High-temperature and pressure-induced ferroelectricity in hydrogen-bonded supramolecular crystals of anilic acids and 2,3-Di(2-pyridinyl)pyrazine

Article abstract of DOI:10.1021/ja400318v

Cocrystallization of anilic acids (H₂xa) and 2,3-di(2-pyridinyl)pyrazine (dppz) affords a variety of molecular geometries, including hydrogen-bonding and supramolecular structures. Proton-transferred 1:1 salts of [H-dppz][Hca] and [H-dppz][Hba] (H₂ca = chloranilic acid, H₂ba = bromanilic acid) were found to host room-temperature ferroelectricity with a spontaneous polarization of 3-4 μC/cm² along the hydrogen-bonded chains. Compared with the Curie points of other supramolecular ferroelectrics, those of the salts are relatively high (402 K and >420 K, respectively) because of the elongated hydrogen bonds, which stabilize the proton-ordered state against thermal agitation. In addition to the ferroelectric black (α) form, dppz and H₂ba gave two different crystal forms with a 2:3 ratio: the brown β form of [H_1.5-dppz] ₂[Hba]₃ and the brownish-red γ form of [H-dppz] ₂[Hba]₂[H₂ba]. Mixed solutions of dppz with the less acidic fluoranilic acid (H₂fa) exhibit valence instability; the H₂fa molecules remain mostly neutral in absolute ethanol, whereas methanol (MeOH) solution apparently increases the deprotonated Hfa^- content. Crystallizations of these solutions gave a neutral [dppz][H ₂fa] cocrystal and ionic [H-dppz⁺][Hfa^-] ·MeOH salt, respectively. The ferroelectricity induced by a modest hydrostatic pressure corroborates the conclusion that the ionic state with a dipolar [H-dppz⁺][Hfa^-] chain is energetically close to the nonpolar neutral ground state of the [dppz][H₂fa] crystal.

Full text of DOI:10.1021/ja400318v

Article
pubs.acs.org/JACS
High-Temperature and Pressure-Induced Ferroelectricity in
Hydrogen-Bonded Supramolecular Crystals of Anilic Acids and 2,3-
Di(2-pyridinyl)pyrazine
Sachio Horiuchi,*,†,‡ Reiji Kumai,‡,§ and Yoshinori Tokura†,∥,⊥
†National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8562, Japan
‡CREST, Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan
§Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator
Research Organization (KEK), Tsukuba 305-0801, Japan
∥Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
⊥Correlated Electron Research Group (CERG) and Cross-correlated Materials Research Group (CMRG), RIKEN Advanced Science
Institute, Wako 351-0198, Japan
S
* Supporting Information
ABSTRACT: Cocrystallization of anilic acids (H2xa) and 2,3-
di(2-pyridinyl)pyrazine (dppz) affords a variety of molecular
geometries, including hydrogen-bonding and supramolecular
structures. Proton-transferred 1:1 salts of [H-dppz][Hca] and
[H-dppz][Hba] (H2ca = chloranilic acid, H2ba = bromanilic
acid) were found to host room-temperature ferroelectricity
with a spontaneous polarization of 3−4 μC/cm2 along the
hydrogen-bonded chains. Compared with the Curie points of
other supramolecular ferroelectrics, those of the salts are relatively high (402 K and >420 K, respectively) because of the
elongated hydrogen bonds, which stabilize the proton-ordered state against thermal agitation. In addition to the ferroelectric
black (α) form, dppz and H2ba gave two different crystal forms with a 2:3 ratio: the brown β form of [H1.5-dppz]2[Hba]3 and the
brownish-red γ form of [H-dppz]2[Hba]2[H2ba]. Mixed solutions of dppz with the less acidic fluoranilic acid (H2fa) exhibit
valence instability; the H2fa molecules remain mostly neutral in absolute ethanol, whereas methanol (MeOH) solution
apparently increases the deprotonated Hfa− content. Crystallizations of these solutions gave a neutral [dppz][H2fa] cocrystal and
ionic [H-dppz+][Hfa−]·MeOH salt, respectively. The ferroelectricity induced by a modest hydrostatic pressure corroborates the
conclusion that the ionic state with a dipolar [H-dppz+][Hfa−] chain is energetically close to the nonpolar neutral ground state of
the [dppz][H2fa] crystal.
such systems until the recent discovery of several supra-
molecular ferroelectrics of 2,5-dihalo-3,6-dihydroxy-p-benzo-
quinones6 (haloanilic acid, hereafter designated as H2xa; Figure
1). Research began with extended chains of alternating proton
donors and acceptors, and the hydrogen-bonded neutral
cocrystal of phenazine (Phz) with chloranilic acid (H2ca) or
bromanilic acid (H2ba) exhibited a spontaneous polarization of
1−2 μC cm−2 and a large dielectric permittivity of up to 2000−
3000.7 Later, we achieved a higher polarization of 3−4 μC cm−2
with a proton-transferred salt derived from 5,5′-dimethyl-2,2′-
bipyridine (55dmbp) and iodanilic acid (H2ia): [H-55dmbp]-
[Hia].8 The Curie points of these supramolecules were
improved to beyond room temperature by deuteration. With
the base 2,3,5,6-tetra(2-pyridinyl)pyrazine (tppz), we found 1:2
salts of [H2-tppz][Hxa]2 with distinct supramolecular archi-
tectures in which the ferroelectricity is driven by a double-
1. INTRODUCTION
The hydrogen-bonding interaction is highly directional and
much stronger than the van der Waals interaction, which
usually favors close molecular packing. Hydrogen-bond
formation is thus often accompanied by some structural
flexibility toward the intermolecular space, which can trigger
structural phase transitions in various ways.1 The ferroelectric
phase transition to a polar structure with bistable polarities has
been one of the most attractive issues because ferroelectrics can
accumulate and store large electronic charges by spontaneous
polarization, sense temperature changes and mechanical stress,
interchange electrical and mechanical energy, modulate light,
and transform optical wavelengths.2 Hydrogen-bonded ferro-
electrics of the KH2PO4 (KDP) family3 are representative
materials that have been extensively studied to understand the
gigantic isotope effect and the role of proton dynamics.2
For organic molecular systems, high-performance ferroelec-
tricity is desired for current and future organic electronics and
optoelectronics.4 However, there were only a few examples5 of
Received: January 10, 2013
Published: February 28, 2013
© 2013 American Chemical Society
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discriminated by their crystal appearances under a microscope
(Figure 2a).
