Distinct formation of a chiral smectic phase in achiral banana-shaped molecules with a central core based on a 2,7-dihydroxynaphthalene unit

Article abstract of DOI:10.1021/ja001370q

We have prepared a novel series of banana-shaped molecules with a central bent core based on a 2,7-dihydroxynaphthalene group, the side wings containing a Schiff's based moiety, and alkoxy flexible tails with carbon numbers n = 6, 8, 10, 12, 14, and 16. Among these, the molecules with n = 8-16 formed a novel smectic phase. Its smectic layer possessed a liquidlike association of the molecules similar to the SmA or SmC phase, although the texture that developed from the isotropic melt was unconventional. Small fractal domains initially grew and then coalesced into several large domains. Very weak birefringence and fine structure without any anisotropy are characteristic of this phase. It is interesting to note that two different domains exist showing the opposite sign of optical rotation. Strong circular dichroism (CD) with a peak at 430 nm was also observed for each domain with the opposite sign. The results suggest a natural occurrence of the helix in these materials. The right-handed and left-handed helices were formed with equal probabilities, but the left-handed helix was selectively formed by the introduction of a chiral dopant. The molecule with n = 6 formed a frustrated smectic phase, which exhibited a simple fan-shaped texture.

Full text of DOI:10.1021/ja001370q

J. Am. Chem. Soc. 2000, 122, 7441-7448
7441
Distinct Formation of a Chiral Smectic Phase in Achiral
Banana-Shaped Molecules with a Central Core Based on a
2
,7-Dihydroxynaphthalene Unit
Jirakorn Thisayukta, Yusuke Nakayama, Susumu Kawauchi, Hideo Takezoe, and
Junji Watanabe*
Contribution from the Department of Organic and Polymeric Materials, Graduate School of Science and
Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
ReceiVed April 19, 2000
Abstract: We have prepared a novel series of banana-shaped molecules with a central bent core based on a
2
,7-dihydroxynaphthalene group, the side wings containing a Schiff’s based moiety, and alkoxy flexible tails
with carbon numbers n ) 6, 8, 10, 12, 14, and 16. Among these, the molecules with n ) 8-16 formed a novel
smectic phase. Its smectic layer possessed a liquidlike association of the molecules similar to the SmA or
SmC phase, although the texture that developed from the isotropic melt was unconventional. Small fractal
domains initially grew and then coalesced into several large domains. Very weak birefringence and fine structure
without any anisotropy are characteristic of this phase. It is interesting to note that two different domains exist
showing the opposite sign of optical rotation. Strong circular dichroism (CD) with a peak at 430 nm was also
observed for each domain with the opposite sign. The results suggest a natural occurrence of the helix in these
materials. The right-handed and left-handed helices were formed with equal probabilities, but the left-handed
helix was selectively formed by the introduction of a chiral dopant. The molecule with n ) 6 formed a frustrated
smectic phase, which exhibited a simple fan-shaped texture.
Introduction
the polar chiral structure of the B2 phase by using the freely
suspended films and thin homogeneous cells. Moreover, anti-
ferroelectricity of the B2 phase was found in their study. Finally,
the B2 phase was found to be an antiferroelectric phase by much
Bent-shaped molecules have been generating much interest
1-6
in the field of liquid crystals since Watanabe et al. first found
ferroelectricity, helicity, and frustration in several B phases in
the homologous series of bent molecules, P-n-O-PIMB. The
1
2-16
appropriate evidence.
In addition to the B2 phase, the compounds of the P-n-O-
PIMB series also exhibited lower temperature smectic phases,
the so-called Hexb and Smblue phases. They were subsequently
nomenclature “B phase” was used after discussion at the
7
“Banana-Shaped Liquid Crystals” workshop. At the beginning,
from an optical microscope observation of the characteristic
8
3 4
designated B and B phases, respectively. Interestingly, the
fringe pattern, Watanabe et al. described the B2 phase (SmAb
9
4
lowest temperature smectic phase (B ) showed an unidentified
phase ) as a helical structure with the helical axis lying
8
,17
texture. It seems to possess a glassy state characteristic and
shows a transparent blue color. Such a color may be due to a
perpendicular to the smectic layer similar to the chiral SC phase.
