Two [Ln4] molecular rings folded as compact tetrahedra.

The design of photoactive ligands may provide an entry into multifunctional molecular nanomagnets. We report here the design and synthesis of a new multitopic chelating ligand, 1,2-bis-(5-(N’-(pyridine-2-yl-methylene)-carbohydrazide)-3-methyl-thien-3-yl)-cyclopentene (H2L), bearing a photoswitchable spacer and demonstrate that it undergoes reversible photo isomerization in solution. H2L reacts with LnCl3 salts (Ln=Dy, Tb) to form the supramolecular assemblies H2O@[Ln4L4Cl4(H2O)4] (Ln=Dy, 1; Tb, 2). Single crystal X-ray diffraction data from 1 and 2 reveal that both compounds consist of highly symmetric metallacycles efficiently folded through intramolecular weak interactions in form of tetrahedra. This arrangement imparts a conformation onto the photochromic spacer of the ligand that inhibits its photoswitching activity. Despite the low symetry of the coordination geometry around the Ln ions, the Dy analog exhibits single molecule magnet behavior. The relaxation rate under various conditions has been investigated, allowing to elucidate the mechanisms operating for this relaxation.