Unique monotropic phase transition behaviors of a butterfly-shaped diphenylpyrimidine molecule

Article abstract of DOI:10.1021/cg501697q

The physical properties of two-dimensional disc-shaped aromatic carbon molecules strongly depend on the molecular packing structures. A butterfly-shaped diphenylpyrimidine molecule (DPP-6C12) was synthesized by covalently attaching two tridodecyl benzoate tails (6C12) at the both sides of the diphenylpyrimidine (DPP) moiety. Unique phase transition behaviors of DPP-6C12 and their origins were investigated with the combined techniques of thermal, scattering, spectroscopic, and microscopic analyses. On the basis of the experimental results and analyses, it was realized that a butterfly-shaped DPP-6C12 formed three ordered phases: a plastic crystal phase (PK), a crystal phase (K), and a liquid crystal phase (Φ). By breaking the molecular symmetry and coplanarity of DPP-6C12, peculiar monotropic phase transition behaviors were observed. The stable Φ mesophase was formed either by a slow heating above the metastable PK phase or by an isothermal annealing between T_Φ and T_K. The stable K phase was only formed by a slow heating from the preordered Φ mesophase, and the formation of the K phase directly from the isotropic state (I) was forbidden because the nucleation barrier from I to K was too high to be overcome via thermal annealing.

Full text of DOI:10.1021/cg501697q

Article
pubs.acs.org/crystal
Unique Monotropic Phase Transition Behaviors of a Butterfly-Shaped
Diphenylpyrimidine Molecule
Minwook Park,† Yu-Jin Choi,† Dae-Yoon Kim,† Seok-Ho Hwang,*,‡ and Kwang-Un Jeong*,†
†Polymer Materials Fusion Research Center & Department of Polymer-Nano Science and Technology, Chonbuk National University,
Jeonju 561-756, Korea
‡Department of Polymer Science and Engineering, Dankook University, Yongin, Gyeonggi 448-701, Korea
S
* Supporting Information
ABSTRACT: The physical properties of two-dimensional disc-shaped aromatic
carbon molecules strongly depend on the molecular packing structures. A
butterfly-shaped diphenylpyrimidine molecule (DPP-6C12) was synthesized by
covalently attaching two tridodecyl benzoate tails (6C12) at the both sides of the
diphenylpyrimidine (DPP) moiety. Unique phase transition behaviors of DPP-
6C12 and their origins were investigated with the combined techniques of
thermal, scattering, spectroscopic, and microscopic analyses. On the basis of the
experimental results and analyses, it was realized that a butterfly-shaped DPP-
6C12 formed three ordered phases: a plastic crystal phase (PK), a crystal phase
(K), and a liquid crystal phase (Φ). By breaking the molecular symmetry and
coplanarity of DPP-6C12, peculiar monotropic phase transition behaviors were
observed. The stable Φ mesophase was formed either by a slow heating above the
metastable PK phase or by an isothermal annealing between TΦ and TK. The
stable K phase was only formed by a slow heating from the preordered Φ
mesophase, and the formation of the K phase directly from the isotropic state (I) was forbidden because the nucleation barrier
from I to K was too high to be overcome via thermal annealing.
