Molecular Motion, Dielectric Response, and Phase Transition of Charge-Transfer Crystals: Acquired Dynamic and Dielectric Properties of Polar Molecules in Crystals

Article abstract of DOI:10.1021/jacs.5b00412

Molecules in crystals often suffer from severe limitations on their dynamic processes, especially on those involving large structural changes. Crystalline compounds, therefore, usually fail to realize their potential as dielectric materials even when they have large dipole moments. To enable polar molecules to undergo dynamic processes and to provide their crystals with dielectric properties, weakly bound charge-transfer (CT) complex crystals have been exploited as a molecular architecture where the constituent polar molecules have some freedom of dynamic processes, which contribute to the dielectric properties of the crystals. Several CT crystals of polar tetrabromophthalic anhydride (TBPA) molecules were prepared using TBPA as an electron acceptor and aromatic hydrocarbons, such as coronene and perylene, as electron donors. The crystal structures and dielectric properties of the CT crystals as well as the single-component crystal of TBPA were investigated at various temperatures. Molecular reorientation of TBPA molecules did not occur in the single-component crystal, and the crystal did not show a dielectric response due to orientational polarization. We have found that the CT crystal formation provides a simple and versatile method to develop molecular dielectrics, revealing that the molecular dynamics of the TBPA molecules and the dielectric property of their crystals were greatly changed in CT crystals. The TBPA molecules underwent rapid in-plane reorientations in their CT crystals, which exhibited marked dielectric responses arising from the molecular motion. An order-disorder phase transition was observed for one of the CT crystals, which resulted in an abrupt change in the dielectric constant at the transition temperature. (Figure Presented)

Full text of DOI:10.1021/jacs.5b00412

Article
pubs.acs.org/JACS
Molecular Motion, Dielectric Response, and Phase Transition of
Charge-Transfer Crystals: Acquired Dynamic and Dielectric
Properties of Polar Molecules in Crystals
Jun Harada,*,†,‡ Masaki Ohtani,‡ Yukihiro Takahashi,†,‡ and Tamotsu Inabe*,†,‡
‡
†Department of Chemistry, Faculty of Science, and Graduate School of Chemical Sciences and Engineering, Hokkaido University,
Sapporo 060-0810, Japan
S
* Supporting Information
ABSTRACT: Molecules in crystals often suffer from severe
limitations on their dynamic processes, especially on those
involving large structural changes. Crystalline compounds,
therefore, usually fail to realize their potential as dielectric
materials even when they have large dipole moments. To
enable polar molecules to undergo dynamic processes and to
provide their crystals with dielectric properties, weakly bound
charge-transfer (CT) complex crystals have been exploited as a
molecular architecture where the constituent polar molecules
have some freedom of dynamic processes, which contribute to
the dielectric properties of the crystals. Several CT crystals of
polar tetrabromophthalic anhydride (TBPA) molecules were prepared using TBPA as an electron acceptor and aromatic
hydrocarbons, such as coronene and perylene, as electron donors. The crystal structures and dielectric properties of the CT
crystals as well as the single-component crystal of TBPA were investigated at various temperatures. Molecular reorientation of
TBPA molecules did not occur in the single-component crystal, and the crystal did not show a dielectric response due to
orientational polarization. We have found that the CT crystal formation provides a simple and versatile method to develop
molecular dielectrics, revealing that the molecular dynamics of the TBPA molecules and the dielectric property of their crystals
were greatly changed in CT crystals. The TBPA molecules underwent rapid in-plane reorientations in their CT crystals, which
exhibited marked dielectric responses arising from the molecular motion. An order−disorder phase transition was observed for
one of the CT crystals, which resulted in an abrupt change in the dielectric constant at the transition temperature.
their molecular-motion-based dielectric properties by incorpo-
rating polar molecules into crystalline environments with
motional freedom.
INTRODUCTION
■
Molecular motion in crystals has been studied in various fields
of chemistry.1 If a molecule does not largely change its structure
in the course of a molecular motion, the process is not
disturbed by the surrounding molecules and proceeds easily
even in crystals. The accumulated information and under-
standing about the relationship between molecular motions and
crystalline environments have paved the way for controlling the
molecular dynamics in crystals by designing molecules and
crystal structures. Remarkable achievements in the design of
crystalline molecules include the realization of molecular
machinery that has some analogy with macroscopic objects
such as gyroscopes, gears, and bearings.2 Also of growing
interest are macroscopic functions of materials related to
molecular motions in crystals. Flipping motion of fluoro-
substituted phenyl groups in crystals has been utilized to create
dielectric and ferroelectric properties of the materials.3
Molecular motions of small organic cations such as pyridinium
or imidazolium ions also gave rise to ferroelectric properties.4
However, only limited variations of molecules and molecular
motions have been explored, aiming for the development of
functional materials with the desired properties. In this paper
we describe the design of crystalline materials and control of
Dielectric materials are electrical insulators that can be
electrically polarized under an applied electric field. In addition
to being of fundamental scientific interest, they are of great
value with a variety of applications as basic components of
electronic devices, and some of them show piezoelectric,
pyroelectric, and ferroelectric properties.5 Dielectric materials
can be characterized by their dielectric constant (electric
permittivity) ε* (ε* = ε′ − iε″, where ε′ and ε″ are the real and
imaginary parts of the complex dielectric constant, respec-
tively). Molecular motion and reorientation of polar molecules
can contribute to the dielectric property of materials in terms of
alignment of their permanent dipoles in response to the applied
electric field. Compounds of molecules with large dipole
moments, therefore, exhibit large dielectric constants in the
liquid phase. In the solid state, however, the restriction of the
molecular motions severely limits the contribution of orienta-
tional polarization. The dielectric constants of materials,
Received: January 14, 2015
Published: March 17, 2015
© 2015 American Chemical Society
4477
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
consequently, tend to exhibit drastic reduction on solid-
ification.6 To obtain crystalline materials with large dielectric
constants based on molecular motions, it is, therefore,
necessary to establish a design guide of crystal structures
where molecules with large dipole moments can reorient and
respond to the external electric field. Appropriate strategies of
crystal engineering combined with designing suitable polar
molecules will also be an important step toward the design of
ferroelectric molecular crystals, which is a rapidly progressing
and promising field of materials science.7
Scheme 1. Motional Restriction of Polar Molecules in the
Single-Component Crystals and Motional Freedom in CT
Crystals
a
We will present a simple and versatile strategy, i.e., formation
of weakly bound charge-transfer (CT) complex crystals, to
provide planar polar molecules with the ability to reorient in
crystals and to respond to the external electric field. CT
complexes are formed by attractive intermolecular forces
between molecules, where a fraction of electronic charge is
transferred from electron-rich donor molecules to electron-
poor acceptor molecules. The crystals of CT complexes are of
great interest and importance in the field of molecular materials
science, showing high electric conductivity,8 superconductivity,9
ferroelectricity,10 and unique magnetic properties.11 These
properties of CT crystals originate from the molecules with
partially occupied molecular orbitals produced by the charge
transfer. Most of the unique features and functions of CT
crystals, therefore, have been limited to those composed of
strong donors and/or acceptors, where the degree of charge
transfer between the components is large. The CT crystals
between weak donors and acceptors, on the other hand, are not
expected to display attractive electric properties because both
donors and acceptors are electrically neutral in the ground state
and their electronic states are almost identical to the ones in the
single-component crystals. We have exploited the weak CT
crystals as a framework in which polar planar molecules can
reorient and respond to the external electric field.
