The effects of molecular structure and functional group of a rodlike Schiff base mesogen on blue phase stabilization in a chiral system

Article abstract of DOI:10.1039/C8NJ04493G

Two types of racemic rodlike Schiff base mesogens with-C═N-(type I) and-N═C-(type III) linkages were prepared. These mesogens possessed either difluoro substitutions at the inner-core position of the phenyl ring or hydroxy group to form intramolecular hyd

Full text of DOI:10.1039/C8NJ04493G

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The effects of molecular structure and functional
group of a rodlike Schiff base mesogen on blue
phase stabilization in a chiral system†
Cite this:DOI: 10.1039/c8nj04493g
Chiung-Cheng Huang, *a Yu-Chang Huang,a Wei-Cheng Hsieh,a Yen-Jung Chen,a
a
Shi-Kai Jiang,
Bo-Hao Chen,bc I.-Jui Hsu*bd and Jey-Jau Leec
Two types of racemic rodlike Schiff base mesogens with –CQN– (type I) and –NQC– (type III) linkages
were prepared. These mesogens possessed either difluoro substitutions at the inner-core position of the
phenyl ring or hydroxy group to form intramolecular hydrogen bonding with an ester or/and imine
linkage. When the appropriate concentration of chiral additive is doped into them, the incorporation of
two fluoro substituents is more useful for blue phase (BP) stabilization than that of a hydroxy group near
the ester linkage in Schiff base mesogens. BPI and BPII can be identified by reflectance spectra and
polarized optical microscope images. BPII emerges easily on cooling when the appropriate chiral dopant
ISO(6OBA)2 or chiral dopant S811 is doped into the Schiff base mesogen having only a hydroxy group
near the ester linkage. Interestingly, BPI can be observed when 10–15 wt% ISO(6OBA)2 was doped into
the difluoro substituted Schiff base mesogen III during a heating process. The experimental and
molecular modeling results indicate that most of the difluorinated Schiff base mesogens with larger
dipole moments exhibit wider BP ranges than their corresponding non-fluorinated homologues under
the same chirality condition. In addition, wide BPs can be induced for racemic rodlike Schiff base
mesogens I in the chiral system and this is easier than that for racemic rodlike Schiff base mesogens III.
In Schiff base mesogens I, the dipole moment is dominant for BP stabilization. However, the fluorine
substituent effect is the main factor in Schiff base mesogens III.
Received 6th September 2018,
Accepted 17th December 2018
DOI: 10.1039/c8nj04493g
rsc.li/njc
can be classified into BPI (body center cubic), BPII (simple
cubic) and BPIII (amorphous) in accordance with the stacking
1. Introduction
Blue phase liquid crystals (BPLCs) have drawn significant research of DTCs in a cubic lattice.13 However, the defect (disclination) of
interest because they possess excellent physical properties such as the fluid three-dimensional periodic structure in a cubic lattice
great potential in wide viewing angle,1,2 fast response3–6 low energy makes BP unstable and this results in a narrow temperature
consumption in liquid crystal displays,7 self-organized optical range. Thus, the practical applicability of a display device based
isotropy,8,9 a short response time down to the sub-millisecond on BPLCs is limited. Recently, a significant number of researchers
range,10 and tunability of soft photonic devices.11,12 Consequently, have been developing many methodologies to overcome these
they have the potential to develop next-generation display disadvantages in temperature stability. Synthetic chemists can
technology, photonics and electro-optic device applications. design novel BP mesogens with optically pure groups and high
BPs are fluid three-dimensional periodic structures between chirality, such as bent-core,14,15 T-shaped,16–19 U-shaped20 and
an isotropic and a chiral nematic phase. BPLCs are considered (ꢀ)-menthol-based biphenyl molecules, which possess a high
to form a helically double twisted cylinder (DTC) structure and twisting power (HTP) to form a helix.21 Other LC research groups
generally utilize convenient blending techniques. Kikuchi et al.
first introduced polymers into the disclination in the cubic lattice
to broaden the BP range which is more than 60 K.22 Subsequently,
the properties and applications of polymer-stabilized BPLCs have
been systematically investigated and demonstrated.23 In addition,
the disclination in cubic lattice BPCLs can also be stabilized by
nanoparticles.24 Besides polymer and nanoparticle stabilizations,
Yang’s group first developed self-assembled LC complexes
a Department of Chemical Engineering, Tatung University, Taipei 104, Taiwan.
E-mail: chchhuang@ttu.edu.tw; Tel: +886-2-77364681
b Department of Molecular Science and Engineering,
National Taipei University of Technology, Taipei 106, Taiwan
c National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
d Research and Development Center for Smart Textile Technology, National Taipei
University of Technology, Taipei 106, Taiwan. E-mail: ijuihsu@mail.ntut.edu.tw;
Tel: +886-2-27712171 # 2420
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj04493g by hydrogen bonding to induce wide BPLCs (23 K).25 On the
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mesogen possessing ester linkage. We proposed that a larger
super-cooling and viscosity effect from the hydroxy group near
the imine linkage of a Schiff base mesogen could result in a broad
BP temperature range. Nevertheless, the effects of molecular
structure and functional group of the Schiff base mesogen on BP
stabilization are still not established thoroughly. Therefore, in this
work, two homologous series of rodlike Schiff base mesogens
with –CQN– (type I) and –NQC– (type III) linkages were
prepared (Fig. 1 and 2).
