Synthesis of symmetric liquid crystal dimers based on azo and imine groups and investigation of phase behaviour by varying alkoxy terminal chain length

Article abstract of DOI:10.1080/15421400802331042

With the objective to study the effect of alkoxy terminal chain length on mesomorphic properties of liquid crystals, we have synthesized two (Azo and Imine) different series of dimesogens by varying terminal alkoxy chain length (n = 6-12) with a short spacer unit in between two mesogens. Transition temperatures and phase characterization were studied by DSC, POM and XRD analysis. It was found that all the dimers show mesomorphic properties and the change in terminal alkoxy chain length has pronounced effect on the smectic phase window. In the DSC cooling scan, the smectic phase window of azo compounds increased from 6.4°C (2A6) to 16.0°C(2A12), whereas in Schiff base compounds, it increased from 9.2°C (2S6) to 31.0°C(2S12). Further, the 2A11 dimer was found to undergo photo induced configurational changes.

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Synthesis of Symmetric
Liquid Crystal Dimers Based
on Azo and Imine Groups
and Investigation of Phase
Behaviour by Varying Alkoxy
Terminal Chain Length
Tae Hyeong Kim a , Chang Sin Lee a , B. Ramaraj a ,
Hye Jin Jeon b , Hyun Hoon Song b , Soo Min Lee a &
Kuk Ro Yoon a
a Organic and Polymer Synthesis Laboratory,
Chemistry Department, Hannam University,
Daejeon, Korea
b Polymer Structural Physics Laboratory, Advanced
Materials Department, Hannam University, Daejeon,
Korea
Version of record first published: 31 Aug 2012.
To cite this article: Tae Hyeong Kim , Chang Sin Lee , B. Ramaraj , Hye Jin Jeon ,
Hyun Hoon Song , Soo Min Lee & Kuk Ro Yoon (2008): Synthesis of Symmetric Liquid
Crystal Dimers Based on Azo and Imine Groups and Investigation of Phase Behaviour by
Varying Alkoxy Terminal Chain Length, Molecular Crystals and Liquid Crystals, 492:1,
102/[466]-116/[480]
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Mol. Cryst. Liq. Cryst., Vol. 492, pp. 102=[466]–116=[480], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802331042
Synthesis of Symmetric Liquid Crystal Dimers Based on
Azo and Imine Groups and Investigation of Phase
Behaviour by Varying Alkoxy Terminal Chain Length
Tae Hyeong Kim1, Chang Sin Lee1, B. Ramaraj1, Hye Jin
Jeon2, Hyun Hoon Song2, Soo Min Lee1, and Kuk Ro Yoon1
1Organic and Polymer Synthesis Laboratory, Chemistry Department,
Hannam University, Daejeon, Korea
2Polymer Structural Physics Laboratory, Advanced Materials
Department, Hannam University, Daejeon, Korea
With the objective to study the effect of alkoxy terminal chain length on meso-
morphic properties of liquid crystals, we have synthesized two (Azo and Imine)
different series of dimesogens by varying terminal alkoxy chain length (n ¼ 6–12)
with a short spacer unit in between two mesogens. Transition temperatures
and phase characterization were studied by DSC, POM and XRD analysis. It
was found that all the dimers show mesomorphic properties and the change
in terminal alkoxy chain length has pronounced effect on the smectic phase
window. In the DSC cooling scan, the smectic phase window of azo compounds
increased from 6.4ꢀC (2A6) to 16.0ꢀC(2A12), whereas in Schiff base compounds, it
increased from 9.2ꢀC (2S6) to 31.0ꢀC(2S12). Further, the 2A11 dimer was found to
undergo photo induced configurational changes.
Keywords: azobenzene; differential scanning calorimetry; liquid crystal; optical
microscopy; Schiff’s base; X-ray diffraction
1. INTRODUCTION
Liquid crystalline dimesogens (dimers) are attracting much attention
because they exhibit variety of phases and serve as useful models for
the semi-flexible, main-chain liquid crystal polymer [1–3]. In typical
dimers where two individual mesogenic entities are attached to each
This research work was supported by Hannam University Research fund.
Address correspondence to Kuk Ro Yoon, Department of Chemistry, Hannam
University, 133 Ojeong-Dong, Daedeuk-gu, Daejeon 306-791, Korea. E-mail: kryoon@
hannam.ac.kr and Soo Min Lee, Organic and Polymer Synthesis Laboratory, Chemis-
try Department, Hannam University, Daejeon 305-811, Korea. E-mail: smlee@
hnu.ac.kr
102=[466]

