Surface-Mediated <i>in Situ</i> Metalation of Porphyrins at the Solid–Vacuum Interface

ConspectusThe investigation of porphyrin derivatives at the solid–vacuum interface has become a vivid research field with the prospect to tailor functional molecular architectures and as prototype examples to study the fundamental properties of porphyrin derivatives in regard to their vital role in many natural processes. The functional properties of the porphyrin derivatives are mainly determined by the central metal atom. Thus, the recent exploration of the surface-confined <i>in situ</i> metalation of porphyrins is an important step toward the realization of molecule-based functional devices. The corresponding metalation reaction of free base porphyrin derivatives can be conveniently realized <i>in situ</i> in ultrahigh vacuum by post- or predeposition of metal atoms or directly with substrate atoms in the so-called self-metalation. Moderate heating above room temperature (RT) might be necessary either to realize the transport of the metal to the porphyrin via diffusion or to overcome an activation barrier determined by the redox reaction itself.Surface science techniques like scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) are powerful tools to scrutinize the reaction and give valuable insights into the metalation process. For example, the completed metalation can be reflected in an enhanced apparent height of the corresponding porphyrin in STM or can be evidenced by characteristic changes in the N 1s region in XPS. These signatures allow monitoring of the progress of the metalation, and it was found that the reaction generally proceeds with very high yield.Surface diffusion of the coadsorbed metal atoms mediates the reaction and is crucial for the high yields of the corresponding reactions with pre- and postadsorbed metals. It was also demonstrated that the completed metalation can indeed significantly alter the adsorption behavior and the electronic properties and thus the functionality of the porphyrin molecules. These alterations can be used to monitor the kinetics of a particular porphyrin self-metalation reaction by STM and to estimate the activation barrier for that reaction based on isothermal measurements at different temperatures. Also TPD measurements of the H₂ and D₂ signals allow for the determination of corresponding activation energies for the metalation of free base porphyrins and their deuterized analogues. Gas phase DFT calculations of the metalation of the “bare” free base porphyrin macrocycle identify intermediate reaction steps with the transfer of the first hydrogen atom to the metal center being the main barrier to overcome. The values from these calculations are in fair agreement with experimentally determined ones. However, TPD based results indicate that exchanges of deuterium and hydrogen between the central nitrogen and the surface occur, which indicate an active role of the surface and challenge the findings from gas phase DFT.The <i>in situ</i> metalation of porphyrins at the solid–vacuum interface is established as a novel and convenient route to tailor functional molecular architectures. With different surface science techniques detailed insights into the surface-mediated metalation reaction were achieved for this class of prototype functional molecules

Coverage Dependent Disorder–Order Transition of 2H-Tetraphenylporphyrin on Cu(111)

In this study, we investigate the peculiar coverage dependent supramolecular arrangement of 2H-tetraphenylporhpyrin (2HTPP) on Cu(111) with scanning tunneling microscopy at room-temperature. At low coverage, “slow” diffusion of individual 2HTPP molecules along the close-packed atomic rows of the substrate is observed, and no supramolecular ordering occurs. However, at higher coverage, the formation of ordered, checkerboard-like domains is found, with two molecules per unit cell at different distances from the surface. This behavior is attributed to a complex interplay of site specific molecule–substrate interaction, mainly the strong interaction between the iminic N atoms and Cu substrate atoms, with intermolecular T-type and π–π interactions

Coverage Dependent Disorder–Order Transition of 2H-Tetraphenylporphyrin on Cu(111)

In this study, we investigate the peculiar coverage dependent supramolecular arrangement of 2H-tetraphenylporhpyrin (2HTPP) on Cu(111) with scanning tunneling microscopy at room-temperature. At low coverage, “slow” diffusion of individual 2HTPP molecules along the close-packed atomic rows of the substrate is observed, and no supramolecular ordering occurs. However, at higher coverage, the formation of ordered, checkerboard-like domains is found, with two molecules per unit cell at different distances from the surface. This behavior is attributed to a complex interplay of site specific molecule–substrate interaction, mainly the strong interaction between the iminic N atoms and Cu substrate atoms, with intermolecular T-type and π–π interactions

Coverage Dependent Disorder–Order Transition of 2H-Tetraphenylporphyrin on Cu(111)

In this study, we investigate the peculiar coverage dependent supramolecular arrangement of 2H-tetraphenylporhpyrin (2HTPP) on Cu(111) with scanning tunneling microscopy at room-temperature. At low coverage, “slow” diffusion of individual 2HTPP molecules along the close-packed atomic rows of the substrate is observed, and no supramolecular ordering occurs. However, at higher coverage, the formation of ordered, checkerboard-like domains is found, with two molecules per unit cell at different distances from the surface. This behavior is attributed to a complex interplay of site specific molecule–substrate interaction, mainly the strong interaction between the iminic N atoms and Cu substrate atoms, with intermolecular T-type and π–π interactions

Coverage Dependent Disorder–Order Transition of 2H-Tetraphenylporphyrin on Cu(111)

In this study, we investigate the peculiar coverage dependent supramolecular arrangement of 2H-tetraphenylporhpyrin (2HTPP) on Cu(111) with scanning tunneling microscopy at room-temperature. At low coverage, “slow” diffusion of individual 2HTPP molecules along the close-packed atomic rows of the substrate is observed, and no supramolecular ordering occurs. However, at higher coverage, the formation of ordered, checkerboard-like domains is found, with two molecules per unit cell at different distances from the surface. This behavior is attributed to a complex interplay of site specific molecule–substrate interaction, mainly the strong interaction between the iminic N atoms and Cu substrate atoms, with intermolecular T-type and π–π interactions