The ferroelectric properties were also similar. Later, the structure
of the B2 phase was investigated by many research groups
helical structure with the helical axis lying parallel to the smectic
layer as in the twisted grain boundary (TGB) phase.1
7,18
This
because of its exclusive behavior.10 Link et al. demonstrated
11
helical structure may possibly be formed from a spontaneous
breaking of the mirror symmetry in this bent shape molecule.
However, there is yet no evidence to clarify the structure of the
B4 phase. Thus, the helicity and chirality in achiral bent shape
molecules have been the topic of much interest in the liquid
crystal field.
*
(
Address correspondence to this author.
1) Niori, T.;Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J.
Mater. Chem. 1996, 6, 1231.
2) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. Mol.
Cryst. Liq. Cryst. 1997, 301, 337.
3) Sekine, T.; Takanishi, Y.; Niori, T.; Watanabe, J.; Takezoe, H. Jpn.
J. Appl. Phys. 1997, 36, L 1201.
4) Watanabe, J.; Niori, T.; Sekine, T.; Takezoe, H. Jpn. J. Appl. Phys.
998, 37, L 139.
5) Choi, S-W.; Zennyoji, M.; Takanishi, Y.; Takezoe, H.; Niori, T.;
Watanabe, J. Mol. Cryst. Liq. Cryst. 1999, 328, 185.
6) Takanishi, Y.; Izumi, T.; Watanabe, J.; Ishikawa, K.; Takezoe, H.;
Iida, A. J. Mater. Chem. 1999, 9, 2771.
7) The nomenclature of the smectic phases in bent-shaped liquid crystals
(
(
(
1
(12) Diele, S.; Grande, S.; Kruth, H.; Lischka, Ch.; Pelzl, G.; Weissflog,
W.; Wirth, I. Ferroelectric 1998, 212, 169.
(13) Grande, S.; Weissflog, W.; Lischka, Ch.; Benne, I.; Scharf, T.; Pelzl,
G.; Diele, S.; Kruth, H. Proc. SPIE 1998, 3319, 14.
(14) Zennyoji, M.; Takanishi, Y.; Ishikawa, K.; Thisayukta, J.; Watanabe,
J.; Takezoe, H. J. Mater. Chem. 1999, 9, 2775.
(
(
(
was suggested at the Workshop on Banana-Shaped Liquid Crystals:
Chirality by Achiral Molecules, held in Berlin, December 1997.
(15) Macdonald, R.; Kentischer, F.; Warnick, P.; Heppke, G. Phys. ReV.
Lett. 1998, 81, 4408.
(8) Sekine, T.; Niori, T.; Watanabe, J.; Furukawa, T.; Choi, S. W.;
(16) Choi, S. W.; Kinoshita, Y.; Park, B.; Takezoe, H.; Watanabe, J.
Jpn. J. Appl. Phys. 1998, 37, 4408.
Takezoe, H. J. Mater. Chem. 1997, 7, 1307.
(
(
(
9) This mnemonic was used before changing to the B2 phase.
10) Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999, 11, 707.
11) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.;
(17) Sekine, T.; Niori, T.; Sone, M.; Watanabe, J.; Choi, S. W.; Takanishi,
Y.; Takezoe, H. Jpn. J. Appl. Phys. 1997, 36, 6455.
(18) Goodby, J. W.; Waugh, M. A.; Stein, S. M.; Chin, E.; Pindak, R.;
Patel, J. S. J. Am. Chem. Soc. 1989, 111, 819.
Korblova, E.; Walba, D. M. Science, 1997, 278, 1924.
1
0.1021/ja001370q CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/25/2000

7442 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Thisayukta et al.
