Synthesis and mesomorphic properties of symmetrical dimers N,N′-bis(3-methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine

Article abstract of DOI:10.1080/15421400500377743

A series of homologous symmetrical dimers N,N′-bis[(3-methoxy-4- alkoxybenzylidene)-1,4-pkenylenediamine with different lengths of terminal alkyl groups of even parity ranging from butyl to octadecyl were prepared and characterized. Spectroscopic methods

Full text of DOI:10.1080/15421400500377743

Mol. Cryst. Liq. Cryst., Vol. 452, pp. 49–61, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500377743
Synthesis and Mesomorphic Properties of Symmetrical
Dimers N,N -Bis(3-Methoxy-4-Alkoxybenzylidene)-
0
,4-Phenylenediamine
1
Guan-Yeow Yeap
Tiang-Chuan Hng
Wan Ahmad Kamil Mahmood
Rohana Adnan
Liquid Crystal Research Laboratory, School of Chemical Sciences,
Universiti Sains Malaysia, Penang, Malaysia
Masato M. Ito
Yamashita Youhei
Department of Environmental Engineering for Symbiosis, Faculty
of Engineering, Soka University, Tokyo, Japan
0
A series of homologous symmetrical dimers N,N -bis[(3-methoxy-4-alkoxybenzy-
lidene)-1,4-phenylenediamine with different lengths of terminal alkyl groups of
even parity ranging from butyl to octadecyl were prepared and characterized.
1
13
Spectroscopic methods FT-IR, H NMR, and C NMR and mass spectrometry
were carried out to determine the physical properties of the target compounds.
The texture with respect to each compound within this series has been investigated
along with their thermal stabilities. A diversity of phase-transition behavior was
observed for the members of this series and explanation was given based on the
possible molecular conformation. Almost all title compounds were nematogenic
except the dimers containing butyl and hexyl chains as well as the longest octa-
decyl dimer in which the mesogenic properties were absent.
0
Keywords: N,N -bis(3-methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine; nemato-
genic; phase transition behaviour; symmetrical dimers; thermal stabilities
1. INTRODUCTION
The majority of the liquid crystalline compounds that have hitherto
been synthesized consist of monomeric structure whereby the
Address correspondence to Guan-Yeow Yeap, currently at Department of Environ-
mental Engineering for Symbiosis, Faculty of Engineering, Soka University, Japan.
E-mail: gyyeap@soka.ac.jp or gy_yeap@yahoo.com
49

5
0
G.-Y. Yeap et al.
aromatic rings that formed the core center were connected to two
flexible terminal chains such as aliphatic and alkoxy groups. Sub-
sequent to monomeric liquid crystals, symmetrical dimers have also
been isolated and characterized. The first dimer possessing liquid
crystal properties was reported by Vorlander in 1927 [1]. Ever since
the discovery of the mesomorphic properties of the dimeric liquid crys-
tal, the importance features of these compounds has continually been
investigated including those reported by Rault et al. in 1975 [2]. Most
of the earlier reported dimers consist of a flexible central spacer con-
nected to mesogenic segments at terminal positions [3]. In the course
of the study, the scientists have also found that dimeric liquid crystals
are known for exhibiting strong odd–even effects in their physical
properties as the function of spacer parity [4,5].
A large number of compounds that exhibit liquid crystalline proper-
ties possess rod-like molecular shape. As a result, the molecules with
para-positioned substituents especially those with elongated tails in
the para-positions of the aromatic ring will generally show liquid crys-
talline behavior. In contrast to this phenomenon, the thermal
depression and the instability of mesophase will be observed if a sub-
stituent such as OCH lies at the ortho and meta positions of the cen-
3
tral aromatic ring system [6]. However, this interesting point does not
apply to compounds with ortho hydroxyl groups with the azomethine
(
CH=N) group within its proximity in which the formation of intramol-
ecular hydrogen bonding generally increases the stability [7].
Over past few years, a number of liquid crystalline compounds with
various lateral substituents have been synthesized and extensively
studied. The typical examples of compounds possess lateral hydrogen
bonded phenolic group and lateral cyano group as reported by Vora
and Gupta [8] and by Sakagami et al. [9], respectively. Similar to
the case of molecular structure of conventional liquid crystalline com-
pounds, molecular breadth is considered an important feature in
relation to its influence on mesophase stability and clearing tem-
peratures [10–12]. Thus, the presence of lateral substituents at the
aromatic rings such as OH and OCH could affect the overall meso-
3
morphic characteristics of the liquid crystalline compounds.
Every member of the homologous series of symmetrical dimeric
liquid crystalline compounds that we documented in this article con-
tains a central aromaticring connected at both para positions by the
azomethine group which in turn bonded to respective aromatic ring
bearing the lateral OCH and the elongated aliphatic chain. Both ali-
3
phatic ends are even-parity flexible alkyl chains ranging from C4 to
C18, leading to the possibility of a nonpolar–polar–nonpolar sequence,
which can enhance the mesomorphic behavior in comparison to the

