Realization of a lateral directional order in nematic and smectic A phases of rodlike molecules by using perfluoroarene-arene interactions

Article abstract of DOI:10.1039/c0sm01459a

In order to realize biaxial mesophases of straight rod-like molecules by introduction of lateral perfluoroarene-arene interactions, non-alkylated rod-like liquid crystalline compounds C₆X₅-COO-C ₆Z₄-C₆

Full text of DOI:10.1039/c0sm01459a

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PAPER
Realization of a lateral directional order in nematic and smectic A phases of
rodlike molecules by using perfluoroarene–arene interactions†
*
Keiki Kishikawa, Sumihiro Aikyo, Seiji Akiyama, Takahiro Inoue, Masahiro Takahashi, Shiki Yagai,
Hiroaki Aonuma and Shigeo Kohmoto
Received 9th December 2010, Accepted 22nd March 2011
DOI: 10.1039/c0sm01459a
In order to realize biaxial mesophases of straight rod-like molecules by introduction of lateral
perfluoroarene–arene interactions, non-alkylated rod-like liquid crystalline compounds C6X5–COO–
C6Z4–C6Z4–OOC–C6Y5, F-BB-F (X ¼ Y ¼ F, Z ¼ H), F-BB-B (X ¼ F, Y ¼ Z ¼ H), B-BB-B (X ¼ Y ¼
Z ¼ H), B-FF-B (X ¼ Y ¼ H, Z ¼ F), F-FF-B (X ¼ F, Y ¼H, Z ¼ F), and F-FF-F (X ¼ Y ¼ Z ¼ F) were
prepared, and their phase transition behaviors were investigated by polarized light optical microscopy
(POM) and differential scanning calorimetery (DSC). From the one- and two-dimensional X-ray
diffraction (1D- and 2D-XRD) studies of the nematic and smectic A phases of F-BB-F, it was clarified
that the smectic A phase had a lateral directional order in the layer and the nematic phase had
a cybotactic smectic A structure.
covalently were also used for generation of a lateral directional
order.18 It is known that shape-assisted approaches are also
effective to generate biaxial smectic A phases19 in which board-
Introduction
Usual nematic liquid crystal (LC) phases (uniaxial nematic (Nu)
phases) of the rod-like molecules are rotating rapidly around
and bent rod-like molecules have a layer structure with biaxial
their molecular long axes, and only have one directional order in
order. However, those molecular shapes have a disadvantage in
their molecular long axes and no directional order in their lateral
changing the directions of their short molecular axes because
directions. In contrast, the rotation movements in biaxial
they have large excluded volumes during the switching process.
nematic (Nb) phases1–5 are slower than those in Nu phases, and
In this study, in order to decrease the excluded volumes, we
the molecules in Nb phases have lateral directional order in
designed straight rod-like molecules to realize Nb and biaxial
addition to the axial directional order. This phase was originally
smectic A phases.
found in lyotropic LC systems,6 and recently a few examples of
Our concept is shown in Fig. 1. We expect that a uniaxial
lyotropic Nb phases were reported.7 However, in thermotropic
mesophase of rod-like molecules (A) can be changed to a biaxial
LC system, realization of Nb phases is still very difficult and the
mesophase (B) by introducing a strong lateral face-to-face (or
methodology has not been established yet. Some of the previ-
edge-to-edge) intermolecular interaction. Control of the direc-
ously reported Nb phases were shown to be Nu phases after
re-evaluation.8,9 These days, many scientists in chemistry,
tional order of their aromatic faces in addition to that of their
molecular long axes generates biaxiality in a nematic phase. We
physics, and materials science have been challenged to realize
selected the face-to-face perfluoroarene–arene (PFA/A)
thermotropic Nb phases, because the phases have very unique
order and a high potentiality for realization of high-responsive
optical switching devices.1,5 Shape-assisted approaches are
thought to be effective to slow down the molecular rotation
movement and to generate a lateral directional order. In those
approaches, bent rod-,9–11 plate-,12 bone-,13,14 spoon-,15 pipette-
like16 molecules, and mixtures of rod- and disk-like molecules17
were examined to bring about Nb phases. Liquid crystalline
oligomers and polymers in which the mesogenic units were linked
Department of Applied Chemistry and Biotechnology, Faculty of
Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba,
263-8522, Japan. E-mail: kishikawa@faculty.chiba-u.jp; Fax: +81-43-
Fig. 1 A conceptual picture of the phase transformation from uniaxial
290-3422; Tel: +81-43-290-3238
mesophase (A) to biaxial mesophase (B) by introduction of the face-to-
face attractive interaction.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0sm01459a
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interaction20 as the attractive interaction. The interaction is
known as a strong attractive force between perfluorinated and
non-fluorinated arenes which is composed of van der Waals and
electrostatic interactions.21 For example, the stabilization energy
between benzene and hexafluorobenzene molecules was
estimated to be 3.7–5.6 kcal molꢀ1 from both theoretical and
experimental studies.22,23 Recently, PFA/A interactions have
mainly been used for crystal engineering,24 and in some cases they
are used for gel-formation,25 double helix-formation,26
self-assembly of polymers,27 stereoselective synthesis28 and
molecular recognition.29 The face-to-face interactions work only
at a cofacial contact distance, because the main part of PFA/A
interaction is dispersion energy which is proportional to
rꢀ6 (r: intercentroid distance between the two benzene rings).
