From Polyphenylenes to Nanographenes and Graphene Nanoribbons

Compared with armchair-edged nanographenes (NGs) and graphene nanoribbons (GNRs), graphene nanostructures with zigzag edge peripheries display unique electronic, magnetic, and photophysical properties resulting from the spin-polarized state at the zigzag edges. More interestingly, some possess prominent biradical/polyradical character in the ground state. Thanks to the development of chemistry in recent decades, such NGs and GNRs with unique zigzag peripheries are now synthetically accessible. This chapter discusses several strategies for the synthesis of zigzag-edged NGs and GNRs, their structural characterization, physical properties, and potential applications.

The review discusses progress in the synthesis of atomically precise graphene nanoribbons from molecularly defined precursors on surfaces. It covers the literature from 2010 through 2016.

Fabrication of covalently bonded molecular structures on the surface of metal single crystals is attracting increased attention because of the special synthetic strategies and potential applications (e.g., in molecular electronics). In contrast to traditional organic synthesis, surface-assisted reactions have the advantages of structural control of the produced polymers/oligomers, understanding of detailed reaction processes, and, most importantly, production of new materials that cannot be synthesized by traditional methods. The types of reactants, choice of metal surface, and initial conditions are critical parameters for control of surface-assisted reactions. Covalent bonds formed in the reaction ensure higher mechanical and thermodynamic stability of the produced structures compared with self-assembled monolayers (SAMs). Some conjugated polymers are ideal candidate organic semiconductors for next-generation carbon-based electronics. In this review, we summarize the surface-assisted reactions reported in recent years, analyzing their mechanisms and comparing them with the corresponding reactions in solution systems. We also discuss the important role that the substrate surface plays in the reaction process.

On-surface polymerization is a novel technique for the fabrication of one- and two-dimensional molecular networks confined on a surface and is a rapidly developing field in surface science. The molecular building blocks exhibit pre-defined connection sites at which, after thermal activation and diffusion on the surface, the molecules are linked in a clean environment. Depending on the position and number of these connection sites, activated molecules polymerize to yield chains or two-dimensional networks. The chemical composition of the resulting polymer is precisely defined by the precursor molecules. We review current developments in the field of on-surface polymerization and present different examples, including the fabrication of graphene nanoribbons. We introduce reductive Ullmann-type coupling as well as Scholl-type cyclodehydrogenation for fabrication of graphene nanoribbons of increasing width. The surface plays a crucial role during the activation and polymerization processes because it serves as a catalyst, promotes molecular diffusion, and has a huge influence on the final molecular architecture. One-dimensional polymers can act as molecular wires and their conductance has been studied at the level of individual chains. In addition, we discuss two-dimensional networks and describe recent progress in attempts to improve their quality using sequential activation or defect-healing.

The sp² carbon-based structures with nanoscale dimension, such as fullerenes, nanographenes, nanoribbons, nanocones, and nanotubes, remain at the forefront of nanotechnology research. These unique structures appear to be the most promising building blocks for future nanotechnology applications and electronic devices. One of the main challenge in this field is to develop facile synthetic methods for production of these unique materials with fully controlled structure on the bulk scale. Here, we review studies on controlled surface-assisted synthesis of fullerenes, buckybowls, and single-walled carbon nanotubes that have been published to date. The relevance and prospects of on-surface synthesis strategies for construction of nonplanar sp² carbon-based nanostructures are highlighted.

Bergman cyclization, an organic reaction that forms 1,4-benzene diradicals via intramolecular cyclization of enediyne (EDY) compounds, is a fascinating reaction and can be used to fabricate polyarylenes with various side chains and construct carbon nanomaterials with diverse morphologies. Bergman cyclization has had a strong impact on a number of fields, including pharmaceutics, polymer chemistry, and materials science. The homopolymerization of EDYs through Bergman cyclization is an ingenious strategy for fabrication of functional polyarylenes and has the advantages of facial operation, high efficiency, tailored structure, and being catalyst-free. Moreover, the obtained functionalized polyarylenes show many remarkable properties, such as excellent thermal stability, good solubility, and processability, which enable these polyarylenes to be further manufactured into carbon nanomaterials. Recently, extensive efforts have been devoted to the application of Bergman cyclization in polymer chemistry, materials science, and nanodevices. This chapter summarizes the synthetic strategies that have been developed for fabrication of structurally unique carbon-rich materials using Bergman cyclization, including formation of nanoparticles by intramolecular collapse of single polymer chains, fabrication of conjugated microporous polymers, and construction of carbon nanomembranes with different morphologies and their applications in nanodevices. The future development of Bergman cyclization in materials science is discussed, especially in the construction and application of carbon nanomaterials by altering the template types and morphologies to precisely control the microstructures and properties of carbon nanomaterials.