Investigation of chelate formation, intramolecular energy transfer and luminescence efficiency and lifetimes in the Eu-thenoyltrifluoroacetone-trioctylphosphine oxide-Triton X-100 system using absorbance, fluorescence and photothermal measurements.

The formation and the photophysical properties of the europium-thenoyltrifluoroacetone (TTA) trioctylphosphine oxide (TOPO)-Triton X-100 chelate were investigated. When the medium is buffered with acetate, there is a strong competition between acetate and TTA for coordination with europium ions. When TOPO is added into the solution, the Eu-TTA-TOPO ternary chelate forms more easily, probably because the coligand acts as a synergic agent and would favour the formation of the enol form of TTA. Although the stoichiometric composition of the chelate is expected to be Eu(TTA)3(TOPO)2, the Eu-TTA and the Eu-TOPO mole ratios may be within 2-3 and 1-2, respectively, depending on the composition of the solution. However, the fluorescent properties of the chelate seem to be mainly dominated by its actual concentration into the solution rather than by its composition. Time resolution of europium emission spectra in the microsecond range has shown that energy transfer occurs from the TTA ligand to the 5D1 level of europium. Then, the emitting 5D0 level is populated through non-radiative deactivation of 5D1. The observed lifetimes of the 5D1 and 5D0 states are 1.25 and 860 micros, respectively. The overall fluorescence quantum yield of the chelate, measured by the photothermal method, is found to be 0.22. On the basis of the time-resolved photothermal experiments, the fluorescence quantum yield of the 5D0 state is expected to be > 0.8 and the energy transfer efficiency < 0.28.