aromatic cores and the nanophase separation between the rigid
disc-shaped core and flexible alkyl or alkoxyl tails.14
Furthermore, the self-assembled nanocolumns can laterally
organize each other and show one- or two-dimensional (1D or
2D) long-range molecular orientational or positional orders. In
this partially ordered thermotropic mesophase, the flexible alkyl
or alkoxyl tails remain in the disordered state. When the
temperature is further decreased, the flexible tails even
crystallize in the phase-separated nanodomains and the highly
ordered columnar crystalline phase is formed.15
Usually, the existence of the low-ordered LC mesophase is
favorable for practical application of the disc-shaped molecules
since the macroscopic molecular orientation even on the meter
length scale can be achieved by applying mechanical, electrical,
or magnetic forces with the support of surface anchoring and
confinement. In organic photovoltaic (OPV) device and
organic thin film transistor (OTFT) applications of disc-shaped
molecules,16−19 the strong π−π interactions between the rigid
cores are favorable to improve the physical device performance,
since the electrical, thermal, and photoconductivity along the
columnar long axis is significantly increased by the π−π strong
interactions between the rigid cores. However, in the organic
light emitting diode (OLED) application,20−22 device engineers
INTRODUCTION
■
The phase transition behavior of soft materials is an interesting
phenomenon when more than one ordered phase exists.1
Especially, liquid crystalline (LC) soft materials show the
enantiotropic LC transition behaviors with the thermodynamic
stability in a temperature region between their melting and
isotropic (I) temperatures. Monotropic LC soft materials, on
the other hand, show metastable states when the crystallization
is bypassed below the LC transition temperature with a fast
cooling rate. Therefore, the formation of the monotropic state
is usually originated from the competition between the crystal
phase and the LC phase.2−8 This type of phase transition allows
us to study the formation of crystals either from the I melt or
from the LC phase.9−12 Additionally, since the physical
properties of LC soft materials strongly depend on molecular
self-assembly, it is essential to control their molecular packing
structure on the different length scales and times from
subnanometer to micrometer.13
The molecular interactions between disc-shaped aromatic
mesogens can be tuned by attaching flexible alkyl or alkoxyl
groups at the periphery of the rigid core with chemical or
physical bonds. The traditional disc-shaped LC mesophases can
be constructed by adjusting the thermal energy in the system.13
Upon decreasing the temperature below the I phase, the aspect
ratios of the self-assembled nanocolumns are suddenly
increased. Here, the formation of nanocolumns is mainly
driven by the strong π−π interactions between disc-shaped
Received: November 21, 2014
Revised: January 7, 2015
© XXXX American Chemical Society
A
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3,4,5-tridodecoxybenzoate (10 g, 15 mmol) in ethanol (200 mL), the
resulting mixture was refluxed for 4 h. After cooling of the mixture to
room temperature, 1 L of deionized water was added. The resulting
mixture was acidified until pH = 1 with HCl. The precipitate was then
filtered, washed with deionized water (50 mL × 2) and diethyl ether
(30 mL), and dried under a vacuum to give a white solid (9.5 g, 94%):
and physicists would like to avoid molecular packings since the
strong π−π interactions between the rigid cores generate
excimers which may decrease the photoefficiency of the
system.23
To understand the self-assembly process of a LC molecule
with the broken symmetry and the low coplanarity, a
diphenylpyrimidine-based molecule (abbreviated as DPP-
6C12, Scheme 1) was newly designed and synthesized by
1
mp 55−57 °C; H NMR (400 MHz, CDCl3, δ): 7.24 (s, 2H; CH),
3.96−4.01 (m, 6H; CH2), 1.81−1.77 (m, 6H; CH2), 1.34−1.46 (m,
6H; CH2), 1.16−1.39(m, 48H; CH2), 0.86 (m, 9H; CH3); 13C NMR
(100 MHz, CDCl3, δ): 14.23 (3C; CH3), 22.90 (3C; CH2), 26.25 (3C;
CH2), 29.61 (3C; CH2), 29.77 (3C; CH2), 29.85 (3C; CH2), 29.87
(3C; CH2), 29.91 (3C; CH2), 30.53 (3C; CH2), 32.14 (3C; CH2),
69.35 (2C; COH2), 73.74 (1C; COH2), 108.70 (2C; CH), 123.85
(1C; C), 143.33 (1C; CO), 153.04 (2C; CO), 172.34 (CO); ESI/
MS: m/z = 675 [M+] (calcd. m/z = 674.58).
