Above Room Temperature Organic Ferroelectrics: Diprotonated 1,4-Diazabicyclo[2.2.2]octane Shifts between Two 2-Chlorobenzoates

Abstract

A pure organic single crystal, [H2dabco]·[2CB]2 ([H2dabco](2+) = diprotonated 1,4-diazabicyclo[2.2.2]octane, 2CB(-) = 2-chlorobenzoate), which undergoes a ferroelectric-to-paraelectric phase transition above room temperature (∼323 K upon heating), was prepared and characterized. This ferroelectric crystal possesses a distinctive supramolecular architecture composed of discrete H-bonded trimeric units (two 2CB(-) anions bridged by one [H2dabco](2+) cation through N-H···O hydrogen bond interactions). In the paraelectric phase, the [H2dabco](2+) cation is rotationally disordered and lies at the symmetric center of the trimer. Upon cooling, it is frozen in an ordered state and deviates toward a 2CB(-) anion at one end along the H-bond. The collective displacement of the cations leads to a polarization of the single crystal along the crystallographic c axis, which is confirmed by the temperature dependence of the second harmonic generation and spontaneous polarization. A significant increase in the phase transition temperature of the deuterated analogue suggests that the proton plays an important role in the ferroelectric phase transition.

Reversible Single-Crystal-to-Single-Crystal Phase Transition of Chiral Salicylidenephenylethylamine

“…A reversible SCSC phase transition can occur when the changes in the molecular conformation and packing arrangement in the crystal are very small at the phase transition, so there are no cracks or breaks in the crystal. Hence, the reversible SCSC phase transition is a fascinating phenomenon because of its potential applications in ferroelectric and switchable dielectric devices [1][2][3]. Several molecular crystals that undergo temperature-induced reversible SCSC phase transition have been reported [4][5][6][7][8][9][10].…”

A Molecular Ferroelectric Showing Room‐Temperature Record‐Fast Switching of Spontaneous Polarization

“…As one of the direct indicators for ferroelectric phase transition, temperaturedependent dielectric constants (e')o f1 along its c-axis direction display sharp peak-like anomalies at 315 K, and the fitting to Curie-Weiss law gives the Curie constants C ferro = 30.1 and C para = 98.7 (at 100 kHz, the inset in Figure 1a). These values are of the same order of magnitude to some molecular ferroelectrics, [13] such as dabco·chlorobenzoate,b is(imidazolium) l-tartrate,a nd RbHSO 4 .I nc omparison with those of b-and c-axes,the dielectric anomalies along c-axis are much stronger,s uggesting ac lear crystallographic anisotropy of dielectric properties ( Figure S5). Besides,b oth crystal and power samples of 1 display smaller should-like dielectric anomalies around 324 K, which relate to acommon dielectric transition below its thermal decomposition temperature ( % 485 K, Figure S6).…”

“…[8] However,e xcept for this exception, the majority of molecular ferroelectrics are only enable to switch the ferroelectric polarization at quite low operating frequencies (< 500 Hz), such as croconic acid at 1Hz, [7] (methylviologen)[BiI 3 Cl 2 ]a t1 0 À3 Hz, [9] diisopropylammonium bromide at 25 Hz, [6c] [Hcha-(18-crown-6)] + [BF 4 ] À (Hcha = cyclohexylammonium) at 50 Hz, [10] [H 2 -TPPZ][H xa ] 2 (TPPZ = 2,3,5,6-tetra(2-pyridyl)pyrazine,H xa = chloranilate or bromanilate) at 1Hz, [11] PHz-H 2 xa (Phz = phenazine) at 10 Hz. [12] To date,few organic ferroelectric crystals have been discovered that have well-defined polarization versus electric field (P--E)h ysteresis loops under high operating frequency at room temperature; [13] the steric difficulty in reorienting dipoles gives rise to slow switching of polarization for molecular ferroelectrics.…”

“…To obtain ad eep insight into its dynamics,w em easured the in situ solid-state NMR spectra for 1.A ss hown in Figure 5a,t he 13 CNMR spectroscopy signals remain almost unchanged from 300 to 313 K, but become sensitive to temperature in the vicinity of T c .A t3 15 K, chemical shifts at 64.9 and 55.2 ppm (assigned to -CH 2 group of cations) show obvious changes,s uch as the enhancement of intensity and narrowing of linewidth. Particularly,t he broad shifts at approximately 64.9 ppm become indistinguishable,a nd the peak at 44.8 ppm (assigned to -CH 3 )e xhibits as mall shift above T c .T his feature behaves as an indicator for the orientational motions of flexible moieties in 1.…”

Designing and Constructing a High‐Temperature Molecular Ferroelectric by Anion and Cation Replacement in a Simple Crown Ether Clathrate

“…Most new molecular ferroelectrics usually come from the known ferroelectric system such as the derivativeo fR ochelle salt, some typical ABX 3 organic-inorganic hybridsa nd the organic amine crathrate etc. [14][15][16][17][18][19][20][21][22][23][24][25][26] How to precisely design and construct new family molecular ferroelectrics through the regulation of an existing molecule system is ag reat challenge. [1,7,[27][28][29] As is known, to be a ferroelectric it must satisfy some basic conditions, i) undergoes as olid reversible phase transition, ii)the ferroelectric should crystallize in the ten polar point groups1 ,m,2 ,mm2, 3, 3m, 4, 4mm, 6, 6mm, iii)exists as pontaneousp olarization which can be changed by the electric field.…”