One Dianionic Luminophore with Three Coordination Modes Binding Four Different Metals: Toward Unexpectedly Phosphorescent Transition Metal Complexes

Abstract This work reports on a battery of coordination compounds featuring a versatile dianionic luminophore adopting three different coordination modes (mono, bi, and tridentate) while chelating Pd(II), Pt(II), Au(III), and Hg(II) centers. An in‐depth structural characterization of the ligand precursor (H₂L) and six transition metal complexes ([HLPdCNtBu], [LPtCl], [LPtCNtBu], [LPtCNPhen], [HLHgCl], and [LAuCl]) is presented. The influence of the cations and coordination modes of the luminophore and co‐ligands on the photophysical properties (including photoluminescence quantum yields (Φ_L), excited state lifetimes (τ), and average (non‐)radiative rate constants) are evaluated at various temperatures in different phases. Five complexes show interesting photophysical properties at room temperature (RT) in solution. Embedment in frozen glassy matrices at 77 K significantly boosts their luminescence by suppressing radiationless deactivation paths. Thus, the Pt(II)‐based compounds provide the highest efficiencies, with slight variations upon exchange of the ancillary ligand. In the case of [HLPdCNtBu]_, both Φ_L and τ increase over 30‐fold as compared to RT. Furthermore, the Hg(II) complex achieves, for the first time in its class, a Φ_L exceeding 60% and millisecond‐range lifetimes. This demonstrates that a judicious ligand design can pave the way toward versatile coordination compounds with tunable excited state properties.