Synthesis of Symmetric Liquid Crystal Dimers
103=[467]
other via flexible polymethylene spacer units, the structure of the
mesophases can be modified through the change in spacer length
and terminal chain length. The mesomorphic behaviour of an organic
compound is basically dependent on its molecular architecture in
which a slight change in the molecular geometry brings about con-
siderable change in its mesomorphic properties. Most of these studies
have been focused on ester-Schiff’s base and azo-dimers. Ester-Schiff’s
base dimers are known to exhibit room temperature nematic phase.
Azo compounds are very attractive for their high thermal stability
and for the photo-induced effects as well as for the potential applica-
tions in liquid crystal displays and devices, reversible optical storage
systems, nonlinear optical waveguides, photo-refractive switches,
and holographic gratings. Reversible optically induced switching effect
has been the basis for these applications [4–8]. Precise placement of
photochromic moieties within the liquid crystal dimesogen also could
lead to reversible photo-induced configurational and constitutional
changes [9]. Where as the schiff’s bases is of particular interest since
the discovery of 4-methoxy benzylidene-40-butylaniline (MBBA), which
exhibits a room temperature nematic phase [10].
We have recently reported the synthesis and mesomorphic properties
of dimeric ester-Schiff bases (2ES-n) and azobenzenes (2An) series in
order to explore the thermal stability of liquid crystals and the relation-
ship with its molecular structures. These two series of the bent-shaped
dimers have a polar hydroxyl group on the center of the short odd-
numbered spacer, different terminal chains and mesogenic structure
[11–13]. With the objective to explore, further, the effect of alkoxy ter-
minal chain length on mesomorphic properties of liquid crystals, we have
synthesized two; azo (2An) and Schiff’s base (2Sn); different series of
dimesogens by varying terminal alkoxy chain length (n¼ 6–12) with a
short (-OCH2CH2 CH2O-) spacer unit in between two mesogens. The
chemical structures of these two series of mesogens were examined by
Fourier transform infrared (FT-IR) spectroscopy and proton nuclear
magnetic resonance (1H NMR) spectroscopy. The mesomorphic proper-
ties and optical textures of the resultant dimers were investigated by dif-
ferential scanning calorimetry (DSC), polarized optical microscopy
(POM) and X-ray diffraction (XRD).
2. EXPERIMENTAL
2.1. Materials
4-Hydroxynitrobenzene, 1,3-bromopropane, hydrazine monohydrate, pal-
ladium on activated carbon (Pd=C, 10%), 1-bromoalkane (1-bromohexane,

104=[468]
T. H. Kim et al.
1-bromoheptane, 1-bromooctane, 1-bromononane, 1-bromodecane,
1-bromoundecane, 1-bromododecane), p-toluenesulfonic acid (PTSA),
anhydrous ethanol were purchased from Aldrich Chemical Co. and
used as received. All other solvents and reagents were purchased
commercially and used.
2.2. Synthesis
2.2.1. Synthesis of Azo (2An) Dimesogens
The 2An dimesogens were synthesized according to the method
described in the literature [11,12,14] as illustrated in Scheme 1 and
1
final products were structurally characterized by IR, H NMR.
2.2.1.1. 1,3-Bis(4-(4-alkyloxyphenylazo)phenoxy)propane[2An]. The
synthetic route used in the preparation of azo dimesogens involved
four steps: (1) first the conventional etherification of 4-hydroxynitro-
benzene with 1,3-dibromopropane in presence of potassium iodide
and potassium carbonate to obtain 1,3-bis(4-nitrophenoxy)propane
[BNPP], (2) then the reduction of nitro group with hydrazine
SCHEME 1 Synthetic pathway for azo dimesogens (2An).

