Coordination Assemblies of [Mn4] Single-Molecule Magnets Linked by Photochromic Ligands: Photochemical Control of the Magnetic Properties

To achieve reversible photoswitching of the magnetic properties of single-molecule magnets (SMMs), coordination assemblies of a mixed valence tetranuclear [(Mn2Mn2III)-Mn-II] complex linked by open-and closed-ring isomers of a photochromic diarylethene ligand were synthesized: [Mn-4(hmp)(6)(dae-o)(2)(ClO4)(2)]center dot 6H(2)O (open-ring form; 1o) and [Mn-4(hmp)(6)(dae-c)(2)(H2O)(2)](ClO4)(2)center dot CH3CN center dot 4H(2)O (closed-ring form; 1c), where Hhmp is 2-hydroxymethylpyridine, and H(2)dae-o and H(2)dae-c are open- and closed-ring isomers of 1,2-bis(5-caxboxyl-2-methyl-3-thienyl)perfluorocyclopentene (H(2)dae), respectively. From X-ray crystallographic analyses, 1o and 1c have one-dimensional (1-D) chain structures with a repeating unit of [-{Mn-4}-(dae)-], in which the [Mn-4] units are bridged by the diarylethene ligands coordinated to the Mn-II sites via carboxylate groups. Magnetic measurements involving direct-current (dc) and alternating-current (ac) techniques showed that each [Mn-4] cluster in the 1-D chains had an S-T = 9 ground state due to two kinds of intracluster ferromagnetic exchange interactions and behaved as SMMs. Both 1o and 1c underwent photochromic reactions in the crystalline phase, and their colors reversibly changed between reddish-brown and blue upon irradiation with ultraviolet (UV) and visible light. For 1o, no significant change in the magnetic properties was observed after the photocyclization of the dae-o moiety by UV irradiation. In contrast, the photocycloreversion of the dae-c moiety by visible irradiation enhanced inter-[Mn-4] unit interactions in 1c, which was triggered by the geometrical change in the diarylethene moiety during the photoreaction and the subsequent change in the inter-[Mn-4] arrangement between the neighboring 1-D chains. This work illustrates the usefulness of photochromic molecules for photoswitching the magnetic behavior of complexes, even superparamagnetic systems, with SMM units.