Add time:08/14/2019         Source:sciencedirect.com

Quenching of the triplet states of nine different organic compounds by 1,3-diketonates of lanthanides was studied by laser flash photolysis in acetonitrile solution. Five different lanthanides were used, but terbium(III) and gadolinium(III) were investigated in detail. For comparison, two 1,3-diketonates of magnesium(II) were also studied. Observed correlations between the quenching rate constants (log kq) and the free energy change for energy transfer processes (ΔGen) allowed a distinction to be made between the energy transfer to the ligand-localized triplet states and energy transfer to the ff* states of the complexes used. The observation of sensitized emission of Tb(III) and a correlation of the donor triplet decay with the rise time of Tb(III) emission gave direct evidence for an energy transfer mechanism of quenching. The quantitative evaluation of the log kqvs. ΔGen correlation led to very small values of the transmission coefficients (κen in the region of 10−6) for energy transfer to the metal-centered ff* states of the lanthanide complexes. These transmission coefficients are from one to three orders of magnitude lower than those obtained for energy transfer to ligand-localized triplet states or to excited dd states of transition metal 1,3-diketonates. An additional quenching process, i.e. electron transfer from the acetylacetonate ligand to the benzophenone triplet state, was observed for benzophenone derivatives as donors.

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