Figure 2. Photographs of cocrystals of dppz. (a) [H-dppz][Hba] (α-
form), [H1.5-dppz]2[Hba]3 (β-form), and [H-dppz]2[Hba]2[H2ba] (γ-
form) crystals. (b) Neutral [dppz][H2fa] and ionic [H-dppz]-
[Hfa]·MeOH crystals.
Figure 1. Chemical structures of the anilic acids (H2xa) and bases.
Protonated dppz and tppz can behave as proton sponges, as
exemplified by H2-tppz2+, or adopt various twisted conformations by
changing the dihedral angles (Ψ and Ψ′) between the pyrazyl and
pyridyl rings.
For the fluorine analogue, black rod crystals of 1:1
[dppz][H2fa] were grown by slow evaporation of ethanol
solution containing a stoichiometric mixture of base and acid,
whereas methanol was solvated as [H-dppz][Hfa]·MeOH to
give brownish plate crystals. These two crystal forms are shown
in Figure 2b. It should be noted that the former crystal
comprises neutral molecules of dppz and H2fa, whereas the
latter is a proton-transferred salt, as demonstrated in the
following experimental results.
2.2. Infrared Absorption Spectra and Ionicity. Remov-
ing hydroxyl protons can transform the anilic acid molecule
from neutral H2xa through the Hxa− monoanion to the xa2−
dianion. Because the pK1 and pK2 values of H2xa15 are similar
to the pK1 value of dppz (2.9−3.1),16 there can be a change in
the degree of proton transfer with a subtle change in the
molecular environments of the cocrystals. Infrared spectroscopy
is an easy method for judging whether the molecules are neutral
or in proton-transferred ionic states; both H2xa and Hxa−
exhibit a strong absorption band associated with the CO
stretching mode at around 1650 cm−1, but only Hxa− and xa2−
show a band associated with the CO− stretching mode at
around 1500−1550 cm−1.7d,17
proton transfer process on the OH···O-bonded cyclic dimer
[Hxa−]2 and the acentric proton locations on the NH···N
hydrogen bridges between the pyridyl units of the proton
sponge H2tppz2+ (Figure 1).9
The underlying principle governing ferroelectricity in these
supramolecules has also been simplified into a single-
component system that can both donate and accept
protons.10,11 A moderate electric field at room temperature
was found to switch a very large polarization: croconic acid10
shows a spontaneous polarization (∼21 μC cm−2) as high as
that of BaTiO3 (26 μC cm−2). These observations encouraged
us to explore electronic functionalities arising from hydrogen-
bonded molecular systems.
With the aim of further improving ferroelectric and related
properties, we have investigated a variety of supramolecules of
H2xa and 2,3-di(2-pyridinyl)pyrazine (dppz). dppz contains
four nitrogen atoms in one pyrazine and two pyridine rings12
and is structurally related to tppz (Figure 1).13 The heterocyclic
rings of dppz can be flexibly twisted with respect to each other,
allowing various intermolecular hydrogen bonds. Two pyridyl
nitrogen atoms can also behave as a proton sponge to capture
protons from the acid,14 as observed for tppz salts. Herein, we
report above-room-temperature ferroelectricity as well as
valence instability and discuss the roles of the supramolecular
structures.
The [H-dppz][Hca] cocrystal and three polymorphs from
H2ba exhibit intense CO− and CO stretching-mode bands
(Figure 3a). This suggests the presence of a deprotonated
species, which is confirmed by the crystal structures discussed
in section 2.7. In addition, the distinct crystal structures of the
three polymorphs are apparent in the different details in the
spectra.
Similar arguments can be applied to the contrasting spectra
of [dppz][H2fa] and its solvated [H-dppz][Hfa]·MeOH form
(Figure 3b). The presence or absence of intense bands around
1500−1550 cm−1 is related to different degrees of deprotona-
tion of the hydroxyl group; the former is a neutral cocrystal,
whereas the latter is a proton-transferred ionic salt. This is
consistent with the formula deduced from the X-ray crystallo-
graphic studies conducted in this work. The infrared spectra of
powdered [dppz][H2fa] also signal its strong hygroscopic
nature (Figure 3c). Compared with the spectrum of the sample
carefully dispersed in KBr under dry atmosphere (curve 1 in
Figure 3c), strong absorption associated with water at 3330
cm−1 appears immediately (curves 2 and 3) after the sample is
ground with KBr particles in wet air (∼60% humidity). The
simultaneous evolution of the CO− stretching band indicates
humidity-mediated proton transfer that forms Hfa− anions. The
fully wet specimen gave a spectrum similar to that of ionic [H-
dppz][Hfa]·MeOH (curve 3). (Note that the [dppz][H2fa]
2. RESULTS AND DISCUSSION
2.1. Formation of Supramolecules and Polymor-
phism. The 1:1 cocrystals of [H-dppz][Hca] and [H-
dppz][Hba] (α form) were obtained as the main products of
diffusing dppz and the corresponding acid in acetone; these
black rod crystals exhibited ferroelectricity as described below.
