Prediction and Description of Molecular Properties by Density Functional Theory
In this thesis density functional theory (DFT) methods were used for the quantum chemical design and description of various types of functional molecules. The topics covered in this work extend from the chemistry of molecules in solution to surface physics. Moreover, DFT calculations provide a very helpful tool for the explanation of hitherto unresolved experimental results. In one of the major topics of this thesis, they were employed to lay the groundwork for efﬁcient spin switches to be used as smart magnetic resonance imaging contrast agents based on the magnetic bistability of Ni(II)-porphyrins. Following the Light-Driven Coordination-Induced Spin-State Switch (LD-CISSS) concept the spin change is achieved by association and dissociation of axial ligands. Consequently, tailor-made molecules have to be quantum chemically designed to realize an efﬁcient LD-CISSS for each approach. Furthermore, DFT methods were used to get a more fundamental understanding of the behavior of functional surfaces. To mount the functional molecules on the surface a platform concept based on triazatriangulenes (TATA) was used. The TATA platforms form highly ordered self-assembled monolayers (SAMs) on Au(111). A precise tuning of the distance between the functionalities can be achieved by modiﬁcation of the lateral spacer side chains of the TATA platform. In this context DFT calculations were used to explain the observed conformational changes of these side chains that are induced upon adsorption on the Au(111) surface. Furthermore, the inﬂuence of the conducting Au(111) surface on the properties of the attached functional group was examined by theoretical simulations. The results of these projects provide the basis for the realization of directed motion on surfaces as well as functional materials that are based on cooperative effects between the surface-mounted functionalities like control over the ﬂuorescence emission of the attached functional group.