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.