Planar disklike molecules, such as naphthalene and pyrene,
often undergo in-plane reorientation in crystals.1b,12 The
reorientation of these molecules is also feasible in their CT
crystals, where the aromatics, working as donors, form CT
complexes with acceptors such as tetracyanobenzene, tetracya-
noethylene, and pyromellitic dianhydride.13 Solid-state NMR
spectroscopic study combined with packing energy calculation
by Fyfe and co-workers revealed that naphthalene and pyrene
exhibited complete reorientation in the CT crystals. The CT
crystals of the rotating molecules often undergo phase
transitions at low temperatures, which are related to the
order−disorder behavior of molecular orientations of the
rotating donors in the crystals.14 Although the molecular
dynamics in some of the CT crystals has been studied
intensively, the dynamic nature of the molecules has not been
employed for designing functional materials. In this study we
have utilized CT crystals as an architecture where polar
molecules, which are stationary in their single-component
crystals, undergo in-plane reorientation and exhibit dielectric
responses to the external electric field (Scheme 1).
a
Red pentagons represent the polar molecule, and blue disks represent
the other component in CT crystals.
with larger donors (3 and 4), TBPA acquired the freedom of
reorientation in the crystalline environment and underwent in-
plane reorientation, which gave rise to dielectric responses. The
crystal of 4 showed a phase transition related to the ordering of
dipolar molecules, which induced significant changes in the
dielectric constants at the transition temperature. Pretransi-
tional crystal structural changes and the corresponding
dielectric responses were also observed and can be interpreted
in terms of a slight difference in the potential energies
corresponding to the two orientations induced by the breaking
of the symmetry of the crystal structure at the phase transition.
EXPERIMENTAL SECTION
■
Materials. TBPA (1), hexamethylbenzene, coronene, and perylene
were purchased from Tokyo Chemical Industry. Compound 1 and
perylene were purified by vacuum sublimation before crystal growth,
while hexamethylbenzene and coronene were used without further
purification. Colorless single crystals of 1 were prepared by slow
evaporation of benzene solution. Single crystals of CT complexes 2, 3,
and 4 (2, yellow; 3, orange; 4, red) were grown from an equimolar
benzene solution of 1 and hexamethylbenzene, coronene, or perylene
by slow evaporation. Diffuse reflectance spectra of crystalline powders
of 2−4 clearly show CT bands in the longer wavelength region
(Supporting Information).
Single-Crystal X-ray Diffraction Analysis. All of the diffraction
measurements were performed on a Bruker SMART APEX II ULTRA
diffractometer with a TXS rotating anode (Mo Kα radiation, λ =
0.71073 Å) and multilayer optics. The data collection was carried out
using a Japan Thermal Engineering DX-CS190LD N2-gas-flow
cryostat. The temperature in the nozzle was held constant within
0.2 K during the measurement. The temperature at the sample
position was calibrated using a thermocouple, and the calibration was
verified by checking the phase transition temperatures, including that
of the potassium dihydrogen phosphate (KDP) crystal (123 K).15 The
structures were solved by direct methods (SHELXT-2014) and refined
We have investigated the crystal structures and dielectric
properties of tetrabromophthalic anhydride (TBPA) (1) and its
CT complexes hexamethylbenzene−TBPA (2), coronene−
TBPA (3), and perylene−TBPA (4). The polar electron-
accepting molecule TBPA (μ = 4.5 D calculated at the MP2/6-
31+G* level) did not show reorientation in the single-
component crystals and in the CT crystal with a small donor
molecule (2), where the TBPA molecule at each crystal site
takes only one orientation. In stark contrast, in the CT crystals
4478
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
Table 1. Crystal Data and Structure Refinement Information for 1−3
TBPA (1)
hexamethylbenzene−TBPA (2)
coronene−TBPA (3)
molecular formula
C8O3Br4
463.72
C20H18O3Br4
625.98
C32H12O3Br4
764.06
fw
cryst syst
monoclinic
P21/n
monoclinic
P21/m
monoclinic
P21/c
space group
temp (K)
300
100
300
100
300
100
a (Å)
12.6749(10)
6.1713(5)
13.4149(11)
90.8147(12)
1049.22(15)
4
12.5164(9)
6.0894(4)
13.3977(10)
90.1220(11)
1021.14(13)
4
9.1304(8)
13.7261(12)
9.4089(8)
118.5970(11)
1035.32(16)
2
9.0493(7)
13.3796(10)
9.3465(7)
118.3590(9)
995.83(13)
2
7.9540(7)
9.5826(8)
16.2147(14)
89.1480(10)
1235.75(18)
2
7.8361(5)
b (Å)
9.6699(7)
15.7801(11)
90.2420(10)
1195.72(14)
2
c (Å)
β (deg)
V (Å3)
Z
no. of reflns collected
no. of independent reflns
Rint
14615
14161
15274
14633
17910
17195
2379
2320
2464
2366
2866
2751
0.0348
0.0287
0.0260
0.0202
0.0249
0.0220
no. of data/restraints/params
goodness-of-fit on F2
R[F2 > 2σ(F2)]
Rw(F2) (all data)
2379/0/137
1.040
2320/0/136
1.136
2464/0/148
1.038
2366/0/148
1.076
2866/307/306
1.039
2751/0/208
1.047
0.0214
0.0173
0.0264
0.0150
0.0404
0.0184
0.0547
0.0426
0.0651
0.0396
0.1173
0.0484
Table 2. Crystal Data and Structure Refinement Information for Perylene−TBPA (4)
molecular formula
C28H12O3Br4
716.02
fw
phase
HTP
LTP
cryst syst
triclinic
triclinic
space group
P1
̅
P1
̅
temp (K)
300
265
240
200
100
a (Å)
7.7233(3)
9.1834(3)
9.1909(3)
66.7020(10)
84.4390(10)
73.0640(10)
572.63(3)
1
10.0091(8)
10.0755(8)
12.5454(10)
85.0856(13)
73.6923(12)
69.0129(12)
1133.55(16)
2
9.9896(9)
10.0635(9)
12.5388(11)
85.3160(10)
73.8500(10)
69.0270(10)
1130.35(17)
2
9.9647(8)
10.0419(9)
12.5208(11)
85.4641(13)
73.8886(12)
68.9570(12)
1123.11(17)
2
9.9226(8)
9.9944(8)
12.4828(10)
85.7400(11)
73.8330(11)
68.8150(11)
1108.14(15)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
no. of reflns collected
no. of independent reflns
Rint
11086
21564
21725
21609
21129
2586
5118
5165
5150
5053
0.0287
0.0334
0.0295
0.0292
0.0253
no. of data/restraints/params
goodness-of-fit on F2
R[F2 > 2σ(F2)]
Rw(F2) (all data)
2586/123/188
1.090
5118/142/299
1.065
5165/0/277
1.033
5150/0/316
1.055
5053/0/316
1.070
0.0494
0.0318
0.0278
0.0220
0.0187
0.1558
0.0912
0.0765
0.0579
0.0484
by full-matrix least-squares on F2 using SHELXL-2014.16 All H atoms
were refined using riding models. For the crystal structures without
disorders, all non-H atoms were refined anisotropically. For the crystal
structures with disorders, details of the refinement were deposited in
the Supporting Information as CIFs embedding the SHELXL-2014 res
files. The crystal and experimental data are summarized in Tables 1
and 2.
