Temperature Switch of LMCT Role: From Quenching to Sensitization of Europium Emission in a ZnII−EuIII Binuclear Complex

The synthesis, structural investigation, and photophysical properties of a new heterobinuclear complex, [Zn(H2O)(valpn)Eu(NO3)3], are reported [H2valpn = 1,3-propanediylbis(2-iminomethylene-6-metoxy-phenol)]. In the absence of the antenna-type sensitization of europium emission at room temperature, the strongest metal-centered emission was obtained following excitation into the 7F0−5D2 transition at 535 nm. In contrast, at 80 K, the strongest emission of europium was obtained by exciting into the maximum of a high-intensity, low-lying ligand-to-metal charge-transfer band (LMCT) located at ∼425 nm. The overall temperature-induced changes of the photophysical properties of europium were assigned to the relative location of the LMCT and 3ππ* ligand states to the europium excited levels. The results may explain the lack of the antenna effects reported for some of the europium complexes with this type of ligand.