π-ConjugationMediates Competing J 12 and J 13 Interactionsat the Nanometric Scale in Linear Fe(III) Porphyrin Trimers

The control of spin–spin interactions is a central challenge in designing materials for quantum technologies. A promising avenue for increasing magnetic coupling between spin centers is to connect them via a delocalized π-conjugated system. Here, we synthesized a family of porphyrins (monomers, dimers, and trimers) with high-spin FeIII centers and studied their magnetic interactions. The linearly fused Fe(III)Cl porphyrin trimers exhibit nearest-neighbor and second-neighbor couplings of the same sign and order of magnitude (J12 = −24.3 GHz; J13 = −10.8 GHz) between spin centers at 0.84 and 1.7 nm separation, respectively. These antiferromagnetic interactions lead to frustration, realizing a nontrivial Heisenberg spin system. The persistence of the long-range exchange coupling (J13) demonstrates that π-mediation is effective despite the spin polarization of the π-system induced by the central iron center. This work establishes a design principle for constructing nontrivial antiferromagnetic Heisenberg spin chains in which metal spin centers interact over several nanometers.