An unusual phase sequence of iso liq-blue phase-smectic A observed for novel binaphthyl mesogenic derivatives

Article abstract of DOI:10.1039/b410931g

A homologous series of novel chiral dimeric compounds, (-R)-2,2′- bis{6-[4-(2-(2-fluoro-4-butyloxyphenyl)pyrimidine-5-yl)phenyloxy]alkyloxy}-1, 1′-binaphthyl, has been prepared and the physical properties investigated. The binaphthyl derivatives with an e

Full text of DOI:10.1039/b410931g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
An unusual phase sequence of iso liq-blue phase-smectic A observed for
novel binaphthyl mesogenic derivatives{
Juri Rokunohe and Atsushi Yoshizawa*
Received 19th July 2004, Accepted 2nd September 2004
First published as an Advance Article on the web 24th September 2004
DOI: 10.1039/b410931g
A homologous series of novel chiral dimeric compounds, (R)-2,29-bis{6-[4-(2-(2-fluoro-4-
butyloxyphenyl)pyrimidine-5-yl)phenyloxy]alkyloxy}-1,19-binaphthyl, has been prepared and the
physical properties investigated. The binaphthyl derivatives with an even number of atoms in the
spacers showed a chiral nematic phase, however, those with an odd number of atoms showed an
unusual direct blue phase to smectic A phase transition.
Investigation of chirality in liquid crystals is one of the most
exciting areas of liquid crystal science. Frustrated phenomena
induced by chirality have been reported. Twist grain boundary
(TGB) phases, blue phases, and smectic Q phases have been
observed and structure–property correlations investigated.1
Blue phases are of particular interest because they have a fluid
lattice structure that is stabilized by lattice defects. Appearance
of blue phases results from the competition between the chiral
twisting force and the desire for molecules to pack in ways
such that they fill space uniformly. Theoretical and experi-
mental works have demonstrated that chiral nematic liquid
crystals of short pitch can form up to three distinct blue
phases, i.e. blue phase I (BPI), blue phase II (BPII) and blue
phase III (BPIII).2–4 Blue phases are normally found in a very
narrow (ca. 1 uC) temperature range between the isotropic
liquid and a chiral nematic (N*) phase of sufficiently short
pitch, except in a few cases where a SmA to BPI transition has
been observed.5–7 Double twist cylinders are believed to exist
in BPI and BPII. Meiboom et al. discovered that the local free
energy of most chiral nematic liquid crystals could be reduced
by a double twist structure.8 Such a structure requires
energetically unfavourable disclinations between separate
double twist regions. As the liquid crystals approach the
isotropic transition, the energy cost of disclinations lowers and
a network of the double twist structures with disclinations
might be more stable than the helical N* phase. Li et al.
reported the observation of a different phase sequence I-BPs-
TGBA-TGBC-SmC* in a fluoro-substituted chiral tolane
derivative.9,10 Recently, there has also been experimental
evidence for a smectic blue phase, which is observed between
the isotropic and smectic phases.11,12 From the viewpoint of
applications, blue phases are interesting for fast light
modulators or tunable photonic crystals. However, the narrow
temperature range is a critical problem. Kikuchi et al. reported
polymer-stabilized blue phases in which a temperature range is
extended to more than 60 uC and they demonstrated a fast
electro-optical switching for the stabilized blue phase.13 On the
other hand, many kinds of liquid-crystalline materials posses-
sing a centre of chirality have been investigated. Recently
compounds possessing an axis or a plane of chirality have been
reported.14–17 A single chiral compound with a wide blue
phase-temperature range has never been obtained. Theoretical
work suggests that biaxiality plays an important role in the
blue phases.18 However, the biaxiality in most chiral nematic
liquid crystals is slight, and as a result the double twist
structure cannot exist in a wide temperature range.
We reported an U-shaped molecule that induces a smectic-
like layer ordering in the nematic phase and noted the
possibility that the nematic (N) phase has biaxiality.19 Thus
we have designed coupling between axial chirality and the U-
shaped system. The designed system is expected to induce
unusual order in the N* phase. In the present study, we have
prepared a homologous series of dimeric liquid-crystalline
compounds possessing a binaphthyl group ((R)-n) and found
an unusual phase sequence of iso-BP- SmA. A few examples of
binaphthyl derivatives have been reported20,21 and some of
them show chiral smectic phases,21 however, a N* phase has
not been observed for binaphthyl mesogenic derivatives.
