Functionalization of Surface Anchored Metalloporphyrins

Nanoarrays of metalloporphyrin molecules self-assembled on metal surfaces are promising candidates for applications in heterogeneous catalysis, chemical sensing and organic photovoltaics. The electronic structure and, hence, its chemical functionality of the metal center within the metalloporphyrin molecule is determined by its interaction with the supporting surface. Due to its intimate relationship with the optical absorption, the electronic structure of metalloporphyrins and its modification upon surface functionalization and chemical interaction can be characterized by optical spectroscopy. Such measurements would allow monitoring the relevant processes in situ and in real time. Here we propose the first systematic study of the optical response in the visible range of metalloporphyrins upon surface anchoring and interaction with subsequently adsorbed molecules such as CO, NO and O 2 . The optical spectra will be correlated directly with the electronic states of metalloporphyrin molecules as characterized by photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). The electronic interaction between metal center and surface will be tuned by anchoring metalloporphyrin molecules onto surfaces with well defined, yet distinct, atomic and electronic structures. For these purpose, metallo-tetraphenyporphyrins hosting different metal centers, namely, Fe, Co and Zn will be deposited onto the bare Cu(110) surface as well as Cu(110)-(2x1)O, Cu(110)-c(6x2)O and Cu(110)-(2x3)N reconstructed surfaces. The effect of such functionalization on the chemical bonding of ligand molecules will be investigated by studying the adsorption and desorption of fundamentally important diatomic molecules like NO, CO and O2 . A deeper understanding of the interfacial electronic interaction and its influence on the novel physico-chemical properties of surface anchored metalloporphyrins is expected by supplementing the proposed experiments with density function theory (DFT) studies that will be conducted in the group of W.G. Schmidt (University of Paderborn). The output of the project will be conducive for the design of surface anchored metalloporphyrin systems with novel functionalities. It will also be essential for establishing optical spectroscopy as a sensitive probe to monitor the key physical/chemical processes under real operating conditions in technical applications, such as redox catalysis, chemical sensing and light harvesting.

The electronic and crystalline structures of organic molecular layers depend strongly on the interactions with substrate surfaces. This interaction can be tuned by adsorbing organic molecules on surfaces with various atomic structures and different chemical compositions. Porphyrin molecules deposited on metal surface exhibit a layer-wise variation of their optical properties. The evolution of the crystalline structure of the wetting layer as a function of its thickness leads to distinct optical properties and allows the fabrication of molecular structures exhibiting novel excitonic properties. For rod like molecules, their orientation in the thin films depends on the balance between molecule-molecule and molecule-substrate interactions. By depositing pentacene, a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings, on the surfaces of various substrates including copper, sapphire and quartz, one can control the molecular orientation from flat-lying gradually to up-right standing. On the other hand, substrate temperature and surface adsorbates influence the molecular orientation strongly, too.Adsorbing on metal surfaces modifies the local electronic structure of organic molecules, which alters in turn their chemical properties. Our result shows that adsorbing organic molecules on metal surfaces may reduce the temperature threshold for their dissociation due to chemical interaction between molecules and the substrate. Based on the fundamental studies and the technical developments involved in this project, we have gained a deep understanding about the interactions between organic molecules and substrate surfaces regarding molecular electronic structure, crystalline structure of the molecular layer and the corresponding exitonic response.