Spectroscopic study of intramolecular energy transfer in a phosphine oxide Eu³⁺ complex: A stepwise process induced by intermediate energy levels

The intramolecular energy transfer process in a ternary Europium(III) complex Eu(TTA)₃(TAPO)₂ (1, TAPO = (4-diphenylamine- phenyl)-diphenylphosphine oxide, TTA = 2-thenoyltrifluoroacetonate) was investigated with both steady-state and time-resolved spectroscopic approaches to figure out the detail of the energy transfer between the neutral and anion ligands. The energy transfer from the first singlet excited energy level (S ₁) of the anion ligand TTA to the first triplet excited energy level (T₁) of TAPO was first proved. This discovery supports that when the neutral ligand has an appropriate T₁ level between the S₁ and T₁ levels of the anion ligand, the energy can be transferred from the S₁ energy level of the neutral ligands to ⁵D ₀ of Eu³⁺ through a stepwise process: S₁ ^TAPO → S₁^TTA → T₁ ^TAPO → T₁^TTA → ⁵D ₀^Eu3+5. This kind of sequential process makes intramolecular energy transfer more efficient, and is one of the most important factors resulting in the great improved photoluminescent performance of 1. It is shown that in ternary lanthanide complexes the efficient energy transfer between the neutral and anion ligands is crucial to the luminescent performance of the complex. And the intramolecular energy transfer can be facilitated by the ladder-like distribution of the excited energy levels.