Accordingly, the molecular shape around the interaction site
(p-face) should be sterically less-hindered so as not to suppress
the intermolecular cofacial contacts, which makes the molecular
design of LC compounds difficult. It forms a striking contrast to
hydrogen bonding which only needs contact between –XH and Y
(X, Y: hetero atoms) and is very useful for design of supramo-
lecular LCs.30 However, we believe that the features of PFA/A
interaction, whose strength is proportional to rꢀ6, has an
advantage during the switching process. The interaction sharply
decreases when the faces of the two aromatic rings separate or
become unparallel each other. Therefore, the rotations of the
rodlike molecules around the long axis are not suppressed by
intermolecular PFA/A interactions. On the other hand, rota-
tions of bulky molecules are suppressed by the intermolecular
steric repulsion during the entire rotation processes. From these
reasons, we believe that this interaction-assisted approach has an
advantage over shape-assisted approaches.
Scheme 1 Molecular structures of F-BB-F, F-BB-B, B-BB-B, B-FF-B, F-
FF-B and F-FF-F.
By studying all the intermolecular interactions except the face-
to-face PFA/A interaction, we estimated the following three
attractive interactions in these LC phases; C–F/H–C interac-
tions between the PFB and NFB,34 F/F interactions between
the two PFBs35 and edge-to-face (C–H/p) interactions between
two NFBs.36 The C–F/H–C and F/F interactions interact as
edge-to-edge interactions between molecules, which also assists
the face-to-face arrangements of these molecules.37 In contrast,
the edge-to-face (C–H/p) interaction between two NFBs (less
than 1 kcal molꢀ136) suppresses the face-to-face interaction, but it
is much weaker than the PFA/A interaction. Accordingly, we
expected that the benzene rings of the molecules should be
arranged by PFA/A, C–F/H–C and F/F interactions in
a face-to-face (and edge-to-edge) manner in the LC phases of
F-BB-F and B-FF-B.
To the best of our knowledge, only one example is known to
use this attractive interaction for stabilization of LC phases of
rod-like molecules. Dai and co-workers reported that the 1 : 1
mixture of 1,4-di(phenylethynyl)benzene and the corresponding
perfluorinated derivative and the 1 : 1 mixture of 1,4-di(phenyl-
Results and discussion
Synthesis of the liquid crystalline compounds
Compounds F-BB-F, F-BB-B and B-BB-B were synthesized by
condensation of 4,40-biphenol and the corresponding carboxylic
acids (pentafluorobenzoic acid and/or benzoic acid), respectively.
With a similar procedure, B-FF-B, F-FF-B and F-FF-F were
prepared from octafluoro-4,40-biphenol and the corresponding
carboxylic acids.
ethynyl)-2,3,5,6-tetrafluorobenzene
and
1,4-di(penta-
fluorophenylethynyl)benzene showed stable LC phases, though
none of those pure compounds give a stable LC phase.31 In non-
calamitic mesophases, there are two examples of the stabilization
of columnar LC phases using the PFA/A interaction. Grubbs’
group reported a stable columnar LC phase of the 1 : 1 mixture
of hexaalkoxytriphenylene and dodecafluorotriphenylene,32 and
our group reported stable columnar LC phases of trialkoxy-
benzyl pentaflurobenzoates possessing one pentafluorobenzene
ring (PFB) and one non-fluorinated benzene ring (NFB) in their
molecules.33 Those examples suggested that the moiety ratio of
1 : 1 for PFBs and NFBs should be important for the generation
of stable LC phases.
Phase identification and DSC of the compounds
The phases of the compounds were identified by POM, and their
transition temperatures and enthalpies were investigated by
DSC. Their phase behaviors are shown in Table 1. Compounds
F-BB-F and F-BB-B showed nematic and smectic A phases. The
non-fluorinated compound B-BB-B did not show any LC phase.
Compounds F-FF-B and F-FF-F showed a nematic phase only on
cooling. We expected that B-FF-B would give stable LC phases
because of the 1 : 1 moiety ratio (PFBs/NFBs). However, it did
not show any LC phase.
In this study, rod-like compounds F-BB-F and B-FF-B were
designed based on our concept (Scheme 1). Our molecules have
the following features to maximize the attractive interaction
between the PFB and NFB; 1) the moiety ratio of PFBs/NFBs in
the molecule is 1 : 1 to obtain the maximum intermolecular
interaction, and 2) the molecules have no alkyl chain in order to
avoid a decrease in intermolecular core–core interactions by
steric repulsion between alkyl chains. Other compounds, F-B-B-
B, B-BB-B, F-FF-B and F-FF-F were prepared to compare their
behaviors with F-BB-F and B-FF-B.
In the POM of the smectic A phases of F-BB-F and F-BB-B,
fan-like textures with dark lines parallel to the directions of the
crossed polarizers were observed and are the typical textures of
smectic A phases (Fig. 2a)38 which indicates that the phases had
layer structures and the molecules were parallel to the layer
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Table
1
Phase transition behaviours of the 4,40-bis(benzoyloxy)-
biphenyl derivativesa
Compound Phase transition (ꢁC, (kcal molꢀ1))
F-BB-F
F-BB-B
B-BB-B
B-FF-B
F-FF-B
F-FF-F
a
The transition temperatures (ꢁC) and enthalpies (in parentheses, kcal
molꢀ1) were determined by DSC (5 ꢁC minꢀ1) and are given above and
below the arrows. Cr1, Cr2 and Cr indicate crystal phases, and SmA,
N, Iso, and X indicate smectic A, nematic, isotropic liquid and
b
unidentified phases, respectively. The peaks could not be separated.
h1 + h2 ¼ 9.5 kcal molꢀ1
c
.
transition temperature could not be measured.
