Abstracts

The bright luminescence of lanthanoid complexes (including enolates) is intrinsically related to the ligand-to-metal energy transfer processes, which are, in turn, dependent on the lifetime of the ligand excited states. Such lifetimes are in itself somewhat challenging to measure through emission spectroscopy when the efficiency of these energy transfer processes are high, and the excited states of the organic moiety are thus almost completely quenched (as is the case of europium(III) 1,3-diketonates). In this context, the femtosecond transient absorption (fs-TA) spectroscopy is a powerful technique which can probe these states even in the presence of energy transfer processes, and that has been gaining attention over the past years in the area of lanthanoid spectroscopy. However, there is still a lack of kinetic data for the most prevalent lanthanoid (III) 1,3-diketonate complexes, such as S₁/T₁ lifetimes, and intersystem crossing/energy transfer rates. In order to fill this gap, we’ve chosen the tetraethylammonium salts of tetrakis(2-thenoyltrifluoroacetonato)europate(III), Et₄N[Eu(tta)₄], and tetrakis(2-thenoyltrifluoroacetonato)lanthanate(III), Et₄N[La(tta)₄], as candidates to determine the lifetimes/kinetic constants. These organic salts are thermodynamically more stable than the tri-coordinated counterpart [Ln(tta)₃(H₂O)₂] (Ln: lanthanoid(III) ion) in acetonitrile solution and are a reliable source of [Ln(tta)₄]⁻ anions. The compounds were prepared following the standard procedure from the literature and characterized via single-crystal X-ray diffraction. The fs-TA measurements showed a S₁ lifetime of ~200 fs followed by the vibrational cooling of the excited T₁ state in ~10 ps in both Eu³⁺ and La³⁺ complexes. These lifetimes agree with the ones obtained from the fs-TA of the free anionic ligand in the potassium 2-thenoyltrifluoroacetonate. This excited state was confirmed to be a triplet as it outlived the duration of the femtosecond-TA experiment (3.3 ns) in the Ktta and Et₄N[La(tta)₄] solutions. In contrast, the excited T₁ state absorption rapidly decayed in the [Eu(tta)₄]⁻ complex with a lifetime of ~320 ps, decay which is attributed to the intramolecular energy transfer process involving the T₁ and the excited states of the 4f⁶ configuration. From these data, we approximate the energy transfer rate (W_ET ~ 3 × 10⁹ s⁻¹) and calculate the lower limit for the energy transfer efficiency at η_ET ≥ 98%. These results demonstrate the power of the femtosecond transient absorption spectroscopy in the analysis of the energy transfer processes in lanthanoid(III) chelates. Furthermore, future measurements of ns-TA (Flash Photolysis) will allow a more precise determination of the W_ET and η_ET and, together with emission quantum yield values, make the calculation of the S₁ → T₁ intersystem crossing rate possible. The values of these rates are important given that the 2-thenoyltrifluoroacetone ligand is among the most widely used in Eu³⁺ chelates and these rates are paramount to the theoretical modelling and future development of such complexes.