Cycloparaphenylenes and related nanohoops

The first synthesis of a cyclic oligophenylene possessing a radial π system was reported in 2008. In the short period that has elapsed since, there has been an ever-increasing level of interest in molecules of this type, as evidenced by the volume of publications in this area. This interest has been driven by the highly unusual properties of these molecules in comparison to their linear oligoarene analogues, as well as the diverse array of potential applications for them. Notably, CPPs and related structures were proposed as viable templates for the bottom-up synthesis of single-walled carbon nanotubes (SWCNTs), a proposition which has recently been realised. This review gives a comprehensive and strictly chronological (by date of first online publication) treatment of literature reports from the inception of the field, with emphasis on both synthesis and properties of CPPs and related nanohoops. (The scope of this review is restricted to molecules possessing a radial π system consisting entirely of subunits which are aromatic in isolation, e.g. CPPs, but not cycloparaphenyleneacetylenes or cyclopolyacetylenes). Cyclic oligo- para -phenylenes ("CPPs") possessing radial π systems have markedly different properties to the analogous linear oligo- para -phenylenes. Their synthesis is challenging due to the strain inherent in their structures, but several distinct strategies to access them have now been described. This review describes the synthesis, properties and applications of CPPs and related variants.