crystals
Article
Reversible Single-Crystal-to-Single-Crystal Phase
Transition of Chiral Salicylidenephenylethylamine
Akifumi Takanabe 1 , Takuro Katsufuji 1 , Kohei Johmoto 2 , Hidehiro Uekusa 2 , Motoo Shiro 3 ,
Hideko Koshima 3, * and Toru Asahi 1,3
1
2
3
*
Department of Advanced Science and Engineering, Graduate School of Advanced Science and Engineering,
Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan; [email protected] (A.T.);
[email protected] (T.K.); [email protected] (T.A.)
Department of Chemistry and Materials Science, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan;
[email protected] (K.J.); [email protected] (H.U.)
Researh Organization for Nano & Life Innovation, Waseda University, 513 Wasedatsurumaki-cho,
Shinjuku-ku, Tokyo 162-0041, Japan; [email protected]
Correspondence: [email protected]; Tel.: +81-3-5286-8307; Fax.: +81-3-5369-7327
Academic Editor: Sławomir J. Grabowski
Received: 3 December 2016; Accepted: 26 December 2016; Published: 29 December 2016
Abstract: The chiral crystal of enantiomeric (S)-N-3,5-di-tert-butylsalicylidene-1-phenylethylamine
in the enol form [enol-(S)-1] undergoes a reversible single-crystal-to-single-crystal (SCSC) phase
transition at Tc ≈ 3 ◦ C from the room temperature α-form in orthorhombic space group P21 21 21
(Z0 = 1) to the low temperature β-form in the monoclinic space group P21 (Z0 = 2) with a thermal
hysteresis of approximately 1.7 ◦ C. A detailed comparison of the crystal structures of the α- and
β-forms revealed that the 5-tert-butyl group of one molecule in the asymmetric unit of the β-form
rotated by ca. 60◦ , and the dihedral angle between the phenyl and salicyl planes increased slightly in
the β-form crystal. However, the changes in the molecular conformation and packing arrangement
are small, which leads to the reversible SCSC phase transition with no destruction of the crystal
lattice. The dielectric constant along the b-axis was small, probably due to the weak intermolecular
interactions in the crystals.
Keywords:
reversible single-crystal-to-single-crystal
(S)-salicylidenephenylethylamine; dielectric properties
phase
transition;
enantiomeric
1. Introduction
A single-crystal-to-single-crystal (SCSC) phase transition is defined as a micro-mutual transformation
in crystal structure with no destruction of the crystal lattice. A reversible SCSC phase transition can
occur when the changes in the molecular conformation and packing arrangement in the crystal are
very small at the phase transition, so there are no cracks or breaks in the crystal. Hence, the reversible
SCSC phase transition is a fascinating phenomenon because of its potential applications in ferroelectric
and switchable dielectric devices [1–3]. Several molecular crystals that undergo temperature-induced
reversible SCSC phase transition have been reported [4–10].
Chiral (S)-N-3,5-di-tert-butylsalicylidene-1-phenylethylamine in the enol-form [enol-(S)-1] has
a photochromic nature in the crystalline state caused by photoinduced proton transfer [11].
The plate-like enol-(S)-1 crystals bend reversibly upon ultraviolet (UV) irradiation [12]. Recently,
we reported the chiroptical and optical anisotropic properties of photomechanical enol-(S)-1 crystals
before and under UV irradiation [13]. In the course of this study, we discovered that the chiral
enol-(S)-1 (Scheme 1) crystal underwent a reversible SCSC phase transition from the room temperature
Crystals 2017, 7, 7; doi:10.3390/cryst7010007
www.mdpi.com/journal/crystals
Crystals 2017, 7, 7
Crystals 2017, 7, 7
Crystals 2017, 7, 7
2 of 10
2 of 10
2 of 10
α-form
to the because
low-temperature
β form
at approximately
3 ◦ C. This reversible
phase
transition
occurrence
it occurs
occurs
between
two enantiomorphic
enantiomorphic
phases,SCSC
which
also
mean
occurrence
because it
between
two
phases,
which
also
mean
is
a
rare
occurrence
because
it
occurs
between
two
enantiomorphic
phases,
which
also
mean
noncentrosymmetric phases.
phases. The
The properties
properties such
such as
as ferroelectricity,
ferroelectricity, piezoelectricity,
piezoelectricity, and
and
noncentrosymmetric
noncentrosymmetric
phases.
The properties
such as ferroelectricity,
piezoelectricity, and
second-order
second-order
optical
nonlinearlity
are
allowed
in
a
noncentrosymmetric
structure.
Other
second-order optical nonlinearlity are allowed in a noncentrosymmetric structure. Other
optical
nonlinearlity are
allowed
in a noncentrosymmetric
Other
noncentrosymmetric
noncentrosymmetric
SCSC
transitions
have been
been previously
previouslystructure.
reported [14].
[14]. We
We
discuss aa possible
possible
noncentrosymmetric
SCSC
transitions
have
reported
discuss
SCSC
transitions
have
been
previously
reported
[14].
We
discuss
a
possible
mechanism
for the
mechanism for
for the
the enantiomorphic
enantiomorphic SCSC
SCSC phase
phase transition
transition based
based on
on the
the crystal
crystal structure
structure changes
changes
mechanism
enantiomorphic
SCSC
phase transition
based on
the crystal of
structure
changes
between
and
between the
the αα- and
and
β-forms.
The temperature
temperature
dependence
the dielectric
dielectric
constant
in the
theαsingle
between
β-forms.
The
dependence
of the
constant
in
the
single
β-forms.
The
temperature
dependence
of
the
dielectric
constant
in
the
single
crystalline
state
was
crystalline state
state was
was also
also measured.
measured.
crystalline
also measured.
Scheme 1.
1. Enantiomeric
Enantiomeric (S)-salicylidenephenylethylamine
(S)-salicylidenephenylethylamine [enol-(S)-1].
[enol-(S)-1].
Scheme
Enantiomeric
(S)-salicylidenephenylethylamine
Scheme
1.
[enol-(S)-1].
2. Results
Results and
and Discussion
Discussion
2.
Results
and
Discussion
2.
The compound
compound enol-(S)-1
enol-(S)-1 was
was synthesized
synthesized as
as reported
reported previously
previously [15].
[15]. Single
Single crystals
crystals were
were
The
The
compound
enol-(S)-1
was
synthesized
as
reported
previously
[15].
Single
crystals
were
obtained
by
slow
evaporation
of
a
solution
in
2-propanol
at
room
temperature.
