Collective In-Plane Molecular Rotator Based on Dibromoiodomesitylene π-Stacks

Article abstract of DOI:10.1021/jacs.5b08215

Interest in artificial solid-state molecular rotator systems is growing as they enable systems to be designed for achieving specific physical functions. The phase transition behavior of four halomesitylene crystals indicated dynamic in-plane molecular rot

Full text of DOI:10.1021/jacs.5b08215

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Article
A Collective In-plane Molecular Rotator
based on Dibromoiodomesitylene #-Stacks
Jun-ichi Ichikawa, Norihisa Hoshino, Takashi Takeda, and Tomoyuki Akutagawa
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Sep 2015
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A Collective In-plane Molecular Rotator based on
Dibromoiodomesitylene π-Stacks
Junꢀichi Ichikawa,† Norihisa Hoshino,†, ‡ Takashi Takeda,†, ‡ and Tomoyuki Akutagawa †, ‡*
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†Graduate School of Engineering, Tohoku University, Sendai 980ꢀ8579, Japan.
‡Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2ꢀ1ꢀ1 Katahira, Aobaꢀku,
Sendai 980ꢀ8577, Japan.
ABSTRACT: Interest in artificial solidꢀstate molecular rotator systems is growing as they enable designing systems for achieving
specific physical functions. The phase transition behaviour of four halomesitylene crystals indicated dynamic inꢀplane molecular
rotator characteristics in dibromoiodomesitylene, tribromomesitylene, and dibromomesitylene crystals. Such molecular rotation in
diiodomesitylene crystals was suppressed by effective I•••I intermolecular interactions. The inꢀplane molecular rotation
accompanied by a change in dipole moment resulted in dielectric phase transitions in polar dibromoiodomesitylene and
dibromomesitylene crystals. No dielectric anomaly was observed for the inꢀplane molecular rotation of tribromomesitylene in the
absence of this dipole moment change. Typical antiferroelectricꢀparaelectric phase transitions were observed in the
dibromomesitylene crystal, whereas the dielectric anomaly of dibromoiodomesitylene crystals was associated with the collective inꢀ
plane molecular rotation of polar πꢀmolecules in the πꢀstack. We found that the singleꢀropeꢀlike collective inꢀplane molecular
rotator was dominated by intermolecular I•••I interactions along the πꢀstacking column of polar dibromoiodomesitylene.
crystalline lattice using intermolecular I•••I interactions has been
utilised for constructing stator environments and fast rotator units
with a rotary speed of 3 GHz at 300 K.16 Another interesting
chemical approach was reported in chargeꢀtransfer complexes
based on discꢀshaped aromatic hydrocarbons such as pyrene,
perylene, and coronene, forming an inꢀplane molecular rotator
along the direction normal to the πꢀplane.17,18 The orderꢀdisorderꢀ
type antiferroelectricꢀparaelectric phase transition was observed in
perylene tetrabromophthalic anhydride crystals, where the inꢀ
plane molecular rotation of the polar structure resulted in a phase
transition to the lowꢀtemperature antiferroelectric state.18
INTRODUCTION
Development of artificial molecular motors has been stimulated,
in particular, by biological molecular motors such as ATPase and
kinesin motors that can be used to conduct precise operations with
high energy conversion efficiencies between chemical and kinetic
energy.1,2 Although the unidirectional motion of artificial
molecular rotators such as triptyceneꢀ,3 catenaneꢀ,4 and rotaxaneꢀ
based molecules5 has been developed using intraꢀ and
intermolecular redox, acidꢀbase, photo, and thermal reactions, the
mechanical motion of these molecular systems is limited in the
solution phase. Therefore, solidꢀstate molecular rotators are key
targets for realising ideal molecular machines.6,7
We previously examined a twoꢀfold flipꢀflop molecular rotator
coupled with ferroelectric bulk physical properties in (mꢀ
FAni+)(dibenzo[18]crownꢀ6)[Ni(dmit)2] crystal (dmit = 2ꢀthioxoꢀ
1,3ꢀdithioleꢀ4,5ꢀdithiolate).19 The presence of the lowꢀsymmetry
polar mꢀfluoroanilinium (mꢀFAni+) cation resulted in different
initial and rotated molecular structures with rightꢀ and leftꢀdipole
orientations, resulting in dipole inversion. The thermal activation
of such dipole inversions above 346 K led to the ferroelectricꢀ
paraelectric phase transition, where the dipoleꢀordering
ferroelectric state was achieved by the collective molecular
rotation of mꢀFAni+ cations arranging the crystal dipole moments.
