Surface functionalization on the nanoscale is the key to tailoring sensors, catalysts, and other devices whose functional principle is based on the interaction of an active solid surface with another (liquid or gaseous) phase. Metalloporphyrins and similar planar metal complexes are particularly suitable for this purpose because they combine a structure forming element - the porphyrin framework - with an active site, the coordinated metal ion. Its planar coordination environment leaves two axial coordination sites available for additional ligands. One of these sites is occupied by the underlying surface. The resulting electronic interaction with the surface can be used to modify the electronic structure and the reactivity of the metal center. The remaining axial site is free for the attachment of molecules (sensor functionality) and can operate as a reaction center (single-site catalysis). Here, we report a fundamental study on the interaction of metalloporphyrins with an Ag(111) surface and the reaction of metalloporphyrin monolayers with small molecules of catalytic and biological relevance. Employing photoelectron spectroscopy (XPS/UPS) and complementary techniques, we focused on: (A) Synthesis of adsorbed metalloporphyrins by direct metalation of tetraphenylporphyrin monolayers with bare iron, cobalt, and zinc atoms deposited from the gas phase. Also, we demonstrated that adsorbed tetraphenylporphyrin molecules "pick up" pre-deposited Zn atoms from the Ag surface. (B) The electronic interaction between a porphyrin-coordinated metal ion and a metal surface in well-defined distances, which were adjusted with different spacer substituents on the periphery of the porphyrin ring. (C) Axial coordination of small molecules such as nitric oxide and ammonia at the metal centers of the adsorbed porphyrins, resulting in competition between the newly formed coordinate bond and the bond between metal ion and surface.
Supported by the Deutsche Forschungsgemeinschaft through SFB 583. |