Hierarchical Frameworks of Metal–Organic Cages with Axial Ferroelectric Anisotropy

Abstract

Designing molecular crystals with switchable dipoles for ferroelectric applications is challenging and often serendipitous. Herein, we show a systematic approach toward hierarchical 1D, 2D and 3D frameworks that are assembled through successive linkage of metal-organic cages [Cu (H O) (TPTA) ] with chloride ions. Their ferroelectric properties are due to the displacement of channel-bound nitrate counterions and solvated water molecules relative to the framework of cages. Ferroelectric measurements of crystals of discrete and 1D-framework assemblies showed axial ferroelectric anisotropy with high remnant polarisation. Both, the reversible formation of cage-connected networks and the observation of ferroelectric anisotropic behaviour are rare among metal-ligand cage assemblies.

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

“…In contrast to the previously reported cages, [26,28] the desolvation assisted dielectric transitions in 1 and 2 occur over much narrower temperature ranges. Variable temperature PXRD measurements on the bulk samples of 1 and 2 indicate loss of crystallinity above the transition temperatures.…”

“…Figures 5a and 5b shows the dielectric curves for 1 and 2 with the anomaly peaks centered on 353 and 363 K, respectively. As with other similar cages, [26] the broad nature of these anomaly peaks is attributed to the desolvation of the framework. The observed ɛ' values at 1 kHz for 1 and 2 at 298 K are 55 and 32, respectively, shooting up to 635 and 111 at their respective anomaly points.…”

“…[6,[27][28] We were able to obtain axially symmetric octahedral M 6 L 8 cages using the tripodal [PS(NH 3 Py) 3 ] ligand and reversibly connect these cages to form hierarchical frameworks of discrete, 1D, 2D and 3D-structures. [26] While the discrete and 1D-frameworks exhibited ferroelectric behavior, this could not be established for the 2D and 3D-frameworks of this series since these were plagued by poor crystal quality that hampers the electrical measurements on them. Hence, we focused on the design of new ligands to improve the structural integrity and the ferroelectric response of these materials.…”

“…[22][23] However, uncovering the mechanism behind their ferroelectric behavior is challenging because of the complexity of their structures. [6,[24][25][26] Our group has been interested in the ferroelectric behavior of metal-organic assemblies supported by di-and tripodal phosphoramide ligands of the type [PhPO(NHPy) 2 ], (Py = 3pyridyl ( 3 Py) or 4-pyridyl ( 4 Py)) and [PS(NH 3 Py) 3 ]. [6,[27][28] We were able to obtain axially symmetric octahedral M 6 L 8 cages using the tripodal [PS(NH 3 Py) 3 ] ligand and reversibly connect these cages to form hierarchical frameworks of discrete, 1D, 2D and 3D-structures.…”

“…Metal‐organic frameworks are promising ferroelectrics because of the many ways in which their structures can be constructed and modified . However, uncovering the mechanism behind their ferroelectric behavior is challenging because of the complexity of their structures …”