Synthesis of liquid crystal molecules based on bis(biphenyl)diacetylene and their liquid crystallinity

Article abstract of DOI:10.1246/cl.2010.513

We synthesized novel liquid crystalline molecules that contain a bis(biphenyl)diacetylene mesogen and confirmed their structures by ¹HNMR, ¹³CNMR, and FT-IR spectroscopy and mass spectrometry. These compounds formed thermotropic liquid crystals in a wide temperature region that was well characterized by optical microscopic and X-ray measurements.

Full text of DOI:10.1246/cl.2010.513

doi:10.1246/cl.2010.513
Published on the web April 17, 2010
513
Synthesis of Liquid Crystal Molecules Based on Bis(biphenyl)diacetylene
and Their Liquid Crystallinity
Makoto Uchimura, Sungmin Kang, Rohei Ishige, Junji Watanabe,* and Gen-ichi Konishi*
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
O-okayama, Meguro-ku, Tokyo 152-8552
(Received March 3, 2010; CL-100200; E-mail: jwatanab@polymer.titech.ac.jp, konishi.g.aa@m.titech.ac.jp)
We synthesized novel liquid crystalline molecules that
C8H17
O
C
C
C
C
OC8H17
contain a bis(biphenyl)diacetylene mesogen and confirmed their
1
structures by H NMR, 13C NMR, and FT-IR spectroscopy and
mass spectrometry. These compounds formed thermotropic
liquid crystals in a wide temperature region that was well
characterized by optical microscopic and X-ray measurements.
During the course of studies to explore new mesogenic
groups, we synthesized liquid crystalline (LC) materials based
on a bis(biphenyl)diacetylene [BBPDA, bis(biphenyl)butadiyne]
moiety. This moiety consists of only hydrocarbons. In other
words, it, irrespective of its remarkably long axial ratio, does not
possess dipolar groups such as esters, azomethine, and amides.
Thus, the interaction between molecules in a liquid crystalline
field may be characteristic and distinct from those in other polar
LC molecule systems. Further, this moiety should afford high
birefringence to the liquid crystal because of its anisotropic
polarization.1-4 To realize these points, in this study, we
synthesized bis(biphenyl)diacetylenes containing alkoxy tail
groups (BBPDA-OCn, n: carbon number in alkoxy tail), and
analyzed their liquid crystalline behavior.
BBPDA-OCns were synthesized according to Scheme 1.
First, 4-alkyloxy-4¤-bromobiphenyls with n = 5-12 were syn-
thesized by the Williamson ether reaction. Second, 4-alkyloxy-
4¤-ethynylbiphenyls were prepared by the palladium-catalyzed
Sonogashira coupling, followed by base-induced hydrolysis.
Finally, BBPDA-OCns were obtained by the Glaser coupling.
The structures of the obtained compounds were confirmed by
1H NMR, 13C NMR, and FT-IR spectroscopy and mass spec-
trometry (see Supporting Infomation).11 The 13C NMR spectrum
of BBPDA-OC8 is shown in Figure 1. Characteristic peaks can
be observed at 82.0 and 74.6 ppm; these are assigned to the
diacetylene unit. Further, we synthesized asymmetric BBPDA-
OC5-11 by using compounds 3 with different carbon numbers
of 5 and 11. The product was separated by HPLC and purified by
recrystallization to obtain pure BBPDA-OC5-11.
Figure 1. 13C NMR spectrum of BBPDA-OC8 (100 MHz,
CDCl3).
40
35
30
25
20
15
10
5
0
-5
-10
-15
-20
50
100
150
200
250
300
Temperature/°C
Figure 2. Typical DSC thermogram of BBPDA-OC8.
calorimetry (DSC). The representative DSC curves of BBPDA-
OC8 are shown in Figure 2 (refer to Figures S6-S12 for other
samples). As observed from the figure, several clear transitions
can be detected. The phase transition temperatures and enthalpy
changes were measured during the cooling scan, and these are
listed in Table 1. In Figure 3, the transition temperatures are
plotted against the carbon number of the alkoxy tail. Four types
of mesophases®nematic, Sm C, Sm I, and Sm J®can be well
distinguished in a wide temperature region, although a nematic-
to-isotropic transition cannot be detected in all these compounds
because of the extremely high transition temperature. Thus, a
high isotropization temperature and wide mesophase temper-
ature region are characteristic of this molecular system. The
former is caused by the high axial ratio of mesogen, and the
latter may be attributable to the nonpolar nature of mesogen.
The shortest homolog, BBPDA-OC5, exhibits only a
nematic (N) LC. BBPDA-OC6 forms N, Sm C, and Sm I
phases. In BBPDA-OCns with n = 7, 8, and 9, the Sm I phase
trasforms to the Sm J phase, and is followed by relatively low
enthalpy changes of around 1.0 kJg¹1. This enthalpy change
decreases with an increase in n; in longer homologs with n = 10,
The thermal behaviors of BBPDA-OCn (n = 5-12) and
BBPDA-OC5-11 were investigated by differential scanning
Br-CnH2n+1/ K2CO3
Br
OCnH2n+1
Br
OH
Acetonitrile
reflux
1
TMS
CuI/Pd(PPh3)4/Et3
N
K
2CO3
TMS
OCnH2n+1
OCnH2n+1
THF
THF/MeOH
3
2
CuCl / TMEDA
O2 / Acetone
H2n+1Cn
O
OCnH2n+1
4
BBPDA-OCn (n = 5-12)
Scheme 1. Synthesis of BBPDA-OCn derivatives.
Chem. Lett. 2010, 39, 513-515
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