Measurements. The real and imaginary parts of the dielectric
constant were measured with an Agilent E4980A precision LCR meter
in the frequency range from 500 Hz to 2 MHz. A homemade cryostat
was used to control the temperature of the samples under a helium
atmosphere between 120 and 360 K. The temperature was measured
using a Si diode thermometer mounted on the sample holder. Single
crystals of 1, 2, and 4 and a compaction pellet of 3 were used as the
samples with painted gold paste as the electrodes. Differential scanning
calorimetry (DSC) was recorded using a Rigaku Thermo Plus
DSC8230 with a heating and cooling rate of 10 K/min to examine
the phase transitions of the materials. Diffuse UV−vis reflectance
spectra were measured on a Jasco V-570 spectrometer equipped with
an integrating sphere accessory (ILN-472). A KBr powder was used as
the diluent and the reference. Powder X-ray diffraction patterns were
measured using a Bruker D8 ADVANCE and confirmed that the
microcrystalline powders used for DSC and dielectric measurements
had crystal structures identical to those obtained by single-crystal X-ray
diffraction analyses.
RESULTS AND DISCUSSION
■
Crystal Structure and Dielectric Property of TBPA (1).
The orientation of TBPA molecules is ordered in their single-
component crystals, which did not show the dielectric response
expected for dipolar reorientation. Crystal structure analysis of
1 was carried out at 300 and 100 K. The observed structures
were essentially identical to the reported one at room
temperature.17 The nearly planar disklike molecules took only
one orientation at each crystal site, and no orientational
disorder was detected at both temperatures (Figure 1). The
ordered structure indicates that the molecular reorientation of
TBPA is not allowed in the crystal. As for the dielectric
4479
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
Figure 2. Perspective views of the crystal structures of hexamethyl-
benzene−TBPA (2) and coronene−TBPA (3). The ellipsoids are
drawn at the 50% probability level. Hydrogen atoms are omitted for
clarity. (a) Projection of 2 at 300 K. (b) Projection of 3 at 100 K along
the b axis. The atoms of one of the two disordered orientations of
TBPA are depicted in white.
Figure 1. (a) Crystal and (b) molecular structures of TBPA (1) at 300
K. The ellipsoids are drawn at the 50% probability level.
property of 1, neither the real nor imaginary part of the
dielectric constant showed significant variation with temper-
ature between 340 and 140 K (Supporting Information). The
observed low and temperature-independent dielectric constant
(ε ≈ 5) clearly shows that, in spite of the large dipole moment
of TBPA molecules, they do not contribute to the dielectric
properties of the crystal in terms of orientational polarization
because they cannot change their orientations in the crystal.
Crystal Structures of Hexamethylbenezene−TBPA (2)
and Coronene−TBPA (3). The TBPA molecules were
incorporated into CT complex crystals as acceptor molecules
with hexamethylbenzene or coronene as donor molecules. Both
aromatic donors have round-shaped structures and undergo in-
plane rotations in their single-component crystals.18 While the
CT crystals of hexamethylbenzene−TBPA (2) did not exhibit
orientational disorders of the TBPA molecule, the TBPA
molecule in coronene−TBPA (3) showed orientational
disorder and its temperature dependence. Crystal structures
of 2 and 3 were determined at 300 and 100 K. Neither of them
showed phase transitions between the temperatures. Both of
the CT crystals 2 and 3 were composed of a 1:1 mixture of the
donor and acceptor molecules and took a mixed-stack
architecture, where the acceptor and donor molecules are
stacked alternately to form one-dimensional columns (Figure
2).
In the crystal 2, the donor molecule hexamethylbenzene lies
on a crystallographic center of inversion, and the atoms of the
acceptor TBPA molecule reside on a mirror plane. Although
the hydrogen atoms of the methyl groups of hexamethylben-
zene showed rotational disorder, no other disorder was not
detected, and the TBPA molecule took only one orientation.
The small values of the dielectric constant of the crystal 2 and
its temperature independence confirmed that the orientational
motion of the polar TBPA molecule did not take place in the
CT crystal 2 (Supporting Information).
Figure 3. Molecular structures of coronene−TBPA (3). The ellipsoids
are drawn at the 50% probability level. (a) Coronene molecules at 300
K. The atoms of orientations A, B, and C are depicted in gray, green,
and blue, respectively. (b) Coronene molecule at 100 K. (c) TBPA
molecules at 300 K. The atoms of the anhydride group (OCO
CO) of orientations B, C, and D are depicted in green, white, and
blue, respectively. (d) TBPA molecules at 100 K. The atoms of
orientation C are depicted in white.
The disorder of the coronene molecule disappeared at 100 K
(Figure 3b), and only one orientation was observed, which
shows that the disorder of coronene is dynamic and is caused
by in-plane reorientation. The TBPA molecules were also
orientationally disordered, and each molecule took four
different orientations (A, B, C, and D) at 300 K (Figure 3c).
