Simple and efficient chiral dopants to induce blue phases and their optical purity effects on the physical properties of blue phases

Article abstract of DOI:10.1021/jp506358h

Blue phases (BPs) have received considerable attention as light shutters in the next generation of liquid crystal (LC) displays. However, no simple and efficient chiral dopant for induction of BPs of commercially available rodlike LC compounds has been re

Full text of DOI:10.1021/jp506358h

Article
pubs.acs.org/JPCB
Simple and Efficient Chiral Dopants to Induce Blue Phases and Their
Optical Purity Effects on the Physical Properties of Blue Phases
Keiki Kishikawa,* Takaaki Sugiyama, Tomohiro Watanabe, Shota Aoyagi, Michinari Kohri,
Tatsuo Taniguchi, Masahiro Takahashi, and Shigeo Kohmoto
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan
S
* Supporting Information
ABSTRACT: Blue phases (BPs) have received considerable attention as light
shutters in the next generation of liquid crystal (LC) displays. However, no simple
and efficient chiral dopant for induction of BPs of commercially available rodlike LC
compounds has been reported. In this study, both (R) and (S) forms of novel chiral
dopants were synthesized, showed extremely high helical twisting power values in
nematic LC compounds, and induced stable BPs with a small amount of our chiral
dopants (3−5 mol %). In enantiomeric excess controlled experiments, we found
novel phenomena in their physical properties, such as generation of a metastable
chiral nematic phase between an isotropic state and a BP.
widening the temperature ranges have been reported. However,
the fifth approach is the most difficult, and the methodology
has not yet been established, though it is the simplest and
easiest way to generate BPs. We believe that the fifth approach
using a commercially available achiral LC compound doped
with a small amount of a single chiral dopant is a tough
scientific challenge and highly valuable.
In this paper, we report chiral dopants 1a−g (Scheme 1)
derived from L- and D-tartaric acids that were highly effective at
inducing stable BPs. The N* phases of 4-n-pentyl-4′-
cyanobiphenyl (5CB) and N-(4-ethoxybenzylidene)-4-n-buty-
laniline (EBBA) doped with 4 mol % and 7 mol % of (R)-1e
gave BPs with a temperature range of 3.6 and 7.2 K on heating,
respectively, and these BPs were not supercooled states but
enantiotropic ones. To our knowledge, the value of 7.2 K on
heating (11.3 K on cooling) in EBBA is the widest temperature
range of a BP of a single rodlike LC compound induced by
addition of a single chiral dopant. Furthermore, the influence of
the enantiomeric excess (ee) of the dopant on the phase
transition behavior, BP temperature range, and selective
reflection wavelength was investigated. This is the first work
exploring the correlation between the physical properties of
BPs and the optical purity of chiral dopant molecules, and we
found several novel phenomena in the BPs.
1. INTRODUCTION
Blue phases (BPs) are liquid crystal (LC) phases that are
observed between the isotropic liquid (Iso) state and the chiral
nematic (N*) phase possessing a relatively short helical pitch.
Two types of cubic BPs (BP-I and BP-II) and one amorphous
type (BP-III) are known.1−6 In the cubic BPs, the LC
molecules are organized into a double twist cylinder (DTC,
Figure 1a) that possesses a helical molecular alignment, and
DTCs with one helical sense are assembled three-dimensionally
into the superstructures of BP-I (Figure 1b) and BP-II (Figure
1c). As shown in Figure 1d, there are disclination lines between
the DTCs because DTCs with one helical sense cannot
continuously fill the space of the unit cell.
BPs have received considerable attention as light shutters in
the next generation of LC displays, fast light modulators,7−25
and tunable photonic crystals.26−28 However, BPs generally
appear over a very narrow temperature range (in most cases,
less than 1 K), which has limited their practical applicability.29
Several theoretical studies have been attempted,30−33 and many
experiments have been carried out to widen the temperature
range of BPs.
There are five types of approaches to widen the BP
temperature range: (1) stabilization of a BP by a polymer
network,34−39 (2) stabilization of a BP by addition of
macromolecules (nanoparticle40 or dendrone molecules41),
(3) induction of BPs by introduction of chirality and
anisotropicity into the LC molecules themselves,42−48 (4)
induction of BPs from an N* phase by addition of achiral
molecules (possessing a bent rod49−56 or rod57 shape), and (5)
induction of a BP from an achiral N phase by addition of chiral
dopant. In the former four approaches, excellent results in
As the chiral dopant for inducing BPs in 5CB, 2,5-bis-[4′-
(hexyloxy)phenyl-4-carbonyl]-1,4;3,6-dianhydride-D-sorbitol
(ISO-(6OBA)2) is often used. However, the temperature range
Received: June 26, 2014
Revised: August 3, 2014
Published: August 5, 2014
© 2014 American Chemical Society
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ranges were induced.59 However, these BPs were induced using
a mixture of 4-n-octyl-4′-cyanobiphenyl (8CB) and plural LC
monomers. Accordingly, it is assumed that the stabilization
cannot exclude the effect of polymer network constructed
during the heating process.
We designed compounds 1−3 (Scheme 1) as chiral dopants
based on the following criteria: (1) the dopant should have a
large helical twisting power (HTP) to allow the induction of
BPs by doping with only a small amount of it, and (2) both
enantiomers of the chiral sources should be commercially
available, which is necessary for the investigation of the relation
between the optical purity of the dopant and the properties of
the BP. On the basis of these criteria, two rodlike moieties were
attached to the rigid five-membered imide ring (succinimide)
with a C2 symmetric geometry to achieve a large HTP, and L-
and D-tartaric acids were selected as the chiral sources because
both L and D forms are commercially available as enantiomeri-
cally pure compounds. As the LC hosts, 5CB, 8CB, 4-(4-n-
pentylcyclohexyl)benzonytrile (PCH-5), N-(4-methoxybenzyli-
dene)-4-n-butylaniline (MBBA), EBBA, N-(4-butoxybenzyli-
dene)-4-n-pentylaniline (BBPA), and 4-(n-octyloxy)phenyl 4-n-
octyloxybenzoate (OPOB) were used (Scheme 2).
Figure 1. Models of (a) a DTC (the cylinder-like superstructure
generated by helically aligned rodlike molecules), (b) the unit cell of
BP-I, (c) the unit cell of BP-II, and (d) the disclination lines in the unit
cell of BP-I. The lattice units are indicated by red lines. The DTCs and
disclination lines are blue and gray, respectively.
Scheme 2. Molecular Structures of LC Compounds (5CB,
8CB, PCH-5, MBBA, EBBA, BBPA, and OPOB) and Their
a
Phase Behaviors
Scheme 1. Molecular Structures of Dopant Compounds (R)-
1a−g, (R)-2, and (R)-3
a
Cr and Iso indicate crystal and isotropic states, and N, SmA, and
SmC indicate N, smectic A, and smectic C phases. The transition
temperatures (°C) were determined by POM and are given above and
below the arrows.
There are two previous reports on chiral succinimide dopants
derived from tartaric acids. Scherowsky et al.60 and Galatina et
al.61 investigated the HTPs of those tartaric imide derivatives in
N* phases. However, they never reported the induction of BPs
in their studies.
2. EXPERIMENTAL SECTION
of BPs of 5CB induced with this dopant is very narrow: the
temperature ranges are only 1.4 K on heating (2.9 K on
cooling) with the addition of 9 mol % of ISO-(6OBA)2.58
Recently, supramolecular dopants derived from isosorbide were
added to N LC mixtures, and BPs with wide temperature
2.1. LC Hosts and Their Phase Behaviors. Molecular
structures and phase transition behaviors of the LC hosts used
in this study are shown in Scheme 2. Their transition
temperatures (5CB, 8CB, PCH-5, MBBA, EBBA, BBPA, and
OPOB) were measured in our laboratory. The LC compounds
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5CB, 8CB, PCH-5, MBBA, EBBA, and BBPA were
commercially available and used without further purification.
OPOB was synthesized by condensation of 4-octyloxyphenol
and 4-octyloxybenozoic acid.
2.1.1. Synthesis of (R)-1a−g, (R)-2, and (R)-3. The synthetic
routes of (R)-1a−g, (R)-2, and (R)-3 are shown in Scheme 3.
octyloxybenzoic acid (10). Condensation of 4-benzyloxyben-
zoic acid (11) with (R)-6 afforded (R)-12. Hydrolysis of (R)-
12 followed by condensation with 9e produced (R)-3.