Synthesis of Symmetric Liquid Crystal Dimers
105=[469]
monohydrate in the presence of palladium on activated carbon to
obtain 1,3-bis(4-amiophenoxy)propane [BAPP], (3) the amine was then
converted into its diazonium chloride and diazonium chloride on
reaction with phenol forms 1,3-bis(4-(4-hydroxyphenylazo)phenoxy)
propane [BHAP], (4) finally the BHAP reacted with different bro-
moalkanes (n ¼ 6–12) to obtain the final products as per the procedure:
BHAP (1.28 mmol) and K2CO3 (4.3 mmol) were dissolved in anhydrous
ethanol (50 mL). 1-Bromoalkane (8.5 mmol) was added very slowly into
the reaction mixture for about 30 min, and then the mixture was stir-
red for 20 h at the room temperature. The resulting precipitate was
isolated by filtration and purified by the recrystallization from chloro-
form to obtain the final products (yellow solid, 87–95%). The IR and
NMR data of final products are listed below.
1
2A6: Yield, 90%. FT-IR (KBr pellet, cmꢁ1): 1580 (-N N-). H-NMR
=
(CDCl3, ppm): d 0.90 (t, 6H, -CH3), 1.31–1.51 (m, 12H, -(CH2)3-), 1.82
(m, 4H, -CH2CH2O-), 2.35 (m, 2H, -OCH2CH2CH2O-), 4.03, (t, 4H,
-CH2CH2O-), 4.27 (t, 4H, -OCH2CH2CH2O-), 6.62–6.65 (dd, 8H,
Ar-H), 7.85–7.89 (d, 8H, Ar-H).
1
2A7: Yield, 87%. FT-IR (KBr pellet, cmꢁ1): 1579 (-N N-). H-NMR
=
(CDCl3, ppm): d 0.90 (t, 6H, -CH3), 1.31–1.51 (m, 16H, -(CH2)4-), 1.82
(m, 4H, -CH2CH2O-), 2.35 (m, 2H, -OCH2CH2CH2O-), 4.03, (t, 4H,
-CH2CH2O-), 4.27 (t, 4H, -OCH2CH2CH2O-), 6.62–6.65 (dd, 8H,
Ar-H), 7.85–7.89 (d, 8H, Ar-H).
1
2A8: Yield, 91%. FT-IR (KBr pellet, cmꢁ1): 1580 (-N N-). H-NMR
=
(CDCl3, ppm): d 0.89 (t, 6H, -CH3), 1.31–1.51 (m, 20H, -(CH2)5-), 1.82
(m, 4H, -CH2CH2O-), 2.35 (m, 2H, -OCH2CH2CH2O-), 4.03, (t, 4H,
-CH2CH2O-), 4.27 (t, 4H, -OCH2CH2CH2O-), 6.62–6.65 (dd, 8H,
Ar-H), 7.85–7.89 (d, 8H, Ar-H).
1
2A9: Yield, 91%. FT-IR (KBr pellet, cmꢁ1): 1580 (-N N-). H-NMR
=
(CDCl3, ppm): d 0.88 (t, 6H, -CH3), 1.25–1.50 (m, 24H, -(CH2)6-), 1.82
(m, 4H, -CH2CH2O-), 2.35 (m, 2H, -OCH2CH2CH2O-), 4.03, (t, 4H,
-CH2CH2O-), 4.27 (t, 4H, -OCH2CH2CH2O-), 6.97–7.04 (dd, 8H,
Ar-H), 7.85–7.89 (d, 8H, Ar-H).
1
2A10: Yield, 93%. FT-IR (KBr pellet, cmꢁ1): 1580 (-N N-). H-NMR
=
(CDCl3, ppm): d 0.88 (t, 6H, -CH3), 1.25–1.50 (m, 28H, -(CH2)7-), 1.82
(m, 4H, -CH2CH2O-), 2.35 (m, 2H, -OCH2CH2CH2O-), 4.03, (t, 4H,
-CH2CH2O-), 4.27 (t, 4H, -OCH2CH2CH2O-), 6.97–7.04 (dd, 8H,
Ar-H), 7.85–7.89 (d, 8H, Ar-H).
1
2A11: Yield, 95%. FT-IR (KBr pellet, cmꢁ1): 1579 (-N N-). H-NMR
=
(CDCl3, ppm): d 0.88 (t, 6H, -CH3), 1.25–1.50 (m, 32H, -(CH2)8-), 1.82
(m, 4H, -CH2CH2O-), 2.35 (m, 2H, -OCH2CH2CH2O-), 4.03, (t, 4H,
-CH2CH2O-), 4.27 (t, 4H, -OCH2CH2CH2O-), 6.97–7.04 (dd, 8H,
Ar-H), 7.85–7.89 (d, 8H, Ar-H).