Differential
scanning
obtained
2-propanol at
Differential scanning
obtained by
by slow
slow evaporation
evaporation of
of aa solution
solution in
in 2-propanol
at room
room temperature.
temperature. Differential
scanning
calorimetry
(DSC)
of
enol-(S)-1
was
performed
over
the
temperature
range
from
10
to
−10
°C◦ Cat
atataa
calorimetry
(DSC)
of
enol-(S)-1
was
performed
over
the
temperature
range
from
10
to
−
calorimetry (DSC) of enol-(S)-1 was performed over the temperature range from 10 to −1010°C
−1
◦
−
1
rate
of
2
°C·min
with
cooling
initially
and
then
heating
(Figure
1).
Cooling
the
crystalline
sample
of
−1 with
arate
rateofof2 2°C·min
C·min
with
cooling
initially
then
heating
(Figure
Cooling
crystalline
sample
cooling
initially
andand
then
heating
(Figure
1). 1).
Cooling
thethe
crystalline
sample
of
◦
◦
enol-(S)-1
had
an
exothermic
peak
at
2.2
°C.
The
phase
transition
started
at
2.7
°C
and
ended
at
of
enol-(S)-1
had
exothermic
peak
2.2°C.C.The
Thephase
phasetransition
transitionstarted
startedat
at 2.7
2.7 °C
C and
and ended
ended at
enol-(S)-1
had
anan
exothermic
peak
atat2.2
at
0.1 ◦°C.
°C.
Heating the
the crystalline
crystalline sample
sample of
of enol-(S)-1
enol-(S)-1 had
had an
an endothermic
endothermic peak
peak at
at 3.9
3.9 ◦°C.
°C.
The reverse
reverse
0.1
C. Heating
of
C. The
0.1
Heating
the
crystalline
sample
enol-(S)-1
had
an
endothermic
peak
at
3.9
The
reverse
◦
◦
phase
transition
began
at
3.6
°C
and
ended
at
4.8
°C.
The
sharp
shapes
of
these
two
peaks
and
the
phase
C and
C. The
phase transition
transition began
began at
at 3.6
3.6 °C
and ended
ended at
at 4.8
4.8 °C.
The sharp
sharp shapes
shapes of
of these
these two
two peaks
peaks and
and the
the
◦
thermal hysteresis
hysteresis of
of 1.7
1.7 °C
°C
reveal aaafirst-order
first-order phase
phase transition.
transition. The
The enthalpy
enthalpy at
at the
the exothermic
exothermic and
and
thermal
C reveal
thermal
hysteresis
of
1.7
reveal
first-order
phase
transition.
The
enthalpy
at
the
exothermic
and
−1, −
1
endothermic
peaks
was
−0.69
and
0.67
kJ·mol
respectively.
This
small
thermal
hysteresis
might
be
−1
endothermic
peakswas
was−0.69
−0.69
and
0.67
kJ·mol, respectively.
, respectively.
small
thermal
hysteresis
might
endothermic peaks
and
0.67
kJ·mol
ThisThis
small
thermal
hysteresis
might
be aa
general
feature
of
SCSC
transition
because
small
hysteresis
was
reported
also
in
the
previous
papers
be
a general
feature
of SCSC
transition
because
hysteresis
was reported
in the previous
general
feature
of SCSC
transition
because
small small
hysteresis
was reported
also in also
the previous
papers
[4,7,10].
DSC
curves
in
wide
temperature
ranges
indicated
no
existence
of
additional
phase
transition
papers
[4,7,10].
DSC
curves
in
wide
temperature
ranges
indicated
no
existence
of
additional
phase
[4,7,10]. DSC curves in wide temperature ranges indicated no existence of additional phase transition
(Figure
S1).
transition
(Figure S1).(Figure S1).
Figure 1. DSC curves of enol-(S)-1 crystal on initial
initial cooling
cooling and
and then
then heating.
heating.
Figure 1. DSC curves of enol-(S)-1 crystal on initial
cooling
and
then
heating.
shows the
the variable-temperature
variable-temperature unit
cell determination
determination of
enol-(S)-1 using
single
Figure
Figure 222 shows
shows
the
variable-temperature
unit cell
cell
determination
of enol-(S)-1
enol-(S)-1
using aa single
single
◦
◦
crystal
over
the
temperature
range
from
−50
to
+50
°C
at
10
°C
intervals
with
initial
heating
and
then
crystal
50 to +50 °C
C at 10 °C
C intervals
intervals with initial heating and then
crystal over
over the
the temperature
temperature range
rangefrom
from−
−50
then
cooling. The
The length
length of
of the
the a-axis
a-axis changed
changed discontinuously
discontinuously between
between 00 and
and 10
10 °C
°C on
on heating
heating and
and
cooling.
cooling;
the
differences
were
0.06
and
0.07
Å,
respectively
(Figure
2a).
Angle
β
also
changed
cooling; the differences were 0.06 and 0.07 Å, respectively (Figure 2a). Angle β also changed
Crystals 2017, 7, 7
3 of 10
cooling.
The 7,length
of the a-axis changed discontinuously between 0 and 10 ◦ C on heating and cooling;
Crystals 2017,
7
3 of 10
the differences were 0.06 and 0.07 Å, respectively (Figure 2a). Angle β also changed reversibly from
◦ to 92◦ between
from 90°
°C and and
10 °Ccooling
on heating
and2d).
cooling
(Figureof2d).
length
of
90reversibly
0 ◦toC 92°
andbetween
10 ◦ C on0heating
(Figure
The length
the The
c-axis
and the
◦
◦
the c-axis
the volume
did notbetween
change 0significantly
0 °C and
10 °C (Figure
2c,e).
In
volume
did and
not change
significantly
C and 10 Cbetween
(Figure 2c,e).
In contrast,
the length
of the
contrast,
the length
of the b-axis
changed
continuously
both heating
cooling
(Figure
2b). The
b-axis
changed
continuously
on both
heating
and coolingon
(Figure
2b). Theand
single
crystal
maintained
its
single
crystal
maintained
its
transparency
without
cracking
after
the
heating
and
cooling
cycles,
transparency without cracking after the heating and cooling cycles, revealing that the phase transition
revealing reversibly
that the phase
proceeded
reversibly via a single crystalline state.
proceeded
via atransition
single crystalline
state.
Figure
Temperature
dependence
of the
constants
of enol-(S)-1
single single
crystalcrystal
on initial
Figure2. 2.
