π‐Enlargement in Porphyrin Macrocycles at Interfaces

) 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.