Comparison of Phosphorescent Pt(II) Complexes with C^N^N vs. N^N^N Chelators and Caffeine-based NHC-co-ligands

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

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 (H2 L) 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 (ΦL ), 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 ΦL and τ increase over 30-fold as compared to RT. Furthermore, the Hg(II) complex achieves, for the first time in its class, a ΦL 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.