Molecular organization of preorganized S-shaped oligomers in the liquid crystalline phases

Article abstract of DOI:10.1080/15421400903054527

We prepared a homologous series of S-shaped oligomers, 4,4′- bis(ω-(2-(ω-(4-(cyanophenyl)phenyloxy)alkyloxy)phenyloxy)alkyloxy) biphenyl [I-(m,n)], and investigated effects of the spacer length on the phase transition behavior. I-(7,7), I-(8,8), I-(9,9) and I-(7,9) showed nematic (N) and smectic A (SmA) phases, whereas I-(6,6) and I-(6,9) showed only an N phase. The melting behavior was found to depend on the parity of the spacer (m). The liquid-crystalline phases of I-(8,8) and I-(6,9) are monotropic, however, those of I-(9,9) and I-(7,9) are enantiotropic. X-ray diffraction measurements of I-(7,7), I-(8,8) and I-(7,9) indicate that the S-shaped oligomers exist as a close-packed structure in the monolayer SmA phase. Copyright Taylor & Francis Group, LLC.

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Molecular Organization
of Preorganized S-Shaped
Oligomers in the Liquid
Crystalline Phases
Yoshihiro Nagashima a , Fumitaka Ogasawara a b
, Jun Yamamoto c , Yoichi Takanishi c & Atsushi
Yoshizawa a
a Department of Frontier Materials Chemistry ,
Graduate School of Science and Technology, Hirosaki
University , Hirosaki, Japan
b Tohoku Chemical Corporation , Hirosaki, Japan
c Department of Physics , Graduate School of
Science, Kyoto University , Kyoto, Japan
Published online: 05 Oct 2009.
To cite this article: Yoshihiro Nagashima , Fumitaka Ogasawara , Jun Yamamoto ,
Yoichi Takanishi & Atsushi Yoshizawa (2009) Molecular Organization of Preorganized
S-Shaped Oligomers in the Liquid Crystalline Phases, Molecular Crystals and Liquid
Crystals, 509:1, 233/[975]-244/[986], DOI: 10.1080/15421400903054527
To link to this article: http://dx.doi.org/10.1080/15421400903054527
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Mol. Cryst. Liq. Cryst., Vol. 509, pp. 233=[975]–244=[986], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903054527
Molecular Organization of Preorganized S-Shaped
Oligomers in the Liquid Crystalline Phases
Yoshihiro Nagashima1, Fumitaka Ogasawara1,2, Jun
Yamamoto3, Yoichi Takanishi3, and Atsushi Yoshizawa1
1Department of Frontier Materials Chemistry, Graduate School of
Science and Technology, Hirosaki University, Hirosaki, Japan
2Tohoku Chemical Corporation, Hirosaki, Japan
3Department of Physics, Graduate School of Science,
Kyoto University, Kyoto, Japan
We prepared a homologous series of S-shaped oligomers, 4,40-bis(x-(2-(x-(4-(cyano-
phenyl)phenyloxy)alkyloxy)phenyloxy)alkyloxy)biphenyl [I-(m,n)], and investigated
effects of the spacer length on the phase transition behavior. I-(7,7), I-(8,8), I-(9,9)
and I-(7,9) showed nematic (N) and smectic A (SmA) phases, whereas I-(6,6) and
I-(6,9) showed only an N phase. The melting behavior was found to depend on
the parity of the spacer (m). The liquid-crystalline phases of I-(8,8) and I-(6,9)
are monotropic, however, those of I-(9,9) and I-(7,9) are enantiotropic. X-ray diffrac-
tion measurements of I-(7,7), I-(8,8) and I-(7,9) indicate that the S-shaped oligo-
mers exist as a close-packed structure in the monolayer SmA phase.
Keywords: liquid crystals; oligomer; phase transition; S shape
1. INTRODUCTION
Oligomeric liquid crystals are current topics in the design of new
liquid crystalline materials because of their abilities to form supramo-
lecular assemblies [1]. In addition, molecular topology has attracted
much attraction as the origin for producing novel self-organizing sys-
tems. One of the initial investigation for oligomeric liquid crystals was
dimer in which mesogenic moieties are connected via a flexible alkyl
spacer. The well-known series of symmetric liquid-crystalline dimer
is the a,x-bis(4-(4-cyanophenyl)phenyloxy)alkanes [2]. This series
This work was partly supported by a Grant-in-Aid for Scientific Research from the
Japan Society for the Promotion of Science (No. 19550175).
Address correspondence to Atsushi Yoshizawa, Department of Frontier Materials
Chemistry, Graduate School of Science and Technology, Hirosaki Univesity,
Bunkyo-cho, Hirosaki 036-8561, Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp
3
233=[975]