2.1.2. Synthesis of (R)-6 and (R)-7. Into a 200 mL three-
necked round-bottom flask equipped with a dropping funnel
filled with molecular sieve 4 Å, a Dimroth condenser, and a
CaCl2 tube were added L-(+)-tartaric acid (2.160 g, 14.39
mmol), p-xylene (30 mL), n-heptylamine (2.13 mL, 14.4
mmol), and the solution was refluxed with stirring for 25 h.
After cooling, the solution was filtered, and the solid was
washed with n-hexane. The residual solid recrystallized from
ethanol/n-hexane to give a white crystal ((R)-6, 1.37 g, yield
41%). Compounds (S)-6 and (R)-7 were also obtained with the
similar procedure.
Scheme 3. Synthetic Routes of (R)-1a−g, (R)-2, and (R)-3
(R)-6 (R = n-C7H15): yield 41%; white crystal; mp 130−136
°C (ethanol/n-hexane); IR (KBr, cm−1) 3398, 2925, 2857,
1705, 1347, 1165, 1084; 1H NMR (CDCl3, 300 MHz): δ = 0.87
(t, J = 6.8 Hz, 3H), 1.28 (m, 8H), 1.60 (m, 2H), 3.12 (br, 2H),
3.52 (m, 2H), 4.58 (s, 2H); 13C NMR ((CD3)2SO, 75 MHz): δ
= 14.3, 22.3, 26.4, 27.4, 28.5, 31.4, 38.0, 74.7, 175.1; HRMS
(ESI) calcd for C11H19O4NNa: 252.1206, found: 252.1205.
(S)-6 (R = n-C7H15): yield: 27%; white crystal; mp 133−135
°C (ethanol/n-hexane); IR (KBr, cm−1) 3398, 2925, 2855,
1705, 1347, 1165, 1084; 1H NMR ((CD3)2SO, 300 MHz): δ =
0.86 (t, J = 6.8 Hz, 3H), 1.24 (m, 8H), 1.47 (m, 2H), 4.31 (m,
2H), 6.26 (m, 2H); 13C NMR ((CD3)2SO, 75 MHz): δ = 14.3,
22.4, 26.4, 27.4, 28.6, 31.5, 38.0, 74.7, 175.1; HRMS (ESI) calcd
for C11H18O4N: 228.1241, found: 228.1247.
(R)-7 (R = n-C12H25): yield 30%; white crystal; mp 134−136
°C (ethanol/n-hexane); 1H NMR (CDCl3, 300 MHz): δ = 0.88
(t, J = 6.6 Hz, 3H), 1.25 (m, 18H), 1.58 (m, 2H), 3.52 (m, 2H),
4.59 (s, 2H).
2.1.3. Synthesis of (R)- and (S)-9f. Into a 200 mL three-
necked round-bottom flask equipped with a CaCl2 tube were
added ethyl 4-(4-hydroxyphenyl)benzoate (0.707 g, 3.10
mmol), triphenylphosphine (0.977 g, 3.72 mmol), and THF
(15 mL), and the solution was stirred at 0 °C for 5 min. Then,
(S)-(+)-2-octanol (0.490 mL, 4.01 g, 3.08 mmol) and a 40%
solution of diethyl azodicarboxylate (DEAD) in toluene (2.7
mol/L, 1.70 mL, 3.4 mmol) were added, and the solution was
stirred at room temperature for 17 h. The solution was
extracted with ethyl acetate (100 mL). The ethyl acetate
solution was washed with water (100 mL × 3), dried over
anhydrous MgSO4, and concentrated in vacuo. The product
was separated by column chromatography on silica gel eluting
with ethyl acetate/n-hexane (1:5) to give a white crystal (ethyl
4-(4-(R)-2-methylheptanoxyphenyl)benzoate, 0.653 g, yield
59%).
Into a 200 mL three-necked round-bottom flask equipped
with a Dimroth condenser were added ethyl 4-(4-(R)-2-
methylheptanoxyphenyl)benzoate (0.653 g, 1.84 mmol),
potassium hydroxide (0.734 g, 13.1 mmol), and a mixture of
ethanol and water (5:1) (50 mL), and the solution was refluxed
with stirring for 4 h. After cooling, potassium hydroxide (0.124
g, 2.21 mmol) was added, and the solution was refluxed with
stirring for 4 h. After cooling, the solution was acidified (pH =
3) with 1 N hydrochloric acid, the resulting precipitation was
obtained by suction−filtration to give a white solid, and the
solid was washed with n-hexane. The solvent was removed
under reduced pressure to give a white crystal ((R)-9f, 0.431 g,
yield 72%). Compound (S)-9f was also obtained with the
similar procedure from (R)-(−)-2-octanol.
Imides (R)-6 and (R)-7 were prepared by condensation of
tartaric acid (R)-5 and alkylamine (R = n-C7H15 or n-
C12H25).62−65 Imide (R)-6 (or (R)-7) was esterified with 4′-
alkoxybiphemylcarboxylic acid 9a−e (R′ = n-C4H9, n-C5H11, n-
C6H13, n-C7H15, and n-C8H17), (R)-9g (R′ = (R)−CH(CH3)-
C6H13) and (S)-9g ((S)−CH(CH3)C6H13) to give the
corresponding dopants (R)-1a−f, (R,R)-1g (R′ = (R)−
CH(CH3)C6H13), and (R,S)-1g (R′ = (S)−CH(CH3)C6H13).
Dopant (R)-2 was obtained by condensation of (R)-6 and 4-
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(R)-9f (R′ = (R)−CH(CH3)C6H13): yield 72%; white
MHz): δ = 0.90 (m, 9H), 1.31−1.50 (m, 20H), 1.77 (m, 6H),
3.67 (m, 2H), 4.00 (t, J = 6.6 Hz, 4H), 5.88 (s, 2H), 6.98 (d, J =
8.7 Hz, 4H), 7.56 (d, J = 9.0 Hz, 4H), 7.64 (d, J = 8.3 Hz, 4H),
8.11 (d, J = 8.7 Hz, 4H); 13C NMR (CDCl3, 75 MHz): δ =
14.5, 22.9, 23.0, 26.1, 27.1, 27.8, 29.2, 29.6, 31.99, 32.04, 40.1,
68.5, 73.9, 115.4, 126.5, 127.0, 128.8, 131.2, 132.2, 146.8, 160.0,
166.0, 170.0; HRMS (ESI) calcd for C49H60O8N: 790.4313,
found: 790.4312.
(R)-1d (R = n-C7H15, R′ = n-C7H15): yield 69%; white
crystal; mp 103.5 °C (n-hexane/ethyl acetate); Rf = 0.46 (n-
hexane/ethyl acetate = 5:1); IR (NaCl, cm−1) 2928, 2855,
1725, 1603, 1272, 1184, 1096, 828, 765; 1H NMR (CDCl3, 300
MHz): δ = 0.90 (m, 9H), 1.31−1.52 (m, 24H), 1.78 (m, 6H),
3.68 (m, 2H), 4.01 (t, J = 6.6 Hz, 4H), 5.87 (s, 2H), 6.99 (d, J =
8.8 Hz, 4H), 7.57 (d, J = 8.7 Hz, 4H), 7.65 (d, J = 8.5 Hz, 4H),
8.12 (d, J = 8.4 Hz, 4H); 13C NMR (CDCl3, 75 MHz): δ =
14.5, 22.97, 23.02, 26.4, 27.1, 27.8, 29.1, 29.5, 29.6, 32.0, 32.2,
40.1, 68.5, 73.9, 115.4, 126.5, 127.0, 128.8, 131.2, 132.2, 146.8,
160.0, 166.0, 170.0; HRMS (ESI) calcd for C51H64O8N:
818.4626, found: 818.4644.
