Magnetic Coupling Control in Triangulene Dimers

Abstract

Metal-free magnetism remains an enigmatic field, offering prospects for unconventional magnetic and electronic devices. In the pursuit of such magnetism, triangulenes, endowed with inherent spin polarization, are promising candidates to serve as monomers to construct extended structures. However, controlling and enhancing the magnetic interactions between the monomers persist as a significant challenge in molecular spintronics, as so far only weak antiferromagnetic coupling through the linkage has been realized, hindering their room temperature utilization. Herein, we investigate 24 triangulene dimers using firstprinciples calculations and demonstrate their tunable magnetic coupling (J), achieving unprecedented strong J values of up to −144 meV in a non-Kekulédimer. We further establish a positive correlation between bandgap, electronic coupling, and antiferromagnetic interaction, thereby providing molecular-level insights into enhancing magnetic interactions. By twisting the molecular fragments, we demonstrate an effective and feasible approach to control both the sign and strength of J by tuning the balance between potential and kinetic exchanges. We discover that J can be substantially boosted at planar configurations up to −198 meV. We realize ferromagnetic coupling in nitrogen-doped triangulene dimers at both planar and largely twisted configurations, representing the first example of ferromagnetic triangulene dimers that cannot be predicted by the Ovchinnikov rule. This work thus provides a practical strategy for augmenting magnetic coupling and open up new avenues for metal-free ferromagnetism.

Unraveling the Nature of Spin Coupling in a Metal-Free Diradical: Theoretical Distinction of Ferromagnetic and Antiferromagnetic Interactions

“…Consequently, we proceed to compute the J values of the supercell junction model shown in Figure (b), which explicitly includes the electrodes. Intriguingly, the PBE functional yields an appreciably positive J at θ = 0, contradicting the Goodenough–Kanamori rule, ,, which typically predicts a negative J at small θ values. To address this discrepancy, we employ the strongly constrained and appropriately normed (SCAN) meta-GGA functional to determine the total energy and the spin coupling between the two radicals within the Au/ 2-OS /Au junction.…”

“…Research on intramolecular electron magnetic interactions is essential in both fundamental science and applied technologies. , Recent efforts have focused on providing direct evidence for carbon-based magnetism and exploring candidate materials with promising practical applications in organic spintronics. , Diradical PAHs, with significant intramolecular ferromagnetic (FM) or antiferromagnetic (AFM) coupling between their radical centers within a reasonable distance, are suitable for fabricating organic magnetic materials. Experimental studies have demonstrated the interconversion between FM and AFM interactions by applying electric and magnetic fields and through the nondestructive rotation of flexible molecules . This unique property of diradical materials holds great potential for various applications.…”

“…Enhancing and tuning spin–spin coupling ( J ) in diradical PAHs have long posed a significant challenge in molecular spintronics. ,, Recently, Yu et al proposed a practical strategy to enhance magnetic coupling by adjusting the potential energy and dynamic exchange through the torsion of molecular fragments . This approach provides control over the sign and strength of J .…”

Spin-State Manipulation in a Luminescent Diradical Polymer

“…Organic diradicals offer the relative independence of the two unpaired electrons within one molecule. This characteristic allows for the control of electron spin–spin interaction through external stimuli such as temperature, pressure, and magnetic fields, rendering diradicals an ideal material system for spin-optical modulation (Figure c). − Nevertheless, a challenge persists as many diradicals are nonluminescent or exhibit poor luminescence efficiency. ,− Additionally, they tend to form dimers or aggregates in the condensed state, posing difficulties in achieving spin-optical modulation.…”

Superexchange Mechanism in Coupled Triangulenes Forming Spin-1 Chains

“…The first step in constructing a synthetic spin-1 chain using triangulene involved combining two triangulene molecules to form a triangulene dimer (TD), as illustrated in Figure (b). This was accomplished experimentally , and investigated theoretically ,,, in recent years. To understand the single-particle properties of the TD, we employ a p z orbital TB model, akin to previous studies. ,− One assumption in these TB models is that the hopping parameters between two sites depend solely on the distance between them.…”