Smart Optical Composite Materials: Dispersions of Metal–Organic Framework@Superparamagnetic Microrods for Switchable Isotropic–Anisotropic Optical Properties

Abstract

A smart optical composite material with dynamic isotropic and anisotropic optical properties by combination of luminescence and high reflectivity was developed. This combination enables switching between luminescence and angle-dependent reflectivity by changing the applied wavelength of light. The composite is formed as anisotropic core/shell particles by coating superparamagnetic iron oxide-silica microrods with a layer of the luminescent metal-organic framework (MOF) [Eu(BDC)]·2DMF·2HO (BDC = 1,4-benzenedicarboxylate). The composite particles can be rotated by an external magnet. Their anisotropic shape causes changes in the reflectivity and diffraction of light depending on the orientation of the composite particle. These rotation-dependent optical properties are complemented by an isotropic luminescence resulting from the MOF shell. If illuminated by UV light, the particles exhibit isotropic luminescence while the same sample shows anisotropic optical properties when illuminated with visible light. In addition to direct switching, the optical properties can be tailored continuously between isotropic red emission and anisotropic reflection of light if the illuminating light is tuned through fractions of both UV and visible light. The integration and control of light emission modes within a homogeneous particle dispersion marks a smart optical material, addressing fundamental directions for research on switchable multifunctional materials. The material can function as an optic compass or could be used as an optic shutter that can be switched by a magnetic field, e.g., for an intensity control for waveguides in the visible range.

Smart Surfaces: Magnetically Switchable Light Diffraction through Actuation of Superparamagnetic Plate‐Like Microrods by Dynamic Magnetic Stray Field Landscapes

“…A precondition for these effects is the reflective property of the plate‐like microrods, as it was observed and discussed in previous work . In a control experiment, spherical particles did not show a pronounced diffraction effect which is most likely related to their diameter (2 µm) being larger than the wavelength of the visible light used for illumination in the experiments as well as a more diffuse scattering by their curved surface and smaller reflection coefficients of the polymer dominating the material mix of the used spherical superparamagnetic particles (see Figure S2, Supporting Information).…”

“…The synthesized rods display superparamagnetism (Figure S1b, Supporting Information) and can be aligned in a liquid along their long axis by a macroscopic external magnetic field. This has been used in preceding work to change the optical properties (Figure S1c,d, Supporting Information), i.e., reflection and scattering of a microrod solution . It is worth noting that the microrods are remarkably little vulnerable toward irreversible agglomeration.…”

“…Synthesis of Superparamagnetic Plate‐Like Microrods : Superparamagnetic plate‐like microrods were obtained from an acid‐stabilized ferrofluid as described in a former publication . In a first step, superparamagnetic iron oxide nanoparticles were synthesized via the coprecipitation method.…”

“…Here, we present an all‐magnetic approach for a diffraction grating for electromagnetic waves in the visible spectrum with a periodically switchable grating constant and on–off functions based on the use of recently synthesized, superparamagnetic plate‐like microrods actuated by dynamically changing engineered magnetic field landscapes (MFLs) …”

Modulation of Crystallinity and Optical Properties in Composite Materials Combining Iron Oxide Nanoparticles and Dye-Containing Covalent Organic Frameworks

“…luminescence [4] and magnetism, can be successfully merged in artificial hybrid materials. In this regard, we recently succeeded in the combination of magnetic iron oxide nanoparticles and luminescent lanthanoid MOFs into nanocomposites for switching isotropic and anisotropic optical properties, [5] white magnetism, [6] shear stress detection [7] or ratiometric water sensing. [8] As another class of porous materials, covalent organic frameworks (COFs) [9] have emerged as porous crystalline materials with potential applications in gas storage, [10] heterogeneous catalysis, [11] or organic electronics.…”