| Tóm tắt: | Functional molecule-based solids built of metal complexes can reveal a great impact of external stimuli upon their optical, magnetic, electric, and mechanical properties. We report a novel molecular material, {[Eu<sup>III</sup>(H<sub>2</sub>O)<sub>3</sub>(pyrone)<sub>4</sub>][Co<sup>III</sup>(CN)<sub>6</sub>]}·<i>n</i>H<sub>2</sub>O (<b>1</b>, <i>n</i> = 2; <b>2</b>, <i>n</i> = 1), which was obtained by the self-assembly of Eu<sup>3+</sup> and [Co(CN)<sub>6</sub>]<sup>3−</sup> ions in the presence of a small 2-pyrrolidinone (pyrone) ligand in an aqueous medium. The as-synthesized material, <b>1</b>, consists of dinuclear cyanido-bridged {EuCo} molecules accompanied by two H-bonded water molecules. By lowering the relative humidity (RH) below 30% at room temperature, <b>1</b> undergoes a single-crystal-to-single-crystal transformation related to the partial removal of crystallization water molecules which results in the new crystalline phase, <b>2</b>. Both <b>1</b> and <b>2</b> solvates exhibit pronounced Eu<sup>III</sup>-centered visible photoluminescence. However, they differ in the energy splitting of the main emission band of a <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> origin, and the emission lifetime, which is longer in the partially dehydrated <b>2</b>. As the <b>1</b> ↔ <b>2</b> structural transformation can be repeatedly reversed by changing the RH value, the reported material shows a room-temperature switching of detailed luminescent features including the ratio between emission components and the emission lifetime values.
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