Electronic correlations revealed by hybrid functional and GW calculations in the anti-aromatic biphenylene molecule

Understanding the fundamental electronic properties of materials is a key step to develop innovations in many fields of technology. For example, this has allowed to design molecular based devices like organic field effect transistors, organic solar cells and molecular switches.In this thesis, the properties of advanced functional materials, in particular metal-organic molecules and molecular building blocks of 2D materials, are investigated by means of Density Functional Theory (DFT), the GW approximation (GWA) and the Bethe-Salpeter equation (BSE), also in conjunction with experimental studies. The main focus is on calculations aiming to understand spectroscopic results.In detail, the molecular architectures of lutetium-bis-phthalocyanine (LuPc2) on clean and hydrogenated vicinal Si(100)2×1, and of the biphenylene molecule on Cu(111) were analysed by means of X-ray Photoelectron spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy; DFT calculations were performed to obtain insights into the atomic and electronic structures. Furthermore, detailed information about the electronic states of the gas phase iron phthalocyanine (FePc) and of the gas phase biphenylene molecule were obtained through XPS and NEXAFS spectroscopy. I have studied by means of DFT, multiplet and GWA calculations the electronic correlation effects in these systems. Also the optical, electronic and excitonic properties of a hypothetical 2D material based on the biphenylene molecule were investigated by GWA and BSE calculations. Monolayers of metal-free phthalocyanine (H2Pc) on Au(111) and of FePc on Au(111) and Cu(100)c(2×2)-2N/Cu(111) with and without pyridine modifier were studied by XPS and final state calculations. A multiplet approach to compute L-edges employing the hybridizations function, known from dynamical mean field theory, was proposed and applied to transition metal oxides.