We also found additional crystal forms with H2ba: The
diffusion procedure or slow concentration of the 1:1
stoichiometric mixture solution occasionally gave dark brown
thin plate crystals of [H1.5-dppz]2[Hba]3 (β form) or reddish-
brown plate crystals of [H-dppz]2[Hba]2[H2ba] (γ form).
(Note that H1.5-dppz1.5+ cation signifies a crystallographically
averaged form between diprotonated H2-dppz2+ and monop-
rotonated H-dppz+ species through the two-site disorder of the
proton.) These 1:1 and 2:3 stoichiometric forms can be readily
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Figure 4. Solvatochromic-like behavior of a 1:1 mixture of dppz and
H2fa. (a) Photographs of the quartz cells containing solutions of
[dppz][H2fa] in ethanol and methanol in comparison with an ethanol
solution of [H-dppz][Hca]. (b) Color change after addition of one
droplet of methanol to the ethanol solution of [dppz][H2fa] (central
cell).
Figure 3. Infrared absorption spectra of powdered cocrystals of dppz
and H2xa (dispersed in a KBr disk). Comparison of the spectra of (a)
[H-dppz][Hba] (α-form), [H1.5-dppz]2[Hba]3 (β-form), and [H-
dppz]2[Hba]2[H2ba] (γ-form) cocrystals and (b) neutral [dppz][H2fa]
and ionic [H-dppz][Hfa]·MeOH. (c) Hygroscopic nature of neutral
[dppz][H2fa] powder. The KBr disks were prepared by grinding the
samples under dry argon (curve 1), quickly in air (curve 2), and finely
in air (curve 3). The arrows indicate the absorption bands of water and
Hfa− ions.
single crystal itself is stable in the bulk and retains its lustrous
surface in air for a prolonged time.) These observations suggest
that [dppz][H2fa] has a valence instability between the neutral
and proton-transferred ionic states. Similar behavior is also
observed for the solution spectra, as described in the next
section.
2.3. Electronic Spectra of [dppz][H2fa] in Solution. The
dppz and H2fa mixture was found to behave similarly to a
solvatochromic dye. The ethanol solution of [dppz][H2fa] is
brownish yellow, and the methanol solution exhibits a reddish
color (Figure 4a). This observation is associated with the
bathochromic shift of the optical absorption around 400−600
nm (Figure 5). Because both neutral dppz and its protonated
form are transparent in the visible region, these bands arise
from H2fa and/or its deprotonated species. For comparison, the
solution spectra of neutral H2fa (λmax = 432 nm) and ionic Hfa−
in the [H-tmbp][Hfa] salt (λmax = 538 nm, tmbp =4,4′,5,5′-
tetramethyl-2,2′-bipyridine18) are also shown. In analogy with
chloranilic acid,19 protonated form H2fa and its conjugate base
Hfa− exhibit distinct absorption spectra (acidichromism20). The
spectral change arises from the different concentrations of the
two species coexisting in solutions of [dppz][H2fa]. Whereas
most of the H2fa molecules remain in a neutral acidic form in
ethanol solution, methanol solution promotes their deproto-
nation. This equilibrium difference in solution is in good
correspondence with the crystallizations of the neutral and
ionic solids from the respective solutions.
Figure 5. Electronic spectra of ethanol and methanol solutions of
[dppz][H2fa] in comparison with those of ethanol solutions of neutral
H2fa and deprotonated Hfa− monoanion (dissolved as [H-tmbp]-
[Hfa]).
We also found that addition of a droplet of methanol or
water immediately changed the solution of [dppz][H2fa] in
ethanol from a brownish to reddish color, as can be seen
around the bottom of the quartz cell in Figure 4b. These
observations can be understood in terms of the polarity of the
additive solvents; the higher solvent polarities of water and
methanol compared to that of ethanol favor the ionic and
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dipolar species and facilitate deprotonation.21 This phenomen-
on is also closely related to the humidity-mediated proton
transfer observed in the solid state. From these observations,
H2fa and dppz are regarded to be the best-balanced
combinations in terms of proton affinity to sufficiently adjust
the degree of proton transfer through a subtle change in the
environment in both the solution and crystalline states.
2.4. Dielectric Properties. Measurement of relative
permittivity is a facile tool for determining the dielectric
response from proton dynamics as well as identifying the Curie
point of ferroelectricity. Among the cocrystals studied, [H-
dppz]2[Hba]2[H2ba] (γ form) and [H-dppz][Hfa]·MeOH did
not display either dipolar fluctuations or phase transitions; the
relative permittivity was small (5−6) and independent of
temperature between 295 and 5 K, when the electric field was
applied normal to the (100) and (101) planes, respectively.
Despite the absence of ferroelectricity, [H1.5-dppz]2[Hba]3 (β
form) revealed a large relative permittivity, the details of which
are provided in the Supporting Information. The detailed
dielectric properties of the ferroelectric crystal forms are
described in the next two sections.
the calorimetric measurements (see section 2.6). The
permittivity of the isomorphous α-[H-dppz][Hba] crystal is
independent of temperature up to 380 K, and the inactive
dipolar fluctuation implies a much higher Curie point. This
behavior is reminiscent of croconic acid, which exhibits a
thermally robust ferroelectric phase up to the sample
degradation temperature (∼450 K). The Curie point of α-
[H-dppz][Hba] (if it exists) far exceeds those of other
supramolecular ferroelectrics.
2.4.2. Neutral [dppz][H2fa] under Ambient and High
Pressures. For the neutral [dppz][H2fa] crystal, the temper-
ature-dependent permittivity does not signify a Curie point at
ambient pressure. Instead, when an electric field (E) is applied
along the hydrogen-bonded chains (E ∥ [101]), εr gradually
increases to 25, implying the presence of some dipolar
fluctuation at elevated temperatures up to 350 K (Figure 6b).
