Charge-Promoted Self-Metalation of Porphyrins on an Oxide Surface

Charge and adsorption height dependence of the self-metalation of porphyrins on ultrathin MgO(001) films

We have experimentally determined the adsorption structure, charge state, and metalation state of porphin, the fundamental building block of porphyrins, on ultrathin Ag(001)-supported MgO(001) films by scanning tunneling microscopy and photoemission spectroscopy, supported by calculations based on density functional theory. By tuning the substrate work function to values below and above the critical work function for charging, we succeeded in the preparation of 2H-P monolayers which contain negatively charged and uncharged molecules. It is shown that the porphin molecules self-metalate at room temperature, forming the corresponding Mg-porphin, irrespective of their charge state. This is in contrast to self-metalation of tetraphenyl porphyrin (TPP), which occurs on planar MgO(001) only if the molecules are negatively charged. The different reactivity is explained by the reduced molecule-substrate distance of the planar porphin molecule compared to the bulkier TPP. The results of this study shed light on the mechanism of porphyrin self-metalation on oxides and highlight the role of the adsorption geometry on the chemical reactivity.

Metamagnetic transition and a loss of magnetic hysteresis caused by electron trapping in monolayers of single-molecule magnet Tb2@C79N

Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb₂@C₇₉N, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb₂@C₇₉N exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5-4 T. Monolayers of Tb₂@C₇₉N were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M_4,5 absorption edge. While in bulk powder samples Tb₂@C₇₉N behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb₂@C₇₉N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb₂@C₇₉N was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb₂@C₇₉N in monolayers is assigned to anionic Tb₂@C₇₉N^- species with doubly-occupied Tb-Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species.

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture

In this Perspective we present a comprehensive study of the multiple reaction products of metal-free porphyrins (2H-Ps) in contact with the rutile TiO₂(110) surface. In the absence of peripheral functionalization with specific linkers, the porphyrin adsorption is driven by the coordination of the two pyrrolic nitrogen atoms of the macrocycle to two consecutive oxygen atoms of the protruding O_br rows via hydrogen bonding. This chemical interaction favours the iminic nitrogen uptake of hydrogen from near surface layers at room temperature, thus yielding a stable acidic porphyrin (4H-P). In addition, a mild annealing (∼100 °C) triggers the incorporation of a Ti atom in the porphyrin macrocycle (self-metalation). We recently demonstrated that such a low temperature reaction is driven by a Lewis base iminic attack, which lowers the energy barriers for the outdiffusion of Ti interstitial atoms (Ti_int) [Kremer et al., Appl. Surf. Sci., 2021, 564, 150403]. In the monolayer (ML) range, the porphyrin adsorption site, corresponding to a TiO-TPP configuration, is extremely stable and tetraphenyl-porphyrins (TPPs) may even undergo conformational distortion (flattening) by partial cyclo-dehydrogenation, while remaining anchored to the O rows up to 450 °C [Lovat et al., Nanoscale, 2017, 9, 11694]. Here we show that, upon self-metalation, isolated molecules at low coverage may jump atop the rows of five-fold coordinated Ti atoms (Ti_5f). This configuration is associated with the formation of a new coordination complex, Ti-O-Ti_5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti-O-Ti_5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti_5f rows is, however, susceptible to the cross-talk between phenyls of adjacent molecules, which ultimately drives the TiO-TPP temperature evolution in the ML range along the same pathway followed by 2H-TPP.

Self-Metalation of Porphyrins at the Solid-Gas Interface

Self‐metalation is a promising route to include a single metal atom in a tetrapyrrolic macrocycle in organic frameworks supported by metal surfaces. The molecule–surface interaction may provide the charge transfer and the geometric distortion of the molecular plane necessary for metal inclusion. However, at a metal surface the presence of an activation barrier can represent an obstacle that cannot be compensated by a higher substrate temperature without affecting the layer integrity. The formation of the intermediate state can be facilitated in some cases by oxygen pre‐adsorption at the supporting metal surface, like in the case of 2H‐TPP/Pd(100). In such cases, the activation barrier can be overcome by mild annealing, yielding the formation of desorbing products and of the metalated tetrapyrrole. We show here that the self‐metalation of 2H‐TPP at the Pd(100) surface can be promoted already at room temperature by the presence of an oxygen gas phase at close‐to‐ambient conditions via an Eley–Rideal mechanism.

Benchmarking atomically defined AFM tips for chemical-selective imaging

Controlling the identity of the tip-terminating atom or molecule in low-temperature atomic force microscopy has led to ground breaking progress in surface chemistry and nanotechnology. Lacking a comparative tip-performance assessment, a profound standardization in such experiments is highly desirable. Here we directly compare the imaging and force-spectroscopy capabilities of four atomically defined tips, namely Cu-, Xe-, CO-, and O-terminated Cu-tips (CuOx-tips). Using a nanostructured copper-oxide surface as benchmark system, we found that Cu-tips react with surface oxygen, while chemically inert Xe- and CO-tips allow entering the repulsive force regime enabling increased resolution. However, their high flexibility leads to imaging artifacts and their strong passivation suppresses the chemical contrast. The higher rigidity and selectively increased chemical reactivity of CuOx-tips prevent tip-bending artifacts and generate a distinct chemical contrast. This result is particularly promising in view of future studies on other metal-oxide surfaces.

Demonstrating the Impact of the Adsorbate Orientation on the Charge Transfer at Organic-Metal Interfaces

Charge-transfer processes at molecule-metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the metal's work function and the adsorbate's electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate's orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic-metal interfaces through the adjustable molecular alignment.

Charge-Promoted Self-Metalation of Porphyrins on an Oxide Surface

Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.