The peaks were overlapping, and the
Comparing the melting points of F-BB-F and F-BB-B with
B-BB-B which possesses a biphenyl group, show that those of
F-BB-F and F-BB-B were much lower than that of B-BB-B. The
melting point of F-BB-F is lower than that of F-BB-B. Intro-
duction of fluorine atoms on the benzoyl–benzene rings were
effective in lowering the melting point. As for B-FF-B, F-FF-B
and F-FF-F which possess an octafluorobiphenyl moiety, their
melting points were lower than those of B-BB-B, F-BB-B and F-
BB-F, respectively. The clearing temperatures of the nematic
phases of F-F-F-B and F-FF-F were lower than those of F-BB-B
and F-BB-F, respectively. The replacement of the biphenyl with
an octafluorobiphenyl moiety was effective for lowering the
melting and clearing points. To investigate the differences in their
conformations, the density functional theory (DFT) calculations
(B3LYP/6-31G(d))39 were performed. The most stable confor-
mations of F-BB-F and B-FF-B are shown in Fig. 3. The
molecular shape of F-BB-F is flat, while that of B-FF-B is twisted.
In the case of F-BB-F, the torsion angles between the two
Fig. 2 Polarized optical micrographs of (a) fan-shaped texture of the
smectic A phase of F-BB-F (190 ꢁC, 600ꢂ), (b) schlieren texture of the
nematic phase of F-BB-F (210 ꢁC, 600ꢂ), and (c) schlieren texture of the
nematic phase of F-FF-F (150 ꢁC, 600ꢂ).
normal. The nematic phases of F-BB-F, F-BB-B, F-FF-B and
F-FF-F exhibited schlieren textures with two and four-brushes
(Fig. 2b) which are typical textures of nematic phases.38
However, in the nematic phase of F-BB-F, each of the four-
brushes has a small bright area at its center, and the lines are very
sharp around the centers. It is known that the core can produce
a bright texture if the inclined region is not too small. Further,
both of the two- and four -brush disclinations have a different
color area to that of their peripheral area (Fig. S1 and Fig. S2 in
the ESI†). The changes of the colours mean that the refractive
indices change significantly at the central areas of those
disclinations. It strongly suggests that the molecular alignments
greatly change in these areas even in the case of two-brush dis-
clinations. This is in contrast to the schlieren texture of F-FF-F
which has no bright area at the disclination and broad lines
around them (Fig. 2c). At the transition from the smectic A phase
to the nematic phase of F-BB-F on heating, smectic-like textures
(fan-like and focal conic-like textures) remained after the tran-
sition up to the clearing temperature (Fig. S3†).
Fig. 3 Most stable conformers of (a) F-BB-F and (b) B-FF-B calculated
by the DFT (B3LYP6-31G(d)) method.39
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Table 2 The XRD peaks of F-BB-F and F-BB-B in the smectic A phases and their intensities
b
e
ꢀ
ꢀ
dobsd (dcalcd)/A
Compound
Mesophasea(Temp/ꢁC)
SmA(190)
Miller index/(hkl)
Relative Intensity (%)
lcalcd/A
F-BB-F
22.1 (22.1)
11.3 (11.1)
7.5 (7.6)
23.6 (23.6)
11.9 (11.8)
7.9 (7.9)
23.4 (23.4)
11.8 (11.7)
7.9 (7.8)
23.5 (23.5)
11.8 (11.8)
7.9 (7.8)
(100)
(200)
(300)
(100)
(200)
(300)
(100)
(200)
(300)
(100)
(200)
(300)
100c
18c
6c
24.2
SmA(200)
SmA(170)
SmA(190)
25d
100d
10d
15d
100d
5d
F-BB-B
24.0
38d
100d
1d
SmA indicates a smectic A phase. b The values dobsd and dcalcd indicate d-spacing observed in the XRD and d-spacing calculated based on the d(100)
distance. c Percentage of the peak intensity divided by that of d(100). d Percentage of the peak intensity divided by that of d(200). e Molecular length
calculated by Chem3D.40
a
benzene rings of the biphenyl group were about 38ꢁ. The p-faces
of the biphenyl–benzene ring and benzoyl–benzene ring linked
with the ester group were almost in the same plane (the torsion
angle between them is about 2ꢁ). On the other hand, the angle of
about 59ꢁ was obtained in B-FF-B between the two per-
fluorinated benzene rings of the octafluorobiphenyl. The torsion
angle between the octafluorobiphenyl–benzene ring and
benzoyl–benzene ring was about 88ꢁ. Because of the bulky
molecular shape, the attractive face-to-face interaction between
the B-FF-B molecules is thought to be weak. These results indi-
cate that molecular flatness should be one of the important
factors for generation of stable LC phases in this series.
1D-X-ray diffraction studies of F-BB-F and F-BB-B on a glass
plate without applying a magnetic field
To investigate the molecular-packing structures in the smectic A
phases of F-BB-F and F-BB-B, their one-dimensional X-ray
diffraction (1D-XRD) profiles were measured (Table 2).
Each of the samples was prepared by spreading it on an
untreated glass plate. The X-ray patterns of these mesophases
gave reciprocal spacings in the ratio 1 : 2 : 3 corresponding to
their lamellar structures. The smectic A phase of F-BB-F at
190 ꢁC (Fig. 4a) shows a large sharp d(100) peak corresponding
to its interlayer spacing. The intensity of the d(100) peak is much
stronger than that of the d(200) peak. However, intensities of
these peaks were reversed at 200 ꢁC (Fig. 4b). The d(200) peak is
more intense than that of the d(100). Further, the shape of the
halo in the XRD at 200 ꢁC is different from that at 190 ꢁC.