The four orientations can be related to one another by the
rotation around the axis that is perpendicular to the molecular
plane and passing through the center of the benzene ring. Each
of two orientations A and B has its crystallographically
equivalent one, i.e., orientations C and D, respectively. Thus,
the orientational disorder of TBPA molecules enables the
noncentrosymmetric molecule to reside on a center of
inversion. The ratio of the populations of orientations A and
In the crystal 3, both coronene and TBPA molecules reside
on centers of inversion. Both the molecules were orientationally
disordered at 300 K. The coronene molecule was refined by
using the model where the molecule with approximate C6
symmetry took three different orientations, and the site
occupation factors of the three orientations (A, B, C) were
refined to be 0.598(9), 0.238(10), and 0.164(9) at 300 K
(Figure 3a).
4480
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
B, which equals that of C and D, was refined to be
0.356:0.144(2) at 300 K. At 100 K the two minor orientations
(B and D) disappeared, and only two orientations (A and C)
were observed (Figure 3d). The results show that TBPA
molecules undergo molecular reorientation and that each of
two pairs of orientations (A−B and C−D) equilibrates through
a 60° jump in orientation. As for the interconversion of TBPA
molecules between orientations A and C or B and D, which can
take place through 180° in-plane rotation, neither the
crystallographic study nor the dielectric measurements shown
in the next section can definitely determine whether it occurs in
the crystal 3, although its occurrence in the crystal 4 was
confirmed by both methodologies (see below).
Dielectric Properties of 3. Dielectric responses due to
molecular reorientation of TBPA were observed for the crystal
of 3. Figure 4 shows the dielectric constant of 3 measured at
several frequencies as a function of temperature. Both the real
(ε′) and imaginary (ε″) parts of the complex dielectric constant
exhibited marked changes with temperature above 150 K, and
the changes were dependent on the frequency of the applied
electric field. These features of dielectric responses are
characteristic of reorientational motion of polar molecules in
crystals and can be explained in terms of dielectric relaxation of
the molecular dipoles.6,19
In general, rotation of the permanent dipole of polar
molecules contributes to the dielectric constant on the
condition that the molecules can reorient fast enough to align
their dipoles in response to the applied alternating electric field.
At low field frequencies, where the molecular reorientation is
faster than the reversals of the electric field, the molecular
dipoles can follow the switching of the electric field and fully
contribute to the dielectric constant, which is observed to have
large values. In the high frequency region, where the molecular
motion is too slow to follow the electric field, the molecular
dipoles are regarded as stationary on the time scale of the ac
field and do not contribute to the dielectric constant, which
should have small values in the region. In the intermediate
frequency region, where the frequencies of the applied field and
molecular reorientation are comparable, the molecular dipoles
contribute to the dielectric constant with a phase lag between
the applied electric field and the induced orientational
polarization. The phase lag results in the absorption of energy
of the applied electric field by the material, which is known as
dielectric loss. The dielectric loss is observed as peaks in the
plot of the imaginary part (ε″) against frequency, and the
relaxation time of the reorientational motion at that temper-
ature can be derived from the plot. The dielectric loss is
negligible in the high and low frequency limits. The ε′ values in
the intermediate frequency region gradually decrease from a
large value at the low frequency limit to a small value at the
high frequency limit. In the case of the crystal of 3, the
observed dielectric responses are attributable to the reorienta-
tion of polar TBPA molecules, and the reorientation of
nonpolar coronene molecules does not contribute to the
orientational polarization. The reorientation of TBPA mole-
cules was much slower than the applied frequencies below
about 150 K and became much faster around 350 K (Figure
4a). In the intermediate temperature region, the real part
increased with temperature and the imaginary part exhibited
absorption peaks (Figure 4b), both of which were dependent
on the frequency of the ac field.
For more quantitative analysis, the real and imaginary parts
of the complex dielectric constant of 3 were plotted against the
frequency of the electric field at several selected temperatures
(Figure 4c,d). The Cole−Cole diagrams (ε″ vs ε′) at two
selected temperatures are also shown in Figure 4e. The Cole−
Cole plots for 3 deviate from the semicircles or circular arcs
expected from the Debye and Cole−Cole relaxation functions,
respectively.20 The skewed circular arcs were well described by
the phenomenological relaxation function known as the
Havriliak−Negami equation:21
ε0 − ε∞
ε*(ω) = ε∞ +
β
[1 + (iωτ)1−α
]
(1)
where ε* is the complex dielectric constant, ε0 and ε∞ are the
low and high frequency limits of the real part of ε*, respectively
(ε0 > ε∞), τ represents the relaxation time, and ω is equal to 2π
times the frequency. The parameters α and β (0 ≤ α, β ≤ 1) are
related to the distribution of relaxation times. The Debye (α =
0, β = 1, a semicircle plot) and the Cole−Cole (0 < α < 1, β =
1, a circular arc plot) expressions are special cases of the
Havriliak−Negami relation. The parameters for 3 were
obtained by fitting the experimental data of the imaginary
part (ε″) using eq 1 (Figure 4d). The real parts and the Cole−
Cole diagrams of 3 were also well represented by eq 1 using the
Figure 4. Dielectric properties of coronene−TBPA (3). Temperature
dependence of (a) the real part (ε′) and (b) the imaginary part (ε″) of
the complex dielectric constant measured at various frequencies.
Frequency dependence of (c) the real part (ε′) and (d) the imaginary
part (ε″) at several selected temperatures. (e) Relationship between ε′
and ε″ (the Cole−Cole diagram). Solid curves in (c)−(e) represent
the fit of the experimental data to the Havriliak−Negami model.
4481
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
parameters obtained. The relaxation time τ estimated at each
temperature followed the Arrhenius equation (eq 2) with good
accuracy, which gave an activation energy Ea of 48 kJ/mol for
the dielectric relaxation (Figure 5).
⎛
⎜
⎝
⎞
⎟
⎠
Ea
kT
τ = C exp
(2)
Figure 5. Arrhenius plot of the relaxation time τ as a function of
inverse temperature for coronene−TBPA (3). The solid line
represents the least-squares fit of the data.
Figure 6. Crystal and molecular structures of perylene−TBPA (4).
The ellipsoids are drawn at the 50% probability level. Hydrogen atoms
in (a) and (c) are omitted for clarity. The atoms of one of the two
disordered orientations of TBPA in (a) and (b) are depicted in white.
(a) Projection of the crystal structure at 300 K along the c axis. The
molecules of two unit cells are presented. (b) TBPA molecules at 300
K. (c) Projection of the crystal structure at 100 K along the [1−11]
direction. (d) TBPA molecule at 100 K.
