Manifestation of Ground-State Baird Aromaticity in a Neutral Hexaazaphenanthrene Derivative with a Topologically...

Controlling molecular spin states is essential for organic electronics, yet achieving high-spin (triplet or higher) ground states in Kekulé-type π-conjugated systems remains a challenge. Achieving high-spin ground states in π-conjugated systems has traditionally relied on two established strategies: implementing specific molecular topologies to create nondisjoint frontier orbitals, or taking advantage of high molecular symmetry (C_n, 3 ≤ n) to induce orbital degeneracy. Consequently, Kekulé-type molecules are overwhelmingly singlet species in their ground state. Here, we report the design and synthesis of o-BenD, a 16π-electron Kekulé-type diradical bridged by an ortho-phenylene unit. Despite lacking conventional structural prerequisites, o-BenD exhibits a robust triplet ground state with strong ferromagnetic coupling (J/k_B = +320 K). This magnetism originates from pseudodegeneracy of the frontier orbitals, controlled by a simple "frontier-orbital engineering" approach guided by topological charge stabilization. This mechanism bypasses topology-based spin-state prediction, providing a new conceptual framework for stabilizing high-spin states. Furthermore, we demonstrated that o-BenD exhibits ground-state Baird aromaticity, the aromaticity of [4n]π-systems in the lowest triplet state, due to the unique combination of the ground triplet nature and 16π-Kekulé-type conjugation system. While Baird aromaticity is typically restricted to short-lived photoexcited states, this work provides a molecular design to favor the Baird aromatic state over the Hückel antiaromatic singlet state. The realization of such a previously unanticipated electronic state expands the accessible chemical space for the development of organic spintronics and quantum information technologies.