Scheme 1
This prompted us to extensively investigate other types of
banana-shaped molecular systems with different mesogenic
cores as shown below. These materials named 2,7-naphthalene
Figure 1. Heating DSC thermograms of the homologous N-n-O-PIMB
compounds.
Table 1. Transition Temperatures and Enthalpies for the
N-n-O-PIMB Series
bis[4-(4-n-alkoxyphenyliminomethyl) benzoate] (N-n-O-PIMB),
have a central core based on the naphthalene group, the side
wings containing a Schiff’s based moiety, and alkoxyl flexible
tails with carbon numbers n ) 6, 8, 10, 12, 14, and 16. It is
interesting that the molecules with n ) 8-16 possessed high-
n
transition temperature °C (enthalpy, kJ/mol)
Cr 237 (12.6) Ia
Sm3 + Cr 222 (10.2) Sm1 237 (5.3) I
Sm3 197 (1.4) Sm2 209 (5.1) Sm1 234 (5.1) I
Sm3 187 (1.3) Sm2 207 (5.0) Sm1 229 (4.3) I
Sm3 179 (1.6) Sm2 204 (5.1) Sm1 225 (4.0) I
Sm3 172 (0.7) Sm2 201 (6.5) Sm1 220 (4.9) I
6
8
10
2
14
6
1
temperature smectic phases with an extraordinary texture, as
well as a low-temperature smectic phase.19 In this report, we
demonstrate the appearance of a helical structure in the smectic
phases by means of optical microscope and circular dichroism
measurements.
1
a
f
Cr 219 (7.9) Sm1 232 (3.6) I: On cooling.
homologous compounds are summarized in Table 1. As shown
Experimental Section
in Table 1, four smectic phases, Sm1, Sm1 , Sm2, and Sm3 are
f
established. Well-defined crystallization takes place for only
The synthesis of the materials used is shown in Scheme 1. The optical
microscopic textures of the materials were examined using a polarizing
microscope (Olympus, BX50) equipped with a hot stage (Mettler Toledo
FP 90 HT). DSC thermogram data were obtained using a Perkin-Elmer
DSC-II differential scanning calorimeter. X-ray diffraction measure-
ments were performed using a Rigaku-Rint-2000 diffractometer with
Cu KR radiation. Circular dichroism (CD) and optical rotation (ORD)
were observed using a JASCO J-720WI spectrometer.
N-6-O-PIMB. N-8-O-PIMB exhibits a crystalline phase on slow
cooling (<10 °C/min). On the other hand, no crystalline phases
could be attained for the other materials under any conditions
nor N-8-O-PIMB when it was subjected to fast cooling. In these
materials, hence, the lowest temperature phase is Sm3. Phase
behavior is shown in Figure 2.
Structural Characteristics of the Sm1 and Sm1f Phases.
Let us consider the highest temperature Sm1 phase (refer to
Figure 2). This phase was observed for the N-n-O-PIMB series
with n ) 8-16. The X-ray patterns exhibited sharp inner
reflections corresponding to the (00l) reflections and an outer
broad reflection corresponding to a spacing of 4.5 Å. Thus, the
X-ray pattern indicates a smectic layer structure similar to the
Results and Discussion
Thermotropic Behavior of N-n-O-PIMB. DSC thermo-
grams are shown in Figure 1. Thermodynamic data of the
(19) Thisayukta, J.; Kamee, H.; Kawauchi, S.; Watanabe, J. Mol. Cryst.
Liq. Cryst. (Anglo-Japanese Seminar on Liquid Crystals ‘99) in press.

