Molecular design of high-temperature organic dielectric switches

Article abstract of DOI:10.1039/c8cc07311b

A design strategy of reducing the molecular symmetry was used to obtain a series of picrate-based high-temperature phase transition compounds. Their dielectric switching behaviours accompanied by phase transitions can be attributed to the order-disorder transitions of the cations and the displacements of both cations and anions.

Full text of DOI:10.1039/c8cc07311b

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Molecular design of high-temperature organic
dielectric switches†
Cite this: Chem. Commun., 2018,
54, 13111
Lin Zhou, Ping-Ping Shi, Xuan Zheng, Fu-Juan Geng, Qiong Ye * and
Received 8th September 2018,
Accepted 26th October 2018
Da-Wei Fu
*
DOI: 10.1039/c8cc07311b
rsc.li/chemcomm
A design strategy of reducing the molecular symmetry was used to on picrate-based phase transition materials are scarce, but have
obtain a series of picrate-based high-temperature phase transition made significant progress recently. For example, Sun et al.
compounds. Their dielectric switching behaviours accompanied by reported a bi-step structural phase transition compound,
phase transitions can be attributed to the order–disorder transitions N-methylcyclohexylamine picrate, which exhibits remarkable
of the cations and the displacements of both cations and anions.
switchable quadratic NLO properties around 240 K.11 More
recently, N-methylmorpholinium picrate was also discovered
Switchable dielectrics, undergoing transitions between different to possess the fastest room-temperature switching of polariza-
dielectric states, have captured great interest owing to their tion (263 kHz) among organic ferroelectrics.12 Unfortunately,
potential applications in data communications, sensing, signal the relatively low Ttr of both compounds may be unfavourable
processing, and rewritable optical data storage, etc.1–3 Essentially, for their practical device applications, which is still a common
the switching of the dielectric constant at the phase transition hindrance to the development of molecular dielectrics.
temperature (Ttr) is caused by the motional changes of polar
Phosphonium ions possess similar structures to ammonium
moieties, such as order–disorder transformations and orienta- ions, but are rarely used to construct molecular phase transi-
tional motions.4–7 Until now, it is the inorganic materials that tion materials. Compared with the N atom, the heavier P atom
mainly dominate the dielectrics market, although most inorganic may make the potential energy barrier of cation motion higher,
materials are of high-cost, require high-temperature processing so replacing the ammonium with a phosphonium cation might
and are environmentally unfriendly. As a matter of fact, organic be a good way to improve Ttr.13,14 Therefore, the [(CH3)4P]+
materials possess the advantages of light weight, metal free cation, as an analogue of the [(CH3)4N]+ cation, was chosen
nature, mechanical flexibility, structural diversity, low cost and initially to obtain tetramethylphosphonium picrate (1). It does
easy processing, and could be potential alternatives or supple- exhibit a phase transition, while its Ttr is just at room tempera-
ments to inorganic dielectric materials.8 These properties allow ture. In order to further increase the potential energy barrier of
organics to hold
a great potential for materials science molecular motion and then tune the Ttr, we proposed to replace
and technology applications. Nevertheless, organic switchable a methyl group of [(CH3)4P]+ with a longer side-chain to disturb
dielectric materials are rarely reported so far. Due to the lack of its spherical symmetry. Herein, we successfully synthesize four
knowledge about control over the dipole motions, it is still high-Ttr picrate-based organic salts: propyl-trimethyl-phosphonium
nontrivial to find several obviously different dielectric states in a picrate (2), methoxymethyl-trimethyl-phosphonium picrate (3),
dielectric material. Experimentally, flexible moieties, whose allyl-trimethyl-phosphonium picrate (4) and (2-hydroxy-ethyl)-
motional states are susceptible to temperature, have been intro- trimethyl-phosphonium picrate (5) (Scheme 1). They display
duced to materials to bring about structural phase transitions and structural phase transitions and switchable dielectric beha-
switchable dielectric properties, such as [Hdabco]+ (dabco is 1,4- viours at 367 K, 393 K, 352 K and 347 K, respectively. After
diazabicyclo[2.2.2]octane) and [(CH3)4N]+ cations, or [ClO4]ꢀ and modifying the phosphonium cations, their Ttr all get greatly
picrate anions (picrate is trinitrophenolate).9,10 Research studies improved, being much higher than those of the reported
picrate salts and 1. Such a molecular design strategy of
Ordered Matter Science Research Center and Jiangsu Key Laboratory for Science
reducing the molecular symmetry of the cations opens a new
pathway to explore high-temperature organic dielectric switches.