Scheme 1. Reagent and Conditions: (i) K2CO3, 1-
Chlorododecane, DMF, Reflux; (ii) KOH, Ethanol, Reflux;
(iii) (1) SOCl2, THF, (2) NaH, THF, Reflux
4,6-(3,4,5-Tris(n-dodecan-1-yloxy)benzoyloxy)-2,5-diphe-
nylpyrimidine (3). N,N-Dimethylformamide was added to a solution
of 3,4,5-tris(n-dodecan-1-yloxy)benzoic acids (5 g 7.4 mmol) and
dried THF (30 mL). After cooling of the reaction flask with an ice
bath, thionyl chloride (0.58 mL, 8.0 mmol) was introduced dropwise.
The reaction mixture was stirred for 3 h after the ice bath was
removed. After filtration and evaporation, sodium hydride (0.16 g, 6.5
mmol) was added to a solution of 4,6-dihydroxy-2,5-diphenylpyr-
imidine (0.81 g, 3.1 mmol) in dried THF (50 mL). After stirring for 1
h at room temperature, 3,4,5-tris(n-dodecan-1-yloxy)benzoic chloride
was added. The solution was refluxed for 12 h. After filtration and
evaporation, the product was purified by silica (SiO2) column
chromatography eluting with a mixture of ethyl acetate and hexane
(4:1, v/v) to give a light yellow powder (2.7 g, 55%): mp 39−43 °C;
1H NMR (400 MHz, DMSO-d6, δ): 8.47 (d, 4H; CH), 7.47 (m, 4H;
CH), 7.25−7.32 (m, 2H; CH), 7.22 (s, 4H; CH), 3.95−3.98 (m, 12H;
CH2), 1.79−1.82 (m, 12H; CH2), 1.39−1.49 (m, 12H; CH2), 1.27 (m,
96H; CH2), 0.89 (m, 18H; CH3); 13C NMR (100 MHz, CDCl3, δ):
14.25 (3C; CH3), 22.84 (3C; CH2), 26.23 (3C; CH2), 29.52 (3C;
CH2), 29.55 (3C; CH2), 29.71 (3C; CH2), 29.79 (3C; CH2), 29.85
(3C; CH2), 30.49 (3C; CH2), 32.08 (3C; CH2), 69.39 (2C; COH2),
73.74 (1C; COH2), 108.97 (4C; CH), 117.49 (1C; C), 122.80 (2C;
C), 128.51 (2C; C), 128.69 (2C; C), 128.89 (2C; C), 129.49 (2C; C),
129.65 (2C; C), 131.66 (1C; C), 133.21 (1C; C), 136.12 (2C; C−O),
143.63 (4C; C−O), 153.02 (1C; N−CN), 163.38 (1C; CO),
165.49 (2C; CN); MALDI-ToF: m/z = 1600.69 [M + Na+] (calcd.
m/z = 1600.23).
chemically attaching two tridodecyl benzoate tails (6C12) at
the both sides of the diphenylpyrimidine (DPP) moiety via
three-step reactions.24−26 Here, 6 and 12 represent the
numbers of alkyl tails of DPP-6C12 and carbon atoms in
each alkyl tail, respectively. Major phase transitions of DPP-
6C12 and their origins were systematically investigated by the
combined techniques of differential scanning calorimetry
(DSC), 1D wide-angle X-ray diffraction (WAXD), polarized
optical microscopy (POM), and solid-state carbon-13 (13C)
nuclear magnetic resonance (NMR) spectroscopy.
RESULTS AND DISCUSSION
■
Programmed Butterfly-Shaped Diphenylpyrimidine
Molecule. A butterfly-shaped DPP molecule having two
tridodecyl benzoate tails (6C12) at both sides of the
diphenylpyrimidine moiety is specifically designed and
synthesized via three-step reactions, as shown in Scheme 1.
