Energy-Transfer Mechanism in Photoluminescent Terbium(III) Complexes Causing Their Temperature-Dependence

Abstract Photoluminescence of Terbium(III) complexes was investigated as a function of temperatures in the range of 80–280 K for [Tb(bfa)3(H2O)2] (bfa: 4,4,4-trifluoro-1-phenyl-1,3-butanedionato), [Tb(hfa)3(H2O)3] (hfa: hexafluoroacetylacetonato), [Tb(tfa)3(H2O)2] (tfa: trifluoroacetylacetonato), [Tb(acac)3(H2O)3] (acac: acetylacetonato), and [Tb(hfa)3(tppo)2] (tppo: triphenylphosphine oxide). These complexes were classified into the two groups with different temperature-dependences. The first group consisting of [Tb(bfa)3(H2O)2], [Tb(tfa)3(H2O)2], and [Tb(acac)3(H2O)3] showed a dependence determined by the energy gap between the excited triplet state of the ligand and the emitting level of terbium(III) ion. In contrast, for [Tb(hfa)3(H2O)3] and [Tb(hfa)3(tppo)2] containing hfa as a ligand, not only the energy gap but also the energy barriers of the “Forward energy transfer” from the ligand to terbium(III) ion and “Back energy transfer” from terbium(III) ion to the ligand were taken into account for understanding their dependences. These results are discussed based on the re-orientation of the complexes accompanied by the forward and back energy transfer processes using DFT calculations.