Theoretical Method for an Accurate Elucidation of Energy Transfer Pathways in Europium(III) Complexes with Dipyridophenazine (dppz) Ligand: One More Step in the Study of the Molecular Antenna Effect

A theoretical protocol to study the sensitization and emission mechanism in lanthanide compounds on the basis of multireference CASSCF/PT2 calculations is proposed and applied to [Eu(NO₃)₃(dppz-CN)] and [Eu(NO₃)₃(dppz-NO₂)] compounds synthesized and characterized herein. The method consists of a fragmentation scheme where both the ligand and the lanthanide fragments were calculated separately but at the same level of theory, using ab initio wave-function-based methods which are adequate for the treatment of quasi-degenerate states. This is based on the fact that the absorption is ligand-localized and the emission is europium-centered. This characteristic allowed us to describe the most probable energy transfer pathways that take place in the complexes, which involved an ISC between the S₁ to T₁ ligand states, energy transfer to ⁵D₂ in the lanthanide fragment, and further ⁵D₀ → ⁷F_J emission. For both compounds, the triplet and ⁵D₂ states were determined at the CASPT2 level to be around ∼26000 and ∼22400 cm^-1, respectively. This difference is in the optimal range for the energy transfer process. Finally, the emissive state ⁵D₀ was found at ∼18000 cm^-1 and the emission bands in the range 550-700 nm, in quite good agreement with the experimental results.