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

TL;DR

This study demonstrates the synthesis and atomic-scale analysis of quantum spin rings made from triangulene units, revealing their magnetic properties and structural effects on quantum phenomena.

Contribution

It introduces a method to create and characterize atomic-scale quantum spin rings from triangulene units, advancing the understanding of correlated magnetism in organic molecules.

Findings

Six-membered ring has a uniform excitation gap described by a Heisenberg model.

Five-membered ring shows structural distortion and asymmetric spin distribution.

The molecular platform enables exploration of quantum spin phenomena with disorder.

Abstract

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.