Gothenburg Chemists Synthesize Diamondiyne, a New 3D Carbon Allotrope

Researchers at the University of Gothenburg have synthesized diamondiyne, a new allotrope of carbon whose atoms form covalent bonds in all three dimensions, as diamond's do — but at relatively low pressure, producing a far more porous crystal. Physical chemist Karl Börjesson and colleagues report the work in Angewandte Chemie, ending a synthesis effort that began more than 30 years ago.

"To our knowledge, diamondiyne is the first allotrope besides diamond in which the carbon atoms form covalent bonds in all three dimensions," Börjesson says.

Why the synthesis took three decades

The pursuit of new carbon allotropes accelerated after graphene was first isolated in 2004. Theoretical chemists have since predicted 1635 candidate structures; most have never been made in a laboratory. Diamondiyne sat among the unmade ones for two reasons.

First, one of its building blocks, tetraethynylmethane, is unstable. Second, diamondiyne's carbon atoms link through robust single carbon–carbon bonds and even stronger triple bonds. Those stiff connections made it difficult to build a crystal ordered in all three dimensions rather than an amorphous or two-dimensional product.

How does the interface reaction work?

Börjesson's group solved the problem with a cascade reaction whose kinetics they could control, running at the interface between two liquids. The system pairs tetra(triethylsilylethynyl)methane with CuF₂. Fluoride ions react with the silane precursor to generate tetraethynylmethane in situ — sidestepping its instability — leaving exposed terminal alkyne groups. Cu²⁺ ions then catalyse coupling between pairs of terminal alkynes.

Because the reaction occurs at the liquid–liquid interface, monomers attach to a growing crystallite one by one. The team kept the precursor concentration relatively low to favour this molecule-by-molecule addition, and a film of diamondiyne crystallites grew perpendicularly out of the interface between the two liquids.

"We have previously shown that under such circumstances, the growth of covalent crystals is favoured kinetically," Börjesson explains.

The resulting structure is a "carbo-mer" of diamond: sp³-hybridized carbons connected through four sp-hybridized carbons, with two triple bonds forming the diyne groups that give the material its name. Liquid–liquid interfaces had previously been used to synthesize graphdiyne — an expanded, sp²/sp-hybrid relative of graphene — and other 2D materials; the Gothenburg result extends the approach into three dimensions.

The route itself was not accidental. The group has spent several years building covalent organic frameworks through carbon–carbon coupling reactions. "We then realized that we could use a similar synthetic methodology to make this new carbon allotrope," Börjesson says.

What has actually been measured?

The measured results, separated from aspiration, remain modest in scale. The finished films cover roughly 5 cm² and range from several nanometres to tens of nanometres thick, depending on reaction time. Transmission electron microscopy confirmed that the diamondiyne crystal structure persists over volumes of 10 nm³ — evidence of genuine long-range order, but at dimensions that still limit bulk property measurements.

The material's electronic, mechanical and optical characteristics remain under investigation. That gap between structural confirmation and measured properties is the practical caveat for any R&D group eyeing applications: no performance figures exist yet because no one has made crystals large enough to measure them accurately.

What comes next

The immediate research priority is scale. Börjesson and his colleagues aim to grow larger crystallites so they can measure diamondiyne's properties with precision. "We will need to do this to find out what we can use the material for," he told Physics World.

Until those measurements arrive, diamondiyne is a structural milestone rather than a materials commercialization story — one confirmed allotrope against 1635 predicted, with a synthesis route that turned a 30-year-old instability problem into a controllable interface reaction.