Flexible H-bonded liquid-crystals with wide enantiotropic blue phases

Article abstract of DOI:10.1039/c3cp55150d

Novel cinnamic-acid-derived H-bonded liquid crystals with enantiotropic BP*s were facilely prepared. When simply modifying the donor-acceptor ratio, the proportion of (S)-4-(2-octanyloxy) cinnamic acid to 4-(4-propylcyclohexyl) phenyl isonicotinate, the g

Full text of DOI:10.1039/c3cp55150d

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Flexible H-bonded liquid-crystals with wide
enantiotropic blue phases†
Wan-Li He,*a Mei-Ju Wei,b Huai Yang,ac Zhou Yang,a Hui Caoa and Dong Wanga
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 5622
Novel cinnamic-acid-derived H-bonded liquid crystals with enantiotropic BP*s were facilely prepared.
When simply modifying the donor–acceptor ratio, the proportion of (S)-4-(2-octanyloxy) cinnamic acid to
4-(4-propylcyclohexyl) phenyl isonicotinate, the generated BP temperature ranges were consequentially
extended with the widest range of about 10 1C, which is seldom found in enantiotropic BP materials.
The optimized results of density-functional-theory calculations show that cinnamic-acid-derived
mesogens have a bent shape and thus could be beneficial to stabilize enantiotropic blue phases.
Received 6th December 2013,
Accepted 25th January 2014
DOI: 10.1039/c3cp55150d
www.rsc.org/pccp
Additionally, BPs are quite different from ordinary liquid
crystals that are liquids with anisotropic physical properties,
Introduction
Liquid crystalline blue phases (BPs) as soft matter materials but optically isotropic, that is, they show no birefringence.
have recently attracted growing attention in the fields of Nevertheless, they show brilliant colours due to the selective
optoelectronics and photonics because of their exotic optical reflection of circularly polarized light. This exotic phase usually
properties, such as non-birefringence and the fast Kerr effect. appears in some cholesteric liquid crystals (CLC) with a suffi-
In contrast to other mesophases, BPs remained a relatively ciently short pitch on the scale of several hundred nanometers,
unknown phenomenon during the first 80 years after Reinitzer’s and typically in a temperature range between the cholesteric
discovery of liquid crystals.1 Due to the fact that they appear only phase and the isotropic phase. The variety of the BPs usually
in highly chiral systems and in a very narrow temperature range, depends on the chirality of LCs and temperature, and appears
BPs were often overlooked or considered as pretransitional during cooling from the isotropic to the chiral nematic phase
phenomena, and generally thought to be of only academic three subphases, in an order, BPIII, BPII, and BPI. Wherein,
interest. Their structure remained a mystery for another dec- BPIII is amorphous with a local cubic lattice structure, while
ade, even after Alfred Saupe (in 1969) had proposed a locally BPII and BPI have simple cubic and body-centered cubic
twisted director field with a periodic superstructure.2 Theore- structures, respectively.
tical work over the past 20 years has indicated that the BP is a
From an applications perspective, BPs can be applied as fast
thermodynamically stable phase, and a frustrated structure light modulators or tunable photonic crystals in the fields of
resulting from the competition between chiral forces and optoelectronics and photonics,4 because BPs are mesophases
packing topology.3 In the cubic lattice of blue phases, the basic that are highly fluid self-assembled three-dimensional (3D) cubic
unit is the double-twisted cylinder (DTC) in which the director defect structures. A self-assembled 3D photonic crystal requires a
is parallel to the axis at the centre, and rotates spatially about relatively simple fabrication process with respect to the ones
any radius. Thus, this superstructure of BPs is quite different prepared from physical manufacturing. Several factors, such as
from the single-twisted structure of the cholesteric phase the variation of temperature, the concentration of chiral dopant,
(or chiral nematic) and no twist arrangement in the electric field,5 laser6 or UV irradiation7 and even boundary
nematic phase.
conditions of substrates,8 can be used to control the photonic
bandgap of BPs. Compared to conventional nematic LCDs,
BPLCs are used as optoelectronic displays exhibiting four trans-
formative performances:9 (1) they do not require any surface
alignment treatment on the display panel and BP transmittance
is insensitive to the cell gap in the in-plane-switching electrode
so that the fabrication process is simple, low-cost and desirable
for fabricating large-panel LCDs; (2) the dark state is optically
isotropic so that the BP-LCD is wide viewing-angle and energy-
saving; (3) the response time is in the sub-millisecond range so
a Department of Materials Physics and Chemistry, School of Materials Science and
Engineering, University of Science and Technology Beijing, Beijing, P. R. China.