In contrast, the random molecular rotation of mꢀFAni+ cations
occurred in the highꢀtemperature paraelectric phase.
Rotation of small structural units in a molecule, such as methyl
groups in hexamethylbenzene crystals, can be thermally activated
at room temperature as the longꢀrange periodic molecular
arrangement of the crystal has no effect rotational motion.8 In
addition, large molecular rotations have been observed in plastic
crystalline materials such as CCl4, adamantane, and C60, where the
isotropic dynamic molecular rotations were observed in the
absence of large steric hindrance.9ꢀ11 The highꢀtemperature
dynamic rotator phase showed a phase transition to the lowꢀ
temperature orientationally ordered static phase upon cooling
where the steric hindrance between the spherical molecules was
small, even in the closeꢀpacking structures. However, the
physical responses of plastic crystalline materials were largely
unaffected by molecular rotations in symmetric apolar molecules.
Therefore, targeted molecular design is important for developing
functional molecular rotator systems.
Our next target is therefore achieving collective molecular
rotation in molecular assemblies, which can be controlled by
specific intermolecular interactions.
intermolecular interactions are possible in simple discꢀshaped πꢀ
molecules. We decided that the introduction of a dipole moment
into the inꢀplane molecular rotation of such structures would
enable us to clarify the relationship between dynamic molecular
motions and dielectric responses in solid materials. We therefore
report studies into halo (Br and/or I)ꢀsubstituted mesitylene
Several types of
Artificial solidꢀstate molecular rotatorꢀstator systems such as
molecular gyroscopes,12,
metalꢀorganicꢀframeworks,14 and
13
supramolecular cationic systems15 have been designed to achieve
specific physical functions. Accurate molecular design for
constructing rotatorꢀstator systems is essential to achieve dynamic
crystalline environments. For example, a twoꢀdimensional rigid
derivatives,
tribromomesitylene
namely
(2),
dibromoiodomesitylene
dibromomesitylene (3),
(1),
and
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273 K (ꢀH = 8.05 kJ molꢀ1), respectively, accompanying a
diiodomesitylene (4) (Figure 1), whose phase transition behaviour
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3
4
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and bulk physical properties are examined to investigate dynamic
molecular behaviour, intermolecular interactions, and dielectric
responses coupled with inꢀplane molecular rotators. In addition,
the intermolecular interactions between iodine atoms are
investigated in terms of the collective molecular rotation in the
dynamic rotator phase.
relatively large thermal hysteresis (>20 K in Figure 2c). This
thermal hysteresis in the heating and cooling processes was not
observed in the S2ꢀM1 phase transitions of 1 or 2, and it was
assumed that 3 showed firstꢀorder transition behaviour from
ordered solid (S3) to plastic crystalline phase (M1).
CH3
CH3
a
b
Br
Br
Br
Br
8
9
H3C
CH3
H3C
CH3
I
Br
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1
3
4
2
Figure 1. Molecular structures. Dibromoiodomesitylene (1),
tribromomesitylene (2), dibromomesitylene (3), and
diiodomesitylene (4). Lower figures were drawn using the CPK
c
model.
RESULTS AND DISCUSSION
The closeꢀpacking structures of 1, 2, 3, and 4 were found to
form van der Waals crystals, where πꢀπ and halogenꢀhalogen
interactions played an important role in constructing the crystal
lattice and in the thermal stability of each crystal. The melting
points of crystals 1, 2, 3, and 4 were 475, 497, 337, and 355 K,
respectively, while under ambient conditions, sublimation was
observed before crystal melting (Figure S2). The initial DSC
heating cycles for crystals 1 and 2 differed from the second
thermal cycles, while no thermal changes were detected in the
DSC cycle of crystal 3 (Figure 2). In the initial heating cycle,
crystals 1 and 2 displayed irreversible endothermic peaks at
approximately 380 and 360 K, with transition enthalpies (ꢀH) of
2.68 and 2.62 kJ molꢀ1, respectively. These transitions were
assigned to the S1ꢀM1 phase transition. Following the second
thermal cycle, the reversible phase transitions of crystal 1 were
observed at 358 K (heating) and 357 K (cooling), with a ꢀH of
~1.2 kJ molꢀ1 (Figure 2a). A trace of the endothermic peak was
observed at ~380 K even in the second heating cycle, which was
dependent on the scan rate of the DSC measurements (Figure S3).