Temperature
dependence
of lattice
the lattice
constants
of enol-(S)-1
onheating
initial
and
then and
cooling;
the (a) of
a-,the
(b)(a)
b-, a-,
and
angle(d)
β, angle
and (e)
heating
thenlengths
cooling;oflengths
(b)(c)b-,c-axes,
and (c)(d)
c-axes,
β, volume.
and (e) volume.
◦ C (Table
X-raycrystallographic
crystallographic analysis of
at at
10 10
°C ◦and
−50 −
°C50(Table
1). The
X-ray
of enol-(S)-1
enol-(S)-1was
wasperformed
performed
C and
1).
◦
12122
1, which
is
similar
to
the
finding
in
α-form
(10(10
°C) C)
crystallizes
in the
orthorhombic
space
group
P2P2
The
α-form
crystallizes
in the
orthorhombic
space
group
2
,
which
is
similar
to
the
finding
1 1 1
◦ C)belongs
inaaprevious
previousreport
report[12].
[12].The
Theβ-form
β-form(−50
(−50°C)
belongstotothe
themonoclinic
monoclinicspace
spacegroup
groupP2
P21.1 .After
Afterthe
the
phase
transition
from
the
α-form
to
the
β-form,
the
lengths
of
the
aand
b-axes
decreased
by
−0.09927
phase transition from the α-form to the β-form, the lengths of the a- and b-axes decreased by −0.09927 Å
Å1.57%)
(−1.57%)
−0.2352
respectively, while
while that
that of
of the
the c-axis
c-axis increased
increased by
(−
andand
−0.2352
ÅÅ
(−(−0.67%),
0.67%), respectively,
by +0.04083
+0.04083 ÅÅ
3 (−1.94%). The powder X-ray diffraction (XRD)
3
(+0.41%).
Overall,
the
volume
decreased
by
−42.51
Å
(+0.41%). Overall, the volume decreased by −42.51 Å (−1.94%). The powder X-ray diffraction (XRD)
patternatatroom
roomtemperature
temperaturewas
wascoincident
coincidentwith
with the
the calculated
calculated XRD
XRD pattern
pattern from
pattern
from the
the single
single crystal
crystal
structure
of
the
α-form,
revealing
no
existence
of
additional
phase
transition
(Figure
S2).
structure of the α-form, revealing no existence of additional phase transition (Figure S2).
Table1.1.Crystal
Crystaldata
datafor
forenol-(S)-1
enol-(S)-1 crystals.
crystals.
Table
Temperature (°C)
Temperature
(◦ C)
Crystal
system
Crystal system
Space
group
Space group
aa(Å)
(Å)
(Å)
bb(Å)
c
(Å)
c (Å)
β (deg)
β (deg) 3
Volume (Å )
3
Volume
Z (Å )
−
Z·cm 3 )
ρcalc (g
−3)
R
[I
> 2σ(I)]
ρcalc1 (g·cm
Goodness of fit
R1 [I > 2σ(I)]
Goodness of fit
α-Form
10
10
Orthorhombic
Orthorhombic
P212121
P21 21 21
6.3058
6.3058 (4)(4)
35.071
35.071(2)(2)
9.8851
9.8851(6)(6)
90
90
2186.1 (2)
2186.1
4 (2)
4
1.025
0.0681
1.025
1.052
0.0681
1.052
α-Form
β-Form
−50
−
50
Monoclinic
Monoclinic
P21
P21
6.20653
6.20653 (11)(11)
34.8358
(7) (7)
34.8358
9.92593
(18)
9.92593 (18)
92.7570 (10)
92.7570 (10)
2143.59 (7)
2143.59
(7)
4
1.0464
0.0475
1.046
1.030
0.0475
1.030
β-Form
Crystals 2017, 7, 7
Crystals 2017, 7, 7
4 of 10
4 of 10
Figure 33 shows
shows the
the Oak
Oak Ridge
Ridge Thermal
Thermal Ellipsoid
Ellipsoid Plot
Plot (ORTEP)
(ORTEP) drawings
drawings and
and molecular
molecular
Figure
conformations
of
the
αand
β-form
crystals.
There
is
one
crystallographically
independent
molecule
conformations of the α- and β-form crystals. There is one crystallographically independent molecule
(A) in
thethe
5-tert-butyl
group
(brown
color)
is disordered
in thein0.217
(A)
in the
the α-form
α-formcrystal
crystal(Figure
(Figure3a);
3a);
5-tert-butyl
group
(brown
color)
is disordered
the
occupancy.
The
β-form
crystal
consists
of
two
crystallographically
independent
molecules
(B and C)
0.217 occupancy. The β-form crystal consists of two crystallographically independent molecules
in and
the C)
asymmetric
unit (Figure
3b,c). The
5-tert-butyl
groupsgroups
(brown
color)color)
of both
molecules
are
(B
in the asymmetric
unit (Figure
3b,c).
The 5-tert-butyl
(brown
of both
molecules
disordered
in the
0.229
(B) (B)
andand
0.245
(C) (C)
occupancies,
respectively.
are
disordered
in the
0.229
0.245
occupancies,
respectively.
Figure
conformations (d–f)
(d–f)
Figure 3.
3. ORTEP
ORTEP drawings
drawings (a–c)
(a–c) at
at the
the 25%
25% probability
probability level,
level, and
and the
the molecular
molecular conformations
of
enol-(S)-1
in
the
αand
β-form
crystals:
(a,d)
A
molecule
of
α-form,
(b,e)
B,
and
(c,f)
C
molecules
of enol-(S)-1 in the α- and β-form crystals: (a,d) A molecule of α-form, (b,e) B, and (c,f) C molecules of
of
β-form.
5-tert-butyl
groups
(brown)
disordered
in the
0.217
0.229
0.245
thethe
β-form.
TheThe
5-tert-butyl
groups
(brown)
are are
disordered
in the
0.217
(A),(A),
0.229
(B),(B),
andand
0.245
(C)
(C)
occupancies,
respectively.
viewpoints
of molecular
conformations
of the
5-tert-butyl
groups
occupancies,
respectively.
TheThe
viewpoints
of molecular
conformations
of the
5-tert-butyl
groups
are
are
displayed
in
the
ORTEP
drawings
with
black
solid
arrows.
The
disordered
5-tert-butyl
groups
displayed in the ORTEP drawings with black solid arrows. The disordered 5-tert-butyl groups in
in
minor
minor occupancies
occupancies and
and all
all hydrogen
hydrogen atoms
atoms in
in (d–f)
(d–f) are
are omitted
omitted for
for clarity.
clarity.