Though no textural change was observed by POM in the
temperature range of the smectic A phase, the XRD profiles
indicated the gradual change in the layer structures.ꢁThe halos at
ꢁ
ꢀ
both 190 and 200 C have a maxima at 2q ¼ 22–24 (3.9–3.8 A)
corresponding to the thickness of the aromatic rings. In general,
smectic A and nematic phases of straight rod-like molecules
consisting of plural benzene rings have a broad maximum at 4.5–
4.8 A corresponding to their average molecular width.41
ꢀ
Accordingly, these results suggest that the free rotation of these
rod-like molecules along their long axis is suppressed in the
smectic A phase of F-BB-F, and the aromatic faces are stacked in
a face-to-face manner and parallel to the glass surface.
Fig. 4 1D-XRD profiles of F-BB-F at (a) 190 ꢁC (smectic A phase), (b)
200 ꢁC (smectic A phase) and (c) 210 ꢁC (nematic phase).
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ꢁ
The nematic phase of F-BB-F at 210 C (Fig. 4c) did not show
small molecular aggregates, even though the textures in POM
belonged to those of nematic phases. To investigate the influence
of the PFA/A interaction, the 2D-XRD profile of F-FF-F was
measured (Fig. 5d). The profile does not have a diffraction
pattern which corresponding to a layer structure, and only shows
a halo corresponding to the molecular width. Accordingly, the
difference between the nematic phases of F-BB-F and F-FF-F
suggests that the PFA/A interaction is necessary to generate
the cybotactic smectic layer structure in the nematic phase
of F-BB-F.
any peaks in the small angle region, and the shape of the halo
became broader at the transition from the smectic A to the
nematic phase. The XRD patterns of F-BB-B in its smectic A
ꢁ
phase at both 170 and 190 C showed that the d(200) peak was
more intense than the d(100) peak. Therefore, it was assumed
that the smectic A phase of F-BB-B maintained one packing
structure over the observed temperature range.
2D-XRD studies with applying magnetic field
To obtain more information about the molecular-packing
structures in the nematic and smectic A phases of F-BB-F, their
two-dimensional X-ray diffraction (2D-XRD) profiles were
measured on the samples sealed in a glass capillary (f ¼ 1.5 mm).
The molecules were aligned by applying a magnetic field
(350ꢁ0 gauss). The 2D-XRDs of the smectic A phases at 190 and
200 C (Fig. 5a and Fig. 5b) showed pairs at the d(100) d(200),
and d(300) reflections corresponding to their interlayer orders
along the equator and the broad reflections corresponded to the
averaged molecular widths at the meridian. These 2D-XRD
patterns indicate that the molecular long axes are parallel to the
Crystal structure of F-BB-F
Single crystals of F-BB-F were obtained by recrystallization from
chloroform–hexane solution, and the single crystal XRD of
F-BB-F were performed to give the molecular packing structure
in the crystal phase as shown in Fig. 6. In Fig. 6a, the C6F5–
groups are stacked in an orderly fashion with a repeat distance of
ꢀ
3.2 A. The molecules are packed in such a way that the PFBs and
NFBs are micro-segregated in a layer as shown in Fig. 6b. It was
assumed that CH/p interaction between the biphenyl moieties
ꢁ
layer normal. Further, in the nematic phase at 210 C (Fig. 5c),
pairs of reflections at d(200) and d(300) are observed along the
equator in the small angle region, which indicates that the
nematic phase also possesses a repeat distance in the direction of
the molecular long axes. This means that the molecules are
packed with a shift of the half molecular length in the level of
Fig. 5 2D-XRD patterns of (a) F-BB-F at 190 ꢁC (smectic A phase), (b)
F-BB-F at 200 ꢁC (smectic A phase), (c) F-BB-F at 210 ꢁC (nematic phase)
and F-FF-F at 150 ꢁC (nematic phase). The direction of the magnetic field
is indicated by an arrow.
Fig.
6 Molecular packing structure (space-fill and ball-and-stick
models) of F-BB-F in the crystal phase. Its (a) side and (b) front views.
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POM and 1D-XRD of F-BB-F on applying a magnetic field
To investigate the biaxiality of the nematic phase of F-BB-F,
POM was performed on the nematic phase homeotropically
aligned by applying a magnetic field (3500 gauss). Compound
F-BB-F was sandwiched between two glass plates whose surfaces
were treated with 1,1,1,3,3,3-hexamethyldisilazane to decrease
interactions between molecules and the glass surface. Surpris-
ingly, under the magnetic field, the nematic phase showed dark
schlieren textures with all-two-brush disclinations (Fig. 7a),
although it showed bright schlieren textures with two- and four-
brush disclinations in the sample sandwiched between two
untreated glass plates in the absence of a magnetic field (Fig. 2b).
In this case, each of the two-brush declinations also has a small
bright area as shown in the 4-brush textures in Fig. 2b. In order
to compare the textures of F-BB-F with those of other nematic
liquid crystalline compounds, the textures of 4-cyano-40-octyl-
biphenyl (8CB) and N-(4-ethoxybenzylidene)-4-n-butylaniline
(EBBA) in the LC phases in a magnetic field on a glass plate
treated with hexamethyldisilazane were investigated by POM.
Those textures (Fig. 7b and c) are also dark, but there are
4-brushes and the brushes do not have bright areas at their
centers. Thus, the texture of F-BB-F is quite different from those
of 8CB and EBBA. It is known that biaxial nematic phases
exhibit three types of 1/2 disclinations (of both signs) corre-
sponding to each director, and one S ¼ 1 disclination42 and the
all-two-brush textures may not be the positive proof.43 However,
it is thought that the all-two-brush textures give us the possibility
of the biaxiality.2 The all-two-brush textures are also reported in
papers of Nb phases in bent rod- and plate-like molecules.11,14,44 It
is assumed that this texture of F-BB-F originated from the lateral
strong intermolecular face-to-face interaction.
