Synthesis and properties of γ-graphite alkyne spoke wheel molecules

On 29 August 2022, Professor Wu Jishan of the National University of Singapore and a team of Professor Sun Zhe of Tianjin University published a study entitled “A graphyne spoked wheel” in the journal Chem.

The results report the synthesis of a γ-graphite alkyne fragment, γ-graphite alkyne radial wheel molecules, reveal their unique electronic properties through theoretical calculations and related characterization, predict the electronic structure and stacking mode of γ-graphite that are completely different from graphene, and provide new ideas for the synthesis of larger γ-graphitene fragments and new graphite dicyne molecules. The corresponding authors of the paper are Wu Jishan and Sun Zhe; The first author is Zhang Boyi.

γ-graphite alkyne is a carbon isomorph formed by alternating benzene rings and three bonds. Due to its high stability, nonlinear optical properties and semiconductor properties, it is considered to be a key material for new electronic and optoelectronic devices. But its preparation has always been a major problem in synthetic chemistry and materials science. Only recently, Zhang Wei’s team achieved the first synthesis of crystalline γ-graphite-graphyne materials through reversible dynamic alkyne recompression reactions (Nat. Synth. 2022,1,449-454)。 However, in order to gain a deep understanding of the basic electronic properties and stacking structure of monolayer γ-graphite alkyne, the synthesis of γ-graphitene fragment molecules with atomic precision remains crucial. Hexahydrobenzo[12]Cyclene (DBA) can be seen as the smallest fragment of γ-graphitene, and its synthesis was first reported in 1966. Since then, several larger-sized γ-graphite clips have been successfully synthesized. Intramolecular cross-coupling reactions (e.g., Sonogashira-Hagihara coupling), Pinocal coupling, and alkyne recombination are often used as critical steps in the construction of these γ-graphite-graphyne structural backbones. However, as the size of the target molecule increases, these methods are often difficult to achieve their synthesis because of their low efficiency, which greatly hinders further exploration of the properties of monolayer γ-graphite alkyne.

Figure 1: Molecular structure of γ-graphite alkyne spoke wheels.

Recently, Professor Wu Jishan’s team at the National University of Singapore adopted the synthesis strategy of the trine intermediate trimer ringing of trine intermediates catalyzed by efficient zero valent cobalt and the 12-fold intramolecular Stille coupling reaction, and for the first time obtained a γ-graphite alkyne radial stripe molecule with 6 symmetries (Figure 1), which solved the difficult synthesis problem for decades and provided a more perfect molecular model compound for understanding the properties of monolayer γ-graphitene. At the same time, the unique electronic properties and stacking methods of γ-graphite alkyne spoke wheel molecules were discussed by comparing them with the graphene fragment molecule hexabenzoanthra (HBC) with six symmetries. Moreover, the significant differences in aromatization and molecular accumulation between γ-graphyne and graphene were predicted by theoretical calculations.

Figure 2: X-ray diffraction crystal structure of γ-graphite-alkyne spoke stripe wheel.

X-ray diffraction crystal tests reveal a slightly curved near-planar structure at its center. Parallel and slippery dimers are formed between the molecules, with a layer spacing of approximately 3.23 Å, which is smaller than the molecular layer spacing of HBC (3.424 Å) (Figure 2). Based on the bond length analysis of γ-graphite radial stripe crystals, it is found that the central benzene ring bond length is longer than the normal benzene ring (about 1.388 Å), and the six benzene rings at the edge have a certain degree of single and double bond alternation, indicating that they have a certain degree of aromaticity. This is also confirmed by the results of the nuclear independent chemical displacement calculation (NICS), but compared with the normal aromatic benzene ring, the aromaticity of the six benzene rings has been weakened to varying degrees, while the central benzene ring is basically non-aromatic. At the same time, the NICS results also show that the 12-member ring in the γ-graphite alkyne spoke wheel has weak anti-aromatic properties, and these properties are very significantly different from HBC.

Figure 3: Anisotropic (ACID) diagram of induced current density of γ-graphite alkyne spoke wheel and HBC and a schematic diagram of a 2/3-dimensional isochemical shielding surface (ICSS).

The calculation of anisotropy (AICD) and 2/3-dimensional isochemical shielding surface (ICSS) of induced current density further analyzes the electronic properties of γ-graphite alkyne spokes (Figure 3). By comparing with HBC’s calculations, the study found that the alternating aromatic rings and anti-aromatic rings in the γ-graphite spoke wheels would interact with each other, so that the aromatic/anti-aromatic rings would be correspondingly reduced, while the central benzene ring showed the weakest aromaticity due to being surrounded by six anti-aromatic 12-member rings, which coincided with the NICS calculations. This also shows that the electronic structure of the γ-graphite alkyne spoke strip wheel is significantly different from that of HBC, which is composed entirely of aromatic rings.

Based on the calculation results of the γ-graphite spoke wheel extension molecules, the authors speculate that in the infinitely extended single-layer γ-graphite-graphite ring, their aromatic/anti-aromatic rings will tend to be homogeneous because the chemical environment of each aromatic/anti-aromatic ring is basically the same (surrounded by the same number of aromatic/anti-aromatic rings). At the same time, due to the simultaneous existence of aromatic rings and anti-aromatic rings, compared with the single layer graphene composed entirely of aromatic rings, there may be a stronger interlayer force between the single layer γ- graphite alkyne, resulting in a tighter stacking structure.

In summary, this work completes the design and synthesis of γ-graphite alkyne spoke wheel molecules, and provides new ideas for the synthesis of larger γ-graphite alkyne fragments and new graphite diyne molecules. At the same time, the study provides some insights into the electronic properties and accumulation structure of γ-graphitene, revealing the fundamental differences between γ-graphyne and graphene in terms of molecular accumulation and aromaticity. (Source: Science Network)

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