The initial S1 phase of 1 was found to transform to the S2 phase
following phase transition to the M1 phase. The first cooling
cycle from the M1 phase resulted in the formation of the
crystalline S2 phase, which differed from the initial S1 phase. In
the second heating cycle, the S2ꢀM1 phase transition was
reversible, and was accompanied by a trace amount of S1ꢀM1
phase transition at ~380 K. Since the size of the small S1ꢀM1
phase transition peak in the second heating cycle at a scan rate of
1 K minꢀ1 was larger than that observed at 5 and 10 K minꢀ1, this
suggested that the S1 phase was a thermodynamically stable
crystal phase. The phase transition behaviours of all crystals were
reproducible following the second thermal cycle in DSC and
dielectric measurements.
Figure 2. Phase transition behaviour. DSC diagrams of crystals
a) 1, b) 2, and c) 3, with a scan rate of 5 K minꢀ1. Notation *
signifies the phase transition of a trace amount of M1 phase in the
second thermal cycle.
a
b
c
d
e
f
Similar phase transition behaviour was observed for crystal 2,
with the reversible S2ꢀM1 transition being observed at 298 K
(heating) and 288 K (cooling), respectively, with a ꢀH of
~0.7 kJ molꢀ1 (Figure 2b). The S1 phase was also detected in the
second DSC cycle as a small endothermic peak. The S2ꢀM1
transition temperature and ꢀH magnitude of 2 were, respectively,
~60 K and 0.5 kJ molꢀ1 lower than those of 1, suggesting weaker
intermolecular interactions in crystal 1 than that in crystal 2.
Figure 3. Polarised optical microscopic images of crystals 1 and 2.
Phase transition behaviour of crystal 1 between a) S1 (T = 370 K),
b) M1 (T = 400 K), and c) S2 (T = 300 K) phases. Phase
transition behaviour of crystal 2 between d) S1 (T = 300 K), e)
M1 (T = 380 K), and f) S2 (T = 300 K) phases.
In contrast, crystal 4 did not exhibit any phase transition from
the solid to the mesophase, displaying only a direct transformation
to the isotropic liquid state. The replacement of two bromine
atoms in 3 with iodine atoms (4), therefore, resulted in the
In crystal 3, the reversible S3ꢀM1 phase transition upon heating
and cooling was observed at 294 K (ꢀH = 7.74 kJ molꢀ1) and
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disappearance of the S3ꢀM1 phase transition, and an increase in
the melting point by ~20 K. This suggested that the
intermolecular interactions between I atoms in the crystals were
stronger than those between Br atoms. Another effective
intermolecular CꢀH•••I (Br) interactions were also played an
important role to generate the attractive force in solids.16b
The crystallographically independent C9H9Br2I unit was found to
1
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3
4
5
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have the Pꢀ1 centric space group in the S1 phase of crystal 1. The
previously reported crystal structure of 2 at 173 K20 was found to
be isostructural to that of 1 at 100 K. The orientational disorder
for two Br and one I atom was observed as an average molecular
structure (Figures 4a, b). The typical CꢀBr and CꢀI bondꢀlengths
inath)e aromatic compounds have previously been reported as 1.85
Figure 3 shows the polarised optical microscopic (POM)
and 2.05 Å, respectively.21 In addition, the bond lengths of Br1ꢀ
C3, Br2ꢀC5, and Br3ꢀC1 in 1 were found to be 1.980(6), 1.988
(9), and 2.017(7) Å, respectively (Figure 4a), which are consistent
with the average bond lengths of two CꢀBr and one CꢀI bond.