In molecule A of the α-form,
α-form, an
an intramolecular
intramolecular hydrogen bond is formed between the N atom
of the C=N Schiff base and the H atom of the 2-OH group of the salicyl ring (black dotted line in
◦ . In molecules
Figure 3a);
3a); the
theN—O
N---Odistance
distanceisis2.591
2.591ÅÅand
and
N---H–O
angle
is 160.40°.
In molecules
B and
of
Figure
thethe
N—H–O
angle
is 160.40
B and
C ofCthe
the
β-form,
similar
intramolecular
hydrogen
bonds
(black
dotted
lines
in
Figure
3b,c)
are
formed
β-form, similar intramolecular hydrogen bonds (black dotted lines in Figure 3b,c) are formed with
◦ and 148.94
◦ , respectively.
with N---O
distances
of 2.596
and 2.591
and N---H–O
of 148.67°
and 148.94°,
respectively.
N—O
distances
of 2.596
and 2.591
Å andÅN—H–O
anglesangles
of 148.67
The dihedral angle between the phenyl and salicyl
salicyl plane
plane in
in molecule
molecule A
A of
of the
the α-form
α-form is 41.06°.
41.06◦ .
The corresponding dihedral angles increase
increase slightly
slightly to
to 46.82
46.82°◦ and 43.97
43.97°◦ in the B and C molecules of
β-form,respectively.
respectively.
The
analogous
salicylideneaniline
crystals
can acause
a photochromic
the β-form,
The
analogous
salicylideneaniline
crystals
can cause
photochromic
reaction
◦
reaction
when
the
dihedral
angle
between
the
phenyl
and
salicyl
planes
exceeds
30°
[16].
Therefore,
when the dihedral angle between the phenyl and salicyl planes exceeds 30 [16]. Therefore, the β-form
the β-form
crystal
should
also have anature
photochromic
nature to the
crystal
should
also have
a photochromic
to the trans-keto-form
withtrans-keto-form
UV irradiation. with UV
irradiation.
The most significant change in the molecular conformation with the phase transition is that
The most significant
change
molecular
conformation
with
the phase
transition
is that 60
the◦
the 5-tert-butyl
group (red
color)inofthe
molecule
C (Figure
3f) in the
β-crystal
rotates
by around
5-tert-butyl
groupposition
(red color)
of molecule
C (Figure
3f) in3d)
thein
β-crystal
rotateswhereas
by around
from the
from
the original
(green)
of molecule
A (Figure
the α-crystal,
the60°
5-tert-butyl
original(green)
position
of Bmolecule
A (Figure
3d)rotate.
in the In
α-crystal,
whereas
5-tert-butyl group
group
of (green)
molecule
(Figure 3e)
does not
comparison,
the the
conformations
of the
(green)
of
molecule
B
(Figure
3e)
does
not
rotate.
In
comparison,
the
conformations
of
3-tert-butyl groups in molecules B and C do not change drastically from that in molecule A after the
3-tert-butyl
groups
intorsion
molecules
B and
do notinchange
phase
transition.
The
angles
areCshown
Table 2.drastically from that in molecule A after the
phase transition. The torsion angles are shown in Table 2.
Crystals 2017, 7, 7
Crystals 2017, 7, 7
5 of 10
5 of 10
Table 2. Intramolecular hydrogen bonds and molecular conformations for enol-(S)-1 crystals.
Table 2. Intramolecular hydrogen bonds
and molecular conformationsβ-Form
for enol-(S)-1 crystals.
α-Form
Molecule A
Intramolecular hydrogen bond
N---O distance (Å)
Intramolecular
bond
N---H−Ohydrogen
angle (deg)
N—O
distance
(Å)
Dihedral
angle
between
N—H−O angle (deg)
phenyl and salicyl plane (deg)
Dihedral angle between
Torsion angle (deg)
phenyl and salicyl plane (deg)
Torsion angle (deg)
5-t-Bu group
5-t-Bu group
3-t-Bu group
3-t-Bu group
Molecule B
α-Form
Molecule
2.591A
Molecule C
β-Form
Molecule
2.596 B
Molecule
2.591 C
160.4
148.67
148.94
2.591
41.06
160.4
2.596
46.82
148.67
43.97
148.94
41.06
61.5
(C5−C6−C11−C13)
61.5
−59.21
(C5−C6−C11−C13)
(C5−C6−C11−C14)
−59.21
178.68
(C5−C6−
C11−C14)
(C5−C6−C11−C12)
178.68
(C5−C6−
C11−C12)
123.57
123.57
(C5−C3−C7−C9)
(C5−C3−
C7−C9)
3.95
3.95
(C5−C3−C7−C10)
(C5−C3−C7−C10)
−−115.50
115.50
(C5−C3−C7−C8)
(C5
−C3−C7−C8)
2.591
46.82
43.97
63.29
(C19−C20−C25−C27)
63.29
−55.24
(C19−C20−C25−C27)
(C19−C20−C25−C28)
−55.24
−175.97
(C19−C20
−C25−C28)
(C19−C20−C25−C26)
−175.97
(C19−C20
−C25−C26)
121.41
121.41
(C19−C18−C21−C23)
(C19−C18
−C21−C23)
1.53
1.53
(C19−C18−C21−C24)
(C19−C18−C21−C24)
−117.63
−117.63
(C19−C18−C21−C22)
(C19−C18−C21−C22)
126.86
(C33−C34−C39−C42)
126.86
6.27
(C33−C34−C39−C42)
(C33−C34−C39−C40)
6.27
−115.53
(C33−C34
−C39−C40)
(C33−C34−C39−C41)
−115.53
(C33−C34
−C39−C41)
125.7
125.7
(C33−C32−C35−C37)
(C33−C32
−C35−C37)
6.05
6.05
(C33−C32−C35−C38)
(C33−C32−C35−C38)
−112.10
−112.10
(C33−C32−C35−C36)
(C33−C32−C35−C36)
In
the α-form crystal, four A molecules (yellow) exist in a unit cell (Figure 4a), and a pair of A
In the α-form crystal, four A molecules (yellow) exist in a unit cell (Figure 4a), and a pair of
molecules
is arranged in a two-fold helical manner along all axes (Figure 4a–c) to form a herringbone
A molecules is arranged in a two-fold helical manner along all axes (Figure 4a–c) to form a herringbone
structure along the a-axis (Figure 4c). In the β-form crystal, two B molecules (light pink) and two C
structure along the a-axis (Figure 4c). In the β-form crystal, two B molecules (light pink) and
molecules (light blue) coexist in a unit cell, and two pairs of B and C molecules are arranged in a
two C molecules (light blue) coexist in a unit cell, and two pairs of B and C molecules are arranged in
two-fold helical manner along the b-axis (Figure 4d). The B and C molecules are arranged alternately
a two-fold helical manner along the b-axis (Figure 4d). The B and C molecules are arranged alternately
along the a-axis to form a pseudo-herringbone structure (Figure 4f).
along the a-axis to form a pseudo-herringbone structure (Figure 4f).
