Crystal structures and the ferroelectric properties of homochiral metal-organic frameworks constructed from a single chiral ligand

A Strategy to Enhance the Ferroelectric Behavior of MOF-802(Hf) via Doping Zr4+ Ions

MOF ferroelectrics, as a crucial member of molecular ferroelectrics, have shown intriguing advantages owing to the designability of structures and tunability of physicochemical properties, which make them an appealing group of ferroelectric materials. However, the weak ferroelectric property still is a huge challenge for further development. Here, a series of Zr-doped MOF-802(Hf)s were successfully synthesized through doping Zr⁴⁺ ions into the parent MOF-802(Hf) to improve ferroelectric properties. The well-shaped P-E hysteresis loops of Zr-doped MOF-802(Hf)s illustrate their ferroelectricity, and ferroelectric properties are effectively enhanced compared with the parent MOF-802(Hf). What's more, remanent polarization reaches 0.511 μC/cm² when the concentration of Zr⁴⁺ ions is 5%, which is 5 times higher than that of the parent MOF-802(Hf) and is on par with some perovskite ferroelectrics. The increased ferroelectric performance is attributed to the enhanced polarity of the whole structure triggered by lattice distortion when Hf⁴⁺ ions of the parent MOF-802(Hf) are substituted by Zr⁴⁺ ions. As far as we know, this is the first report on Hf-MOF exhibiting improved ferroelectric behaviors through doping metal ions into lattice nodes. This work demonstrates that introducing the second metal ions into lattice nodes of MOFs is an efficacious approach for exploiting MOF ferroelectrics with superior performance.

Effective Enhancement of the Ferroelectric Performance of Polar Co-Gallate MOF by Doping M2+ Ions (M = Mg, Mn, Ni) into Framework Nodes

MOF ferroelectrics have been demonstrated to be a promising candidate owing to various structures and controllable properties. However, weak ferroelectricity hampers their boom. Herein, a convenient strategy, doping metal ions into the framework nodes of parent MOF, is adopted to enhance ferroelectric performance. A series of M-doped Co-Gallate (M = Mg, Mn, Ni) were synthesized to improve ferroelectric properties. The electrical hysteresis loop demonstrated its ferroelectric behaviors, exhibiting obviously improved ferroelectric properties compared with the parent Co-Gallate. The remanent polarization was enhanced by two times for Mg-doped Co-Gallate, six times for Mn-doped Co-Gallate, and four times for Ni-doped Co-Gallate. The promoted ferroelectric performances are ascribed to the enhanced polarity of the overall structure triggered by framework distortion. Intriguingly, ferroelectric behaviors increase in the order Mg < Ni < Mn, displaying the same tendency as the difference value in the ionic radius between Co²⁺ ions and M²⁺ metal ions (M = Mg, Mn, Ni). These results demonstrate doping of metal ions is a valid strategy to enhance ferroelectric performances, which may serve as a guide in modulating ferroelectric behaviors.

Crystal morphology tuning and green post-synthetic modification of metal organic framework for HPLC enantioseparation

Chiral metal-organic frameworks (CMOFs) served as chiral stationary phases (CSPs) show great potential in enantioseparation field. However, their performance improvement are still hindered by the difficult column packed and high back pressure due to the irregular morphology and broad size scope of CMOF particles. Here, the size and morphology of achiral Co-MOF-74 were effectively adjusted by controlling the synthetic route, temperature, the ratio of reactants and the amount of 2-methylimidazole (2-MI) at first. As a result, the uniformly spherical crystals in size of about 5 μm with good dispersion were obtained. Subsequently, a simple, green post-synthetic modification strategy was proposed for the fabrication of l-tyrosine functionalized Co-MOF-74, namely Co-MOF-74-L-Tyr in H₂O by incorporating l-tyrosine into the parent framework of Co-MOF-74 to construct chiral microenvironment. The homochiral Co-MOF-74-L-Tyr CSP gave superior enantioseparation performance for the eight chiral drugs and drug intermediates, such as nitrendipine, nimodipine, benzoin, 2,2'-furoin and bi-2-naphthol to the commercial columns under normal phase condition. The good repeatability and stability of this CSP was verified by the replicate enantioseparation for nimodipine and flavanone. Furthermore, the Co-MOF-74-L-Tyr packed column was successfully applied to detect the product N-1-(1-naphthyl)ethyltosylamide (HR-8) in the asymmetric reductive amination reaction. The size/morphology-controlled synthesis coupled with the green post-synthetic modification approach paves the way to fabricate target chiral MOFs with pre-designed functional groups, which is an effective complement for the preparation of CSPs in chiral chromatography.

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

The ferroelectric performance of tetranuclear copper clusters can be modulated by altering the free halogen anions existing in the crystal structure.