Carbon Nanobelts:Brief History and Perspective

Carbon nanotubes(CNTs)are an emerging nanomaterial because of their outstanding performance in various applications.In recent years,the segment molecules of CNTs,referred to as carbon nanorings(CNRs)or carbon nanobelts(CNBs),have gained attention for their unique structures and properties,as well as their potential as seed molecules for the precise synthesis of CNTs.CNBs are rigid and thick segments of CNTs whose synthesis has been addressed by scientists fascinated by the uniqueness of CNBs long before the discovery of CNTs.After 60 years of efforts by synthetic chemists all over the world,the synthesis of the first CNB,(6,6)CNB,was achieved by our group in 2017.Since this milestone,diverse types of nanobelts have been synthesized through various synthetic routes,thereby demonstrating their photophysical,magnetic,and redox properties derived from rigid belt structures.The applications of CNBs have also been introduced recently.The formation of the host−guest complex,transformation to three-dimensional molecules,and measurement of conductivity have been reported for CNBs.This paper summarizes the brief history and perspective of CNBs.Further synthetic campaigns and aggressive application of CNBs would create novel and groundbreaking scenes in materials science.

Chemical Synthesis of Carbon Nanorings and Nanobelts

CONSPECTUS:The discovery and creation of new forms of carbon have always transformed the scientific landscape.For example,the discoveries of fullerenes,carbon nanotubes(CNTs),and graphenes have opened doors to the science of nanometersized carbon allotropes,otherwise known as nanocarbons.Since then,researchers worldwide have unveiled their outstanding physical and chemical properties,and a number of applications and technologies have arisen in not only materials science but also biological research fields.The synthesis and study of this privileged class of“single-molecule”compounds has become one of the most engaging subjects in chemistry and holds huge promise to establish new fields in molecular science.However,there have been huge gaps between established small-molecule chemistry and nanocarbon science.In the particular case of CNTs,it is still not possible to access structurally uniform CNTs.Although a wide range of synthetic methods have been reported,CNTs are generally accessed as a mixture of various structures.One logical strategy to achieve full synthetic control over CNTs is to build up from a template molecule with structural precision(the so-called“growth-from-template”strategy),where a short CNT segment molecule represents an initial synthetic target.To this end,organic synthesis techniques are our most powerful tools to synthesize short CNT segments such as carbon nanorings and carbon nanobelts.This Account highlights our 16-year campaign in the synthesis and application of carbon nanorings and carbon nanobelts.The first topic is the synthesis of carbon nanorings(cycloparaphenylenes)as substructures of CNTs.The second topic is the synthesis of armchair and zigzag carbon nanobelts,which consist solely of fully fused hexagonal rings and provide a continuous double-stranded cylindrical framework.The third topic is the synthesis of methylene-bridged cycloparaphenylene,an aromatic belt containing nonhexagonal rings,in which the cyclic paraphenylene chain is ladderized by methylene bridges.This nonalternant aromatic belt can be regarded as segments of nonconventional CNTs.During our extensive investigation,we found that the careful design of strainless macrocyclic precursors is crucial to the success of the synthesis of these curvedπ-conjugated nanorings and nanobelts.In the final section,some of the representative size-dependent properties of these nanorings/belts,including their HOMO−LUMO energies,strain energies,and photophysical properties,are summarized.In addition to basic properties,the utilization of these compounds as supramolecular hosts and organic materials is also briefly introduced.We hope this Account will inspire the development of new forms of nanocarbon molecules that would open doors to new fields and applications.

Side-Chain Type Ferrocene Macrocycles

A class of side-chain type ferrocene macrocycles with a radially conjugated system is introduced in this study.The stereo configurations of these ferrocene rings were determined through single-crystal X-ray diffraction analysis.Notably,in the solid state,the ferrocene rings exhibit a distinctive herringbone stacking pattern imposed by a ferrocene-to-ring host−guest interaction.Through UV−vis absorption spectroscopy,electrochemical measurements,and theoretical calculations,valuable insights into the electronic properties of these rings were obtained.In addition,the single crystal of macrocycle A_(2)B demonstrates a second-order nonlinear optical response.As a class of organometallic nanorings,this work holds great potential for further exploration in the fields of organometallic chemistry,molecular electronics,and host−guest chemistry.