Computational Insights into Magnetic and Optical Interactions in Molecular Qubits: From Isolated Phase to Condensed Phases

Quantum information architectures based on spin qubits implement gate operations through controllable magnetic exchange interactions, (J), which mediate entanglement between neighbouring spins, while the coherence of both spin and optical states must be preserved across different environments. This thesis presents methodological and conceptual advances for the calculation and interpretation of magnetic exchange and optical excitations in molecular qubits, considering photoexcited states, condensed phases, and hybrid interfaces. The first part introduces a broken-symmetry formalism for electronically excited states and applies it to porphyrin-based qubit–chromophore systems containing paramagnetic ions such as VIV=O/CuII. By identifying the relevant excitation and applying the broken-symmetry method at the level of the excited state obtained via TD-DFT, it has been possible to directly extract magnetic exchange couplings in the photoexcited state, providing an accurate tool for quantifying photoinduced modifications (also on the order of 10-3 cm-1) in spin-spin interactions in molecular qubits. The second part focuses on the first application of many-body perturbation theory to molecular paramagnetic systems (qubit–chromophore). Using the G0W0-BSE formalism, the thesis shows that MBPT can be applied reliably to open-shell porphyrinic systems, providing an accurate description of band excitations in the region defined as the Q band and of their excitonic fingerprints in regimes where standard TD-DFT is qualitatively inadequate. The third part proposes a study of the evolution of magnetic exchange interactions across different phases using density functional theory under periodic conditions and within the plane-wave formalism. Again, employing the broken-symmetry approach, exchange interactions were calculated in heterometallic porphyrin dimers, progressing from isolated molecules to crystalline packing and adsorption on the Au(111) surface. This analysis highlights how intermolecular contacts and surface-induced charge redistribution reshape superexchange magnetic interactions, both in magnitude and in sign. Within the same methodological framework, a second class of molecular qubits was explored through the sulfur-rich [Cu(dttt)2] complex. By tracing exchange interactions from the isolated molecule to the bulk lattice and to graphene-supported phases, this thesis work shows how strong antiferromagnetic coupling can emerge from interactions dominated by van der Waals forces, emphasizing the collective role of weak interactions in stabilizing a strong magnetic exchange interaction. Overall, the results obtained highlight how the different computational protocols employed have proven sufficiently accurate to reproduce how electronic state, packing, and photophysics govern exchange interactions in complex systems such as molecular qubits, providing useful indications for the design of chemically tunable platforms for quantum connectivity and the implementation of gate operations.