106=[470]
T. H. Kim et al.
1
2A12: Yield, 95%. FT-IR (KBr pellet, cmꢁ1): 1580 (-N N-). H-NMR
(CDCl3, ppm): d 0.88 (t, 6H, -CH3), 1.25–1.50 (m, 36H, -(CH2)9-), 1.82
(m, 4H, -CH2CH2O-), 2.35 (m, 2H, -OCH2CH2CH2O-), 4.03, (t, 4H,
-CH2CH2O-), 4.27 (t, 4H, -OCH2CH2CH2O-), 6.97–7.04 (dd, 8H,
Ar-H), 7.85–7.89 (d, 8H, Ar-H).
=
2.2.2. Synthesis of Schiff Base (2Sn) Dimesogens
The 2Sn dimesogens were synthesized according to the method
described in the literature [11,12,14] as illustrated in Scheme 2 and
1
final products were structurally characterized by IR and H NMR.
2.2.2.1. 1,3-Bis[4-((4-alkyloxyphenylimino)methyl)phenoxy]-propane
[2Sn]. The synthetic route used in the preparation of Schiff
base dimesogen series also involved four steps: (1) conventional etheri-
fication of 4-nitrophenol with an alkyl halide (n ¼ 6–12) to produce
the 4-alkoxynitrobenzenes (ANBn), (2) then reduction of nitro group
with hydrazine monohydrate to produce 4-alkoxyanilines (AAn),
(3) reaction of 4-hydroxybenzaldehyde (2 eq.) with 1,3-dibromopropane
(1 eq.) to produce 1,3-bis(4-formylphenoxy)-propane (FPP-4), and
(4) finally condensation reactions of FPP-4 with AAn yield the diimine
compounds as per the details; 1,3-bis(4-formylphenoxy) propane
SCHEME 2 Synthetic pathway for Schiff base dimesogens (2Sn).

