Graphene, in which carbon atoms are linked in a hexagonal mesh, has a strong and stable structure. For this reason, it has long been considered impossible to cut open the “interior” of an already-assembled carbon mesh and rearrange it into a different form. Until now, chemists have built up target nanocarbons by joining small building blocks edge to edge, little by little. However, a research team at Nagoya University has overturned this half-century-old convention and established a method for directly “editing” the interior of a molecule. By applying an approach used in pharmaceutical development to giant carbon molecules, they have created unprecedented three-dimensional and functional structures.
Cutting Open a Robust Carbon Skeleton from the Inside
Nanocarbons are a general term for molecules formed when countless carbon atoms are joined together by strong covalent bonds, as typified by fullerenes and carbon nanotubes. Among these, planar nanographenes are attracting attention as a foundation for next-generation electronic devices and optical materials because of their thinness and unique electronic properties. The history of nanocarbons, beginning with the discovery of fullerenes in the 1980s, has also been a history of chemists’ ceaseless challenge: how to synthesize carbon molecules with exactly the intended shape and size.
Until now, nanocarbon synthesis has relied on a “bottom-up” strategy, much like joining jigsaw puzzle pieces together from the outer edges. Bonds at the periphery (the edges) are relatively easy to react and modify with other molecules. However, rearranging the robust mesh sitting at the center of a molecule after the fact has been extremely difficult. If one tries to cleave the tightly bonded internal carbon-carbon linkages, fatal distortion arises throughout the entire molecule, halting the reaction itself. Forcibly breaking such bonds also risked decomposing the whole molecule. As a result, the premise that the interior of a molecule is not a target for transformation had become firmly established in the chemistry world.
A team led by Associate Professor Norito Fukui of the Graduate School of Engineering at Nagoya University took on this challenge. What they focused on was “skeletal editing,” a technique that has recently drawn attention in fields such as pharmaceutical synthesis. This is a technology that pinpoints a specific part of a completed molecule’s skeleton, cleaves it, and then inserts a different atom or alters its shape. By carefully selecting appropriate starting materials and precisely controlling reaction conditions, the research team succeeded in cleaving the robust internal bonds of a nanocarbon and then repairing them anew. This is a feat comparable to reconfiguring only the interior pillars of a completed building into a different shape without demolishing its walls. In the world of nanocarbon architecture—where extension had only ever been possible from the outside—the significance of making interior renovation possible cannot be overstated.
Overcoming Strain to Create Ten-Membered Rings and Chiral Structures
With this new tool of skeletal editing in hand, structures that had previously been impossible to make began to become a reality, one after another. Foremost among these achievements is the introduction of a “ten-membered ring” composed of ten carbon atoms.
Due to constraints on bond angles, carbon in nature most readily favors forming hexagonal meshes. In recent years, structures containing seven- and eight-membered rings have also been synthesized, but incorporating a ten-membered ring into a giant nanocarbon had been an extraordinarily difficult task. It is akin to casting ripples from a ten-sided shape into the calm sea of a robust hexagonal lattice, bringing intense structural strain to the entire molecule. Nevertheless, by making full use of skeletal editing, the research team succeeded in stably incorporating this ten-membered ring into a giant molecule composed of as many as 170 carbon atoms. This represents a level of precision capable of rearranging, exactly as intended, only a specific bond deep within an extremely complex giant molecule.
This achievement carries significance that goes beyond mere structural novelty. It has opened the door to asymmetric synthesis—that is, to selectively producing “chiral” nanocarbons, which exist in mirror-image relationships like a right hand and a left hand. Chiral molecules with complex three-dimensional structures interact in distinctive ways with light and electrons. The team succeeded in synthesizing chiral nanocarbons containing ten-membered rings, selectively producing only the intended right-handed or left-handed form as desired. Reportedly, asymmetric synthesis has succeeded in such giant nanocarbons only twice before in history.
| Comparison of Synthesis Methods | Conventional Method (Bottom-Up) | New Method (Skeletal Editing) |
|---|---|---|
| Reaction Region | Molecular periphery (edges) | Internal molecular skeleton |
| Introduction of Ten-Membered Rings | Fundamentally difficult | Possible with high precision |
| Control of Three-Dimensional Structure | Only 2 prior successful cases in giant molecules | Precise selective production of right- and left-handed forms (asymmetric synthesis) possible |
A Future Where 280°C Heat Resistance and Porous Crystals Self-Assemble
The newly synthesized chiral nanocarbons possess a number of distinctive physical properties.
The first to note is their remarkably high thermal stability. Normally, three-dimensional chiral molecules bearing unreasonable strain will either collapse in shape when heated, or fail to maintain their original form and invert into the opposite enantiomer. The energy barrier that prevents this inversion is called the racemization barrier. One of the molecules created in this study maintained its chiral shape even under the harsh conditions of 280°C. To put that in perspective, 280°C exceeds even the temperature used in the soldering process for ordinary electronic circuit boards. Being able to withstand such heat means that the structure does not break down even when electrons are added to or removed from the molecule—an ideal property for long-lifespan organic electronic materials.
Molecular stability is an essential condition when designing next-generation electronic devices, particularly circuits requiring low power consumption and transistors at extremely small scales. A molecule’s resistance to breakdown directly translates into an extension of a device’s own lifespan.
The optical properties are also noteworthy. The synthesized molecules exhibited highly efficient “circularly polarized luminescence,” in which light propagates in a helical fashion. This is a property expected to find applications in next-generation 3D displays and in advanced encrypted communication technologies that utilize the rotational direction of light. It was confirmed that the twist of the molecule itself also twists the trajectory of the emitted light.
Furthermore, molecules with a double-helical shape spontaneously assembled to form porous structures containing countless microscopic pores within. This is a property that could be described as a relative of metal-organic frameworks (MOFs), which won the 2025 Nobel Prize in Chemistry. This crystal has been confirmed to function as a gas-adsorbing material capable of selectively capturing and releasing carbon dioxide. The fact that a sophisticated filter was created using carbon alone, without any metal, hints at applications in environmentally low-impact gas separation technology.
The interior of a molecule is no longer an untouchable sanctuary. With the demonstration of the skeletal editing method, chemists can now draw up blueprints for carbon materials on an entirely new dimension. Going forward, questions remain as to what other molecular structures this method can be applied to, and how challenges of scalability and mass production can be overcome. And there is the question of how the newly created, unknown nanocarbons will find their way to social implementation as low-power devices and environmental technologies. The evolution of next-generation materials woven from carbon has taken its first step into uncharted territory.