Topological effect in dimeric liquid crystals

Article abstract of DOI:10.1080/15421400590957585

We have prepared some new dimeric liquid crystals in which two phenylpyrimidine moieties are connected via (±)-trans-1,2-cyclohexanediol or p-toluenesulfon amide and investigated the transition behaviour. (±)-trans-1,2-Bis(6-[4-(5-octylpyrimidin-2-yl)phen

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Topological Effect in Dimeric
Liquid Crystals
a
a
A. Yoshizawa & T. Kawaguchi
a
Department of Materials Science and Technology ,
Faculty of Science and Technology, Hirosaki
University , Hirosaki, Japan
Published online: 31 Aug 2006.
To cite this article: A. Yoshizawa & T. Kawaguchi (2005) Topological Effect in Dimeric
Liquid Crystals, Molecular Crystals and Liquid Crystals, 439:1, 97/[1963]-105/[1971],
DOI: 10.1080/15421400590957585
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Mol. Cryst. Liq. Cryst., Vol. 439, pp. 97=[1963]–105=[1971], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590957585
Topological Effect in Dimeric Liquid Crystals
A. Yoshizawa
T. Kawaguchi
Department of Materials Science and Technology, Faculty of Science and
Technology, Hirosaki University, Hirosaki, Japan
We have prepared some new dimeric liquid crystals in which two phenylpyrimi-
dine moieties are connected via (Æ)-trans-1,2-cyclohexanediol or p-toluenesulfon
amide and investigated the transition behaviour. (Æ)-trans-1,2-Bis{6-[4-(5-octyl-
pyrimidin-2-yl)phenoxy]hexyloxy}cyclohexane 1 was found to show enantiotropic
nematic (N) and smectic A (SmA) phases with a low melting point. On the other
hand, N-p-toluensulfonyl-bis{6-[4-(5-octylpyrimidin-2-yl)phenoxy]hexyloxy}amine
2
showed only a monotropic SmA phase. We discuss topological effect of the
connecting group on the phase transition behaviour.
Keywords: dimer; liquid crystals; phase transition; smectic phase
1. INTRODUCTION
The driving force of mesophase formation is a fundamental topic in the
investigation of molecular assembly. Recently molecular topology has
attracted much attention as the origin for producing novel self-organiz-
ing systems [1]. Dimeric liquid crystals are attractive because they
exhibit different properties from the corresponding low-molecular
mass mesogens [2–4]. On the other hand, we have investigated the
Ã
Ã
microscopic organization of molecules in SmA, SmC and SmC phases
A
by means of C-13 NMR [5–7]. The NMR studies suggest that cooperat-
ive motion for the core parts contributes to the orientational order of
molecules in each layer. We prepared a U-shaped molecule in which
two phenylpyrimidine moieties are connected via catechol (BOPPHB).
This work was supported by Aomori Prefecture Collaboration of Regional Entities for
the Advancement of Technological Excellence, JST.
Address correspondence to Atsushi Yoshizawa, Department of Materials Science and
Technology, Faculty of Science and Technology, Hirosaki University, 3, Bunkyo-cho,
Hirosaki 036-8561, Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp
97=[1963]

