Mesomorphic behavior of bent-shaped molecules with non-aromatic central core based on bis(p-hydroxyphenyl)methane

Article abstract of DOI:10.1246/cl.2008.1134

New homologous series of bent-shaped molecules with non-aromatic central core based on bis(p-hydroxyphenyl)methane were studied comparing with the conventional bent-shaped molecules with aromatic central core. Copyright

Full text of DOI:10.1246/cl.2008.1134

1134
Chemistry Letters Vol.37, No.11 (2008)
Mesomorphic Behavior of Bent-shaped Molecules with Non-aromatic Central Core
Based on Bis(p-hydroxyphenyl)methane
Seng Kue Lee,ꢀ Lu Shi, Masatoshi Tokita, and Junji Watanabe
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
(Received August 22, 2008; CL-080803; E-mail: sklee@polymer.titech.ac.jp)
New homologous series of bent-shaped molecules with non-
Table 1. Phase-transition temperature (ꢁC) and enthalpies (ꢀH,
kJ molꢂ1) for the (CH2)-On series (on cooling at a rate of
aromatic central core based on bis(p-hydroxyphenyl)methane
were studied comparing with the conventional bent-shaped
molecules with aromatic central core.
10 ꢁC minꢂ1
)
n
Transition temperature (enthalpy)
4
7
10
13
16
Iso 253.1 (14.5) B1rev 208.9 (52.3) Cr1
Iso 241.5 (21.9) B7 184.0 (51.9) Cr1
Iso 228.8 (20.6) B7 175.8 (47.7) Cr2 127.4 (1.88) Cr1
Iso 220.5 (18.4) B7 168.6 (43.7) Cr2 135.3 (10.7) Cr1
Iso 216.5 (26.0) B7 164.0 (63.1) Cr2 138.3 (13.2) Cr1
Since discovery by Watanabe et al. for the first time,1 bent-
shaped molecules (so-called banana molecules) have attracted
great attention in the field of innovative liquid crystal research.
Their unique properties of spontaneous chiral symmetry break-
ing and decoupling of polar and tilt order have been extensively
studied for the last decade.2–4 A number of bent-shaped mole-
cules have been synthesized and characterized, and at least eight
different phases have been identified.5 Particularly, the struc-
ture–property relationship in bent-shaped molecules is a very
important factor for the development of polar order and chirality,
and it can cause considerable interlayer interaction, influencing
the constitution for the Bn phases.5,6 For this reason, many ap-
proaches have been made to vary the chemical structure, espe-
cially the central bent core position. Nevertheless, most bent-
shaped molecules are usually designed by using 1,3-disubstitut-
ed aromatics or heteroaromatic units, or by joining two road-like
molecules by noncyclic spacers with an odd number of atoms.
However, the introduction of non-aromatic central core units
has been rarely reported. Thus, we have synthesized a new ho-
mologous series of bent-shaped molecules [(CH2)-On, see
Scheme 1] with non-aromatic central core based on bis(p-hy-
droxyphenyl)methane and side wings containing Schiff base
moieties.10 Electro-optical and X-ray diffraction measurements
have been utilized for the investigation of their mesomorphic
properties and phase structures.
All the compounds exhibited enantiotropic transitions, and
their mesophase behavior was different from each other in ac-
cordance with the length of terminal alkyl chain. The mesomor-
phic transition temperature and enthalpy change for the final
products were collected from DSC in conjunction with POM.
They are listed in Table 1.
On cooling from the isotropic liquid compound (CH2)-O4
formed a fan-shaped texture along with a mosaic pattern in some
regions (Figure 1a). This fan-shaped texture was sensitive to
change in applied electric field. Optical birefringence in fan-
shaped textures suddenly increased and a smooth texture was
obtained (Figure 1b). However, the texture remained unchanged
Figure 1. Photomicrographs of the frustrated smectic phases
observed for compound (CH2)-O4. (a) B1rev phase obtained on
cooling the liquid isotropic phase, (b) B1rev phase during the
DC field (7.8 V/mm) application, and (c) B1rev phase observed
upon the off-state of the field. White arrows indicate polarizer
directions. (d) Proposed schematic representation.
after terminating the electric field with only a small decrease of
the birefringence (Figure 1c). Both field-on and field-off states
had the same extinction directions, which means that the field-
induced texture change is irreversible and the principal axes of
the optical dielectric tensor do not change after the electric field
is applied. However, polarization reversal current was not found
at this phase for field lower than 30 V/mm. The X-ray diffraction
pattern was obtained using X-ray beam perpendicular to cells
sandwiched between thin glasses. Two sharp inner reflections
and a broad outer reflection were observed, indicating intralayer
liquid-like order. These characteristics of the diffraction come
from the two-dimensional (2D) frustrated smectic phase struc-
ture, and the original and additional peaks correspond to (002)
and (101), respectively. Thus, this diffraction geometry of
the frustrated phase can be explained nicely by 2D lattice of
˚
˚
the non-tilted B1rev phase with a ¼ 44:7 A and c ¼ 40:6 A
(Figure 1d) in which 2D density modulations are in a plane per-
pendicular to the spontaneous polarization vector.7 The molecu-
lar length (40.5 A) calculated by Spartan’02 program is consis-
tent with the layer spacing by X-ray diffraction experiment
˚
O
O
˚
(40.6 A).
As shown in Figures 2a–2c, a variety of texture variants to
O
O
N
N
the B7 phase was observed in compounds (CH2)-O7 and
(CH2)-O10 when the cell was slowly cooled from the isotropic
phase. Spiral, myelinic-like, banana-leaf-like, oval-like, and
(CH2)-On (n = 4, 7, 10, 13, and 16)
H2n+1CnO
OCnH2n+1
Scheme 1.
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.11 (2008)
1135
lations by Spartan’02 program. It was shown that replacement of