E-mail: hewanli@mater.ustb.edu.cn; Fax: +86-10-62333759; Tel: +86-10-62333759
b School of Chemistry and Biological Engineering, University of Science and
Technology, Beijing, P. R. China
c Department of Materials Science and Engineering, College of Engineering,
Peking University, Beijing, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cp55150d
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that it enables field-sequential color display without using color
filters replaced by using back RGB light emitting diodes.
Unfortunately, the narrow stable temperature range in
which BPs could exist, usually less than a few degrees, has
been one of the major obstacles to their potential applications.
Many efforts have thus been devoted to stabilize BP structures
over a broader thermal phase range, as well as to shift the range
including the room temperature. One notable way has been the
‘guest component’ strategy, in which a second component is
introduced into the BPLC that collects in the high energy defect
region. By filing these defects and thus reducing the overall free
energy of the phase, the guest component could increase the
blue phase ranges. This stabilization method was pioneered by
Kikuchi et al. who reacted monomers inside the BPLC to form a
mixed polymer–liquid-crystal system.10 Other groups have since
extended this work to use nanoparticles,9e,11 like Au, ZnS, and
BaTiO3 nanoparticles. Another method is the special molecular
design for broadening the BP temperature range. Coles et al.
reported BPI in a eutectic mixture of fluorinated mesogen
dimers with high flexoelectricity,5a in which the temperature
range was 16–60 1C. And some other special molecules like
tolane cyano derivatives,12 chiral dimeric binaphthyl compounds,13
discotic triphenylene derivatives,14 bent-shaped15 and T-shaped
compounds16 were investigated to have a positive effect on extend-
ing the BP temperature ranges.
Fig. 1 Synthetic routes of the H-bonded LC complexes. (a) SOCl2; (b) alkyl-
trans-cyclohexyl phenol, NEt3, THF; (c) K2CO3, (R)-octan-2-yl-4-methyl-
benzenesulfonate, butan-2-one; (d) NaOH, H2O; HCl; (e) THF.
circles are unusual. This interesting finding indicates that a
flexible structure may play a role in stabilizing a frustrated
phase, and also demonstrates that H-bond self-assembly is a
simple and promising way to explore the BP*s.
Experimental
Synthesis
Nevertheless, a more convenient procedure for stabilizing
BPs without a conventional time-consuming and expensive
chemical synthesis or sophisticated device fabrication is the The studied H-bonded LCs were constructed from non-mesogenic
urgent need for applications in fast light modulators or tunable 4-(4-trans-propylcyclohexyl) phenol isonicotinate (PPI) and (S)-4-(2-
photonic crystals. Fortunately, hydrogen-bonded self-assembly octanyloxy) cinnamic acid (SOCA) or (S)-4-(2-octanyloxy) benzoic
has been proven to be one of the most facile approaches to acid (SOBA) according to the method reported before.18 The
prepare materials or even devices.17 Benefitted from the facile- general procedure is as follows: a requisite amount of chiral
ness of self-assembly preparation and the convenience to adjust alkoxy benzoic acids and the achiral acceptor were dissolved in
composition, a wide BP range could be simply achieved in the 1.0 mL of THF and then heated to 60.0 1C for several minutes.
