School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China.
Acc Chem Res. 2023 Feb 7;56(3):363-373. doi: 10.1021/acs.accounts.2c00767. Epub 2023 Jan 26.
ConspectusUnderstanding and harnessing the properties of nanoscale molecular entities are considered as new frontiers in basic chemistry. In this regard, synthetic nanographene with atomic precision has attracted much attention recently. For instance, taking advantage of the marvelous bonding capability of carbon, flat, curved, ribbon-type, or cone-shaped nanographenes have been prepared in highly controllable and elegant manner, allowing one to explore fascinating molecular architectures with intriguing optical, electrochemical, and magnetic characteristics. This stands in stark contrast to other carbon-rich nanomaterials, such as graphite oxides or carbon quantum dots, which preclude thorough investigations because of complicate structural defects. Undoubtedly, synthetic nanographene contributes strongly to modern aromatic chemistry and represents a vibrant field that may deliver transforming functional materials crucial for optoelectronics, nanotechnologies, and biomedicine.Nonetheless, in many cases, synthesis and characterization of nanographene compounds are highly demanding. Low solubility, high molecular strain, undesired selectivity, as well as incomplete or excessive C-C bond formation are common impediments, that require formidable efforts to control the molecular geometry, to modulate the edge structure, to achieve accurate doping, or to push the upper size boundary. These endeavors are indispensable for establishing structure-property relationships, and lay down foundation for exploring synthetic nanographenes at a high level of sophistications.In this Account, we summarize our contributions to this field by presenting a series of helical synthetic nanographenes, such as hexapole [7]helicene (H7H), nitrogen-doped H7H, hexapole [9]helicene (H9H), superhelicene, and supertwistacene. This kind of giant synthetic nanographene reaches the size domain of carbon quantum dots, albeit has precise atomic structure. It provides a unique platform to study aromatic chemistry and chirality at the nanoscale. We discuss synthetic methods and point out, in particular, the strengths and pitfalls of Scholl oxidation, which are expected to be valuable for making synthetic nanographenes in general. In addition, we illustrate their exciting electrochemical and photophysical performance, which include, but are not limited to, reversible multielectron redox chemistry, record high panchromatic absorption, impressive photothermal behavior, and extremely strong Cotton effect. These unusual characteristics are convincingly traced back to their three-dimensional conjugated architectures, highlighting the critical roles of π-electron delocalization, heteroatom-doping, substitution, and molecular symmetry in determining nanographenes' properties and functions. Lastly, we put forward our understanding on the challenges and opportunities that lies ahead and hope this Account will inspire ever more ambitious achievements from this attractive area of research.
概述
理解和利用纳米尺度分子实体的性质被认为是基础化学的新前沿。在这方面，具有原子精度的合成纳米石墨烯最近引起了广泛关注。例如，利用碳的奇妙键合能力，可以以高度可控和优雅的方式制备平坦、弯曲、带状或锥形纳米石墨烯，从而可以探索具有迷人光学、电化学和磁特性的分子结构。这与其他富碳纳米材料形成鲜明对比，例如石墨氧化物或碳量子点，由于结构缺陷复杂，这些材料无法进行彻底研究。毫无疑问，合成纳米石墨烯为现代芳香族化学做出了重要贡献，代表了一个充满活力的领域，可能会为光电、纳米技术和生物医学提供改变功能的材料。
然而，在许多情况下，纳米石墨烯化合物的合成和表征要求很高。低溶解度、高分子应变、不理想的选择性以及不完全或过度的 C-C 键形成是常见的障碍，需要付出巨大的努力来控制分子几何形状、调节边缘结构、实现精确掺杂或推动上尺寸边界。这些努力对于建立结构-性质关系是不可或缺的，并为在更高水平上探索合成纳米石墨烯奠定了基础。
在本账目中，我们通过介绍一系列螺旋合成纳米石墨烯，如六极[7]螺旋烯（H7H）、氮掺杂 H7H、六极[9]螺旋烯（H9H）、超螺旋烯和超扭曲烯，总结了我们在这一领域的贡献。这种巨型合成纳米石墨烯达到了碳量子点的尺寸范围，尽管具有精确的原子结构。它为在纳米尺度上研究芳香族化学和手性提供了独特的平台。我们讨论了合成方法，并特别指出了 Scholl 氧化的优缺点，这有望对一般的合成纳米石墨烯的制备有价值。此外，我们还说明了它们令人兴奋的电化学和光物理性能，包括但不限于可逆的多电子氧化还原化学、记录高全光吸收、令人印象深刻的光热行为和极强的 Cotton 效应。这些异常特性可以令人信服地追溯到它们的三维共轭结构，突出了π电子离域、杂原子掺杂、取代和分子对称性在决定纳米石墨烯性质和功能方面的关键作用。最后，我们提出了我们对未来面临的挑战和机遇的理解，并希望本账目的内容能激发人们对这一极具吸引力的研究领域的更大野心。
