funCOS 6: Theory

Adsorption of functional molecular structures on nanostructured metal oxide surfaces, thin films grown on a metal substrate, or nanocrystalline assemblies are pivotal for accessing novel functional materials for a wide range of applications, including photovoltaic, sensing, or catalysis. Molecular landscaping is the vision of the multidisciplinary research unit Functional Molecular Structures on Complex Oxide Surfaces (funCOS). Its aim is to understand the adsorption of functional molecular structures on the basis of a surface science approach and, to this end, explore site selective adsorption and formation of molecular assemblies. Means are the substitution of organic molecules with specific linker groups and tuning of the adsorbate-adsorbate interaction or the electronic and chemical properties via functional groups. The theory area in the funCOS research unit pursues theoretical modeling of these topics based on density-functional theory, ab initio molecular dynamic simulations, and additional high-level electronic structure methods and complements the experimental funCOS projects. Challenges for the theory area lie in the development of atomic-scale models in close collaboration with experiment and their rationalization in terms of a structure-property relationship. Modeling of adsorbate-substrate and adsorbate-adsorbate interaction will follow the funCOS approach from small to large and will take the knowledge gained in the first funding period of funCOS on linker molecules, networks of smaller organic molecules, and metalation reactions to functionalized porphyrins. Metal adatoms in combination with functionalized organic molecules may open new opportunities to create 2D metal-organic networks as a step in Molecular Landscaping. The metal oxide substrate is a key element of structure formation and thus a topic of investigations. For instance, novel nanoparticle systems will be addressed that feature surfaces of different orientation and connecting elements as adsorption sites for functional molecules. Deposition of large organic molecules from solution is a very appealing, experimentally easy to handle alternative to the gas phase preparation. To take ab initio modeling beyond the surface science approach to the metal oxide/liquid interface is a huge step. The funCOS theory area will investigate interactions of liquids with metal oxides and chemical reactions at liquid/oxide interface.

Adsorption of functional molecular structures on nanostructured metal oxide surfaces or nanocrystalline assemblies are pivotal for accessing novel functional materials for a wide range of applications, including photovoltaic, sensing, or catalysis. "Molecular landscaping" is the vision of the multidisciplinary research unit "Functional Molecular Structures on Complex Oxide Surfaces" (funCOS). Its aim is to understand the adsorption of functional molecular structures based on a surface science approach and, to this end, explore site selective adsorption and formation of molecular assemblies. The theory area in the funCOS research unit pursues theoretical modeling of these topics based on quantitative first principles methods and complements the experimental funCOS projects. Three subprojects teamed up in funCOS 6 focusing on (A) the structure and vibrational properties of molecular adsorbates, (B) the interaction of porphyrins with low coordinated surface sites as well as their electronic/optical properties, and (C) dynamical behavior of molecules at the solid/liquid interface. Challenges for the theory area lie in the development of atomic-scale models in close collaboration with experiments, the subsequent verification of the models via calculated with experimental spectral fingerprints, and their rationalization in terms of a structure-property relationship. Porphyrins are key elements in organic-inorganic hybrid systems for a wide range of applications. Understanding their interaction with the substrate gives a handle on structural and electronic device properties. The Austrian subproject B has investigated the transition-metal porphyrin Co(II)-tetraphenylporphyrin (CoTPP) on the MgO(100) surface linking to our earlier study on the adsorption of the free-base tetraphenylporphyrin (2HTPP) and its metalation. Driven by experiments in the funCOS consortium on the CoTPP adsorption on MgO films on silver substrates, we investigated the porphyrin adsorption on the prestine surface and at low-coordinated sites. For the identification of adsorption sites, we used simulated photoemission spectra in comparison to measured adsorption-related features in such spectra. We demonstrated the relevance of morphology-related low-coordinated sites for the adsorption of CoTPP on the MgO(100) surface. We also discussed scenarios for metal-exchange reactions. Motivated by funCOS experiments on multilayer porphyrin films, we investigate the impact of crystallization of prototypical porphyrins on the electronic and optical properties. Our calculations enabled a quantitative understanding of the experiments on the multilayer films. Despite the huge effect of crystallization on the electronic properties as compared with the gas phase, we find that such strong effects are absent in optical spectra. This topic is extended towards different porphyrin crystal structures and metal porphyrins MTPP with M=Fe, Co, Cu, Ni, Zn. Thin porphyrin films are employed in modern optical devices or photovoltaic applications. The insight contributed by our work may pave way to an engineering of structural, electronic, and optical properties via ligand or substituent control.