Morphology-Dependent Interaction between Tetraphenylporphyrins and a Metal Oxide Surface: Electronic Signature, Photo Physics, and Chemical Interaction

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Porphyrin-substrate hybrid systems are the building blocks in a series of materials, such as organic light-emitting diodes, chemical sensors, dye-sensitized solar cells and catalysts. Functional layers and 2D nanostructures on surfaces enable sensors, and nanoscale optical and magnetic materials. 3D porphyrin structures are for instance used in controlling and manipulating light at subwavelength dimensions. Understanding and correctly describing the way molecules interact with the substrate and in thin-film structures gives a handle on geometrical and electronic device properties. Indeed, adsorption and film-formation have a relevant effect on the molecular electronic structure even in the absence of strong covalent bonding, namely in terms of level-alignment with substrate states in the former and polarization-induced renormalization of molecular band gaps in the latter.Using the case of Co(II)-tetraphenylporphyrin (CoTPP) on the MgO(100), we shine light on the adsorption at morphology-related low-coordinated surfaces sites and the effect of multilayer film formation from a theoretical perspective in a frame work of hybrid density functional theory and many-body perturbation theory. We simulate the photoemission spectra for relevant adsorption sites as a fingerprint with distinct site-related features in both valence and core-level regions of the electronic structure that are also observed in experiments on MgO(100) films on Ag(100) and thus aid the identification of sites. Effects of the formation of porphyrin films on the electronic and optical properties will be outlined for H2TPP and MgTPP.We also highlight the self-metalation of H2TPP at morphology-related low-coordinated sites. Metalation is an important pathway for functionalization of porphyrin-based organic-inorganic structures. We address the self-metalation mechanism using ab initio molecular dynamics simulation at room temperature. While H2TPP is mobile on the pristine surface as the steric hindrance by phenyl rings prevents the physisorption of the macrocycle at a specific site, step edges or kink sites provide anchor points exposing low-coordinated, reactive oxygen-sites to hydrogens of the macrocycle. We report a spontaneous proton transfer at these sites forming an intermediate complex before the metalation occurs. This highlights the role of low-coordinated surface sites for the chemical interaction with the porphyrin.