging molecules in π-conjugated chemistry and materials science. Since the synthesis of cycloparaphenylene (CPP), which is the simplest nanoring, has been achieved in 2008, creation of diverse types of nanorings and their unique properties have been reported in this decade. In contrast to nanorings, the molecules called nanobelts have structures where aromatic rings are fused to be tubular shape. A number of chemists have attempted the synthesis of nanobelts from long time ago even before that of CPP; however, it has never been achieved until 2017. The breakthrough synthesis of (6,6)carbon nanobelt has led huge progress of nanobelt chemistry in recent years. On the other hand, synthesis of nanobelts often requires multiple reaction steps resulting in low yields of target products. These drawbacks have prevented to produce large amount of nanobelts and derivatives with diverse belt sizes. In 2020, we have developed a new nanobelt, methylene-bridged [6]cycloparaphenylene ([6]MCPP), where neighboring phenylene units in [6]CPP are respectively connected by a methylene moiety. The synthesis of [6]MCPP needs only three steps from readily available pillar [6]arene, which has enabled its rapid commercialization. [6]MCPP has unique properties such as the smallest diameter and small HOMO-LUMO gap, encouraging us to further study MCPPs. This article describes the chemistry and applications of methylene-bridged nanobelts including other belt sizes of MCPPs, methylene-bridged cyclonaphthylene (MCN) and a water-soluble [6]MCPP derivative.