Although all molecular motions should be frozen at 100 K, the
dynamic disordered state for the orientation of Br and I atoms was
expected in the highꢀtemperature M1 phase. The temperatureꢀ
dependent powder Xꢀray diffraction (PXRD), differential
scanning calorimetry (DSC), and dielectric measurements of 1
strongly supported the dynamic orientational disorder of Br and I
atoms in the M1 phase.
images of crystals 1 and
2 with a crossꢀNichol optical
arrangement. The green/blue mosaic domains of 1 in the S1
phase (T = 370 K) were not present in the M1 phase (400 K),
while a uniform pink domain was observed in the S2 phase when
the temperature was decreased to 300 K (Figures 3a, b, and c).
Similar phase transition behaviour was also observed in the POM
images of 2. The blue/violet mosaic domains in the S1 phase (T =
300 K) changed to uniform violet domains in the M1 phase (T =
380 K), and uniform pink/violet domains in the S2 phase (T =
300 K), as shown in Figures 3d, e, and f. The POM images of 3
also displayed changes in the domain shape and colour around the
S3ꢀM1 phase transition (Supporting Figure S4).
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Figure 4c shows the unit cell of crystal 1 viewed along the a
axis, where iodine was ideally assigned at the position of the Br3
atom to clarify the molecular orientation. The length of the b axis
(9.2673(3) Å) was 0.02 Å shorter than that of the c axis
(9.2879(3) Å), suggesting broken symmetry and anisotropic
molecular orientation at 100 K. Nonꢀuniform πꢀstacking structure
was observed along the a axis of 1 (Figure 4d), where the two
types of interplanar distances, d1 = 3.64 and d2 = 3.66 Å, were
defined by the mean interplanar distances between the two
benzene planes. The nonꢀuniform πꢀstacking structure of 1 was
associated with a significant H•••H interaction between ꢀCH3
groups, which affected the dynamic behaviour of individual
molecules and the magnitude of rotational barriers as the
repulsive interaction.16b Since the Xꢀray diffuse scattering of 1
from the superstructure of the heavy iodine arrangement was not
observed in the S1 phase, the disordered iodine atoms were
distributed three positions in the lattice.
a
b
Br
I
I
Br
Br
Br
Br
I
Br
c
d
a
c
d1
d2
The isostructural crystal structure of 2 at 173 K showed a ~5%
contraction of the unit cell volume compared to that of 1 due to
the replacement of an
I atom with a smaller Br atom.
b
o
Orientational disorder of Br atoms was not observed in crystal 2.
It was difficult to obtain highꢀquality diffraction data in the
singleꢀcrystal Xꢀray analysis of 3 due to a phase transition at room
temperature. However, the PXRD patterns of 3 at 263 K were
consistent with that of the S3 phase of 4 (Figures S7 and S8),
indicating a similar herringꢀbone packing structure. In addition,
the S3ꢀM1 phase transition of 3 at 294 K upon heating was
suppressed in 4. The PXRD pattern of S2 phase in crystal 1 was
depended on the annealing procedures due to the coexistence of
S1 phase. Since similar intermolecular interactions and packing
structures were expected in these two crystals, we then chose to
examine the crystal structure of 4. Figures 4e and 4f show the
unit cell of crystal 4 viewed along the b and c axes, respectively.
e
f
a
a
c
o
b
o
Figure 4. Crystal structures of the S1 phase of 1 and 4 (T =
100 K). a) Average molecular structure for orientationally
disordered Br and I atoms. b) Three possible molecular
orientations were possible with every 120° rotation of the benzene
ring along the direction normal to the πꢀplane. c) Unit cell of
crystal 1 viewed along the a axis. The I atom was ideally
assigned at the position of Br3 atom. d) The πꢀstacking structure
of molecule 1 along the a axis. Two types of interplanar
interactions were observed. e) Unit cell of 4 viewed along the b
axis. The I•••I interaction (represented by dashed lines) was
elongated along the c axis. f) Herringꢀbone arrangement of 4
within the ab plane, where molecule 4 was oriented in the
ferroelectric arrangement. Hydrogen atoms are omitted for
clarity.
