Modulating the ferroelectric performance by altering halogen anions in the crystals of tetranuclear copper-clusters

Abstract

Molecular ferroelectrics as a class of functional materials have broad applications in information storage and molecular devices. It is necessary to expand the ferroelectric family or modulate ferroelectricity in order to better utilize ferroelectric materials. Herein, two pairs of chiral tetranuclear copper-clusters containing different halogen anions formulated as [Cu₄(L²⁻)₂(HL⁻)₂(MeOH)₂]²⁺·2Cl⁻ (complexes 1 and 2) and [Cu₄(L²⁻)₂(HL⁻)₂(H₂O) (MeOH)]²⁺·2Br⁻ (complexes 3 and 4) were successfully synthesized through a pair of chiral Schiff-base ligand L/D-H₂L (H₂L = 2-(((1-hydroxy-3-phenylpropan-2-yl)imino)-methyl-6-methoxyphenol)), coordinate with CuCl₂·2H₂O and CuBr₂ respectively. The solid-state circular dichroism (CD) spectra excellently demonstrate that complexes 1 and 2, and complexes 3 and 4 are a pair of enantiomers, respectively. It is noteworthy to mention that the halogen anions contained in the crystal structure have a crucial effect on the ferroelectricity of tetranuclear copper-clusters. Complexes 1 and 3 can exhibit different ferroelectricity, which is verified by the hysteresis loop. When test frequency and voltage are 1 Hz and 800 V, respectively, the remanent polarization and coercive field of complex 1 are 0.68 μC cm⁻² and 1.43 kV cm⁻¹, respectively, while those of complex 3 are 0.12 μC cm⁻² and 3.45 kV cm⁻¹, respectively. The ferroelectric performance of complex 1 is superior to that of complex 3. Moreover, polarization is at the same level to that of classical ferroelectric material Rochelle salt (0.25 μC cm⁻²) and is better than some ferroelectrics incorporating halogen in coordination compounds. Importantly, this study provides a route to enlarge the ferroelectric material family or further modulate ferroelectricity by altering halogen anions in the crystal structure.