CCSChemistry|When fullerene meets nanographene


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Professor Wang Zhaohui of Tsinghua University and others developed a new type of cross-dimensional molecular carbon material system. Porphyrin has adjustable electronic structure and assembly structure.

As a bright pearl in the material world, molecular carbon materials have excellent electrical, optical, and magnetic properties, and have good application potential in the fields of energy, environment, electronics, and biology. From the topological classification, molecular carbon includes zero-dimensional fullerene, one-dimensional carbon nanotubes and two-dimensional graphene. As the earliest discovered allotrope of molecular carbon, fullerene's unique spherical structure and physicochemical properties have become the focus of research in optoelectronics and nanoscience. Over the past three decades, many synthetic strategies have been developed for the functional modification of the fullerene family, especially C60 and C70, such as free radical reaction, cycloaddition reaction, palladium-catalyzed cyclization reaction, and decarboxylation cyclization reaction. Nevertheless, fine-tuning the fullerene's chemical structure, electronic structure, and processing performance is still full of challenges, hindering its further application in the field of optoelectronic devices. Combining the topological structure characteristics of different dimensions, the precise synthesis of cross-dimensional molecular carbon, such as the fullerene/nanographene hybrid material system, to explore its unique and cross-border material properties is a fertile ground to be developed.

The edge structures of nano-graphene, such as armchair and zigzag structures, have very different reactivity and physicochemical properties. Based on this, Professor Wang Zhaohui (Tsinghua University), Associate Researcher Jiang Wei (Institute of Chemistry, Chinese Academy of Sciences) and Associate Professor Wang Dong (Tsinghua University) selected the lysine unit with chair and zigzag edge structure as the model system and developed A new cross-dimensional fullerene/nanographene hybrid molecular carbon material reveals the effect of edge structure hybridization on molecular structure and properties.

Figure 1. The synthesis route of Fuller-Porphyrin (Fuller-PMI and Fuller-PDI)

Figure 1. The synthesis route of Fuller-Porphyrin (Fuller-PMI and Fuller-PDI)
In this paper, the authors selected perylene monoimide (PMI) with a zigzag edge structure and perylene diimide (PDI) with a chair edge structure as monomers, through a one-step palladium-catalyzed [3+2] or [4+2] A new type of cross-dimensional molecular carbon material was constructed for the first time in the cyclization reaction: spherical edge C60 molecules and different edge structures of planar lysine molecules perylene were condensed at the peri or bay positions to obtain fullerene/nanographene The hybrid model compound (Fuller-Leline) is named Fuller-PMI and Fuller-PDI, respectively (Figure 1). The synthetic method is efficient, easy to separate and purify, and the product has good solubility and excellent thermal stability.

Figure 2. Theoretical calculation of the energy distribution of the frontier orbit
Density functional theory calculations show that the two molecules exhibit completely different orbital distributions. The Fuller-PMI condensed through the peri position shows the obvious "orbital partition" property, that is, the HOMO orbit is distributed on the PMI, and the LUMO orbit is distributed on the C60, showing the "donor-acceptor" characteristic; The HOMO orbits of the combined Fuller-PDI are mainly distributed on C60, most of the LUMO orbits are on PDI, and a small part is on C60. This is mainly because the front-line energy levels of PDI and C60 are closer to each other, resulting in a stronger intramolecular Electronic coupling (Figure 2).
The UV-Vis absorption spectrum and electrochemical properties were further studied (Figure 3). The absorption spectrum of Fuller-PMI is mainly the superposition of the spectra of the subunits PMI and C60. It shows enhanced absorption in the visible region of 400-600 nm, and the maximum absorption peak is 25 nm red shifted from the parent PMI, which is mainly due to The push-pull electronic effect of C60 to PMI after the "orbit partition". In comparison, Fuller-PDI exhibits broad and complex absorption peaks in the 400-650 nm region due to the strong electronic coupling within the molecule. In addition, the experimental frontal orbital energy levels are in agreement with the theoretical calculations. The HOMO/LUMO energy levels of Fuller-PMI are close to PMI and C60, respectively, while the LUMO energy level of Fuller-PDI is as low as -3.88 eV, indicating a strong Electronic affinity.

Figure 3. (a) Ultraviolet-visible absorption spectrum; (b) Electrochemical test curve

Single crystal diffraction analysis clearly revealed the hybrid structure of C60 and perylene molecules with different edge structures. At the same time, due to the "concave-convex" interaction, the perylene skeleton in both showed a slight bowl-shaped curve, causing it to appear weak Chirality. In addition, unlike the dense sphere π-π interaction in Fuller-PMI crystals, Fuller-PDI only has a π-π interaction between the fullerene sphere and the lyleline surface (Figure 4).

Figure 4. Fuller-PMI (a), (b), (c) and (d) and Fuller-PDI (e), (f), (g) and (h) single crystal structure diagram
In summary, this research work provides a model system to construct cross-dimensional fullerene/nanographene hybrid molecular carbon materials. Through the hybridization of fullerene and lysoline with different edge structures, a regulated electronic structure and assembly structure were obtained, which laid the foundation for the study of new molecular carbon materials with different dimensional topologies. The research work was supported by the National Key Research and Development Program, the National Natural Science Foundation of China's major projects and key projects. The work was published in CCS Chemistry in the form of Research Article and was recently launched on the "Just Published" section of the CCS Chemistry official website.