Radical‐Based On‐Surface Transformation of Nonplanar Aromatics Into Nonbenzenoid Nanographenes

ABSTRACT On‐surface synthesis has emerged as a powerful tool for atomically precise C─C bond formation, enabling access to low‐dimensional carbon‐based materials, often unattainable by conventional solution‐based chemistry. This approach gave rise to a novel class of magnetic materials, namely open‐shell magnetic nanographenes, whose magnetism originates primarily from unpaired electrons. Despite this progress, a fundamental understanding of selectivity and specificity of surface‐confined radical chemistry remains limited. Here, we demonstrate that nonplanar hydrocarbons such as helicenes undergo highly selective intramolecular radical‐driven bond formation and reorganization on an Au(111) surface, yielding nonbenzenoid nanographenes incorporating 5‐, 6‐, and 7‐membered rings. The products are identified using time‐of‐flight secondary ion mass spectrometry, scanning tunneling microscopy, and non‐contact atomic force microscopy, which collectively support a radical‐mediated cyclization pathway. The mechanism is distinct from the Diels–Alder cycloaddition and cyclodehydrogenation reactions previously reported for helicenes on surfaces.