A
single C9H10I2 molecule was the crystallographically
independent structural unit, where two CꢀI bonds of 2.116(6) and
2.109(7) Å were observed in the orientationally ordered state at
100 K. The herringꢀbone packing structure in 4 was observed in
the ab plane, as opposed to the πꢀstacking structure of 1 and 2.
The ferroelectric molecular orientation of 4 in the ab plane was
alternately arranged along the c axis to cancel the total dipole
moment with the antiferroelectric arrangement (Figure S6). In
addition, the effective I•••I interaction (3.945(2) Å) was uniformly
elongated along the c axis, which enhanced the intermolecular
interactions and thermal stability of 4, thus resulting in the
absence of the S3ꢀM1 phase transition. Since the magnitude of
the Br•••Br interaction was lower than that of I•••I,22 the inꢀplane
molecular rotation of 3 could be thermally activated at relatively
low temperatures (~280 K). Furthermore, the motional freedom
of 3 differed from those in πꢀstacking molecules 1 and 2, which
Structural analysis by singleꢀcrystal Xꢀray diffraction of the S1
phase of crystal 1 was only successful upon cooling of the crystals
(Figures 4a, b, c, and d), and the thermal cycle of 1 through the
M1 phase was particularly damaging to the crystalline S2 phase.
3
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was consistent with the difference in the phase transition
single crystal 1, respectively. The second thermal cycles were
1
2
3
4
5
6
7
8
behaviour observed in the DSC traces.
plotted as dashed lines in the absence of ε1 anomaly and thermal
hysterics. b) The phase transition of compressed pellet 3 from the
lowꢀtemperature antiferroelectric phase to the highꢀtemperature
paraelectric phase. c) Phase transition from the antiferroelectric
static ordered S3 phase to the paraelectric dynamic disordered M1
phase, accompanied by thermal hysteresis. d) Static disordered πꢀ
stacking columnar structure of polar molecule 1, where the
orientation of iodine atoms was disordered, generating the macro
dipole moment and dipole fluctuation. Phase transition from the
static disordered S1 phase to the dynamic collective rotator phase
M1 occurred at 380 K, and the static ordered S2 phase was
formed from the M1 phase upon cooling.
Based on DFT calculations, the inꢀplane dipole moments of 1,
2, 3, and 4 were found to be 0.13, 0, 1.28, and 1.44 Debye,
respectively. The dielectric constant of ε = ε1 + iε2 was sensitive
to the dipole change in solids, where the ε1 and ε2 were
corresponded to the real and imaginary parts. The former ε1
drastically changed at dielectric phase transitions such as
ferroelectric and/or antiferroelectric–paraelectric ones. The
motional freedom of apolar molecule 2 could not be detected in
the dielectric spectra as no change in dipole moment took place
(Figure S8). Figures 5a and b show the temperature and
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frequencyꢀdependent real part dielectric constant (ε1) of single
crystal 1 and the compressed pellet 3. The πꢀstacking direction of
crystal 1 along the a axis was silent for the dielectric measurement,
whereas the ε1 response along the bc plane normal to the πꢀ
stacking axis showed a clear ε1 response. This is consistent with
the inꢀplane molecular rotation of polar molecule 1 within the πꢀ
stacking column. The S1ꢀM1 phase transition of 1 appeared as a
sharp drop in the ε1 measurement at 380 K along the bc plane
upon heating, whereas the dielectric ε1 jump was observed at
360 K in the cooling cycle at the M1ꢀS2 phase transition. It
should be noted that the ε1 anomaly was not observed in the
second thermal cycle (300ꢀ380 K), although the reversible S2ꢀM1
phase transition was clearly observed at 360 K by DSC
measurements. The inꢀplane molecular rotation of 1 in the πꢀ
stacking column exhibited a change in dipole moment at the S1ꢀ
M1 phase transition but not at the S2ꢀM1 transition.
The temperature and frequencyꢀdependent ε1 of 3 was found to
be in accordance with the DSC measurements. A rapid ε1 jump
from 3.0 to 4.3 occurred at 295 K upon heating, while a sudden
drop was observed at 278 K upon cooling. In addition, the
frequency independent reversible dielectric phase transition
corresponded to dielectric ordering at the S3ꢀM1 phase transition
temperature, where the lowꢀtemperature antiferroelectric phase
transformed to the highꢀtemperature paraelectric one.
