Modulating Diradical Spin Coupling via Isomeric Tetra‐Thiophene Bridges: Dominance of Super‐Exchange Versus Spin‐Polarization Mechanisms

ABSTRACT Molecular bridge isomerism plays a critical role in regulating electron transport and magnetic exchange within diradical systems of organic spintronics, yet polyfused thiophenic molecular bridges have not been thoroughly investigated. We report the design and synthesis of four novel diradical compounds employing nitronyl nitroxide (NN) radicals as spin sources and structurally defined tetra‐thiophene (Tt) isomers as molecular bridges. The influence of the four isomeric Tt bridge with different alternative and spiral fused structure on magnetic coupling of Tt‐NNs were systematically investigated through electron paramagnetic resonance (EPR), superconducting quantum interference device (SQUID) magnetometry, and density functional theory (DFT) calculations. DFT calculations reveal a direct correlation between spin interaction mechanism and magnetic coupling strength. Experimental results demonstrate that the isomeric arrangement of the tetra‐thiophene bridge significantly modulates spin distribution in the ground state and electron delocalization capability. The Kekulé‐type tetra‐thiophene bridge (Tt‐A), characterized by a continuous conjugation pathway, exhibits superior spin delocalization efficiency and strong antiferromagnetic (AFM) coupling ( J /k B = −17.79 K, SQUID). In contrast, the nonKekulé‐type isomers (Tt‐B, Tt‐C, and Tt‐D) with disrupted conjugation show relatively weaker coupling, governed primarily by super‐exchange interactions. Moreover, in bridge structures, a higher proportion of nonKekulé regions correlates with lower coupling strength.