Distinct Formation of a Chiral Smectic Phase
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7443
Table 2. Layer Spacings for the N-n-O-PIMB Series
N-6-O-PIMB
34.8 22.0a
101) (002)
N-8-O-PIMB
N-10-O-PIMB
N-12-O-PIMB
N-14-O-PIMB
N-16-O-PIMB
Sm1
f
(
Sm1
Sm2
40.0 19.8
42.2 20.6
44.8
22.3
47.3 23.2
54.1
47.3 32.9 27.2c 53.2
34.9 29.2d 55.9 36.0 31.0e 58.3 37.9 32.9f
(
101) (301) (002) (101) (301) (002) (101) (301) (002) (101) (301) (002)
Sm3
Cr
48.7 24.4 16.4 54.4 27.2 17.7 57.3
29.2 18.9 63.0 31.0 20.5 65.0 32.9 21.7
b
34.8 22.0 17.3 40.0 23.8 15.7
101) (002) (103) (101) (002) (103)
(
a
1
4.1 11.1
(202) (004)
a-f
The indicies are based on the frustrated structures with the two-dimensional lattices: (a) a ) 59 Å, c ) 45 Å; (b) a ) 71 Å, c ) 47 Å; (c)
a ) 66 Å, c ) 54 Å; (d) a ) 67 Å, c ) 58 Å; (e) a ) 67 Å, c ) 62 Å; (f) a ) 71 Å, c ) 66 Å. For other reflections, they can be indexed into
the 00l series.
polarizer or analyzer from the cross-polarization position.
Transmittance of light from one domain is increased by a
clockwise rotation of the analyzer (see Figure 4a), while from
the other domain the transmittance is increased by a counter-
clockwise rotation (see Figure 4b). Using crossed polarizers,
there is no way to distinguish the two domains, except for the
fact that they are separated by a dark boundary (see Figure 3c).
When the texture development was carefully observed by
rotating the analyzer by 5° from the cross-polarization position,
it was found that one fractal nucleus with a specific sign of
optical rotation developed into a large domain with preservation
of the sign of optical rotation. The coalescence of two domains
with the same sign of optical rotation yielded no boundary, while
the coalescence of domains with opposite signs gave a clear
boundary (see Figure 3c). These results suggest the formation
of a helix or chirality.
To confirm the occurrence of a helical or chiral domain, a
circular dichroic (CD) measurement was carried out. Figure 5
shows the CD spectrum observed for the N-12-O-PIMB sample
with several small domains. A small but clear CD peak was
observed with a maximum peak around 430 nm. Since the
Figure 2. Phase behavior of the homologous compounds. The transition
absorption position of the material was in the wavelength region
less than 360 nm, this peak is not due to an induced circular
temperatures are based on heating DSC data. N-6-O-PIMB forms the
Sm1 phase only on cooling (refer to Table 1).
f
dichroism, but from reflection of the circularly polarized light.3
We note that the sign of the CD was altered from positive to
negative, depending on the preparation of sample. For this
reason, the CD spectra measurements were repeated after heating
the sample into the isotropic liquid. After 20 runs, the average
of the intensity became almost zero although the maximum
wavelength scarcely changed as shown in Figure 6a. This
indicates an equal occurrence of the right-handed and left-
handed helices; the observed CD effect comes from an unbal-
anced formation of the two domains.
21
SmA or SmC phases, which consist of a liquidlike association
of bent molecules within a layer. The Sm1f phase was formed
from N-6-O-PIMB only on cooling from the isotropic liquid
(refer to Table 1). It is also composed of a liquidlike association
of molecules within a layer, but has a frustrated structure with
a layer undulation.4 A two-dimensional lattice with a ) 59 Å
and c ) 45 Å was established. The spacings of the inner
reflections are listed in Table 2. It should be noted here that
the similar diffraction profile has also been assigned to the
,6
1
0
columnar phase with two-dimensional rectangular lattice.
The size of the domains varied according to the preparation
condition. It increased with a decrease in the cooling rate
because the number of nuclei is decreased. Large domains with
a size larger than 2 mm can be prepared by the gradual
development of a few nuclei attained by very slow cooling from
isotropic melt (less than 1 °C/min). Thus, we can focus the light
on each domain. The result is shown in Figure 7. It is clearly
seen that domains A and B showing opposite signs of optical
rotation give large CD peaks with opposite signs of dichroic
ratio. The CD intensity is comparable with those observed for
the chiral SmC* and chiral nematic phases, indicating selective
reflection of the circularly polarized light due to a certain helical
structure.