(R)-1e (R = n-C7H15, R′ = n-C8H17): yield 79% (97.2% ee);
white crystal; mp 102.2 °C (n-hexane/ethyl acetate); Rf = 0.42
(n-hexane/ethyl acetate = 5:1); IR (NaCl, cm−1) 2925, 2855,
1723, 1606, 1272, 1185, 1096, 828, 767; 1H NMR (CDCl3, 300
MHz): δ = 0.89 (m, 9H), 1.29−1.47 (m, 28H), 1.77 (m, 6H),
3.68 (m, 2H), 4.00 (t, J = 6.6 Hz, 4H), 5.88 (s, 2H), 6.99 (d, J =
8.8 Hz, 4H), 7.56 (d, J = 8.8 Hz, 4H), 7.64 (d, J = 8.4 Hz, 4H),
8.12 (d, J = 8.4 Hz, 4H); 13C NMR (CDCl3, 75 MHz): δ =
14.5, 23.0, 23.1, 26.4, 27.1, 27.8, 29.2, 29.6, 29.8, 32.0, 32.2,
40.1, 68.6, 73.9, 115.4, 126.5, 127.0, 128.8, 131.2, 132.2, 146.8,
160.0, 166.0, 170.0; HRMS (ESI) calcd for C53H67O8NNa:
868.4759, found: 868.4772.
(S)-1e (R = n-C7H15, R′ = n-C8H17): yield 72% (100.0% ee);
white crystal; mp 98.6 °C (n-hexane/ethyl acetate); Rf = 0.43
(hexane/ethyl acetate = 5:1); IR (NaCl, cm−1) 2924, 2855,
1725, 1602, 1272, 1184, 1096, 825, 768; 1H NMR (CDCl3, 300
MHz): δ = 0.89 (m, 9H), 1.27−1.47 (m, 28H), 1.77 (m, 6H),
3.68 (m, 2H), 4.01 (t, J = 6.6 Hz, 4H), 5.88 (s, 2H), 6.99 (d, J =
8.7 Hz, 4H), 7.56 (d, J = 8.6 Hz, 4H), 7.64 (d, J = 8.4 Hz, 4H),
8.12 (d, J = 8.4 Hz, 4H); 13C NMR (CDCl3, 75 MHz) δ 14.5,
23.0, 23.1, 26.4, 27.1, 27.8, 29.2, 29.7, 29.8, 32.0, 32.2, 40.1,
68.6, 73.9, 115.4, 126.5, 127.0, 128.8, 131.2, 132.2, 146.8, 160.0,
166.0, 170.0; HRMS (ESI) calcd for C53H68O8N: 846.4939,
found: 846.4933.
(R)-1f (R = n-C12H25, R′ = n-C8H17): yield 33%; pale-brown
crystal; mp 104.8 °C (n-hexane/ethyl acetate); Rf = 0.44 (n-
hexane/ethyl acetate = 5:1); IR (NaCl, cm−1) 2924, 2855,
1723, 1601, 1272, 1184, 1098, 825, 768; 1H NMR (CDCl3, 300
MHz): δ = 0.89 (m, 9H), 1.27−1.47 (m, 38H), 1.78 (m, 6H),
3.68 (m, 2H), 4.00 (t, J = 6.6 Hz, 4H), 5.88 (s, 2H), 6.99 (d, J =
8.6 Hz, 4H), 7.57 (d, J = 8.7 Hz, 4H), 7.65 (d, J = 8.3 Hz, 4H),
8.12 (d, J = 8.3 Hz, 4H); 13C NMR (CDCl3, 75 MHz): δ =
14.6, 23.11, 23.15, 26.5, 27.2, 27.9, 29.55, 29.67, 29.69, 29.8,
29.9, 30.0, 30.1, 32.2, 32.4, 40.1, 68.5, 73.9, 115.4, 126.5, 127.0,
128.8, 131.2, 132.2, 146.8, 160.0, 166.0, 170.0; HRMS (ESI)
calcd for C58H78O8N: 916.5722, found: 916.5734.
(R,R)-1g (R = n-C7H15, R′ = (R)−CH(CH3)C6H13): yield
50%; white crystal; mp 117.4 °C (n-hexane/ethyl acetate); Rf =
0.31 (n-hexane/ethyl acetate = 5:1); IR (NaCl, cm−1) 2929,
2855, 1725, 1603, 1272, 1185, 1096, 828, 765; 1H NMR
(CDCl3, 300 MHz): δ = 0.89 (m, 9H) 1.29−1.61 (m, 32H),
1.73 (m, 4H), 3.68 (m, 2H), 4.41 (sext, J = 6.1 Hz, 2H), 5.88
(s, 2H), 6.98 (d, J = 7.0 Hz, 4H), 7.56 (d, J = 8.8 Hz, 4H), 7.65
1
crystal; H NMR ((CD3)2CO, 300 MHz): δ = 0.90 (t, J =
6.6 Hz, 3H), 1.28−1.80 (m, 13H), 4.56 (sext, J = 6.0 Hz, 1H),
7.07 (d, J = 8.9 Hz, 2H), 7.70 (d, J = 8.9 Hz, 2H), 7.78 (d, J =
8.4 Hz, 2H), 8.11 (d, J = 8.3 Hz, 2H).
(S)-9f (R′ = (S)−CH(CH3)C6H13): yield 95%; white crystal;
1H NMR ((CD3)2SO, 300 MHz): δ = 0.86 (t, J = 7.0 Hz, 3H),
1.25−1.78 (m, 13H), 4.50 (sext, J = 6.0 Hz, 1H), 7.03 (d, J =
8.8 Hz, 2H), 7.67 (d, J = 8.8 Hz, 2H), 7.75 (d, J = 8.5 Hz, 2H),
7.99 (d, J = 8.4 Hz, 2H).
2.1.4. Synthesis of (R)-1a−g. Into a 100 mL three-necked
round-bottom flask equipped with a Dimroth condenser and a
CaCl2 tube were added compound 9a (0.360 g, 1.42 mmol),
thionyl chloride (0.510 mL, 7.07 mmol), and CH2Cl2 (10 mL),
and the solution was refluxed with stirring for 5 h. After
removal of the solvent and an excess amount of thionyl chloride
under reduced pressure using a evaporator, compound (R)-6
(0.149 g, 0.650 mmol), triethylamine (0.590 mL, 0.431 g, 4.26
mmol), DMAP (0.0403 g, 0.0330 mmol), and THF (10 mL),
and the solution was stirred at room temperature for 15 h. The
solution was extracted with ethyl acetate (100 mL); the ethyl
acetate solution was washed with a saturated sodium hydrogen
carbonate solution (100 mL × 3). The ethyl acetate solution
was dried over anhydrous MgSO4, and the solution was
concentrated in vacuo. The product was separated by column
chromatography on silica gel eluting with n-hexane/ethyl
acetate (10:1) to give compound (R)-1a (0.340 g, yield
71%). Compounds (R)-1b−g were also obtained with the
similar procedure. Further, compound (S)-1e was obtained
from 9e and (S)-6. The enantiomeric purities of (R)-1e and
(S)-1e were measured by high-performance liquid chromatog-
raphy (HPLC) (column: CHIRALPAL IB (Daicel Corpo-
ration), column size: ϕ0.46 cm × 15 cm; detector: UV 300 nm;
flow rate: 2.0 mL/min; solvent: EtOH/hexane = 4:6) to give
97.3% and 100% ee, respectively.
(R)-1a (R = n-C7H15, R′ = n-C4H9): yield 71%; white crystal;
mp 102.6 °C (n-hexane/ethyl acetate); Rf = 0.47 (n-hexane/
ethyl acetate = 5:1); IR (NaCl, cm−1) 2928, 2874, 1725, 1603,
1272, 1189, 1091, 825, 765; 1H NMR (CDCl3, 300 MHz): δ =
0.90 (t, J = 6.7 Hz, 3H), 0.99 (t, J = 7.4 Hz, 6H), 1.34 (m, 8H),
1.52 (sext, J = 7.4 Hz, 4H), 1.78 (m, 6H), 3.68 (m, 2H), 4.02
(t, J = 6.5 Hz, 4H), 5.88 (s, 2H), 6.99 (d, J = 8.8 Hz, 4H), 7.57
(d, J = 8.7 Hz, 4H), 7.65 (d, J = 8.5 Hz, 4H), 8.12 (d, J = 8.5
Hz, 4H); 13C NMR (CDCl3, 75 MHz): δ = 14.3, 14.5, 19.7,
23.0, 27.1, 27.8, 29.2, 31.7, 32.0, 40.1, 68.2, 73.9, 115.4, 126.5,
127.0, 128.8, 131.2, 132.2, 146.8, 160.1, 166.0, 170.0; HRMS
(ESI) calcd for C45H52O8N: 734.3687, found: 734.3686.
