Enhanced Luminescence of Asymmetrical Seven‐Coordinate EuIII Complexes Including LMCT Perturbation

Abstract

Luminescent mononuclear seven‐coordinate EuIII complexes, with monocapped‐octahedral (point group: C3v), monocapped‐trigonal‐prismatic (C2v), and pentagonal‐bipyramidal (D5h) coordination structures, are reported. The complexes each consist of a EuIII 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 EuIII complexes were evaluated by single‐crystal X‐ray diffraction analysis and shape‐factor calculations. The radiative rate constant of the EuIII 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 EuIII complexes are discussed with TD‐DFT calculations and Arrhenius analysis of the ligand‐to‐metal charge transfer.