Orbital Topology Overcomes Energetic Constraints: Conjugation Expansion Breaks the Inverse Relationship between SOMO Splitting and Spin Coupling for Giant Ferromagnetic Coupling in Organic Diradicals

The commonly assumed inverse relationship between magnetic exchange coupling (J) and the energy gap between two singly occupied molecular orbitals (ΔESS) has long guided the design of organic diradicals. Contrary to this well-established doctrine, we demonstrate an unprecedented violation: the concurrent increase in both ΔESS and J through conjugation expansion engineering in the PAH-bridged diradicals, unveiling an "inverted SOMO splitting effect". Across a controlled PAH size series, J surges from 585 cm⁻¹ to over 2147 cm⁻¹ (nearly a 3.7-fold increase) even as ΔESS widens from 0.10 to 1.15 eV. We propose a novel mechanism of synergistic orbital separation and spin delocalization. Extended conjugation induces orbital mixing between two symmetry-adapted radical-group orbitals and HOMO or HOMO-1 of the coupler, yielding a low-lying local SOMO between two radical groups and a high-lying SOMO with a delocalized distribution on the coupler as spin-transport network-channels. This cooperative action of symmetry-governed orbital separation and spin network effectively overcomes the energy penalty associated with enlarged ΔESS, resulting in a remarkable enhancement of J. Substituent perturbation analysis further shows that J becomes progressively less sensitive to local electronic effects as conjugation increases, supporting a transition from an energetic to orbital-topological control. These results establish orbital spatial topology engineering as a practical molecular design strategy for achieving simultaneously large SOMO splitting and strong magnetic coupling in organic diradicals.