The results show that the TBPA molecules, which are
stationary in the crystals of 1 and 2, acquired freedom of
reorientation in the CT crystals of 3 with a reasonably small
value of the activation energy.22 As for the mode of
reorientation of the TBPA molecules, 60° jump and 180°
jump of molecules are not distinguishable from the present
dielectric measurements using a compaction pellet of crystalline
powders, which lost the information on the anisotropy of the
crystal structure and molecular arrangement that can identify
the directions of the molecular dipole moments and the course
of their changes in the sample.
Crystal Structure and Structural Phase Transition of
Perylene−TBPA (4). The CT crystal of perylene−TBPA (4)
exhibited a structural phase transition related to order−disorder
of the orientation of TBPA molecules. The CT crystal 4
consists of a 1:1 mixture of the donor perylene and the acceptor
TBPA molecules, which are stacked alternately as the donors
and acceptors in the crystals of 2 and 3 (Figure 6a). The crystal
remained identical with that in HTP (Table 2, Figure 6a,c).
The overall arrangement of molecules in the crystal did not
change significantly at this phase transition. In LTP, there are
two crystallographically independent perylene molecules in the
asymmetric unit and both of them reside on inversion centers.
There is a single TBPA molecule in the asymmetric unit, which
does not lie on any crystallographic symmetry element. Thus,
the orientational disorder of TBPA observed in HTP
disappeared at 100 K (Figure 6d), and the phase transition
can be defined as the order−disorder type. In the disordered
HTP, the polar TBPA molecule is on the inversion center and
the dipole moment of each molecule is canceled at each site of
the crystal. In the LTP, on the other hand, the TBPA molecule
at each site has its polarity. Although the changes in the degrees
of order of the orientation and dipole at each site satisfy some
of the prerequisites for ferroelectric phase transitions, the
4 belongs to the triclinic P1 space group at 300 K (high-
̅
temperature phase, HTP). Both perylene and TBPA molecules
lie on centers of inversion at that temperature, and TBPA is
orientationally disordered over two sites with equal occupancies
similarly to the TBPA molecule in 3 at 100 K (Figure 6b). As
will be shown below, the dielectric measurements confirmed
that the disorder of the TBPA molecules was dynamic and
originated from a 180° in-plane rotation.
centrosymmetry of the crystal (nonpolar space group P1) in
̅
LTP rules out the possibility of ferroelectric transition. On the
basis of the observed structure changes, the phase transition of
4 is expected to be a para- to antiferroelectric one.
The crystal 4 exhibited a structural phase transition when it
was cooled to 100 K. DSC measurement of the microcrystalline
sample of 4 showed that the crystals underwent a phase
transition at 268 K (Supporting Information). The transition
enthalpy ΔH = 1.56 kJ/mol and entropy ΔS = 5.80 J/(K·mol)
were calculated from the DSC peak. The magnitude of
transition entropy, ΔS = R ln(2.01), where R is the gas
constant, is consistent with the order−disorder phase transition
with two different orientations observed by crystallographic
study; see below. X-ray diffraction analysis at 100 K revealed
that the cell volume of the crystal 4 was almost doubled in the
It should be noted that the orientational disorder of TBPA
molecules in 3 and 4 was related to crystallographic symmetry
elements and that the appearance and disappearance of the
disorder dominated the phase transition of 4. Although
dynamic disorders of molecules often disappear at low
temperature, most of them are not related to phase transitions.
This is especially the case for orientational disorders of small
molecules in clathrate crystals or disorders of only a part of the
whole molecule, such as alkyl chains or tertiary butyl groups.
Even the orientational disorders of whole molecules are not
necessarily related to phase transitions.23 Introduction of a new
low-temperature phase (LTP), while the space group (P1)
̅
4482
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
symmetry element (i.e., an inversion center) by the disorder of
TBPA molecules in the CT crystals indicated that the
orientational order of the molecules, although it varies with
crystals, is not a trivial matter to the crystals and that the order
and disorder of the orientation can switch the symmetry of the
crystal site between pseudocentrosymmetric and centrosym-
metric and toggle the polarity and nonpolarity of the site.
Dielectric Properties of 4. Dielectric measurements of the
crystal of 4 confirmed that the TBPA molecules underwent in-
plane reorientation in the crystal, and the dielectric properties
exhibited marked changes at the order−disorder phase
transition. Figure 7 shows the dielectric constant of 4 measured
dropped at the transition temperature. The dielectric constant
further decreased with lowering of the temperature in LTP. As
discussed in the next section, the smaller values of the dielectric
constant in LTP in the vicinity of the phase transition
temperature reflect a suppressed dielectric response due to
the energy difference between the two orientations in the
crystal lattice.
The frequency dependence of the complex dielectric
constant of 4 in HTP (Figure 7c,d) and the symmetric circular
arcs of its Cole−Cole diagram (Figure 7e) were represented
well by the Cole−Cole equation (eq 3),20 which is a special
case (β = 1) of the Havriliak−Negami equation (eq 1).
ε0 − ε∞
ε*(ω) = ε∞ +
1 + (iωτ)1−α
(3)
The Arrhenius fitting of the estimated relaxation time τ
yielded the activation energy 56 kJ/mol for the dielectric
relaxation (Figure 8). These results clearly show that the TBPA
Figure 8. Arrhenius plot of the relaxation time τ as a function of
inverse temperature for perylene−TBPA (4). The solid line represents
the least-squares fit of the data.
molecules of 4 in HTP also exhibited in-plane reorientation and
dielectric responses and that the change in the degree of
orientational order at low temperature induced a dielectric
phase transition.
Pretransitional Crystal Structure Changes and Mech-
anism of the Order−Disorder Phase Transition of 4. To
gain further understanding of the order−disorder phase
transition of 4, we examined the pretransitional behavior of
the molecules in the crystal and found that the order−disorder
transition was induced by changes not in the molecular
dynamics but in the relative stability of the two orientations. X-
ray diffraction analysis of 4 in LTP was carried out at several
temperatures in the vicinity of the transition temperature.
Figure 9 shows the difference electron density maps on the
least-squares mean plane of the atoms of the TBPA molecule.
At 200 K only one orientation of TBPA was observed, and the
map shows no significant residual peaks that can belong to the
atoms of the molecule with another orientation (Figure 9a). At
240 K, however, two residual peaks appeared and can be
assigned to the two Br atoms of the minor orientation (Figure
9b). The intensities of the peaks increased at 265 K, a few
kelvin below the transition temperature, and other peaks of the
atoms of the minor orientation were detected at this
temperature (Figure 9c). The ratio of the populations of the
major and the minor orientations was refined to be
0.9167:0.0833(8) at 265 K.