Synthesis of Symmetric Liquid Crystal Dimers
107=[471]
(FPP-4) (1.6 mmol) was dissolved in anhydrous ethanol (50 mL).
4-Alkyloxyaminobenzene (3.2 mmol) and catalytic quantity of
a
PTSA were mixed and stirred for 20 h at the room temperature. The
resulting precipitate was isolated by filtration and purified by
the recrystallization from ethanol. Final products were obtained as
white solid (60–83%). The IR and NMR data of final products are listed
below.
2S6: Yield, 80%. FT-IR (KBr pellet, cmꢁ1): 1508, 1604 (-CH N-).
=
1H-NMR (CDCl3, ppm): d 0.91 (t, 6H, -CH3), 1.29–1.35 (m, 12H,
-(CH2)3), 1.71 (m, 4H, -CH2CH2O-), 2.33 (m, 2H, -OCH2CH2CH2O-),
3.97 (t, 4H, -CH2CH2O-), 4.25 (t, 4H, -OCH2CH2CH2O-), 6.93 (d, 4H,
Ar-H), 7.01 (d, 4H, Ar-H), 7.18 (d, 4H, Ar-H), 7.81 (d, 4H, Ar-H),
=
8.40 (s, 2H, -N CH-).
2S7: Yield, 81%. FT-IR (KBr pellet, cmꢁ1): 1508, 1604 (-CH N-).
1H-NMR (CDCl3, ppm): d 0.90 (t, 6H, -CH3), 1.25–1.32 (m, 16H,
-(CH2)4), 1.81 (m, 4H, -CH2CH2O-), 2.33 (m, 2H, -OCH2CH2CH2O-),
3.97 (t, 4H, -CH2CH2O-), 4.25 (t, 4H, -OCH2CH2CH2O-), 6.93 (d, 4H,
Ar-H), 7.01 (d, 4H, Ar-H), 7.18 (d, 4H, Ar-H), 7.81 (d, 4H, Ar-H),
=
=
8.41 (s, 2H, -N CH-).
2S8: Yield, 83%. FT-IR (KBr pellet, cmꢁ1): 1508, 1604 (-CH N-).
1H-NMR (CDCl3, ppm): d 0.89 (t, 6H, -CH3), 1.27–1.46 (m, 20H,
-(CH2)5), 1.79 (m, 4H, -CH2CH2O-), 2.33 (m, 2H, -OCH2CH2CH2O-),
3.97 (t, 4H, -CH2CH2O-), 4.25 (t, 4H, -OCH2CH2CH2O-), 6.93 (d, 4H,
Ar-H), 7.01 (d, 4H, Ar-H), 7.18 (d, 4H, Ar-H), 7.81 (d, 4H, Ar-H),
=
=
8.40 (s, 2H, -N CH-).
2S9: Yield, 82%. FT-IR (KBr pellet, cmꢁ1): 1508, 1604 (-CH N-).
1H-NMR (CDCl3, ppm): d 0.89 (t, 6H, -CH3), 1.28–1.46 (m, 24H,
-(CH2)6), 1.79 (m, 4H, -CH2CH2O-), 2.33 (m, 2H, -OCH2CH2CH2O-),
3.97 (t, 4H, -CH2CH2O-), 4.25 (t, 4H, -OCH2CH2CH2O-), 6.93 (d, 4H,
Ar-H), 7.01 (d, 4H, Ar-H), 7.18 (d, 4H, Ar-H), 7.84 (d, 4H, Ar-H),
=
=
8.41 (s, 2H, -N CH-).
2S10: Yield, 72%. FT-IR (KBr pellet, cmꢁ1): 1510, 1604 (-CH N-).
1H-NMR (CDCl3, ppm): d 0.89 (t, 6H, -CH3), 1.28–1.48 (m, 28H,
-(CH2)7), 1.79 (m, 4H, -CH2CH2O-), 2.33 (m, 2H, -OCH2CH2CH2O-),
3.97 (t, 4H, -CH2CH2O-), 4.25 (t, 4H, -OCH2CH2CH2O-), 6.93 (d, 4H,
Ar-H), 7.01 (d, 4H, Ar-H), 7.18 (d, 4H, Ar-H), 7.81 (d, 4H, Ar-H),
=
=
8.40 (s, 2H, -N CH-).
2S11: Yield, 60%. FT-IR (KBr pellet, cmꢁ1): 1510, 1604 (-CH N-).
1H-NMR (CDCl3, ppm): d 0.88 (t, 6H, -CH3), 1.27–1.48 (m, 32H,
-(CH2)8), 1.79 (m, 4H, -CH2CH2O-), 2.33 (m, 2H, -OCH2CH2CH2O-),
3.97 (t, 4H, -CH2CH2O-), 4.25 (t, 4H, -OCH2CH2CH2O-), 6.93 (d, 4H,
Ar-H), 7.01 (d, 4H, Ar-H), 7.18 (d, 4H, Ar-H), 7.81 (d, 4H, Ar-H),
=
=
8.41 (s, 2H, -N CH-).