(R)-1b (R = n-C7H15, R′ = n-C5H11): yield 84%; white
crystal; mp 117.4 °C (n-hexane/ethyl acetate); Rf = 0.43 (n-
hexane/ethyl acetate = 5:1); IR (NaCl, cm−1) 2929, 2866,
1725, 1603, 1276, 1186, 1096, 824, 765; 1H NMR (CDCl3, 300
MHz): δ = 0.92 (m, 9H), 1.31−1.49 (m, 16H), 1.78 (m, 6H),
3.67 (m, 2H), 4.01 (t, J = 6.2 Hz, 4H), 5.87 (s, 2H), 6.99 (d, J =
8.8 Hz, 4H), 7.57 (d, J = 8.8 Hz, 4H), 7.65 (d, J = 8.4 Hz, 4H),
8.12 (d, J = 8.4 Hz, 4H); 13C NMR (CDCl3, 75 MHz): δ =
14.5, 22.9, 23.0, 27.1, 27.9, 28.6, 29.2, 29.4, 32.1, 40.1, 68.6,
73.9, 115.4, 126.5, 127.0, 128.8, 131.2, 132.2, 146.8, 160.0,
166.0, 170.0; HRMS (ESI) calcd for C47H56O8N: 762.4000,
found: 762.3997.
(R)-1c (R = n-C7H15, R′ = n-C6H13): yield 76%; white
crystal; mp 116.9 °C (n-hexane/ethyl acetate); Rf = 0.40 (n-
hexane/ethyl acetate = 7:1); IR (NaCl, cm−1) 2929, 2864,
1725, 1603, 1272, 1184, 1096, 828, 765; 1H NMR (CDCl3, 300
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Article
(d, J = 8.4 Hz, 4H), 8.12 (d, J = 8.5 Hz, 4H); 13C NMR
(CDCl3, 75 MHz): δ = 14.5, 20.1, 23.00, 23.04, 26.0, 27.1, 27.9,
29.2, 29.7, 32.1, 32.2, 36.9, 73.9, 74.4, 116.6, 126.4, 127.0,
128.9, 131.2, 132.1, 146.9, 159.2, 166.0, 170.0; HRMS (ESI)
calcd for C53H68O8N: 846.4939, found: 846.4943.
(R,S)-1g (R = n-C7H15, R′ = (S)−CH(CH3)C6H13): yield
44%; white crystal; mp 144.6 °C (n-hexane/ethyl acetate); Rf =
0.40 (n-hexane/ethyl acetate = 5:1); IR (NaCl, cm−1) 2928,
2855, 1723, 1603, 1272, 1185, 1095, 828, 765; 1H NMR
(CDCl3, 300 MHz): δ = 0.88 (m, 9H), 1.29−1.61(m, 32H),
1.73 (m, 4H), 3.68 (m, 2H), 4.41 (sext, J = 6.0 Hz, 2H), 5.88
(s, 2H), 6.98 (d, J = 8.8 Hz, 4H), 7.56 (d, J = 8.7 Hz, 4H), 7.65
(d, J = 8.4 Hz, 4H), 8.12 (d, J = 8.4 Hz, 4H); 13C NMR
(CDCl3, 75 MHz): δ = 14.5, 20.2, 23.00, 23.04, 26.0, 27.1, 29.2,
29.7, 32.1, 32.2, 73.9, 74.4, 116.6, 126.4, 127.0, 128.9, 131.2,
132.1, 146.9, 159.2, 166.0, 170.0; HRMS (ESI) calcd for
C53H68O8N: 846.4939, found: 846.4957.
2.1.5. Synthesis of (R)-2. Into a 100 mL three-necked round-
bottom flask equipped with a Dimroth condenser and a CaCl2
tube were added 4-octyloxybenzoic acid (10) (0.402 g, 1.61
mmol), thionyl chloride (0.580 mL, 8.04 mmol), and CH2Cl2
(10 mL), and the solution was refluxed with stirring for 4 h.
After removal of the solvent and an excess amount of thionyl
chloride under reduced pressure using an evaporator,
compound (R)-6 (0.165 g, 0.720 mmol), triethylamine
(0.670 mL, 0.489 g, 4.83 mmol), DMAP (0.0403 g, 0.0330
mmol), and THF (10 mL) were added into the flask. The
solution was stirred at room temperature for 23 h. The solution
was extracted with ethyl acetate (100 mL), and the ethyl acetate
solution was washed with a saturated sodium hydrogen
carbonate solution (100 mL × 3). The ethyl acetate solution
was dried over anhydrous MgSO4 and concentrated in vacuo.
The product was separated by column chromatography on
silica gel eluting with n-hexane/ethyl acetate (10:1) to give (R)-
2 (0.381 g, yield 76%).
δ = 0.89 (t, J = 6.8 Hz, 3H), 1.33 (m, 8H), 1.70 (quint, J = 7.0
Hz, 2H), 3.65 (m, 2H), 5.14 (s, 4H), 5.78 (s, 2H), 7.01 (d, J =
8.7 Hz, 4H), 7.38 (m, 10H), 8.03 (d, J = 8.6 Hz, 4H); 13C
NMR (CDCl3, 75 MHz): δ = 14.5, 23.0, 27.1, 27.8, 29.2, 32.1,
40.0, 70.6, 73.8, 115.1, 121.1, 127.9, 128.7, 129.1, 132.8, 136.4,
163.7, 165.6, 170.2; HRMS (ESI) calcd for C39H40O8N:
650.2748, found: 650.2742.
2.1.7. Synthesis of (R)-13. Into a flask for hydrogenation
were added compound (R)-12 (0.391g, 0.602 mmol), 10% Pd−
carbon (0.120 g), and ethyl acetate (10 mL). Under hydrogen
atmosphere, the solution was vigorously stirred at room
temperature. After 27.0 mL of hydrogen was used for the
reaction, the stirring was stopped. The suspended solution was
filtered with Celite, and the filtrate was concentrated in vacuo.
The product was separated by column chromatography on
silica gel eluting with n-hexane/ethyl acetate (1:1) to give
compound (R)-13 as white crystals (0.252 g, yield 89%).
(R)-13: yield 89%; white crystal; Rf = 0.43 (n-hexane/ethyl
acetate = 1:1); IR (KBr, cm−1) 3406, 2929, 2855, 1705, 1610,
1
1272, 1167, 1096, 851, 765; H NMR (CD3OD, 300 MHz): δ
= 0.95 (t, J = 6.8 Hz, 3H), 1.39 (m, 8H), 1.70 (m, 2H), 3.65
(m, 2H), 5.93 (s, 2H), 6.88 (d, J = 9.0 Hz, 4H), 7.96 (d, J = 8.7
Hz, 4H); 13C NMR (CD3OD, 75 MHz): δ = 14.9, 24.1, 28.1,
28.8, 30.4, 33.3, 40.7, 75.1, 116.8, 120.9, 133.9, 164.8, 167.4,
172.3; HRMS (ESI) calcd for C25H28O8N: 470.1809, found:
470.1808.
2.1.8. Synthesis of (R)-3. Into a 100 mL three-necked round-
bottom flask equipped with a Dimroth condenser and a CaCl2
tube were added 9e (0.182 g, 0.558 mmol), thionyl chloride
(0.200 mL, 2.77 mmol), and CH2Cl2 (10 mL), and the solution
was refluxed with stirring for 3 h. After removal of the solvent
and an excess amount of thionyl chloride under reduced
pressure using an evaporator, compound (R)-13 (0.118 g,
0.252 mmol), triethylamine (0.230 mL, 0.174 g, 1.72 mmol),
DMAP (0.0158 g, 0.129 mmol), and THF (10 mL) were added
into the flask, and the solution was stirred at room temperature
for 18 h. After cooling, the solution was extracted with ethyl
acetate (100 mL), the ethyl acetate solution was washed with
water (100 mL × 3) and dried over anhydrous MgSO4, and the
solution was concentrated in vacuo. The product was separated
by column chromatography on silica gel eluting with n-hexane/
ethyl acetate (10:1) to give compound (R)-3 (0.0532 g, yield
19%).
