Stable Negatively Curved Polycyclic Hydrocarbon Radical: A Molecular Segment of Magnetic Carbon Schwarzite

While stable, negatively curved nanographene radicals have long been theorized as molecular models to understand the magnetism predicted for carbon schwarzites, their synthesis and isolation have remained elusive. Here, we report the first synthesis and isolation of a stable, negatively curved nanographene radical in the crystalline phase. This structure serves as a molecular segment of magnetic schwarzite, exhibiting good geometric similarity. In the crystal lattice, both an achiral, saddle-shaped C_s conformer and a chiral, twisted C₂ conformer are observed. These conformers assemble into distinct π-dimers, driven by spin pairing and shape complementarity. Depending on the solvent system, crystallization-induced σ-dimerization is observed, while in solution, the dimer undergoes bond cleavage upon exposure to heat or light. One-electron oxidation of the radical yields a stable cation, which exhibits prominent near-infrared II absorption at 1238 nm. This cation demonstrates outstanding photothermal properties, with a photothermal conversion efficiency of 90% and a maximum temperature (T_max) reaching 332.6 °C, enabling its use in a laser welding application. These results not only establish negatively curved nanographene radicals as potential building blocks for magnetic carbon schwarzites but also open new avenues for the development of high-performance photothermal materials.