Abstract
The rate of the light-induced spin transition in a coordination polymer network solid dramatically increases when included as the core in mesoscale core-shell particles. A series of photomagnetic coordination polymer core-shell heterostructures, based on the light-switchable Rb aCo b[Fe(CN)6] c· mH2O (RbCoFe-PBA) as core with the isostructural K jNi k[Cr(CN)6] l· nH2O (KNiCr-PBA) as shell, are studied using temperature-dependent powder X-ray diffraction and SQUID magnetometry. The core RbCoFe-PBA exhibits a charge transfer-induced spin transition (CTIST), which can be thermally and optically induced. When coupled to the shell, the rate of the optically induced transition from low spin to high spin increases. Isothermal relaxation from the optically induced high spin state of the core back to the low spin state and activation energies associated with the transition between these states were measured. The presence of a shell decreases the activation energy, which is associated with the elastic properties of the core. Numerical simulations using an electro-elastic model for the spin transition in core-shell particles supports the findings, demonstrating how coupling of the core to the shell changes the elastic properties of the system. The ability to tune the rate of optically induced magnetic and structural phase transitions through control of mesoscale architecture presents a new approach to the development of photoswitchable materials with tailored properties.
Highlights
Interest in solid state spin transition materials has long been associated with their spin-state bistability and the potential to record information readable with magnetic or colorimetric detection.[1−4] Beyond memory effects, the sensitivity of spin transitions to chemical changes, often when included in porous networks, has led to studies aimed at potential uses in chemical or environmental sensing and storage.[5−11] an often overlooked feature of spin transition materials is the significant volume change associated with altered d-orbital occupancy and metal−ligand bonding in the two spin states
The ability to change volume or shape in response to a stimulus opens the possibility of using spin transition materials as components of artificial muscles[32] as was recently shown by Gural’skiy et al.[17] through the development of an electromechanical actuator made of particles of the spin crossover complex [Fe(Htrz)2(trz)](BF4) (Htrz = 1,2,4-4H-triazole and trz = 1,2,4triazole) dispersed in a poly(methyl methacrylate) matrix
Pawley refinements of powder X-ray diffraction (PXRD) patterns of each sample are consistent with two face centered cubic lattices, corresponding to the RbCoFe-PBA cores and the KNiCr-PBA shells, Figure S1
Summary
Interest in solid state spin transition materials has long been associated with their spin-state bistability and the potential to record information readable with magnetic or colorimetric detection.[1−4] Beyond memory effects, the sensitivity of spin transitions to chemical changes, often when included in porous networks, has led to studies aimed at potential uses in chemical or environmental sensing and storage.[5−11] an often overlooked feature of spin transition materials is the significant volume change associated with altered d-orbital occupancy and metal−ligand bonding in the two spin states. Using light as the stimulus, the direct conversion of light to work was demonstrated by coupling spin transition materials with an inert material in bilayer cantilevers,[15,18] which flex as the light induces the spin transition because of different coefficients of expansion. In another example, controlled heating with light was used to adjust the position of the interface between high spin and low spin domains in a single crystal, demonstrating the potential to use spin transition materials for precise actuation and for micropositioning.[19] Such ideas represent innovative strategies for light energy harvesting and for photomechanically responsive materials
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