Systematic Introduction of Aromatic Rings to Diphosphine Ligands for Emission Color Tuning of Dinuclear Copper(I) Iodide Complexes

We
have newly synthesized two solution-stable luminescent dinuclear
copper­(I) complexes, [Cu2(μ-I)2(dpppy)2] (Cu-py) and [Cu2(μ-I)2(dpppyz)2] (Cu-pyz), where dpppy =
2,3-bis­(diphenylphosphino)­pyridine and dpppyz = 2,3-bis­(diphenylphosphino)­pyrazine,
using chelating diphosphine ligands composed of N-heteroaromatic rings.
X-ray analysis clearly indicates that the molecular structures of Cu-py and Cu-pyz are almost identical with that
of the parent complex, [Cu2(μ-I)2(dppb)2] [Cu-bz; dppb = 2,3-bis­(diphenylphosphino)­benzene].
Complexes Cu-py and Cu-pyz exhibit luminescence
[emission quantum yield (Φem) = 0.48 and 0.02, respectively]
in the solid state at 298 K. A wide emission color tuning, from 497
to 638 nm (energy = 0.55 eV, with an emission color ranging from green
to reddish-orange), was achieved in the solid state by the introduction
of pyridinic N atoms into the bridging phenyl group between the two
diphenylphosphine groups. Density functional theory calculations suggest
that the emission could originate from the effective combination of
the metal-to-ligand charge-transfer excited state with the halide-to-ligand
charge-transfer excited state. Thus, the emission color change is
due to stabilization of the π* levels of the central aryl group
in the diphosphine ligand. Furthermore, these copper­(I) complexes
exhibit thermally activated delayed fluorescence at 298 K because
of the small singlet–triplet energy difference (ΔE = 523 and 564 cm–1 for Cu-py and Cu-pyz, respectively). The stability of these complexes
in chloroform, due to the rigid bonds between the diphosphine ligands
and the CuI ions, enables the preparation of emissive poly­(methyl
methacrylate) films by the solution-doping technique.