Figure 7. Dielectric properties of perylene−TBPA (4). Temperature
dependence of (a) the real part (ε′) and (b) the imaginary part (ε″) of
the complex dielectric constant measured at various frequencies. The
phase transition temperature is shown by arrows. Frequency
dependence of (c) the real part (ε′) and (d) the imaginary part
(ε″) at several selected temperatures. (e) Relationship between ε′ and
ε″ (the Cole−Cole diagram). Solid curves in (c)−(e) represent the fit
of the experimental data to the Cole−Cole model.
at several frequencies (1 kHz to 2 MHz) as a function of
temperature. The electric field was applied along the crystal b
axis of the unit cell of HTP, which is parallel to the direction of
the dipole moment vector of TBPA molecules in the crystal. In
HTP (above around 270 K), both the real (ε′) and imaginary
(ε″) parts of the dielectric constant showed temperature
dependence similar to that observed for 3 (Figure 7a,b). As the
temperature was decreased, the dielectric constant abruptly
4483
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
of two orientations decreases the amount of the minor
orientation to be undetectable by X-ray diffraction analysis.27
Ordered structures in the low-temperature phase at order−
disorder transitions are likely to be related to the energy
difference between structures responsible for the disorder
rather than the freezing of their dynamic exchanges.
Halogen Bonding in the Crystals 1 and 2. The ordered
orientation of TBPA molecules in the crystals of 1 and 2 can be
rationalized in terms of the preference of one orientation due to
intermolecular halogen bonding.28 Figure 10 illustrates the
arrangements of TBPA molecules in the crystals of 1 and 2.
Figure 9. Difference electron density maps of perylene−TBPA (4).
Each map is the least-squares mean plane of the atoms of the TBPA
molecule. The contour lines are at 0.2 e Å−3 intervals. Negative
contours are indicated by broken lines. (a) At 200 K. (b) At 240 K.
The residual peaks corresponding to the atoms of the minor
orientation are indicated by arrows. (c) At 265 K. The molecular
skeleton of the minor orientation is drawn in red.
Figure 10. TBPA molecules linked by the C−Br···O halogen bonds in
crystals. The ellipsoids are drawn at the 50% probability level. (a)
TBPA (1) at 300 K. (b) Hexamethylbenzene−TBPA (2) at 300 K.
These results clearly show that the orientation of the TBPA
molecule was not completely ordered in the “ordered” LTP but
mostly biased toward one of the two orientations. The
disappearance of the minor orientation below 200 K indicates
the following: The two orientations were equilibrating with
each other through molecular reorientation even in LTP at least
near the phase transition temperature, and the amount of the
minor orientation decreased to be negligible as the temperature
was lowered according to the Boltzmann distribution law. The
small but significant dielectric response observed in LTP
(Figure 7a,b) is consistent with the occurrence of reorienta-
tional motion of TBPA and concomitant dipolar relaxation in
LTP.24
The observed changes in the crystal structure of 4 between
300 and 100 K are characteristic of order−disorder phase
transitions, which tend to be interpreted as follows: In the high-
temperature phase, the molecule dynamically changes its
orientation between the two crystallographically equivalent
ones and the observed structure is therefore disordered. In the
low-temperature phase, in contrast, the molecular reorientation
is frozen and each molecule is confined to one orientation that
corresponds to a minimum of the rotational potential energy
surface defined by the crystal lattice. The pretransitional
structural changes and the dielectric response of 4, however,
clearly show that the molecular reorientation of TBPA was not
frozen in LTP.26 Only a small difference in the relative stability
Short and almost linear contacts of (C−)Br···O (1 at 300 K,
3.1525(26) Å, 175.65(10)°; 2 at 300 K, 2.9993(26) Å,
174.29(11)°) are indicative of halogen bonding,29 and TBPA
molecules in both crystals are connected to the neighboring
ones by the halogen bonds. The intermolecular halogen
bonding should make the observed orientation energetically
much more favorable than the other possible ones. The
reorientation of TBPA molecules and dielectric response of the
crystals are not allowed because the residence time of molecules
at any other disfavored orientations is negligibly short even if
the process of molecular reorientation is not inhibited by the
surrounding molecules. In contrast, no halogen bonding or
other significant intermolecular contact, except CT interactions,
was detected in the crystal structures of 3 and 4, where the
TBPA molecules undergo molecular reorientation.
CONCLUSION
■
We have found that the dynamic nature of TBPA molecules in
crystals and their dielectric responses were drastically enhanced
by formation of CT crystals and that, despite their static nature
in the single-component crystal, the polar molecules acquired
dynamic properties in their CT crystals. In the single-
component crystal of TBPA (1) and the CT crystal with
hexamethylbenzene (2), each TBPA molecule took only one
orientation that was energetically favored due to intermolecular
4484
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
halogen bonding, and the crystals did not show any dielectric
anomaly. In the CT crystals with coronene (3) and perylene
(4), in contrast, each TBPA molecule was orientationally
disordered at room temperature and dynamic exchange
between different orientations resulted in marked dielectric
responses of the crystals. The changes in the molecular
dynamics and orientational order of TBPA molecules are likely
to be brought about by additional free space available in the CT
crystals and/or centrosymmetric environment of TBPA in the
crystals induced by the centrosymmetric donor molecules. The
present results tend to show that the larger the donors, the
more mobility and freedom of orientation TBPA acquires in the
CT crystals.
ASSOCIATED CONTENT
* Supporting Information
Diffuse reflectance spectra of crystalline powders of 1−4,
hexamethylbenzene, coronene, and perylene, temperature
dependence of the complex dielectric constants of 1 and 2,
DSC of 4, and X-ray crystallographic data (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
■
Corresponding Authors
*junharada@sci.hokudai.ac.jp
*inabe@sci.hokudai.ac.jp
Notes
The present study has demonstrated that formation of
weakly bound CT crystals can provide a simple and versatile
method to modify the dynamic properties of polar compounds
and also a design guide to construct dielectric crystalline
materials. Only if a polar molecule of interest has some electron
donor or acceptor character is a wide range of choice available
for the constituent donor or acceptor molecules to form CT
crystals. The formation of CT crystals of polar molecules is,
therefore, a promising approach to develop dielectric materials
with desired properties, in a way similar to the successful
development of organic conductive and superconductive
materials.
