Orbital-symmetry effects on magnetic exchange in open-shell nanographenes

Abstract

Open-shell nanographenes appear as promising candidates for future applications in spintronics and quantum technologies. A critical aspect to realize this potential is to design and control the magnetic exchange. Here, we reveal the effects of frontier orbital symmetries on the magnetic coupling in diradical nanographenes through scanning probe microscope measurements and different levels of theoretical calculations. In these open-shell nanographenes, the exchange energy exhibits a remarkable variation between 20 and 160 meV. Theoretical calculations reveal that frontier orbital symmetries play a key role in affecting the magnetic coupling on such a large scale. Moreover, a triradical nanographene is demonstrated for investigating the magnetic interaction among three unpaired electrons with unequal magnetic exchange, in agreement with Heisenberg spin model calculations. Our results provide insights into both theoretical design and experimental realization of nanographene materials with different exchange interactions through tuning the orbital symmetry, potentially useful for realizing magnetically operable graphene-based nanomaterials.

Designer Spin Models in Tunable Two-Dimensional Nanographene Lattices

“…We choose as our S = 1 / 2 units phenalenyl molecules, 27 known to form open-shell dimers 12 and trimers. 13 In each site of the honeycomb lattice, we place three phenalenyls, coupled via a benzene linker that, as we show, induces a weak ferromagnetic exchange. Each phenalenyl in a trimer is coupled with one additional molecule from a different site of the honeycomb lattice, in a configuration known to give antiferromagnetic exchange.…”

“…Open-shell nanographenes are planar carbon molecules with a finite spin S in the ground state. The formation of supramolecular structures, such as dimers, , trimers, rings, chains, and two-dimensional arrays, , that preserve the open-shell nature of the building blocks has recently been demonstrated. This bottom-up approach permits one to design − two-dimensional carbon crystals with very narrow bands, linear combinations of carbon p z orbitals, and strong electron correlations, complementing the top-down approach of magic angle twisted bilayer graphene. , …”

“…For that matter, we propose a system that realizes the so-called star lattice, a triangle-decorated honeycomb graph, with S = 1 / 2 spins (See Figure ). We choose as our S = 1 / 2 units phenalenyl molecules, known to form open-shell dimers and trimers . In each site of the honeycomb lattice, we place three phenalenyls, coupled via a benzene linker that, as we show, induces a weak ferromagnetic exchange.…”

Atomic-Limit Mott Insulator in [4]Triangulene Frameworks

“…Due to the topological frustration of carbon π-bonds, the zigzag-edged nanographenes have been intensively studied to reveal their exotic magnetic properties over the past decade, − which are promising for next-generation spintronic and quantum computing applications. As one fascinating class of these nanographenes, [ n ]­triangulene is an open-shelled triangular graphene molecule with a zigzag edge of n -unit length. , Since all edge atoms of [ n ]­triangulene belong to the same graphene sublattice, based on the Lieb theorem, its three edges prefer a ferromagnetic coupling , and hold a spin moment linearly increased with the edge length .…”

Topological Structure Realized in Cove-Edged Graphene Nanoribbons via Incorporation of Periodic Pentagon Rings

“…Besides, the measured excitation threshold is estimated to be 19 meV, which fits well with the energy of calculated singlet-to-triplet energy gap. In this regard, we attribute this feature to spin excitation from the spin singlet ( S = 0) to the spin triplet first excitation state ( S = 1). , The corresponding d I /d V maps were taken at the spin excitation threshold of 19 meV, reflecting the spin density distributions on C-5-Seg2 (Figure d). In the case of C-5-Seg3, the symmetric step feature can also be observed in the d I /d V spectrum near the Fermi level, while its excitation threshold value is reduced to 6 meV (Figure e).…”