234=[976]
Y. Nagashima et al.
FIGURE 1 Molecular structure of the S-shaped liquid crystal oligomer.
exhibited a namatic (N) phase, and pronounced odd-even effects asso-
ciated with spacer length were observed for their melting and
nematic-isotropic transition temperatures. These dependences could
be explained by using zig-zag shape for the even-numbered dimer
and bent shape for odd-numbered dimer. Then, trimer [3] and tetra-
mer [4] have ever been reported.
We have designed novel preorganized systems, e.g., a flexible dime-
sogenic compound [5], a U-shaped molecule [6], a binaphthyl deriva-
tive [7], a k-shaped molecule [8], V-shaped [9], T-shaped [10] and
Y-shaped molecules [11]; the preorganized supermolecules were found
to induce unusual ordering in the supramolecular liquid crystalline
phase. Recently, we reported pronounced odd-even effects in the tran-
sition behavior of U-shaped systems [12]. Moreover, we have reported
the synthesis and transition properties of S-shaped molecules in which
two U-shaped mesogenic molecules are connected [13].
For the present study, we prepared a homologous series of 4,40-
bis(x-(2-(x-(4-(cyanophenyl)phenyloxy)alkyloxy)phenyloxy)alkyloxy)-
biphenyl [I-(m,n)] and investigated effects of spacer length on their
liquid-crystalline properties. Figure 1 shows molecular structure of
the S-shaped liquid-crystalline oligomer. We discuss packing and con-
formation for the S-shaped oligomers in their liquid crystalline phases.
2. EXPERIMENTAL
2.1. Preparation of Materials
Purification of the final products was carried out using HPLC (Japan
Analytical Industry Co., Ltd., LC-9101, JAIGEL-2H column); chloroform
was used as eluant. Detection of products was achieved by UV irradiation
(k ¼ 254 nm). Recrystallization from ethanol gave the final products.
Structures were elucidated using infrared (IR) spectroscopy (FTS-30;
Bio Red Laboratories, Inc.) and proton nuclear magnetic resonance