(R)-2: yield 76%; pale-yellow crystals; mp 42.5 °C (n-
hexane/ethyl acetate); Rf = 0.28 (n-hexane/ethyl acetate =
10:1); IR (NaCl, cm−1) 2929, 2855, 1723, 1606, 1254, 1167,
1
1094, 842, 765; H NMR (CDCl3, 300 MHz): δ = 0.89 (m,
9H), 1.29−1.47 (m, 28H), 1.77 (m, 6H), 3.65 (m, 2H), 4.01 (t,
J = 6.5 Hz, 4H), 5.78 (s, 2H), 6.92 (d, J = 8.9 Hz, 4H), 8.01 (d,
J = 8.9 Hz, 4H); 13C NMR (CDCl3, 75 MHz): δ = 14.5, 23.0,
23.1, 26.4, 27.1, 27.8, 29.2, 29.4, 29.6, 29.7, 32.0, 32.2, 68.7,
73.8, 114.4, 120.5, 132.5, 164.2, 165.8, 17.03; HRMS (ESI)
calcd for C41H60O8N: 694.4313, found: 694.4313.
2.1.6. Synthesis of (R)-12. Into a 200 mL three-necked
round-bottom flask equipped with a CaCl2 tube were added
compound (R)-6 (0.417 g, 1.82 mmol), 4-benzyloxybenzoic
acid (11) (0.913 g, 4.00 mmol), and CH2Cl2 (20 mL), and the
solution was stirred at room temperature for 5 min. Then,
DIPC (0.620 mL, 0.502 g, 3.98 mmol) and DMAP (0.0457 g,
0.374 mmol) were added, and the solution was stirred at room
temperature for 61 h. The solution was extracted with
chloroform (100 mL), and the chloroform solution was washed
with a saturated sodium hydrogen carbonate solution (100 mL
× 3). The chloroform solution was dried over anhydrous
MgSO4, and the solution was concentrated in vacuo. The
product was separated by column chromatography on silica gel
eluting with chloroform to give compound (R)-12 (0.967 g,
yield 82%).
(R)-3: yield 19%; white crystal; mp 155.4 °C (n-hexane/
ethyl acetate); Rf = 0.18 (n-hexane/ethyl acetate = 10:1); IR
(NaCl, cm−1) 2929, 2855, 1731, 1603, 1260, 1189, 1163, 1068,
1
828, 765; H NMR (CDCl3, 300 MHz): δ = 0.90 (m, 9H),
1.25−1.52 (m, 28H), 1.77 (m, 6H), 3.68 (m, 2H), 4.02 (t, J =
6.5 Hz, 4H), 5.88 (s, 2H), 7.01 (d, J = 8.7 Hz, 4H), 7.38 (d, J =
8.7 Hz, 4H), 7.60 (d, J = 8.7 Hz, 4H), 7.71 (d, J = 8.4 Hz, 4H),
8.19 (d, J = 8.7 Hz, 4H), 8.24 (d, J = 8.4 Hz, 4H); 13C NMR
(CDCl3, 75 MHz): δ = 14.5, 23.0, 23.1, 26.5, 27.1, 27.9, 29.1,
29.7, 29.8, 32.1, 32.3, 40.2, 68.6, 74.0, 115.4, 122.6, 126.1,
127.1, 127.2, 128.9, 131.3, 132.2, 132.4, 146.8, 156.0, 160.1,
164.8, 165.3, 169.8; HRMS (ESI) calcd for C67H75O12N:
1108.5181, found: 1108.5206.
3. RESULTS AND DISCUSSION
3.1. Investigation of the UV and Circular Dichroism
Spectra and ee of (R)-1e and (S)-1e. To confirm that the
(R)-1e and (S)-1e obtained by our procedure were a pair of
enantiomers, their circular dichroism (CD) spectra were
measured. As shown in Figure 2a, the UV spectrum of (R)-
(R)-12: yield 82%; pale yellow oil; Rf = 0.42 (n-hexane/
chloroform = 1:1); IR (NaCl, cm−1) 2929, 2855, 1723, 1602,
1254, 1167, 1096, 842, 765, 695; 1H NMR (CDCl3, 300 MHz):
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Table 1. HTPs of Dopant Compounds (R)-1a−g, (R)-2, and
a
(R)-3 in 5CB (EBBA)
compd
HTP (μm−1
)
(R)-1a
(R)-1b
(R)-1c
(R)-1d
(R)-1e
(S)-1e
(R)-1f
(R,R)-1g
(R,S)-1g
(R)-2
−148 (−60)
−154 (−58)
−144 (−59)
−143 (−62)
−147 (−62)
−149 (−67)
−145 (−58)
−115 (−38)
−106 (−36)
−77 (−25)
−144 (−71)
(R)-3
a
The HTPs were measured at T = TIN − 5 (°C) using a wedge-cell.
The first number is the molecular HTP in 5CB, and the number in the
parentheses is the molecular HTP in EBBA. The molecular HTP is
defined by the following equation: HTP = (∂p−1/∂x)x→0 (p: pitch
length; x: mole fraction). The helical senses were determined using a
contact experiment with cholesteryl oleyl carbonate possessing M
helicity.67
Figure 2. (a) UV spectrum of (R)-1e and (b) CD spectra of (R)-1e
(red solid line) and (S)-1e (blue solid line) in THF solution (1.18 ×
10−5 M). (The optical purities of (R)-1e and (S)-1e measured using
chiral chromatography were 97.3% and 100.0% ee, respectively.)
1e in THF (1.18 × 10−5 M) has a peak at 306 nm. The CD
spectra of (R)-1e and (S)-1e (Figure 2b) in THF (1.18 × 10−5
M) show peaks that have equal magnitudes in opposite
directions. This means that the molecules are a pair of
enantiomers. The CD spectrum of each compound has a large
peak at 315 nm, which originates from the UV absorption of
the two −O−CO−C6H4−C6H4−OC8H17 moieties at approx-
imately 306 nm. The ee of each enantiomer was measured
using HPLC on a chiral column, and the purities of (R)-1e and
(S)-1e were 97.3% ee and 100.0% ee, respectively.
3.2. Investigation of the HTPs of the Chiral Dopants in
N* Phases and Substituent Effects of the Dopant on the
HTP. The HTPs of compounds doped with our chiral dopants
were measured in the N* phase at T = TIN − 5 (°C) using a
wedge-cell.66 The helical senses were investigated using a
contact experiment67 with cholesteryl oleyl carbonate that
possessed a left-handed helical sense (as shown in Figures S2
and S3, Supporting Information). All the HTPs were negative
because, in all cases, the textures of the N* LC phases did not
disappear at the contact area between the cholesteryl oleyl
carbonate and the LC compound doped with our dopant, as
shown using polarized-light optical microscopy (POM). In
Table 1, the HTPs of dopants (R)-1a−g, (R)-2, and (R)-3 in
the N* phases of 5CB and EBBA are shown. The HTPs in 5CB
(−77 to −154 μm−1) are much larger than the HTP of the
chiral dopant that is generally used, ISO-(OBA)2 (HTP in
5CB: 42.1 μm−1).68 In all cases, the HTP in 5CB is much
higher than that in EBBA, although the substituent effect of the
dopant on the HTP is similar in both LC hosts.
half of that of (R)-1e, whereas the HTP of (R)-3 with its two
long rods is almost the same as that of (R)-1e. This indicates
that the length of the rod in (R)-1e is enough to generate a
strong helicity in the N* phases. On the basis of these results,
(R)-1e and (S)-1e were selected as the dopants for further
experiments in this study.
