Quenching of triplet states of organic compounds by 1,3-diketonate transition-metal chelates in solution. Energy and/or electron transfer

Abstract

1,3-Diketonate transition-metal chelates are shown to be excellent model compounds to study the interaction between excited states of organic compounds and coordination complexes in solution. They are particularly useful for studying both energy- and electron-transfer quenching. The results of laser flash and steady-state photolysis measurements for quenching of triplet states of organic compounds by acetylacetonates (acac) and hexafluoroacetylacetonates (hfac) of transition metals (Cu(II), Ni(II), Cr(III), and Fe(III)) and lanthanides (Sm(III), Eu(III), Gd(III), Th(III), and Dy(III)) and Mg(II) in solution (acetonitrile and benzene) are reviewed. No sensitized luminescence (except for 1,3-diketonates of some lanthanides) was found. In addition, no short-lived transients were found that could be ascribed to sensitized excited states of 1,3-diketonate complexes or to electron-transfer intermediates. However, much useful indirect evidence concerning the nature of the triplet quenching was obtained. The methodology involved first measuring the quenching rate constants of a series of organic triplet states by selected 1,3-diketonate complexes and then analyzing the correlations between these rate constants and the standard free-energy changes for energy or electron transfer. With a classical framework for energy and electron transfer, these correlations were used to evaluate appropriate intrinsic barriers and transmission coefficients for both processes. The quenching data were interpreted in terms of energy and electron transfer alone or in terms of both as competing processes. It was shown that the quenching of most of the organic triplet states by 1,3-diketonates of Ni(II), lanthanides, and Mg(II) (as well as Cr(acac) 3 and Fe(acac) 3) was adequately described by the energy transfer to the ligand-localized triplet states or to the metal-centered excited dd or ff states of the complexes. The transmission coefficients of these processes were in the range of 10 −3, 10 −3–10 −5, and 10 −6, respectively. Quenching by Cu(acac) 2 was suggested to occur mainly by electron transfer, whereas quenching by Cu(hfac) 2 was shown to involve energy transfer to the ligand-localized triplet state in competition with electron transfer from the organic triplet states to the Cu(II) complex. An additional quenching process, that is electron transfer from the acetylacetonate ligand to the benzophenone triplet state, was observed for triplet states of benzophenone derivatives. The latter process led to the sensitized photochemistry of Cu(II) and Ni(II) acetylacetonates, in hydrogen-donating solvents.