9
8=[1964]
A. Yoshizawa and T. Kawaguchi
Strong correlation of rotation around the long axis of each mesogenic
group is expected. BOPPHB was found to have smectic-like layer
ordering in the N phase, and unusual enthalpy changes were observed
in the N phase on heating from the monotropic SmC phase [8].
In the present study, we have designed some novel dimeric liquid
crystals in which two phenylpyrimidine moieties are connected via
(
ꢀ)-trans-1,2-cyclohexanediol or p-toluenesulfon amide. Topological
effect of the connecting group on phase transition behaviour of the
dimeric liquid crystals has been investigated.
2
2
(
. EXPERIMENTAL
.1. Materials
ꢀ )-trans-1,2-Bis{6-[4-(5-octylpyrimidin-2-yl)phenoxy]hexyloxy}
cyclohexane, 1
To a solution of 5-octyl-2-(4-hydroxyphenyl)pyrimidine (569 mg,
2
.0 mmol) and 1,6-dibromohexane (736 mg, 3.0 mmol) in cyclohexa-
none (10 mL) was added potassium carbonate (280 mg, 2.0 mmol).
ꢁ
The reaction mixture was stirred at 70 C for 4 h. After the filtration
of participate, the solvent was removed by evaporation. The residue
was washed with ethanol and purified by column chromatography
on silica gel with dichloromethane. 5-Octyl-2-[4-(6-bromohexyl)phe-
nyl]pyrimidine was obtained. Yield: 559 mg (45%).
To a solution of (ꢀ)-trans-1,2-cyclohexanediol (67 mg, 0.57 mmol)
and 5-octyl-2-[4-(6-bromohexyl)phenyl]pyrimidine (1.08 g, 2.4 mmol)
in a dimethylsulfoxide–toluene (1:4) mixture (10 mL) was added pot-
assium hydroxide (200 mg, 3.6 mmol). The reaction mixture was stir-
ꢁ
red at room temperature for 6 hr and then stirred at 60 C for 25 h.
After the filtration of participate, the solvent was removed by evapor-
ation. The residue was purified by column chromatography on silica
gel with a dichloromethane–ethylacetate (14:1) mixture as a eluent.
The desired product was obtained. Yield: 41 mg (8%).

Topological Effect in Dimeric Liquid Crystals
99=[1965]
1
d
HNMR (270 MHz, solvent CDCl , standard TMS) H: 8.56 (s, 4H),
3
8
.34 (d, 4H, J ¼ 8.9 Hz), 6.96 (d, 4H, J ¼ 8.9 Hz), 4.01 (t, 4H,
J ¼ 6.5 Hz), 3.56 (t, 4H, J ¼ 6.3 Hz), 3.17–3.10 (m, 2H), 2.59 (t, 4H,
ꢂ1
J ¼ 7.6 Hz), 1.27–1.97 (m, 48H), 0.88 (t, 6H, J ¼ 6.8 Hz); IR (KBr, cm )
2
928, 2855, 1609, 1585, 797.
N-p-toluensulfonyl-bis{6-[4-(5-octylpyrimidin-2-yl)phenoxy]
hexyloxy}amine, 2
To a stirred solution of p-toluensulfonamide (77 mg, 0.45 mmol) in
N, N-dimethyformamide (DMF, 10 mL) was added 60% sodium
ꢁ
hydride (52 mg, 1.3 mmol) at 90 C and the reaction mixture was stir-
ꢁ
red at 90 C for 1 h. Cooling the reaction mixture to room temperature,
5
-octyl-2-(4-hydroxyphenyl)pyrimidine (440 mg, 1.0 mmol) was added
to the mixture and then the reaction mixture was stirred at room
temperature for 5 h. DMF was removed under reduced pressure and
solid residue was obtained. Dichloromethane was added to the residue
and the suspension was stirred. After the filtration of participate, the
solvent was removed by evaporation. The residue was purified by
column chromatography on silica gel with a dichloromethane-ethyl-
acetate (40:1) mixture as the eluent. Recrystallization from ethanol
gave the desired product, yield 265 mg (65%).
1
d
HNMR (270 MHz, solvent CDCl , standard TMS) H: 8.57 (s, 4H),
3
8
8
.35 (d, 4H, J ¼ 8.9 Hz), 7.68 (d, 2H, J ¼ 8.4 Hz), 7.27 (d, 2H, J ¼
.6 Hz), 6.96 (d, 4H, J ¼ 9.2 Hz), 4.00 (t, 4H, J ¼ 7.7 Hz), 2.59 (t, 4H,
J ¼ 6.2 Hz), 2.40( s, 3H), 1.27–1.83 (m, 48H), 0.88 (t, 6H, J ¼ 6.8 Hz);
ꢂ1
IR (KBr, cm ) 2928, 2854, 1586, 800.
2.2. Liquid-Crystalline Properties
The initial assignments and corresponding transition temperatures
for the final product were determined by thermal optical microscopy
using a Nikon Optiphoto POL polarizing microscope equipped with a
Mettler FP82 microfurnance and FP80 control unit. The heating and
ꢁ
ꢂ1
cooling rates were 5 C min . Temperatures and enthalpies of tran-
sition were investigated by differential scanning calorimetry (DSC)
using a Seiko DSC 6200.
3
3
. RESULTS AND DISCUSSION
.1. Liquid-Crystalline Properties
Molecular structures of the novel dimeric materials and the corre-
sponding monomeric compound (8-PYP-6O) are shown in Figure 1.

