Growing γ-graphyne

Reversible alkyne metathesis proves its worth in the bulk synthesis of γ-graphyne, an sp–sp²-hybridized carbon allotrope. The ability to synthesize graphynes in bulk is an important step towards harnessing the properties of these graphene-like allotropes.

Carbon allotropes comprise atoms linked together by single, double or triple bonds and which can be sp³-, sp²- or sp-hybridized. Many well-known carbon allotropes consist of a single type of hybridized carbon; for example, in graphene, all carbon atoms are arranged in a two-dimensional sheet and exhibit only sp²-hybridization. However, there are also many carbon allotropes that are composed of carbon atoms with a mixture of sp³-, sp²- and sp-hybridizations. One such class of carbon allotropes are graphynes.

γ-Graphyne. Reproduced from the Article by Hu et al., Springer Nature Ltd.

Graphynes are similar to graphene in that they are two-dimensional and have been predicted to have comparable electronic, mechanical and optical properties. However, graphynes consist of a combination of sp- and sp²-hybridized carbon atoms with double and triple bonds in their frameworks. Interestingly, this structural difference is expected to allow control over the direction of electron conduction in some graphynes in a defined direction, rather than the multidirectional conduction in graphene.

Several different types of graphyne structures have been proposed in the past few decades, for example, α-graphyne, β-graphyne and γ-graphyne, to name a few (J. Kang et al., ACS Appl. Mater. Interfaces 11, 2692–2706; 2019). These structures have diverse arrangements of atoms and symmetries as well as varying ratios of sp- and sp²-hybridized atoms. So far, a range of graphyne fragments with low molecular weights have been synthesized, which have shown interesting optoelectronic properties, complementing the properties predicted by theoretical calculations. These fragments therefore offer a tantalizing glimpse into the possible applications of this promising class of materials. However, the preparation of large-scale graphynes with long-range crystallinity has proven very challenging. Featured on the cover of this issue and in an Article by Hu et al., alkyne metathesis of two structurally related monomers is used for the bulk synthesis of γ-graphyne and allows the study of this material.

Alkyne metathesis permits reversible cleavage and reformation of bonds between sp-hybridized carbon atoms. These reactions proceed through either a productive pathway where new products are formed along with small alkyne by-products or through a non-productive pathway where the ends of the two alkynes are exchanged without forming new structures. Crucially, this reversibility of the metathesis reactions enables any errors within the growing polymeric structures to self-correct.

Hu et al. use two types of hexa-alkynyl-substituted benzenes simultaneously to grow a polymer network with periodicity. The use of two monomers proved essential because when only one monomer was used, the polymerization proceeded by kinetic growth, allowing undesired and disordered bonds to form because the volatile 2-butyne by-product was unavailable to correct these errors. The addition of a second, structurally related, monomer meant that these errors could be corrected through reversible reactions, allowing large, ordered structures to grow by balancing kinetic and thermodynamic control of the process.

As mentioned in a News & Views article by Michael Haley, previous attempts to synthesize γ-graphyne focussed on the use of irreversible cross-coupling reactions and were met with limited success. Often these reactions suffer from low yields and incomplete cross-coupling, as well as the formation of defects in the developing structures with no way to correct them.

Hu et al. study the prepared γ-graphyne, determining the material has an electronic bandgap of 0.93 eV, placing this experimental value in reasonable agreement with early theoretical predictions. They also find that γ-graphyne adopts an ABC staggered interlayer stacking arrangement, rather than AA (eclipsed) or AB (partial staggered) arrangements. Additionally, investigation of the folding behaviour of the few-layer graphyne material reveals a step edge within a single graphyne flake of 9 nm.

Alkyne metathesis therefore provides access to γ-graphyne in bulk and our understanding of the stacking and folding behaviour of this material has been expanded. Now, the door is open for further research into the optical, electronic and mechanical properties of this exciting carbon allotrope and we look forward to seeing what’s next for graphyne.