How to get deeper insights into the optical properties of lanthanide systems: a computational protocol from ligand to complexes

Theoretical calculations are proving as an essential tool to understand luminescence processes even for systems including lanthanide (Ln3+) ions. As such, the aim of this study is that of presenting a general and comprehensive theoretical protocol based on DFT calculations to rationalize and possibly drive the design of new luminescent Ln3+ complexes through the ab initio determination of the electronic properties of a ligand and two Eu3+ complexes. Different theoretical methodologies have been combined to look into the excited state energies, the luminescence quantum yield, and the energy transfer processes. The protocol has been validated for a β-diketone ligand and two Eu3+ complexes, which contain, in addition to the main ligand, ethanol or triphenylphosphine oxide. Moreover, by starting from the geometry optimization up to the estimation of the ligands' singlet and triplet lowest energy states, theoretical results quantitatively agree with luminescence experimental parameters, providing at the same time insights into the different energy transfer processes. The different quantum yields of the two complexes have been correctly reproduced.