Very similar behavior was reported for the pressure-induced
ferroelectric Phz-H2fa crystal.22
Considering the origins of solvatochromic-like behavior, we
anticipated that some external stimuli such as hydrostatic
pressure would likely transform neutral [dppz][H2fa] into
dipolar ionic [H-dppz+][Hfa−] accompanied by the emergence
of ferroelectricity. As depicted in the inset in Figure 6b, the
permittivity increased with pressure in the low-pressure range.
With a load of 0.53 GPa, a sharp peak suddenly emerged at 65
K, which is indicative of the expected pressure-induced phase
transition. Application of a higher hydrostatic pressure rapidly
increased both the height and temperature of the peak anomaly.
The ferroelectricity with spontaneous polarization along the
hydrogen-bonded chain was manifested in the divergently
increasing εr value up to 1300. The permittivity peaks were
accompanied by a slight thermal hysteresis characteristic of a
first-order phase transition.
2.4.1. [H-dppz][Hca] and [H-dppz][Hba] (α Form). Figure
6a displays the temperature dependence of the relative
Figure 7 displays the pressure−temperature phase diagram
compiled from the peak temperature and pressure of εr, the
Figure 6. Temperature dependence of the relative permittivity εr. (a)
[H-dppz][Hca] and [H-dppz][Hba] (α-form) crystals measured with
an ac field of various frequencies applied along the hydrogen-bonded
chain (parallel to the crystallographic c axis). (b) One [dppz][H2fa]
crystal under various pressures. The ac field (f = 100 kHz) was applied
along the crystal [101] direction. The inset shows the data in the low-
pressure range.
Figure 7. Temperature−pressure phase diagram of [dppz][H2fa]
crystal.
latter of which was corrected by thermal changes in the
contracting pressure-transmitting medium23 (see Experimental
Section). The upward curvature of the phase boundary is very
similar to that of the paraelectric−ferroelectric phase boundary
of Phz-H2fa.22 However, unlike Phz-H2fa, the [dppz][H2fa]
crystal undergoes a first-order ferroelectric phase transition
without passing through any additional lower-temperature
phases. In addition, the pressure-induced phases of the two
crystals should be different in nature. Phz-H2xa cocrystals
exhibit face-to-face π-molecular stacks that would increase the
strength of the repulsive Coulomb interactions upon ionization.
Consequently, proton transfer from acid to base is incomplete
and related to the appearance of lattice-modulated phases. In
permittivity εr for [H-dppz][Hca] and the Br-substituted
isomorph (α form) measured with the electric field applied
along [001], which is parallel to the hydrogen-bonded chains.
For [H-dppz][Hca], εr, which is as high as 20−30 at room
temperature, decreases with increasing ac frequency f and
monotonically increases with temperature; at f = 1 kHz, εr
exceeds 400 at the high-temperature limit of the measurements
(T = 380 K). For this reason, the thermal fluctuations of the
dipoles and/or polar domains in the cocrystal are expected to
become significant as the temperature approaches the Curie
point, and a phase transition was in fact detected at 402 K in
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contrast, the dppz (or H2fa) molecule is well separated from its
neighboring dppz (or H2fa) molecules, and their diminished
repulsion might allow complete proton transfer under high
pressure. This conjecture will require future validation from
elaborate structural characterizations of the pressurized crystal.
2.5. Ferroelectric Properties. The [H-dppz][Hca] and α-
[H-dppz][Hba] crystals revealed ferroelectricity along the
hydrogen-bonded chains (i.e., E∥c) at room temperature, as
evidenced by the parallelogram-shaped hysteresis loops in the
electric polarization (P) versus electric field (E) diagrams
(Figure 8). At the lowest ac frequency, relatively high remanent
Figure 9. Fatigue behavior of the ferroelectricity in [H-dppz][Hba]
crystal at room temperature. Changes in the remanent polarization Pr
(solid circles) and coercive field Ec (open squares) as a function of the
(total) number of continuously applied voltage pulses (rectangular
waveform, fpulse = 100 Hz, Epulsemax = 10 kV/cm > Ec). P−E hysteresis
measurements with the triangular waveform voltage (f = 1 Hz, Emax
=
13 kV/cm) were repeated with a constant interval on a logarithmic
time scale. Inset: P−E hysteresis loops before and after application of
the first 105 pulses.
(Figure S1, Supporting Information). We found a simple linear
P−E curve, which indicates neither ferroelectricity nor
antiferroelectricity, even with an applied maximum field of
100 kV/cm for both crystal directions.
Figure 8. Electric polarization (P)−electric field (E) hysteresis loops
of (a) [H-dppz][Hca] and (b) [H-dppz][Hba] (α-form) crystals at
room temperature. A triangular ac electric field of various frequencies
was applied along the crystal c direction.
2.6. Thermal Properties. The ferroelectric [H-dppz][Hca]
and [H-dppz][Hba] salts decompose at 448−455 and 440−444
K, respectively. Differential scanning calorimetry (DSC) tests
indicated that the Curie point might be far above room
temperature. The [H-dppz][Hca] salt exhibits a pair of
exothermic and endothermic peaks indicative of the phase
transition around T = 402 K (Figure S2, Supporting
Information), which can be defined as the Curie point, as
suggested by the rapid change in the permittivity (Figure 6a).