The Sm1 phase shows an unconventional texture. As depicted
in Figure 3, several small fractal nuclei20 initially appear from
the isotropic melt on cooling. They gradually associate into
several large domains (Figure 3b,c). The texture of each domain
exhibits a very weak birefringence and no anisotropy. It looks
somewhat like a sandy or foggy texture with a fine structure.
In contrast to the unusual texture of the Sm1 phase, the frustrated
Sm1f merely displays a normal fan-shaped texture (see Figure
3
d).
Interestingly, the neighboring domains of the Sm1 phase
separated by a clear boundary show opposite optical rotations
see Figure 4a,b). This can be recognized by rotation of the
(
Structural Characteristics of the Sm2 and Sm3 Phases.
In the lower temperature region, two other phases, Sm2 and
(20) In fact, the fractal dimension D of 1.8 was successfully determined
with the box-counting technique (Harrison, A. Fractals in Chemistry; Oxford
University Press: Oxford, 1995; p 16); Thisayukta, J.; Kawauchi, S.;
Watanabe, J. To be submitted for publication.
(21) Saeva, F. D. Liquid Crystals-The Fourth State of Matter; Saeva,
F. D., Ed.; Marcel Dekker, Inc.: New York, 1979; Chapter 6.

7444 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Thisayukta et al.
Figure 3. Photomicrographs of the textures observed for the highest temperature Sm1 phase of N-12-O-PIMB at 210 °C under crossed polarizers.
Initially, the small fractal nuclei appear in (a), grow up with the similar shape in (b), and finally coalesce to the large fractal domain with clear
boundary in (c). In a comparison, the typical fan texture observed for the Sm1 phase of N-6-O-PIMB is shown in (d).
f
Figure 4. The texture of the Sm1 phase observed at the same position as in Figure 3c, but by the rotation of analyzer from the cross-polarization
position. Two different domains with opposite sign of optical rotation can be detected by (a) the clockwise rotation and (b) counterclockwise
rotation. The rotation angle is 5°.
Sm3, are observed (refer to Figure 2). These phases have a
transparent blue color similar to the Smblue (B4) phase in the
P-n-O-PIMB series.3 The DSC thermogram shows a clear
transition from Sm1 to these phases, but the overall domain
texture and fine texture inside the domains are not altered at all
from those of the Sm1 phase. Only a slight blue shift of the
color was observed. Similar CD spectra were also observed;
the shape of the CD peak scarcely changes, but the peak

Distinct Formation of a Chiral Smectic Phase
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7445
two-dimensional lattice data of the frustrated structure is given
in Table 2. The small transition enthalpies between the Sm2
and Sm3 phases suggest a small change in their structures (refer
to Figure 1 and Table 1).
Effect of Chiral Dopant. To further investigate the occur-
rence of helical domains, we synthesized the chiral N-4-O-
PIMB2* with a terminal (S)-(+)-2-methylbutoxy group as
shown below. This chiral compound, however, does not form
2
Figure 5. Typical CD spectrum observed in the Sm1 phase composed
of the small fractal domains.
a liquid crystal; the melting temperature of the crystal is 247
°C. Thus, we prepared blended samples of N-4-O-PIMB2* and
N-12-O-PIMB with various weight fractions of chiral dopant
from 0% to 30%. Figure 8 shows the optical microscopic
textures of the Sm1 phase for these blended samples. We found
that one of the two different chiral domains, the areas of which
maximum was shifted slightly to the blue side. The maximum
wavelength was around 400 and 420 nm for the Sm3 and Sm2
phases, respectively. Thus, a preservation of the macroscopic
chiral structure is likely in these phases.
Three smectic phases, Sm1, Sm2, and Sm3 phases, can be
clearly distinguished from the X-ray patterns. The X-ray
diffraction data of these phases are summarized in Table 2. As
mentioned above, the Sm1 phase shows a typical smectic layer
structure with liquidlike association within the smectic layer,
while in the Sm2 and Sm3 phases a few outer reflections appear,
suggesting two-dimensional positional ordering within a layer.