S-Shaped Liquid Crystals
235/[977]
spectroscopy (JMN-GX270; JOEL, JMN-A400; JOEL, or JMN-ECA500;
JPEL). The analyses of the structures of the products by spectroscopic
methods were found to be consistent with the predicted structures.
2.1.1. 4,40-Bis(8-(2-(8-(4-(cyanophenyl)phenyloxy)
octyloxy)phenyloxy)octyloxy)biphenyl, I-(8,8)
Potassium carbonate (2.41 g, 17.5 mmol) was added to a solution of
4-cyano-40-hydroxybiphenyl (2.92 g, 15 mmol) and 1,8-dibromohexane
(5.40 g, 20 mmol) in cyclohexanone (45 mmol). The resulting mixture
was stirred at 70ꢀC for 10 h. After filtration of the precipitate, the sol-
vent was removed by evaporation. The residue was purified by column
chromatography on silica gel with toluene. Recrystallization from
ethanol gave 4-cyano-40-(8-bromooctyloxy)biphenyl; yield 2.93 g (51 %).
Potassium carbonate (0.69 g, 5.0 mmol) was added to a solution of
4-cyano-40-(8-bromooctyloxy)biphenyl (1.93 g, 5.0 mmol) and 1,2-dihy-
droxybenzene (1.65 g, 15 mmol) in cyclohexanone (15 ml). The result-
ing mixture was stirred at 70ꢀC for 11 h. After filtration of the
precipitate, the solvent was removed by evaporation. The residue
was purified by column chromatography on silica gel with dichloro-
methane. Recrystallization from ethanol gave 2-(8-(4-(cyanophenyl)-
phenyloxy)octyloxy)phenol; yield 2.93 g (51%).
Potassium carbonate (1.74 g, 12.5 mmol) was added to a solution of
4,40-dihydroxybiphenyl (1.86 g, 10 mmol) and 1,8-dibromooctane
(10.95 g, 40 mmol) in acetone (50 ml). The resulting mixture was
stirred under reflux for 12 h. After filtration of the precipitate, the
solvent was removed by evaporation. The residue was reprecipitated
with a mixture of chloroform and ethanol to give 4,40-bis(8-bromoocty-
bromooctyloxy)biphenyl; yield 1.40 g (25%).
Potassium carbonate (0.14 g, 1.0 mmol) was added to a solution of
2-(8-(4-(cyanophenyl)phenyloxy)octyloxy)phenol (0.42 g, 1.0 mmol)
and 4,40-bis(8-bromooctyloxy)biphenyl (0.23 g, 0.50 mmol) in cyclohex-
anone (10 ml). The resulting mixture was stirred at 120ꢀC for 10 h.
After filtration of the precipitate, the solvent was removed by evapora-
tion. The residue was washed with dichloromethane and purified
using HPLC with chloroform as eluant. Recrystallization from ethanol
gave the desired product; yield 90 mg (14%).
1H NMR (500 MHz, solvent CDCl3, standard TMS) dH=ppm: 7.67 (d,
4H, Ar–H, J ¼ 8.6 Hz), 7.62 (d, 4H, Ar–H, J ¼ 8.55 Hz), 7.50 (d, 4H, Ar–
H, J ¼ 8.55 Hz), 7.43 (d, 4H, Ar–H, J ¼ 8.6 Hz), 6.97 (d, 4H, Ar–H,
J ¼ 9.2 Hz), 6.92 (d, 4H, Ar–H, J ¼ 8.55 Hz), 6.89–6.87 (m, 8H, Ar–
H), 4.01–3.95 (m, 16H, Ar–OCH2–). 1.86–1.74 (m, 16H, aliphatic–H).
1.53–1.45 (m, 16H, aliphatic–H). 1.43–1.36 (m, 16H, aliphatic–H). IR
(KBr) n=cmꢁ1: 2936, 2856 (C–H), 2224 (CꢂN), 1602, 1497 (C¼C).

