Tuning the Spin Interaction in Nonplanar Organic Diradicals through Mechanical Manipulation

Area of Science:

• Materials Science
• Organic Chemistry
• Surface Science
• Quantum Mechanics

Background:

• Open-shell polycyclic aromatic hydrocarbons (PAHs) are key building blocks for novel carbon-based magnetic materials.
• Their magnetic properties arise from unpaired electrons in radical states, leading to spin delocalization and collective states.
• High reactivity of radical states necessitates protective structural strategies for practical applications.

Purpose of the Study:

• To investigate the stability and magnetic properties of an open-shell diradical PAH, specifically 2-OS, on a Au(111) substrate.
• To explore the influence of molecular conformation on the spin coupling and magnetic state of the diradical.
• To demonstrate the potential of PAHs as all-carbon spin-crossover materials.

Main Methods:

• Utilized scanning tunneling microscopy (STM) to localize interacting spins and probe electronic states.
• Employed mean-field Hubbard simulations to understand the exchange coupling dependence on molecular conformation.
• Performed in-situ manipulation of molecular conformation using the STM tip to observe changes in spin excitation spectra.

Main Results:

• The open-shell diradical 2-OS on Au(111) forms a global singlet state with antiparallel coupled spins.
• An excited triplet state was detected slightly above the singlet ground state, indicating spin-crossover potential.
• Exchange coupling strength was found to be highly dependent on torsional angles, controllable via structural changes.

Conclusions:

• The nonplanar geometry of 2-OS stabilizes its open-shell diradical character against environmental reactions.
• Molecular conformation directly influences the magnetic properties, enabling control over spin states.
• These findings highlight the promise of functionalized PAHs as tunable, all-carbon spin-crossover materials.