Figure 4.
Crystal shape
shape and
and molecular
molecular packings
packings of
of the
the (a–c)
(a–c) α-form
α-form and
and (d–f)
(a,d) (010),
(010),
Figure
4. Crystal
(d–f) β-form:
β-form: (a,d)
(b,e) (100), and (c,f) (001) faces. The disordered 5-tert-butyl groups in minor occupancies and all
(b,e) (100), and (c,f) (001) faces. The disordered 5-tert-butyl groups in minor occupancies and all
hydrogen atoms are omitted for clarity.
hydrogen atoms are omitted for clarity.
Crystals 2017, 7, 7
6 of 10
Crystals 2017, 7, 7
6 of 10
We discuss a possible mechanism for the reversible SCSC phase transition, based on the
changes in the molecular conformation and packing arrangement on the (001) face of the α- and
β-form
5). mechanism
The 5-tert-butyl
groups
of theSCSC
A molecules
of the right
column
by
Wecrystals
discuss (Figure
a possible
for the
reversible
phase transition,
based
on therotate
changes
approximately
(red circular
5a) at the
phase
Tcβ-form
≈ 3 °Ccrystals
under
in
the molecular60°
conformation
andarrow,
packingFigure
arrangement
on the
(001)transition
face of thepoint
α- and
cooling;5).consequently,
thegroups
A molecules
the right of
column
change
into
C molecules
(Figure 5b).
(Figure
The 5-tert-butyl
of the Aofmolecules
the right
column
rotate
by approximately
60◦
◦
However,
thearrow,
A molecules
of the
left phase
column
do not rotate
c under
cooling;
consequently,
the A
(red
circular
Figure 5a)
at the
transition
pointat
TcT≈
3 C under
cooling;
consequently,
molecules
of theofleft
change
into Binto
molecules
(Figure
5b). 5b).
Hence,
two crystallographically
the
A molecules
thecolumn
right column
change
C molecules
(Figure
However,
the A molecules of
independent
molecules
(B andatC)Tcare
generated
by consequently,
these asymmetrical
a pair
the
left column
do not rotate
under
cooling;
the Aconformation
molecules ofchanges
the left in
column
of A molecules.
change
into B molecules (Figure 5b). Hence, two crystallographically independent molecules (B and C)
As
described
above,
the dihedralconformation
angle 41.6° between
salicyl planes in molecule
are generated
by these
asymmetrical
changesthe
in aphenyl
pair ofand
A molecules.
◦
A increases
slightly
to
46.82°
and
43.97°
in
molecules
B
and
C,
respectively.
Accordingly,
the salicyl
As described above, the dihedral angle 41.6 between the phenyl and salicyl
planes in molecule
A
◦
◦
rings of the
A molecules
slightly
so that they
are almost
perpendicular
to the (001)
face (light
increases
slightly
to 46.82 rotate
and 43.97
in molecules
B and
C, respectively.
Accordingly,
the salicyl
rings
blue
arrow,rotate
Figure
5a). The
dihedral
angle
between
the salicyltoplane
andface
the (light
(001) blue
face
of
thecircular
A molecules
slightly
so that
they are
almost
perpendicular
the (001)
increasesarrow,
from 80.94°
85.90°
(B) and
81.39°
(C). These
conformation
changes
fromface
A toincreases
B and C
circular
Figure(A)
5a).toThe
dihedral
angle
between
the salicyl
plane and
the (001)
◦ (A)of
◦ (C).
cause80.94
the loss
along
the
a- and
c-axes,
which induces
thefrom
reordering
of the
packing
from
tohelical
85.90◦ axes
(B) and
81.39
These
conformation
changes
A to B and
C cause
the
arrangement
from
the
orthorhombic
space
group
P2
1
2
1
2
1
(α-form)
to
the
monoclinic
space
group
P21
loss of helical axes along the a- and c-axes, which induces the reordering of the packing arrangement
(β-form).
from
the orthorhombic space group P21 21 21 (α-form) to the monoclinic space group P21 (β-form).
Figure 5.5.Molecular
with
the intermolecular
interactions
on the on
(001)
face
of the
(a) αMoleculararrangements
arrangements
with
the intermolecular
interactions
the
(001)
face
of and
the
(b)
β-form
crystals.
The
A
molecules
of
the
left
column
are
converted
into
the
B
molecules
after
(a) α- and (b) β-form crystals. The A molecules of the left column are converted into the B molecules
the
transition.
TheThe
A molecules
of of
thethe
right
molecules.
afterphase
the phase
transition.
A molecules
rightcolumn
columnare
areconverted
convertedinto
into the
the C
C molecules.
The
The disordered
disordered 5-tert-butyl
5-tert-butyl groups
groups in
in minor
minor occupancies
occupanciesare
areomitted
omittedfor
forclarity.
clarity.
The rotation
rotationofofthe
the
salicyl
rings
toward
the perpendicular
the face
(001)
facetoleads
closer
The
salicyl
rings
toward
the perpendicular
to theto(001)
leads
closertopacking
packing
in
the
direction
along
the
a-axis.
As
a
result,
the
length
of
the
a-axis
shrinks
discontinuously
in the direction along the a-axis. As a result, the length of the a-axis shrinks discontinuously from
from 6.3058
to 6.2065
Å transition
at the transition
Tc. Before
the phase
transition,
weak intermolecular
6.3058
to 6.2065
Å at the
point, point,
Tc . Before
the phase
transition,
a weakaintermolecular
CH–π
CH–π interaction
forms between
the base
Schiffand
base
the phenyl
the A molecules
in the
interaction
forms between
the Schiff
theand
phenyl
plane ofplane
the Aofmolecules
in the α-form;
α-form;
the distance
is (black
2.881 Ådotted
(blackline,
dotted
line, 5a
Figures
5a and
6a).
thetransition
phase transition
to the
the
distance
is 2.881 Å
Figures
and 6a).