Especially, an esterification reaction in the presence of 1,3-
dicyclohexyl carbodiimide (DCC) and 4-dimethylaminopyr-
idine (DMAP) is conducted. The butterfly-shaped DPP
molecule is abbreviated as DPP-6C12. Here, 6 stands for the
number of alkyl tails with 12 carbons in each tail. The
synthesized DPP-6C12 compound was extensively purified by
column chromatography several times. The chemical structure
and purity were confirmed by MALDI-ToF mass spectroscopy
(Figure 1) in addition to NMR. The molecular weight of DPP-
6C12 was determined to be 1578.4 g/mol.
EXPERIMENTAL SECTION
■
The general procedures of chemical treatments and characterizations
were described in the Supporting Information. Additionally, the
experimental conditions and sample preparation procedures of DSC,
1D WAXD, POM, MALDI-ToF, and solid-state 13C NMR are
explained in detail in the Supporting Information.
Methyl 3,4,5-Tris(n-dodecan-1-yloxy)benzoate (1). After
K2CO3 (44.5 g, 330 mmol) was added into the solution of methyl
3,4,5-trihydroxybenzoate (10 g, 54 mmol) in DMF (100 mL), 1-
chlorododecane (51.2 mL, 217 mmol) was then slowly introduced.
The mixture was additionally stirred for 12 h at 100 °C. The gummy
solid obtained by filtration and evaporation was dissolved in
chloroform. The resulting mixture was washed with brine and
deionized water (50 mL × 3) and dried over anhydrous Mg2SO4,
and then filtrated. The crude product was purified by column
chromatography (SiO2) with a mixture of dichloromethane and
hexane (4:1, v/v), followed by recrystallization from acetone to give a
Thermodynamic Transitions of DPP-6C12. As repre-
sented in Figure 2, the DSC experiment is first conducted to
obtain information on the quantitative thermodynamic proper-
ties of the DPP-6C12. At different cooling rates from 0.5 to 40
°C/min in the investigated temperature range, DSC thermo-
grams exhibit only a single exothermic transition around −10
°C. Upon increasing the cooling rate, the onset temperature
shifts to lower temperatures and the heat (ΔH) release at the
1
white solid (28.3 g, 76%): H NMR (400 MHz, CDCl3, δ): 7.24 (s,
2H; CH), 4.00 (m, 6H; CH2), 3.88 (s, 3H; CH3), 1.81−1.77 (m, 6H;
CH2), 1.45−1.24 (m, 54H; CH2), 0.87 (t, 9H; CH3).
3,4,5-Tris(n-dodecan-1-yloxy)benzoic Acids (2). After the
addition of KOH (1.63 g, 30 mmol) into the solution of methyl
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processes, thermal transition behaviors are quite complicated
below the 10 °C/min heating rate, as shown in Figure 2b.
During the subsequent heating at 0.5 °C/min, an endothermic
thermal transition is first occurred at −1.8 °C with 43 kJ/mol,
whose transition corresponds to the exothermic transition
detected during the previous cooling process. At different
heating rates from 0.5 to 2.5 °C/min, two endothermic
transitions and two exothermic transitions are observed. When
the heating rate is increased from 0.5 to 2.5 °C/min, the onset
temperature of the first exothermic peak shift to higher
temperatures and the heat (ΔH) release is reduced.
The peculiar thermal transition behaviors depending on the
rate of heating process could be originated from the metastable
phase transitions, which are related with phase transformation
kinetics.13,29,30,40 In order to prove that there is a trans-
formation from the metastable/unstable phases to the stable
phases, specifically designed experiments were conducted. The
DPP-6C12 sample is cooled to 7 °C (Figure 3a) or 30 °C
Figure 1. MALDI-ToF spectrum of DPP-6C12 molecule.