3.3. Effect of the LC Hosts on the HTP of (R)-1e. To
investigate the effect of the LC hosts on the magnitude of the
HTP, 0.5 mol % of (R)-1e was added to 5CB, 8CB, PCH-5,
MBBA, EBBA, BBPA, and OPOB. The results are shown in
Table 2. Within this group, 5CB had the largest value (−147
a
Table 2. HTP and TIN of (R)-1e in Various LC Hosts
LC host
HTP (μm−1
)
TIN (°C)
5CB
−147
−127
−137
−68
−62
−45
35.3
41.5
58.0
48.2
83.0
88.0
94.4
8CB
PCH-5
MBBA
EBBA
BBPA
OPOB
−14
a
TIN (°C) is the transition temperature from the nematic phase to the
isotropic liquid. The HTP was measured at T = TIN − 5 (°C) using a
wedge-cell. The helical senses were determined by a contact
experiment using cholesteryl oleyl carbonate possessing M helicity.67
On the basis of the comparison of (R)-1a through (R)-1e, it
is clear that there is almost no influence of the length of the R′
group on the HTP. Moreover, on the basis of the comparison
between (R)-1e (R = n-C7H15) and (R)-1f (R = n-C12H24), it is
assumed that the length of the N-alkyl chain (R group) does
not affect the HTP. Because the HTP of both (R,R)-1g (R′ =
(R)−CH(CH3)C6H13) and (R,S)-1g (R′ = (S)−CH(CH3)-
C6H13) are smaller than that of their structural isomer (R)-1e, it
is thought that the introduction of a bulky R′ group decreases
the HTP. Furthermore, the introduction of chirality into the R′
group is not an effective means of obtaining higher values of the
HTP, because the HTP of (R,R)-1g is very similar to that of
(R,S)-1g. The HTP of (R)-2 with its two short rods is nearly
μm−1), and OPOB had the smallest value (−14 μm−1). The LC
compounds with lower TIN values had higher intensities within
this group of LC hosts. The HTPs in 8CB and PCH-5, which
possess a cyano group, were −127 and −137 μm−1,
respectively, which were close to the HTP in 5CB. The
HTPs in MBBA, EBBA, and BBPA, which possess an imine
group, had rather small values of −68, −62, and −45 μm−1,
respectively. The LC compounds possessing a cyano group had
relatively large HTPs within this group. In addition, a
comparison of 5CB and 8CB (or a comparison of MBBA,
EBBA, and BBPA) showed that LC compounds with a shorter
alkyl chain might have a slight advantage at obtaining a larger
HTP value.
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3.4. BP Induction in 5CB (and EBBA) Doped with
Dopants (R)-1a−g, (R)-2, and (R)-3. As shown in Table 3,
Table 4. Phase Behavior of EBBA Doped with (R)-1a−g,
a
(R)-2, and (R)-3
entry
1
compd
mol %
5
phase behavior
Table 3. Phase Behaviors of 5CB Doped with (R)-1a−g, (R)-
56.2
61.2
a
(R)-1a
2, and (R)-3
N* XoooY BP XoooY Iso
54.0
52.0
60.9
entry
1
compd
mol %
3
phase behavior
N* XoooY BP ⎯⎯⎯⎯⎯5⎯⎯6⎯.⎯8⎯⎯→⎯ Iso
2
3
(R)-1b
(R)-1c
5
5
20.3
23.4
←⎯⎯⎯⎯N*←⎯⎯⎯⎯
(R)-1a
46.8
56.7
60.7
56.8
N* XoooY BP XoooY Iso
19.5
20.0
23.4
23.0
55.9
N* XoooY BP ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ Iso
2
(R)-1b
(R)-1c
(R)-1d
(R)-1e
(R)-1e
(R)-1e
(R)-1f
(R,R)-1g
(R,S)-1g
(R)-2
3
3
3
2
3
4
3
3
3
3
1
←⎯⎯⎯⎯N*←⎯⎯⎯⎯
54.1
N* XoooY BP XoooY Iso
60.3
60.7
18.5
21.6
22.9
24.0
47.9
53.1
4
(R)-1d
(R)-1e
(R)-1e
(R)-1e
(R)-1f
(R,R)-1g
(R,S)-1g
(R)-2
5
3
5
7
5
5
5
3
1
3
N* XoooY BP XoooY Iso
N* XoooY BP XoooY Iso
45.2
53.1
20.2
21.5
23.8
24.0
68.3
N* XoooY Iso
68.3
5
4
N* XoooY BP XoooY Iso
20.7
23.9
55.3
59.8
6
27.9
N* XoooY Iso
27.1
5
N* XoooY BP XoooY Iso
49.8
47.3
59.8
54.5
7
22.6
25.0
6
N* XoooY BP XoooY Iso
N* XoooY BP XoooY Iso
43.1
49.8
54.4
55.7
21.9
24.9
20.1
8
N* 1Xo6o.o5Y BP XoooY Iso
7
N* XoooY BP XoooY Iso
44.3
55.8
15.6
23.3
19.7
25.3
59.5
N* XoooY Iso
59.3
9
8
N* XoooY BP XoooY Iso
22.6
25.2
56.2
N* XoooY Iso
56.0
10
11
12
N* 1Xo7o.o9Y Iso
9
17.5
62.5
N* XoooY Iso
62.2
N* 1Xo9o.o2Y Iso
10
11
12
19.0
75.5
N* XoooY Iso
75.2
(R)-3
22.0
N* XoooY Iso
21.6
a
32.5
N* XoooY Iso
(R)-3
N* and Iso indicate an N* phase and an Iso state, respectively, and
BP indicates cubic blue phases. The temperature change rate was 0.1
°C/min. The transition temperatures (°C) were determined by POM
and are given above and below the arrows.
a
N* and Iso indicate an N* phase and an Iso state, respectively, and
BP indicates cubic blue phases. The temperature change rate was 0.1
°C/min. The transition temperatures (°C) were determined by POM
and are given above and below the arrows.
that doping with 5 mol % of (R)-1e (entry 6) (4.5 K on heating
and 10.0 K on cooling).
5CB doped with 3 mol % of (R)-1a through (R)-1e (entries 1−
4 and 6) exhibited a BP between an N* phase and an Iso state.
The addition of less than 2 mol % of (R)-1e (entry 5) did not
induce a BP. 5CB doped with 4 mol % of (R)-1e (entry 7)
showed a wider temperature range of the BP (3.6 K on heating
and 4.1 K on cooling) than 5CB doped with 3 mol % of (R)-1e
(entry 6) (2.4 K on heating and 3.0 K on cooling). 5CB doped
with 3 mol % of (R)-1f (entry 8), possessing an N-dodecyl
group, also exhibited a BP, and its range was 2.0 K.
However, the addition of 3 mol % of (R,R)- and (R,S)-1g did
not result in a BP (entries 9 and 10). It is thought that bulky R′
groups are not suitable for the induction of a BP. In the cases of
(R)-2 and (R)-3, the maximum soluble amounts in 5CB were 3
mol % and 1 mol %, respectively. Neither 5CB doped with 3
mol % of (R)-2 (entry 11) nor 5CB doped with 1 mol % of
(R)-3 (entry 12) exhibited a BP.
As shown in Table 4, heating EBBA doped with 5 mol % of
(R)-1a through (R)-1e (entries 1−5) induced a BP with
temperature ranges of 4.5−5.2 K. In the cases of (R)-1b and 1c,
a monotropic N* phase was observed at the temperature above
that of the BP. More than 5 mol % of each dopant was mixed
with EBBA because the solubilities of these dopants in EBBA
was higher than those in 5CB. The addition of less than 3 mol
% of (R)-1e (entry 5) did not induce a BP, and doping with 7
mol % of (R)-1e (entry 7) resulted in a wider temperature
range of the BP (7.2 K on heating and 11.3 K on cooling) than
Heating EBBA doped with 5 mol % of (R)-1f (entry 8),
which possesses an N-dodecyl group, also induced a BP with a
range of 5.9 K (11.5 K on cooling). The dopants (R,R)-1g
(entry 9), (R,S)-1g (entry 10), (R)-2 (entry 11), and (R)-3
(entry 12) did not induce a BP in the N* phase of EBBA.
3.5. Estimate of the Molecular Shape of the Dopant
and Its Role in Formation of the Stable BP. To estimate
the molecular shape of the dopant, the most stable
conformation of (2R,3R)-bis(4-phenylbenzoyloxy)-N-methyl-
succimide was calculated by the density functional theory
(DFT) method with B3LYP functional and 6-31G(d) basis
set.69 As shown in Figure 3, the angle generated by the two
biphenyl-long axes is 110° (Figure 3a), and the angle generated
by each biphenyl-long axis and the succinimide plane is 45°
(Figure 3b).
Two main reasons that compounds (R)-1 are suitable chiral
dopants for inducing stable BPs are as follows. First, these
dopants have large HTP values. As shown in Figure 4a, the
chiral dopant molecule possesses two rodlike units with C2
symmetry, and in the N* phase (Figure 4b), the LC molecules
around the two rods are aligned in parallel to each rodlike unit.
Furthermore, the imide-ring plane also arranges the LC
molecules. Accordingly, these two types of molecular arrange-
ment sites in (R)-1 generate M helicity in N* LC phases.
