Energy transfer processes in the quenching of triplet states of organic compounds by 1,3-diketonates of lanthanides( III) and magnesium( II) in acetonitrile solution. Laser flash photolysis studies

Abstract

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 k q) and the free energy change for energy transfer processes (Δ G en) 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 k q vs. Δ G en correlation led to very small values of the transmission coefficients (κ en 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.