108=[472]
T. H. Kim et al.
2S12: Yield, 77%. FT-IR (KBr pellet, cmꢁ1): 1508, 1604 (-CH N-).
1H-NMR (CDCl3, ppm): d 0.88 (t, 6H, -CH3), 1.27–1.46 (m, 36H,
-(CH2)9), 1.79 (m, 4H, -CH2CH2O-), 2.33 (m, 2H, -OCH2CH2CH2O-),
3.97 (t, 4H, -CH2CH2O-), 4.25 (t, 4H, -OCH2CH2CH2O-), 6.93 (d, 4H,
Ar-H), 7.01 (d, 4H, Ar-H), 7.18 (d, 4H, Ar-H), 7.81 (d, 4H, Ar-H),
=
=
8.40 (s, 2H, -N CH-).
2.3. Characterization Methods
FT-IR spectra were recorded with a Perkin-Elmer 1000 FT-IR spectro-
photometer on KBr pellets. 1H NMR spectra were recorded by using a
Varian Unity 300 (300 MHz) NMR spectrometer. DSC measurements
were performed using a TA instrument 910S DSC apparatus under
dry nitrogen flow at the heating rate of 10ꢀC=min. The transition tem-
peratures were taken at the maxima of peaks for each sample. Optical
micrographs were obtained by using a Nikon Labophot-2 polarizing
microscope fitted with a RTC-1 temperature controller (Instec Inc.,
Broomfield, Co.) and a Mettler FP-82HT hot stage. The UV irradiation
was carried out by 10 mW=cm2 UV lamp with a 365 nm peak
wavelength. XRD measurements were performed at the 3C2 and
4C1 beamline in pohang Accelerator Laboratory.
3. RESULTS AND DISCUSSION
The synthetic routes used for the preparation of dimesogens (azo
and imine group) are outlined in Scheme 1 and 2. The mesomorphic
properties for these two (2An & 2Sn) series of dimesogens were determ-
ined by means of DSC, POM and XRD. DSC is a valuable aid by which
phase transition temperature can be conveniently measured. This
techniques offers a direct and complimentary (to microscopy) evalu-
ation of thermal behaviour. Figure 1 presents the DSC heating and
cooling traces of dimesogens (2An, n ¼ 6–12). In the heating and cool-
ing scans (Fig. 1), all the compounds in this series shows a crystal to
smectic phase transition and a smectic to isotropic phase transition.
The smectic phases are identified as smectic A on the basis of baton-
nets and broken fan textures observed when viewed through micro-
scope. It can be seen from the Table 1, the crystal to smetic phase
transition at 167.4ꢀC (2A6) decreased to 152.6ꢀC (2A12) with increase
in chain length as expected; this clearly shows the dependence of ther-
mal transition temperature on terminal chain length n of dimesogens
for all 2An series. The heating scans of n ¼ 6–9 show only a single melt-
ing transition temperature, but those of n ¼ 10–12 showed smectic
phases. In case of cooling scans, smectic phases are visible in all

Synthesis of Symmetric Liquid Crystal Dimers
109=[473]
FIGURE 1 Differential scanning calorimetry thermograms for azo dimeso-
gens (2An; n ¼ 6–12) in the heating and cooling scans (10ꢀC=min).
dimers (n ¼ 6–12), where crystallization temperature decreased from
152.8ꢀC (n ¼ 6) to 138.0ꢀC (n ¼ 12). The smectic phase window is
increased from 6.4ꢀC (n ¼ 6) to 16.0ꢀC (n ¼ 12). Since the molecules are
bent-shaped dimesogens with two terminal chains, increasing the
terminal chain length is anticipated to increase significantly the
TABLE 1 Thermal Transition Temperatures of Azo-Dimesogen
(2An) Series Compounds
Heating cycle
thermal transitions (ꢀC)
Cooling cycle
thermal transitions (ꢀC)
Number of carbons
in 2An series
Crystallization Smectic Crystallization Smectic
6
7
8
167.4
165.3
162.2
161.0
157.4
156.2
152.6
–
–
–
152.8
152.0
151.5
149.5
141.0
139.3
138.0
159.2
156.4
157.3
157.8
158.4
153.9
154.0
9
–
10
11
12
161.1
161.0
160.9

