Chemical Synthesis of Carbon Nanorings and Nanobelts

Abstract

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 nanometer-sized 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 curved π-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.

Intense Molar Circular Dichroism in Fully Conjugated All‐Carbon Macrocyclic 1,3‐Butadiyne Linked pseudo‐meta [2.2]Paracyclophanes**

“…Hence, when plotting the Δɛ max values against the number of PCP subunits and compare it to a linear relationship obtained by extrapolating the values of (R p ) 3 -1 3 , no hyperbolic behaviour is observed. The Cotton effect first increases 2.3 times for (R p ) 4 -1 4 , identically to the absorption spectra, followed by only a very minor increase for (R p ) 5 -1 5 , and a strong drop for (R p ) 6 -1 6 , which noteworthy matches the value of the before mentioned linear relationship. However, this behaviour might be coincidental.…”

“…The number of reports on carbon‐based macrocyclic structures has seen a tremendous upward trend in recent years due to the promising photophysical properties and topological beauty such π‐conjugated structures exhibit [1–3] . The access to these macrocycles has also greatly improved due to the development of new synthetic methodologies [4–6] . The necessity for strain‐overcoming synthetic strategies becomes clear when considering the planar and linear conformation that para ‐substituted benzenes exhibit, which is the most prevalent building block in π‐conjugated macrocycles.…”

Synthesis and Chiral Resolution of Twisted Carbon Nanobelts

“…Recently, three types of aesthetically appealing carbon nanostructures have attracted intense interest. One is the cylinder-shaped carbon nanobelts (CNBs), which can be regarded as well-defined side-wall fragments of carbon nanotubes (CNTs). Solution-phase synthesis of this type of molecules is extremely challenging, mainly due to their large strain.…”

Molecular Dynamics Study of Collective Behavior of Carbon Nanotori in Columnar Phase

“…At present, a large number of ring nanostructures have been experimentally discovered [47][48][49]. Theoretically, their ring shape suggests similar dynamic properties in supramolecular systems.…”