It is interesting that the layer spacing of the Sm3 phase is
somewhat larger than that of the Sm1 phase. This suggests that
the molecules in the Sm1 phase are appreciably tilted to the
layer. Under the assumption that a similar conformation and
molecular length are maintained in these phases, and that the
molecules in the Sm2 and Sm3 phases lie perpendicular to the
layer, a tilt angle of the molecules was elucidated as ap-
proximately 40°. According to this argument, no tilting of the
molecules is observed in the frustrated Sm1f phase.
+
-
are A and A , gradually decreases with an increase in the
amount of the chiral dopant and the texture becomes composed
of a single chiral domain for mixtures with 20-30% chiral
content. This trend is clearly observed in Figure 9 where the
+
-
+
relative difference between the two areas, |A - A |/|A
+
-
A |, is plotted against the chiral content. A similar trend can
be detected from the CD measurement. Figure 6b shows a plot
of the maximum intensity and wavelength of the CD spectra
obtained by 20 observations of the mixture with a chiral content
of 20%. It is clear that the intensity of the CD is fairly large
and the sign becomes biased to the positive side. In contrast,
the wavelength of the CD peak scarcely changes. Thus, we
conclude that the chiral dopant does not affect the twisting power
of the helix, or in other words, the helical twisting is due to the
inherent nature of such banana-shaped molecules. The only
effect of the chiral dopant is to dominate the helical sense. A
similar effect of chiral dopants on circularly polarized scattered
A difference in the X-ray patterns between the Sm2 and Sm3
phases was observed for the inner reflection profiles; the inner
reflections of the Sm3 phase can be indexed to a series of 00l
reflections, but other h0l reflections are observed for the Sm2
1
7
light was made in the B4 phase of P-8-O-PIMB.
Figure 10 shows the typical CD and ORD spectra observed
for a single chiral domain of the blended sample with a 20%
4
phase, which can be explained by frustration of the layer. The
Figure 6. Peak wavelength and intensity of CD collected for the Sm1 phase by 20 runs: (a) pure N-12-O-PIMB and (b) N-12-O-PIMB containing
0% chiral N-4-O-PIMB2*.
2

7446 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Thisayukta et al.
Figure 7. (a) Photomicrographs for large A and B domains of the
Sm1 phase of N-12-O-PIMB with the different optical rotation and (b)
CD spectra taken by focusing the light on each A or B domain. The
texture was observed by rotating the analyzer clockwise by 5° from
the cross-polarization position. The different sign of CD can be clearly
observed between A and B domains.
chiral content. These are to be expected from a left-handed
helical structure.22,23
Origin of the Helix. Two basic questions arise with respect
to the spontaneous formation of a helix in achiral molecules:
(1) What is the structure of the helix? (2) Why can a helical
structure be formed from achiral molecules?
In response to the first question, the texture exhibited a very
weak birefringence and no anisotropy as mentioned above. This
character was the same regardless of the sample preparation,
for example, cooling rate, sample thickness, glass surface
treatment, etc. Fractral appearance indicates a completely
different structure from that of the conventional smectic phase,
suggesting the formation of a three-dimensional helical structure
24,25
like a blue phase or foggy phase.
We have tried to directly
measure the helical structure by means of electron microscopy
and atomic force microscopy. Unfortunately, however, we failed
to obtain a satisfactory result.
It should be noted here that there are several other examples
26,27
suggesting the helicity in the smectic phase.
One of the
Figure 8. Photomicrographs of the Sm1 phase observed for N-12-O-
PIMB with the contents of a chiral dopant of (a) 3%, (b) 10%, and (c)
20%. The texture was observed by rotating the analyzer clockwise by
(
22) Chandrasekhar, S. Liquid Crystals; Cambridge University Press:
New York, 1992; Chapter 4.
(
23) Watanabe, J.; Nagase, T. Macromolecules 1988, 21, 171.