1
00=[1966]
A. Yoshizawa and T. Kawaguchi
FIGURE 1 Molecular structures for the novel dimeric liquid crystals and its
corresponding monomeric compound.
Temperatures and enthalpies of transition for the compounds and
BOPPHB determined by optical microscopy and differential scanning
microscopy (DSC) are compared in Table 1.
BOPPHB possessing catechol as a connecting group showed N and
SmC phases, however, compound 1 was found to show enantiotropic N
and SmA phases with markedly low melting temperature. Compound
2
showed only a monotropic SmA phase. Temperature range for the
liquid-crystalline phases for compound 1 is similar to that for the
monomeric compound, 8-PYP-6O.

Topological Effect in Dimeric Liquid Crystals
101=[1967]
ꢂ1
ꢁ
TABLE 1 Transition Temperatures ( C) on Cooling and Enthalpies (kJmol )
of Transition (in brackets) for Compounds under Investigation
Compound
SmC
SmA
N
Iso
mp
1
2
. 66.3 (1.2)
. 48.4 (10.6)
. 70.9 (3.4)
.
.
.
.
34.5(30.1)
90.1(60.9)
76.0(57.7)
27.0(27.5)
BOPPHB
. 43.4(0.3)
. 44.5
. 83.3 (3.5)
. 65.0 (1.7)
8
-PYP-6O
. 57.5(0.4)
3.2. Binary Mixture Studies
We investigated the transition behaviour on cooling of binary mixtures
between each novel dimeric compound and its corresponding mono-
meric compound (Fig. 2). There was no biphasic temperature region
for the binary mixtures. Figure 2(a) shows a binary phase diagram
for mixtures of 8-PYP-6O and compound 1. The N and SmA phases
of both materials proved to be miscible across the full composition
range. Furthermore, the SmC phase was observed even in the mixture
containing 7 wt% of compound 1. The mixtures between 8-PYP-6O
ꢁ
and compound 1 were not crystallized on cooling to ꢂ30 C. Figure
2
(b) shows a binary phase diagram for mixtures of 8-PYP-6O and com-
pound 2. The SmA phase of both materials proved to be miscible, how-
ever, the SmC phase disappeared in the mixture containing 25 wt% of
compound 2. Transition temperatures on heating of the mixture con-
ꢁ
ꢁ
ꢁ
taining 25 wt% of compound 2 were Cryst 25 C SmA 47 C N 60 C
Iso, however, the mixture containing 75 wt% of compound 2 did not
show a liquid-crystalline phase on heating.
Then, we investigated the transition behaviour on cooling of binary
mixtures between each dimeric compound and BOPPHB (Fig. 3).
There was no biphasic temperature region for the binary mixtures.
Figure 3(a) shows a binary phase diagram for mixtures of BOPPHB
and compound 1. The SmC phase was observed even in the mixture
of containing 75 wt% of compound 1. The SmA and SmC phases
observed for the cooling phase diagram did not appear on heating.
Transition temperatures on heating of the mixture containing
ꢁ ꢁ
5
0 wt% of compound 1 were Cryst 66 C N 79 C Iso. Figure 3(b) shows
a binary phase diagram for mixtures of BOPPHB and compound 2.
The SmA phase was stable even in the mixture containing 90 wt%
of BOPPHB, however, the SmC phase of BOPPHB was markedly
depressed by addition of compound 2. The SmA and SmC phases did
not appear on heating. The mixture containing 25 wt% of compound
2
showed the N phase on heating, however, the mixture containing
5
0 wt% of compound 2 did not show a liquid-crystalline phase.