110=[474]
T. H. Kim et al.
length-to-breadth ratio in resulting liquid crystal phases, and parti-
cularly smectic phases, being stabilized at higher temperatures for
higher homologues. We also note that the temperature range of the
smectic mesophase increased with the terminal chain length. The com-
pound of 2A12 shows the largest temperature interval extending to 16ꢀC.
These results demonstrate that the tendency toward smectic meso-
morphism and the thermal stability of tilted smectic phase increased
with increasing terminal alkoxy chain length. This may be the result
of the terminal alkoxy chains lying at an angle to the long molecular axis
inducing the molecular tilt between neighboring molecules. The effects
of the terminal chain length on the transition temperatures and phases
behavior observed in this series are in accordance with those observed
for conventional low molar mass mesogens.
Optical micrographs of 2A11 and 2A12 obtained during the cooling
cycle are shown in Figure 2. The micrograph of 2A11 exhibit baton-
net texture at 142ꢀC and crystal texture at 115ꢀC, similarly the
FIGURE 2 Optical photomicrographs of 2An (a) batonnets texture (Sm,
cooling, 142ꢀC), (b) crystal texture (Cr, cooling, 115ꢀC) of 2A11 and (c) broken
fan shape texture (Sm, cooling, 151ꢀC), (d) crystal texture (Cr, cooling,
125ꢀC) of 2A12 (10ꢀC=min, ꢂ 100).

Synthesis of Symmetric Liquid Crystal Dimers
111=[475]
micrograph of 2A12 exhibit broken fan texture at 151ꢀC and crystal
texture at 125ꢀC. Further analysis of the mesophases was performed
with X-ray diffraction.
As a representative case of azo-dimesogens, an X-ray diffraction
study was carried out on 2A12 to prove the presence of smectic phases
beyond doubt. Figure 3 shows the X-ray diffractograms obtained at
various temperatures for 2A12. At 118ꢀC, it shows one sharp reflection
in the small angle region and an array of small peaks in the wide angle
regions corresponding to the formation of crystalline and various
smectic phases. At higher temperatures, reflections corresponding to
the smectic phases diffused, finally at 172ꢀC, the crystalline phase also
changes to isotropic liquid state. A clear change in the diffraction pat-
tern of the sample could be observed. The sharp reflection in the small
angle region confirms the formation of a highly ordered smectic A
phase. In addition, small peaks, most probably a second order diffrac-
tion is seen in 118, 124, and 158ꢀC diffractograms of the compound in
the liquid crystalline phase.
FIGURE 3 X-ray diffraction curves of azo-dimesogen (2A12).

112=[476]
T. H. Kim et al.
A reversible photochromic behavior of solution was evidenced
when the sample 2A11 was exposed to UV irradiation. Dark incubation
of a dichloromethane solution (50 mM=mL) of 2A11 has a maximum
absorbance at 360 nm corresponding to the trans-azobenzene chromo-
phore. Irradiation of this solution with 365 nm light resulted in photo-
isomerization to cis-2A11, as evidenced by a decrease in the absorbance
at 360 nm and an increase in absorbance at 451 nm and at 314 nm
(Fig. 4). The stationary population of trans and cis isomers is a func-
tion of irradiation intensity. A photo stationary state from trans
2A11 to cis isomer was reached within approximately 50s under the
irradiation at 365 nm.
New bent-shaped dimesogens, 1,3-bis(4-((4-alkyloxyphenylimino)
methyl)phenoxy) propane (2Sn; n ¼ 6–12), were synthesized starting
from
dialdehyde
derivative,
1,3-bis(4-formylphenoxy)-propane
(FPP-4). The dimesogens, 2Sn, were then obtained by the condensation
reaction between amines and dialdehyde (see Scheme 2). Figure 5
presents the DSC heating and cooling traces of dimesogens (2Sn,
n ¼ 6–12). In the heating and cooling scans, all the compounds in this
FIGURE 4 UV-VIS absorption spectra of 2A11 under irradiation (365 nm, at
0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 s).