236/[978]
Y. Nagashima et al.
The other compounds presented in this paper were obtained by a
similar method to that for I-(8,8). Analytical data for other compounds
are given below.
2.1.2. 4,40-Bis(9-(2-(9-(4-(cyanophenyl)phenyloxy)
nonyloxy)phenyloxy)nonyloxy)biphenyl, I-(9,9)
1H NMR (500 MHz, solvent CDCl3, standard TMS) dH=ppm: 7.67 (d,
4H, Ar–H, J ¼ 8.6 Hz), 7.62 (d, 4H, Ar–H, J ¼ 8.6 Hz), 7.50 (d, 4H, Ar–
H, J ¼ 9.15 Hz), 7.43 (d, 4H, Ar–H, J ¼ 9.15 Hz), 6.97 (d, 4H, Ar–H, J ¼
9.16 Hz), 6.92 (d, 4H, Ar–H, J ¼ 8.59 Hz), 6.88 (m, 8H, Ar–H,
J ¼ 8.59 Hz), 4.02–3.94 (m, 16H, Ar–OCH2–), 1.86–1.75 (m, 16H, alipha-
tic–H), 1.52–1.43 (m, 16H, aliphatic–H), 1.42–1.32 (m, 24H, aliphatic–H).
IR (KBr) n=cmꢁ1: 2928, 2852 (C–H), 2227 (CꢂN), 1603, 1507 (C¼C).
2.1.3. 4,40-Bis(6-(2-(9-(4-(cyanophenyl)phenyloxy)nonyloxy)
phenyloxy)hexyloxy)biphenyl, I-(6,9)
1H NMR (500MHz, solvent CDCl3, standard TMS) dH=ppm: 7.67 (d,
4H, Ar–H, J ¼ 7.45 Hz), 7.62 (d, 4H, Ar–H, J ¼ 8.05 Hz), 7.50 (d, 4H,
Ar–H, J ¼ 8.55 Hz), 7.43 (d, 4H, Ar–H, J ¼ 8.05 Hz), 6.97 (d, 4H,
Ar–H, J ¼ 8.6 Hz), 6.92 (d, 4H, Ar–H, J ¼ 8.6 Hz), 6.87 (m, 8H, Ar–H),
4.05–3.95 (m, 16H, Ar–OCH2–), 1.91–1.76 (m, 16H, aliphatic–H), 1.62–
1.53 (m, 8H, aliphatic–H), 1.52–1.43 (m, 8H, aliphatic–H), 1.42–1.31 (m,
12H, aliphatic–H). IR (KBr) n=cmꢁ1: 2937, 2855 (C–H), 2227 (CꢂN),
1603, 1507 (C¼C).
2.1.4. 4,40-Bis(7-(2-(9-(4-(cyanophenyl)phenyloxy)
nonyloxy)phenyloxy)hepyloxy)biphenyl, I-(7,9)
1H NMR (500 MHz, solvent CDCl3, standard TMS) dH=ppm: 7.67 (d,
4H, Ar–H, J ¼ 8.6 Hz), 7.62 (d, 4H, Ar–H, J ¼ 8.6 Hz), 7.50 (d, 4H, Ar–
H, J ¼ 8.55 Hz), 7.43 (d, 4H, Ar–H, J ¼ 9.2 Hz), 6.97 (d, 4H, Ar–H,
J ¼ 8.6 Hz), 6.92 (d, 4H, Ar–H, J ¼ 9.15 Hz), 6.89 (m, 8H, Ar–H),
4.04–3.95 (m, 16H, Ar–OCH2–), 1.88–1.76 (m, 16H, aliphatic–H),
1.55–1.42 (m, 20H, aliphatic–H), 1.42–1.32 (m, 12H, aliphatic–H). IR
(KBr) n=cmꢁ1: 2934, 2854 (C–H), 2226 (CꢂN), 1603, 1500 (C¼C).
2.1.5. 4,40-Bis(6-(2-(6-(4-(cyanophenyl)phenyloxy)
hexyloxy)phenyloxy)hexyloxy)biphenyl, I-(6,6)
1H NMR (270 MHz, solvent CDCl3, standard TMS) dH=ppm: 7.65 (d,
4H, Ar–H, J ¼ 8.6 Hz), 7.59 (d, 4H, Ar–H, J ¼ 8.9 Hz), 7.48 (d, 4H, Ar–
H, J ¼ 8.9 Hz), 7.41 (d, 4H, Ar–H, J ¼ 8.4 Hz), 6.97–6.88 (m, 16H, Ar–
H), 4.04–3.94 (m, 16H, Ar–OCH2–), 1.85–1.52 (m, 12H, aliphatic–H).
IR (KBr) n=cmꢁ1: 2940, 2861 (C–H), 2224 (CꢂN), 1601, 1498 (C¼C).