Figure 2. Sets of DSC cooling (a) and subsequent heating (b)
thermodiagrams for DPP-6C12 at scanning rates ranging from 0.5 to
40 °C/min.
transition is reduced. At the 0.5 °C/min cooling rate, the onset
temperature of the exothermic transition and corresponding
heat is −6.5 °C and −27.3 J/g (−43 kJ/mol), respectively,
which are reduced to −14.49 °C and −14.8 J/g (−23 kJ/mol)
at 40 °C/min. From the linear dependence on the cooling
rate,27−39 the onset transition temperature and heat at the
equilibrium state are estimated to be −5.5 °C and −27.5 J/g
(−43.4 kJ/mol), respectively. The dependence of the transition
temperature on the cooling rate often means that the
exothermic transition is related with a crystallization
process.34,36 Even though the thermodynamic properties
obtained from the subsequent heating process above 10 °C/
min are consistent with those from the corresponding cooling
Figure 3. Sets of DSC thermodiagram by cooling DPP-6C12 from the
I state at a rate of 10 °C/min to 7 °C (a) and 30 °C which are the
onset temperature of the first and second exothermic peaks during
heating, and then staying there for different time periods before the
sample was heated at 10 °C/min to above the melting temperature.
(Figure 3b) at 10 °C/min and isothermally annealed for the
different periods of time. As shown in Figure 3a, the annealed
DPP-6C12 samples at 7 °C for the different periods of time are
heated to above the highest endothermic transition temperature
(45 °C) at 10 °C/min. The heating diagram of the annealed
DPP-6C12 samples at 28 °C for 10 min does not exhibit any
thermal transition (Figure 3a), which is identical to the heating
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diagrams obtained by scanning above 10 °C/min (Figure 2b).
After 20 min of annealing, two endothermic transition peaks at
28 and 46 °C start to emerge, and their heats of transitions are
saturated after the 120 min annealing.
and 2θ = 21.53°, respectively, and the half width of these
scatterings are dramatically decreased. However, the correlation
lengths of these scatterings are still in the quasi-long-range
order. Additionally, weak diffraction peaks are observed at 2θ =
22.36°, 2θ = 24.29°, and 2θ = 25.64°. These weak diffractions
should relate to the molecular packing of alkyl tails.
Note that, in the DPP-6C12 samples annealed for more than
60 min, an exothermic thermal transition appears right after the
endothermic transition at 45 °C. To know whether there is a
phase transformation from the metastable/unstable phases to
the stable phases at 45 °C, similar isothermal annealing
experiments are conducted and their results are shown in
Figure 3b. In the experimental time scale, any thermal
transition is not detected. This indicates that the stable phase
between 30 and 40 °C is only formed by a slow heating from
the preordered phase observed between 40 and 50 °C, and the
formation of the stable phase directly from the I state is
forbidden because the nucleation barrier from the I state to the
stable crystalline phase is too high to be overcome via thermal
annealing in the experimental time scales.
To confirm this speculation, the solid-state 13C NMR spectra
with CP/MAS/DD (Figure 5a) and Bloch decay (Figures 5b
Monotropic Phase Evolutions of DPP-6C12. The 1D
WAXD experiments are conducted at different temperatures to
identify the corresponding structural evolutions of DPP-6C12.
Figure 4 shows a set of 1D WAXD powder patterns of DPP-
Figure 4. A set of 1D WAXD powder patterns of DPP-6C12 during
cooling and subsequent heating at a rate of 1.0 °C/min at different
temperatures between −30 and 100 °C.
Figure 5. Solid-state 13C NMR spectra of DPP-6C12 during heating
from −30 to 70 °C at a rate of 1.0 °C/min: CP/MAS (a) and Bloch
decay (b).
6C12 at different temperatures between −30 and 100 °C
during cooling and subsequent heating at the rate of 1 °C/min.
In this figure, structures can be identified on two different
length scales. One is on the nanometer scale between 2θ = 1.5°
and 7°, and the other is on the subnanometer scale above 2θ =
7°.
Upon cooling DPP-6C12, it is obvious that phase transition
takes place when the temperature reaches −8 °C, which agrees
well with crystallization behavior observed in DSC (Figure 2).
