Lanthanide thiocyanate complexes: a combined TD-DFT, CASSCF, and spectroscopic study of their luminescent properties

Eu(III) complexes containing NCS⁻ frequently exhibit weak luminescence, and the origin of this behavior remains insufficiently understood. In this work, time-dependent density functional theory (TD-DFT) and complete active space self-consistent field (CASSCF) calculations are combined with spectroscopic data to investigate the electronic structure of lanthanide thiocyanate complexes. Particular attention is given to the relative energies of the first excited singlet (S₁) and triplet (T₁) states and to low-energy ligand-to-metal charge-transfer (LMCT) states, aiming to clarify their roles in nonradiative deactivation and luminescence quenching. The combined theoretical and experimental approach provides valuable insights into the excited-state relaxation pathways governing the luminescent properties of these compounds.