Environmentally Friendly Mechanochemical Syntheses and Conversions of Highly Luminescent Cu(I) Dinuclear Complexes

Luminescent
dinuclear Cu­(I) complexes, [Cu2X2(dpypp)2] [Cu-X; X = Cl, Br, I; dpypp = 2,2′-(phenylphosphinediyl)­dipyridine],
were successfully synthesized by a solvent-assisted mechanochemical
method. A trace amount of the assisting solvent plays a key role in
the mechanochemical synthesis; only two solvents possessing the nitrile
group, CH3CN and PhCN, were effective for promoting the
formation of dinuclear Cu-X. X-ray analysis revealed
that the dinuclear structure with no Cu···Cu interactions,
bridged by two dpypp ligands, was commonly formed in all Cu-X species. These complexes exhibited bright green emission in the
solid state at room temperature (Φ = 0.23, 0.50, and 0.74; λem = 528, 518, and 530 nm for Cu-Cl, Cu-Br, and Cu-I, respectively). Emission decay measurement
and TD-DFT calculation suggested that the luminescence of Cu-X could be assigned to phosphorescence from the triplet metal-to-ligand
charge-transfer (3MLCT) excited state, effectively mixed
with the halide-to-ligand charge-transfer (3XLCT) excited
state, at 77 K. The source of emission changed to thermally activated
delayed fluorescence (TADF) with the same electronic transition nature
at room temperature. In addition, the CH3CN-bound analogue,
[Cu2(CH3CN)2­(dpypp)2]­(BF4)2, was successfully mechanochemically
converted to Cu-X by grinding with solid KX in the presence
of a trace amount of assisting water.