Dielectric-Optical Switches: Photoluminescent, EPR, and Magnetic Studies on Organic-Inorganic Hybrid (azetidinium)2MnBr4

Abstract

A new organic-inorganic hybrid, AZEMnBr, has been synthesized and characterized. The thermal differential scanning calorimetry, differential thermal analysis, and thermogravimetric analyses indicate one structural phase transition (PT) at 346 and 349 K, on cooling and heating, respectively. AZEMnBr crystallizes at 365 K in the orthorhombic, Pnma, structure, which transforms to monoclinic P2₁/n at 200 K. Due to the X-ray diffraction studies, the anionic MnBr₄^2- moiety is discrete. The azetidinium cations show dynamical disorder in the high-temperature phase. In the proposed structural PT, the mechanism is classified as an order-disorder type. The structural changes affect the dielectric response. In this paper, the multiple switches between low- and high- dielectric states are presented. In addition, it was also observed that the crystal possesses a mutation of fluorescent properties between phase ON and OFF in the PT's point vicinity. We also demonstrate that EPR spectroscopy effectively detects PTs in structurally diverse Mn(II) complexes. AZEMnBr compounds show DC magnetic data consistent with the S = 5/2 spin system with small zero-field splitting, which was confirmed by EPR measurements and slow magnetic relaxation under the moderate DC magnetic field typical for a single-ion magnet behavior. Given the above, this organic-inorganic hybrid can be considered a rare example of multifunctional materials that exhibit dielectric, optical, and magnetic activity.

Figure 1

Single crystals of AZEMnBr crystallized from aqueous solution (a) under ambient light and (b) under UV light.

Figure 2

Packing of the AZEMnBr structure at (a) 200 K (phase II) and (b) 365 K (phase I). Displacement ellipsoids are plotted at the 50% probability level.

Figure 3

Distortion models for N1 and N5 cations at 200 (phase II) and 365 K (phase I). Displacement ellipsoids are plotted at the 50 and 10% probability level for phase II and I, respectively. At phase I, the orientation of pseudo-2-fold axes (pseudo D_4h symmetry) generates overall disorder of the N1 and N5 cations.

Figure 4

(a) DSC runs measured for AZEMnBr (solid line) upon heating (red) and cooling (blue) run. The observation of the crystal under the polarized microscope at (b) 313 K (phase II) and (c) 363 K (phase I).

Figure 5

(a) Temperature dependence of the real (ε′) part of the complex dielectric constant (ε*) measured for AZEMnBr. The inset presents the heating and cooling cycles for f = 24 kHz, 218 kHz, and 2 MHz. (b) Cycles of switching ON and OFF of ε′ between 300 and 370 K measured at 2 MHz.

Figure 6

UV–Vis spectra: (a) absorption, (b) excitation luminescence, and (c) emission of the AZEMnBr monocrystals.

Figure 7

(a) Dependency of normalized emission intensity versus temperature. (b) Cycles of switching ON and OFF of emission spectra at 313 K (phase II) and 363 K (phase I) for an excitation wavelength of 530 nm.

Figure 8

Temperature-dependent EPR spectra of Mn(II) ions in AZEMnBr (a) and simulations of the spectra recorded at 200 (b) and 360 K (c).

Figure 9

DC magnetic data for AZEMnBr. (a) Thermal dependencies of χ_MT (half -open circles) and χ_M (open circles); the insets show thermal dependencies of inverse magnetic susceptibility; and (b)—field dependence of the magnetization per formula unit. The solid lines (on both graphs) are calculated using the spin Hamiltonian given in eq 4.

Figure 10

(a) Argand diagram for AZEMnBr. (b) Frequency dependence of the AC susceptibility components for AZEMnBr at B_DC = 0.1 T and fixed temperature. Lines—fitted.