The transition entropy, ΔS, was estimated by integrating the
anomalous part of the heat flow and then dividing by the molar
amount of sample and transition temperature. For order−
disorder phase transitions having g possible configurations, the
entropy change is expressed by
polarizations of 2.7 μC cm−2 (f = 0.1 Hz) and 4.0 μC cm−2 (f =
0.5 Hz) and small coercive fields of 0.7 and 4 kV cm−1,
respectively, were observed. As Ec increases with increasing f,
the P−E loops are deformed (Figure 8) simply because of the
high f exceeding the polarization reversal rate. Specifically, the
amplitude of the applied electric field strongly affects the
dynamics of the domain walls, that is, the boundaries between
the ferroelectric domains of different polarities.
To examine the electric endurance of the high-temperature
ferroelectric [H-dppz][Hba], the ferroelectric fatigue properties
were investigated by continuously loading a rectangular pulse
voltage (f = 100 Hz, amplitude Ep = 10 kV cm−1 > Ec) and
repeating the P−E hysteresis measurements with a triangular
waveform voltage (f = 1 Hz, Emax= 13 kV cm−1) and a constant
interval on a logarithmic time scale. Figure 9 depicts the change
in the remanent polarization, Pr, and coercive field, Ec. Both Pr
and Ec increased gradually as the cycle number N increased for
N < 104 (the N = 105 curve from the initial run is shown in the
inset). The electric oscillation possibly releases some domains
that were originally pinned around impurities or defects and
then develops Pr. Beyond N = 105 cycles, Pr suddenly started to
decrease because the gradually increasing coercive field
approached the maximum applied field value. Ferroelectric
fatigue with hardening of the polarization reversal might
originate from degradation near the interface with the
electrodes, for example, impurity injection, charge trapping,
and/or diffusion loss of protons. It should be noted that the
hysteresis curves shown in Figure 8 are those obtained with
nearly maximized polarizations after these electric treatments.
The behavior observed here is quite similar to the observations
in croconic acid, a hydrogen-bonded ferroelectric.
ΔS = R ln g
where R = 8.31 J K−1 mol−1.
(1)
With ΔS = 1.01 J K−1 mol−1, we obtained a relatively large g
value (1.13) that suggests an order−disorder character mainly
in the present phase transition but that is much smaller than 2,
which is expected for the disordered hydrogen-bonded protons
between the proton donor and acceptor molecules. This would
reflect the strong correlations of multiple protons interacting
through the π-electron system. By contrast, for [H-dppz][Hba],
the absence of corresponding peak anomalies upon heating to
420 K indicates a persistent ferroelectric phase in agreement
with the observation of the flat ε−T curves.
2.7. Structural Properties. To understand the microscopic
origin of ferroelectricity and the nature of polymorphism and
solvatochromic-like behavior, the crystal structures were
determined by X-ray diffraction experiments. Table S1
(Supporting Information) summarizes the structural data for
the six cocrystals obtained at room temperature.
2.7.1. Molecular Structures and Geometric Change.
2.7.1.1. Acid Molecules (H2xa/Hxa−/xa2−). The molecular
geometry of each acid molecule in the cocrystals is summarized
in Table S2 (Supporting Information) with the neutral,24
dianion,25 and proton-transferred monovalent state ([H-
The P−E curves were also examined at room temperature in
[H1.5-dppz]2[Hba]3 (β form), which revealed a relatively large
permittivity for the E ∥ [101] and E ⊥ (101) configurations
̅
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44dmbp][Hca]26 and [H-tmbp][Hfa]; 44dmbp =4,4′-dimeth-
yl-2,2′-bipyridine) shown for reference and comparison. Note
that the substitution of halogen atoms in each state barely
changes the bond geometries except those involving the
substituents. The neutral form was found in [H-
dppz]2[Hba]2[H2ba] (γ form) and 1:1 [dppz][H2fa]. In these
crystals, the H2xa molecules occupy an inversion center and
hence are exactly planar by symmetry. The CO, CO, C
C, and CC bond distances agree closely with those in the
neutral H2xa crystals, as shown in Table S2 (Supporting
Information), and the molecular geometry adopts the typical
para-quinoid structure.
2.7.2. Ferroelectric [H-dppz][Hca] and [H-dppz][Hba] (α
Form). The crystal structures of [H-dppz][Hca] and [H-
dppz][Hba] (α form) are isomorphous and belong to a
monoclinic system with acentric symmetry [space group Cc
(No. 9), Figure 10a]. Both the H-dppz+ cation and the Hxa−
The monovalent Hxa− form is included in all of the
cocrystals except [dppz][H2fa]. The cocrystallization has little
affect on the CO and CC bond lengths, thus preserving
the para-quinoid molecular form (see bond lengths f, f′, and h
in Table S2, Supporting Information). The Hxa− anions adopt
an asymmetric geometry upon deprotonation of either hydroxyl
group in the ferroelectric [H-dppz][Hca] and [H-dppz][Hba]
(α-form), as well as in the [H-dppz]2[Hba]2[H2ba] (γ-form)
crystal. Compared with the neutral form, the bond geometry
around the proton-transferred hydroxyl oxygen exhibits a
substantially shortened CO bond and a lengthened CC
bond (see e′ and h′ in Table S2, Supporting Information),
whereas the corresponding bonds are almost unaffected around
the hydroxyl oxygen. The [H1.5-dppz]2[Hba]3 (β form)
contains both asymmetric and symmetric Hba− anions. The
latter anion allocates protons with one-half site occupancy
equally to both oxygen sides, and the local π-bond geometry
around the oxygen is nearly intermediate between those with
and without protons. These observations indicate clearly that
the π-bond geometries are closely interrelated with the degree
of protonation.
Figure 10. Crystal structures of [H-dppz][Hca] cocrystal. (a)
Molecular packing projected along the crystallographic b direction.
