Modulation of Through-Space Magnetic Exchange in Cyclophane-Bridged Benzotriazinyl Diradicals

Most research on organic magnetic materials focus on maximizing ferromagnetic interactions. However, systems with tunable antiferromagnetic exchange (J < 0) have potential for applications where controllable singlet-triplet gaps enable coherent manipulation. Although 1,2,4-benzotriazinyl (Blatter) radicals are exceptionally stable, the model diradical studied here, 1-meta, possesses negligible intramolecular exchange due to the disjoint nature of the SOMOs across its C(3)-meta-phenylene-C(3') connectivity. Here, we investigate a topological strategy to induce antiferromagnetic exchange by enforcing π-stacking within cyclophane scaffolds. Using SA-CASSCF/NEVPT2 and SF-TD-DFT, we show that linear meta-phenylene-bridged diradical has small singlet-triplet gaps (|2J| < 19 cm^-1) due to insufficient orbital overlap. In contrast, cyclophane-bridged diradicals display persistent through-space coupling caused by direct SOMO overlap. By modulating the cyclophane linkage (syn- versus anti- stacking), the coupling is tuned from -4 to -1674 cm^-1. This spans three distinct physical regimes: the weak-coupling limit spectrally addressable with high-field EPR, the intermediate coupling regime with potential for thermal switching and finally, the strong-coupling limit of incipient covalent bonding. Therefore, although [2.2]paracyclophane scaffolds enforce antiferromagnetic ground states, the magnitude of the interaction is sensitive to the orbital overlap dictated by the conformation of the radical decks. Strategies to further lower 2J to -1.8 cm^-1 are discussed.