Enhanced Luminescence of Asymmetrical Seven-Coordinate Eu-III Complexes Including LMCT Perturbation

Luminescent mononuclear seven-coordinate Eu-III complexes, with monocapped-octahedral (point group: C-3v), monocapped-trigonal-prismatic (C-2v), and pentagonal-bipyramidal (D-5h) coordination structures, are reported. The complexes each consist of a Eu-III ion, three tetramethylheptanedionates, and a phosphine oxide derivative. Controlling steric hindrance by means of introducing methyl groups into the phosphine oxide ligands resulted in the formation of three types of coordination polyhedral structures. The coordination geometrical structures of the Eu-III complexes were evaluated by single-crystal X-ray diffraction analysis and shape-factor calculations. The radiative rate constant of the Eu-III complex with a monocapped-octahedral structure was larger than those with monocapped-trigonal-prismatic and pentagonal-bipyramidal structures. Characteristic photophysical properties of the seven-coordinate Eu-III complexes are discussed with TD-DFT calculations and Arrhenius analysis of the ligand-to-metal charge transfer.