π-Conjugation Mediates Competing J12 and J13 Interactions at 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.