22 August 2019

Largest-made segments of the elusive carbon allotrope 6,6,12-graphyne

In a paper just published in Nature Communications, the syntheses and properties of the largest fragments of the elusive carbon allotrope 6,6,12-graphyne are reported. The work is the result of joint experimental and theoretical efforts between researchers at Department of Chemistry, University of Copenhagen and at Department of Chemistry, University of Alberta in Canada.

Graphene – the two-dimensional monolayer of graphite – has for long been termed a ”wonder material” on account of  its mechanical and electronic properties. Nevertheless, several computer simulations have suggested that the electronic properties of so-called graphynes could be even superior to those of graphene.

Graphynes are a class of carbon allotropes that result by inserting sp-hybridized carbon atoms into the sp2-carbon framework of graphene. One particular variant of graphyne has been identified as particularly interesting from computational work, namely the so-called 6,6,12-graphyne. Despite several theoretical predictions of superior properties of this carbon allotrope, it has never been made. There are good reasons for this! It has a rather complicated symmetry, which makes it particularly challenging to synthesize. Much work has today moved towards on-surface polymerization for the synthesis of graphene nanoribbons. This is, however, currently not a viable approach for the complicated structure of 6,6,12-graphyne, which calls for a more classical synthetic approach. And now significant steps have been made. Thus, in their joint paper, the Danish and Canadian groups report the to-date largest segments, prepared by stepwise acetylenic coupling reactions. The segments can be described as oligomers of so-called radiaannulene macrocycles that are hybrid structures between dehydrobenzannulenes and expanded radialenes. This hybrid structure means that the cyclic core has both endocyclic and exocyclic carbon-carbon double bonds, and, in consequence, it contains both linearly conjugated bonds (alternating single and double or triple bonds) and cross-conjugated double bonds (double bonds branching off from the ring).

The combination of linearly and cross-conjugated double bonds in the radiaannulene cores provides the oligomers with interesting electronic properties. Thus, electrochemical studies reveal that they are particularly good electron acceptors; they are in other words good ”electron reservoirs”. In their joint paper, the authors explain this ability to accommodate electrons by the generation of a row of large aromatic, hence stable, rings with a negative charge at each end of the oligomer. In fact, extrapolating from this observation suggests that neutral graphyne nanoribbons may have radical character at the edges and large aromatic rings in-between.  

The work has focused on elongation of the model molecules in one direction. To reach larger two-dimensional graphyne nanoribbons and ultimately 6,6,12-graphyne, the radiaannulene oligomers have to be extended in the other direction as well. This is a huge synthetic challenge for future work.

Link to the paper: https://www.nature.com/articles/s41467-019-11700-0.pdf