{ Electronic supplementary information (ESI) available: Analytical
data for compounds (S)-9, (R)-6, (R)-7, (R)-8, (R)-10, (R)-11 and (R)-
12. Photomicrograph of a contact region between compounds (R)-9
(right-hand side) and (R)-10 (left-hand side) at 118.5 uC.
Photomicrograph of a contact region between mixtures of achiral
host material, 5-octyl-2-(4-hexyloxyphenyl)pyrimidine, and 5 wt% of
each chiral compound, i.e. compound (R)-9 (left-hand side) and
compound (R)-10 (right-hand side) at 63.5 uC in the N* phase. See
http://www.rsc.org/suppdata/jm/b4/b410931g/
Experimental
Spectroscopic analysis
Purification of final products was carried out using column
*ayoshiza@cc.hirosaki-u.ac.jp
chromatography over silica gel (63–210 mm) (Kanto Chemical,
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J. Mater. Chem., 2005, 15, 275–279 | 275

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Tokyo, Japan) using dichloromethane or a dichloromethane–
ethyl acetate mixture as the eluent, followed by recrystalliza-
tion from ethanol. The purities of final compounds were
checked by thin layer chromatography (TLC, aluminium
sheets, silica gel 60 F254 from Merck, Elmsford, NY, USA).
Dichloromethane was used as the solvent. Detection of
products was achieved by UV irradiation (l = 254 and
365 nm). The purities of the final compounds were also
checked by normal phase HPLC (Intersil SIL 150A-5 column).
A dichloromethane–isopropylalcohol (85 : 15) mixture was
used as eluent. Detection of products was achieved by UV
irradiation (l = 254 nm).
Potassium carbonate (0.14 g, 1.0 mmol) was added to a
solution of 5-{4-(6-bromononyl)}phenyl-2-(2-fluoro-4-buty-
loxyphenyl)pyrimidine (0.54 g, 1.0 mmol) and (R)-2,29-dihy-
droxy-1,19-binaphthyl (0.14 g, 0.50 mmol) in cyclohexanone
(10 ml). The reaction mixture was stirred at 140 uC for 9 h.
After the filtration of precipitate, the solvent was removed by
evaporation. The residue was purified by column chromato-
graphy on silica gel with (39 : 1) dichloromethane–ethylacetate.
Recrystallization from ethanol gave the desired product. Yield
0.23g (0.19 mmol, 38%).
1H NMR (270 MHz, solvent CDCl3, standard TMS) dH/
ppm: 9.00(s, 4H, Ar–H), 8.12(t, 2H, Ar–H, J = 8.9 Hz), 7.90(d,
2H, Ar–H, J = 8.9 Hz), 7.83(d, 2H, Ar–H, J = 8.1 Hz), 7.54(d,
4H, Ar–H, J = 8.6 Hz), 7.39(d, 2H, Ar–H, J = 8.9 Hz), 7.32–
7.16(m, 6H, Ar–H), 7.03(d, 4H, Ar–H, J = 8.6 Hz), 6.84–
6.71(m, 4H, Ar–H), 4.03(t, 4H, Ar–OCH2–, J = 6.5 Hz),
4.00–3.84(m, 4H, –CH2O–), 4.00(t, 4H, Ar–OCH2–, J =
The structures of the final products were elucidated by
infrared (IR) spectroscopy (BIO RAD FTS-30) and proton
nuclear magnetic resonance (1H NMR) spectroscopy (JEOL
JNM-GX270). Elemental analysis (Perkin Elmer 2400II, CHN
analyzer) was obtained for each final compound.
6.5 Hz), 1.85–0.89 (m, 42H, aliphatic–H); IR (KBr) nmax/cm21
:
Preparation of materials
2932, 2855, 1621, 1439, 1430, 1248; Purity: 100%. Elemental
analysis: Found: C, 77.35; H, 6.89; N, 4.75. Calc. for
C78H84F2N4O6: C, 77.26; H, 6.93; N, 4.62%.
