Study on photophysical properties of Eu(III) complexes with aromatic β-diketones – Role of charge transfer states in the energy migration

We synthesized a set of aromatic β-diketones and measured the photophysical properties of their europium(III) complexes. According to these photophysical properties, the europium complexes can be divided into two groups: the complexes with or without the freely rotating amino-group (FRAG). On the basis of the experimental results, it can be concluded that in the FRAG complexes, the ligand-centered excitation energy is most probably transferred from a ligand to a coordinated europium via the intraligand charge transfer (ILCT) state. The temperature dependency of the lifetimes of the emissive 5D0 state revealed that in the FRAG complexes, the energy of the emissive 5D0 state is back-transferred to the ligand-to-metal charge transfer (LMCT) state and in the non-FRAG complexes, to the triplet state of the ligand. The most efficient complex synthesized was the europium complex of carbazole derivative L6 with the quantum yield of 47% and molar absorption coefficient of 70,400 M−1cm−1.