Azo-Label Heterometal-Organic Rhomboids Exhibiting Photoswitchable NIR Luminescence in Crystalline State.

Photochromism is an important strategy for realizing reversible light-controllable fluorescence switching. In spite of several reports on fluorescence switching via a photochromic process, the success of photochromic multimetallic complexes reversibly showing fluorescence switching in the solid or crystalline state has been limited for their application importance. Here, we report a photoswitchable near-infrared (NIR) fluorescence based on photochromism in the azo-label 3d/4f heterometal-organic rhomboids, (Ln = Eu (), Yb (), and Er ()), in the crystalline state. An individual metallorhomboid contains up to four azobenzene fragments, which is prepared via the three-component assembly of a -azobenzene-grafted multifunctional ligand, and 3d and 4f metal ions. The photoisomerization quantum yields of complexes can be retained or even higher when compared to the free ligand due to the modification of electronic structure. The impressive crystalline-state NIR luminescence is observed for the complexes of azo-ZnYb () and azo-ZnEr () at room temperature. Intriguingly, the switchable NIR luminescence can be effectively regulated by photochromism in the crystalline state. These features endow the self-assembly of the 3d/4f metallorhomboid with synergetic multifunctional behavior between photochromism and NIR luminescence.