Effect of Water Coordination on Luminescent Properties of Pyrazine-Bridged Dinuclear Cu(I) Complexes.

Two luminescent pyrazine-bridged dinuclear Cu(I) complexes, namely, [{Cu(PPh3)2(H2O)}(μ-MeOpyz){Cu(PPh3)2(CH3CN)}](BF4)2 and [{Cu(PPh3)2(H2O)}(μ-MeOpyz){Cu(PPh3)2(H2O)}](BF4)2 (H2O-Cu2-AN and H2O-Cu2-H2O; PPh3 = triphenylphosphine, MeOpyz = 2-methoxypyrazine), were successfully synthesized and characterized by single-crystal X-ray diffraction and luminescence measurements. X-ray analysis revealed that the water molecules are coordinated to both Cu(I) ions to form almost the same P2N1O1 coordination structure in H2O-Cu2-H2O, whereas one of the two Cu ions in H2O-Cu2-AN was coordinated by acetonitrile instead of water to form a different P2N2 coordination environment. The asymmetric H2O-Cu2-AN exhibits very bright yellow-green emission with a high emission quantum yield (λem = 550 nm, Φ = 0.70) at room temperature in the solid state in spite of the coordination of water molecule, which usually tends to deactivate the emissive state through O-H vibration. The intense emission at room temperature is a result of thermally activated delayed fluorescence, and the remarkable temperature dependence of emission lifetimes indicates the existence of unique multiple emission states for the asymmetric dinuclear complex. In contrast, the emission of H2O-Cu2-H2O was observed at longer wavelengths with remarkably a lower quantum yield (λem = 580 nm, Φ = 0.05). Time-dependent density functional theory calculations suggested that the emission could result from the metal-to-ligand charge-transfer transition state. However, it could be rapidly deactivated by the structural distortion around the Cu ion with a less-bulky coordination environment in H2O-Cu2-H2O.