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The paper reports the on-surface synthesis and characterization of antiferromagnetic S=1/2 quantum spin rings composed of [2]triangulene units on Au(111). Cyclic five- and six-membered spin rings were constructed using stepwise on-surface synthesis followed by STM tip-induced dehydrogenation, and their spin states were probed using scanning probe microscopy and multireference calculations. The six-membered ring exhibits a uniform excitation gap well-described by a Heisenberg spin model, while the five-membered ring shows asymmetric spin distributions due to structural distortion-induced degeneracy lifting.
Precisely engineered quantum spin rings made of triangulene units reveal distinct magnetic behaviors depending on ring size and geometry, opening avenues for exploring correlated magnetism at the molecular level.
Quantum spin rings represent fundamental model systems that exhibit distinctive quantum phenomena-such as quantum critical behavior and quasiparticle excitations-arising from their periodic boundary conditions and enhanced quantum fluctuations. Here, we report the on-surface synthesis and atomic-scale characterization of antiferromagnetic S=1/2 quantum spin rings composed of pristine and unmodified [2]triangulene units on a Au(111) surface. Using stepwise on-surface synthesis followed by STM tip-induced dehydrogenation, we precisely constructed cyclic five- and six-membered spin rings and investigated their spin states via scanning probe microscopy and multireference calculations. Nc-AFM imaging reveals that the six-membered ring retains a planar geometry, whereas the five-membered ring exhibits pronounced structural distortion. The six-membered ring hosts a uniform excitation gap that can be accurately described by a Heisenberg spin model and multireference CASCI calculations. In contrast, the distorted five-membered ring displays spin ground states with asymmetric spatial distributions due to degeneracy lifting induced by structural distortion. Our findings establish a versatile molecular platform for exploring correlated magnetism and quantum spin phenomena in cyclic organic magnetic architectures with disorder.