Aromatic Ring Fusion to Benzoporphyrin γ- 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 π-system of palladium tetraphenyltetrabenzoporphyrins (Pd-TPTBP) surface-assisted γ- 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 microscopy with a CO-functionalized tip. We captured a series of γ cyclodehydrogenation products, wherein the maximum extent to which the reaction can progress is associated with 7-fold C-C formation to afford nearly planar γ- fused porphyrins with 66 conjugated π-electrons. In addition, a small number of molecules undergo C-C bond dissociation of -phenyl at elevated temperature, producing fully planar γ- fused products lacking one or two phenyl moieties. Scanning tunneling spectroscopy measurements and DFT calculations suggest the electronic gap of the γ fused porphyrin decreases compared to that of the precursor. The HOMO and LUMO of the planar γ- fused products are localized on the partially fused benzo moieties and the -position, respectively.