The binaphthyl derivative (R)-n was prepared by the synthesis
outlined in Scheme 1. Optically active (R)-2,29-dihydroxy-
1,19-binaphthyl and (S)-2,29-dihydroxy-1,19-binapthyl were
obtained from Aldrich Chemical Company, Milwaukee, WI,
The other compounds presented in this paper were obtained
by a similar method to that for (R)-9. Analytical data for the
other compounds are listed in Electronic Supplementary
Information.{
USA.
5-(4-Hydroxy)phenyl-2-(2-fluoro-4-butyloxyphenyl)-
pyrimidine was obtained from Midori Kagaku Co., Ltd.,
Tokyo, Japan.
Liquid-crystalline and physical properties
(R)-2,29-Bis{6-[4-(2-(2-fluoro-4-butyloxyphenyl)pyrimidine-5-
yl)phenyloxy]nonyloxy}-1,19-binaphthyl, (R)-9
The initial assignments and corresponding transition tempera-
tures for the final products were determined by thermal optical
microscopy using a Nikon Optiphot POL polarizing micro-
scope equipped with a Mettler FP82 microfurnace and FP80
Potassium carbonate (0.41 g, 3.0 mmol) was added to a
solution of 5-(4-hydroxy)phenyl-2-(2-fluoro-4-butyloxyphe-
nyl)pyrimidine (1.0 g, 3.0 mmol) and 1,9-dibromononane
(1.1 g, 4.0 mmol) in cyclohexanone (15 ml). The reaction
mixture was stirred at 80 uC for 6 h. After the filtration of
precipitate, the solvent was removed by evaporation. The
residue was purified by column chromatography on silica gel
with dichloromethane. The intermediate product, 5-{4-(6-
bromononyl)}phenyl-2-(2-fluoro-4-butyloxyphenyl)pyrimidine,
was obtained. Yield 0.79 g (1.4 mmol, 49%).
control unit. The heating and cooling rates were 5 uC min21
,
unless otherwise indicated. Temperatures and enthalpies of
transition were investigated by differential scanning calorime-
try (DSC) using a Seiko DSC 6200 calorimeter. The material
was studied at a scanning rate of 5 uC min21, for both heating
and cooling cycles, after being encapsulated in aluminium
pans. The helical pitch in the N* phase was measured by the
Cano wedge method for a chiral nematic mixture consisting of
Scheme 1 Synthesis of compound (R)-n.
276 | J. Mater. Chem., 2005, 15, 275–279
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nematic liquid-crystalline material, 4-hexyl-49-cyanobiphenyl
(6CB) (purchased from BDH, Dorset, UK) and each chiral
additive. The mixtures were studied using the contact method
and their chiral nematic helical twist senses were established.
The helical pitches were measured at room temperature. The
definition of the helical twist senses used in this article is the
same as that of Goodby.22
Results and discussion
The phase transition behaviour of compound (R)-9 was
investigated by optical microscopy. On cooling (5 uC min21
)
the isotropic liquid exhibited a blue colour of low birefringence
and then showed a platelet texture composed of blue and slight
red plates. This transition behaviour is indicative of blue phase
I (Fig. 1a). On further cooling the BP changed to a smectic A
(SmA) phase without appearance of a N* phase (Fig. 1b). The
temperature range of the BP was about 4 uC. On heating from
the SmA phase, a SmA to BP phase transition was observed.
We investigated the effect of chirality on the transition
behaviour of compound (R)-9. A contact study between
compound (R)-9 and its (S)-isomer was carried out. The
texture in the contact region is shown in Fig. 2. A Schlieren
Fig. 2 Photomicrograph of a contact region between compounds
(R)-9 and (S)-9 at 118 uC. Magnification 2006.
texture is seen in the central region, indicating that this region
is an N phase. On decreasing the optical purity, the blue phase
of the single compound changed to the N* phase and then to
the N phase in its racemic mixture. Transition behaviour of the
racemic mixture of compounds (R)-9 (50 wt%) and (S)-9
(50 wt%) on cooling was I 121 uC N 115 uC SmA.
We investigated the effect of spacers on the phase transition
behaviour of the binaphthyl derivatives. Temperatures and
enthalpies of transition determined for the (R)-n compounds
by optical microscopy and differential scanning calorimetry
(DSC) are compared in Table 1. The binaphthyl derivatives
with an even number of atoms in the spacers showed a chiral
nematic phase, whereas those with an odd number of atoms
showed a blue phase. The remarkable odd–even effect on
chirality-dependent phase transition was observed for the
binaphthyl mesogenic derivatives. As the spacer length
increases, the stability of the SmA phase increases.