H-bonded complex of chiral fluoro-substituted benzoic acid Then the THF was slowly removed under reduced pressure, the
and pyridine derivatives.18 Guo and co-workers also success- resulting solid complex was dried under vacuum at room tem-
fully made use of H-bonded self-assembly to stabilize blue perature for about 24.0 hours. The PPI and SOCA as well as SOBA
phases in bent-shaped and T-shaped molecules.16a However, were obtained according to the routine shown in Fig. 1, and used
most reported BPs only existed during the cooling process. as the acceptor and the donor, respectively. More details about the
Evidence indicated that such so-called wide BPs could have preparation are provided in the ESI.†
resulted from a possible supercooling effect due to the viscosity
SOCA: yield: 79.5%. Purity: 98.6% by HPLC. FT-IR (KBr cmÀ1):
or other reasons.19 Chojnowska et al. found a slow crystalli- 3250–2632 (acidic –OH stretching), 2915, 2849 (–CH2–, –CH3
zation in a so-called wide polymer-stabilized BP system after stretching), 1718 (CQO stretching), 1631, 1571, 1447 (aromatic
several days.20 Although this result has not been verified by CQC), 1398, 1334, 1312, 1249 (C–O–Ar stretching), 1155, 1048, 839,
other researchers, a phase extended to several tens of degrees in 769, 677, 622, 579, 526. 1H NMR (400 MHz, CDCl3): 0.89–0.92
temperature due to supercooling effects can easily be achieved (t, 3H, –(CH2)5CH3), 1.31–1.40 (m, 11H, –OCH(CH3)CH2(CH2)4CH3),
in the quench experiments on some reported mesophases. 1.57–1.77 (m, 2H, Ar–OCH(CH3)CH2–), 4.41–4.45 (m, 1H,
So the observed temperature range by supercooling could not Ar–OCH(CH3)CH2–), 6.31–6.35 (d, 1H, ethylene), 6.90–6.92
be the real range for stable BPs.21
(d, 2H, ArH), 7.50–7.52 (d, 2H, ArH), 7.75–7.79 (d, 1H, ethylene).
Herein, we report enantiotropic blue phases in a flexible Elemental analysis: calcd for C17H24O3: C 73.88%, H 8.75%;
H-bonded liquid crystal system, as shown in Fig. 1. Additionally, found C 73.70%, H 8.70%.
when changing the donor–acceptor ratio, the resulting blue
SOBA: yield: 85.5%. Purity: 98.0% by HPLC. FT-IR (KBr, cmÀ1):
phase temperature ranges will consequentially be extended with 3100–2666 (acidic –OH stretching), 2930, 2856 (–CH2–, –CH3
the widest range of about 10 1C. Although the BP* range of this stretching), 2666, 2544, 1677 (CQO stretching), 1606, 1513 (aromatic
studied material was not as wide as reported before, to our CQC), 1426, 1257 (C–O–Ar stretching), 1173, 1120, 1031, 936,
knowledge, the BPs observed both in the heating and the cooling 848, 777, 697, 643, 551. 1H NMR (400 MHz, CDCl3, d): 0.89–0.92
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(t, 3H, –(CH2)5CH3), 1.29–1.44 (m, 11H, –OCH(CH3)CH2(CH2)4CH3),
1.59–1.61 (m, H, Ar–OCH(CH3)CH2–), 1.73–1.77 (m, H,
Ar–OCH(CH3)CH2–), 4.44–4.48 (m, 1H, Ar–OCH(CH3)CH2–),
6.89–6.92 (d, 2H, ArH), 8.03–8.05 (d, 2H, ArH). Elemental
analysis: calcd for C15H22O3: C 71.97%, H 8.86%; found C
71.91%, H 8.90%.
The phase transition behaviours of H-bonded LCs
The formation of the H-bonded complexes could be directly
confirmed by IR spectra using a FT-IR spectrometer (Nicolet
510P). Two H-bonded characteristic peaks centered at about
2500 cmÀ1 and 1900 cmÀ1 which were absent in the precursors
SOBA, SOCA and PPI, indicate the existence of a hydrogen bond
(–OHÁ Á ÁN) between the proton donor and the acceptor in self-
assembly complexes. More information about infrared spectra
of complexes is provided in the ESI.†
Each H-bonded complex was thermally stable and exhibited
H-bond induced mesogenic behaviour, which was not observed in
the precursors. The phase transition temperatures of precursors and
the H-bonded complexes were recorded by using the differential
scanning calorimetry (DSC, Mettler DSC822e) at a rate of 5 1C minÀ1
under a dry nitrogen purge, as summarized in Table 1.