(b) Extended supramolecular chain with intermolecular hydrogen
bonds (dotted lines). (c) Structural changes of the hydrogen-bonded
supramolecular chain from the paraelectric state (T = 410 K, left) to
the ferroelectric state (295 K, right). The molecules are viewed along
2.7.1.2. Base Molecules. Protonation should also perturb the
molecular geometry of dppz. This effect is most obvious in the
CNC angle, and it has been demonstrated for pyridine
molecules that the CNC angle (116−117° for neutral
nitrogen) widens to 121−123° upon protonation.27 For dppz
molecules, the angle is 116.4° for the neutral form12b and
122.6° and 121.7° for the divalent state.14 For all of the
compounds investigated here, the corresponding angles (ϕ) of
the neutral OH···N and ionic NH+···O− hydrogen bond
are compared in Table S2 (Supporting Information). These
different bonding types are distinguished clearly by the value of
ϕ with a borderline at 120° for all of the compounds studied.
The dppz molecule can adopt a planar configuration, as seen
for its tetraphenylborate (BPh4) salt,14 when its two copolanar
pridyl rings behave as a proton sponge by capturing the proton
into a short NHN bridge. Apart from this case, the
molecule is typically twisted at the CC bonds between the
pyridyl and pyrazyl rings. The dihedral angle between these
planar rings is 41.9° in the neutral dppz crystal14 and ranges
from 22° to 57° in the cocrystals with anilic acids. As listed in
Table S3 (Supporting Information), protonated H-dppz+
molecules tend to adopt a larger dihedral angle (Ψ′) at the
protonated pyridyl site than that (Ψ) at the unprotonated
pyridyl site. This would be because steric hindrance is enhanced
with ring protonation. The difference, ΔΨ, between Ψ and Ψ′
is very small for the ferroelectric cocrystals, especially for [H-
dppz][Hca] (3.8°). This molecular geometry is associated with
the pseudosymmetry of 2-fold rotation, which would be
retained at temperatures above the Curie point. Note that
ΔΨ is the largest for the ionic [H-dppz][Hfa]·MeOH crystal.
the crystal [110] direction.
̅
anion are located in the general positions. Without the
hydrogen-bonding protons, the crystal structure seems to
gain pseudosymmetries for which the cation and anion have 2-
fold rotation and inversion symmetries, respectively. In fact, a
least-squares refinement of the parameters reduced R to some
extent under the corresponding centric space group C2/c: R =
0.066 for 50 parameters on 575 reflections [I > 2σ(I) for 2θ <
55°]. That is, the polar proton ordering must be the major
cause of symmetry breaking.
In both crystals, each H-dppz+ cation constructs bifurcated
hydrogen bonds with the neighboring Hxa− anion. The
unprotonated pyridyl ring forms OH···N bonds, whereas
the protonated ring forms NH+···O− bonds with a bifurcated
character. At first glance, the molecular packing seems to be
rather complicated in crystal structures containing dppz
molecules in a twisted conformation. In contrast, the
hydrogen-bonded network can be extracted easily as a linear
supramolecular chain of acid and base molecules alternating
along the crystal [001] direction (Figure 10b).
Importantly, the polarities of the chains are aligned in the
same direction (Figure 10a), which generates spontaneous
polarization. Furthermore, each chain has a bistable polarity
that depends on the locations of the protons (Figure 11). The
observed ferroelectricity can thus be explained by cooperative
proton transfer in an analogous way to that of the ferroelectric
[H-55dmbp][Hia] salt (Tc = 269 K)8 and the KDP (KH2PO4)
family. The stability of the ferroelectric state in the KDP family
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and N···HO bond configurations are thermally averaged by
the proton dynamics over single or shallow double potential
minima.
2.7.4. β and γ Forms. The crystal structures of both [H-
dppz]2[Hba]2[H2ba] (β form) and [H1.5-dppz]2[Hba]3 (γ
form) exhibit centric crystal symmetry, in agreement with the
absence of ferroelectricity. The full details of molecular
arrangements are provided in the Supporting Information.
2.7.5. [dppz][H2fa] (Neutral Form). The crystal structure of
[dppz][H2fa] belongs to the monoclinic system with centric
symmetry [space group C2/c (No. 15), Figure 12a]. The dppz
and H2fa molecules have 2-fold rotation and inversion
symmetry, respectively.
Figure 11. Chemical structures of hydrogen-bonded chains of
alternating acid and base molecules.
is governed by the potential barrier height for proton transfer,
and the Curie point is well correlated with the hydrogen-bond
geometry.28 The ferroelectric dppz salts actually exhibit much
longer distances for both OH···N and NH+···O− bonds
(dO···N(1) = 2.75−2.77 Å and dN···O(1) = 2.63 Å for the O···N and
N···O distances, respectively; see also Table S3, Supporting
(1)
Information) than [H-55dmbp][Hia] (dO···N = 2.61−2.70 Å,
(1)
dN···O = 2.54−2.55 Å) with a lower Tc of 269 K. The Curie
(1)
point is also elevated with an increase of 0.01−0.02 Å in dO···N
(1)
and dN···O from [H-dppz][Hca] (Tc = 402 K) to [H-
Figure 12. Crystal structures of [dppz][H2fa] cocrystal. (a) Molecular
packing projected along the crystallographic b direction. (b) Extended
supramolecular chain with intermolecular hydrogen bonds (dotted
lines).
dppz][Hba] (Tc > 420 K). Therefore, some steric hindrance for
elongating hydrogen bonds should be a promising approach to
thermally robust ferroelectricity for such KDP-like organic
crystals.