To the best of our knowledge, these Schiff base mesogens
that possessed intramolecular hydrogen bonding via hydroxy
groups with ester or imine linkages, and difluoro substitutions
at the inner-core position of the phenyl ring have never been
reported up to now. The mesomorphism and thermal behaviours of
these Schiff base mesogens were investigated by X-ray diffraction
Fig. 1 Chemical structures of chiral dopants S811 and ISO(6OBA)2.
other hand, Yang’s group found that lateral fluoro substituents of (XRD), differential scanning calorimetry (DSC) and the characteristic
the mesogen play crucial roles in broadening the BP temperature texture of polarized optical microscopy (POM). Subsequently, we
range.26 Moreover, bent-core molecules with giant flexoelectricity, doped appropriate concentrations of chiral dopants S811 and
which were doped into chiral nematic LCs to increase the elastic ISO(6OBA)2 into these Schiff base mesogens to investigate the
constant K33 and improve the thermal stability, have been effects of molecular structure and functional group of these rodlike
confirmed.27–31 Furthermore, the photoresponsive bent-core Schiff base mesogens on BP stabilization in a chiral system.
molecule with azo linkage can be photoinduced to fill in the
disclination in 3D cubic lattices and induce wider BP ranges.32–36
Accordingly, these studies proposed that chirality, elasticity, flexo-
2. Experimental
2.1. Spectroscopic analysis
electricity, lateral fluoro-substitution, and molecular biaxiality play
important roles in BP stabilization.16
Imine or Schiff base (azomethine) as a linking group is widely The chemical structures of the target materials were identified
utilized in many types of LC molecular structure. Its derivative, by proton nuclear magnetic resonance (1H NMR) spectroscopy
salicylaldimine[N-(2-hydroxy-4-alkoxybenzylidene) aniline], possesses using a Bruker Avance DRX 500 NMR spectrometer (Bruker Co.,
the ability to coordinate metals to form metallomesogens as a result Karlsruhe, Germany). The purity of the final compounds was
of the presence of the hydroxy group.37 Recently, Schiff bases, such assessed by thin layer chromatography (TLC), and further
as ferrocene-based Schiff base metallomesogen,38 chiral three-ring confirmed by elemental analysis using a Heraeus Vario EL III
calamitic Schiff base mesogens,39 unsymmetrical and symmetric analyzer (Elementar Analysenyteme GmbH Co., Hanau, Germany).
Schiff base dimers40,41 and photoresponsive bent-core molecules,33 Variable-temperature XRD experiments were performed at the
have also been widely applied in BPLCs. Some of these BPLCs wiggler beam-line BL17A at the National Synchrotron Radiation
exhibit wide BP ranges and are stable at room temperature. How- Research Center (NSRRC), Taiwan. The experimental wave-
ever, only a few simple rodlike Schiff base molecules were utilized to length is 1.3214 Å, and the sample was packed into a 0.5 mm
induce wide BPs. Recently, we induced an unprecedented wide BP capillary. A heat gun was equipped at this beamline and the
temperature range (430 K) by doping the chiral dopant S811, at ca. temperature controller was programmable by a PC with a
35 wt%, into the racemic rodlike Schiff base mesogen with alkynyl PID feedback system. The experimental XRD pattern was
linkage.42,43 The BP temperature range can also be extended by indexed by DICVOL program to obtain the crystal system and
adding 10 wt% ISO(6OBA)2 into the racemic rodlike Schiff base cell constants.44–46
Fig. 2 Molecular structures of Schiff base mesogens.
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2.2. Liquid-crystal and physical properties
Synthesis of A. To a solution of 4-(hexyloxy)-2-hydroxybenzoic
acid or 2,3-difluoro-4-(hexyloxy)benzoic acid (15.2 mmol) and
4-hydroxybenzaldehyde or 2,4-dihydroxybenzaldehyde (10.1 mmol)
in dichloromethane (10 ml), N,N0-dicyclohexylcarbodiimide
(30.3 mmol) and 4-dimethylaminopyridine (1.1 mmol) were added
in an ice bath. The solution mixture was stirred at room
temperature for 2 days. Precipitates were removed by filtration.
The solvent was removed and the obtained residue was purified
by column chromatography over silica gel using EtOAc/hexane
(1 : 4) as the eluent. The crude product was further purified
by repeated recrystallization from methanol to give a white
solid.
Synthesis of Schiff base mesogens (type I). A mixture of A
(9.0 mmol) and 4-(1-methylheptoxy)aniline (9.0 mmol) in ethanol
(10 mL) was refluxed for 4 h. After cooling to room temperature, a
yellow precipitate was obtained and collected by filtration. The
crude product was further purified by repeated recrystallization
The initial phase sequence and corresponding transition tem-
perature of the compounds were determined by POM. Meso-
phases were principally identified by the microscopic texture of
the materials sandwiched between two glass plates under a
crossed polarizing microscope using a Nikon Microphoto-FXA
optical microscope in conjunction with a hot stage controlled
by a control processor. The phase transition temperatures and
corresponding phase transition enthalpies of compounds were
determined by DSC using a Perkin Elmer Diamond calorimeter
under running rates of 3 1C minꢀ1. The Bragg reflection spectra
of BPs were recorded with a USB2000 spectrometer in reflection
mode, and the temperature of the samples was controlled
accurately by the hot stage.
2.3. Preparation of materials
All of the starting materials were purchased from Sigma-Aldrich from ethanol to give a yellow powder in 50–60% yield.
with purity greater than 99%. Thin layer chromatography was Synthesis of compound B. It was synthesized by using the
performed using TLC sheets coated with silica; spots were detected same synthetic method as that described for compound A. The
by UV irradiation. Silica gel (Merck silica gel 60, 63-200 mesh) was crude product was further purified by repeated recrystallization
used for column chromatography. The organic solvents were dried from methanol to give a white solid in 55–60% yield.
and distilled before use. 4-(Hexyloxy)-2-hydroxybenzoic acid, 2,3-
Synthesis of compound C. To a solution of compound B
difluoro-4-(hexyloxy)benzoic acid and 4-(hexyloxy)benzoic acid were (5.5 mmol) in dry ethyl acetate (10 mL), Pd/C (0.1 g, 1.1 mmol)
synthesized according to the literature.47,48 Detailed synthetic pro- was added and stirred under a hydrogen atmosphere (balloon)
cedures for the intermediates and target materials are described for 24 h (monitored by TLC). The reaction mixture was then
below. In addition, the molecular structures of the target products filtered through a celite bed. The filtrate was concentrated
and the intermediates were confirmed by spectroscopic analysis and under reduced pressure to give a yellow or white solid in
microanalytical data, as described in the ESI.†
90–95% yield.