Further, F-FF-F showed schlieren textures typical of nematic
phases on applying a magnetic field (Fig. 7d). The texture has
two- and four-brushes with broad lines. This also suggests that
the PFA/A interaction is necessary to generate the all-two-
brush textures, because F-FF-F molecules do not have
intermolecular PFA/A interactions. Although we also tried
a conoscopic study, we could not get a large enough area of the
homeotropic thin-layered sample.
Fig. 7 Dark schlieren textures in the nematic phases of (a) F-BB-F
(210 ꢁC), (b) 4-cyano-40-octylbiphenyl (8CB) (30 ꢁC), (c) N-(4-ethoxy-
benzylidene)-4-n-butylaniline (EBBA) (76 ꢁC), and (d) F-FF-F (150 ꢁC).
The samples were observed under a magnetic field (3500 gauss) between
two glass plates treated with 1,1,1,3,3,3-hexamethyldisilazane.
and the F/F interactions between the NFBs took place during
the crystallization process. The F/F interaction should be the
main intermolecular interaction because each fluorine atom
contacts efficiently with the fluorine atoms of other molecules. It
is assumed that this edge-to-edge interaction organizes the
p-faces of the PFBs in parallel in the crystal phase. The layer
structure is almost the same as that estimated from the 1D-XRD
of F-BB-F at the lower temperature region of its smectic A phase.
Fig. 8 Equipment for measuring the XRD of F-BB-F under a magnetic
field. The X-ray beam irradiated the sample in a glass capillary which was
set between a pair of magnets. The XRD intensity against the 2q was
measured. The angle (q ¼ 90ꢁ) of the glass capillary to the irradiated
X-ray beam was fixed.
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We tried to measure homeotropically aligned textures of the
smectic A phase of F-BB-F by applying a magnetic field.
However, it was difficult to align the smectic A phase by using
magnets to give the fan-shaped textures (Fig. S4†). Without
applying a magnetic field, a homeotropically aligned part around
the focal-conic was observed in some cases (Fig. S5†). However,
it is very difficult to distinguish it from the usual homeotropic
textures.
nematic phase. At 240 ꢁC (nematic phase), one broad peak at
ꢁ
ꢀ
around 5.0 A (b: 2q ¼ 17.5 ) and one shoulder broad peak at
ꢁ
ꢀ
3.9 A (a: 2q ¼ 22.5–23 ) are observed. It is assumed that these
two broad peaks correspond to two kinds of repeat distances in
In order to investigate information on the repeat distances in
the direction of the molecular short axis, the XRD profiles of
F-BB-F in the isotropic liquid (290, 270 and 2ꢁ55 ꢁC), nematic
ꢁ
(240 and 210 C), and smectic A (200 and 190 C) phases were
measured on applying a magnetic field. The equipment is
depicted in Fig. 8. The LC sample was sealed in a glass capillary
(f ¼ 1.5 mm), and a magnetic field (about 3500 gauss) was
applied vertically to the glass capillary. The X-ray beam was
irradiated horizontally to the LC sample, and the intensity of the
X-ray diffraction against 2q was measured by a scintillation
counter. The XRD profiles are shown in Fig. 9. From the XRD
profiles of the smectic A phases, it is clear that the rod-like
molecules are well-aligned in parallel to the magnetic field
because the large peaks of the inter-layer distances are not
observed and the broad peaks based on the molecular widths are
emphasized. In the isotropic liquid phase (270 and 255 ꢁC), there
ꢁ
ꢀ
is a small broad peak around 11.8 A (2q ¼ 7.5 ) corresponding to
the d(200) reflection. The intensities of the broad peaks at wide
angle region increased on cooling from the isotropic liquid to the
Fig. 10 XRD profiles of F-BB-F in the smectic A phase at (a) 190 ꢁC and
(b) 200 ꢁC and nematic phase at (c) 210 ꢁC under the following two
experimental conditions: 1) capillary method in which the sample was
packed in a glass capillary (f ¼ 1.5mm) and a magnetic field (3500 gaus)
was applied (the profile was indicated by blue lines) and 2) glass plate
method in which the sample was spread on a glass plate (the profile was
indicated by red lines).
Fig. 9 XRD profiles of F-BB-F under a magnetic field showing broad
peaks a, b and d(200) at 290 (Iso), 270 (Iso), 255 (Iso), 240 (N), 210 (N),
200 (SmA) and 190 (SmA) ꢁC. Iso, N and SmA indicate isotropic liquid,
nematic and smectic A phases, respectively. Peak x is a noise peak.
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the lateral directions. At 210 ꢁC (nematic phase), the peak at
Estimation of the superstructures
ꢁ
ꢀ
ꢀ
5.0 A (b: 2q ¼ 18 ) and the shoulder peak at 3.9 A (a: 2q ¼ 22.5–
From these results, the superstructures of F-BB-F in the smectic
A and nematic phases were proposed as depicted in Fig. 11. In
the smectic A phase of F-BB-F, it was suggested that the mole-
cules were biaxially packed in the layers and the direction of their
aromatic faces were controlled by the interaction with the glass
surface. This means that the free rotation of the molecules
around the long axes is suppressed on the glass plate and close to
the glass surface as shown in Fig. 12a. In the low temperature
region of the smectic A phase, it is expected that the PFBs and
NFBs are micro-segregated as shown in Fig. 11b (d z l; d: layer
distance; l: molecular length), which is similar to the molecular
packing structure in the crystal. The edge-to-edge F/F inter-
actions control the directions of the p-faces of the penta-
23ꢁ) were also observed. Lowering the temperature to 190 ꢁC
ꢁ
ꢀ
(smectic A phase), the broad peak (b: 5.0 A, 2q ¼ 18 ) became
sharper and the broad shoulder peak a was also observed. These
XRD profiles are compared with those measured using the glass-
plate method (Fig. 10). In the smectic A phase both at 190 and
200 ꢁC (Fig. 10a and Fig. 10b), the broad peak reached
ꢀ
a maximum at 3.8–3.9 A (peak a) and is emphasized in the glass-
plate method. The molecules on the glass plate are more aligned
in their lateral direction than those in the glass capillary. In the
nematic phase (Fig. 10c), these two methods gave similar profiles.