S-Shaped Liquid Crystals
237/[979]
2.1.6. 4,40-Bis(7-(2-(7-(4-(cyanophenyl)phenyloxy)
heptyloxy)phenyloxy)heptyloxy)biphenyl, I-(7,7)
1H NMR (400 MHz, solvent CDCl3, standard TMS) dH=ppm: 7.66 (d,
4H, Ar–H, J ¼ 8.3 Hz), 7.60 (d, 4H, Ar–H, J ¼ 8.3 Hz), 7.49 (d, 4H, Ar–
H, J ¼ 8.3 Hz), 7.42 (d, 4H, Ar–H, 8.8 Hz), 6.97–6.89 (m, 16H, Ar–H),
4.02–3.95 (m, 16H, Ar–OCH2–), 1.85–1.52 (m, 40H, aliphatic–H). IR
(KBr) n=cmꢁ1: 2947, 2856 (C–H), 2225 (CꢂN), 1602, 1499 (C¼C).
2.2. Liquid-Crystalline and Physical Properties
The initial phase assignments and corresponding transition tempera-
tures for the final products were determined by optical polarized light
microscopy using
a Nikon Optiphot-pol polarizing microscope
equipped with a Mettler FP82 hot stage and FP90 control processor.
The heating and cooling rates were 5ꢀC minꢁ1. The photomicrographs
were taken using a camera (OLYMPUS DIGITAL CAMERA C-5050
ZOOM) in conjunction with a Nikon Optiphot-pol polarizing micro-
scope. Temperatures and enthalpies of transition were investigated
by differential scanning calorimetry (DSC) using a Seiko DSC6200
calorimeter. The compounds were studied at a scanning rate of
5ꢀC minꢁ1, for both heating and cooling cycles, after being encapsu-
lated in aluminium pans. X-ray diffraction patterns of the sample on
cooling process were obtained using a real-time X-ray diffractometer
(D8 Discover; Bruker AXS GmbH) equipped with a hot stage and a
temperature-control processor. A sample was put on a convex lens,
which was placed in a custum-made temperature stabilized holder
(stability within ꢃ0.1ꢀC). The X-ray apparatus was equipped with a
¨
cross-coupled Gobel mirror on a platform system with a two-
dimensional position-sensitive proportional counter (PSPC) detector
(HI-Star; Bruker AXS GmbH). X-rays were generated at 40 kV and
40 mA; a parallel Cu Ka X-ray beam was used to irradiate the sample.
Each diffraction pattern was obtained using the PSPC detector at a
camera distance of 150 mm for a short counting time of 30 s.
2.3. Molecular Shape
Figure 2 shows MM2 models for I-(m,n) based on the assumption that
intramolecular interactions between the cyanobiphenyl and biphenyl
moieties cause the molecules to form a close-packed structure.
Although a structure of a mesogenic molecule based on the MM2
calculation does not reflect the real conformation in the
liquid-crystalline phase, the structure can give some information to
discuss the observed structure-property relationships.

238/[980]
Y. Nagashima et al.
FIGURE 2 MM2 models for I-(m,n).
3. RESULTS AND DISCUSSION
3.1. Phase Transion Properties
Transition temperatures and transition enthalpies for S-shaped liquid-
crystalline oligomers I-(m,n) measured by optical polarized light
microscopy and differential scanning calorimetry (DSC) are listed in
TABLE 1 Phase Transition Temperature (ꢀC), Enthalpies (kJ molÀ1) of
Transition (in Brackets) and Transition Entropies (DS=R) for S-Shaped
Compounds I-(m,n) on Cooling
Compound Phase transition tmperatures=ꢀC (DH=kJ molÀ1
)
DSN-SmA=R DSI-N=R
I–(6,6)
I–(7,7)
I–(8,8)
I–(9,9)
I–(6,9)
I–(7,9)
Cr 154 [N 139 (0.71)] Iso
0.21
0.31
1.36
Cr 85 SmA 100 (3.38) N 105 (0.95) Iso
Cr 139 [SmA 115a N 132 (4.59)] Iso
Cr 89 SmA 107b N 108b Iso
Cr 120 [N 119 (2.76)] Iso
Cr 82 SmA 97 (3.27) N 102 (1.11) Iso
1.09
0.85
0.35
1.07
aThe N-SmA transition enthalpy was too small to be detected.
bThe Iso-N and N-SmA transitions occurred simultaneously. The total value of both
transition enthalpies was 11.2 kJ molÀ1
.

S-Shaped Liquid Crystals
239/[981]
Table 1. Compounds I-(6,6) and I-(6,9) exhibited a nematic (N) phase,
whereas compounds I-(7,7), I-(8,8), I-(9,9), and I-(7,9) exhibited
nematic and smectic A (SmA) phases. Pronounced odd-even effects
were observed for the Iso-N and N-SmA transition temperatures of
the S-shaped oligomers as the parity of the spacer was varied.
The melting behavior was found to depend on the parity of the
spacer. DSC thermograms for I-(6,6) and I-(7,7) are shown in
Figure 3. Compound I-(6,6) showed a Cry to Iso transition at 154ꢀC,
and exhibited a monotropic N phase on cooling. There is marked hys-
teresis in phase transition behavior between heating and cooling pro-
cesses of compound I-(6,6). On the other hand, there is no significant
difference in transition behavior between heating and cooling pro-
cesses of compound I-(7,7). DSC measurements of all the compounds
revealed that the melting behavior depends on the parity (m) of the
spacer. Lower melting temperatures for compounds I-(7,7), I-(9,9)
FIGURE 3 DSC thermograms for (a) I-(8,8) and (b) I-(7,7).