Eventually, compound (R)-12 showed only a SmA phase.
In order to understand the spacer effect on the transition
behaviour, we investigated twisting power of each chiral
compound. The helical pitch values and their helical twist
senses in the N* phases are listed in Table 2.
The results of induced pitches in a host nematic liquid
crystal doped with each chiral material indicate that twisting
power of the odd dimers is smaller than that of the even
dimers. Helical twist sense for compounds (R)-6, (R)-7, (R)-8
and (R)-9 was right-handed, however, that for compounds
Table 1 Transition temperatures (uC) on cooling and enthalpies (kJ
mol21) of transition (in brackets) for compounds (R)-n
n
SmA
N*
BP
I
mpa
6
7
8
9
. 63 (0.7)
. 94 (6.7)
. 105 (5.2)
. 116 (8.6)
. 108 (3.5)
. 127b
. 113 (1.4)
. 124 (3.2)
. 122 (2.5)
.
.
.
.
.
.
.
99
54
100
46
41
62
74
. 103(2.0)
. 120 (2.2)
. 127b
10
11
12
a
. 135 (9.4)
b
The melting points were measured by DSC. The I-BP and BP-
SmA transitions occurred simultaneously. The total value of both
Fig. 1 Photomicrographs of (a) the blue phase at 118 uC and (b) the
blue phase to SmA phase transition at 116 uC of a homogeneously
aligned sample of compound (R)-9. Magnification 1006.
transition enthalpies was 5.9 kJ mol21
.
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Table 2 Helical pitch in the N* phase for a chiral nematic mixture of
6CB and 2 wt% of each chiral compound and the helical twist senses,
i.e. right-handed (RH) or left-handed (LH)
n
Pitch/mm
Sense
6
7
8
9
10
11
12
8.4
21.9
RH
RH
RH
RH
LH
LH
LH
8.5
too long to be observed
8.2
too long to be observed
16.1
(R)-10, (R)-11 and (R)-12 was left-handed. The chiral materials
producing the opposite handedness in the mixtures, i.e.,
compounds (R)-9 and (R)-10, were contacted. The contact
region between the blue phase of compound (R)-9 and the N*
of compound (R)-10 was discontinuous.{ In order to confirm
the opposite handedness, mixtures composed of achiral host
material, 5-octyl-2-(4-hexyloxyphenyl)pyrimidine, and 5 wt%
of each chiral compound, i.e. compound (R)-9 and compound
(R)-10 were also contacted. A nematic phase was found to be
induced in the central region between the two mixtures in the
N* phase, indicating that the helical senses for compounds
(R)-9 and (R)-10 are opposite in the N* phase.{ Further
investigation is necessary to clear origin of the helical twist
inversion.
Fig. 3 MM2 models of (a) compound (R)-6 with even-numbered
spacers and (b) compound (R)-7 with odd-numbered spacers.
configuration of the two 2-fluorophenyl-5-phenylpyrimidine
moieties can produce the blue phase. The present chiral U-
shaped system is thought to be a new molecular design for the
appearance of blue phases. Furthermore, a new type of helical
twist inversion was observed for the novel chiral system.
Interesting odd–even effects were observed for chiral
properties of dimeric liquid crystals.23–25 Blatch, Fletcher and
Luckhurst reported that blue phase I behaviour was observed
in a very narrow temperature range (0.6 uC) between isotropic
liquid and N* phases for the odd non-symmetric dimers,
whereas a direct I–N* transition was observed for the even
non-symmetric dimers.26 There is no marked difference in
twisting power between the odd and even dimers, whereas the
helical pitch for the odd dimer in its N* phase is significantly
smaller than for the even dimers. The authors noted that the
odd–even effect on the appearance of blue phase I is related to
the smaller twist elastic constant of the odd dimers.
Acknowledgement
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science,
Sports, Culture, and Technology in Japan (No. 15550156).
Juri Rokunohe and Atsushi Yoshizawa*
Department of Materials Science and Technology, Faculty of Science
and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561,
Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp; Fax: +81 172 39 3558;
Tel: +81 172 39 3558
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