However, the shapes of these two profiles are slightly different.
The profile of the glass-plate method includes the component of
ꢁ
ꢀ
the broad peak (peak a) at 3.8–3.9 A (2q ¼ 22.5–23 ) more than
ꢀ
fluorophenyl groups and have a repeat distance of 3.8 A. As the
that of the capillary method.
temperatures rise, molecular movements such as vibration and
rotation increase, and the apparent volume of the terminal
pentafluorophenyl groups increases compared to those of the
biphenyl benzene rings. To minimize the intermolecular steric
repulsions, the molecules are organized into a superstructure in
which the molecules are packed with a shift of the half molecular
length in the direction of the molecular long axes ((d z l/2),
Fig. 11c). In this superstructure, a face-to-face PFA/A inter-
ꢀ
action takes place between the molecules with a 3.9 A p–p
stacking distance (Fig. 12b). Thus, it is assumed that the equi-
librium between the above-mentioned two superstructures in the
smectic A phase is controlled thermodynamically. These lateral
ꢀ
repeat distances 3.8–3.9 A are very closed to the intercentroid
ꢀ
distance 3.77 A between benzene and hexafluorobenzene mole-
cules in the cocrystal.22 Though the nematic phase (Fig. 11d) does
not show typical optical textures of layered structures, it is
assumed that small clusters with the layer structures are laterally
ordered (Fig. 11e) and are formed as observed in the 2D-XRD.
Considering the observation of biaxiality in the smectic A phase
in the 1D-XRD study, the results suggest that the molecules in
the nematic phase are organized biaxially in the level of small
Fig. 11 Schematic representation of F-BB-F and its superstructures. (a)
the molecular structure of F-BB-F and its schematic model, (b) layer
structure (d z l, d: layer distance, l: molecular length) in the smectic A
phase at 190 ꢁC, (c) layer structure (d z l/2) in the smectic A phase at 200
ꢁC, (d) the molecules in the nematic phases and (e) small molecular
aggregate packed with a shift of the half molecular length in the nematic
phase.
Fig. 12 Face-to-face molecular packing models (side view) of F-BB-F on
the glass plate in the smectic A phases at (a) 190 and (b) 200 ꢁC.
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molecular aggregates by the strong lateral attractive face-to-face
PFA/A interaction, which might be the origin of the schlieren
textures with all-two-brush disclinations and of the difference
between the 1D-XRD profiles of the glass plate and capillary
methods.
sulfate. After filtration, the solvent was removed by evaporation.
The product was separated by silica gel column chromatography
eluting with methanol/chloroform (¼ 1 : 9) to give 4-benzoyloxy-
40-hydroxybiphenyl as a white solid (0.240 g, 54.7%).
4-Benzoyloxy-40-hydroxybiphenyl (0.14 g, 0.48 mmol), pen-
tafluorobenzoic acid (0.12 g, 0.58 mmol), and dichloromethane
(20 mL) were placed in a 100 mL round-bottom flask, and the
mixture was cooled to 0 ꢁC. To the mixture were added DMAP
(0.01 g, 0.07 mmol) and DIPC (0.36 g, 0.29 mmol), and then the
mixture was warmed to room temperature and stirred for 24 h.
After quenching with 1 N HCl (20 mL), the organic phase was
washed with water (100 mL) and saturated NaHCO3 (aq.)
(50 mL), and dried over anhydrous sodium sulfate. The solvent
was removed by evaporation. The crude product was purified by
silica gel column chromatography eluting with chloroform/
hexane (¼ 1 : 1).
Conclusions
In summary, we demonstrated that the simple non-alkylated rod-
like molecules possessing PFBs and NFBs were self-organized to
give stable mesophases by using F/F, CH/F and PFA/A
interactions. Especially, compound F-BB-F consisting of
a biphenyl moiety and two terminal pentafluorobenzoyl groups
showed stable unique superstructures in its nematic and smectic
A phases. The interlayer distances (d(200), d(300) and d(400))
were observed even in the nematic phase and the two XRD
methods (glass plate and capillary methods) showed differences
in the lateral repeat distances in both nematic and smectic A
phases. For the stabilization of the liquid crystalline compounds
possessing PFBs and NFBs, we emphasize that the following
factors are important; 1) the 1 : 1 moiety ratio of PFBs and NFBs
and 2) the flat molecular shape.
F-BB-B
Yield 80%; nmax (KBr)/cmꢀ1 3066, 1750, 1732, 1496, 1219, 1004;
dH(400 MHz; CDCl3; Me4Si) 7.31 (d, J ¼ 8.7 Hz, 4H), 7.54 (t, J ¼
8.3 Hz, 4H), 7.65 (d, J ¼ 8.7 Hz, 4H), 7.67 (t, J ¼ 8.3 Hz, 2H),
8.24 (d, J ¼ 8.3 Hz, 4H); dC(125.65 MHz; CDCl3; Me4Si) 121.60,
122.16, 128.27, 128.43, 128.62, 129.43, 130.22, 133.70, 137.86,
139.16, 149.45, 150.64, 157.53 (C]O), 165.20 (C]O) (The
carbon atoms bearing a fluorine atom could not be; FT-HRMS
(ACPI) 484.0729 (M+. C26H13F5O4 requires 484.0718); EA
(C26H13F5O4) calcd C 64.31, H 2.49, found C 64.47, H 2.71%.