Coverage Dependent Disorder–Order Transition of 2H-Tetraphenylporphyrin on Cu(111)

In this study, we investigate the peculiar coverage dependent supramolecular arrangement of 2H-tetraphenylporhpyrin (2HTPP) on Cu(111) with scanning tunneling microscopy at room-temperature. At low coverage, “slow” diffusion of individual 2HTPP molecules along the close-packed atomic rows of the substrate is observed, and no supramolecular ordering occurs. However, at higher coverage, the formation of ordered, checkerboard-like domains is found, with two molecules per unit cell at different distances from the surface. This behavior is attributed to a complex interplay of site specific molecule–substrate interaction, mainly the strong interaction between the iminic N atoms and Cu substrate atoms, with intermolecular T-type and π–π interactions

Abrupt Coverage-Induced Enhancement of the Self-Metalation of Tetraphenylporphyrin with Cu(111)

Using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), the coverage-dependent self-metalation of 2<i>H</i>-tetraphenylporphyrin (2HTPP) with Cu on Cu(111) at 400 K has been studied. At low coverages the porphyrin molecules are adsorbed as isolated molecules, and the rate of metalation is slow. As the coverage is increased beyond ∼0.36 molecules/nm², a supramolecular checkerboard structure is formed, with every second molecule slightly elevated above the surface. The appearance of this checkerboard structure coincides with a dramatic increase in the rate of metalation. This enhancement is attributed to a smaller activation barrier for the elevated molecules, which have an internal conformation similar to that of the free molecule, whereas the less reactive molecules in direct contact with the surface are strongly distorted

Surface-Anchored Metal–Organic Frameworks as Versatile Resists for Gas-Assisted E‑Beam Lithography: Fabrication of Sub-10 Nanometer Structures

We demonstrate that surface-anchored metal–organic frameworks (SURMOFs) are extraordinary well-suited as resists for high-resolution focused electron beam induced processing (FEBIP) techniques. The combination of such powerful lithographic protocols with the huge versatility of MOF materials are investigated in respect to their potential in nanostructures fabrication. The applied FEBIP methods rely on the local decomposition of Fe­(CO)₅ and Co­(CO)₃NO as precursors, either by the direct impact of the focused electron beam (electron beam induced deposition, EBID) or through the interaction of the precursor molecules with preirradiated/activated SURMOF areas (electron beam induced surface activation, EBISA). We demonstrate the huge potential of the approach for two different types of MOFs (HKUST-1 and Zn-DPDCPP). Our “surface science” approach to FEBIP, yields well-defined deposits with each investigated precursor/SURMOF combination. Local Auger electron spectroscopy reveals clean iron deposits from Fe­(CO)₅; deposits from Co­(CO)₃NO contain cobalt, nitrogen, and oxygen. EBISA experiments were successful with Fe­(CO)₅. Remarkably EBISA with Co­(CO)₃NO does not result in deposit formation on both resists, making the process chemically selective. Most importantly we demonstrate the fabrication of “nested-L” test structures with Fe­(CO)₅ on HKUST-1 with extremely narrow line widths of partially less than 8 nm, due to reduced electron proximity effects within the MOF-based resists. Considering that the actual diameter of the electron beam was larger than 6 nm, we see a huge potential for significant reduction of the structure sizes. In addition, the role and high potential of loading and transport of the precursor molecules within the porous SURMOF materials is discussed

Electron Beam-Induced Writing of Nanoscale Iron Wires on a Functional Metal Oxide

Electron beam-induced surface activation (EBISA) has been used to grow wires of iron on rutile TiO₂(110)-(1 × 1) in ultrahigh vacuum. The wires have a width down to ∼20 nm and hence have potential utility as interconnects on this dielectric substrate. Wire formation was achieved using an electron beam from a scanning electron microscope to activate the surface, which was subsequently exposed to Fe­(CO)₅. On the basis of scanning tunneling microscopy and Auger electron spectroscopy measurements, the activation mechanism involves electron beam-induced surface reduction and restructuring

Role of Specific Intermolecular Interactions for the Arrangement of Ni(II)-5, 10, 15, 20-Tetraphenyltetrabenzoporphyrin on Cu(111)

We have studied the coverage-dependent adsorption behavior of Ni­(II)-5,10,15,20-tetra­phenyl­tetra­benzoporphyrin on Cu(111) by scanning tunneling microscopy (STM) at room temperature. At medium coverages, the molecules self-assemble into two-dimensional islands, due to mutual stabilization through intermolecular interactions. Altogether, three different supramolecular arrangements coexist at low-to-medium coverages. On the basis of high-resolution STM images and density functional theory calculations, models for the three arrangements and the corresponding intramolecular conformations of the individual molecules are proposed. The observed polymorphism is attributed to a complex interplay of specific T-type and π–π stacking interactions between the phenyl groups. For Ni­(II)-<i>meso</i>-tetrakis (4-<i>tert</i>-butylphenyl) benzoporphyrin, in which the aromatic periphery is modified by the attachment of <i>tert</i>-butyl groups, only one supramolecular arrangement on Cu(111) is found. This difference highlights the fact that the choice of peripheral ligands of the porphyrin derivatives plays an important role in the fabrication and tailoring of functional molecular architectures