Benzo[ cd ]Perylenyl: A π‐Expanded Phenalenyl Radical With Enhanced Aggregation Enthalpy and Stability

We report the synthesis and characterization of a π‐expanded neutral PAH radical, benzo[ cd ]perylenyl ( 1 ) derived from the phenalenyl radical ( 2 ). Hückel molecular orbital (HMO) analysis reveals a uniformly distributed SOMO that enables effective two‐electron 20‐center bonding in a π‐dimer. ESR measurements of 1 show a long half‐life of 63 days in air‐saturated toluene at room temperature. Temperature‐dependent ESR and absorption spectra as well as DFT calculations confirm the preferential formation of a π‐dimer over a σ‐dimer with an association energy of approximately −65 kJ mol⁻¹, driven by enhanced dispersion interactions. These findings demonstrate that π‐expansion stabilizes organic radicals without bulky substituents while preserving intermolecular interactions offering new prospects for advanced materials in electronics and spintronics.