Radical-Based On-Surface Transformation of Nonplanar Aromatics Into Nonbenzenoid Nanographenes.

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.