Second, the dopant molecule has a bent shape with a large
lateral dipole (3.0 D). In previous reports, bimesogenic
molecules with a large flexoelectricity were reported by Coles
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Table 5. Phase Behaviors of 8CB, PCH-5, MBBA, BBPA,
a
and OPOB Doped with (R)-1e
b
LC host
8CB
mol %
3
phase behavior
28.9
32.8
SmA XoooY N* XoooY BP ⎯⎯⎯⎯⎯3⎯⎯4⎯.⎯3⎯⎯→⎯ Iso
←⎯⎯⎯⎯N*←⎯⎯⎯⎯
28.3
32.2
34.0
34.2
41.3
PCH-5
MBBA
BBPA
4
3
5
5
N* XoooY Iso
41.2
23.4
N* XoooY Iso
23.2
66.5
N* XoooY Iso
66.3
53.4
63.8
84.0
OPOB
Cr XoooY SmC XoooY N* XoooY Iso
49.5
63.4
83.7
a
N*, SmA, and SmC indicate N*, smectic A, and smectic C phases,
respectively. Cr and Iso indicate crystal and Iso states, respectively, and
BP indicates cubic blue phases. The temperature change rate was 0.1
°C/min. The transition temperatures (°C) were determined by POM
b
and are given above and below the arrows. The concentration is the
maximum concentration of the dopant in the LC compound.
Although the phase behaviors of the LC compounds doped
with less than this maximum amount were also observed, none
of these LC compounds exhibited a BP. Only 8CB doped with
3 mol % of (R)-1e exhibited a BP (1.5 K on heating). The
other LC compounds (PCH-5, MBBA, BBPA, and OPOB)
doped did not induce a BP. This confirmed that 5CB and
EBBA are more suitable for inducing a BP than the other LC
compounds.
Figure 3. Most stable conformation of (2R,3R)-bis(4-phenylbenzoy-
loxy)-N-methylsuccinimide calculated by the DFT method with
B3LYP functional and 6-31G(d) basis set: (a) side and (b) bottom
views.69 The long axis of each biphenyl group is indicated by a broken
black line. The direction passing the centers of the two carbonyl
oxygen atoms is indicated by a solid blue line.
In Figure 5, polarized microphotographs of 5CB doped with
3 mol % of (R)-1e at 24.5−11.7 °C are shown. On cooling to
24.5 °C, the green and blue platelet texture of a BP-I (Figure
5a) appeared, indicating a transition from the Iso state. At 23.8
°C (Figure 5b), the color changed dramatically from green and
blue to dark purple, which is assumed to occur because of a BP-
I → BP-II transition. At 22.8 °C (Figure 5c) and 22.0 °C
(Figure 5d), the blue and green colors of a BP-I appeared again,
and finally, an N* phase (Figure 5e) was observed.
On cooling, Iso → BP-II → BP-I → N* is the expected order
in the phase transitions of LC compounds that exhibit both BP-
I and BP-II.70,71 However, the order in this experiment was Iso
→ BP-I → BP-II → BP-I → N*, which is the first example of
this order in the transitions of BPs to be reported, to the best of
our knowledge.
In Figure 6, the polarized microphotograph of EBBA doped
with 5 mol % of (R)-1e is shown. The colorful platelet texture
of a BP-I was observed at 57.4 °C (Figure 6a). Figure 6b, c
shows microphotographs of EBBA doped with 7 mol % of (R)-
1e. On cooling from the Iso state, the microphotograph
exhibited the dark platelet texture of a BP-I at 51.8 °C (Figure
6b) and then gradually exhibited the typical texture of a BP-I at
48.3 °C, as shown in Figure 6c.
3.7. Investigation of the Effect of the Optical Purity of
the Dopant on the Phase Behavior of an N* LC Doped
with (R)-1e. To investigate the effect of ee on phase behaviors,
the phase diagram of 5CB doped with 3 mol % of 1e at
different ee values was plotted. The enantiomeric purities of
(R)-1e were adjusted to values between 0% and 97% ee by
mixing (R)-1e (97.3% ee) and (S)-1e (100.0% ee). The
diagrams on heating and cooling are shown in Figure 7a and b,
respectively. The Iso−N* transition temperature increased
gradually from 0% to 48% ee. A BP is induced at 58−97% ee on
heating and at 68−97% ee on cooling. The BP−Iso transition
Figure 4. Estimated molecular alignment of a rodlike LC compound
doped with (R)-1e: (a) schematic models of (R)-1e; (b) molecular
alignment model of LC molecules with M helicity in the N* phase (the
yellow arrows indicate the directors of the molecules). The small blue
and dark-blue cylinders in (b) indicate rodlike LC molecules.
and Pivnenko as BP materials with a broad temperature
range.42 They stated that the disclinations of the BPs were
stabilized by the large localized flexoelectric polarization
generated by the director distortions of the bimesogenic
molecules. We also think that the interaction of the dipoles at
the disclinations is the most significant factor in the stabilization
of the BPs, although it is difficult to know the amount of the
stabilization energy.
3.6. Investigation of the Effect of the LC Hosts on the
Induction of BPs. To investigate the effect of the LC hosts on
the induction of BPs, the phase behaviors of 8CB, PCH-5,
MBBA, BBPA, and OPOB doped with (R)-1e were investigated
using POM. The results are shown in Table 5. The mol % of
the dopant in each entry is the maximum soluble amount in the
LC host.
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Figure 5. Polarized microphotographs of 5CB doped with 3 mol % of (R)-1e at (a) 24.5 °C (BP-I), (b) 23.8 °C (BP-II), (c) 22.8 °C (BP-I), (d)
22.0 °C (BP-I), and (e) 11.7 °C (N*) (200×, on cooling).
temperature increased from 58% to 97% ee on heating. To our
surprise, at 68% ee on cooling, the texture indicating an N*
phase appeared at 24.5 °C from the Iso state, and this texture
slowly changed to the platelet texture of a BP at approximately
24 °C. On cooling to 22 °C, a second N* phase was observed.
In general, it is known that BPs appear at the temperature
above that of the N* phase.
The phase diagram of EBBA doped with 5 mol % of 1e at
different ee percentages was plotted (Figure 8). As shown in
Figure 8a, b, in the case of EBBA, the N*−Iso transition
temperature is almost constant in the range 0−58% ee. On
heating (Figure 8a), more than 63% ee was necessary to induce
a BP. On cooling (Figure 8b), in the range 63−83% ee, the
above-mentioned high-temperature N* phase was again
observed. At a constant temperature, the typical texture of an
N* phases gradually changed to that of a BP. This result
indicates that the high-temperature N* phase is metastable and
that the BP is thermodynamically stable at the temperature. As
shown in Table 4, the monotropic N* phase was observed at
the temperature above that of the BP in EBBA doped with each
of optically pure (R)-1b and (R)-1c. However, in the cases of
Figures 7b and 8b, monotropic N* phases were observed only
at lower optical purities and were not observed at high optical
purities.
Figures 7 and 8 also indicate the relation between the optical
purity and the temperature range of BPs. For example, in the
case of EBBA on heating (Figure 8a), the temperature ranges of
the BP are almost constant (approximately 4 K) in the range
68−97% ee. It is thought that dopant with a 68% ee is
composed of 68% optically pure (R)-1e and 32% racemic 1e (a
1:1 mixture of (R)- and (S)-1e), whereas dopant with a 97% ee
is composed of 97% optically pure (R)-1e and 3% racemic 1e.
Accordingly, the effect of the 29% (= 32% − 3%) racemic 1e on
the expansion of the BP temperature range is almost the same
as the effect of 29% (= 97% − 68%) optically pure (R)-1e. This
strongly suggests that the effect on expansion of the BP
temperature range at 68−97% ee does not come from the
optical purity of the dopant but from its molecular shape and/
or electrostatic properties, although at least 68% ee is necessary
for generating the helical structures in the BPs.
3.8. Mechanism Postulated for the Induction of the
Metastable N* Phase. The Iso → high-temperature N* →
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Figure 7. Plots of the transition temperature of 5CB versus the ee of
the dopant (R)-1e on (a) heating and (b) cooling. The transition
temperatures were measured in the N* phases of 5CB doped with 3
mol % of dopant 1e. The ee was adjusted by mixing (R)- and (S)-1e.
Figure 6. Polarized microphotographs of (a) EBBA doped with 5 mol
% of (R)-1e at 57.4 °C (BP-I, 500× ) and EBBA doped with 7 mol %
of (R)-1e at (b) 51.8 °C (BP-I, 200× ) and (c) 48.3 °C (BP-I, 200× )
on cooling.
does occur in the range 63−83% ee. This phenomenon clearly
originates from the optical purity of the dopant.