Besides, the switchable quadratic NLO properties observed in 5
and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189,
People’s Republic of China. E-mail: yeqiong@seu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details and
near Ttr suggests its potential application as a dual-switching
material.
characterization data. CCDC 1866561–1866566. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c8cc07311b
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angles see Table S4, ESI†), while the two have similar flexibility,
resulting in the isostructure. When the methyl group of
the [(CH3)4P]+ cation is replaced with an allyl, 4 crystallizes in
the orthorhombic space group Pcab (point group mmm). The
–CH2CHQCH2 group is more rigid and shorter than the
–CH2OCH3 and –CH2CH2CH3 groups, and consequently, 4
shows a higher crystallographic symmetry compared to 2 and 3.
As for 5, its second harmonic generation (SHG) activity at
room temperature (vide infra) helps to define it in the non-
centrosymmetric space group P1 (point group 1). It is the one
Scheme 1 Chemical structures of the (a) propyl-trimethyl-phosphonium,
(b) methoxymethyl-trimethyl-phosphonium, (c) allyl-trimethyl-phosphonium,
(d) (2-hydroxy-ethyl)-trimethyl-phosphonium cations in 2–5, respectively.
Yellow crystals of 1–5 were obtained by the evaporation of a with the lowest symmetry among the four compounds.
mixed acetonitrile aqueous solution containing an equimolar The asymmetric units of 2–4 all contain one phosphonium
amount of picric acid and the respective phosphonium halide at cation and one picrate anion, while 5 contains two inequivalent
room temperature. Infrared spectra and powder X-ray diffraction phosphonium cations and two picrate anions (Fig. S3, ESI†).
(PXRD) measurements confirm the phase purities of 1–5 (Fig. S1, For the picrate anion of 2, the oxygen atoms of the para-NO2
S2 and S8, ESI†). Differential scanning calorimetry (DSC) measure- group are located on the benzene ring plane, while those of the
ments were performed to detect the phase transitions. As shown in ortho-NO2 groups deviate away from the benzene ring plane,
Fig. 1 and Fig. S1 (ESI†), pairs of endothermic/exothermic peaks especially the O6 and O7 atoms. Similar conformations of
were observed at 305/293 K (Ttr1), 367/348 K (Ttr2), 393/370 K (Ttr3), picrate anions were also found in 3–5 and other picrate
352/325 K (Ttr4) and 347/298 K (Ttr5) upon heating/cooling process salts.11,16 The picrate anions of 2, 3 and 5 arrange into planar
for 1–5, respectively, indicating the reversible phase transitions. It is layers alternately stacking in an ABAB. . . sequence, while those
clear that 2–5 all exhibit higher Ttr than 1, indicating that the of 4 arrange into a set of zig-zag sheets perpendicular to the
design strategy works. Despite the similar shape and size of the a-axis (Fig. 2 and Fig. S4–S6, ESI†). This indicates that the
cations in 2–5, the different Ttr values suggest that the variation of rigidity of the side-chain has a significant effect on the mole-
the side-chain could make a distinct difference in the thermal cular geometries. For 2, the adjacent picrate anions from the
properties. The corresponding thermal hystereses of 2–5 are A and B layers adopt antiparallel orientations, among which
observed at 19, 23, 27 and 49 K, respectively, indicating the first the obvious p–p stacking interactions (Cg–Cg: d1 = 3.5977 Å and
order characteristic of these transitions.15 The large enthalpy d2 = 3.5025 Å, Cg is the centroid of the benzene ring) were
changes (DH) and entropy changes (DS) are shown in Table S1 found, forming one-dimensional infinite anion chains extending
(ESI†), implying severe structural transformations during the phase along the a-axis. These chains are surrounded and connected by
transition. For convenience, the phases below and above Ttr are the propyl-trimethyl-phosphonium cations via weak C–Hꢁ ꢁ ꢁO
designated as a phase and b phase, respectively.