Furthermore, the thermally activated inꢀplane molecular rotation
of 3 in the M1 phase resulted in the highꢀtemperature dynamic
disordered paraelectric phase being observed in the herringꢀbone
packing structure. The inꢀplane molecular rotation was found to
be suppressed by Br•••Br interactions in the S3 phase, forming an
antiparallel molecular arrangement to cancel the net dipole
moment in the crystal. It was therefore clear that the dielectric
responses of 1 and 3 were not comparable.
The dielectric phase transition mechanism of 1 and 3 was
discussed in terms of the rotational freedom and packing
a
b
structures.
temperatureꢀdependent
The phase transition of 3 showed
a
typical
the
dielectric response from
antiferroelectric to paraelectric phases (Figure 5c). The larger
dipole moment of 3 compared to that of 1 effectively stabilised
the antiꢀparallel molecular arrangement and cancelled the net
a
b
// bc
dipole moment of the crystal.
The ferroelectric dipole
arrangement of polar 3 was observed in the ab plane, which
interacted with the antiꢀparallel dipole arrangement along the
interlayer c axis. An increase in temperature up to 294 K
generated the inꢀplane molecular rotation and formed the
paraelectric phase. The independent inꢀplane molecular rotations
of 3 contributed to the dielectric phase transition in the absence of
πꢀstacking interactions, with Br•••Br interactions playing an
important role in revealing the antiferroelectricꢀparaelectric phase
transition.
// a
The temperatureꢀdependent dielectric response of 1 exhibited
rather complex behaviour. An increase in temperature from the
S1 phase resulted in a sudden drop in ε1 at the S1ꢀM1 phase
transition temperature, which was not consistent with typical
c
294 K
273 K
b
antiferroelectricꢀparaelectric
phase
transition
behaviour.
ꢁ
Furthermore, no 1 responses were observed following the second
ε
1.28 D
c
thermal cycles despite 1 exhibiting a permanent dipole moment.
Since the dipoleꢀdipole interaction between the molecules of 1
was smaller than that of molecule 3, both the πꢀstacking
interactions and steric hindrance within the πꢀstack played an
important role in determining the dielectric phase transition
behaviour. The sudden drop in ε1 at the S1ꢀM1 phase transition
was associated with the existence of intrinsic static disorder of I
atoms in the S1 πꢀstack, where the orientationally disordered I
atom enhanced the polarisation magnitude and ε1 values (Figure
5d). The inꢀplane molecular rotation of 1 in the M1 phase relaxed
the intrinsic orientationally disorder of I atoms, and resulted in a
o
3
1
M1: Paraelectric
S3: Antiferroelectric
d
360 K
380 K
*
ꢁ
0.13 D
S2
S1: Disorder M1: Collective rotator
sudden drop in ε1 at 380 K. Cooling from the M1 to the S2 phase
Figure 5. Temperatureꢀ and frequencyꢀdependent real part
dielectric constants ( 1) and phase transition mechanism. a) Two
fixed the orientation of the I atoms of 1 in the πꢀstack, resulting in
the disappearance of the net dipole moment of one πꢀstacking
column. The dielectric anomaly at the S2ꢀM1 phase transition
ε
types of measurement axes along the a axis and the bc plane
corresponded to the parallel and normal to the πꢀstacking axis of
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was found to disappear in the absence of the dipole moment
the fullꢀmatrix leastꢀsquares method on F2. The crystal structure
1
2
3
4
5
6
7
8
change, due to the collective inꢀplane rotation of 1 in the πꢀstack,
resulting in constant ε1 values at this phase transition. A helical
arrangement of 1 was possible, with 31 symmetry along the πꢀ
stack, where I•••I interactions maintained each molecular
orientation. The I•••I distance of ~4.2 Å for the 31ꢀhelix
arrangement of 1 in the πꢀstack resulted in a collective inꢀplane
molecular rotation in the πꢀstacking column. The weaker Br•••Br
interaction is not sufficient to achieve a collective inꢀplane
molecular rotation of 2 in the πꢀstacking column. Therefore, the
dynamic M1 phase of 1 was considered the collective ordered πꢀ
stacking rotator phase, where the molecular orientation was
collectively activated in a single ropeꢀlike long range assembly.
data for 1 and 4 can be accessed from the Cambridge
Crystallographic Data Centre (CCDC, www.ccdc.cam.ac.uk) and
have been allocated accession numbers CCDC 1406424 and
1406425.