We have also found that the CT crystal formations tend to
impose centrosymmetry or pseudocentrosymmetry on the site
of the TPBA molecule, the symmetry of which can be switched
with phase transitions. The crystal 4 showed an order−disorder
phase transition, where the dielectric property was abruptly
changed. The pretransitional changes in the crystal structure
were observed, and the transition was interpreted not in terms
of freezing of the molecular reorientation but in terms of the
energy difference between the two interconverting orientations
in the low-temperature phase. Although the crystal structure
changes indicate that the transition of 4 belongs to a para- to
antiferroelectric one, several features of the transition satisfied
some of the conditions required for para- to ferroelectric phase
transitions.
In phase transitions of crystalline dielectric materials, the
nature of the transitions, such as ferroelectric or antiferro-
electric, is mainly determined by the symmetry and polarity of
the crystals, regulation of which requires the control of the
crystal structures. Although controlling the crystal structures
and phase transitions is, in general, not possible at present, the
arrangement of molecules in the weakly bound CT crystals is
largely predetermined: i.e., the donor and acceptor molecules
are alternately stacked to form one-dimensional columns, in
which the molecular dipoles stack parallel or antiparallel to each
other. Only a limited number of interactions have to be
considered to understand the structures and to shift the course
of phase transitions of the CT crystals. We are now undertaking
a systematic study of the CT crystals of TBPA or its chloro
analogue, tetrachlorophthalic anhydride (TCPA), using differ-
ent aromatic hydrocarbons as donors. DSC and X-ray
diffraction studies have revealed that many of them undergo
phase transitions below or above room temperature,31 which
has made this class of CT crystals a promising target of
exploration. Further investigation of CT crystals of polar
molecules with interchangeable donor or acceptor component
molecules will serve to significantly advance the development of
crystalline dielectric materials.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. We are grateful to Dr.
Kobayashi at Hokkaido University for permission to use the
Bruker D8 ADVANCE powder X-ray diffractometer.
REFERENCES
■
(1) (a) Gavezzotti, A.; Simonetta, M. Chem. Rev. 1982, 82, 1−13.
(b) Fyfe, C. A. Solid State NMR for Chemists; CFC Press, Guelph,
Canada, 1983. (c) Dynamics of Molecular Crystals. Studies in Physical
and Theoretical Chemistry 46; Lascombe, J., Ed.; Elsevier: Amsterdam,
1987. (d) Gavezzotti, A.; Simonetta, M. In Organic Solid State
Chemistry. Studies in Organic Chemistry 32; Desiraju, G. R., Ed.;
Elsevier: Amsterdam, 1987; Chapter 11. (e) Dunitz, J. D.; Maverick, E.
F.; Trueblood, K. N. Angew. Chem., Int. Ed. Engl. 1988, 27, 880−895.
(f) Harris, K. D. M.; Aliev, A. E. Chem. Br. 1995, 132−136. (g) Burgi,
̈
H. B. Annu. Rev. Phys. Chem. 2000, 51, 275−296. (h) Harada, J.;
Ogawa, K. In Topics in Stereochemistry; Denmark, S. E., Siegel, J. S.,
Eds.; John Wiley & Sons, Inc.: 2006; Vol. 25, p 31. (i) Vogelsberg, C.
S.; Garcia-Garibay, M. A. Chem. Soc. Rev. 2012, 41, 1892−1910.
(2) (a) Garcia-Garibay, M. A. Proc. Natl. Acad. Sci. U.S.A. 2005, 102,
10771−10776. (b) Karlen, S.; Garcia-Garibay, M. Top. Curr. Chem.
2005, 262, 179−227. (c) Khuong, T.-A. V.; Nunez, J. E.; Godinez, C.
E.; Garcia-Garibay, M. A. Acc. Chem. Res. 2006, 39, 413−422.
(d) Sokolov, A. N.; Swenson, D. C.; MacGillivray, L. R. Proc. Natl.
Acad. Sci. U.S.A. 2008, 105, 1794−1797. (e) Karlen, S. D.; Reyes, H.;
Taylor, R. E.; Khan, S. I.; Hawthorne, M. F.; Garcia-Garibay, M. A.
Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 14973−14977. (f) Lemouchi,
́ ̀
C.; Meziere, C.; Zorina, L.; Simonov, S.; Rodríguez-Fortea, A.;
Canadell, E.; Wzietek, P.; Auban-Senzier, P.; Pasquier, C.; Giamarchi,
T.; Garcia-Garibay, M. A.; Batail, P. J. Am. Chem. Soc. 2012, 134,
7880−7891. (g) Sato, S.; Yamasaki, T.; Isobe, H. Proc. Natl. Acad. Sci.
U.S.A. 2014, 111, 8374−8379.
(3) (a) Horansky, R. D.; Clarke, L. I.; Price, J. C.; Khuong, T.-A. V.;
Jarowski, P. D.; Garcia-Garibay, M. A. Phys. Rev. B: Condens. Matter
Mater. Phys. 2005, 72, 014302. (b) Horansky, R. D.; Clarke, L. I.;
Winston, E. B.; Price, J. C.; Karlen, S. D.; Jarowski, P. D.; Santillan, R.;
Garcia-Garibay, M. A. Phys. Rev. B: Condens. Matter Mater. Phys. 2006,
74, 054306. (c) Akutagawa, T.; Koshinaka, H.; Sato, D.; Takeda, S.;
Noro, S.-I.; Takahashi, H.; Kumai, R.; Tokura, Y.; Nakamura, T. Nat.
Mater. 2009, 8, 342−347.
(4) (a) Czarnecki, P.; Nawrocik, W.; Pająk, Z.; Wąsicki, J. J. Phys.:
Condens. Matter 1994, 6, 4955−4960. (b) Wąsicki, J.; Czarnecki, P.;
Pająk, Z.; Nawrocik, W.; Szczepan
576−579. (c) Pajak, Z.; Czarnecki, P.; Szafran
́
̧
́
́
Fojud, Z. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 132102.
(d) Przesławski, J.; Kosturek, B.; Dacko, S.; Jakubas, R. Solid State
Commun. 2007, 142, 713−717. (e) Zhang, Y.; Ye, H.-Y.; Cai, H.-L.;
Fu, D.-W.; Ye, Q.; Zhang, W.; Zhou, Q.; Wang, J.; Yuan, G.-L.; Xiong,
R.-G. Adv. Mater. 2014, 26, 4515−4520.
4485
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486

Journal of the American Chemical Society
Article
(5) Lines, M. E.; Glass, A. M. Principles and Applications of
Ferroelectrics and Related Materials; Clarendon Press: Oxford, U.K.,
1977.