Scheme 1 Synthetic route of Schiff base mesogens of type I.
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Synthesis of compound D. A solution of diisopropyl azodi- set at the center of the phenyl ring, and the x-axis is parallel with
carboxylate (7.2 mmol) and 4-hydroxybenzaldehyde or 2,4-di- the longitudinal direction and z-axis is perpendicular to the
hydroxybenzaldehyde (7.2 mmol) in 10 mL of anhydrous THF phenyl plane. The molecular structures, dipole moments, and
was added drop by drop to a solution of triphenylphosphine isosurface plots of the molecular orbitals were generated using
(7.2 mmol) and 2-octanol (7.2 mmol) in 15 mL of anhydrous GaussView5.0.52
THF at room temperature with stirring for 24 h. After a work-up
procedure, this product was isolated by column chromatography
over silica gel using toluene as eluent. A colorless oil was obtained
in 50–60% yield.
Synthesis of Schiff base mesogens (type III). A mixture of C
(9.0 mmol) and D (9.0 mmol) in ethanol (10 mL) was refluxed
3. Results and discussion
3.1. Synthesis and molecular structure characterization
of rodlike Schiff base mesogen
for 4 h. After cooling to room temperature, a yellow precipitate The synthetic steps of ten rodlike Schiff base mesogens are
was obtained and collected by filtration. The crude product was shown in Schemes 1 and 2. At first, four compounds A were
further purified by repeated recrystallization from ethanol to prepared by Steglich esterification reaction of 4-hydroxybenzalde-
give a yellow or white powder in 50–60% yield.
hyde or 2,4-dihydroxybenzaldehyde with 4-(hexyloxy)-2-hydroxy-
Theoretical calculation. All density functional theory (DFT) benzoic acid or 2,3-difluoro-4-(hexyloxy)benzoic acid in the presence
calculations were performed in the Gaussian09 package.49 The of dicyclohexylcarbodiimide (DCC) as a coupling reagent and
calculation strategy is based on our previous report.42,43 The 4-dimethylaminopyridine (DMAP) as a catalyst. Similarly, three
coordinates used for geometry optimization of all Schiff base compounds B could be obtained by Steglich esterification
compounds were initially built in the Avogadro program50 then condition. Subsequently, these nitro materials were hydrogenated
optimized by the long-range corrected hybrid functional CAM- using hydrogen gas and Pd/C as a catalyst to obtain compound C
B3LYP51 in the G09 program. The basis set of 6-311G(d,p) was with an amino group. Compounds D could be synthesized by
used to calculate the dipole moment and polarizability. In order Mitzunobu reaction. In the final step, compounds A were
to compare the dipole moment of all molecules, the origin was condensed with 4-(1-methylheptoxy)aniline in ethanol to obtain
Scheme 2 Synthetic route of Schiff base mesogens of type III.
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the targeted racemic rodlike Schiff base mesogens I with the characteristic transition of nematic to smectic C phase. At
–CQN– linkage. Racemic rodlike Schiff base mesogens III with room temperature, the schlieren texture of four brush singularities
–NQC– linkage could be prepared from anilines C and aldehyde for the SmC phase can be observed. In addition, the incorporation
1
D by the same method. In the H NMR spectra of Schiff base of difluoro substitutions into the inner-core position of the phenyl
mesogens OH-EI6-OH with two hydroxy groups, the chemical ring or intramolecular hydrogen bonding via the hydroxy group
shift of the –OH hydrogen adjacent to the ester group appears with the ester results in the clearing point of the Schiff base
at ca. 10.5 ppm and one characteristic peak in the range of mesogen declining. Furthermore, the Schiff base mesogen that
13–14 ppm indicates another proton that forms the intramolecular possessed a hydroxy group near the ester linkage to form intra-
hydrogen bond with the imine linkage (Fig. S1a, ESI†). Similar molecular hydrogen bonding exhibits a lower clearing point in
chemical shifts can be observed in the 1H NMR spectrum of comparison with its homologue that possessed intramolecular
OH-EIII6-OH (Fig. S1b, ESI†). In addition, the existence of the hydrogen bonding next to the imine linkage. On the other hand,
intramolecular hydrogen bonding between the hydroxy group the incorporation of intramolecular hydrogen bonding between
and ester linkage in these Schiff base mesogens can be con- the hydroxy group and ester linkage in the Schiff base mesogen
firmed by CQO with a lower frequency (ca. 1680 cmꢀ1) than the could reduce the molecular interaction and stacking, which may
general ester group in the FT-IR spectra (Fig. S2, ESI†).
result in the temperature range of the nematic phase increasing
and that of the smectic phase decreasing for LC properties.
However, two fluoro substituents protrude from the side of the
mesogenic core to shorten the temperature range of the nematic
phase and widen that of the smectic phase. Notably, only Schiff
base mesogen H-EIII6-OH shows enantiotropic nematic phases
and no smectic mesomorphism, and is similar to biphenyl
mesogens CnOBiPhI-OH possessing a hydroxy group near the
ester linkage.53
3.2. The phase transition behaviors of ten Schiff base mesogens
The phase transition temperatures and enthalpies of ten Schiff
base mesogens shown in Table 1 were determined by DSC
(Fig. S3, ESI†), POM (Fig. 3 and Fig. S4–S7, ESI†) and XRD
(Fig. S8, ESI†). Most of these rodlike Schiff base mesogens
exhibit nematic and smectic mesophoric behaviors. Notably,
Schiff base mesogen OH-EI6-OH possessing two hydroxy groups
exhibits a wider interval of the nematic phase (83.6 K).