5° from the cross-polarization position. It can be observed that the area
(24) Flack, J. H.; Crooker, P. P.; Johnson, D. L.; Long, S. Liquid Crystals
of one domain becomes predominant with the increase of chiral dopant.
and Ordered Fluids; Griffin, A. C., Johnson, J. F., Eds.; Plenum: New
York, 1984; Vol. 4, p 901.
typical examples is observed in the smectic B7 phase. This phase
appears as elongated germs with a clearly spiral or double spiral
character. The germs finally coalesce to form fanlike or circular
(
25) Yang, D. K.; Crooker, P. P. Phys. ReV. A 1988, 37, 4001.
(26) Pelzl, G.; Diele, S.; Lischka, Ch.; Wirth, I.; Weissflog, W. Liq. Cryst.
1
999, 26, 1, 135.
27) Thisayukta, J.; Nakayama, Y.; Watanabe, J. Liq. Cryst. In press.
8
,26,27
(
domains including the fringe pattern.
The texture is

Distinct Formation of a Chiral Smectic Phase
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7447
Figure 9. Relative difference in the area between the two domains
+
-
+
-
with opposite optical rotation, |A - A |/|A + A |, plotted against
the chiral content. The data are collected from the photomicrographs
of Figure 8.
Figure 11. Illustration of (a) the most stable twist conformation
calculated for the banana-shaped molecule with 1,3-dihydroxybenzene
as a core, (b) escape from spontaneous polarization, and (c) tilting of
the molecule to the layer normal producing the mirror image.
central phenyl ring, i.e. the molecule should be twisted (con-
formational chirality). In addition, a significant result was found
from the conformational calculation study of a bent-shaped
model compound, 1,3-benzenediol dibenzoate in which a 1,3-
dihydroxybenzene core was linked to two phenyl rings via an
2
9
ester bond linkage. The result shows that the most stable
conformation was produced as a result of the two carbonyl
groups of the two side-wings twisting to each other as illustrated
in Figure 11a. This twisted conformation was supported by
3
0
31
recent polarizing FT-IR measurements in pure and doped
systems. In this study, the 1,3-dihydroxybenzene is altered to
the 2,7-dihydroxynaphthalene, but the similar twist conformation
can be expected. In general, however, the spontaneous resolution
of twist conformation is unlikely since it is contrary to
thermodynamic principles in the fluid liquid crystals. Possibly
the confined packing of molecules due to their bent shape may
enhance the differences in conformation energy as in the solid
Figure 10. Typical CD and OR curves for the simple chiral domain
of the Sm1 phase prepared in the blended sample of N-12-O-PIMB
and N-4-O-PIMB2* with the chiral content of 20%.
completely different from the present one, but it is obvious that
the helical formation is spontaneously induced. The study is
still proceeding in order to clarify the helical structure.
For the second question, there are three possibilities that
should be taken into account (Figure 11a-c).
2
8
state.
The twisting power can be induced by an escape from the
1
9,32
spontaneous polarization as illustrated in Figure 11b.
proposed by Watanabe et al.,
As
1,33,34
the bent molecules are forced
to be packed with the same bent directionality, leading to a
spontaneous polarization within each layer. Here, we have to
say that the system tends to escape from the resulting polariza-
We first consider the molecular chirality that is caused by
28
the twist conformation as in the crystals. This consideration
13
is based on the solid state C NMR study of the P-n-O-PIMB
compounds.17 In this study, the most remarkable feature was
the signal from the carbonyl carbons in the ester linkage. Its
signal appeared as a doublet in the SmAb (B2) phase (164.3
and 164.0 ppm), whereas it appeared as a singlet in the isotropic
tion. Two ways are observed: one is the antiferroelectric
structure which cancels the polarity between layers1
2-16,34
and
another is the frustrated structure which extinguishes the in-
(
29) Imase, T.; Kawauchi, S.; Watanabe, J. J. Phys. Chem. Submitted
for publication.
30) Zennyoji, M.; Takezoe, H. To be submitted for publication.