514
Table 1. Phase transition temperature (°C) and enthalpies (¦H,
kJ mol¹1) (in italics) for the BBPDA-OCn series and BBDPDA-
OC5-11 obtained from cooling DSC thermograms at a rate of
(a)
(b)
¹1
10 °C min
n
Phase behavior and transition temperature (enthalpy)
5
6
N 211.2 (44.61) Cr
N 213.9 (1.74) Sm C 201.3 (7.43) Sm I 195.0 (28.7) Cr
N 236.0 (1.62) Sm C 207.7 (6.47) Sm I 197.6 (0.76) Sm J
179.0 (32.2) Cr
N 250.0 (2.27) Sm C 210.2 (5.67) Sm I 191.8 (0.60) Sm J
164.1 (23.5) Cr
7
8
(c)
(d)
N 257.7 (3.19) Sm C 210.3 (5.32) Sm I 182.4 (0.22) Sm J
154.2 (21.1) Cr
9
Sm C 209.4 (5.93) Sm I (or Sm J) 148.0 (19.8) Cr1 127.6
(7.47) Cr2
Sm C 207.5 (4.07) Sm I (or Sm J) 143.3 (16.5) Cr1 111.7
(5.35) Cr2
Sm C 205.7 (4.34) Sm I (orSm J) 138.4 (18.1) Cr1 116.3
(14.4) Cr2
N 239.8 (2.78) Sm C 199.9 (12.7) Sm I 171.5 (0.42) Sm J
137.2 (48.3) Cr
10
11
12
5-11
Figure 4. WAXD patterns of BBPDA-OC7. (a) Nematic
(245 °C), (b) Sm C (220 °C), (c) Sm X1 (200 °C), and (d) Sm
X2 (186 °C). Here, the X-ray patterns were obtained for a
homeotropically oriented sample on a glass plate with the beam
parallel to the glass surface. The arrow indicates the glass
surface direction.
260
I (N)
250
240
phase is likely; the nematic LC is formed by cybotactic groups
composed of about 102 moelcules with the molecular center in
each groups arranged in layers.6
230
220
210
200
190
180
170
160
150
140
130
120
110
II (Sm C)
III (Sm I)
Here, it is noteworthy that the director orients in a tilted
direction to the glass surface. This situation can be easily
understood from a comparison with the X-ray pattern of nematic
LC aligned by the magnetic field shown in Figure S1, where the
director corresponds to the magnetic field direction. One of the
maximum positions in an inner streak is on a meridional
direction and an outer broad reflection is observed 50° apart
from the equatorial line. Such an unusual orientation is
attributable to the cybotactic association of molecules in such
a manner that the pseudo layer formed by the cybotactic groups
lies parallel to the glass surface.6 On reflecting this orientation,
the Sm C phase appearing on cooling exhibits a more concrete
orientation with layer reflection on a meridional direction and a
broad outer reflection 40° apart from the equatorial direction
(Figure 4b). In the Sm I phase, the layer reflection on the
meridian becomes narrower, indicating that the orientation of the
layer is improved. Simultaneously the outer reflections become
sharper and appear at six positions (Figure 4c). Three sets of
these reflections have similar spacings of 4.35 ¡. Among them,
two sets of reflections are observed at the same angular position
as that observed in the Sm C phase, and another set is observed
along the equator. Such a pattern can be explained by the
improved packing order within a layer; the molecules within a
layer are packed with a more definite hexagonal lattice and the
molecular tilting is directed toward the apex of the hexagonal
lattice, as illustrated in Figure 5. This is characteristic of the Sm
I phase.7,8 The X-ray pattern of the Sm J phase is essentially
similar to that of the Sm I phase, except that it is more improved
so that all of the reflections appear as very sharp spots
(Figure 4d). The additional sharp spots observed immediately
above the six (100) reflections are assigned to (101). This
IV (Sm J)
Cr1
Cr2
5
6
7
8
9
10
11
12
n
Figure 3. Phase behavior of BBPDA-OCns.
11, and 12, no distinct transition can be detected in the DSC
thermogram although some change can be detected by polarized
optical microscopy. Asymmetric BBPDA-OC5-11 exhibited
four phase transitions. Its transition behavior is similar to that of
BBPDA-OC8, which has a similar molecular length, however
the mesophase temperature region is wider than those of
symmetric compounds.
In order to reveal the detailed mesophase structure, all the
compounds were analyzed by wide angle X-ray diffractometry
(WAXD). Typical WAXD patterns obtained for the mesophases
of BBPDA-OC7 are shown in Figure 4. Here, the sample was
aligned homeotropically on a glass plate that is surface-treated
by organo-silane coupling agents,5 and then the X-ray beam is
irradiated parallel to the glass surface.
The WAXD pattern shown in Figure 4a is that for the
highest-temperature nematic phase. It exhibits an inner streak
with a spacing of 34.4 ¡ and an outer broad reflection with a
spacing of 4.5 ¡, characteristic of a nematic phase. Because the
streaks in the inner zone are clearly split, a cybotactic nematic
Chem. Lett. 2010, 39, 513-515
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

