Multiscale Simulation of Photochemical and Irradiation-Driven Processes

Photoassisted chemical vapour deposition (PACVD) is a novel near-room-temperature deposition method in which gas-phase precursors are activated by photons rather than by thermal energy. The technique can be employed in the controlled coating of thermosensitive substrates with metals, forming thin films and nanostructured patterns through area-selective deposition (ASD). This work aims towards a comprehensive multiscale simulation of a PACVD process for the deposition of ruthenium on functionalised alkanethiol self-assembled monolayers (SAMs) on gold from Ru-carbonyl precursors. Ruthenium π-diene carbonyl complexes have emerged as promising precursors for the PACVD of Ru metal, yet their photodissociation mechanisms remain unclear. The first part of this thesis utilises quantum-chemical calculations to map the excited-state dissociation landscape of (η4-diene)Ru(CO)3 precursors [diene = butadiene (BuD), isoprene or 2-methyl-1,3-butadiene (MBuD), cyclooctatetraene (COT), 1,3-cyclohexadiene (CHD), cyclobutadiene (CBuD)]. Ground-state density functional theory (DFT) optimisations and Ru-ligand scans are combined with time-dependent DFT (TDDFT) excitations and adiabatic singlet potential energy surface profiles to identify CO- and diene-loss pathways, while energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) and natural transition orbital (NTO) analyses rationalise the Ru-diene bonding and the character of the excited states that drive reactivity. Calculated UV-vis spectra reproduce experimental trends, including the markedly higher absorptivity and extended wavelength coverage of the COT-containing complex. CO loss is generally enabled by a range of localized metal-ligand or charge-transfer excitations and internal conversion-assisted channels. In contrast, diene loss is strongly ligand dependent: BuD, MBuD, and CHD show accessible pathways, CBuD remains resistant, and COT exhibits additional complexity due to ligand flexibility and coordination changes, all consistent with experiment. These results outline favourable stepwise routes towards unsaturated fragments and provide a basis for subsequent nonadiabatic dynamics, irradiation-driven molecular dynamics (IDMD), time-resolved spectroscopy, and multiscale simulations of the PACVD processes. Besides their importance for various technological applications such as electroanalytical sensors, organic electronic devices, and catalysts, SAMs of alkanethiols on gold surfaces are excellent models for organic thin films and can be functionalised to attain ASD under PACVD of metals. However, providing a consistent computational description of the unique structural features of these systems, such as adsorption patterns, chain conformations, and superlattice arrangements, is challenging, particularly within a versatile computational framework that can simulate both the structural features of these systems and their irradiation-driven chemical transformations. The second part of this thesis systematically analyses molecular mechanics (MM) force field (FF) parameters for bonded and nonbonded (van der Waals and Coulomb) interactions in alkanethiol SAMs with different terminal groups. Using structure optimisation and energy decomposition analysis, we assess the impact of FF parameters on key properties, such as the equilibrium tilt angle, ligand packing density, and nanoscale structural organisation. Based on this detailed benchmarking, we identified an optimal set of FF parameters that reproduces the experimentally determined structural and energetic properties of alkanethiol SAMs and ensures their dynamic stability at room temperature. This provides a validated general framework for simulating pristine and functionalised alkanethiol-coated substrates under thermal conditions relevant to experimental applications. The synthesis of these two bodies of work into a multiscale simulation of the PACVD deposition of Ru on alkanethiol SAM substrates utilising the IDMD strategy implemented in the MBN Explorer software package is discussed in the final part. We conclude by proposing a MM model of the Ru-containing precursors and a strategy to extract photodissociation probabilities for the IDMD simulations from the richness of data generated by the investigations mentioned above. We also discuss what is available and what is yet needed to construct the final multiscale simulation of the PACVD process, both from the theoretical and from the experimental sides. Finally, we critically assess our method and offer a direction for future improvements that will increase its predictive power and accuracy.