Aromatic Ring Fusion to Benzoporphyrin <i>via</i> γ-<i>ortho</i> Cyclodehydrogenation on a Ag(111) Surface.

Abstract

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) <i>via</i> surface-assisted γ-<i>ortho</i> 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<i>-ortho</i> 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-<i>ortho</i> fused porphyrins with 66 conjugated π-electrons. In addition, a small number of molecules undergo C-C bond dissociation of <i>meso</i>-phenyl at elevated temperature, producing fully planar γ-<i>ortho</i> fused products lacking one or two phenyl moieties. Scanning tunneling spectroscopy measurements and DFT calculations suggest the electronic gap of the γ<i>-ortho</i> fused porphyrin decreases compared to that of the precursor. The HOMO and LUMO of the planar γ-<i>ortho</i> fused products are localized on the partially fused benzo moieties and the <i>meso</i>-position, respectively.