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

On-Surface Synthesis of Square-Type Porphyrin Tetramers with Central Antiaromatic Cyclooctatetraene Moiety

“…Figure S7 displays calculated differential charge density, which reveals a presence of charge transfer from the metallic substrate to the molecule. Even though PBE was successfully applied to describe experimental findings for several molecules on metallic surfaces (see, e.g., refs − ), it is well known that GGA XC functionals tend to significantly underestimate the molecular band gap. This tendency may depreciate a correct description of the charge transfer across organic/metallic interface. , Figure S8 shows calculated projected density of states (PDOS) of both T × and T ◇ models in the gas phase, revealing a strong difference between the band gap obtained from the GGA-PBE (∼0.5 eV) and hybrid PBE0 XC-functional (∼few eV).…”

Orbital and Skeletal Structure of a Single Molecule on a Metal Surface Unveiled by Scanning Tunneling Microscopy

“…To meet these challenges, sharper p -orbitals of functionalized probes as well as thin buffer films or noble metal surfaces (mostly Au and Ag) have been used to access the nearly free electronic structure of adsorbed molecules in STM experiments. − On Cu(111), the imaging of a pure hydrocarbon molecule, pentacene, was achieved with submolecular resolution using pentacene-terminated and CO-terminated STM tips. Very often, resonant tunneling of charge carriers into the orbitals is required to this end.…”

On-Surface Synthesis of Pentagon-Incorporated Graphene-Like Nanoribbons with Multiple Precursors

“…On-surface synthesis of low-dimensional nanostructures has been extensively studied because it has many advantages as a result of the surface confinement effect compared to the traditional solution method. − The bottom-up assembled strategy guarantees the precision of on-surface synthesis, which involves multiple reaction types, for instance, Ullmann coupling, − Glaser coupling, − Sonogashira coupling, , cross coupling, − cyclodehydrogenation, etc. − On the other hand, graphene nanoribbons (GNRs) have received tremendous attention from researchers among many graphene materials, because they exhibit a wide range of tunable bandgaps that facilitate the applications in molecular electronics and related single-molecule devices, − like organic field-effect transistors, − reversible photoswitching, tunable electroluminescence, etc. − In particular, GNRs with various widths, edge structures, and even different cavities have been fabricated through Ullmann coupling and further cyclodehydrogenation based on rational design and utilization of a single precursor. − There exist many factors affecting the above-mentioned reaction processes and final products, such as the template effect of substrates, , active sites and symmetry of the precursors, ,, thermodynamics, and kinetics . However, the controlled synthesis of GNR derivatives by multiple precursors with different symmetries has been scarcely investigated, which may bring great diversity to the final products.…”