2.7.3. [H-dppz][Hca] at High Temperature (HT Form). The
structural change of [H-dppz][Hca] at the phase transition (Tc
= 402 K) was examined by synchrotron X-ray diffraction. We
first checked the degree of crystal symmetry breaking in the
bulk by probing the intensity difference between the Bijvoet
Along the [101] direction, the acid and base molecules
alternate in a linear chain with intermolecular OH···N bonds
(O···N distances of 2.636 Å). The overall molecular arrange-
ment in each chain is very similar to those in the ferroelectric
[H-dppz][Hca] and [H-dppz][Hba] (α-form) crystals (com-
pare Figures 10a and 12a). The only difference is that the
neutral chains in the former compound are nonpolar and the
ionic chains in the latter are dipolar and ferroelectric (Figure
11). Therefore, the dipolar fluctuation observed in the
permittivity along the supramolecular chain of [dppz][H2fa]
is likely to be related to the response from thermally excited
dipolar ionic domains (or strings), considering the neutral−
ionic instability in both the solution and solid forms.
pair (hkl versus h̅kl̅ )̅ reflections (respective intensities of I+ and
I−), which arises from the anomalous X-ray scattering effect. If
the crystal has centric symmetry or the acentric crystal has
equal volume fractions of domains of opposite polarities (i.e., a
Flack parameter equal to 0.5), it must obey Friedel’s law: I+/I−
= 1. The [H-dppz][Hca] crystal used in this study exhibited an
obvious deviation of I /I from unity for the Bijvoet pair of 604
̅
+
−
and 604 at room temperature (Figure S3, Supporting
̅
Information). Thus, the crystal must be polar, in agreement
with the observation of ferroelectricity. The deviation decreases
with temperature and disappears at Tc, indicating a structural
change to centric symmetry as expected for the paraelectric
state.
At T = 410 K > Tc, the crystal structure was satisfactorily
solved with the centric symmetry of the space group C2/c (the
minimal supergroup of Cc symmetry). The cation and anion
acquire 2-fold rotation and inversion symmetries, respectively,
and all of the hydrogen-bonding sites become crystallo-
graphically equivalent. A differential Fourier synthesis gave a
single maximum in the electron density between the hydrogen-
bonded oxygen and nitrogen, as shown by the hydrogen
locations in the Figure 10c. This high-temperature structure
thus represents the paraelectric form in which the NH+···O−
2.7.6. [H-dppz][Hfa]·MeOH (Ionic Form). The crystal
structure of [H-dppz][Hfa]·MeOH belongs to the monoclinic
system (Figure 13a). All of the molecules are located in general
positions. The space group is acentric Cc (No. 9), identical to
that of the ferroelectric [H-dppz][Hca] crystal. However, the
structure of the Hfa salt does not show any additional
pseudosymmetry even without protons and is therefore not
ferroelectric but simply dipolar. An infinite supramolecular
chain is constructed along the crystal b direction from
alternating Hfa− anions and methanol molecules with hydrogen
bonds of very short OH···OMeOH (2.584 Å) and O−···H
OMeOH (2.723 Å) distances (Figure 13b). The protonated
pyridyl ring of the H-dppz+ cation forms a NH+···O− bond
with the deprotonated oxygen of the Hfa− anion. The
unprotonated pyridine and pyrazine rings, however, are not
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H2ba, dppz, and 44dmbp were purified by repetitive recrystallization
from ethanol or methanol and then twice by gradient sublimation
under vacuum. The base tmbp was synthesized according to the
literature method18 and also purified by gradient sublimation under
vacuum.
Diffusion of dppz and H2ca (or H2ba) in acetone solution gave
black rod crystals of ferroelectric [H-dppz][Hca] (or the α form of
[H-dppz][Hba]), which decomposes at 175−182 °C (or 167−171
°C). Two additional nonferroelectric crystal forms were also found for
H2ba. Slow evaporation of an ethanol solution of a 1:1 stoichiometric
mixture under a stream of argon gas gave dark brown thin-plate
crystals of [H1.5-dppz]2[Hba]3 (β form). This crystal form was also
obtained as a minor product of the diffusion procedure. Upon similar
slow evaporation of methanol or ethanol solution, we also found
reddish-brown plate crystals of [H-dppz]2[Hba]2[H2ba] (γ form).
Black rod crystals of [dppz][H2fa] were obtained by slow evaporation
of an ethanol solution containing a stoichiometric mixture of base and
acid. With the use of methanol, we obtained brownish plate crystals of
the solvated form [H-dppz][Hfa]·MeOH.
Figure 13. Crystal structures of [H-dppz][Hfa]·MeOH cocrystal. (a)
Molecular packing projected along the crystallographic b direction. (b)
Extended supramolecular chain of alternating Hfa− and MeOH
molecules with a H-dppz+ cation attached as a pendant.
[H-44dmbp][Hca] and [H-tmbp][Hfa] cocrystals were prepared to
identify the typical optical spectra and molecular geometry of the
monovalent Hca− and Hfa− anions. The former salt was obtained as a
single crystal by the slow evaporation of its methanol solution and the
latter by diffusing tmbp and H2fa in ethanol solution.
4.2. Spectroscopy and Calorimetry. Infrared absorption spectra
of the samples dispersed in a KBr disk were recorded at room
temperature with a JASCO FTIR-200 spectrometer over a wavelength
range of 4000−400 cm−1 at a resolution of 4 cm−1. Electronic
absorption spectra in solution were measured with a Hitachi U-3500
UV−vis spectrometer. The melting or decomposition points were
checked on a Yanako MP-500D apparatus prior to differential
scanning calorimetry. The thermal analysis was performed using a
Seiko Instruments DSC220C differential scanning calorimeter. The
sample was encapsulated in an aluminum pan and heated at a rate of 5
K min−1. The temperature was calibrated using the melting point of
indium (429.8 K). Synthetic sapphire was used as the standard for the
determination of the heat capacity.