3.3. Effect of the hydroxy group position and fluoro
substituents on BP ranges
Fig. 3 shows its POM textures on cooling. The schlieren
texture that can be observed at 103.3 1C indicates the appearance
of a nematic phase. On further cooling to near room temperature In accordance with previous reports,42,43 BPs could emerge
(33.6 1C), the striated texture or transition bar appears, indicating after 20.0 wt% chiral dopants S811 or 5 wt% ISO(6OBA)2 are
Table 1 Phase transition temperature and corresponding transition enthalpies of Schiff base mesogensa
Phase sequenceb (1C, DH/kJ molꢀ1
)
Compounds
Cooling
Heating
H-EI6-OH
H-EI6-F
OH-EI6-OH
OH-EI6-F
H-EIII6
H-EIII6-OH
H-EIII6-F
OH-EIII6
Iso 109.9(2.40) N 34.8(0.83) SmC o 20 Cr
Iso 94.8(0.24) N 76.4(0.62) SmC o 20 Cr
Iso 111.6(0.56) N 33.0(0.36) SmC o 20 Cr
Iso 124.6(0.25) N 96.1(2.42) SmC o 20 Cr
Iso 126.1(0.59) N 58.5(1.15) SmC o 20 Cr
Iso 96.0(0.74) N o 20 Cr
Cr 85.0(36.76) N 112.2(0.58) Iso
Cr 30.7(11.26) SmC 77.5(0.37) N 109.7(0.17) Iso
Cr 41.7(8.44) N 112.4(0.42) Iso
Cr 65.8(22.36) SmC 95.3(0.77) N 121.8(0.53) Iso
Cr 75.2(33.69) N 117.67(0.82) Iso
Cr 42.71(21.35) N 97.8(0.50) Iso
Iso 125.6(0.72) N 72.4(0.80) SmC o 20 Cr
Iso 154.1(1.10) N 128.9(0.40) SmC o 20 Cr
Iso 104.6(0.43) N 73.3(0.24) SmC o 20 Cr
Iso 141.0(1.20) N 120.0(1.32) SmC 64.6(35.9) Cr
Cr 69.3(22.90) N 124.0(0.70) Iso
Cr 65.8(26.63) SmC 129.7(0.48) N 154.7(1.14) Iso
Cr o 20 SmC 66.7(0.30) N 103.9(0.59) Iso
Cr 74.0(37.30) SmC 120.5(1.44) N 141.5(1.18) Iso
OH-EIII6-OH
OH-EIII6-F
a
b
Peak temperature in the DSC profiles obtained during the second heating and cooling cycles at a rate of 3 1C minꢀ1
.
N = nematic; SmC = smectic C;
Cr: crystal.
Fig. 3 Microphotographs for compound OH-EI6-OH: (a) N texture at 103.3 1C; (b) N–SmC texture at 33.6 1C; (c) SmC texture at 30.0 1C.
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added into the racemic rodlike Schiff base mesogens. And, we
Fig. 4 shows the phase diagrams of rodlike Schiff base
have demonstrated that the stabilization of blue phases for mesogens I doped with different concentrations (20–40 wt%)
rodlike Schiff base mesogens in binary mixture systems is not of chiral additive S811. With a hydroxy group at the inner-core
correlated with HTP. Moreover, we also found that the hydroxy phenyl ring to form intramolecular hydrogen bonding with the
group in the molecular structures of rodlike Schiff bases ester linkage, Schiff base mesogens H-EI6-OH and OH-EI6-OH
dominated BP stabilization. As most of these rodlike Schiff exhibited a lower BP emerging temperature and narrower BP
base mesogens possess a hydroxy group, it is necessary to figure range in comparison with their homologues H-EI6 and OH-EI6 under
out the effect of the hydroxy group position of rodlike Schiff the same chirality condition. However, with the incorporation of two
base mesogens on BP ranges in chiral systems. The phase fluoro substituents into the molecular structure, Schiff base meso-
transition temperatures of the binary mixtures were identified gens H-EI6-F and OH-EI6-F exhibit a wide BP range (ca. 17–18 K)
by POM during the cooling process at 0.2 1Cꢁminꢀ1
.
in chiral systems with S811 even though their BP emerging
Fig. 4 Phase diagrams of (a) H-EI6-OH; (b) H-EI6-F; (c) OH-EI6-OH; (d) OH-EI6-F when doped with different concentrations of chiral dopant S811 at a
cooling rate of 0.2 1C minꢀ1. Iso, BP, N*, SmX* and Cr indicate isotropic phase, blue phase, cholesteric phase, smectic phase and crystal phase,
respectively.
Table 2 Phase transitions in binary mixture of Schiff base mesogens of type I doped with different concentrations of chiral dopant ISO(6OBA)2 at a
cooling rate of 0.2 1C minꢀ1 a
Compound wt% Phase behavior
DBPcooling (K) Compound wt% Phase behavior
DBPcooling (K)
o 27:0
Ð
o 27:0
98:4
ꢂ Ð Iso
o 27:0
Ð
o 27:0
107:8
ꢂ Ð Iso
H-EI6-OH
H-EI6-F
a
5
10
15
5
0.0
5.5
OH-EI6-OH
5
10
15
5
0.0
Cr
Cr
Cr
Cr
Cr
Cr
N
N
N
N
N
N
Cr
Cr
Cr
Cr
Cr
Cr
N
N
N
N
N
N
98:6
107:3
o 27:0
Ð
o 27:0
87:6
ꢂ Ð BP
91:3
Ð
91:1
o 27:0
Ð
o 27:0
88:5
ꢂ Ð BP
95:2
Ð
95:0
9.2
0.0
0.0
0.0
0.0
Iso
Iso
Iso
85:6
85:8
o 27:0
Ð
o 27:0
77:4
ꢂ Ð BP
79:4
Ð
83:2
o 27:0
Ð
o 27:0
70:2
ꢂ Ð Iso
6.7
76:5
81:6
o 27:0
Ð
o 27:0
96:5
ꢂ Ð Iso
o 40:0
Ð
o 40:0
115:8
ꢂ Ð Iso
0.0
OH-EI6-F
96:1
115:4
o 27:0
Ð
o 27:0
63:7
ꢂ Ð BP
83:1
Ð
77:2
84:3
Ð
75:5
34:6
Ð
34:6
111:6
ꢂ Ð Iso
10
15
21.8
21.1
10
15
Iso
Iso
55:4
111:4
o 27:0
Ð
o 27:0
62:8
ꢂ Ð BP
32:0
Ð
32:0
105:6
ꢂ Ð Iso
54:4
105:0
Iso = isotropic; BP = blue phase; N* = chiral nematic; Cr = crystal phase.
