Theoretical insight in highly luminescent properties of Eu(III) complex with phenanthroline

Highlights: • Theoretical and experimental study of phen and luminescent Eu(phen)₂(NO₃)₃. • DFT/TDDFT, Judd-Ofelt, and Malta's models calculations. • Energy diagrams of absorption and excited state transitions in gas phase/DMF. • Prediction of energy transfer rates and T₁(L) → ⁵D₀ (Eu) energy transfer channel. • Key factors responsible to effective luminescent quantum yield of Eu(III). - Abstract: The complete photophysical process, from absorption in the UV region, excited state energy transitions, ligand → Eu(III) energy transfer to luminescence of Eu(phen)₂(NO₃)₃ in Vis region, is elucidated by means of two theoretical approaches and in close relation to the experiment. The energy diagram, S₁ and T₁ excited state character and the concurrent energy transitions in phen and its Eu(III) complex in gas phase and in DMF solution are predicted by DFT-TDDFT/PBE1PBE calculations. The contributions of phen and NO₃ ligands to the excited state energies are evaluated in detail. The high population of T₁ state (due to effective intersystem crossing) and its closeness in energy to the ⁵D₀ level of the Eu(III) (0.4 eV), make T₁ state the key donor for the excited state ligand-to-metal ion energy transfer. The T₁ state energy of phen chromophore determines this one in its Eu(III) and Gd(III) complexes. The solvent DMF effect on the lowest triplet excited state is insignificant. Providing similar accuracy for the vertical excited state energy as the estimates for the TDDFT/PBE1PBE energy of T₁ minimum and the experimental phosphorescence data, INDO/S-CIS method could be reliably used for further prediction of the luminescent properties. A computational strategy based on Sparkle/RM1 geometry, INDO/S-CIS excitation energies, Judd-Ofelt parameters, obtained with the QDC model and energy transfer rates predicted T₁ → ⁵D₀ as the most probable channel of the energy transfer process and luminescent quantum yield of 32% close to the experimental result of 35%.