A groundbreaking method to incorporate copper into graphene-like coordination polymers (GCPs) can unlock exciting possibilities for electronics, energy storage and materials science.

Carbon atoms in graphene come together in a distinctive honeycomb pattern, a unique 2D atomic structure that gives the material strength, as well as exceptional electrical and heat conductivity. Graphene-like 2D coordination polymers mimic this structure, presenting exciting prospects in electronics, energy storage and materials science.

He-Kuan Luo, a Principal Scientist and Group Leader at A*STAR's Institute of Sustainability for Chemicals, Energy and Environment (ISCE2) noted that graphene-like 2D coordination polymers (GCPs) have garnered significant interest over the last decade.

However, incorporating different copper ions into graphene-like structures has challenged materials scientists. Luo explained that according to classical coordination chemistry, the desirable Cu(I) form, known for its electrical configurations, does not naturally lend itself to forming flat, layer-like structures required for GCPs, unlike the Cu(II) form.

To address this, Luo teamed up with Shuo-Wang Yang, a Senior Principal Scientist at A*STAR's Institute of High-Performance Computing (IHPC); colleagues from A*STAR’s Institute of Materials Research and Engineering (IMRE); and researchers from Xiamen University and Jilin University, China; and University of Newcastle, Australia. They hypothesised that a chemical linker called benzenehexathiol (BHT) can facilitate the creation of graphene-like scaffolds with both Cu(I) and Cu(II) and form isostructural 2D GCPs.

“We applied a novel strategy to synthesise Cu(I)-BHT 2D GCP by using in situ-generated [CuI2]- anions that dramatically slow down the coordination process,” Luo explained. This approach enables Cu(I) ions to diffuse into BHT ligand micro-crystals, forming a 2D structure under spatially constrained conditions.

The team successfully created two isostructural 2D GCPs with Cu(I) and Cu(II). Interestingly, both GCPs contain Cu(I) and Cu(II) in near 1:1 ratios—a feat seemingly impossible according to traditional chemistry rules. They also identified an intramolecular pseudo-redox mechanism whereby BHT alters the copper forms (by either adding or removing an electron), resulting in materials with identical appearances but distinct electrical properties.

"For the first time, we elucidate how a charge-neutral periodic atomic structure can host a different number of electrons, potentially marking a new milestone in chemistry, physics and materials science,” remarked Luo. These advancements hold promise for revolutionising various fields, including catalysis, energy storage and electronic device development.

For now, Luo and colleagues are keen to contribute to both fundamental and translational research efforts aimed at advancing mixed-valence 2D GCPs for industrial applications.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2), Institute of High Performance Computing (IHPC) and the Institute of Materials Research and Engineering (IMRE).