Tunable Molecular Spins in Polyradical Nanographenes Offer Quantum Computing Potential.

Researchers synthesised two extended nanographene structures, C62H22 and C76H26, exhibiting robust, correlated tetraradical character and a singlet ground state. Vertical extension increased zero-energy modes, while lateral extension enhanced electron interactions, yielding tunable spin and resilience to external magnetic fields, validated by theoretical calculations.

The pursuit of stable, multi-radical organic molecules represents a significant challenge in materials science, with potential applications ranging from quantum computing to novel magnetic materials. Researchers are now demonstrating precise control over the creation of these complex structures, achieving robust spin entanglement and resilience to external interference. A collaborative team, comprising En Li, Manish Kumar, Xinnan Peng, Tong Shen, Diego Soler-Polo, Yu Wang, Yu Teng, Haoyu Zhang, Shaotang Song, Jishan Wu, Pavel Jelinek and Jiong Lu, from the National University of Singapore and the Czech Academy of Sciences, detail their approach in the article ‘Designer polyradical nanographenes with strong spin entanglement and perturbation resilience via Clar’s goblet extension’. They present a predictive design strategy for synthesising two novel nanographene homologues, exhibiting correlated tetraradical character and a stable ground state, achieved through controlled extensions of a parent ‘Clar goblet’ structure.

Novel Nanographenes Exhibit Tunable, Stable Polyradical Character

Researchers have successfully synthesised and characterised two novel polyradical nanographenes, C₆₂H₂₂ and C₇₆H₂₆, demonstrating a predictive strategy for designing molecules with tailored magnetic properties. These compounds, built upon the ‘Clar goblet’ structure, consistently exhibit robust, correlated tetraradical character and a singlet ground state – properties of interest for potential applications in molecular spintronics and quantum information processing.

The Clar goblet refers to a specific arrangement of fused benzene rings, possessing unique electronic properties. The research team constructed the nanographenes directly on a surface, systematically extending this foundational structure both laterally – by adding rings to the sides – and vertically – by stacking rings on top of each other. This precise control over the molecular architecture allows for manipulation of the resulting electronic and magnetic characteristics. Vertical extension increases the number of topologically protected, zero-energy modes – electronic states that are robust against perturbations – while lateral extension amplifies electron-electron interactions.

Computational modelling, utilising density functional theory with accurate Van der Waals corrections, played a crucial role in understanding and guiding the synthesis. These calculations accurately predicted the emergence of a correlated tetraradical state – a state where four unpaired electrons interact strongly with each other – confirming the stability and nature of the observed magnetic states.

The nanographenes were synthesised using on-surface chemistry, a technique where molecules are assembled directly on a solid surface under carefully controlled conditions. Scanning tunnelling microscopy and spectroscopy were then employed to confirm the successful synthesis and reveal the unique electronic and magnetic properties of the resulting materials.

Both synthesised structures consistently exhibit this correlated tetraradical character, converging on a many-body singlet ground state – a state where the total spin is zero, resulting in no net magnetic moment. Importantly, these nanographenes demonstrate resilience to external perturbations, maintaining their magnetic properties under various conditions.

To experimentally validate the predicted magnetic states, researchers employed nickelocene-functionalised scanning probe microscopy. This technique allows for high-resolution imaging and probing of the electronic and magnetic properties at the molecular level, providing direct evidence for the existence of the correlated tetraradical state and confirming the stability of the magnetic moments.

The research demonstrates a generalisable approach to designing polyradical nanographenes with tunable spin numbers and enhanced stability. By controlling the molecular architecture, researchers achieve precise control over the magnetic properties, potentially enabling the exploration of novel correlated spin phases and advancement of molecular-scale information technologies. The study establishes a clear link between molecular architecture and magnetic properties, providing a rational design strategy for highly polyradical nanographenes and enabling the creation of materials with precisely controlled magnetic characteristics. Researchers manipulate the vertical and lateral dimensions of these structures, tuning both the spin number and the stability of the resulting many-body spin states, offering unprecedented control over the magnetic properties of molecular materials.