Closed- vs Open-Shell: Unlocking Doublet Emission in Persistent Phosphorus-Containing Polycyclic Aromatic Hydrocarbon Radicals

Abstract Persistent organic radicals embedded in conjugated materials are extremely rare due to the scarcity of suitable structural motifs capable of stabilizing unpaired spins. Here, we present a phosphorus-incorporated polycyclic aromatic hydrocarbon scaffold, along with its respective chemically reduced open-shell relatives, that provides access to these radical species. Two strategically placed keto groups activate new conjugation pathways that, coupled with two phosphoryl moieties, gives rise to a strongly electron-accepting molecular architecture. The resulting radical species demonstrate persistence in THF up to a week and are remarkably stable in ethanol, a feature rarely observed for π-system radicals. Despite the presence of the keto groups in the framework, the P-based closed-shell compounds display intense fluorescence with quantum yields of up to 54%. More importantly, the corresponding radicals are also emissive, constituting the first isolated luminescent radicals in conjugated organophosphorus materials. Intriguingly, these species feature dual doublet emission at room temperature that occurs through two distinct radiative pathways: one following Kasha’s rule and the other constituting a rare anti-Kasha process.