After
theAfter
phase
to the β-form,
β-form,
the distances
of theinteractions
CH–π interactions
the
and C molecules
shorten
to 2.823
the
distances
of the CH–π
betweenbetween
the B and
C Bmolecules
shorten to
2.823 and
2.846and
Å,
2.846 Å, respectively
(Figures
5bHowever,
and 6b). However,
theinchanges
in molecular
conformation
and the
respectively
(Figures 5b
and 6b).
the changes
molecular
conformation
and the reordering
reordering
of the
packing arrangement
arewhich
very leads
small,towhich
leads to SCSC
the reversible
SCSC phase
of
the packing
arrangement
are very small,
the reversible
phase transition
with
transition
with of
nothe
destruction
of the crystal lattice.
no
destruction
crystal lattice.
Crystals
Crystals 2017,
2017,7,
7,77
Crystals 2017, 7, 7
77 of
of 10
10
7 of 10
Figure6.
6.CH
CH−π
interaction in
α-form and
β-form crystals.
Figure
−π interaction
in the
the (a)
(a) α-form
α-form
and (b)
(b) β-form
β-form
crystals.
Figure
6.
CH−π
The SCSC
SCSC phase
phase transition
transition should
should lead
lead to
to an
an anomalous
anomalous change
change in
in the
the dielectric
dielectric constant.
constant.
The
constant.
Millimeter-size
square
plate
α-form
crystals
of
enol-(S)-1
were
obtained
by
slow
evaporation
of the
the
Millimeter-size square plate α-form
α-form crystals of enol-(S)-1 were obtained by slow evaporation of
2-propanolsolution
solutionatat
atroom
room
temperature.
The
top
surfaces
were
identified
as(010)
the (010)
(010)
and (0–10)
(0–10)
2-propanol
solution
room
temperature.
The
surfaces
were
identified
as
the
and
2-propanol
temperature.
The
toptop
surfaces
were
identified
as the
and (0–10)
faces
faces
by
X-ray
crystallography
analysis.
Hence,
the
dielectric
constant
of
the
depth
direction
along
faces
by crystallography
X-ray crystallography
analysis.
the dielectric
of the
depth direction
along
by
X-ray
analysis.
Hence, Hence,
the dielectric
constantconstant
of the depth
direction
along the b-axis
the b-axis
b-axis
was A
measured.
A square
square
plate
crystal
(5.4mm
4.8
0.7
mm ×
long
wide
thick) was
was
the
was
measured.
A
(5.4
×× 4.8
×× 0.7
mm
long
×× wide
×× thick)
was
measured.
square plate
crystalplate
(5.4 ×crystal
4.8 × 0.7
long
× wide
thick)
was fabricated
by
fabricated
by
coating
the (010)
(010)
and (0–10)
(0–10)
top
surfaces with
with
silver-based
conductive
paste,
and
fabricated
coating
the
and
top
surfaces
aa silver-based
paste,
and
coating
theby
(010)
and (0–10)
top surfaces
with
a silver-based
conductive
paste,conductive
and enameled
copper
enameled
copper wires
wires
were
attached(Figure
to both
both
topThe
surfaces
(Figure
7a,b). The
The
temperature
enameled
were
attached
to
top
surfaces
(Figure
7a,b).
wires
were copper
attached
to both
top surfaces
7a,b).
temperature
dependence
of temperature
the
real part
dependence
of
the
real
part
of
the
dielectric
constant
ε
r was measured along the b-axis at 1 kHz
dependence
of the
real part
of the
dielectric
constant
εr was
along
the b-axis at
1 kHz
of
the dielectric
constant
εr was
measured
along
the b-axis
at 1measured
kHz (see the
Experimental
Section).
(seeT–ε
the Experimental
Experimental
Section).
The T–ε
T–εdielectric
r curve revealed two step-like dielectric anomalies
atcooling
aroundand
3.4
◦ C uponat
(see
the
Section).
The
r
curve
revealed
two
step-like
dielectric
anomalies
around
3.4
The
curve
revealed
two
step-like
anomalies
at
around
3.4
and
3.9
r
and
3.9
°C
upon
cooling
and
then
heating,
respectively
(Figure
7c),
in
agreement
with
the
DSC
results
and 3.9
°C upon
cooling and
then 7c),
heating,
respectively
7c),results
in agreement
with
the DSC
results
then
heating,
respectively
(Figure
in agreement
with(Figure
the DSC
(Figure 1).
These
results
also
(Figure
1).
These
results
also
indicate
that
the
transition
is
a
first-order
structural
phase
transition.
(Figure 1).
These
alsoisindicate
that the
transition
is a first-order
indicate
that
the results
transition
a first-order
structural
phase
transition.structural phase transition.
Figure
of of
enol-(S)-1
with
application
of silver
pastepaste
and enameled
coppercopper
wires:
Figure 7.
7.Square
Squareplate
platecrystal
crystal
enol-(S)-1
with
application
of silver
silver
and enameled
enameled
Figure
7.
Square
plate
crystal
of enol-(S)-1
with
application
of
paste and
copper
(a)
the (010)
top(010)
face top
and face
(b) the
side
face (the
1 mm);
temperature
dependence
wires:
(a) the
the
and(001)
(b) the
the (001)
(001)
sidescale
face bar
(theisscale
scale
barand
is 11(c)
mm);
and (c)
(c) temperature
temperature
wires:
(a)
(010) top
face and
(b)
side
face
(the
bar
is
mm);
and
of
dielectric constant
for the
enol-(S)-1
crystal
along thecrystal
b-axis.along
The cooling
scan The
was cooling
switchedscan
intowas
the
dependence
of
dielectric
constant
for
the
enol-(S)-1
the
b-axis.
dependence of dielectric
constant for the enol-(S)-1 crystal along the b-axis. The cooling scan was
◦ C.
heating
scan
at
1.4
switched into
into the
the heating
heating scan
scan at
at 1.4
1.4 °C.
°C.
switched
The dielectric
dielectric constant
constant εεrr increased
increased from
from 2.636
2.636 to
to 2.671
2.671 (+1.33%)
(+1.33%) on
on cooling
cooling from
from 5.7
5.7 °C
°C to
to 1.3
1.3 °C
°C
The
(Figure
7c).
