Molecular motion is one of the structural foundations for the development of functional molecular materials such as artificial motors and molecular ferroelectrics. Herein, we show that pendulum-like motion of the terminal group of a molecule causes a ferroelectric phase transition. Complex 4-methoxyanilinium tetrafluoroborate-18-crown-6 ([C(7)H(10)NO(18-crown-6)](+)[BF(4)](-), 1) shows a second-order ferroelectric phase transition at 127 K, together with an abrupt dielectric anomaly, Debye-type relaxation behavior, and the symmetry breaking confirmed by temperature dependence of second harmonic generation effect. The origin of the polarization is due to the order-disorder transition of the pendulum-like motions of the terminal para-methyl group of the 4-methoxyanilinium guest cation; that is, the freezing of pendulum motion at low temperature forces significant orientational motions of the guest molecules and thus induces the formation of the ferroelectric phase. The supramolecular bola-like ferroelectric is distinct from the precedent ferroelectrics and will open a new avenue for the design of polar functional materials.

A simple organic salt, diisopropylammonium chloride, shows the highest ferroelectric phase transition temperature among molecule-based ferroelectrics with a large spontaneous polarization, making it a candidate for practical technological applications.

Recently, very intensive research interests have focused on the exploration of metal-organic frameworks (MOF) as potential ferroelectrics and multiferroics, which exhibit the coexistence of magnetic and ferroelectric ordering. [1][2][3][4][5][6] One example is the class of MOFs with the formula [{cationic guest molecule}{(metal ion)(formate) 3 }], [1][2][3][4] which closely resemble pure inorganic ferroelectric BaTiO 3 and multiferroic BiFeO 3 with perovskite ABO 3 -type structures in which A is the cationic guest molecule, the B is the metal ion, and the anion O is replaced by formate.It is known that most ABO 3 -type perovskite compounds display sequential structural phase transitions. [7] For example, BaTiO 3 exhibits three structural phase transitions: a paraelectric-ferroelectric transition at 393 K (cubic m3m to tetragonal 4mm phase), a ferroelectric-ferroelectric transition at 278 K (tetragonal 4mm to orthorhombic mm2 phase), and a ferroelectric-ferroelectric transition at 180 K (orthorhombic mm2 to rhombohedral 3m phase). A similar sequence is also found in KNbO 3 , that is, a polar tetragonal phase between 498 and 708 K, an orthorhombic phase between 263 and 498 K, and a rhombohedral phase below 263 K. For comparison, the recently reported MOFs based on metal formate as anionic frameworks all display only one structural phase transition with low-temperature magnetic ordering. Pioneering work on magnetic properties, dielectric behaviors, and ferroelectric explorations of metal formates has been done by the groups of Wang and Gao, [4] Kobayashi, [1] and Cheetham.[2] To our knowledge, no structural phase transition above room temperature has been reported for these metal formates, although Kobayashi and co-workers have investigated the effect of deuteration of the neutral guest molecules in MOF [Mn 3 (formate) 6 ], which did not result in a structural phase transition above room temperature. [1] Guest molecules undergoing order-disorder motions are responsible for the inducement of (or are the driving force for) structural phase transitions, which are probably distinct from hydrogen-related order-disorder motions. Usually, replacement of a hydrogen atom by a deuterium atom results in two significant changes in physical properties: 1) sharp enhancement of phase-transition temperature (T C ), such as in the ferroelectric compounds KH 2 PO 4 and KD 2 PO 4 with DT C % 90 K and antiferroelectric compounds NH 4 H 2 PO 4 and ND 4 D 2 PO 4 with DT C % 94 K; 2) enhancement of the dielectric constant by one to two orders of magnitude or striking enhancement of the saturated polarization (P s ). Consequently, molecules or cations confined in cage-like frameworks probably only display order-disorder features like the typical ferroelectric NaNO 2 , even if the disordered atoms are bonded to H atoms. To testify this assertion, it is very important and interesting to explore perdeutero MOFs to see how deuteration affects T C and the dielectric constant, that is, the isotopic effect. Encouraged by pioneering work on...

Piezoelectric sensors that can work under various conditions with superior performance are highly desirable with the arrival of the Internet of Things. For practical applications, a large piezoelectric voltage coefficient g and a high Curie temperature T c are critical to the performance of piezoelectric sensors. Here, we report a two-dimensional perovskite ferroelectric (4-aminotetrahydropyran)2PbBr4 [(ATHP)2PbBr4] with a saturated polarization of 5.6 μC cm–2, high T c of 503 K [above that of BaTiO3 (BTO, 393 K)], and extremely large g 33 of 660.3 × 10–3 V m N–1 [much beyond that of Pb­(Zr,Ti)­O3 (PZT) ceramics (20 to 40 × 10–3 V m N–1), more than 2 times higher than that of poly­(vinylidene fluoride) (PVDF, about 286.7 × 10–3 V m N–1)]. Combined with the advantages of molecular ferroelectrics, such as light weight, easy and environmentally friendly processing, and mechanical flexibility, (ATHP)2PbBr4 would be a competitive candidate for next-generation smart piezoelectric sensors in flexible devices, soft robotics, and biomedical devices.

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