A team of researchers from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN), Jagiellonian University, and AGH University of Kraków has developed a switchable composite material that combines the remarkable properties of molecular magnets with the mechanical robustness and flexibility of polymer fibers. The paper was published in the journal Nanoscale.
A key component of the composite is the coordination chain {NH₄[Ni(cyclam)][Fe(CN)₆]·5H₂O}ₙ, which exhibits reversible intermetallic charge transfer between iron and nickel centers. Changes in temperature, humidity, or pressure can induce a transition between different states of the material, accompanied by a pronounced and reversible change in color as well as changes in its magnetic properties. This enables the material to function simultaneously as an active magnetic component and as a visual indicator of the transition taking place within the material.
An important challenge in the practical application of such materials is their physical form. The crystalline coordination compound is brittle and has limited stability when exposed to polar liquids, particularly water, which can lead to its degradation and loss of its desired properties. To address this issue, the researchers developed a composite in which the molecular magnetic chain was incorporated into polymer fibers produced by electrospinning. Two polymer matrices were used: poly(ε-caprolactone) (PCL) and a poly(2-vinylpyridine-co-styrene) copolymer (P2VP-PS). This approach preserved the switchable properties of the coordination compound while converting it into a flexible fibrous mat with improved mechanical properties. In particular, the use of hydrophobic PCL proved highly beneficial, providing protection against the adverse effects of water.
Characterization of the resulting materials required several complementary experimental techniques, with X-ray absorption spectroscopy (XAS) playing a particularly important role. This technique makes it possible to investigate the local atomic environment of selected elements and their electronic states. XAS measurements were performed at the SOLARIS National Synchrotron Radiation Centre in Kraków, on the ASTRA beamline, by recording spectra around the K-absorption edges of iron and nickel. These measurements provided important complementary information to the magnetic and spectroscopic studies, making it possible to correlate the observed changes in the material’s properties with changes in its electronic structure.
Read more on the SOLARIS website
Image: Temperature-dependent XAS spectra recorded at the Fe and Ni K-edges for the coordination polymer {NH4[Ni(cyclam)][Fe(CN)6]·5H2O}n in the low-temperature (LT, 260 K) and high-temperature (HT, 330 K) states. Changes in the shape and position of the absorption edges indicate a change in the electronic configuration of the Fe and Ni centers induced by temperature change. The middle panel shows the structure of the coordination polymer {NH4[Ni(cyclam)][Fe(CN)6]·5H2O}n, together with a schematic representation of electron transfer between the metal centers. The bottom panel presents the electronic configuration of the coordination polymer {NH₄[Ni(cyclam)][Fe(CN)₆]·5H₂O}ₙ in the LT and HT states. Photographs of the samples clearly show the change in color occurring between the low-temperature (LT) and high-temperature (HT) states.