Experimental
Synthesis of 4,40-bis(pentafluorobenzoyloxy)biphenyl (F-BB-F)
4,40-Biphenol (0.30 g, 1.6 mmol), pentafluorobenzoic acid
(0.68 g, 3.2 mmol), and dichloromethane (20 mL) were placed in
a 100 mL round-bottom flask, and the mixture was cooled to
0 ꢁC. To the mixture were added DMAP (0.02 g, 0.2 mmol) and
DIPC (0.24 g, 1.9 mmol), and then the mixture was warmed to
room temperature and stirred for 24 h. After quenching with 1 N
HCl, the organic phase was washed with water (100 mL) and
saturated NaHCO3 (aq.) (50 mL), and dried over anhydrous
sodium sulfate. The solvent was removed by evaporation. The
residual crude product was purified by silica gel column
chromatography eluting with chloroform/hexane (¼ 1 : 1).
Synthesis of 4,40-bis(benzoyloxy)biphenyl (B-BB-B)
4,40-Biphenol (0.50 g, 2.7 mmol), benzoic acid (0.82 g, 6.8 mmol),
and dichloromethane (20 mL) were placed in a 100 mL round-
bottom flask, and the mixture was cooled to 0 ꢁC. To the mixture
were added DMAP (0.06 g, 0.5 mmol) and DIPC (0.82 g,
6.4 mmol), and then the mixture was warmed to room temper-
ature and stirred for 24 h. After quenching with 1 N HCl
(20 mL), the organic phase was washed with water (100 mL) and
saturated NaHCO3 (aq) (50 mL), and was dried over anhydrous
sodium sulfate. The solvent was removed by evaporation. The
crude product was purified by silica gel column chromatography
eluting with chloroform/hexane (¼1 : 1).
F-BB-F
Yield 40%; nmax (KBr)/cmꢀ1 3059, 1752, 1532, 1221, 1005; dH(400
MHz; CDCl3; Me4Si) 7.34 (d, J ¼ 8.7 Hz, 4H), 7.65 (d, J ¼ 8.7
Hz, 4H); dC(125.65 MHz; CDCl3; Me4Si) 121.70, 128.47, 138.79,
149.61, 157.52 (C]O) (The carbon atoms bearing a fluorine
atom could not be observed); FT-HRMS (atmospheric pressure
chemical ionization (ACPI)) 574.0247 (M+. C26H8F10O4 requires
574.0257); EA (C26H8F10O4) calcd C 54.37, H 1.40, found C
54.21, H 1.11%.
B-BB-B
Yield 22%; nmax (KBr)/cmꢀ1 3071, 1733, 1496, 1199, 1003; dH
(400 MHz; CDCl3; Me4Si) 7.31 (d, J ¼ 8.7 Hz, 4H), 7.54 (t, J ¼
8.3 Hz, 4H), 7.65 (d, J ¼ 8.7 Hz, 4H), 7.66 (t, J ¼ 8.3 Hz, 2H),
8.24 (d, J ¼ 8.3 Hz, 4H); dC (125.65 MHz; CDCl3; Me4Si) 122.05,
128.23, 128.58, 129.45, 130.20, 133.65, 138.21, 150.42, 165.21
(C]O); FT-HRMS (ACPI): 393.1140 ((M–1)ꢀ. C26H17O4
requires 393.1121); EA (C26H18O4) calcd C 79.17, H 4.60, found
C 78.91, H 4.35%.
Synthesis of 4-benzoyloxy-40-(pentafluorobenzoyloxy)-biphenyl
(F-BB-B)
4,40-Biphenol (0.281 g, 1.51 mmol), benzoic acid (0.220 g,
1.80 mmol), and DMAP (18 mg, 0.15 mmol) were dissolved in
dry THF (20 mL). DIPC (0.230 g, 1.82 mmol) was added into the
solution with stirring. The solution was stirred at room temper-
ature for 17 h. The reaction was quenched with 1 N HCl (20 mL).
The solution was extracted with chloroform (20 mL ꢂ 3) and the
combined organic solutions were dried over anhydrous sodium
Synthesis of 4,40-bisbenzoyloxyoctafluorobiphenyl (B-FF-B)
Octafluoro-4,4-biphenol (0.25 g, 0.75 mmol), benzoic acid
(0.23 g, 1.90 mmol), and dichloromethane (20 mL) were placed
in a 100 mL round-bottom flask, and the mixture was cooled to
0
ꢁC. To the mixture were added DMAP (0.02 g, 0.2 mmol)
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and DIPC (0.23 g, 1.8 mmol), and then the mixture was
warmed to room temperature and stirred for 24 h. After
quenching with 1 N HCl (20 mL), the organic phase was
washed with water (100 mL) and saturated NaHCO3 (aq)
(50 mL), and was dried over anhydrous sodium sulfate. The
solvent was removed by evaporation. The crude product was
purified by silica gel column chromatography eluting with
chloroform/hexane (¼ 1 : 1).
Synthesis of 4,40-bis(pentafluorobenzoyloxy)octafluoro-biphenyl
(F-FF-F)
Octafluoro-4,4-biphenol (0.25 g, 0.75 mmol), pentafluorobenzoic
acid (0.40 g, 1.9 mmol) and dichloromethane (20 mL) were
placed in a 100 mL round-bottom flask, and the mixture was
cooled to 0 ꢁC. To the mixture were added DMAP (0.02 g,
0.2 mmol) and DIPC (0.23 g, 1.8 mmol), and then the mixture
was warmed to room temperature and stirred for 24 h. After
quenching with 1 N HCl (20 mL), the organic phase was washed
with water (100 mL) and saturated NaHCO3 (aq) (50 mL), and
dried over anhydrous sodium sulfate. The solvent was removed
by evaporation. The crude product was purified by silica
gel column chromatography eluting with chloroform/hexane
(¼ 1 : 1).