From
the
temperature
dependence
of
the
lattice
constants
in
Figure
2,
the
surface
area
of
(Figure 7c). From the temperature dependence of the lattice constants in Figure 2, the surface area of
Crystals 2017, 7, 7
8 of 10
The dielectric constant εr increased from 2.636 to 2.671 (+1.33%) on cooling from 5.7 ◦ C to 1.3 ◦ C
(Figure 7c). From the temperature dependence of the lattice constants in Figure 2, the surface area
of the (010) face should decrease (−0.83%) and the thickness along the b-axis should also decrease
(−0.09%) in the phase transition from the α-form (10 ◦ C) to the β-form (0 ◦ C). The calculated change
in the dielectric constant caused by the slight decreases in surface area and depth is +0.74% from
Equation (1), indicating a slight increase. Hence, the estimated real change in the dielectric constant
corrected with the crystal volume shrinkage is +0.59% (=1.33% − 0.74%) from Equation (1). This small
change in the dielectric constant at the phase transition is derived from the small changes in the
molecular conformation and packing arrangement between the α- and β-form crystals. The maximum
εr value (2.67) of the β-form crystal of enol-(S)-1 along the b-axis at 1.5 ◦ C is similar to that (2.7) of
benzyl at −189 ◦ C [17] but is lower than those reported for ferroelectric organic crystals [1]. As shown
in the packing diagrams (Figures 4 and 5), the intermolecular interactions in both the α- and the
β-form crystals are the van der Waals force and CH–π interactions alone. Such weak intermolecular
interactions might have induced the small dielectric constant εr . In fact, a large εr value (11) at room
temperature has been reported for the croconic acid crystal, in which strong hydrogen-bonded sheets
are formed [18].
3. Experimental Section
3.1. Materials
The compound enol-(S)-1 was synthesized according to the literature procedure [14]. Single crystals
were obtained by slow evaporation of the solution in 2-propanol at room temperature within several
days. For X-ray crystallographic analysis and dielectric measurements, several single crystals were put
into paraffin oil to wash them and were dried. Remaining crystals were used for DSC.
3.2. Differential Scanning Calorimetry
DSC runs of enol-(S)-1 crystals (6.58 mg) were recorded using a DSC8500 (PerkinElmer, Norwalk,
CT, USA) in the temperature range from 10 ◦ C to −10 ◦ C with a rate of 2 ◦ C·min−1 on cooling and
heating under nitrogen at atmospheric pressure in aluminum crucibles with covers.
3.3. Crystallography
α-form: Single-crystal X-ray diffraction data were collected at 10 ◦ C with R-AXIS RAPID II
(Rigaku, Tokyo, Japan) using Cu Kα radiation (λ = 1.54186 Å). The integrated and scaled data were
empirically corrected for absorption effects with ABSCOR (Rigaku, Tokyo, Japan) [19]. The initial
structures were solved by direct methods with SHELXT 2014 (Göttingen, Germany) [20] and refined
on F0 2 with SHELXL 2016 (Göttingen, Germany) [21]. With the exception of the disordered C atoms of
the tert-butyl group in minor occupancies, the non-H atoms were refined anisotropically, and all of the
H atoms were obtained geometrically and included in the calculations using the riding atom model.
β-form: Single-crystal X-ray diffraction data were collected at −50 ◦ C with R-AXIS RAPID II
(Rigaku) using Cu Kα radiation (λ = 1.54186 Å). The integrated and scaled data were empirically
corrected for absorption effects with NUMABS (Rigaku, Tokyo, Japan) [22]. The initial structures
were solved by direct methods with SHELXS 97 (Göttingen, Germany) [23] and refined on F0 2 with
SHELXL 97 (Göttingen, Germany) [23]. With the exception of the disordered tertiary C atoms of the
tert-butyl group, the non-H atoms were refined anisotropically, and all of the H atoms were obtained
geometrically and included in the calculations using the riding atom model.
Crystals 2017, 7, 7
9 of 10
3.4. Dielectric Measurements
The dielectric constant εr is defined by the equation below:
εr =
Cd
,
ε0A
(1)
where C is the capacitance, in farads; A is the area of a crystal surface, in square meters; ε0 is the electric
constant (ε0 ≈ 8.854 × 10−12 Fm−1 ); and d is the thickness of plate crystal, in meters. The square
plate crystal was fabricated by application of silver-based conductive paste on the (010) and (0−10)
top surfaces. The (010) area and the thickness of the crystal were measured by a digital high-speed
microscope (VHX-5000; Keyence, Itaska, IL, USA). Capacitance was measured using with a capacitance
bridge (1 kHz, AH 2550A; Andeen-Hagerling, Solon, OH, USA).
4. Conclusions
The chiral crystal of enantiomeric (S)-salicylidenephenylethylamine enol-(S)-1 undergoes
a reversible SCSC phase transition at Tc ≈ 3 ◦ C from the room temperature α-form in orthorhombic
space group P21 21 21 to the low temperature β-form in monoclinic space group P21 with a thermal
hysteresis of around 1.7 ◦ C. The most significant conformation change is the approximately 60◦ rotation
of the 5-tert-butyl group of one molecule in the asymmetric unit of the β-form. The small changes in
the molecular conformation and packing arrangement lead to the reversible SCSC transformation with
no destruction of the crystal lattice. The dielectric constant along the b-axis is small, probably due to
the weak intermolecular interactions.
Supplementary Materials: Supporting results (DSC and XRD) and CIF files (CCDC 1518678, 1518679) are
available online at www.mdpi.com/2073-4352/7/1/7/s1. Figure S1: DSC curves of the enol-(S)-1 crystal over the
temperature range from (a) −50 to +50 ◦ C and (b) −10 to +110 ◦ C on initial heating and then cooling; Figure S2:
X-ray diffraction (XRD) patterns of the enol-(S)-1 crystal: powder XRD pattern measured at room temperature
(black line) and calculated XRD pattern from the single crystal structure of the α-form (red line).
Acknowledgments: This work was supported by the JSPS Scientific Research in the Challenging Exploratory
Research, and the grant-in-aid from Mitsubishi Materials Corporation (Tokyo, Japan). We acknowledge Hiroaki Ikeda
at Waseda University for his useful suggestions. Akifumi Takanabe acknowledges the Leading Graduate Program
in Science and Engineering, Waseda University from MEXT, Japan.
Author Contributions: Collection and assembly of data were conducted by Akifumi Takanabe. The draft of the
paper was written by Akifumi Takanabe and Hideko Koshima. Takuro Katsufuji supported the dielectric constant
measurement. The crystal structures were analyzed by Motoo Shiro, Kohei Johmoto, and Hidehiro Uekusa.
