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

Abstract

The synthesis of two-dimensionally extended polycyclic heteroatomic molecules keeps attracting considerable attention. In particular, frameworks bearing planar cyclooctatetraenes (COT) moieties can display intriguing properties, including antiaromaticity. Here, we present an on-surface chemistry route to square-type porphyrin tetramers with a central COT ring, coexisting with other oligomers. This approach employing temperature-induced dehydrogenative porphyrin homocoupling in an ultrahigh vacuum environment provides access to surface-supported, unsubstituted porphyrin tetramers that are not easily achievable by conventional synthesis means. Specifically, monomeric free-base (2H-P) and Zn-metalated (Zn-P) porphines (P) were employed to form squaretype free-base and Zn-functionalized tetramers on Ag( 100). An atomiclevel characterization by bond-resolved atomic force microscopy and scanning tunneling microscopy and spectroscopy is provided, identifying the molecular structures. Complemented by density functional theory modeling, the electronic structure is elucidated, indeed revealing antiaromaticity induced by the COT moiety. The present study thus gives access, and insights, to a porphyrin oligomer, representing both a model system for directly fused porphyrins and a potential building block for conjugated, extended two-dimensional porphyrin sheets.

Zinc-Porphine on Coinage Metal Surfaces: Adsorption Configuration and Ligand-Induced Central Atom Displacement

“…27,29,34,35 Also the nc-AFM images show a central depression (persisting in a large range of tip-molecule distances, see Figure S12) and thus do not allow for a direct identification of the Zn center. This finding is reminiscent of bond-resolved AFM data on selected metalated tetrapyrroles, featuring a central depression (e.g., planarized Zn-porphyrin, 77 square-type Zn-porphyrin tetramers, 78 Zn-Pc/NaCl 79 Ag-Pc/ Ag(111), 80 and CuPorA2 21 ), whereas other metalated, as well as free-base, tetrapyrroles show faint cross-like signatures at the macrocycle center. 18,21,23,26,60 Generally, the contrast is influenced by electrostatic interaction between the COfunctionalized tip and the central ion (expected to be attractive for Zn 2+ ) and by Pauli repulsion affected by the d-orbital occupancy and the vertical position of the atom/ion.…”

“…The appearance of Zn-P on Cu(111) and Ag(111) in STM images, featuring a central depression at low bias voltages is expected for Zn. ,− The (approximate) d 10 element occupation discriminates Zn-P from other metal porphines, such as Co-P and Cu-P, which typically show a central protrusion. ,,, Also the nc-AFM images show a central depression (persisting in a large range of tip-molecule distances, see Figure S12) and thus do not allow for a direct identification of the Zn center. This finding is reminiscent of bond-resolved AFM data on selected metalated tetrapyrroles, featuring a central depression (e.g., planarized Zn-porphyrin, square-type Zn-porphyrin tetramers, Zn-Pc/NaCl Ag-Pc/Ag(111), and CuPorA2), whereas other metalated, as well as free-base, tetrapyrroles show faint cross-like signatures at the macrocycle center. ,,,, Generally, the contrast is influenced by electrostatic interaction between the CO-functionalized tip and the central ion (expected to be attractive for Zn 2+ ) and by Pauli repulsion affected by the d-orbital occupancy and the vertical position of the atom/ion. AFM image simulations based on the DFT-optimized geometry (see Scheme ) indeed reflect these competing contributions.…”

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.…”

Concealed antiaromaticity

“…Besides monocyclic antiaromatic molecules with two degenerate SOMOs and a diradical character y0 of 1 (referred to as truly antiaromatic molecules in this article), there are molecules with a formal 4n π-electron system but reduced diradical character (between 0 and 1). 14 These molecules -including pentalene, dibenzopentalene, s-indacene, and biphenylene (Figure 1c) and their derivatives as well as different cyclooctatetraene derivatives -are often also referred to as antiaromatic, [15][16][17][18][19][20][21][22][23] even when the diradical character is low. In contrast to truly antiaromatic molecules, which may only be isolated under cryogenic conditions, these polycyclic molecules are more stable and isolated more readily.…”