The world of molecular nanocarbon science is only limited by our imagination and creativity, and we have a bright future ahead of us!

The world of molecular nanocarbon science is only limited by our imagination and creativity, and we have a bright future ahead of us! Carbon is one of the central and essential elements supporting all life on earth. The structure and alignment of carbon atoms in molecules have large effects on their properties. Looking back in history, the discovery and creation of new forms of carbon and hydrocarbons have always opened doors to new science and technology. Diamond and graphite are classic examples, and during the last three decades nanometer-sized allotropes of carbon, otherwise known as nanocarbons, have emerged and completely transformed the landscape of carbon-based materials.(1) For example, spherical fullerene C₆₀ was discovered in 1985(2) and cylindrical carbon nanotubes (CNTs) were discovered in 1991.(3) In 2004, a new form of nanocarbon, graphene, was isolated as a single layer from graphite and quickly gained much attention from around the world.(4) Nanocarbons have potential applications in nearly all areas of material science with particular promise in organic electronics and biology.(1) The importance of these elegant carbon structures was recognized by the awarding of Nobel Prizes for the discovery of fullerene and graphene in 1996 and 2010, respectively. Nanocarbons are the most promising materials of the future; the discovery of new forms of carbon will open new doors to advanced technologies and many exciting developments in this area are expected. While we have appreciated the various discoveries and applications of many of these nanocarbons, we recognize that there remain two critically important problems (unmet needs) in the field: the “mixture” problem and “unsynthesized” problem.(5) In the field of nanocarbon science, the synthesis of structurally uniform, single-molecule nanocarbons is the greatest challenge and is crucial for the development of functional materials in nanotechnology, electronics, optics, and biomedical applications.(5) At present, however, synthetic routes to nanocarbons usually lead to mixtures of molecules with a range of different structures and properties, and these cannot be easily separated or refined into pure forms. One-dimensional nanocarbons such as CNTs and graphene nanoribbons (GNRs) have suffered from this “mixture” problem for some time; physical synthetic methods cannot access structurally uniform CNTs and GNRs.(5) The only logical way to achieve full synthetic control over these structures is to draw inspiration from organic synthesis, where a target molecular entity is built up from a template (seed) molecule with structural precision. To achieve this intuitive “growth-from-template” strategy, the synthesis of target seed molecules and the development of precise template-elongating chemical reactions are needed. In the last 10–15 years, there have been significant achievements in the synthetic chemistry field toward this end. For example, a number of long sought-after CNT partial structures such as carbon nanobelts (fully fused ultrashort CNTs),(6) carbon nanorings (cycloparaphenylenes),(7) and CNT end-caps(8) have been unearthed, enabled by precision organic synthesis methodology. Moreover, these small molecular nanocarbons can be used as initiators (templates) to grow into structurally well-defined CNTs by somewhat harsher “physical” methods, which represent an important landmark in the quest to control the structure of CNTs.(9,10) It should be noted that the synthesis of carbon nanobelts and nanorings has been one of the holy grails in chemistry, even before the discovery of CNTs in 1991. These highly strained and unique molecular structures are of great interest in their own right, providing significant opportunity in a range of applications.