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temperatures increased. In addition, Table 2 displays the phase composed of difluoro-substituted Schiff base mesogen OH-EI6-F.
transition temperature and BP range for Schiff base mesogens I This phenomenon is similar to the binary mixture containing
doped with 5–15 wt% chiral dopant ISO(6OBA)2. In the case of a Schiff base mesogen OH-EI6 under the same chirality condition.
binary mixture consisting of 90 wt% Schiff base mesogen H-EI6-F However, a BP emerges again (9.2 K) in the case of Schiff base
and 10 wt% ISO(6OBA)2, the BP range can be increased to more mesogen OH-EI6-OH possessing two intramolecular hydrogen
than 20 K as well as its BP emerging temperature being lower than bonds by two hydroxy groups with the ester and imine linkages.
that of the binary mixture consisting of no fluoro-substituted
Fig. 5 shows the phase diagrams of rodlike Schiff base
H-EI6. However, no BP can be observed in the binary mixture mesogens III mixed with 20–40 wt% concentrations of chiral
Fig. 5 Phase diagrams of (a) H-EIII6; (b) H-EIII6-OH; (c) H-EIII6-F; (d) OH-EIII6; (e) OH-EIII6-OH; (f) OH-EIII6-F when doped with different
concentrations of chiral dopant S811 at a cooling rate of 0.2 1C minꢀ1. Iso, BP, N*, SmX* and Cr indicate isotropic phase, blue phase, cholesteric
phase, smectic phase and crystal phase, respectively.
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Table 3 Phase transitions in a binary mixture of Schiff base mesogens of type III doped with different concentrations of chiral dopant ISO(6OBA)2 at a
cooling rate of 0.2 1C minꢀ1 a
Compound wt% Phase behavior
DBPcooling (K) Compound wt% Phase behavior
DBPcooling (K)
o 27:0
Ð
o 27:0
116:5
ꢂ Ð Iso
o 27:0
Ð
o 27:0
112:2
SmXꢂ Ð
143:8
ꢂ Ð Iso
H-EIII6
H-EIII6-OH
H-EIII6-F
a
5
10
15
5
0.0
0.0
OH-EIII6
5
10
15
5
0.0
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
N
N
N
N
N
N
N
N
N
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
N
N
N
114:4
114:5
145:1
o 27:0
Ð
o 27:0
104:9
ꢂ Ð Iso
o 27:0
Ð
o 27:0
103:7
SmXꢂ Ð
141:5
ꢂ Ð Iso
135:5
0.0
0.0
5.4
0
103:3
102:5
140:1
38:1
Ð
38:1
95:0
ꢂ Ð Iso
o 27:0
Ð
o 27:0
98:6
SmXꢂ Ð
0.0
ꢂ Ð Iso
99:5
Ð Iso
93:2
97:7
135:1
o 27:0
Ð
o 27:0
81:0
ꢂ Ð Iso
o 27:0
Ð
o 27:0
95:6
0.0
OH-EIII6-OH
OH-EIII6-F
N
N
N
ꢂ Ð BP
82:1
93:9
99:3
o 27:0
Ð
o 27:0
75:7
ꢂ Ð Iso
o 27:0
Ð
o 27:0
81:0
10
15
5
0.0
10
15
5
ꢂ Ð Iso
75:7
80:6
o 27:0
Ð
o 27:0
53:9
ꢂ Ð Iso
o 27:0
Ð
o 27:0
59:5
0.0
0.0
0.0
14.2
15.5
ꢂ Ð Iso
50:9
55:0
o 27:0
Ð
o 27:0
105:0
ꢂ Ð Iso
62:5
Ð
62:5
91:7
126:0
ꢂ Ð Iso
0.0
SmXꢂ Ð
SmXꢂ Ð
SmXꢂ Ð
N
N
N
105:7
91:1
128:5
o 27:0
Ð
o 27:0
78:5
ꢂ Ð BP
89:4
Ð
89:5
91:0
Ð
85:0
62:5
Ð
62:2
93:1
118:0
ꢂ Ð BP
122:5
Ð
124:7
110:0
Ð
107:5
10
15
14.2
21.8
10
15
Iso
Iso
Iso
Iso
75:3
92:5
110:5
o 27:0
Ð
o 27:0
81:5
ꢂ Ð BP
61:3
Ð
61:3
89:0
101:3
ꢂ Ð BP
63:2
88:2
92:0
Iso = isotropic; BP = blue phase; N* = chiral nematic; SmX* = unidentified chiral smectic phase; Cr = crystal phase.
