The Interesting Properties of Functionalized Macromolecules on Metal Surfaces

Eva Rauls

University of Paderborn, Germany

Macromolecules offer a wide range of possible applications, like e.g. as functional agents in (nano-)medicine, catalysis, sensors or in photovoltaics. Furthermore, once synthesized, they are typically quite easy to manipulate and functionalize in order to prepare the desired property.

Porphyrins belong to this class of molecules, and upon varying them and studying model systems of these molecules in gas phase, as monolayers or as multilayers on a surface or in crystalline phases, we can learn much about the systematics and effects of their functional parts.

Corroles are structurally closely related to porphyrins, they have a lower symmetry and smaller cavities, enabling them to stabilize metal ions in exceptionally high oxidation states. This makes them highly interesting for a variety of applications especially in catalysis.

As one example, I will present our dispersion-corrected density-functional theory (DFT) calculations supported by scanning tunneling microscopy (STM) measurements and X-ray photoelectron spectroscopy (XPS) measurements on the adsorption of the free-base 5,10,15-tris(pentafluorophenyl)corrole on Ag(111) surfaces. We studied both single adsorbed molecules as well as monolayer and multilayer thin films. Single molecules adsorb with its macrocycle tilted with respect to the surface, contrasting the typical saddle-shape conformation of related porphyrin species. The tilted adsorption geometries enable the molecules to aggregate in nontrivial interwoven monolayer structures.

Our novel method to simulate the X-ray photoelectron spectroscopy (XPS) of the 1s edges of C, F, and N in conjunction with simulated STM data and the corresponding measurements nicely confirm the molecular structures concluded from the totalenergy calculations and predict the new molecular chemical states obtained after annealing at elevated temperatures. Strong changes in the symmetry of the adsorbed molecular film are observed. At 330 K, our calculations confirm the experimental observation of a singly deprotonated 2H-C species with a near-to-planar macrocycle tilted approximately 20o with respect to the surface plane. At 430 K, the calculations predict an evidence of a ring–closure reaction between a phenyl ring of the molecule and a pyrrole moiety accompanied by hydrogen and fluorine abstraction. Furthermore, polymerization reactions between adjacent molecules start to happen at these temperatures and are currently under investigation.