Above −8 °C, the DPP-6C12 molecules are in the I state,
exhibiting only two amorphous halos at 2θ = 3.6° (d = 2.45
nm) and 2θ = 20.2° (d = 0.44 nm), which correspond to the
average periodicity of electron density fluctuations between the
nanophase separated DPP cores and alkyl tails and to the
average distance among the amorphous alkyl chains,
respectively. Below −8 °C, the amorphous halos at 2θ = 3.6°
and 2θ = 20.2° in the I phase are suddenly shifted to 2θ = 3.06°
and 6) were acquired at different temperatures to selectively
investigate the rigid and mobile components, respectively. The
solid-state 13C NMR spectra with CP/MAS/DD and Bloch
decay below 4 °C are not much changed in the chemical shift
from 100 to 200 ppm, compared with those of the I state. The
chemical shift at 30.0 ppm in the Bloch decay 13C solid-state
NMR spectra of DPP-6C12 (Figure 6) is originated from the
carbon atoms in the disordered, statistically distributed trans
and gauche conformations. On the other hand, the chemical
shift at 32.5 ppm represents the carbon atoms in the long trans
zigzag segments of the alkyl tails. Below −10 °C, more than
70% of CH2 carbon atoms are in the relatively long trans zigzag
conformations, while the percentage of trans conformations in
the I phase is below 10%. This solid-state 13C NMR results
combined with those of 1D WAXD indicate that the nanophase
separated DPP mesogens are in the solidified and quasi-long-
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flexible alkyl tails as well as by the molecular stacking among
aromatic DPP cores. This molecular stacking among aromatic
DPP cores increases the amount of CH2 long trans zigzag
conformations, but the alkyl tails are not in the 3D crystalline
phase, similar to the traditional mesophases.
After continually increasing temperature up to 40 °C (Figure
2b), an endothermic thermal transition was detected at 28 °C
right after the exothermic transition at 32 °C. By heating the
stable Φ mesophase, the molecular mobility gradually
increased, and the thermal fluctuation was enough to overcome
the nucleation barrier. This explanation is supported by the
structure-sensitive 1D WAXD. As shown in Figure 4, a lot of
diffractions are detected not only in the small angle but also in
the wide angle regions, which represent a 3D ordered
crystalline structure of DPP-6C12. It is worth mentioning
that the percentage of the CH2 long trans zigzag conformations
is slightly decreased from 60% (Φ mesophase) to 55%, even
though the rigidity of aromatic carbons is increased compared
with those of the Φ mesophase. This crystalline phase is
identified as the K phase. Since the 2D WAXD pattern from a
macroscopically oriented sample and selected area electron
diffraction from a single crystal are required to identify the
detail molecular packing in the K phase, the identification of
ordered K phase is under investigation. From the experimental
results, it is found that the strong molecular packing among
DPP mesogens can distort molecular ordering of alkyl tails.
Therefore, it is concluded that the PK phase is due to the alkyl
chain crystallization, while the K phase is induced by the strong
molecular packing among DPP mesogens. Note that the K
phase could not be formed directly from the I state but from
the preordered Φ mesophase because the nucleation barrier
from the I state to the K phase is too high to be overcome via
thermal annealing in experimental time scales. The unique
phase transition behaviors of DPP-6C12 should originate from
the distorted mesogens which make it difficult to pack by
themselves.
Morphological Phase Transition Behaviors of DPP-
6C12. The peculiar phase transformation behaviors of DPP-
6C12 are further investigated by POM texture changes, which
can provide morphological information on the micrometer
length scale depending on phase evolutions. Figure 7 shows the
POM micrographs of DPP-6C12 samples at −30, 20, and 35
°C after cooling from the I state at rate of 1 °C/min. No
birefringence is detected in the PK phase. Only alkyl chains of
DPP-6C12 compound are crystallized in the PK phase, a fact
clearly identified based on the 1D WAXD combined with DSC
and solid-state 13C NMR. However, the synergistic organization
of alkyl chains is not strong enough to generate detectable
birefringences in the POM experiments. By increasing temper-
ature at 1 °C/min, a POM texture of the stable Φ mesophase
was obtained between 7 and 25 °C, as shown in Figure 7b.
