Silicone chemistry is devoted to the synthesis and research of various organic compounds containing silicon. Among them, silabenzene is a class of heterocyclic aromatic compounds, which can be regarded as one or more carbon atoms in the benzene ring substituted by silicon atoms. Heterocyclic compounds containing atoms such as nitrogen, oxygen, and sulfur have long been known to chemists, but compounds like silabenzenes are considered unstable and difficult to synthesize and isolate. The Si-C π bond of silabenzene is highly polar and easily cleaved, and is also sensitive to light. Since the 1970s, scientists have tried to synthesize silabenzene compounds, but the synthesized silabenzenes are almost unstable and react at extremely low temperatures to form other compounds. However, there are very few examples of synthesizing stable silabenzene compounds in general, let alone using them as monomers to construct more complex macromolecules or functional nanostructures. An alternative approach is to prepare structures such as stabilized silabenzenes by surface synthesis techniques.
On-surface synthesis is a method and technology for the preparation of functional nanomaterials developed in the past ten years: precursor molecules are activated on the solid surface and then coupled to each other into zero-, one-, and two-dimensional molecular or nanostructures with atomic precision. Scientists can prepare various functional nanomaterials by designing suitable precursor molecules. At present, on-surface synthesis is increasingly used to prepare various low-dimensional carbon-containing functional materials and novel nanostructures.
Recently, new studies have demonstrated the successful preparation of stable 1,4-disilabenzene-bridged two-dimensional covalent organic frameworks (COFs) and one-dimensional graphene nanoribbons on gold substrates using surface synthesis techniques. The researchers deposited silicon atoms and hexabromotriphenylene (HBTP) molecules on the surface of gold (111), respectively. After heating to 580 K, the HBTP molecules underwent debromination and formed C-Si covalent bond. Since each benzene ring of the HBTP molecule contains two bromine (Br) atoms, two Si atoms can be combined after debromination, and these two Si atoms can form bonds with another HBTP free radical, and finally can leading to the formation of rings of C4Si2. After debromination of HBTP, there are six active sites, combined with six Si atoms, and finally can be extended into a two-dimensional COF structure with 1,4-disilabenzene as the link point, and each Si atom will be connected to a Br atom.
To demonstrate the applicability of this method, the researchers designed another brominated molecule (Tetrabromopyrene, TBP). Under heating at 420 K, this molecule and Si atoms generate 1,4-disilabenzene-bridged Cove-type GNRs on the surface of gold (111), and under heating at 580 K, the structure can be further transformed as the bright dots at the edges form a zigzag arrangement along the longitudinal axis. This result successfully generalizes the reaction to other molecular systems, indicating the broad applicability of this synthetic reaction.
In conclusion, the researchers perfectly synthesized 1,4-disilabenzene-bridged 2D covalent networks and 1D GNR structures by developing novel surface chemistry reactions. This chemical reaction opens the door to the field of surface organosilicon chemistry and provides a reaction basis for the synthesis of more silicon-containing organics in the future.
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Reference
- On-surface synthesis of disilabenzene-bridged covalent organic frameworks.
Kewei Sun, Orlando J. Silveira, Yujing Ma, Yuri Hasegawa, Michio Matsumoto, Satoshi Kera, Ondřej Krejčí, Adam S. Foster & Shigeki Kawai.