4.3. Electric Measurements. All electric measurements were
obtained from as-grown single crystals with painted gold or silver
electrodes. The dielectric constant was measured with an LCR meter
(HP 4284A). Hydrostatic pressure was generated in a clamp-type
high-pressure cell using a pressure-transmitting oil (Idemitsu Kosan
Daphne 7373). The applied pressure at each transition point was
obtained by correcting the thermal change of pressure with 7.6 × 10−4
GPa K−1 (for T > 90 K) due to contraction of the medium.23 The
cooling and heating rates were 1−2 K min−1, and the accuracy in the
temperature was ∼0.05 K.
involved in the hydrogen bonds. Thus, the H-dppz+ cation
appears as a pendant simply bonded to the linear supra-
molecular chain.
3. CONCLUSIONS
The dppz molecule has proven to be an excellent building block
for a variety of supramolecular structures with various twist
conformations. Its combination with anilic acid is delicately
balanced in terms of the pK value, and cocrystallization was
found to regulate the degree of proton transfer from the neutral
to ionic crystalline forms depending on the acidity. We found
room-temperature ferroelectricity in the 1:1 proton-transferred
salts of [H-dppz][Hca] and [H-dppz][Hba]. Through the
twisted conformation of dppz, these salts construct rather
simple linear supramolecular chains of alternating acid and base
molecules, which are bistable in polarity. Compared with other
supramolecular ferroelectrics, the robust nature of the
ferroelectricity to high temperatures of 400 K or more can be
attributed to protons in the elongated hydrogen bonds favoring
the ordered state.
The electric polarization (P)−electric field (E) hysteresis curves
were collected on a ferroelectrics evaluation system (Toyo
Corporation, FCE-1) consisting of a current/charge−voltage converter
(Toyo Corporation, model 6252), an arbitrary waveform generator
(Biomation 2414B), an analog-to-digital converter (WaveBook 516),
and a voltage amplifier (NF Corporation, HVA4321). The measure-
ments were performed at room temperature with a high-voltage
triangular wave field and various alternating frequencies. The
ferroelectric hysteresis loops were successfully obtained in air for
both [H-dppz][Hca] and [H-dppz][Hba] crystals having a modest
coercive field. The [H1.5-dppz]2[Hba]3 (β-form) crystal, for which no
dielectric hysteresis loops were identified, was examined by immersing
the crystal in silicon oil to prevent atmospheric discharge under a high
electric field (80−100 kV cm−1).
4.4. Crystallographic Studies. The crystallographic data and
experimental details for the cocrystals of dppz are summarized in
Table S1 (Supporting Information). Both the X-ray diffraction data
collection at room temperature and the assignment of the crystallo-
graphic axes of the bulk single crystals were completed using a Rigaku
AFC7R four-circle diffractometer equipped with a Mercury CCD area
detector (graphite-monochromated Mo Kα radiation).
The combination of fluoranilic acid (H2fa) and dppz was
found to be the best balanced in terms of proton affinity to
sufficiently adjust the degree of proton transfer to dppz in both
solution and crystalline states. The equilibrium between the
neutral and proton-transferred ionic species depends on the
solvent polarity, and the consequent spectral changes with
different alcohols also affect the crystallization with or without
proton transfer: [dppz][H2fa] and [H-dppz+][Hfa−]·MeOH.
The linear-chain supramolecular structure of paraelectric
[dppz][H2fa] is very similar to that of ferroelectric [H-
dppz][Hca], except for the degree of proton transfer. The
ferroelectricity induced with a modest hydrostatic pressure
corroborates the conclusion that the ionic state with a dipolar
[H-dppz+][Hfa−] chain is energetically close to the nonpolar
neutral ground state of the [dppz][H2fa] crystal.
4. EXPERIMENTAL SECTION
4.1. Materials. Fluoranilic acid was prepared from p-fluoranil
according to the literature method15 and purified twice by temper-
ature-gradient sublimation under vacuum. The purity was confirmed
by elementary analysis and mass spectra. Commercially available H2ca,
The temperature-dependent structural changes were also examined
for the ferroelectric [H-dppz][Hca] crystal. Data were collected at
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various temperatures above room temperature with a Rigaku DSC
imaging plate system using Si double-crystal monochromatized
synchrotron radiation (λ = 0.6876 Å) at the BL-8A beamline of the
Photon Factory (PF), High-Energy Accelerator Research Organization
(KEK). The monochromatized beam was focused using a bent
cylindrical mirror made of a Si crystal coated with Rh to produce a
focused beam size of 0.3 mm (vertical) × 0.7 mm (horizontal). The
crystal attached to a glass fiber was heated by flowing nitrogen gas. The
full data collection for the structural analysis was obtained at 295 and
410 K to compare the structural changes before and after the phase
transition at Tc = 402 K.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed characterization (dielectric properties and crystal
structures) for the nonferroelectric polymorphs, calorimetric
data (DSC) for [H-dppz][Hca], crystallographic details for all
of the dppz cocrystals along with CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
s-horiuchi@aist.go.jp
■
Notes
(20) Bouas-Laurent, H.; Durr, H. Pure Appl. Chem. 2001, 73, 639−
̈
The authors declare no competing financial interest.
665.
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(22) Kumai, R.; Horiuchi, S.; Fujioka, J.; Tokura, Y. J. Am. Chem. Soc.
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ACKNOWLEDGMENTS
■
This work was partially supported by KAKENHI (No.
20110003 from MEXT, No. 23340111 from JSPS) and by
the Japan Society for the Promotion of Science (JSPS) through
“Funding Program for World-Leading Innovative R&D on
Science and Technology (FIRST Program)” initiated by the
Council for Science and Technology Policy (CSTP). The
synchrotron X-ray study was performed with the approval of
the Photon Factory Program Advisory Committee (No.
2009S2-003).
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