In order to understand the effect of the donor–acceptor ratio
on the mesophase, especially on the BP* range of the H-bonded
complexes, a binary phase diagram of SOCA vs. PPI was plotted
as shown in Fig. 2. Besides the appearance of the BP*, smectic A
(SmA), twisted-grain-boundary A (TGBA*) and the chiral nematic
phase (N*) were observed in the H-bonded complexes, and each
mesophase was carefully identified by using the thermal optical
microscopy under different boundary conditions using a polariz-
ing microscope (POM, Olympus BX-51) equipped with a hot
stage calibrated to an accuracy of Æ0.1 1C (Linkam LK-600PM),
with more information provided in the ESI.† Because enthalpy
changes in the phase transitions of SmA-TGBA*, TGBA*-N* and
N*-BP* were too weak to be measured by our DSC, these
transition temperatures thus were recorded using the POM at
a cooling rate of less than 1 1C minÀ1. BP*s were found in a
broad composition range from about 45.0 mol% to 83.5 mol% of
SOCA in the complex and also could be observed during the
heating process of some complexes high in SOCA content. When
the content of SOCA reached about 66.7 mol% (molar ratio of
SOCA/PPI = 2/1), the generated BP* range on the cooling circle
Fig. 2 The binary phase diagrams of SOCA vs. PPI. K: crystalline; SmA:
smectic A; TGBA*: twisted-grain-boundary A; N*: chiral nematic; BP*: blue
phase; M: mesophase; I: isotropic.
was widest extended up to about 10 1C, which is seldom found in
enantiotropic BP materials.
The results imply that the chirality as an indispensable
factor had a considerable effect on the structure of BP*, and a
higher chirality was also beneficial to achieve a stable or wide
BP* in H-bonded LCs as well as in conventional LCs. BP* is a
frustrated phase resulting from the competition between the
chiral twisting force and the packing topology.3b The stability of
the double-twisted structure in the BP* increased upon increasing
the twisting force, in other words, upon increasing the content
of the chiral dopant (SOCA). The domain size and the conforma-
tion of the BP platelets were obviously varied with the change in
the donor–acceptor ratio as shown in Fig. 3, which demonstrated
that H-bond self-assembly is a simple way to generate BP*s.
The induced BP*s were observed in a relatively broad
concentration region of PPI vs. SOCA both during the heating
and the cooling cycles, while monotropic BP*s were found in
the PPI vs. SFBA system,18 whereas no BP* was found in the PPI
vs. SOBA complexes. To analyze this phenomenon, density-
functional-theory (DFT) was applied to calculate the molecular
structure of H-bonded complexes in the minimum free energy
state. DFT is a quantum mechanical modelling method widely used
in materials physics and chemistry to investigate the electronic
structure (principally the ground state) of many-body systems,
Table 1 Phase transition temperature of 1 : 1 molar ratio H-bonded
complexes
Compound
Phase transition behavioura,b cooling/heating (1C)
SOCA
SOBA
PPI
K81.2(71.5)I
K67.2(61.88)I
K118.1(72.9)I
SOCA–PPI
K74.2SmA*115.3TGBA*116.5N*136.5BP*137.5I*
K103.3(45.87)SmA*117.8(0.61)N*138.3(1.95)I*
K42.7SmA*112.2TGBA*113.0N*120.0I
K71.8(47.3)SmA*115.2(2.29)N*122.2(1.167)I
SOBA–PPI
a
K = crystal; SmA* = smectic A; TGBA* = twisted-grain-boundary A; N* =
b
Fig. 3 The POM images of the blue phase in the complexes with the
molar ratio of SOCA vs. PPI (A) 1/1; (B) 2/1 and (C) 3/1.
chiral nematic phase; BP* = blue phase; I = isotropic. Corresponding
enthalpy changes in J gÀ1
.
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Fig. 5 Schematic illustration of the formation of the double-twisted structure
in flexible H-bonded mesogens.
core of SOCA–PPI (23.076 Å) is longer than that of SOBA–PPI
(21.052 Å), which means that the SOCA–PPI molecule could be
more easily affected by the external environment than that of
SOBA–PPI.
Fig. 4 Optimized molecular structure of the studied H-bonded com-
plexes (a) intra-plane bending and (b) inter-plane bending.