phenyls with –CH2– reduce the angle between side wings by
about 7ꢁ. Moreover, the band linkage between –CH2– core and
phenyl can make rotation somewhat free in the lowest energy
conformation, thus the twist angle of the phenyl becomes 90ꢁ.8
Therefore more flexible conformations are allowed in the liquid
crystalline region, which leads to essentially different mesophas-
es from that based on the aromatic central core. In fact, all of our
molecules shows mesophase at a temperature region below the
isotropic melt different from that of the conventional phenyl cen-
tral core molecules, P-n-O-PIMB, despite that the chemical
structure is almost the same except in the central core.9 For ex-
ample, most of the conventional phenyl central core molecules
show the B2 phase at a temperature region below the isotropic
melt, but our molecules show the B7 phase on cooling from
the isotropic liquid. Another difference is that our molecules
do not show the chiral segregated domains as was observed in
the chiral blue phase. Thus, these results indicate that the mod-
ification of central bent core plays a major role for the emergence
of mesophase structures. Especially the formation of the B7
phase may be more advantageous in non-aromatic central core
molecules. We are now proceeding with a decisive study of
the properties of the B7 phase in non-aromatic central core
systems and more comprehensive results will be reported in
due course.
Figure 2. Typical photomicrographic textures observed in the
B7 phase: (a) spiral and banana-leaf-like, (b) two-dimentional
periodic pattern domains, and (c) fan-shaped domain.
Figure 3. X-ray intensity profiles of compound (CH2)-O10.
circular domain textures were observed. One can also observe
fan-shaped textures with different colors, in which the brushes
of the extinction cross are parallel to the polarizer and analyzer
directions. This mesophase was not switchable despite little
change in birefringence. No switching current peak was recorded
even at high voltage. X-ray diffraction measurement was also
performed, as shown in Figure 3. A diffuse scattering in the wide
angle region is observed indicating a liquid-like order within the
smectic layer as in the B2 phase and sharp reflections are detect-
ed in the small angle region.
In compound (CH2)-O10, on slowly cooling from the B7
phase, all domains appeared to be broken with sudden birefrin-
gent change. This phase shows no distinct difference between
this intermediate phase and the lower temperature crystal phase
although a transition peak is clearly observed by DSC. Thus, this
phase can be characterized to be crystalline phase (Cr2 phase)
and the two crystal phases existing at the lower temperature
region below the B7 phase. On continuous cooling to the lower-
temperature-crystalline phase (Cr1 phase), only a little change
in birefringence was observed without significant change in
the photomicrograph. X-ray patterns and layer spacing (Cr2
In summary, the mesomorphic behavior of new homologous
series of bent-shaped molecules with non-aromatic central core
based on bis(p-hydroxyphenyl)methane has been studied. All
compounds exhibited the well-defined fluid smectic mesophase,
B1rev or B7 phase, depending on the length of alkyl terminal
chain. However, the phase sequence is totally different from
the conventional aromatic central core molecules. This differ-
ence may attribute to the structural properties of central bent-
core; the free rotation and somewhat smaller bent angle between
the side wings.
References and Notes
1
T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe,
J. Mater. Chem. 1996, 6, 1231.
2
3
G. Pelzl, S. Diele, W. Weissflog, Adv. Mater. 1999, 11, 707.
M. Nakata, D. R. Link, J. Thisayukta, Y. Takanishi, K.
Ishikawa, J. Watanabe, H. Takezoe, J. Mater. Chem. 2001,
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˚
and Cr1 phase are 58.9 and 58.1 A, respectively) were also
almost the same.
4
D. A. Coleman, J. Fernsler, N. Chattham, M. Nakata, Y.
Takanishi, E. Korblova, D. R. Link, R.-F. Shao, W. G. Jang,
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Watanabe, D. M. Walba, H. Takezoe, N. A. Clark, Science
2003, 301, 1204.
(CH2)-O13 and (CH2)-O16 also show a similar phase tran-
sition to (CH2)-O10. But the B7 phase shows quite different
domain. From the isotropic melt, spherulitic domains slowly ap-
peared and coalesced to fan-shaped domains. X-ray diffraction
data for this phase supported the B7 phase with layer spacings
˚
of 53.3 and 61.5 A. This B7 phase was also unswitchable despite
5
6
7
H. Takezoe, Y. Takanishi, Jpn. J. Appl. Phys. 2006, 45, 597.
R. A. Reddy, C. Tschierske, J. Mater. Chem. 2006, 16, 907.
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A. Anwer, R. Lovell, A. H. Windle, in Computer Simulation
of Polymers, ed. by R. J. Ros, 1991, Chap. 3.
little change of birefringence observed. On cooling from the B7
phase, a somewhat broken circular shape texture of Cr2 phase
was also observed as well as Cr1 phase.
As mentioned in the introduction section, the variation of the
central bent core is very important for interlayer interaction,
which is related to the constitution of specific Bn mesophase.
In this study, we used the non-aromatic –CH2– unit for central
bent core instead of aromatic ones. This central core makes
the bent angle between side wings smaller compared with the ar-
omatic one. This assumption was confirmed by molecular simu-
8
9
T. Sekine, Y. Takanishi, T. Niori, J. Watanabe, H. Takezoe,
Jpn. J. Appl. Phys. 1997, 36, L1201.
10 Supporting Information is also available electronically on
the CSJ-Journal Web site, http://www.csj.jp/journals/chem-
lett/index.html.
Published on the web (Advance View) October 11, 2008; doi:10.1246/cl.2008.1134