hydrogen bond interactions. The p–p stacking interactions also
To reveal what differences the side-chain will make in the exist in 3–5, and the corresponding distances are depicted in
crystal structures, single-crystal X-ray diffraction analyses were Fig. S5, S6 (ESI†) and Fig. 2. Different from 2–4, the p–p stacking
performed on 2–5 at room temperature (a phase). In the a interactions in 5 are intermittent and exist only between each
phase, both 2 and 3 crystallize in the monoclinic space group pairs of the neighbouring A and B layers (Cg–Cg = 3.4887 Å). The
P21/c (point group 2/m). The –CH2OCH3 group in 3 is a little introduction of 2-hydroxy-ethyl groups gives rise to O–Hꢁ ꢁ ꢁO
shorter than the –CH2CH2CH3 group in 2 (for bond lengths and hydrogen bonds between the O atoms of the (2-hydroxy-ethyl)-
trimethyl-phosphonium cations and the O1 atoms of the picrate
anions in 5 (dO–O = 2.760 Å), forming hydrogen-bonding dimers.
Sometimes, the formation of a strong hydrogen bond between
molecules with well-matched proton affinities might be a useful
strategy for designing molecular functional dielectrics.17,18
To sum up, the variation of the side-chain results in quite a
different structure.
Although the structures of 2–5 in the a phases are clarified
well, their structures in the b phases are hard to determine due
to their weak diffraction at high temperature. The structure of 5
in the b phase was roughly refined at 353 K to depict the
structural changes, which could help understand the origins
of the phase transitions in this series of compounds. At 353 K
(5b phase), 5 transforms to a centrosymmetric orthorhombic
space group Cmcm (point group mmm), and the thermal ellipsoids
of all atoms are much bigger than those of the 5a phase, demon-
strating the severe molecular motions at high temperature (Fig. S3,
ESI†). The relationship of the unit cell axes is a5a (10.232(2)) E a5b
Fig. 1 DSC curves of 2 (a), 3 (b), 4 (c) and 5 (d).
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PXRD patterns was found, demonstrating the higher symmetries of
the crystal structures at high temperature.20 These changes confirm
the occurrence of the phase transitions, agreeing well with the DSC
and structural analyses. The possible cell parameters and space
groups of the 2b, 3b and 4b phases suggested by the indexing of the
PXRD patterns and Pawley refinement are listed in Table S3 (ESI†).
Generally, the presence of the pseudo center of symmetry
always makes the crystal symmetry difficult to be accurately
determined. Thus, the SHG effect that cannot exist in the 11
centrosymmetric point groups is an effective supplementary
method to detect inversion symmetry breaking.21 At room
temperature, 2–4 are not SHG-active, while 5 shows SHG signals
approximately 0.4 times that of potassium dihydrogen phosphate
(KDP), manifesting its non-centrosymmetric crystal structure in
the 5a phase.22 Variable-temperature SHG measurements were
performed on 5 within 250–388 K. As shown in Fig. 3, upon
heating, the SHG signals maintain a relatively stable value until
about 340 K (SHG-ON state), then a step-like collapse appears
and the SHG signals are negligible above 352 K (SHG-OFF state).
