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Materials science

Organic analogues of graphene

Chemists have long aspired to synthesize two-dimensional polymers that are fully conjugated — an attribute that imparts potentially useful properties. Just such a material has been prepared using a solid-state polymerization reaction.

One of the greatest breakthroughs in materials science was the isolation of graphene1 — a two-dimensional carbon crystal, obtained by stripping off (exfoliating) a single layer of atoms from graphite using adhesive tape. This discovery prompted the emergence of 2D materials as a new paradigm in science and technology. Nevertheless, graphene's potential use as a semiconductor in electronic devices has been limited because its bandgap (a property that controls the conductivity of materials) is zero, which enforces metal-like conduction. This problem has stimulated growing interest in synthesizing organic analogues of graphene that might exhibit useful properties, including a tunable bandgap2,3. Writing in Nature Chemistry, Liu et al.4 report a breakthrough in this field: the synthesis of an organic graphene analogue known as a 2D-conjugated aromatic polymer (2D-CAP).

Graphene is the largest 2D system in nature to be conjugated — that is, some of its electrons (its π-electrons, in scientific jargon) are delocalized within a planar framework of alternating single and double bonds. This conjugation is responsible for the material's remarkable mechanical, electronic and optoelectronic properties5. Synthesizing extended (micrometre-scale), fully conjugated and ordered 2D structures has been a long-standing goal for polymer scientists. Until now, the main approaches pursued to address this challenge have fallen into three categories6: the formation of 2D covalent organic frameworks (COFs; porous crystalline solids constructed from organic building units connected by strong covalent bonds); surface-mediated polymerization; and solid-state topochemical polymerization. In each approach, the type and strength of the bonds formed between reacting molecules determine the robustness of the final product7.

COFs are typically made using dynamic and reversible covalent-bond-forming reactions, which enables self-correction of any structural defects that form during the synthesis6. Unfortunately, most COFs have limited chemical stability, and tend to decompose under ambient conditions. However, 2D-conjugated COFs made using irreversible covalent double bonds were recently reported8, which form as crystalline flakes up to 100 μm long. In parallel, surface-confined reactions have been used to make flat, conjugated polymers in which the largest ordered domains are of the order of just a few square nanometres9,10. The third approach — solid-state topochemical polymerization — takes place when monomers form a crystal in which the molecules have the proximity and orientation needed to react with each other. Several groups have reported the crystal-to-crystal transformation of monomers into layered polymers using this strategy6. However, these polymers were not fully conjugated 2D polymers, and could be exfoliated only by applying heat or certain solvents, or both.

Enter Liu and colleagues. The basic building block of their 2D-CAP is a planar molecule that contains several aromatic rings — rings of atoms whose stability is enhanced by complete delocalization of their π-electrons. The authors judiciously designed this aromatic monomer so that it can form an ordered arrangement that, when heated, undergoes a crystal-to-crystal solid-state polymerization (Fig. 1).

Figure 1: Synthesis of a two-dimensional conjugated aromatic polymer.
figure1

M. Ebrahimi & F. Rosei

a, b, Liu et al.4 prepared an aromatic monomer (a) that crystallizes in a packing arrangement (b) that pre-organizes the molecules for a polymerization reaction. c, When the crystals were heated, polymerization occurred to form a material composed of flat, stacked layers; side and top views are shown. Individual layers of the polymer can be peeled off the stacked system. Carbon atoms, brown; nitrogen, blue; bromine, red; hydrogen, pink. Structures were prepared using VESTA 3 software11.

Liu et al. first attempted the polymerization of their monomer on the surfaces of single crystals of gold, using a process in which the monomer lost its bromine atoms on heating and then formed covalent carbon–carbon bonds with its neighbours at the positions where their bromine atoms were detached. This yielded an undesirable mixture of 1D and 2D polymers. But when the authors preordered the monomer by crystallizing it, the reaction yielded a monocrystalline polymer that was fully conjugated and formed of planar layers. The layers are stacked with a distinct lamellar structure that can be readily exfoliated into micrometre-sized sheets just 1 nm thick. The fact that individual layers could be exfoliated using adhesive tape suggests that 2D-CAP is a close analogue of graphene.

The building block's shape and the stacking of the polymers result in the formation of uniform 1D channels (approximately 0.6 nm in diameter) through the 2D-CAP. Liu et al. demonstrated that these aligned channels can be used to store sodium ions in an energy-storage device that can be quickly charged and discharged at room temperature. In other words, the polymer can be used as an organic anode in a sodium-ion battery.

Less than a decade ago, extended, planar, fully conjugated polymers were considered to be just a dream2. The exciting properties of Liu and colleagues' 2D-CAP open up new horizons in the field of 2D materials and will inspire future efforts in this arena. A potential drawback of the authors' approach, if applied to other monomers, is that, when heated, some monomers might decompose before polymerization occurs. A variety of monomers must therefore be designed that polymerize at temperatures low enough to prevent decomposition. Flexible synthetic routes are also needed that allow different monomers and polymers to be made easily, because this in turn will allow the properties of 2D polymers to be tuned. Another promising direction of research could be to choose two different monomers that can co-crystallize before reacting to form a single polymer.

The 2D-CAP reported by Liu et al. might have promising properties in addition to those described. For example, its electronic and optoelectronic properties are yet to be measured. Such measurements will require exfoliated polymer sheets to be overlaid onto different substrates, so that they can be used as the active layer in devices such as field-effect transistors.

Liu and colleagues' approach paves the way for a general, controlled synthesis of extended crystalline 2D-conjugated materials, and might form the basis of a new branch of crystal engineering. Such organic analogues of graphene would constitute a class of material that might be useful in various technologies, thanks to the enhancement of electronic properties that arises from π-conjugation in two dimensions. Applications might include organic transistors faster than those available today, and highly efficient solar cells and sensors.Footnote 1

Notes

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Correspondence to Maryam Ebrahimi or Federico Rosei.

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Ebrahimi, M., Rosei, F. Organic analogues of graphene. Nature 542, 423–424 (2017). https://doi.org/10.1038/nature21503

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