(
164.6 ppm) and crystal phases (162.5 ppm). A similar doublet
was also observed in the SmBlue (B4) phase. Therefore it has
been concluded that the two carbonyl groups connecting the
central core of the molecule are under different electronic
circumstances. In this particular conformation, hence, the two
carbonyls have to make different angles with respect to the
(
(31) Gorecka, E.; Nakata, M.; Mieczkowski, J.; Takanishi, Y.; Ishikawa,
K.; Watanabe, J.; Eichhorm, S. H.; Swager, T. M. Phys. ReV. Lett. Submitted
for publication.
(
(
32) Khachaturyan, A. G. J. Phys. Chem. Solids 1975, 36, 1055.
33) Watanabe, J.; Hayashi, M.; Nakata, Y.; Niori, T.; Tokita, M. Prog.
Polym. Sci. 1997, 22, 1053.
(34) Niori, T.; Choi, S. W.; Takanishi, Y.; Takezoe, H.; Watanabe, J.
Jpn. J. Appl. Phys. 1998, 37, L401.
(
28) Casarini, D.; Lunazzi, L.; Pasquali, E.; Gasparrini, F.; Villani, C.
J. Am. Chem. Soc. 1992, 114, 6521.

7448 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Thisayukta et al.
plane polarity.4
,6,35-37
Thus, it is reasonable to consider a TGB-
For the Sm1 phases of the n ) 8, 10, 12, 14, and 16
homologues, an unconventional texture was observed. Initially
on cooling from the isotropic liquid, small-fractal nuclei
appeared and coalesced to form several large domains. Very
weak birefringence and no anisotropy are characteristic of this
phase although X-ray data showed a liquidlike packing of
molecules in the smectic layers. Most interesting is that two
different domains having an opposite optical rotation were
observed under uncrossed polarizers. In addition, the circular
dichroisms (CD) with a peak around 430 nm and with the
opposite signs were observed for these two domains. The results
strongly suggest the formation of a helical structure in these
phases. Of course, the two domains of right- and left-handed
helices are simultaneously formed.
like helix as a third type of escape from the polarization.
As reported by Heppke and Moro,38 an ordering of bent-
shaped molecules by tilting with respect to the layer normal
can generate a mirror image so that it would be able to build
up an enantiomorphic structure by chirality. In other words, the
C2V symmetry is altered to C2 symmetry by tilting as illustrated
in Figure 11c. In fact, the chiral phases observed so far are
formed by a tilted association of molecules.1
1,19,26,38
Unique determination of the origin of the helicity will be the
next important subject.
Conclusion
We have synthesized a series of banana-shaped molecules,
N-n-O-PIMB with n ) 6-16 and the 2,7-dihydroxynaphthalene
unit, as the mesogenic bent core. They all exhibit the high-
temperature Sm1 phase as well as the low-temperature Sm3
phase except for N-6-O-PIMB, which forms only a frustrated
Sm1f phase (from X-ray data). Moreover, a frustrated Sm2 phase
was also observed between the Sm1 and Sm3 phases for the
materials where n ) 10, 12, 14, and 16.
A chiral dopant was used to obtain more information about
this helix. We found that the helical sense was naturally affected
by the doping agent. However, the doping agent did not
influence the twisting power. This would be, therefore, an
example of a distinct formation of a helical structure in an achiral
molecular system. Many reasons have been considered for this
formation of a helix in these achiral materials as discussed
above. However, to fully understand this phenomenon we need
to collect more significant and decisive data. Our progress will
be reported in due course.
(
35) Nakata, Y.; Shimizu, K.; Watanabe, J. J. Phys. II (France) 1994,
4
, 581.
(
(
36) Watanabe, J.; Nakata, Y. Polym. J. 1997, 29, 193.
37) Watanabe, J.; Izumi, T.; Niori, T.; Zennyoji, M.; Takanishi, Y.;
Takezoe, H. Mol. Cryst. Liq. Cryst. In press.
38) Heppke, G.; Moro, D. Science 1998, 279, 1872.
(
JA001370Q