A Stable Triindenobenzotrithiophene Triradical with Equilateral Ferromagnetic Spin Coupling

Abstract Persistent carbon‐centered polyradicals with high‐spin ground states have received increasing attention for their unique magnetic and quantum technological applications. Yet, balancing the high spin density, favoring robust ferromagnetic (FM) coupling, with chemo‐stability presents a significant challenge. Here, a molecule featuring a central benzotrithiophene fused with three indeno‐radicals is designed to address this dilemma. The initially synthesized trianthryl‐substituted triradical ( TR1 ) undergoes rapid cyclo‐dimerization and yields a stable σ‐dimeric diradical. Upon further structural modifications to enhance the steric blocking effect, dimerization is successfully prevented, and a new persistent triradical TR2 is achieved, exhibiting a half‐life time of 191 h in air‐saturated solution and 398 h in solid state. The continuous‐wave (cw) and pulsed electron paramagnetic resonance (EPR) spectroscopy, along with superconducting quantum interference device (SQUID) magnetometry, unambiguously verify a quartet ( S = 3/2) ground state with Δ E D−Q of 0.41 kcal mol −1 . Notably, TR2 exhibits a zero‐field splitting parameter | D | of up to 115 MHz, affording well‐resolved and individually addressable spin transitions, as confirmed by nutation experiments. These features offer a viable organic platform for quantum spin‐manipulation studies.