240/[982]
Y. Nagashima et al.
and I-(7,9) are thought to be attributed to loose molecular packing in
the crystalline phases.
We compared the transition properties of I-(8,8) with those of
I-(7,7). Both compounds exhibited a phase sequence of Iso-N-SmA.
Their transition entropies (DS=R) are listed in Table 1. Surprisingly,
the associated entropy change at the N–SmA transition of I-(8,8)
was negligible small. The associated entropy change at the Iso-N tran-
sition of I-(8,8) was almost corresponding to the total value of both the
I-N and N-SmA transition entropies of I-(7,7). The results suggested a
possibility that the N phase of I-(8,8) has a smectic-like layer ordering.
We will discuss it later.
FIGURE 4 (a) Relationship between TI-N and L=D ratio for compound
I-(m,n). (b) Relationship between TN-SmA and L=D ratio for compound I-(m,n).

S-Shaped Liquid Crystals
241/[983]
3.2. Relationship Between Molecular Shape and Transition
Temperature
Isotropic-nematic transition temperature (TI-N) is plotted against L=D
for the S-shaped oligomers in Figure 4(a). As increasing L=D ratio, TI-N
FIGURE 5 X-ray diffraction patterns in the SmA phase of (a) I-(7,7), (b)
I-(8,8) and (c) I-(7,9).

242/[984]
Y. Nagashima et al.
TABLE 2 Molecular Length (l) and Layer Spacing (d) in the
SmA Phase of I-(m,n)
a
b
˚
˚
Compound
l =A
d =A
I–(7,7)
I–(8,8)
I–(7,9)
35
39
35
32
34
33
aThe estimated molecular length from MM2 calculation.
bThe layer spacing obtained from X-ray diffraction measurements.
increases. Therefore, the excluded volume effects due to molecular
shape promote formation of the N phase. Figure 4(b) shows relation-
ship between namatic-smectic A transition temperature (TN-SmA) and
L=D ratio. There is good linearity between them. Compounds with
even-membered (m) spacers have larger L=D ratio than those with
odd-membered (m) spacers. The odd-even effects for the Iso-N and
N-SmA transition temperatures of the S-shaped oligomers are explain-
able in terms of L=D ratio. However, TN-SmA=(L=D) is smaller than TI-N
=
(L=D). Not only the excluded volume effect but also an electrostatic
interaction plays an important role in formation of the SmA phase.
FIGURE 6 Possible model of the molecular organization in the SmA phase of
I-(8,8).

S-Shaped Liquid Crystals
243/[985]
3.3. X-ray Diffraction Studies
Figure 5 shows X-ray diffraction patterns in the small angle region in
the SmA phase of (a) I-(7,7), (b) I-(8,8) and (c) I-(7,9). The estimated
molecular length (l) from MM2 calculation and the layer spacing (d)
obtained from the XRD measurements are summarized in Table 2.
Although DS=R at the N-SmA transition for I-(8,8) is negligible small,
XRD studies indicate that there is no significant positional order in
the N phase of I-(8,8). The obtained layer spacings of the SmA phase
˚
for I-(7,7), I-(8,8), and I-(7,9) were about 32, 34, and 33 A, respec-
tively. Figure 6 shows a possible model for molecular organization in
the SmA phase of I-(8,8). The model suggests that: (1) the oligomer
forms a close-packed S-shaped structure, (2) antiparallel interactions
between neighboring cyanobiphenyl groups exist in each layer, and
3) interlayer interactions between the phenyl units occur.
4. CONCLUSIONS
We prepared a homologous series of S-shaped oligomers and investi-
gated their phase transition behavior. All S-shaped oligpmers exhibited
an N phase. In addition, compounds I-(7,7), I-(8,8), I-(9,9), and I-(7,9)
also exhibited an SmA phase. Pronounced odd-even effects were observed
for melting behavior, TI-N and TN-SmA of the S-shaped oligomers. Further-
more, XRD measurements indicate that the SmA phase has a monolayer
structure composed of the close-packed S-shaped oligomers.
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