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

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 te rbium(III) and gadolinium(III) were investigated in detail. For compar ison, 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 (Delta G(en)) allowed a distinction to be made between the energy transfer to the ligand-lo calized triplet states and energy transfer to the ff states of the co mplexes used. The observation of sensitized emission of Tb(III) and a correlation of the donor triplet decay with the rise time of Tb(III) e mission gave direct evidence for an energy transfer mechanism of quenc hing. The quantitative evaluation of the log k(q) vs. Delta G(en) corr elation led to very small Values of the transmission coefficients (kap pa(en), in the region of 10(-6)) for energy transfer to the metal-cent ered ff states of the lanthanide complexes. These transmission coeffi cients are from one to three orders of magnitude lower than those obta ined for energy transfer to ligand-localized triplet states or to exci ted dd states of transition metal 1,3-diketonates. An additional quenc hing process, ie. electron transfer from the acetylacetonate ligand to the benzophenone triplet state, was observed for benzophenone derivat ives as donors.