A robust spin-coupled tetraradical via nonalternant design

Polyradicals in open-shell polycyclic hydrocarbons (PHs) offer access to multiple spin states and hold great promise for spintronics and quantum technologies. Here we report a nonalternant, air-stable tetraradical featuring two fused 7-5-7 rings and a strongly correlated singlet ground state. This molecule displays an exceptional tetraradical character (y₀ = 1.00, y₁ = 0.89) and exhibits long ambient stability (t_1/2 = 33 days). Comprehensive electronic structure and magnetic response analyses reveal that the spin topology is governed by both ferromagnetic and antiferromagnetic interactions, with robust electron correlation (J₁ = +321 K, J₂ = -44 K, and J₃ = -29 K). The unique nonalternant design suppresses hyperfine and vibrational decoherence, giving rise to long spin relaxation and coherence time. These findings demonstrate that embedding nonalternant motifs into PH polyradicals provides a powerful strategy to simultaneously promote molecular stability, spin-exchange coupling, and spin robustness-key features for materials in the rising field of quantum information science.