Synthesis of Symmetric Liquid Crystal Dimers
113=[477]
FIGURE 5 Differential scanning calorimetry thermograms for Schiff base
dimesogens (2Sn; n ¼ 6–12) in the heating and cooling scans (10ꢀC=min).
series also shows a crystal to smectic phase transition and a smectic
to isotropic phase transition. The thermal transition temperatures
for dimesogens are summarized in Table 2. It can be seen from the
Table 2, the crystal to smectic phase transition at 158.4ꢀC (2S6)
TABLE 2 Thermal Transition Temperatures of Schiff Base Dimesogen (2Sn)
Series Compounds
Heating cycle
thermal transitions (ꢀC)
Cooling cycle thermal
transitions (ꢀC)
Number of carbons
in 2An series
Crystallization
Smectic
Crystallization
Smectic
6
7
8
158.4
153.4
149.6
145.4
143.0
140.3
138.5
167.6
168.5
172.0
171.6
171.1
170.3
169.5
144.1
142.3
137.3
134.3
129.6
128.8
127.8
164.5
166.0
168.7
168.8
169.0
166.7
167.0
9
10
11
12

114=[478]
T. H. Kim et al.
decreased to 138.5ꢀC (2S12) with increase in chain length. In contrast
to azo-mesogens, here the smectic phase transitions are visible for all
the Schiff base (2Sn) compounds in the heating cycle also. The smectic
phase window is increased from 9.2ꢀC (n ¼ 6) to 31.0ꢀC (n ¼ 12). Simi-
larly in the cooling scan also the crystal to smectic phase transition at
144.1ꢀC (2S6) decreased to 127.8ꢀC (2S12) and the smectic phase win-
dow is also increased from 13.4ꢀC (n ¼ 6) to 39.2ꢀC (n ¼ 12) with
increase in chain length.
Optical micrographs of 2S11 and 2S12 obtained during the cooling
cycle are shown in Figure 6. The micrograph of 2S11 exhibited fan
texture with spherulitic domains at 131.4ꢀC and crystal texture at
119.4ꢀC similarly the micrograph of 2S12 exhibited fan texture with
spherulitic domains at 142ꢀC and crystal texture at 116.8ꢀC. Further
analysis of the mesophases was performed with X-ray diffraction.
As a representative case of Schiff’s base dimesogens, an X-ray dif-
fraction study was carried out on 2S12 to prove the presence of smectic
phases beyond doubt. Figure 7 shows the X-ray diffractograms
FIGURE 6 Optical photomicrographs of 2Sn (a) fan texture with spherulitic
domains (Sm, cooling, 131.4ꢀC), (b) crystal texture (Cr, cooling, 119.4ꢀC) of
2S11 and (c) fan texture with spherulitic domains (Sm, cooling, 142ꢀC), (d) crys-
tal texture (Cr, cooling, 116.8ꢀC) of 2S12 (10ꢀC=min, ꢂ100).

Synthesis of Symmetric Liquid Crystal Dimers
115=[479]
FIGURE 7 X-ray diffraction curves of Schiff base dimesogen (2S12).
obtained at various temperatures for 2S12. It shows one sharp
reflection at 120ꢀC and 138ꢀC in the small angle region and an array
of small peaks in the wide angle regions corresponding to the forma-
tion of crystalline and various smectic phases. With increase in tem-
perature, at 146, 150, and 170ꢀC reflections corresponding to the
smectic phases diffused, finally at 178ꢀC, the crystalline phase also
diffused and changed to isotropic liquid state. The sharp reflection
in the small angle region confirms the formation of a highly ordered
smectic A phase. In addition, small peaks, most probably a second
order diffraction is seen in only at 120 and 138ꢀC diffractograms of
the compound in the liquid crystalline phase.
4. CONCLUSIONS
New series of symmetric azo and imine liquid crystal dimesogens have
been prepared by varying terminal alkoxy chain length (n ¼ 6–12) with
a short spacer unit in between two mesogens. The chemical structures
of these two series of dimesogens were examined by FT-IR and
1H NMR spectroscopy. The mesomorphic properties and optical

116=[480]
T. H. Kim et al.
textures of the resultant dimers were characterized by DSC, POM and
XRD. The change in terminal alkoxy chain length has pronounced
effect on the temperature range of smectic phase. In the DSC cooling
scan, the smectic phase window of azo compounds increased from
6.4ꢀC (2A6) to 16.0ꢀC (2A12), whereas in Schiff base compounds, it
increased from 9.2ꢀC (2S6) to 31.0ꢀC (2S12). Further, the 2A11 dimer
was found to undergo photo induced configurational changes.
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