Interfacial complexation reactions in vacuo: a self-terminating protocol via metal-organic chemical vapor deposition

The engineering of coordination complexes and metal-organic architectures at well-defined interfaces holds great promise in important research areas such as single-site catalysis and molecular magnetism. Here the ability to incorporate metal centers in unique coordination environments can convey specific functionalities to tailor the physical and chemical properties. Usually, such systems are prepared in vacuo by grafting prefabricated metal-organic complexes on surfaces or by co-deposition of molecular linkers and metal adatoms followed by complexation reactions. In the case of transition metals such as W, Ru and Ir, the latter method can be severely limited by the high sublimation temperature of these materials, whereas the former relies on the availability of established organic synthesis protocols. This prompted us to investigate an alternative pathway based on metal-organic chemical vapor deposition, exemplified here by the reaction of porphyrins with a ruthenium carbonyl precursor, Ru₃(CO)₁₂, on the Ag(111) surface. Our experiments were based on X-ray photoelectron spectroscopy (XPS) and complementary Near Edge Absorption Fine Structure (NEXAFS) with synchrotron radiation and were performed at the Materials Science Beamline. Additional support was provided by scanning tunneling microscopy (STM) measurements carried out at the Technische Universität München.
Porphyrins are key components in many biological systems and thanks to their thermal stability and chemical versatility are also widely used in technological applications, e.g. in gas sensors and dye-sensitized solar cells. Following an established procedure, first reported by G. Di Santo et al. (Chem. Eur. J. 17, 14354 (2011)) we prepared a layer of free-base porphyrins on Ag(111) by heating a condensed multilayer of meso-tetraphenylporphyrin. At ~550 K the molecules at the organic-metal interface undergo intramolecular cyclodehydrogenation reactions which result in a modified porphyrin species. High resolution STM identified the dominant product of this annealing step as the one depicted in Fig. 1a (right), and NEXAFS confirmed that the latter adsorbs almost flat on the surface. XPS measurements were acquired before and after exposing this porphyrin layer to the vapors of the Ru precursor at 300 K. The N 1s photoemission signal shows that while at this temperature the organic layer is not affected by the adsorption of Ru₃(CO)₁₂ (Figs. 1a,b), upon annealing the porphyrins readily undergo metalation (Fig. 1c). In fact, the spectrum of the free-base molecules displays two components associated with the pyrrolic and iminic N atoms of the macrocycle (green and orange in Fig. 1), while the appearance of a third chemically-shifted component at intermediate binding energy signals the incorporation of Ru into the macrocycle. Further exposure of the film to Ru₃(CO)₁₂ followed by annealing as before leads to a fully metalated layer characterized by a single component in the N 1s spectrum (Fig. 1d). Notably, by combination of quantitative XPS and STM analysis we found that the reaction proceeds without build-up of surplus material and without any surface byproducts. Moreover the 3d_5/2 signal from the Ru centers is peaked at 279.6 eV (Figs. 1c,d), typical of metallic Ru rather than Ru²⁺ as expected for an isolated Ru porphyrin. This effect in the apparent oxidation state is ascribed to charge transfer from the Ag substrate and highlights the importance of molecule-substrate hybridization and the resulting novel physical properties of organic monolayers on solid surfaces.

Figure 1: Sequential N 1s and Ru 3d_5/2 XP spectra and corresponding cartoon of the porphyrin Ru metalation process on Ag(111): (a) Monolayer of the porphyrin on Ag(111). (b) Following exposure to the Ru precursor at 300 K. (c) After annealing to ~550 K. (d) After further exposure and annealing. From A.C. Papageorgiou et al., ACS Nano 7, 4520 (2013) with permission. Copyright 2013 American Chemical Society.

In conclusion, this work demonstrates the viability of a novel approach towards the design of interfacial coordination systems. A facile, reproducible protocol was developed for the on-surface metalation of porphyrins without the need of controlling precisely the metal coverage and with no build-up of undesirable surface byproducts. This represents a versatile and simple method for tailoring the functionality of a wide variety of metal centers and for the manufacturing of composite materials.

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