π‐Enlargement in Porphyrin Macrocycles at Interfaces

ABSTRACT Porphyrins are essential heteroatomic macrocycles, fundamental to both biological systems and advanced technology. Their unique molecular architecture enables key functions in nature, such as oxygen transport in hemoglobin and light harvesting in chlorophyll, and inspires cutting‐edge applications in chemical sensing, catalysis, renewable energy conversion, and optoelectronics. Consequently, significant efforts are dedicated to develop their solution‐phase chemistry, particularly by strategically modifying the macrocyclic structure to tailor their properties. Inspired by the field of on‐surface covalent synthesis, we introduce a pioneering strategy to tailor porphyrin macrocycles at interfaces, specifically expanding an 18‐π porphyrin into a 20‐π system. Such transformation is achieved by depositing a porphyrin precursor, equipped with two trifluoromethyl (‐CF 3 ) functional groups in a trans configuration, onto a hot Ag(111) surface. By combining scanning probe microscopy and spectroscopy, complemented with density‐functional theory calculations, we confirm the successful formation of the 20‐π free‐base expanded porphyrin, exhibiting potential high antiaromaticity attributed to the preservation of planar conformation at the interface according to theoretical calculations. The transformation occurs through precursor dehalogenation and subsequent insertion of two carbon atoms into the macrocycle, driving its expansion, and affording a narrow bandgap of ∼0.2 eV. Furthermore, we demonstrate its coordinative capabilities toward cobalt, forming a unique two‐fold coordination node within the expanded core. Our findings pave the way for engineering expanded porphyrins at interfaces enabling enhanced antiaromaticity and narrow bandgaps, while affording the design of novel coordination motifs, and, simultaneously, demonstrating the capabilities of surface science in exploring such expanded macrocyclic architectures at the atomic scale.