Quantum Spin-1/2 Rings Built From [2]Triangulene Molecular Units

Quantum spin rings represent fundamental model systems that exhibit distinctive quantum phenomena 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 [2]triangulene units on Au(111). Using stepwise on-surface synthesis followed by scanning tunneling microscopy 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. Bond-resolved noncontact atomic force microscopy 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, although an ideal C₅-symmetric pentamer is theoretically expected to host a degenerate, frustrated ground state, the experimentally realized five-membered ring is structurally distorted, which lifts this degeneracy and produces asymmetric spatial distributions of the spin ground state. Our findings establish a versatile molecular platform for exploring correlated magnetism and quantum spin phenomena in cyclic organic magnetic architectures.