Synthesis of molecule with high magnetic spin

NUS chemists have synthesised and characterised π-extended triangulene molecules with ferromagnetically coupled edge states for molecular spintronic devices.

When triangular motifs are clipped along the zigzag orientation of graphene, a family of zigzag-edged triangular graphene molecules (ZTGMs) can be created. These molecules, also known as non-Kekulé polynuclear benzonoid compounds, are predicted to have multiple unpaired π-electrons and high-spin ground states with large net spin. An example is the high-spin π-extended [5]triangulene molecule (N=5, where N represents the number of carbon atoms at the zigzag edges (Figure (A)). Due to their ferromagnetic properties, these molecules are considered potential candidates for the development of next generation molecular spintronic devices. However, the direct chemical synthesis of ZTGMs using a wet-chemistry approach has been a challenge due to their high chemical instability.

A research team led by Prof LU Jiong and Prof WU Jishan, both from the Department of Chemistry, NUS has developed a bottom-up synthetic approach to synthesise π-extended [5]triangulene molecules on copper and gold substrates under ultra-high vacuum condition. The π-extended [5]triangulene molecule consists of 15 benzene rings fused in a triangular fashion  (Figure (B)). To achieve on-surface synthesis of this high-spin molecule, the researchers designed a unique molecular precursor that contained a central triangular core with six hexagonal benzene rings and three 2,6-dimethylphenyl substituents attached at the meso-positions of the core. At elevated temperatures on a catalytic metallic surface, the precursor molecule undergoes cyclodehydrogenation and ring closure reactions to produce the π-extended [5]triangulene molecules.

Using non-contact atomic force microscopy (nc-AFM) with a carbon monoxide (CO)-functionalised tip (Figure (C)), the team was able to map out the molecular skeleton of the [5]triangulene molecule at a temperature of 4.5 Kelvin, showing 15 fused benzene rings arranged in a triangular manner with zigzag topology at the edges (Figure (D)). Scanning tunnelling spectroscopy measurements showed that the [5]triangulene molecule had edge-localised electronic states, which follow theoretical predictions.

Prof Lu said, “These findings enable the electronic and magnetic properties of the π-extended [5]triangulenes to be explored at the single molecule level. They also open up a new methodology for the fabrication of larger triangular zigzag-edged graphene quantum dots for spin and quantum transport applications.”

The research team plans to continue developing similar materials systems with different molecular sizes and high-spin states to understand their electronic and magnetic properties on a variety of substrates for future spintronic devices.

Figure (A) shows open-shell zigzag-edged triangular graphene molecules (ZTGMs) with a different number of zigzag carbon atoms (N) and predicted spin multiplicity (S). The molecules are in yellow: Monoradical phenalenyl (N=2), red: Biradical triangulene (N=3), violet: π-extended triradical [4]triangulene (N=4) and blue: π-extended tetraradical [5]triangulene (N=5).

Figure (B) shows (left) the precursor molecule and (right) the π-extended [5]triangulene molecule.

Figure (C) is a schematic illustration of the non-contact atomic force microscopy (nc-AFM) imaging technique used in the study. It uses a carbon monoxide molecule-terminated qPlus sensor.

Figure (D) shows the corresponding nc-AFM image of the π-extended [5]triangulene molecule on copper (111) surface.

[Credit: Science Advances]

Reference

Su J; Telychko M; Hu P; Macam G; Mutombo P; Zhang HJ; Bao Y; Cheng F; Huang ZQ; Qiu ZZ; Tan SJR; Lin H; Jelinek P*; Chuang FC*; Wu JS*; Lu J*, “Atomically precise bottom-up synthesis of pi-extended [5]triangulene” SCIENCE ADVANCES Volume: 5 Issue: 7 Article Number: eaav7717 DOI: 10.1126/sciadv.aav7717 Published: 2019.