dopant S811 and Table 3 displays their phase transition tempera- ester linkage. In addition, a similar selective reflection peak of
tures and BP ranges with 5–15 wt% concentrations of chiral dopant BPII can also be observed during cooling and heating processes
ISO(6OBA)2. The results show that the BP emerging temperatures in when difluoro substituted Schiff base mesogen H-EI6-F is doped
these binary mixtures are higher than those containing Schiff base with 30 wt% S811 (Fig. S9 and S10, ESI†). In the case of Schiff
mesogens I. Schiff base mesogen H-EIII6 possessing no hydroxy base mesogen OH-EI6-F being doped with 30 wt% S811, only a
group exhibits a BP range of approximately 15 K in the binary single reflection band around 450 nm shown in Fig. 7 can be
mixture where chiral dopant S811 is increased to 40 wt%. Notably, observed at 75 1C, indicating that BPII with the (100) direction
Schiff base mesogen OH-EIII6 cannot induce a BP under any exists in the higher BP temperature range. On further cooling to
chirality condition even though its molecular structure is similar 70 1C, another broad reflection band appeared at approximately
to Schiff base mesogen OH-EI6, possessing intramolecular hydro- 510 nm and it exhibits an obvious red shift from 510 nm to
gen bonding between the hydroxy group and the imine linkage. In 655 nm during cooling, indicating the characteristics of the BPI
the case of the binary mixture with chiral dopant ISO(6OBA)2, a BP phase caused by the multi-lattice plane orientations of the
can be observed only in the binary mixture composed of Schiff different platelet domains. The two reflection bands are asso-
base mesogens H-EIII6-F, OH-EIII6-OH or OH-EIII6-F having two ciated with the (200) and (110) directions of the cubic lattice
hydroxy or fluoro substituents. The widest BP range that can be vectors for nanostructured BPI.13 Therefore, the pictures of POM
induced is more than 20 K when 15 wt% ISO(6OBA)2 is added into shown in the insets of Fig. 7b gradually change from a blue
Schiff base mesogen H-EIII6-F. And the doping concentration of platelet texture to a grazed colorful platelet texture during the
chiral dopant ISO(6OBA)2 that can induce a wider BP range cooling process. Notably, in the case of Schiff base mesogen
(420 K) for Schiff base mesogen H-EIII6-F is higher than that OH-EI6-OH being doped with 25 wt% S811, two selective reflection
for Schiff base mesogen H-EI6-F (10 wt% ISO(6OBA)2).
peaks appear as BPI characteristics in the higher temperature
The study on the phase transition between BPI and BPII with range (Fig. S11, ESI†). On further cooling to 77 1C, broader
reflection measurements. To precisely identify the boundaries multiple reflection peaks can be observed possibly due to the
of cubic BPs, the temperature dependence of the Bragg reflection multi-lattice plane orientations of the different platelet domains
spectrum for these BP binary mixtures was examined. Fig. 6–9 in the lower temperature range.54–56 As for Schiff base mesogens
and Fig. S9–S27 (ESI†) display the typical temperature depen- I being doped with chiral additive ISO(6OBA)2, only BPII is
dence of the Bragg reflection wavelength for two types of Schiff present in the binary mixture consisting of Schiff base meso-
base mesogens being doped with appropriate concentrations of gens H-EI6-OH, OH-EI6-OH and H-EI6-F both on cooling and
chiral dopants S811 or ISO(6OBA)2 during cooling (0.2 1C minꢀ1). heating (Fig. S12–S16, ESI†).
According to our previous report,42,43 BPI and BPII can be
As for Schiff base mesogens III, Schiff base mesogens
observed when the binary mixture is composed of 35 wt% S811 H-EIII6-OH and OH-EIII6-OH possessing a hydroxy group near
and 65 wt% Schiff base mesogen OH-EI6 possessing a hydroxy the ester linkage exhibit BPII on cooling after an appropriate
group near the imine linkage. However, Fig. 6a shows only a concentration of S811 is added into them (Fig. S18 and S19,
single reflection band around 420 nm during cooling, indicating ESI†). Similar characteristics of BPII can be observed in the case
only BPII is present when chiral dopant S811 is doped into Schiff of difluoro substituted Schiff base mesogen OH-EIII6-F both on
base mesogen H-EI6-OH possessing a hydroxy group near the cooling and heating (Fig. S20 and S21, ESI†). However, in the
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Fig. 6 (a) Typical shapes of the reflection spectra. (b) The temperature dependence of the Bragg reflection wavelength for the blending mixture
consisting of H-EI6-OH + 40% S811 at a cooling rate of 0.2 1C minꢀ1. Insets show the POM images of BPs at different temperatures.
Fig. 7 (a) Typical shapes of the reflection spectra. (b) The temperature dependence of the Bragg reflection wavelength for the blending mixture
consisting of OH-EI6-F + 30% S811 at a cooling rate of 0.2 1C minꢀ1. Insets show the POM images of BPs at different temperatures.
case of difluoro substituted Schiff base mesogen H-EIII6-F being cases of the difluoro substituted Schiff base mesogens OH-EIII6-F
doped with 35% S811, Fig. 8b obviously shows a discontinuous and H-EIII6-F.
jump at around 63 1C and a selective reflection peak exhibits an
Theoretical simulation. According to our previous investiga-
apparent red shift from 460 nm to 510 nm in the temperature tions for the factors of BP stabilization, the range-corrected
dependence on cooling, indicating a typical transition between functional CAM-B3LYP51 was applied to analyze the structural
BPI and BPII. In contrast to OH-EI6-F, the BPI of Schiff base variations of Schiff base mesogens and obtain the stable molecular
mesogen H-EIII6-F shows that only one reflection band at structure, dipole moment, polarizability and biaxiality. The
approximately 510 nm is associated with the (200) direction.13 optimized molecular structures together with the vector of
However, one reflection band with another direction (110) of dipole moments are shown in Fig. 10. The correlations of BP
BPI cannot be observed possibly due to the weaker intensity of stabilization with the differences of biaxiality, modulus of
the grazed iridescent platelet for the BP domain of Schiff base dipole moments and polarizability in the optimized Schiff base
mesogen H-EIII6-F.54 When Schiff base mesogens III are doped mesogens are shown in Tables 4 and 5. The biaxiality (W1/W2) is
with chiral dopant ISO(6OBA)2, BPII can be observed only in the defined as the ratio of two distinguishable short axes of the
cases of H-EIII6-F, OH-EIII6-OH and OH-EIII6-F on cooling. molecule. W1 is the width along the short axis parallel to the
Interestingly, Fig. 9 and Fig. S26 (ESI†) exhibit two reflection middle phenyl ring and W2 is the width along the short axis
bands as BPI characteristics can be observed on heating in the normal to the middle phenyl ring. In addition, the detailed
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Fig. 8 (a) Typical shapes of the reflection spectra. (b) The temperature dependence of the Bragg reflection wavelength for the blending mixture
consisting of H-EIII6-F + 35% S811 at a cooling rate of 0.2 1C minꢀ1. Insets show the POM images of BPs at different temperatures.