Strong birefringence starts to emerge against the dark
background. As mentioned above, DPP meogens in the Φ
mesophase are stacked each other, while the alkyl chains are
more or less in a molten state. From this result, it is realized
that the major cause of strong birefringence in the Φ
mesophase is from the strong molecular packing among DPP
mesogens of DPP-6C12 compound rather than the organ-
ization of alkyl chains. After continuously increasing temper-
ature above 30 °C, the K crystalline phase was obtained, and its
POM texture is represented in Figure 7c. The strong
birefringence in the K phase should be mainly originated
Figure 6. Solid-state 13C NMR spectra of DPP-6C12 during heating
from −30 to 70 °C at a rate of 1.0 °C/min: Bloch decay spectra
between 10 and 45 ppm.
range ordered state below −10 °C, but the individual carbons
in DPP mesogens are quite mobile compared with those of the
well-ordered phases.
After subsequent heating of the DPP-6C12 sample from −30
to 5 °C, the amorphous halos at 2θ = 3.06° (d = 2.88 nm) and
2θ = 21.53° (d = 0.41 nm) shifted to 2θ = 3.11° (d = 2.85 nm)
and 2θ = 21.02° (d = 0.42 nm) (Figure 4), respectively, and the
half width of these scatterings suddenly increased. Additionally,
weak diffractions observed at 2θ = 22.36°, 2θ = 24.29° and 2θ =
25.64° disappeared. Therefore, the diffraction pattern at 5 °C is
very similar to that of the I state. This result indicates that the
endothermic transition at 2 °C is originated to the ordered alkyl
chain melting. Again, this explanation can be supported by
solid-state 13C NMR spectra with CP/MAS/DD (Figure 5a)
and Bloch decay (Figures 5b and 6) at 4 °C. The 70% of CH2
carbon atoms in the relatively long trans zigzag conformations
is suddenly decreased to 30% at 4 °C. Additionally, the carbon
atoms in DPP mesogens are increased by melting alkyl tails of
DPP-6C12. Since the thermal transition below 5 °C is due to
the alkyl tail crystallization and melting of DPP-6C12, this
phase is identified as the plastic crystal (PK) phase.
By annealing the DPP-6C12 sample between 5 and 25 °C,
the metastable I state is gradually transformed to a stable
mesophase. As shown in Figure 4, the amorphous halos at 2θ =
3.11° and 2θ = 21.02° at 5 °C gradually shifted to 2θ = 2.83° (d
= 3.12 nm) and 2θ = 21.5° (d = 0.43 nm), respectively, and the
half width of these scatterings decreased. This gradual phase
transformation from the metastable I state to a stable
mesophase between 5 and 25 °C was also detected by solid-
state 13C NMR spectra. As represented in solid-state 13C NMR
spectra with CP/MAS/DD (Figure 5a) and Bloch decay
(Figures 5b and 6), the mobility of carbon atoms in DPP
mesogens considerably increased and the percentage of the
CH2 long trans zigzag conformations again increased up to
60%. Note that this percentage is a little bit lower than that of
the PK phase. On the basis of the experimental results of 1D
WAXD and solid-state 13C NMR, it is concluded that this phase
is a traditional columnar LC phase (Φ) which is originated by
the nanophase separation between aromatic DPP cores and
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DPP-6C12 at the rate of 0.5 °C/min (represented as red
arrows), the stable Φ mesophase gradually emerged above TPKΦ
and below TΦK. This means that, between TPKΦ and below TΦK
,
the superheated I state is in a metastable state, and the stable Φ
mesophase can be obtained by providing enough time for DPP-
6C12 to find a pathway into the stable Φ mesophase. As
represented before, this explanation is further confirmed by the
isothermal annealing (Figure 3a). When this stable Φ
mesophase is heated up to TΦK, the K crystalline phase is
formed. In this stable K phase, both DPP mesogens and alkyl
tails of DPP-6C12 are closely packed into the 3D long-range
ordered lattice. From the isothermal annealing experiments
(Figure 3b), it is realized that the stable K phase is only formed
by a slow heating from the preordered Φ mesophase, and the
formation of the K phase directly from the I state is forbidden
because the nucleation barrier from the I state to the K phase is
too high to be overcome via thermal annealing. Finally, the K
crystal is melted at TKI and goes back to the I state.