Moreover, the SHG signals obtained upon cooling show a
reversible change and almost recover to their previous value
below 300 K. The reversible thermal-induced ‘on–off’ SHG
switching makes 5 to be a promising candidate as a solid-state
SHG switch. The whole SHG curve upon a heating and cooling
cycle exhibits a SHG bistability with a large hysteresis window
of B50 K.
Fig. 2 (a) Packing view of the 2a phase along the a-axis and p–p stacking
interactions. (b) Planar picrate anions layers in the 2a phase. (c) The zig-zag
anion sheets in the 4a phase. (d) Packing view of the crystal structure of 5a,
showing the layered structure. The cell edges of the 5a and 5b phases are
drawn in black and pink, respectively. For the cations, only P atoms are
drawn for clarity. (e) p–p stacking interactions along the c-axis in the 5a
and 5b phases. For each compound, the anions are drawn in two colours
to show the neighbouring sheets A and B, respectively, and the cation
adopts the same colour as that of its nearest anion sheet. All hydrogens are
omitted for clarity.
Considering that the dielectric constant e (e = e0 ꢀ ie00, where
e0 is the real part and e00 is the imaginary part) is sensitive to
structural changes, 2–5 are expected to exhibit interesting
dielectric responses with the phase transitions. As illustrated
in Fig. 4, the values of e0 for 2–5 are around 5.0 at 1 MHz and
(11.5686), b5a (9.883(2)) E a5b/2 + b5b/2 (11.5684), c5a (8.5961(17)) E change little below Ttr, corresponding to the a phases and the
c5b (7.0578), and the unit cell angles of 102.92(3)1, 99.20(3)1 and low dielectric states. Then, they all display a fast increase near
68.34(3)1 in the 5a phase correspond to those of 90.0001, 90.0001 364 K for 2, 393 K for 3, 352 K for 4 and 345 K for 5 upon
and 60.0001 in the 5b phase, respectively. The significant change heating, which is in good agreement with the DSC results
of the unit cell is due to the tilting of the picrate anions and the mentioned above. The e0 quickly reaches a high value and
shifts of the cations as shown in Fig. S7 (ESI†). The most obvious increases slowly with the increase of the temperature in the b phases
structural changes take place in the stacking pattern of the (high-dielectric states), corresponding to a step-like dielectric
picrate anions, which are neatly lined up along the c-axis after switching behaviour.5,23 For 2, the value of e0 increases from
the phase transition. Consequently, the intermittent p–p stacking
interactions become successive in the 5b phase (Cg–Cg = 3.5294 Å),
leading to the shrinkage in cell parameter c (Fig. 2e). Similar
changes of the molecular stacking patterns occur in 7,7,8,8-
tetracyanoquinodimethane-p-bis(8-hydroxyquinolinato)copper(II)
accompanied by dimensional changes under mechanical
stimulation.19 The rearrangement of the anions in 5 should
be aroused by the highly disordered motions of the (2-hydroxy-
ethyl)-trimethyl-phosphonium cations at high temperature,
which are the main driving force for the phase transition.
Due to the difficulties in obtaining the complete crystal structures
of the b phases, variable-temperature PXRD measurements were
performed on 2–5 to further verify the phase transitions (Fig. S8,
ESI†). Upon increasing the temperatures above the phase transition
points, some diffraction peaks vanish and new peaks emerge in the
b phases compared with the data obtained at 293 K. And the
significant decreases in the numbers of peaks in the measured
Fig. 3 Temperature dependence of the SHG effects of 5 (inset: oscillo-
scope traces of SHG signals for 5 and KDP).
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This work was supported by the National Natural Science
Foundation of China (21471032, 21771037, 21673038 and
21805033) and the Natural Science Foundation of Jiangsu
Province (JSNSF) (BK20170659).
Conflicts of interest
There are no conflicts to declare.
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phase transition materials by the symmetry reduction of
spherical moieties, and will promote the exploration and con-
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13114 | Chem. Commun., 2018, 54, 13111--13114
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