Fig. 9 (a) A typical shape of the reflection spectrum. (b) The temperature dependence of the Bragg reflection wavelength for the blending mixture
consisting of OH-EIII6-F + 10% ISO(6OBA)2 at a heating rate of 0.2 1C minꢀ1. Insets show the POM images of BPs at different temperatures.
information of each component of dipole moment, polarizability, condition. Similar trends can be observed in the binary mixtures
and the energy of the HOMO and LUMO are listed in Table S1 composed of two fluoro substituted Schiff base mesogens III
(ESI†). To facilitate the comparisons with those factors of BP possessing no hydroxy group near the imine linkage. With a
stabilization based on structural similarity, the molecular hydroxy group near the ester linkage, the dipole moment of Schiff
structures of Schiff base mesogens H-EI6 and OH-EI6 that have base mesogen H-EI6-OH (2.80 D) is smaller than those of Schiff
been prepared previously were also calculated and their factors base mesogens H-EI6 (2.95 D) and H-EI6-F (4.58 D). Subsequently,
are also listed in Tables 4 and 5, respectively.
Schiff base mesogen H-EI6-OH exhibits a narrower BP range than
Table 4 shows the correlations of BP range with the differences H-EI6 and H-EI6-F. Notably, when Schiff base mesogen H-EIII6 is
of biaxiality, modulus of dipole moments and polarizability for doped with 30 wt% S811, it exhibits a BP range (9.4 K) due to its
optimized Schiff base mesogens possessing no hydroxy group near large biaxiality (1.45) even though it possesses a small dipole
the imine linkage. In comparison with Schiff base mesogen H-EI6, moment (0.64 D). However, the dipole moment and polarizability
two fluoro substituted Schiff base mesogen H-EI6-F possesses a of H-EIII6 are smaller than those of H-EIII6-F, so that a BP cannot
larger dipole moment and polarizability due to the incorporation be induced in the binary mixture with 10 wt% ISO(6OBA)2. In
of two fluorine atoms. Therefore, H-EI6-F in the binary mixture addition, the dipole moment and polarizability of Schiff base
system containing 30 wt% S811 or 10 wt% ISO(6OBA)2 shows a mesogen H-EIII6-OH are larger than those of its homologues
wider BP range than that of H-EI6 under the same chirality H-EIII6 and H-EIII6-F. Nevertheless, when H-EIII6-OH is doped with
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Fig. 10 The optimized molecular structures of Schiff base mesogens (a) H-EI6; (b) H-EI6-F; (c) H-EI6-OH; (d) OH-EI6; (e) OH-EI6-F; (f) OH-EI6-OH;
(g) H-EIII6; (h) H-EIII6-F; (i) H-EIII6-OH; (j) OH-EIII6; (k) OH-EIII6-F; (l) OH-EIII6-OH together with their each corresponding molecular dimensions and
dipole moment indicated by the blue arrow.
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Table 4 The correlations of BP range with the differences of biaxiality, modulus of dipole moments and polarizability for optimized Schiff base
mesogens possessing no hydroxy group near the imine linkage
Compound
H-EI6
2.95
445.84
4.7
H-EI6-F
H-EI6-OH
H-EIII6
H-EIII6-F
H-EIII6-OH
Dipole moment (Debye)
Polarizability
4.58
448.08
5.0
2.80
450.73
4.7
0.64
444.02
4.3
1.57
445.19
4.5
1.90
449.49
4.8
a
W1b (Å)
W2 (Å)
6.3
6.2
6.2
6.3
6.4
6.5
Biaxiality W1/W2
0.75
35.1
12.1
7.1
0.81
35.2
17.0
21.8
0.77
35.1
3.5
1.45
35.1
9.4
0.70
35.0
10.6
14.2
0.74
34.8
3.4
Lc (Å)
The BP range (K) with 30 wt% S811
The BP range (K) with 10 wt% ISO(6OBA)2
5.5
0.0
0.0
a
b
c
W1: width along the short axis parallel to the middle phenyl ring. W2: width along the short axis normal to the middle phenyl ring. L: length
along the long axis.
Table 5 The correlations of BP range with the differences of biaxiality, modulus of dipole moments and polarizability for optimized Schiff base
mesogens possessing a hydroxy group near the imine linkage
Compound
OH-EI6
OH-EI6-F
OH-EI6-OH
OH-EIII6
OH-EIII6-F
OH-EIII6-OH
Dipole moment (Debye)
Polarizability
2.67
465.55
4.8
4.69
453.81
4.8
3.83
457.82
4.8
2.48
453.06
4.6
2.71
454.44
4.5
0.49
458.46
4.6
a
W1b (Å)
W2 (Å)
6.6
6.5
6.3
6.6
6.5
6.5
Biaxiality W1/W2
0.73
35.0
7.4
0.74
35.0
18.3
0.0
0.76
35.0
4.3
0.70
34.8
0.0
1.45
35.0
5.6
0.70
34.8
4.9
Lc (Å)
The BP range (K) with 30 wt% S811
The BP range (K) with 10 wt% ISO(6OBA)2
0.0
9.2
0.0
14.2
0.0
a
b
c
W1: width along the short axis parallel to the middle phenyl ring. W2: width along the short axis normal to the middle phenyl ring. L: length
along the long axis.