CONCLUSIONS
■
A butterfly-shaped diphenylpyrimidine molecule (abbreviated
as DPP-6C12) was newly designed and successfully synthe-
sized. By the combination of thermal, scattering, spectroscopic,
and microscopic experiments, it was realized that DPP-6C12
formed a plastic crystal phase (PK), a crystal phase (K), and a
liquid crystal phase (Φ). The stable Φ mesophase was formed
either by a subsequent slow heating above the PK phase or by
an isothermal annealing between TΦ and TK. The stable K
phase was only formed by a slow heating from the preordered
Φ mesophase, and the formation of the K phase directly from
the isotropic state (I) is forbidden because the nucleation
barrier from the I state to the K phase is too high to be
overcome via thermal annealing in experimental time scales.
This peculiar monotropic phase transition behaviors of DPP-
6C12 should be originated from the nonplanarity of DPP-6C12
aromatic mesogen. From the 1D WAXD results combined with
those of DSC and solid-state 13C NMR, it was realized that the
driving forces of the PK phase formation are the nanophase
separation between alkyl tails and rigid aromatic cores as well as
the crystallization of alkyl tails. In the PK phase, aromatic cores
did not participate in the crystallization and remained as a
disordered state. On the other hand, the aromatic cores in the
Φ mesophase were ordered to form the self-assembled
nanocolumns by π−π interactions, but the alkyl tails were in
a molten state.
Figure 7. POM image of DPP-6C12 at different temperatures during
heating from −30 to 35 °C at a rate of 1.0 °C/min: −30 °C (a), 20 °C
(b), and 35 °C (c).
from the DPP mesogens, which are closely packed in the 3D
long-range ordered crystalline lattice.
Monotropic Phase Transition Diagram of DPP-6C12.
To understand the peculiar monotropic phase transition
behaviors of DPP-6C12,13,29,30,40 the free energy versus
temperature at constant pressure is schematically illustrated in
Figure 8. As represented as blue arrows in Figure 8, upon
ASSOCIATED CONTENT
* Supporting Information
■
S
General experimental procedures and operating methods of
equipment. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 8. Illustration of the free energy versus temperature at constant
pressure for DPP-6C12. Here, blue and red arrows represent the
cooling and heating tracks.
AUTHOR INFORMATION
Corresponding Authors
*(K.-U.J.) E-mail: kujeong@jbnu.ac.kr.
*(S.-H.H.) E-mail: bach@dankook.ac.kr.
■
cooling DPP-6C12, the I phase transfers to the PK phase which
possesses the lowest free energy phase below TPKI. During the
cooling process from the I state, any phase transformation does
not occur above TPKI. This means that the nucleation barriers
to form any mesophases or crystalline phases are too high so
that no phase transitions are observed even at 0.5 °C/min
cooling. With subsequently heating DPP-6C12 at the rate of 10
°C/min, only TPKI transition is detected by melting alkyl chain
crystals, as show in Figure 2. However, with slowly heating
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work has been supported by the Basic Science Research
Program (2013R1A1A2007238), KIST institutional program
■
F
DOI: 10.1021/cg501697q
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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DOI: 10.1021/cg501697q
Cryst. Growth Des. XXXX, XXX, XXX−XXX