Interpretation of data was conducted by Akifumi Takanabe, Hideko Koshima, and Toru Asahi. All authors have
given approval to the final version of the paper.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
Horiuchi, S.; Tokura, Y. Organic ferroelectrics. Nat. Mater. 2008, 7, 357–366. [CrossRef] [PubMed]
Yao, Z.S.; Yamamoto, K.; Cai, H.L.; Takahashi, K.; Sato, O. Above Room Temperature Organic Ferroelectrics:
Diprotonated 1,4-Diazabicyclo[2.2.2]octane Shifts between Two 2-Chlorobenzoates. J. Am. Chem. Soc. 2016,
138, 12005–12008. [CrossRef] [PubMed]
Tang, Y.Z.; Gu, Z.F.; Xiong, J.B.; Gao, J.X.; Liu, Y.; Wang, B.; Tan, Y.H.; Xu, Q. Unusual Sequential Reversible
Phase Transitions Containing Switchable Dielectric Behaviors in Cyclopentyl Ammonium 18-Crown-6
Perchlorate. Chem. Mater. 2016, 28, 4476–4482. [CrossRef]
Kaftory, M.; Botoshansky, M.; Kapon, M.; Shteiman, V. Irreversible single-crystal to polycrystal and reversible
single-crystal to single-crystal phase transformations in cyanurates. Acta Crystallogr. Sect. B Struct. Sci. 2001,
57, 791–799. [CrossRef] [PubMed]
Fernandes, M.A.; Levendis, D.C.; Schoening, F.R.L. A new polymorph of ortho-ethoxy-trans-cinnamic acid:
Single-to-single-crystal phase transformation and mechanism. Acta Crystallogr. Sect. B Struct. Sci. 2004, 60,
300–314. [CrossRef] [PubMed]
Crystals 2017, 7, 7
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
10 of 10
Takahashi, H.; Ito, Y. Low-temperature-induced reversible single-crystal-to-single-crystal phase transition of
3,4-dichloro-20 ,40 ,60 -triethylbenzophenone. CrystEngComm 2010, 12, 1628–1634. [CrossRef]
Schmidt, A.; Kababya, S.; Appel, M.; Khatib, S.; Botoshansky, M.; Eichen, Y. Measuring the Temperature
Width of a First-Order Single Crystal to Single Crystal Phase Transition Using Solid-State NMR: Application
to the Polymorphism of 2-(2,4-Dinitrobenzyl)-3-methylpyridine. J. Am. Chem. Soc. 1999, 121, 11291–11299.
[CrossRef]
Das, D.; Engel, E.; Barbour, L.J. Reversible single-crystal to single-crystal polymorphic phase transformation
of an organic crystal. Chem. Commun. 2010, 46, 1676–1678. [CrossRef] [PubMed]
Girard, J.; Fromm, K. Single crystal to single crystal polymorphic phase transition of a silver nitrate
24-crown-8 complex and its pseudo-polymorphism. CrystEngComm 2012, 14, 6487–6491. [CrossRef]
Pal, R.; Reddy, M.B.M.; Dinesh, B.; Balaram, P.; Row, T.N.G. Temperature-Induced Reversible First-Order
Single Crystal to Single Crystal Phase Transition in Boc-γ4 (R)Val-Val-OH: Interplay of Enthalpy and Entropy.
J. Phys. Chem. A 2014, 118, 9568–9574. [CrossRef] [PubMed]
Kawato, T.; Koyama, H.; Kanatomi, H.; Shigemizu, H. Difference in the Rate of Photo-induced Unimolecular
Motion of Chiral Salicylideneamines in the Chiral Crystal Environments. Chem. Lett. 1997, 26, 401–402.
[CrossRef]
Koshima, H.; Matsuo, R.; Matsudomi, M.; Uemura, Y.; Shiro, M. Light-Driven Bending Crystals of
Salicylidenephenylethylamines in Enantiomeric and Racemate Forms. Cryst. Growth Des. 2013, 13, 4330–4337.
[CrossRef]
Takanabe, A.; Tanaka, M.; Johmoto, K.; Uekusa, H.; Mori, T.; Koshima, H.; Asahi, T. Optical Activity and
Optical Anisotropy in Photomechanical Crystals of Chiral Salicylidenephenylethylamines. J. Am. Chem. Soc.
2016, 45, 15066–15077. [CrossRef] [PubMed]
Centore, R.; Jazbinsek, M.; Tuzi, A.; Roviello, A.; Capobianco, A.; Peluso, A. A series of compounds forming
polar crystals and showing single-crystal-to-single-crystal transitions between polar phases. CrystEngComm
2012, 14, 2645–2653. [CrossRef]
Smith, H.E.; Cook, S.L.; Warren, M.E., Jr. Optically Active Amines. II. The Optical Rotatory Dispersion
Curves of the N-Benzylidene and Substituted N-Benzylidene Derivatives of Some Open-Chain Primary
Amines. J. Org. Chem. 1964, 29, 2265–2272. [CrossRef]
Johmoto, K.; Ishida, T.; Sekine, A.; Uekusa, H.; Ohashi, Y. Relation between photochromic properties and
molecular structures in salicylideneaniline crystals. Acta Crystallogr. Sect. B Struct. Sci. 2012, 68, 297–304.
[CrossRef] [PubMed]
Almeida, A.; Dos Santos, M.L.; Chaves, M.R.; Amaral, M.H.; Tolédano, J.C.; Périgaud, A.; Savary, H.
Pyroelectric effect in benzil. Ferroelectrics 1988, 79, 253–256. [CrossRef]
Horiuchi, S.; Tokunaga, Y.; Giovannetti, G.; Picozzi, S.; Itoh, H.; Shimano, R.; Kumai, R.; Tokura, Y.
Above-room-temperature ferroelectricity in a single-component molecular crystal. Nature 2010, 463, 789–792.
[CrossRef] [PubMed]
Rigaku. ABSCOR; Rigaku Corporation: Tokyo, Japan, 1995.
Sheldrick, G.M. SHELXT—Integrated space-group and crystal-structure determination. Acta Crystallogr. Sect.
A Found. Adv. 2015, 71, 3–8. [CrossRef] [PubMed]
Sheldrick, G.M. Crystal Structure Refinement with SHELXL. Acta Crystallogr. Sect. C Struct. Chem. 2015, 71,
3–8. [CrossRef] [PubMed]
Rigaku. NUMABS; Rigaku Corporation: Tokyo, Japan, 1999.
Sheldrick, G.M. A Short History of SHELX. Acta Crystallogr. Sect. A Found. Adv. 2008, 64, 112–122. [CrossRef]
[PubMed]
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).