3.9. Investigation of the Influence of Temperature on
the Selective Reflection Wavelength of BPs. In Figure 9a,
the selective reflection wavelength of the BPs of 5CB doped
with 3 mol % of (R)-1e is plotted against the temperature on
cooling. On cooling in the range 24.5−23.5 °C, the selective
reflection wavelength of d(110) of the BP-I decreased. At 23.5
°C, the color quickly changed from green and blue to dark
purple, and the reflection wavelength changed discontinuously.
A selective reflection wavelength of 450 nm was observed in the
range 23.5−22.9 °C and was assumed to be d(100) of the BP-
II. On cooling in the range 22.7−20.9 °C, d(110) of the BP-I
appeared again, and the wavelength increased. In general, BP-
IIs appear at the temperature above that of a BP-I. However, in
this case, the order was BP-I ↔ BP-II ↔ BP-I during both
heating and cooling. This behavior is unique to this BP system.
These results are explained as follows. On cooling in the
range 24.5−23.5 °C, the selective reflection wavelength of
d(110) of the BP-I decreases, because the interaction of the
dopant molecules with the LC molecules becomes stronger,
which makes the helical pitch shorter. The decrease in the
helical pitch induces the BP-II, and its d(100) reflection
wavelength is observed.
BP → low-temperature N* phase transitions on cooling should
be explained by the difference in the distribution states of the
dopant molecules (R)- and (S)-1e in the Iso state, N* phase,
and BP. Both (R)- and (S)-1e molecules in the Iso state and N*
phase are uniformly distributed in the LC hosts. However, the
distribution states of (R)- and (S)-1e molecules in the BP are
greatly different from the distribution states in the Iso state and
N* phase, because cubic BPs have the two different areas, DTC
and disclination areas as shown in Figure 1. All the DTCs in the
BP have a helical molecular alignment with one handedness
generated by the major enantiomer (which is (R)-1e in this
case). It is assumed that (S)-1e molecules are excluded from
the inside of the DTCs and consigned to the disclination areas
because (S)-1e molecules sterically mismatch the M helicity
generated by (R)-1e molecules. This separation process occurs
during the Iso → BP or high-temperature N* → BP transition.
If the transfer of the dopant molecules is slow, the Iso → N*
transition occurs directly without the induction of the BP
transition and then the high-temperature N* → BP transition
accompanied. This explanation is also supported by the
observation that this unusual transition does not occur when
the dopant (R)-1e with high enantiomeric purity (87−97% ee
in the case of EBBA doped, as shown in Figure 8b) is used but
It is known that BP-IIs often appear at the temperature above
that of an N* phase that have a relatively short helical pitch.65
On cooling in the range 22.7−20.9 °C, the BP-I appeared again,
and its d(110) reflection wavelength increased because of the
increase in the helical pitch. It is assumed that the neighboring
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As shown in Figure 9b, the selective reflection wavelengths of
d(110) of the BP-I of EBBA doped with 5 mol % of (R)-1e are
plotted against the temperature on cooling. The plot of the
wavelength versus temperature was similar to that of 5CB
doped with (R)-1e except that the BP-II was not observed. On
cooling in the range 59.4−57.3 °C, the selective reflection
wavelength of the BP-I decreased. Then, in the range 56.6−52.3
°C, the selective reflection wavelength increased greatly.
3.10. Investigation of the Influence of the ee of the
Dopant on the Selective Reflection Wavelength of the
BP. To know the relation between the lattice parameter a (≈
one helical pitch length in the case of BP-Is) and the ee of the
dopant in the BP, the selective reflection wavelength of d(110)
was measured in the BP-I of 5CB doped with 3 mol % of (R)-
1e. In the case of BPs, the wavelength of the selective reflection
λhkl is expressed by the following equation: λhkl = 2na/√(h2 +
k2 + l2) (λhkl: selective reflection wavelength; n: refractive index;
h, k, l: Miller index). From the equation λ110 = (√2)·n·a, the
lattice constant a is proportional to λ110
.
Figure 10 shows a plot of the selective reflection wavelength
versus the ee of the dopant. The observed wavelength (solid
Figure 8. Plots of the transition temperature of EBBA versus the ee of
the dopant (R)-1e on (a) heating and (b) cooling. The transition
temperatures were measured in the N* phases of EBBA doped with 5
mol % of dopant 1e. The ee was adjusted by mixing (R)- and (S)-1e.
Figure 10. Plot of the selective reflection wavelength of d(110) of the
BP-I versus ee in 5CB doped with 3 mol % of (R)-1e at the highest
temperature in each of the BPs (filled diamonds and solid line). The
open circles and broken line indicate the estimated values based on the
selective reflection (705 nm) at 68% ee.
line) decreases with an increase in chirality of the dopant in the
range 68−97% ee. The broken line indicates the wavelength
estimated from the wavelength (705 nm) at 68% ee, assuming
that the refractive index n is constant in the wavelength range72
(the change in n in the range between 725 and 475 nm is less
than 0.3%73). In the lower ee range, the observed value is close
to the estimated one. However, in the higher ee range, the value
is far from the estimated value. It is assumed that there may be
a limitation in shortness of the helical pitch in this system.
3.11. Estimate of the Effects of Mother LC Molecules
on the Stabilization of the BPs. Some papers have reported
that the ratio of the elastic constants K33/K11 was important to
stabilize BPs (K11: splay elastic constant; K33: bend elastic
constant). It is thought that a decrease in K33/K11 lowers the
energy of the disclinations in the BPs.74,75 However, another
paper has reported that chiral LC compounds with a larger K33/
K11 ratio gave a wider BP temperature range.76 Thus, the
relation between K33/K11 and BP stability has not been clarified
yet. In our case, the clear correlation between the K33/K11 ratio
and the stability of the BP was not observed. Both 5CB and
8CB, possessing a rather small K33/K11 ratio, exhibited the BP
Figure 9. Plots of the selective reflection wavelength versus
temperature in the BPs of (a) 5CB doped with 3 mol % of (R)-1e
and (b) EBBA doped with 5 mol % of (R)-1e. The filled and open
diamonds show the plots of BP-I and BP-II, respectively.
LC molecules in the helical superstructure are organized in a
more parallel manner at the temperatures close to the BP−N*
transition temperature because of an increase in intermolecular
interaction among the LC molecules.
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when doped with (R)-1e. The K33/K11 ratio of 5CB is in the
range 1.2−1.4 at TIN − T = 2.6−15.0 K,77−80 and that of 8CB is
the range 0.9−1.1 at TIN − T = 1−5 K81,82 However, PCH-5
(K33/K11 = 1.96 at TIN − T = 16 K83 and K33/K11 = 2.02 at TIN
− T = 19 K84) doped with (R)-1e did not exhibit BPs. These
results indicate that a K33/K11 < 1.4 is suitable for inducing
stable BPs and that a K33/K11 > 1.9 is too large for it. However,
it is difficult to explain the difference between EBBA and
MBBA on the induction of a BP using a comparison of their
K33/K11 ratios. EBBA was the most suitable LC compound to
induce BPs with wide temperature ranges, whereas MBBA did
not show BPs. We could not find a reference for the K33/K11 of
EBBA, but it is estimated to be similar to that of its homologue,
MBBA (K33/K11 = 1.2−1.3 (TIN − T = 4−30 K)85).
Then, to estimate the influence of molecular dipoles of the
LC hosts on BP stability, the molecular dipoles in their most
stable conformers were calculated using the AM1 method.86
The dipole sizes are ordered as follows: 5CB (4.1 D (debye)) ≈
8CB (4.1 D) > PCH5 (4.0 D) > OPOB (2.6 D) ≫ BBPA (0.7
D) > EBBA (0.6 D) > MBBA (0.3 D). The dipole directions of
5CB, 8CB, and PCH-5 are parallel to their molecular long axis,
and that of OPOB is between the directions of its molecular
short and long axes. However, BBPA, EBBA, and MBBA have
very weak molecular dipoles in their lateral directions. We
could not find a relation between the molecular dipole and the
BP stabilization.
ACKNOWLEDGMENTS
■
We thank Dr. H. Ozaki who saved K.K.’s life by an operation
for brain haemorrhage during the term of this project. This
work was supported by Japan Society for the Promotion of
Science KAKENHI Grant Numbers 24655166, 25288088.
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We demonstrated that our chiral dopants 1a−g are effective for
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Corresponding Author
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