Calculations. The dipole moments of molecules 1, 2, 3, and 4
were obtained by density functional theory (DFT) calculations
using the B3LYP 6ꢀ31G (d, p) basis set in Gaussian 09W.24 The
potential energy curves for the inꢀplane rotations of molecules 1,
2, and, 4 were obtained in the atomic coordinates based on the
single crystal Xꢀray structural analyses. The calculations were
performed for the πꢀtrimer units of (1)3 and (2)3, whereas the
rotation of 4 was examined on the partial herringꢀbond
arrangement structure of the (4)3 unit. The singleꢀpoint energy
was obtained at 30° rotation intervals from the central molecules
based on an RHF / 6ꢀ31(d, p) basis set.
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CONCLUSIONS
The phase transition from the static solid phase to the thermally
activated dynamic inꢀplane rotator phase was activated by
molecular rotation along the direction normal to the crystal πꢀ
plane. The isostructural crystal structures of 1 and 2 indicated the
formation of a πꢀstacking columnar structure, whereas a herringꢀ
bone molecular arrangement was observed for crystal 4. Effective
I•••I interactions in 4 suppressed the phase transition to the inꢀ
plane rotator phase, whereas the weak Br•••Br interactions in 3
resulted in the formation of the rotator phase before crystal
melting occurred. No dielectric response was observed in apolar
molecule 2 due to inꢀplane rotations, whereas those of asymmetric
polar molecules 1 and 3 were responsible for their temperature
ASSOCIATED CONTENT
Supporting Information.
IR spectra at KBr pellet, TG charts, POM images of 3 and 4, DSC
charts of 1 at different scan rate, crystal structure of 4, dielectric
properties of 1, 2, and 4 on compressed pellet, PXRD pattern of 4,
potential energy calculation for 1 with different orientations.
These materials are available free of charge via the Internet at
http://pubs.acs.org.
and frequencyꢀdependent dielectric spectra.
Typical
AUTHOR INFORMATION
Corresponding Author
antiferroelectricꢀparaelectric phase transition behaviour was
observed in the temperatureꢀdependent ε1 of 3, whereas the
temperatureꢀdependent ε1 of 1 could be accounted for by phase
transition from the static disordered phase to the dynamic ordered
collective rotator phase. In this case, the inꢀplane molecular
rotation was collectively activated in the πꢀstacking columnar
structure, and macro dipole moments were cancelled out. Dipole
relaxation from the static disordered lowꢀtemperature phase to the
dynamic ordered collective inꢀplane rotator phase resulted in a
dielectric anomaly in 1 during the first heating cycle, with the
collective inꢀplane molecular rotation of 1 occurring in a single
ropeꢀlike πꢀstacking columnar assembly. No correlation was
observed between the molecular rotator columns, and the
realisation of collective inꢀplane rotation for all columns in the
bulk has the potential to achieve unidirectional molecular motion
for constructing molecular motors in the future.
akuta@tagen.tohoku.ac.jp
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENT
This work was supported by a GrantꢀinꢀAid for Science Research
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan, and by a grant from the Management
Expenses Grants for National Universities of Japan.
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Graphical abstract
CH3
Br
Br
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H3C
CH3
I
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Orpen, A. J. J. Chem. Soc. Perkin Trans. 2, 1987, S1ꢀS19.
(22) Pedireddi, V. R.; Reddy, D. S.; Goud, B. S.; Craig, D. C.;
Rae, A. D.; Desiraju, G. R. J. Chem. Soc., Perkin Trans. 2, 1994,
2353ꢀ2360.
(23) Kajigaeshi, S.; Kakinami, T.; Moriwaki, M.; Tanaka, T.;
Fujisaki, S.; Okamotoet, T. Bull. Chem. Soc. Jap. 1989, 62, 439ꢀ443.
(24) M. J. Frisch, et al. Gaussian 09, Revision C 01, Gaussian,
Inc., Wallingford CT (2009).
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