Topologically Unique Molecular Nanocarbons.

As new forms of carbon are unearthed, they invariably transform the scientific landscape. Numerous researchers have been inspired to discover the unique characteristics of these fascinating materials, consistently leading to the development of important technological innovations in materials science. Recently, studies on the preparation of molecular nanocarbons (small molecule analogues of larger carbon nanostructures) by precision organic synthesis have attracted much attention. Cycloparaphenylene (CPP), the substructure of carbon nanotubes (CNTs), is the oldest of such organic molecules, and since 2008 the successful synthesis of CPP dramatically advanced the synthetic chemistry of molecular nanocarbons. In fact, as pioneering research, we succeeded in producing carbon nanotubes using seed CPP molecules in 2013. This method represented an important landmark in the quest for controlling the diameter of CNTs via utilization of a well-defined small molecule as a template. Other avenues of research on graphene nanoribbons and partial structures of fullerenes such as corannulene and sumanene are also highly active at the current time. On the other hand, carbon forms with nontrivial topologies, i.e., topological nanocarbons, are virtually unexplored. In addition to the 3D network structures represented by the Mackay crystal, many topologically complex structures have been envisioned. To date, there is no rational approach toward the bottom-up synthesis of these carbon structures. As with the case of fullerenes and CNTs, access to these unique carbon structures should undoubtedly revolutionize a wide range of sciences. This Account highlights our efforts toward the synthesis of topologically unique molecular nanocarbons. Starting from CPP as the topologically simple subunit, we have successfully created novel molecular nanocarbons that have more complexed topologies. The first topic is carbon nanobelts, fully fused cylinder-shaped molecular nanocarbons representing the segment structure of armchair-type CNTs. The second topic is carbon nanocages, molecular nanocarbons having a "three-holed" topology representing the joint unit of branched CNTs. The third and fourth topics are all-benzene catenanes consisting of two CPP rings and an all-benzene trefoil knot topologically related to a carbon nanotorus. The world of nanocarbon molecules is only limited by our imagination and creativity. As history has proved, the synthesis of new forms of carbon and topologically complex molecules has always subsequently led to new fields and applications associated with their unforeseen properties and functions.

Quantum Interference Enhanced Chemical Responsivity in Single-Molecule Dithienoborepin Junctions.

Providing a chemical control over charge transport through molecular junctions is vital to developing sensing applications at the single-molecule scale. Quantum-interference effects that affect the charge transport through molecules offer a unique chance to enhance the chemical control. Here, we investigate how interference effects can be harnessed to optimize the response of single molecule dithienoborepin (DTB) junctions to the specific coordination of a fluoride ion in solution. The single-molecule conductance of two DTB isomers is measured using scanning tunneling microscopy break-junction (STM-BJ) before and after fluoride ion exposure. We find a significant change of conductance before and after the capture of a fluoride ion, the magnitude of which depends on the position of the boron atom in the molecular structure. This single-molecule sensor exhibits switching ratios of up to four orders of magnitudes, suggesting that the boron-fluoride coordination can lead to quantum-interference effects. This is confirmed by a quantum chemical characterization, pointing toward a cross-conjugated path through the molecular structure as the origin of the effect.

Thiophene-fused borepins as directly functionalizable boron-containing π-electron systems.

Synthetic protocols were developed for the gram-scale preparation of two isomeric dithienoborepins (DTBs), boron-containing polycyclic aromatics featuring the fusion of borepin and thiophene rings. DTBs exhibit reversible cathodic electrochemistry and boron-centered Lewis acidity in addition to enhanced electronic delocalization relative to benzo-fused analogues. Boron's precise position within the conjugation pathway of DTBs significantly affected electronic structure, most clearly demonstrated by the variation in spectroscopic responses of each isomer to fluoride ion binding. In addition to excellent stability in the presence of air and moisture, DTBs could also be subjected to electrophilic aromatic substitution and metalation chemistry, the latter enabling the direct, regiospecific functionalization of the unsubstituted thiophene rings. Subsequent tuning of molecular properties was achieved through installation of donor and acceptor π-substituents, leading to compounds featuring multistep electrochemical reductions and polarizable electronic structures. As rare examples of directly functionalizable, π-conjugated, boron-containing polycyclic aromatics, DTBs are promising building blocks for the next generation of organoboron π-electron materials whose development will demand broad scope for molecular diversification in addition to chemical robustness.

Meta-B-entacenes: new polycyclic aromatics incorporating two fused borepin rings.

The synthesis of new boron-containing acenes (meta-B-entacenes) is reported. These compounds exhibit slightly non-planar core geometries with blue-shifted spectral properties and more negative electrochemical reduction potentials relative to known para isomers. Polarizable π-extended architectures were realized via cross-coupling procedures with chloro-functionalized precursors.