Enhanced luminescence properties of mononuclear lanthanide complexes with asymmetric seven‐coordination structures are reported for the first time. The lanthanide complexes are composed of a lanthanide ion (EuIII or TbIII), three tetramethyl heptanedionato ligands, and one triphenylphosphine oxide ligand. The coordination geometries of the lanthanide complexes have been evaluated by using single‐crystal X‐ray analyses and shape‐measurement calculations. The complexes are categorized to be seven‐coordinate monocapped octahedral structures (point group C3v). The seven‐coordinate lanthanide complexes show high intrinsic emission quantum yields, extra‐large radiative rate constants, and unexpected small nonradiative rate constants. The brilliant luminescence properties have been elucidated in terms of the asymmetric coordination geometry and small vibrational quanta related to thermal relaxation.

This work reports on lanthanide complexes with seven‐coordination structures that show brilliant luminescence properties owing to their asymmetric geometrical structure. The complexes exhibit extra‐large radiative rate constants and unexpected small nonradiative rate constants.