Aromatic Ring Fusion to Benzoporphyrin via ?-ortho Cyclodehydrogenation on a Ag(111) Surface

Aromatic ring fusion to porphyrins and their derivatives represents an attractive route to tune the molecular conjugation and thus expand their functionalities. Here, we report the expansion of the aromatic pi-system of palladium tetraphenyltetrabenzoporphyrins (Pd-TPTBP) via surface-as-sisted gamma-ortho cyclodehydrogenation on Ag(111). The chemical transformation of Pd-TPTBP into different products at an elevated temperature of 600 K was revealed at the single-molecule level using bond-resolved scanning tunneling micros-copy with a CO-functionalized tip. We captured a series of gamma- ortho cyclodehydrogenation products, wherein the maximum extent to which the reaction can progress is associated with 7-fold C-C formation to afford nearly planar gamma-ortho fused porphyrins with 66 conjugated pi-electrons. In addition, a small number of molecules undergo C-C bond dissociation of meso-phenyl at elevated temperature, producing fully planar gamma-ortho fused products lacking one or two phenyl moieties. Scanning tunneling spectroscopy measurements and DFT calculations suggest the electronic gap of the gamma-ortho fused porphyrin decreases compared to that of the precursor. The HOMO and LUMO of the planar gamma-ortho fused products are localized on the partially fused benzo moieties and the meso-position, respectively.