B-FF-B
Yield 40%; nmax(KBr)/cmꢀ1 3075, 1764, 1484, 1236, 1003; dH (400
MHz; CDCl3; Me4Si) 7.58 (t, J ¼ 8.3 Hz, 4H), 7.73 (t, J ¼ 8.3 Hz,
2H), 8.25 (d, J ¼ 8.3 Hz, 4H); dC (125.65 MHz; CDCl3; Me4Si)
126.94, 128.94, 130.81, 134.48, 162.49 (C]O) (The carbon atoms
bearing a fluorine atom could not be observed); FT-HRMS
(ACPI): M+ and (M–1)ꢀ were not observed; EA (C26H10F8O4)
calcd C 58.01, H 1.87, found C 57.78, H 1.64%.
F-FF-F
Yield 45%; 13C NMR (125.65 MHz, CDCl3, 25 C): all carbon
ꢁ
atoms could not be observed; nmax (KBr)/cmꢀ1 1780, 1493, 1185,
1008 cmꢀ1; FT-HRMS (ACPI): M+ and (M–1)ꢀ were not
observed; EA (C26F18O4) calcd C 43.48, H 0.00, found C 43.39,
H ꢀ0.19%.
Synthesis of 4-benzoyloxy-40-(pentafluorobenzoyloxy)
octafluobiphenyl (F-FF-B)
Octafluoro-4,40-biphenol (0.885 g, 2.68 mmol), benzoic acid
(0.327 g, 2.68 mmol), and DMAP (33 mg, 0.27 mmol) were dis-
solved in dry THF (20 mL). DIPC (0.406 g, 3.22 mmol) was
added into the solution with stirring. The solution was stirred at
room temperature for 17 h. The reaction was quenched with 1N
HCl (20 mL). The solution was extracted with chloroform (20
mL ꢂ 3) and the combined organic solutions were dried over
anhydrous sodium sulfate. After filtration, the solvent was
removed by evaporation. The product was separated by silica gel
column chromatography eluting with methanol/chloroform (¼
1 : 9) to give 4-benzoyloxy-40-hydroxyoctafluorobiphenyl as
a white solid (0.150 g, 12.9%).
Single crystal data of F-BB-F
Chemical formula: C26H8F10O4, formula weight: 574.32,
ꢀ
Temperature: 173 K, wavelength: 0.71073 A, crystal system:
monoclinic, space group: C2/c, Unit cell parameters: a ¼ 45.474
ꢁ
ꢀ
ꢀ
ꢀ
(4) A, b ¼ 7.4583(7) A, c ¼ 6.3948(6) A, b ¼ 91.1620(10) , V ¼
2168.4(3) A , Z ¼ 4, D (calcd) ¼ 1.759 mg mꢀ3, absorption
3
ꢀ
coefficient: 0.175 mmꢀ1, F(000) ¼ 1144, crystal size: 0.30 ꢂ
0.20 ꢂ 0.05 mm3, crystal description: plate, crystal colour: no
colour, theta range for data collection: 2.69 to 22.80ꢁ, index
ranges: ꢀ33 # h # 59, ꢀ9 # k # 9, ꢀ6 # l # 8, independent
reflections: 5939 [R(int) ¼ 0.0304], absorption correction:
empirical, max. and min. transmission: 0.9494 and 0.9913,
refinement method: Full-matrix least-squares on F2, data/
restraints/parameters: 2496/0/198, goodness-of-fit: 0.928, final R
indices [I > 2sigma(I)]: R1 ¼ 0.0424, wR2 ¼ 0.1254, R indices (all
data): R1 ¼ 0.0658, wR2 ¼ 0.1468.
4-Benzoyloxy-40-hydroxyoctafluorobiphenyl (0.20 g, 0.46 mmol),
pentafluorobenzoic acid (0.12 g, 0.55 mmol), and dichloromethane
(20 mL) were placed in a 100 mL round-bottom flask, and the
ꢁ
mixture was cooled to 0 C. To the mixture were added DMAP
(0.01 g, 0.01 mmol) and DIPC (0.06 g, 0.6 mmol), and then the
mixture was warmed to room temperature and stirred for 24 h. After
quenching with 1 N HCl (20 mL), the organic phase was washed with
water (100 mL) and saturated NaHCO3 (aq) (50 mL), and dried over
anhydrous sodium sulfate. The solvent was removed by evaporation.
The crude product was purified by silica gel column chromatography
eluting with chloroform/hexane (¼ 1 : 1).
Acknowledgements
We are grateful to the Futaba Electronics Memorial Foundation,
Mukai Science and Technology Foundation, the Asahi Glass
Foundation for the funds, and we thank Grant-in-Aid for
Scientific Research (B) 19350090.
Notes and references
F-FF-B
1 G. R. Luckhurst, Thin Solid Films, 2001, 393, 40; D. W. Bruce, Chem.
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Yield 80%; nmax (KBr)/cmꢀ1 3041, 1765, 1495, 1235, 1004; dH
(400 MHz; CDCl3; Me4Si) 7.32 (t, J ¼ 8.7 Hz, 4H), 7.54 (t, J ¼
8.3 Hz, 2H), 7.65 (m, 5H), 8.24 (d, J ¼ 8.0 Hz, 2H); dC (125.65
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