(6) Wright, J. D. Molecular Crystals, 2nd ed.; Cambridge University
Press: Cambridge, U.K., 1995.
(7) (a) Horiuchi, S.; Ishii, F.; Kumai, R.; Okimoto, Y.; Tachibana, H.;
Nagaosa, N.; Tokura, Y. Nat. Mater. 2005, 4, 163−166. (b) Horiuchi,
S.; Tokura, Y. Nat. Mater. 2008, 7, 357−366. (c) Horiuchi, S.;
Tokunaga, Y.; Giovannetti, G.; Picozzi, S.; Itoh, H.; Shimano, R.;
Kumai, R.; Tokura, Y. Nature 2010, 463, 789−792. (d) Horiuchi, S.;
Kagawa, F.; Hatahara, K.; Kobayashi, K.; Kumai, R.; Murakami, Y.;
Tokura, Y. Nat. Commun. 2012, 3, 1308.
(d) Lemouchi, C.; Vogelsberg, C. S.; Zorina, L.; Simonov, S.; Batail,
P.; Brown, S.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2011, 133,
6371−6379. (e) Lemouchi, C.; Yamamoto, H. M.; Kato, R.; Simonov,
S.; Zorina, L.; Rodríguez-Fortea, A.; Canadell, E.; Wzietek, P.;
Iliopoulos, K.; Gindre, D.; Chrysos, M.; Batail, P. Cryst. Growth Des.
2014, 14, 3375−3383.
(29) The sum of the van der Waals radii of Br and O atoms is 3.37
Å.30
(30) Bondi, A. J. Phys. Chem. 1964, 68, 441−451.
(31) For example, the chloro analogue of 2, i.e., hexamethylbenzene−
TCPA, underwent a phase transition at 401 K. The chloro analogue of
4, perylene−TCPA, crystallized in two forms, both of which also
showed a phase transition (at 233 and 148 K, respectively).
(8) (a) Ferraris, J.; Cowan, D. O.; Walatka, V.; Perlstein, J. H. J. Am.
Chem. Soc. 1973, 95, 948−949. (b) Saito, G.; Yoshida, Y. Bull. Chem.
Soc. Jpn. 2007, 80, 1−137.
́
(9) Jerome, D. Science 1991, 252, 1509−1514.
(10) (a) Tokura, Y.; Koshihara, S.; Iwasa, Y.; Okamoto, H.; Komatsu,
T.; Koda, T.; Iwasawa, N.; Saito, G. Phys. Rev. Lett. 1989, 63, 2405−
2408. (b) Okamoto, H.; Mitani, T.; Tokura, Y.; Koshihara, S.;
Komatsu, T.; Iwasa, Y.; Koda, T.; Saito, G. Phys. Rev. B: Condens.
Matter Mater. Phys. 1991, 43, 8224−8232. (c) Kobayashi, K.; Horiuchi,
S.; Kumai, R.; Kagawa, F.; Murakami, Y.; Tokura, Y. Phys. Rev. Lett.
2012, 108, 237601.
(11) (a) Miller, J. S.; Calabrese, J. C.; Rommelmann, H.; Chittipeddi,
S. R.; Zhang, J. H.; Reiff, W. M.; Epstein, A. J. J. Am. Chem. Soc. 1987,
109, 769−781. (b) Miller, J. S. J. Mater. Chem. 2010, 20, 1846−1857.
(12) Boyd, R. K.; Fyfe, C. A.; Wright, D. A. J. Phys. Chem. Solids
1974, 35, 1355−1365.
(13) (a) Fyfe, C. A. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1633−
1641. (b) Fyfe, C. A. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1642−
1649. (c) Fyfe, C. A.; Harold-Smith, D.; Ripmeester, J. J. Chem. Soc.,
Faraday Trans. 2 1976, 72, 2269−2282.
(14) Herbstein, F. H. Crystalline Molecular Complexes and
Compounds; Oxford University Press: Oxford, U.K., 2005; Vol. 2,
Chapter 16.
(15) Tomaszewski, P. E. Phase Transitions 1992, 38, 127−220.
(16) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(17) Sake Gowda, D. S.; Rudman, R. Acta Crystallogr. Sect. B 1982,
38, 2842−2845.
(18) (a) Allen, P. S.; Cowking, A. J. Chem. Phys. 1967, 47, 4286−
4289. (b) Van Steenwinkel, R. Z. Naturforsch., A 1969, 24, 1526−1531.
(c) Fyfe, C. A.; Dunell, B. A.; Rimeester, J. Can. J. Chem. 1971, 49,
3332−3335.
(19) Kao, K. C. Dielectric Phenomena in Solids: With Emphasis on
Physical Concepts of Electronic Processes; Elsevier Academic Press: San
Diego, 2004.
(20) Cole, K. S.; Cole, R. H. J. Chem. Phys. 1941, 9, 341−351.
(21) Havriliak, S.; Negami, S. Polymer 1967, 8, 161−210.
(22) The activation energies of in-plane reorientation of
hexamethylbenzene, coronene, and pyrene in crystals were reported
to be 28.1, 25.2, and 60.9 kJ/mol, respectively.1b
(23) Harada, J.; Ogawa, K. Chem. Soc. Rev. 2009, 38, 2244−2252.
(24) The small dielectric response in LTP is consistent with Debye-
type orientational polarization.25 The energy difference in the two
orientations without an electric field decreases the number of
molecular dipoles that change their orientation in response to the
applied electric field.
(25) Debye, P. J. W. Polar Molecules; Chemical Catalogue Co.: New
York, 1929.
(26) From the Arrhenius relation obtained in Figure 8, the relaxation
time τ at the transition temperature is calculated to be 1.1 × 10−5 s−1.
(27) (a) Harada, J.; Ogawa, K. J. Am. Chem. Soc. 2001, 123, 10884−
10888. (b) Harada, J.; Harakawa, M.; Ogawa, K. CrystEngComm 2008,
10, 1777−1781.
(28) (a) Metrangolo, P.; Resnati, G. Chem.Eur. J. 2001, 7, 2511−
2519. (b) Desiraju, G. R.; Ho, P. S.; Kloo, L.; Legon, A. C.; Marquardt,
R.; Metrangolo, P.; Politzer, P.; Resnati, G.; Rissanen, K. Pure Appl.
Chem. 2013, 85, 1711−1713. (c) Priimagi, A.; Cavallo, G.;
Metrangolo, P.; Resnati, G. Acc. Chem. Res. 2013, 46, 2686−2695.
4486
DOI: 10.1021/jacs.5b00412
J. Am. Chem. Soc. 2015, 137, 4477−4486