Excited-state dissociation of (η 4 -diene)Ru(CO) 3 precursors for photo-assisted chemical vapour deposition

Ruthenium π-diene carbonyl complexes have emerged as promising precursors for photo-assisted chemical vapour deposition (PACVD), yet their photodissociation mechanisms remain unclear. Here we use static quantum-chemical calculations to map the excited-state dissociation landscape of (η4-diene)Ru(CO)3 precursors (diene = BuD, MBuD, CHD, COT, CBuD). Ground-state DFT optimisations and Ru–ligand scans are combined with TD-DFT excitations and adiabatic singlet potential energy surface profiles to identify CO- and diene-loss pathways, while EDA–NOCV and natural transition orbital analyses rationalise the Ru–diene bonding and the character of the bright 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 ligand-field 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 future non-adiabatic dynamics, irradiation-driven molecular dynamics, time-resolved spectroscopy, and multiscale simulations of PACVD processes.