Hierarchical Frameworks of Metal-Organic Cages with Axial Ferroelectric Anisotropy

From individuals to families: design and application of self-similar chiral nanomaterials

Establishing an intimate relationship between similar individuals is the beginning of self-extension. Various self-similar chiral nanomaterials can be designed using an individual-to-family approach, accomplishing self-extension. This self-similarity facilitates chiral communication, transmission, and amplification of synthons. We focus on describing the marriage of discrete cages to develop self-similar extended frameworks. The advantages of utilizing cage-based frameworks for chiral recognition, enantioseparation, chiral catalysis and sensing are highlighted. To further promote self-extension, fractal chiral nanomaterials with self-similar and iterated architectures have attracted tremendous attention. The beauty of a fractal family tree lies in its ability to capture the complexity and interconnectedness of a family's lineage. As a type of fractal material, nanoflowers possess an overarching importance in chiral amplification due to their large surface-to-volume ratio. This review summarizes the design and application of state-of-the-art self-similar chiral nanomaterials including cage-based extended frameworks, fractal nanomaterials, and nanoflowers. We hope this formation process from individuals to families will inherit and broaden this great chirality.

Chiral Self-Sorting, Spontaneous Resolution, and Hierarchical Self-Assembly in Metal-Organic Cages

The ability to collectively program chiral recognition and the hierarchical self-assembly of molecular and supramolecular building blocks into complex higher-order superstructures is a significant goal in supramolecular chemistry. Metal-organic cages are excellent model systems to examine chiral self-sorting and build hierarchical self-assembly. Herein, details on how limiting the conformational flexibility and incorporating hydrogen bonding functional groups in the ligands can influence chiral self-sorting and hierarchical self-assembly of metal-organic cages are reported. The urea-functionalized axially chiral bis-pyridyl ligands afford high-fidelity in chiral self-sorting in Pd2L4 cages, when they have fewer conformations. Ligand L1, with more conformations, affords mixture of heterochiral and homochiral cages (≈70:30). Among them, the heterochiral cage adopts unusual twisted conformation and self-assembles into 2D sheets, linked by anion coordination between urea and nitrate. Ligand L2, with fewer conformations, affords homochiral cages via high-fidelity chiral self-sorting. The choice of counter anions influences further self-sorting in the solid state: racemate with PF6 - and spontaneously resolves conglomerate with BF4 -. Urea-BF4 hydrogen bonding directs hierarchical self-assembly of the Pd2L4 metal-organic cages into super-cubic networks. The study introduces a new approach in hierarchical self-assembly of metal-organic cages into higher-order networks aided by hydrogen bonding anion coordination with functional ligands.

Visualization of domain structure and piezoelectric energy harvesting in a ferroelectric metal-ligand cage

The ferroelectric behaviour of an octahedral cage [[Ni₆(H₂O)₁₂(TPTA)₈]·(NO₃)₁₂·36H₂O] (1) exhibiting high remnant polarization of 25.31 μC cm^-2 is discovered. For the first time, clear domain structures and the characteristic electromechanical responses are demonstrated using piezoresponsive force microscopy for a thin film of 1. Owing to its mechanical energy conversion capability, polymer composites of 1 were employed as efficient piezoelectric nanogenerators.

Ferroelectric Behavior of an Octahedral Metal-Ligand Cage and Its 2D-Connected Cage Framework

Supramolecular systems hold great potential as ferroelectric materials because they are easy to prepare and do not require toxic and environmentally damaging elements. However, directing the self-assembly process of a supramolecular array to yield polarizable solids is still challenging. Here, we describe induced ferroelectricity in a supramolecular framework of metal-organic cages that are supported by a flexible tripodal ligand (NHCH₂ -(3-Py))₃ PO (TPPA). Ferroelectric responses on the discrete cage [Cu₆ (H₂ O)₁₂ (TPPA)₈ ](NO₃ )₁₂  ⋅ 45H₂ O (1) and its 2D-connected framework [{Cu₆ Cl₄ (H₂ O)₆ (TPPA)₈ }(NO₃ )₈  ⋅ 60H₂ O]_n (2) yielded well-resolved rectangular hysteresis loops at room temperature with remnant polarization values of 27.27 and 29.09 μC/cm² , respectively. Thermal hysteresis measurements (THM) and capacitance-voltage (C-V) plots further corroborate the ferroelectric behavior in these compounds. The polarization in them is due to the displacements of solvated molecules and nitrate ions in the pockets of these frameworks.

Two 1D homochiral heterometallic chains: crystal structures, spectra, ferroelectricity and ferromagnetic properties

Two new homo chiral Cu-Ln (Ln = Gd and Ho) compounds bearing a chiral Schiff base ligand (1R,3S)-N',N''-bis[3-methoxysalicylidene]-1,3-diamino-1,2,2-trimethylcyclopentane (H2L) have been synthesized and characterized by elemental analysis, IR spectroscopic and single-crystal X-ray diffraction techniques. The compounds were found to exhibit 1D zig-zag skeletons with double μ-1,5 bridging dicyanamide anions. Circular dichroism (CD) spectra have been used to verify their chiroptical activities. Magnetic studies suggest that 1 and 2 hold the same magnetic behavior with the dinuclear compounds presenting ferromagnetic interaction. Furthermore, both compounds show ferroelectricity with the remnant polarization (P r) value of 0.23 and 0.18 μC cm-2 at room temperature, respectively.

Photochemical hydrogen evolution from water by a 1D-network of octahedral Ni6L8 cages

A self-assembled M6L8 type cage-connected 1D-coordination network of formula {[Ni6(MeSi(3py)3)8Cl9(H2O)2]Cl3·16H2O}∞ (1) was obtained from a 3-pyridyl substituted silane ligand MeSi(3py)3. This complex shows significantly high performance for the electrocatalytic and photocatalytic hydrogen evolution reaction (HER) in water. A maximum turnover number (TON) of 2824 has been observed for photocatalytic HER after 69 h.

Hydrogen-bonded organo-amino phosphonium halides: dielectric, piezoelectric and possible ferroelectric properties

Molecular ferroelectric materials are an exciting class of materials for potential applications in energy and electronics. Herein, we report examples of hydrogen-bonded binary salts of diphenyl diisopropylamino phosphonium halides [Ph2(iPrNH)2P]·X [DPDP·X, X = Cl, Br, I] which show dielectric, piezoelectric and NLO properties and some potentially ferroelectric attributes at room temperature. The phosphonium bromide salt was prepared by bromination of the phosphine precursor Ph2PCl and its subsequent treatment with isopropyl amine. The chloride and iodide salts were synthesized by the halogen exchange reaction of the bromide salt. The variable temperature single crystal X-ray analysis indicates the retention of the polar non-centrosymmetric phase of these materials for a wide range of temperatures from 100 to 400 K and above. All these assemblies were shown to exhibit 1D H-bonded chain structures along the crystallographic b-axis. The P-E loop measurements of these salts gave curves similar to those of non-linear leaky dielectric materials. However, the vertical piezoresponse force microscopy (V-PFM) analyses showed the existence of polarizable domain inversions indicating the possibility of ferroelectric behaviour in these materials. The temperature dependent dielectric measurements on these salts support the absence of phase transition temperatures in these assemblies. Also, bias-dependent PFM studies reveal their piezoelectric nature as the obtained converse piezoelectric coefficients are consistent with the d33 values obtained by the direct quasi-static methods.