Energy Transfer Mechanism and Quantitative Modeling of Rate from an Antenna to a Lanthanide Ion

The excitation energy transfer (ET) pathway and mechanism from an organic antenna to a lanthanide ion has been the subject of discussion for many decades. In the case of europium (Eu³⁺), it has been suggested that the transfer originates from the ligand singlet state or a triplet state. Taking the lanthanide complex Eu(TTA)(HO) as an example, we have investigated the spectra and luminescence kinetics, mainly at room temperature and 77 K, to acquire the necessary experimental data. We put forward an experimental and theoretical approach to measure the energy transfer rates from the antenna to different Eu³⁺ levels using the Dexter formulation. We find that transfer from the ligand singlet state to Eu³⁺ may account for the ET pathway, by combined electric dipole-electric dipole (ED-ED) and ED-electric quadrupole (EQ) mechanisms. The contributions from the triplet state by these mechanisms are very small. An independent systems rate equation approach can effectively model the experimental kinetics results. The model utilizes the cooperative processes that take place on the metal ion and ligand and considers S₀, S₁, and T₁ ligand states in addition to ⁷F_0,1, ⁵D₀, ⁵D₁, and ⁵DJ (=⁵L₆, ⁵D₃, ⁵D₂ combined) Eu³⁺ states. The triplet exchange ET rate is estimated to be of the order 10⁷ s^-1. The observation of a nanosecond risetime for the Eu^3+ 5D₁ level does not enable the assignment of the ET route or the mechanism. Furthermore, the ⁵D₁ risetime may be contributed by several processes. Observation of its temperature dependence and also that of the ground-state population can supply useful information concerning the mechanism because the change in metal-ion internal conversion rate has a greater effect than changes in singlet or triplet nonradiative rates. A critical comparison is included for the model of Malta employed in the online software LUMPAC and JOYSpectra. The theoretical treatment of the exchange mechanism and its contribution are now being considered.