(11) New molecular nanocarbons have fundamentally transformed the scientific landscape and inspired legions of forward-thinking researchers; the discovery of new function is simply a natural outcome of the intense research stimulated by these fascinating materials. GNRs, 1D graphene strips, are another newer form of carbon that are attracting significant attention. Like in the case of CNTs, the properties of GNRs, such as conductivity/semiconductivity, charge mobility, and on/off ratio, depend heavily on their width, length, and edge structure.(12) Recently, there has been significant progress in accomplishing the controlled and predictable synthesis of structurally well-defined GNRs.(12) A worldwide campaign by chemists and physicists has resulted in the development of powerful new methodologies, such as on-surface polymerization(13) and living annulative π-extension polymerization,(14) rendering custom-made precise synthesis of a range of GNRs possible. Another problem in nanocarbons is the “unsynthesized” problem. In a sense, the world of nanocarbons can be compared to the art of origami. Small manipulations can bring alternate forms and functions. Indeed, many nanocarbon structures have been predicted by theoreticians and mathematicians but are yet to be synthesized. Unlike paper, however, it is extremely difficult to synthesize these atomic-scale nanocarbons, and controlling their shape and structure with atom-by-atom precision holds even greater challenges. Those currently hypothetical, wondrous nanocarbon structures include 3D periodic carbon crystals with negative Gaussian curvatures (Mackay crystals)(15) and carbon nanotorus. Although many of the actual properties of these frameworks are still in question, most nanocarbon studies have historically shown that the emergence of distinct geometries and morphologies of carbon leads to the discovery of functions and applications that are neither initially predicted nor expected. Therefore, we believe that there is a great motivation of making “unsynthesized” nanocarbons for future breakthroughs in science and technology. Although there is no rational approach toward the synthesis of these unexplored carbon structures at present, some significant progress was made by applying organic synthesis techniques. In the past decade, a variety of 3D nanographenes with negative Gaussian curvatures have been reported, for example, warped nanographene,(16) saddle-shaped nanographene,(17) and twisted nanographene.(18) It is also notable that the synthesis of distinct all-benzene nanorings comprising catenane and trefoil-knot structures has been accomplished recently.(19) These molecules represent cornerstone objects for topological molecular nanocarbons. In this Viewpoint, we have pointed out the two problems arising from current nanocarbon science and introduced significant progress and breakthroughs toward solving each. By applying the principle of precision organic synthesis, we are now steadily approaching the ultimate goals of accessing structurally uniform nanocarbons and creating new nanocarbon forms. However, there are several challenges for pushing this field further forward. The accessible sizes and shapes of molecular nanocarbons are still limited and most of the synthesis still requires lengthy synthetic protocols and complicated experimental operations. In addition, structural analysis of molecular nanocarbons often involves challenges in the determination of their structures with atomic precision, particularly in the case of scaffolds that have never existed before. Thus, the establishment of new methodologies and techniques for molecular synthesis and structural analysis will remain of vital importance. In addition to their synthesis, we believe that the next 10–20 years of molecular nanocarbon science will see significant progress in a wide range of applications including optoelectronic devices and biomedical functions. Despite several molecular nanocarbons in hand, there is no reliable method to fine-tune their functionalities for suitability in each application. The development of techniques to modify molecular nanocarbons