1
02=[1968]
A. Yoshizawa and T. Kawaguchi
FIGURE 2 (a) Binary phase diagram for mixtures of 8-PYP-6O and com-
pound 1. (b) Binary phase diagram for mixtures of 8-PYP-6O and compound
2.
Transition behaviour of a 1:1 mixture of compounds 1 and 2 on cool-
ꢀ
ꢀ
ꢀ
ing was Iso 58.3 C N 51.8 C SmA À4.0 C Cryst, indicating that there
is no marked difference in microscopic structure of the SmA phase
between compounds 1 and 2. The binary phase diagrams for mixtures
between each dimeric compound and 8-PYP-6O or BOPPHB suggest
that there is difference in microscopic environment between com-
pounds 1 and 2 in the smectic phases of the binary mixtures.

Topological Effect in Dimeric Liquid Crystals
103=[1969]
F IG URE 3 (a) Binary phase diagram for mixtures of B O P PH B and com-
pound 1. (b) Binary phase diagram for mixtures of B O P PH B and compound
2
.
3.3. Topological Effect
Let us discuss topological effect of the connecting group on transition
behaviour for the novel dimeric system. There are two conformers, i.e.,
diequatrial and diaxial conformers, for trans-1,2-disubstituents on
chair cyclohexane. MM2 models for the two conformers of compound

1
04=[1 9 7 0 ]
A. Yoshizawa and T. Kawaguchi
F IG URE 4 A M M2 m ode l f o r t h e die q ua t ria l co n f o rm at io n o f co m po u nd 1 ( a )
a nd t h at f or t h e dia x ia l co n f or ma t ion o f 1c o m( b p) .o u nd
1
ar e s ho w n in F ig ur e 4 . E ner g y v al ue s w er e est i me t ed , fro m t he MM 2
À1
À1
mo d e l s , t o be ab o ut 5 7 k J mo l a nd 5 8 k J m ol fo r t he d i eq u a t ori a l con-
f o rm e r (F ig . 4( a ) ) a nd t he d i ax i al on e ( Fi g. 4 (b )) , res pe ct i v el y . T he d i f-
f e re nc e in e ne r g y v al ue s b et w ee n t he t w o conf or mer s i s no t so l a rg e,
t hus t he c o n fo r m a t io na l cha ng e ca n oc cur b et w een t hem i n ord e r t o
st a bi liz e t he SmC p ha se of i t s b i na ry mi x t ures w i t h 8 -P YP -6 O or
B OP P H B . O n t he ot h er ha nd , t he co nne ct i ng g r oup of com p ou nd 2

Topological Effect in Dimeric Liquid Crystals
10 5=[ 19 71 ]
F IG URE 5A M M 2 m ode l f o r co m p2 o. u n d
is t ho ug ht t o be ri g i d . A M M 2 mo d el f or com p ound 2 i s show n i n
Fi gu r e 5. T he r ig id co nf or ma t i on ma y d es t a b i li z e t he SmC p ha se of
it s bi nar y m ix t ure s w i th 8 - PY P - 6 O or B OPP H B .
4. CONCLUSION
We ha ve p re p ar ed no ve l sy mm et ri c d i mer i c com p oun d s i n w hi c h t w o
p h e ny lp yr im i d in e mo ie t ie s a re co nne ct ed v i a (Æ)-trans-1 , 2 -cy cl ohe xa -
ne d io l o r p-t o lu e ne sul f on a m i d e. C omp o u nd 1 p osse ssi n g cy cl ohe x a ne
a s t he c o nne c t in g g ro up w a s f ound t o sho w en a nt i ot r op i c N a nd SmA
p h as e s w it h lo w me lt i ng t em p er a t ure . Th e b i n a ry mi x t ure st ud i es
su g ge s t t h at c o m p ou nd 1 ca n st a b il i ze a SmC p ha se of t he mi x t ure.
Fl e xi ble c o nfo r m at i on of co mp o un d 1 d ue t o t he cy cl oh ex a ne ri n g i s
t h o ug ht t o c ha ng e i t s mo le c ul ar sha p e i n or d er t o st a b i li z e t ot a l fre e
e n e r gy o f t he m ol ec ul ar a sse mb l y i n t he sy s t em.
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