Center for Advancing Electronics Dresden (cfaed) & Faculty of Chemistry and Food Chemistry, Technische Universität Dresden, Mommsenstrasse 4, 01062 Dresden, Germany.
Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany.
Acc Chem Res. 2022 Dec 6;55(23):3322-3333. doi: 10.1021/acs.accounts.2c00550. Epub 2022 Nov 15.
Graphene nanoribbons (GNRs)─quasi-one-dimensional graphene cutouts─have drawn growing attention as promising candidates for next-generation electronic and spintronic materials. Theoretical and experimental studies have demonstrated that the electronic and magnetic properties of GNRs critically depend on their widths and edge topologies. Thus, the preparation of structurally defined GNRs is highly desirable not only for their fundamental physicochemical studies but also for their future technological development in carbon-based nanoelectronics. In the past decade, significant efforts have been made to construct a wide variety of GNRs with well-defined widths and edge structures via bottom-up synthesis. In addition to extensively studied planar GNRs consisting of armchair, zigzag, or gulf edges, curved GNRs (cGNRs) bearing cove ([4]helicene unit) or fjord ([5]helicene unit) regions along the ribbon edges have received increasing interest after we presented the first attempt to synthesize the fully cove-edged GNRs in 2015. Profiting from their novel edge topologies, cGNRs usually exhibit an unprecedented narrow band gap and high carrier transport mobility in comparison to the planar GNRs with similar widths. Moreover, cGNRs with particular out-of-plane-distorted structures are expected to provide further opportunities in nonlinear optics and asymmetric catalysis. However, the synthesis of cGNRs bearing cove or fjord edges remains underdeveloped due to the absence of efficient synthetic strategies/methods and suitable molecular precursor design.In this Account, we present the recent advances in the bottom-up synthesis and characterization of structurally defined cGNRs containing cove or fjord edges, mainly from our research group. First, the synthetic strategies toward cGNRs bearing cove edges are described, including the design of molecular monomers and polymer precursors as well as the corresponding polymerization methods, such as Ullmann coupling, Yamamoto coupling, AB-type Diels-Alder polymerization, followed by Scholl-type cyclodehydrogenation. The synthesis of typical model compounds is also described to support the understanding of the related cGNRs. In addition, the synthesis of cGNRs containing fjord edges from other research groups via the regioselective Scholl reaction, Hopf cyclization or regioselective photochemical cyclodehydrochlorination approach is presented. Second, we discuss the optoelectronic properties of the as-synthesized cGNRs and reveal the design principle to obtain cGNRs with high charge carrier mobilities. Finally, the challenges and prospects in the design and synthesis of cGNRs are offered. We anticipate that this Account will further stimulate the development of cGNRs through a collaborative effort between different disciplines.
石墨烯纳米带（GNRs）——准一维石墨烯的切条——作为下一代电子和自旋电子材料的候选材料，引起了越来越多的关注。理论和实验研究表明，GNRs 的电子和磁性能取决于其宽度和边缘拓扑结构。因此，制备结构明确的 GNRs 不仅对于它们的基础物理化学研究非常重要，而且对于它们在基于碳的纳米电子学中的未来技术发展也非常重要。在过去的十年中，通过自下而上的合成方法，已经做出了很大的努力来构建具有明确定义的宽度和边缘结构的各种 GNRs。除了我们在 2015 年首次尝试合成全 Cove 边缘 GNRs 后，受到越来越多关注的具有 Cove([4]螺旋单元)或 Fjord([5]螺旋单元)区域的沿边缘的弯曲 GNRs(cGNRs)外，还研究了由扶手椅、锯齿形或海湾边缘组成的平面 GNRs。受益于它们新颖的边缘拓扑结构，与具有相似宽度的平面 GNRs 相比，cGNRs 通常表现出前所未有的窄带隙和高载流子迁移率。此外，具有特定面外扭曲结构的 cGNRs 有望在非线性光学和不对称催化中提供更多机会。然而，由于缺乏有效的合成策略/方法和合适的分子前体设计，具有 Cove 或 Fjord 边缘的 cGNRs 的合成仍然不发达。
在本报告中，我们主要介绍了我们研究小组在自下而上合成和表征具有 Cove 或 Fjord 边缘的结构明确的 cGNRs 方面的最新进展。首先，描述了具有 Cove 边缘的 cGNRs 的合成策略，包括分子单体和聚合物前体的设计以及相应的聚合方法，如 Ullmann 偶联、Yamamoto 偶联、AB 型 Diels-Alder 聚合，然后是 Scholl 型环脱氢。还描述了典型模型化合物的合成，以支持对相关 cGNRs 的理解。此外，还介绍了通过区域选择性 Scholl 反应、Hopf 环化或区域选择性光化学环脱氢氯化方法，由其他研究小组合成具有 Fjord 边缘的 cGNRs。其次，我们讨论了所合成的 cGNRs 的光电性质，并揭示了获得具有高电荷载流子迁移率的 cGNRs 的设计原则。最后，提出了在 cGNRs 的设计和合成方面面临的挑战和展望。我们期望，通过不同学科之间的合作，本报告将进一步推动 cGNRs 的发展。
