A Stable Triindenobenzotrithiophene Triradical with Equilateral Ferromagnetic Spin Coupling

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 ΔE_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.