515
Table 2. Carbon number dependence of tilt angle
Layer spacing/¡
Tilt angle
n
o
/
Experimental
Calculated
6
7
8
28.6
30.1
30.8
32.3
34.6
36.7
38.9
37.9
39.9
42.8
44.8
47.7
49.7
52.6
41.1
41.1
44.0
43.9
43.5
42.4
42.3
9
10
11
12
Figure 5. Structure of Sm I phase where the molecules tilt to
the layer and are locally packed into a hexagonal lattice.
40
35
30
molecule. Such an interesting association is observed in the
crystal structure of bis(biphenyl)diacetylene reported by Con-
stable et al.,10 suggesting strong interaction between biphenyl
and diacetylene moieties.
In summary, we synthesized bis(biphenyl)diacetylenes con-
taining an alkoxy group (BBPDA-OCns) and analyzed their
liquid crystalline behaviors. The long nonpolar mesogen
provides a wide mesogenic temperature region, including the
nematic, Sm C, Sm I, and Sm J phases. Irrespective of this
polymorphism, the tilting association of molecules giving a
large tilt angle of 40° is invariably observed; this is due to the
specific interaction between phenyl and acetylene groups.
Sm J
SmC
Sm I
N
160
180
200
220
240
260
280
Temperature/°C
References and Notes
Figure 6. Temperature dependence of layer spacing of
BBPDA-OC9 through the transitions from nematic to Sm J
phases.
1
2
3
4
5
H. Takatsu, K. Takeuchi, Y. Tanaka, M. Sasaki, Mol. Cryst.
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improved diffraction profile indicates the long-range periodic
ordering within the layers and between the layers. These are
characteristics of the Sm (or crystal) J phase.9
It should be noted that the spacing of the layer reflection is
maintained through the phase transition from the nematic to the
Sm J phase. This can be seen in Figure 6, which shows the
temperature dependence of the layer spacing. The collected
spacings for all compounds are compared with the molecular
lengths in Table 2. The tilt angles elucidated from a comparison
of the two parameters are also listed in Table 2. They do not
depend on the carbon number of the alkoxy tail, and are found
to range from 41 to 44°. These values correspond to those
elucidated (36 to 40°) from the X-ray patterns shown in
Figures 4b-4d.
6
7
A. de Vries, Mol. Cryst. Liq. Cryst. 1970, 10, 219.
A. J. Leadbetter, J. P. Gaughan, B. Kelly, G. W. Gray, J.
Goodby, J. Phys. (Paris) 1979, 40, 178.
8
9
P. A. C. Gane, A. J. Leadbetter, P. G. Wrighton, Mol. Cryst.
Liq. Cryst. 1981, 66, 247.
J. Goodby, in Handbook of Liquid Crystals, ed. by D.
Demus, J. Goodby, G. W. Gray, H.-W. Spiess, Wiley-VCH,
Weinheim, 1998, Vol. 2A, p. 17.
The large tilt angle and its independency of the temperature
and alkoxy tail length are probably caused by the special
interaction between mesogenic moieties. Figure 5 shows
BBPDA-OCn molecules associated into a layer with a tilt angle
of 40°. It is obvious that in this association, the phenyl groups of
one molecule are near the acetylene groups in the neighboring
10 E. C. Constable, D. Gusmeroli, C. E. Housecroft, M.
Neuburger, S. Schaffner, Acta Crystallogr., Sect. C 2006, 62,
o505.
11 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Chem. Lett. 2010, 39, 513-515
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/