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.

The emergence of quantum magnetism in nanographenes provides ample opportunities to fabricate purely organic devices for spintronics and quantum information. Although heteroatom doping is a viable way to engineer the electronic properties of nanographenes, the synthesis of doped nanographenes with collective quantum magnetism remains elusive. Here, a set of nitrogen-doped nanographenes (N-NGs) with atomic precision are fabricated on Au(111) through a combination of imidazole [2+2+2]-cyclotrimerization and cyclodehydrogenation reactions. High-resolution scanning probe microscopy measurements reveal the presence of collective quantum magnetism for nanographenes with three radicals, with spectroscopic features which cannot be captured by mean-field density functional theory calculations but can be well reproduced by Heisenberg spin model calculations. In addition, the mechanism of magnetic exchange interaction of N-NGs has been revealed and compared with their counterparts with pure hydrocarbons. Our findings demonstrate the bottom-up synthesis of atomically precise N-NGs which can be utilized to fabricate low-dimensional extended graphene nanostructures for realizing ordered quantum phases.

Open-shell nanographenes appear as promising candidates for future applications in spintronics and quantum technologies. How to engineer and obtain large magnetic exchange coupling in nanographenes is crucial for realizing their applications at room temperature. Here, we reveal a mechanism of engineering magnetic exchange coupling in nanographenes through tailoring their frontier orbital symmetries, investigated by combining scanning probe microscope measurements and different levels of theoretical calculations. In nanographenes of phenalenyl spin dimers, their exchange interaction can be widely tuned from 20 meV to 160 meV. Theoretical calculations reveal that frontier orbital symmetries play a key role in engineering magnetic exchange coupling on such a large scale. Moreover, a spin trimer is demonstrated for investigating the magnetic interaction among three spins with unequal magnetic exchange coupling, in agreement with Heisenberg spin chain model calculations. Our results provide insights into both theoretical design and experimental realization of nanographene materials with widely tunable exchange interactions, potentially useful for realizing magnetically operable graphene nanomaterials.