BP* is a well-known cubic structure built from hundreds of
double twisted structures (DTC) resulting from highly chiral
twisting force. It could be speculated that it is a smaller
twisting-force needed to make flexible mesogens to twist biaxially,
than to make rigid mesogens in DTC arrangement, as shown in
Fig. 5. The SOCA–PPI mesogens and the SOCA–SOCA dimers
twisted together by 901 along their radii. While on the outermost
circumference of a cylinder, the H-bonded LC molecules were
twisted by 451 with respect to the cylinder axis, from À451 at one
end to +451 at another end. Although both H-bonded complexes
were not built from strong covalent bonds but via weak non-
covalent bonds, the flexible ethylene segment in the SOCA–PPI
unit makes it easier to deform than the SFBA–PPI or SOBA–PPI
unit, which may lead to decrease in interfacial elastic energy and
increase in the structure stability. Additionally, the cinnamic-
acid-derived H-bonded LCs with a relatively longer conjugate
structure could enhance the length-to-width ratio of the complex
(L/W ratio 6.95), therefore could benefit the mesogenic behaviours
of complexes. In a word, the flexible cinnamic-acid-derived
mesogens could effectively enable a stable BP behaviour in
the H-bonded complexes.
in particular atoms, molecules, and the condensed phases. DFT
has also been the dominant method for the quantum mechanical
simulation of periodic systems for the past 30 years.22 It was
reported that B97-D belongs to one of the most accurate general
purpose GGAs, and its performance for non-covalently bonded
systems including many pure van der Waals complexes is excep-
tionally good, reaching on the average accuracy of CCSD-T (a highly
accurate but time-consuming calculation method).23 Herein, the
B97D method was used for the computation of molecular geo-
metries in the GAUSSIAN 09 program. Due to the reason that
calculation of the H-bonded complex is more complicated than
that of covalent compounds, thus three split-polarization basis set
is respectively employed for description of a better structure of
hydrogen bonds, that is, the hydrogen in the H-bond with 6-311++G
(d,2pd), the nitrogen in the H-bond with 6-311+G (2df) and the
other atoms with the 6-311G (d,p) basis set.
It is found from the geometry optimized calculation that the
SOCA–PPI mesogen shows a bent-core shape in the minimum
free energy state presented in Fig. 4. A SOCA–PPI molecule shows
not only an intra-plane bending constitution (about 18.71), but
also gives an inter-plane-bending core of about 51. Whereas
SOBA–PPI and SFBA–PPI18 have only a small intra-plane bending
constitution (about 101). That might be the reason for the SOCA–
Conclusions
PPI mesogens exhibiting an enantiotropic and wide blue phase In conclusion, cinnamic-acid-derived H-bonded liquid crystals
ranges. The finding is consistent with the bent-core BPLC with enantiotropic BP*s were facilely prepared. When changing
reported previously.24 From the perspective of the molecular the donor–acceptor ratio, the generated BP temperature ranges
structure, bent-shape mesogens can be seen as biaxial struc- were consequentially extended with the widest range of about
tures. Many reports have stated that molecular biaxiality plays 10 1C. The structure optimized calculation using DFT shows that
an important role in the formation of the blue phase.25
flexible cinnamic-acid-derived mesogens have a bent shape, which
Another explanation for the enantiotropic phenomenon may is different from the structure of benzoic-acid-derived mesogens.
be the flexible nature of the cinnamic-acid-derived mesogenic This finding indicates that flexible structure of cinnamic-acid-
core. From the result of optimized calculation, it was found that derived mesogen may play a positive role in stabilizing BPs and
the H-bond length of SOCA–PPI (O–HÁ Á ÁN 1.743 Å) is larger thus it could be used to prepare enantiotropic BP materials in the
than that of SOBA–PPI (O–HÁ Á ÁN 1.733Å), and the mesogenic fields of photonic crystals and optoelectronic.
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2010, 81, 041703; (d) L. Wang, W. L. He, Q. Wang, M. N. Yu,
X. Xiao, Y. Zhang, M. Ellahi, D. Y. Zhao, H. Yang and L. Guo,
J. Mater. Chem. C, 2013, 1, 6526.
Acknowledgements
The authors thank College of Chemistry in Beijing Normal
University for performing the geometry optimized calculation in
the GAUSSIAN 09 program. The research was financially sup-
ported by National Natural Science Foundation of China (Grant
no. 61370048, 51173017, 51203011, 51103010), the Fundamental
Research Funds for the Central Universities of China (Grant
no. FRF-TP-12-035A) and the National Natural Science Founda-
tion Youth Fund of China (Grant no. 61007016). Wan Li He, Huai
Yang, Zhou Yang, Hui Cao, and Dong Wang are mainly in charge
of the preparation of BPLC mixtures, analyzing and summarizing
the experimental data as well as discussing and writing the paper.
Meiju Wei is mainly in charge of the computational calculation of
molecular structures of all the H-bonded compounds.
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