Frustration-Induced Many-Body Degeneracy in Spin −1/2 Molecular Quantum Rings

Frustrated spin systems, where competing interactions prevent conventional magnetic ordering, provide a platform for uncovering emergent quantum phases and exotic many-body phenomena. Particularly, low-dimensional and symmetric geometries without boundary conditions allow us to study unconventional spin states. Here, we present S = 1/2 antiferromagnetic Heisenberg cyclic pentamer and hexamer via homocoupling of air-stable phenalenyl derivatives on Au(111). With a combination of scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) at 4.3 K and comprehensive theoretical simulations, we found that while large magnetic exchange interactions exist in both rings, the pentamer features an increased geometric frustration of the system. This frustration induces rotational symmetry in the spin wave function, leading to a 4-fold degenerate ground states of the pentamer. The interplay between molecular geometry and magnetic interactions creates a unique quantum spin environment. Our findings offer a powerful approach for constructing spin-frustrated molecular architectures, allowing precise control over quantum magnetic interactions.