with atomic precision will significantly contribute to expanding the range of applications. Moreover, we expect that the integration of techniques for synthesis, analysis, and modification of nanocarbons will lead to new fields and applications associated with their unforeseen properties and functions. Molecular nanocarbons have gradually become not only a new standard to bring out the full potential of carbon materials but also an opportunity for various researchers to reaffirm the fundamental importance and impact of conducting science with molecules. Novel functions will emerge from pure, perfect, and often beautiful structures. The impact and possibilities that molecular nanocarbon science can bring are similar to those of the discovery of the molecular structure of DNA, which has brought dramatic advances to molecular biology. The authors declare no competing financial interest. We thank all the present and past members of the Itami group for exploring molecular nanocarbon science and sharing excitement during the last 15 years and Dr. Iain A. Stepek for fruitful comments. This work was supported by JST ERATO Grant JPMJER1302 (K.I.), Japan and JSPS KAKENHI Grant 19H05463 (K.I.). ITbM is supported by the World Premier International Research Center Initiative (WPI), Japan. This article references 19 other publications.

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分子纳米碳科学：现在和未来。

分子纳米碳科学的世界仅受我们的想象力和创造力限制，我们的前途一片光明！

分子纳米碳科学的世界仅受我们的想象力和创造力限制，我们的前途一片光明！碳是支持地球上所有生命的主要要素之一。分子中碳原子的结构和排列对其性能有很大影响。回顾历史，发现和创造新形式的碳和碳氢化合物一直为新科学和技术打开大门。金刚石和石墨是典型的例子，在过去的三十年中，出现了纳米级的同素异形体碳，也称为纳米碳，并完全改变了碳基材料的面貌。（1）例如球形富勒烯C ₆₀在1985年发现了碳纳米管（2），在1991年发现了圆柱形碳纳米管（CNT）。（3）2004年，从石墨中分离出了一种新形式的纳米碳石墨烯，即单层石墨，并迅速受到了全世界的关注（4）纳米碳几乎在材料科学的所有领域都有潜在的应用，特别是在有机电子学和生物学领域具有广阔的前景。（1）这些优雅的碳结构的重要性被诺贝尔化学奖授予了诺贝尔奖，该奖用于发现富勒烯和石墨烯。分别是1996年和2010年。纳米碳是未来最有前途的材料。新形式碳的发现将为先进技术打开新的大门，并有望在这一领域取得令人振奋的发展。尽管我们赞赏其中许多纳米碳的各种发现和应用，我们认识到，该领域仍然存在两个至关重要的问题（未满足需求）：“混合物”问题和“未合成”问题。（5）在纳米碳科学领域，结构均匀的单分子纳米碳的合成是（5）然而，目前，合成纳米碳的途径通常会导致具有各种不同结构和性质的分子混合物，并且这对于纳米技术，电子，光学和生物医学应用中功能材料的开发至关重要。这些不容易分离或提炼成纯净的形式。一维纳米碳，例如碳纳米管和石墨烯纳米带（GNR）已经遭受这种“混合”问题一段时间了。物理合成方法无法访问结构均匀的CNT和GNR。（5）对这些结构进行完全合成控制的唯一合乎逻辑的方法是从有机合成中汲取灵感，在有机合成中，目标分子实体由具有结构精度的模板（种子）分子构建而成。为了实现这种直观的“从模板生长”策略，需要合成目标种子分子并开发精确的模板延伸化学反应。在过去的10-15年中，为此目的在合成化学领域取得了重大成就。例如，已经出土了许多长期追捧的CNT部分结构，例如碳纳米带（完全融合的超短CNT），（6）碳纳米环（环对亚苯基），（7）和CNT端盖（8）。精密有机合成方法。此外，这些小分子纳米碳可用作引发剂（模板），可以通过更苛刻的“物理”方法成长为结构明确的CNT，这代表了控制CNT结构的重要里程碑。（9,10）值得注意的是，甚至在1991年发现CNT之前，碳纳米带和纳米环的合成就一直是化学领域中的圣地。这些高度应变且独特的分子结构本身就引起了人们的极大兴趣，这为碳纳米带和纳米环的发展提供了巨大的机会。 （11）新的分子纳米碳从根本上改变了科学前景，激发了一批有远见的研究人员；新功能的发现仅仅是这些引人入胜的材料所激发的大量研究的自然结果。GNR，一维石墨烯条，是另一种引起广泛关注的新型碳。像CNT一样，GNR的性质（例如电导率/半导电性，电荷迁移率和开/关比）在很大程度上取决于其宽度，长度和边缘结构。（12）最近，在CNTs方面取得了重大进展。 （12）化学家和物理学家在全球范围内开展的运动导致了强有力的新方法的发展，例如表面聚合（13）和活泼的π-延伸聚合， （14）使得定制的一系列GNR精确合成成为可能。纳米碳的另一个问题是“未合成的”问题。从某种意义上讲，纳米碳的世界可以与折纸艺术相提并论。小型操作可以带来其他形式和功能。实际上，理论家和数学家已经预测了许多纳米碳结构，但尚未合成。但是，与纸张不同，要合成这些原子级的纳米碳非常困难，以原子对原子的精度控制其形状和结构面临着更大的挑战。目前那些假想的，奇妙的纳米碳结构包括具有负高斯曲率的3D周期性碳晶体（Mackay晶体）（15）和碳纳米层。尽管这些框架的许多实际属性仍存在疑问，但大多数纳米碳研究历史上都表明，碳的不同几何形状和形态的出现导致人们发现了最初并未预测或预期的功能和应用。因此，我们认为，制造“未合成的”纳米碳有极大的动力，以推动科学技术的未来突破。尽管目前尚无合理的方法来合成这些未探索的碳结构，但通过应用有机合成技术已取得了一些重大进展。在过去的十年中，已经报道了多种具有负高斯曲率的3D纳米石墨烯，例如翘曲的纳米石墨烯，（16）鞍形纳米石墨烯，（17）和扭曲的纳米石墨烯。（18）最近已经完成了由链烷和三叶结结构组成的独特的全苯纳米环。（19）这些分子代表了拓扑分子纳米碳的基石对象。在这个观点上，我们指出了当前纳米碳科学中出现的两个问题，并介绍了解决每个问题的重大进展和突破。通过应用精密有机合成原理，我们现在正在稳步实现获取结构均匀的纳米碳并创建新的纳米碳形式的最终目标。但是，要推动该领域的发展还存在一些挑战。分子纳米碳的可达到的尺寸和形状仍然受到限制，并且大多数合成仍需要冗长的合成方案和复杂的实验操作。另外，分子纳米碳的结构分析通常在以原子精度确定其结构时通常会遇到挑战，特别是在以前从未存在的支架的情况下。从而，建立分子合成和结构分析的新方法和技术将仍然至关重要。除了合成以外，我们相信分子纳米碳科学的未来10–20年将在包括光电器件和生物医学功能在内的广泛应用中看到重大进展。尽管手头上有几种分子纳米碳，但尚无可靠的方法来微调它们的功能性以适合每种应用。以原子精度修饰分子纳米碳的技术的发展将大大有助于扩大应用范围。此外，我们希望将综合技术进行综合，分析，纳米碳的修饰和修饰将带来与其不可预料的特性和功能相关的新领域和应用。分子纳米碳已逐渐不仅成为发挥碳材料的全部潜力的新标准，而且还为各种研究人员重申以分子进行科学的根本重要性和影响提供了机会。新颖的功能将来自纯净，完美且通常美丽的结构。分子纳米碳科学可以带来的影响和可能性与DNA分子结构的发现相似，而DNA的发现为分子生物学带来了巨大的进步。作者宣称没有竞争性的经济利益。我们感谢Itami小组的所有现任和过去成员在过去15年中探索分子纳米碳科学并分享激动之情，并感谢Iain A. Stepek博士的富有成果的评论。这项工作得到了日本JST ERATO Grant JPMJER1302（KI）和JSPS KAKENHI Grant 19H05463（KI）的支持。ITbM得到了日本世界总理国际研究中心计划（WPI）的支持。本文引用了其他19个出版物。