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

TLDR

To achieve reversible photoswitching of the magnetic properties of single‑molecule magnets, coordination assemblies of a mixed‑valence tetranuclear [MnII2MnIII2] complex linked by open‑ and closed‑ring isomers of a photochromic diarylethene ligand were synthesized. X‑ray crystallography revealed 1‑D chain structures of [Mn4] units bridged by diarylethene ligands coordinated to MnII via carboxylates, and dc/ac magnetic measurements showed each cluster has an ST = 9 ground state from intracluster ferromagnetic exchange, behaving as SMMs. Both 1o and 1c undergo reversible photochromic color changes between reddish‑brown and blue upon UV/visible irradiation; 1o shows no magnetic change after UV‑induced photocyclization, whereas visible‑induced photocycloreversion of 1c enhances inter‑[Mn4] interactions, altering the magnetic behavior.

Abstract

To achieve reversible photoswitching of the magnetic properties of single-molecule magnets (SMMs), coordination assemblies of a mixed valence tetranuclear [MnII2MnIII2] complex linked by open- and closed-ring isomers of a photochromic diarylethene ligand were synthesized: [Mn4(hmp)6(dae-o)2(ClO4)2]·6H2O (open-ring form; 1o) and [Mn4(hmp)6(dae-c)2(H2O)2](ClO4)2·CH3CN·4H2O (closed-ring form; 1c), where Hhmp is 2-hydroxymethylpyridine, and H2dae-o and H2dae-c are open- and closed-ring isomers of 1,2-bis(5-caxboxyl-2-methyl-3-thienyl)perfluorocyclopentene (H2dae), respectively. From X-ray crystallographic analyses, 1o and 1c have one-dimensional (1-D) chain structures with a repeating unit of [−{Mn4}−(dae)−], in which the [Mn4] units are bridged by the diarylethene ligands coordinated to the MnII sites via carboxylate groups. Magnetic measurements involving direct-current (dc) and alternating-current (ac) techniques showed that each [Mn4] cluster in the 1-D chains had an ST = 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-[Mn4] 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-[Mn4] 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.