Structural and Photophysical Studies on Geometric (Er<sub>2</sub>Yb<sub>2</sub>/Yb<sub>2</sub>Er<sub>2</sub>) and Configurational (EuTb<sub>3</sub>/Eu<sub>3</sub>Tb) Isomers of Heterotetranuclear Lanthanide(III) Complexes

Heterotetranuclear geometrical (Er<sub>2</sub>Yb<sub>2</sub>/Yb<sub>2</sub>Er<sub>2</sub>) and configuational (EuTb<sub>3</sub>/Eu<sub>3</sub>Tb) isomeric lanthanide­(III) complexes have been synthesized and characterized by spectroscopy as well as X-ray crystallography. The geometric Er<sub>2</sub>Yb<sub>2</sub>/Yb<sub>2</sub>Er<sub>2</sub> isomers exhibit dual emissions from both erbium­(III) and ytterbium­(III) ions. For the EuTb<sub>3</sub>/Eu<sub>3</sub>Tb configurational isomers, the Tb<sup>III</sup> subunits transfer energy to the Eu<sup>III</sup> centers in the EuTb<sub>3</sub> complex, whereas the Tb<sup>III</sup> ion in the TbEu<sub>3</sub> complex serves mainly as a structural stabilizer.