30 wt% S811, it exhibits a narrow BP range (3.4 K). And no BP can be
induced when H-EIII6-OH is doped with 10 wt% ISO(6OBA)2.
4. Conclusion
In conclusion, two series of racemic rodlike Schiff base meso-
gens that possess –CQN– (type I) and –NQC– (type III) linkages
were synthesized, respectively. Their mesophases were con-
firmed by variable-temperature XRD, DSC and the character-
istic texture of POM. Subsequently, the effects of intramolecular
hydrogen bonding via the hydroxy group with ester or Schiff
base linkages, and difluoro substitutions at the inner-core
position of the phenyl ring on BP stabilization were investi-
gated when they were blended with chiral dopants S811 or
ISO(6OBA)2. The incorporation of difluoro substitutions into
the inner-core position of the phenyl ring or intramolecular
hydrogen bonding via the hydroxy group with the ester in Schiff
base mesogens results in the BP formation temperature
decreasing in the binary mixture. The presence of two fluoro
substituents protruding from the side of the mesogenic core is
more beneficial for BP stabilization than that of intramolecular
hydrogen bonding between a hydroxy group and the ester
linkage in the Schiff base mesogen. The phase transition
between BPI and BPII can be confirmed by measuring the
temperature dependence of the Bragg reflection and POM.
Schiff base mesogens possessing only a hydroxy group near
the ester linkage easily exhibit BPII in the binary mixture
containing chiral dopant S811 on cooling. In addition, only
BPII can be induced when chiral dopant ISO(6OBA)2 is doped
into the Schiff base mesogen during the cooling process.
Interestingly, BPI can be observed in the binary mixture of
two difluoro substituted Schiff base mesogens III being doped
In the case of Schiff base mesogen I possessing a hydroxy
group the near imine linkage, Schiff base mesogens OH-EI6-F
and OH-EI6-OH show larger dipole moments in comparison
with Schiff base mesogen OH-EI6 by reason of the incorporation
of two fluorine atoms and a hydroxy group near the ester linkage,
respectively. Thus, Schiff base mesogen OH-EI6-F in the binary
mixture system with 30 wt% S811 shows a wider BP range (18.3 K)
than that of OH-EI6 (7.4 K) under the same chirality condition.
However, Schiff base mesogen OH-EI6-F cannot induce a BP when
it is doped with ISO(6OBA)2. This phenomenon is similar to that
of OH-EI6 under the same chirality condition. Unexpectedly, a BP
(9.2 K) emerges when Schiff base mesogen OH-EI6-OH is doped
with 10 wt% ISO(6OBA)2 possibly due to its larger biaxiality. As for
Schiff base mesogens III, two fluoro substituted Schiff base meso-
gen OH-EIII6-F possesses larger biaxiality, dipole moments and
slightly lager polarizability than Schiff base mesogens OH-EIII6
and OH-EIII6-OH. Thus, a BP emerges easily when two fluoro
substituted Schiff base mesogen OH-EIII6-F is doped with 30 wt%
S811 or 10 wt% ISO(6OBA)2. However, in comparison with Schiff
base mesogens I, the dipole moment of Schiff base mesogen
OH-EIII6-OH (0.49 D) possessing two hydroxy groups is smaller
than that of OH-EI6-OH (3.83 D), so that the BP disappears when
OH-EIII6-OH is doped with chiral dopant ISO(6OBA)2. Accordingly,
except for OH-EI6 doped with ISO(6OBA)2, the main factor in BP
stabilization is the dipole moment for Schiff base mesogens I in the
chiral system. However, for Schiff base mesogens III, the fluorine
substituent effect is dominant for BP stabilization.
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13 H. Kikuchi, Struct. Bonding, 2008, 128, 99–117.
with 10–15 wt% chiral dopant ISO(6OBA)2 on heating. Based on
the molecular modeling, we demonstrated that a wider BP
range can be induced when two difluoro substituted Schiff
base mesogens that possess larger dipole moments are doped
with appropriate concentrations of chiral dopants S811 or
ISO(6OBA)2. In addition, the dipole moments of Schiff base
mesogens I are larger than those of Schiff base mesogens III.
Consequently, it is concluded that BPs can be induced for
racemic rodlike Schiff base mesogens I in a chiral system and
it is easier than that for racemic rodlike Schiff base mesogens
III. In addition, the presence of two fluoro substituents in the
racemic rodlike Schiff base mesogens is the main factor that
affects BP stabilization for Schiff base mesogens III in a chiral
system. This work reports the effects of molecular structure and
functional group of racemic rodlike Schiff base mesogens on
BP stabilization under chiral conditions. Moreover, it also offers
possible approaches for the use of the difluoro substituted Schiff
base mesogen H-EI6-F as a host to be diluted with nematic LCs,
which may be used for future applications of room-temperature
eutectic liquid crystal mixtures with wide BP ranges.
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Conflicts of interest
There are no conflicts to declare.
27 F. Castles, S. M. Morris, E. M. Terentjev and H. J. Coles,
Phys. Rev. Lett., 2010, 104, 157801.
28 H. Wang, Z. Zheng and D. Shen, Liq. Cryst., 2012, 39, 99–103.
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Acknowledgements
This research was supported by the Ministry of Science and
Technology through MOST 106-2113-M-036-001, MOST 106-2113-M-
027-002, and Tatung University through B107-C04-010, Taiwan. In
addition, we particularly thank Mr Chang-Yu Chou and Ms Pei-Yi
Shih for assisting in the synthesis of some precursors of these Schiff
base mesogens.
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