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

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Abstract
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 C 5 ‐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.
Funding Information
  • National Natural Science Foundation of China (22325203)
  • National Natural Science Foundation of China (92577203)
  • National Natural Science Foundation of China (22272050)
  • National Natural Science Foundation of China (92580139)
  • National Natural Science Foundation of China (92365302)
  • National Natural Science Foundation of China (92265105)
  • National Natural Science Foundation of China (92065201)
  • National Natural Science Foundation of China (12074247)
  • National Natural Science Foundation of China (12174252)
  • National Natural Science Foundation of China (12304230)
  • National Natural Science Foundation of China (22161160319)
  • National Key Research and Development Program of China (2024YFF0727103)
  • National Key Research and Development Program of China (2020YFA0309000)
  • Science and Technology Commission of Shanghai Municipality (2019SHZDZX01)
  • Science and Technology Commission of Shanghai Municipality (20QA1405100)
  • Science and Technology Commission of Shanghai Municipality (24LZ1401000)
  • Science and Technology Commission of Shanghai Municipality (22QA1403000)

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