Molecular materials, particularly those based on magnetic molecules, are a class of promising candidates for new and alternative (spin)electronic materials thanks to the interesting interplay between their conductive and magnetic properties. In this context, chemistry plays an important role in the design and construction of molecular devices thanks to the possibility of rationally tuning the physicochemical properties of such compounds. Thus, magnetic molecules can be a source of basic computational units, such as memory devices, or be used in modern computation architecture, providing examples of spin valves in spintronics or qubits in quantum technologies. They are also susceptible to being exploitable as active elements in higher complexity structures, such as multifunctional materials. Computational algorithms based on Quantum-mechanics equations allow for comprehensive explorations of the physicochemical properties of molecular materials. The ultimate objective of Computational chemistry is the extensive screening and discovery of new compounds with targeted properties. In this thesis, by means of computational studies based on first principles calculations, we assess some of the most relevant aspects of these current challenges while rationally designing molecular (spin)electronic devices. In the field of spintronics, we rationalize which are the most crucial parameters determining conductance in single molecule nanojunctions, such as the anchoring geometry between the molecule and electrodes or the interelectrode distance. We predict that the molecular design of rigid linkers is more relevant than the design of univocal anchoring positions. We con-duct a second study exploring the selective spin transport properties of three highly anisotropic Er(III)-based Single Ion Magnets coordinated by bulky organic ligands. Results show that single-molecule magnet behavior is maintained in the proposed nanodevices but different donor/acceptor character of the ligands will determine its transport properties. In the second chapter, we give some insights into the intricate relation between magnetic properties and molecular structure of single-ion molecular magnets suitable for spin qubits. We estimate the influence of physical parameters governing magnetic anisotropy in such compounds aiming for the protection and enhancement of targeted EPR- accessible spin transitions, allowing for coherent control of spin states known as Clock Transitions (CTs). We demonstrate the robustness of the hyperfine coupling in Vanadyl-type leading to well-protected spin transitions, rationalize how changes in the crystal field of a classic polyoxometallate (POM) complex may affect its previously characterized CTs and finally how strong molecular distortions can destroy or create new accessible spin levels in another POM complex with different coordination symmetry. In a second study, we apply statistical inference methodologies to uncover correlations between chemical features, such as coordination geometry and axial compression/elongation with experimentally measured spin dynamics properties. Results show that the Arrhenius energy barrier correlates remarkably well with Raman mechanisms in terms of magnetization reversal energies and that compressed pentagonal bipyramids, even based on lighter lanthanides, are very likely to show good protection of the spin dynamics against magnetic noise. In the last chapter, we performed a computational assessment accounting for electronic structure calculations, energy prediction, and interaction with semiconducting surfaces of Spin Crossover compounds (SCO) based onFe(II). We aim to present systematic theoretical procedures while studying this type of compound with a variety of purposes. We discuss the limitations of Theoretical chemistry methods in addressing low-energy spin splittings and transitions. Finally, it is shown that SCO properties are very susceptible to spin-orbit coupling effects arising from the interaction with a metallic surface in their processing as functional devices.
