Thermally Activated Self-metalation of Carboxy-functionalized Porphyrin Films on MgO Nanocubes

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.

Self-Metalation of Anchored Porphyrins on Atomically Defined Cobalt Oxide Surfaces: In situ Studies by Surface Vibrational Spectroscopy

Metalation of anchored porphyrins is essential for their functionality at hybrid interfaces. In this work, we have studied the anchoring and metalation of a functionalized porphyrin derivative, 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin (MCTPP), on an atomically-defined CoO(100) film under ultrahigh vacuum (UHV) conditions. We follow both the anchoring to the oxide surface and the self-metalation by surface Co2+ ions via infrared reflection absorption spectroscopy (IRAS). At 150 K, MCTPP multilayer films adsorb molecularly on CoO(100) without anchoring to the surface. Upon heating to 195 K, the first layer of porphyrin molecules anchors via formation of a bridging surface carboxylate. Above 460 K, the MCTPP multilayer desorbs and only the anchored monolayer resides on the surface up to temperatures of 600 K approximately. The orientation of anchored MCTPP depends on the surface coverage. At low coverage, the MCTPP adopts a nearly flat-lying geometry, whereas an upright standing film is formed near the multilayer coverage. Self-metalation of MCTPP depends critically on the surface temperature, the coverage and on the molecular orientation. At 150 K, metalation is largely suppressed, while the degree of metalation increases with increasing temperature and reaches a value of around 60 % in the first monolayer at 450 K. At lower coverage higher metalation fractions (85 % and above) are observed, similar as for increasing temperature.

Approaches to synthesize MgO nanostructures for diverse applications

Magnesium oxide remained interesting from long time for several important phenomena like; defect induced magnetism, spin electron reflectivity, broad laser emission etc. Moreover, nanostructures of this material exhibited suitability for different kinds of applications ranging from wastewater treatment to spintronics depending upon their shape and size. In this way, researchers had grown nanostructures in the form of nanoparticles, thin films, nanotubes, nanowalls, nanobelts. Though nanoparticles and thin films are well known form of nanostructures and wide variety of synthesis approaches are available, however, limited methodology for other nanostructures are available. In order to grow these nanostructures in an optimized way an understanding of these methods is essential. Thus, this review article depicts an overview of various approaches for design of different kinds of nanostructures.

Anchoring of carboxyl-functionalized porphyrins on MgO, TiO2, and Co3O4 nanoparticles

Hybrid materials consisting of functional organic molecules on metal oxide nanomaterials are key components in emerging technologies, for example in energy conversion and molecular electronics. In this work, we present the results of a comparative study of carboxyl-functionalized porphyrins on different oxide nanomaterials. Specifically, we investigated the interaction of 5(3-carboxyphenyl)-10,15,20-triphenyl-21,23H-porphyrin (2H-3-MCTPP) and 5(4-carboxyphenyl)-10,15,20-triphenyl-21,23H-porphyrin (2H-4-MCTPP), on MgO, TiO2, and Co3O4 nanoparticles (NPs) using isothermal and temperature-programmed diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). We show that both porphyrins bind to the NPs, yielding stable monolayer films consisting of tilted surface carboxylates. In all cases, anchoring through the carboxylic acid group suppresses self-metalation of the porphyrin unit. Upon annealing, all anchored porphyrin films undergo metalation. The position of the acid group has no major influence on the reactivity. The same is true for the nature of the metal oxide, suggesting that the observed behaviour is general for most anchored porphyrin films on oxide nanomaterials.