0
N,N -bis(3-Methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine
51
polar-nonpolar-polar sequence described by Eidenschink [13] and
Cerrada et al. [14]. The interesting aspects of the molecular structure
were taken into account, which can be associated with the nematic
mesophase and their relatively low clearing temperatures.
2. EXPERIMENTAL
Vanillin, p-phenylenediamine, potassium carbonate, 1-bromobutane,
1
1
-bromohexane, 1-bromooctane, 1-bromodecane, 1-bromododecane,
-bromotetradecane, 1-bromohexadecane and 1-bromooctadecane
were used directly from the freshly opened bottles.
IR spectra were recorded using a Perkin Elmer 2000 FT-IR spectro-
ꢀ1
photometer in the frequency range of 4000–400 cm . The spectra of all
compounds were obtained and measured from samples in the form of
1
13
KBr pellets. H NMR and C NMR spectra were recorded on a Bruker
TM
4
00-MHz Ultrashield
FT-NMR spectrometer using CDCl3 as
solvent and TMS as the internal standard. CHN microanalyses were
carried out using a Perkin Elmer 2400 LS Series CHNS=O analyzer.
Mass spectrometry was performed on a Hewlett-Packard 5989 A mass
spectrometer.
All mesophase textures observation was carried out using a Carl
Zeiss Axioskop 40 polarizing optical microscope equipped with a Met-
tler FP52 hot stage in the Liquid Crystals Research Laboratory,
School of Chemical Sciences, USM. Differential scanning calorimetry
was carried out using Seiko DSC120 model 5500 in Faculty of Engin-
ꢁ
ꢀ1
eering, Soka University, at heating and cooling rates of 5 C min and
ꢁ
ꢀ1
ꢀ
5 C min , respectively.
2.1. Synthesis
The syntheses of the intermediate compound 1 and its derivatives of
the homologous series, 2–9 (n ¼ 4–18), were carried out as summar-
ized in Scheme 1.
2
.1.1. Synthesis of Compound 1
A solution of p-phenylenediamine (2.163 g, 2 ꢂ 10 mol) in ethanol
was added dropwise to a stirring 50-mL ethanolic solution of vanillin
ꢀ
2
ꢀ2
(
6.542 g, 4.3 ꢂ 10 mol) in a round-bottom flask. The reaction mixture
ꢁ
was then heated at 70 C for 3 h before it was left to evaporate off at
room temperature. The precipitate thus isolated was then purified
twice with ethanol to yield the desired intermediate compound 1. Yield
6
0%. Elemental analysis: found C 70.42, H 5.49, N 7.41; calculated
ꢀ1
(
C H N O ) C 70.20, H 5.36, N 7.44. IR (KBr), 3373 cm (OH),
22 20 2 4
ꢀ
1
1
620 cm (C=N).

5
2
G.-Y. Yeap et al.
SCHEME 1
2
.1.2. Synthesis of Compound 2
ꢀ
3
In a round-bottom flask, 1 (0.640 g, 1.7 ꢂ 10 mol) was dissolved in
0
a
N,N -dimethylformamide solution containing K CO (1.175 g,
2
3
ꢀ
3
ꢁ
8
.5 ꢂ 10 mol). The stirring solution was heated to 40 C. 1-Bromo-
ꢀ
3
butane (0.507 g, 3.7 ꢂ 10 mol) was then added dropwise, and the
ꢁ
mixture thus obtained was heated at 70 C for 7 h before being cooled
down to room temperature. The resulting solution was then poured

0
N,N -bis(3-Methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine
53
into 70 mL of water, and the precipitate thus formed was filtered off
0
and dried. The precipitate was purified with N,N -dimethylformamide
and recrystallized with ethyl acetate to yield the desired product. Yield
7
3%. Elemental analysis: found C 74.63, H 7.60, N 5.69; calculated
þ
(
C H N O ) C 73.74, H 7.43, N 5.73. MS m=z: 488 (M ). IR (KBr),
3
0
36
2
4
ꢀ1
ꢀ1
ꢀ1
1
2
959–2872 cm (CꢀH alkyl), 1616 cm (C=N), 1268 cm (OꢀCH ). H
2
NMR d (CDCl ), 0.99–1.04 ppm (t, 6H, CH ), 1.50–1.94 ppm (m, 8H,
3
3
CH ), 3.99 ppm (s, 6H, OꢀCH ), 4.11 ppm (t, 4H, OꢀCH ), 6.96 ppm
2
3
2
(
2
d, 2H, Ar), 7.29 ppm (m, 6H, Ar), 7.63 ppm (s, 2H, Ar), 8.43 ppm (s,
1
3
H, CH=N). C NMR d (CDCl ), 14.26 ppm (CH ), 19.60–31.50 ppm
3 3
(
1
CH ), 56.49–69.14 ppm (OꢀCH ), 109.73–129.82 ppm (ArꢀC), 150.20–
2
2
52.09 ppm (ArꢀCꢀO), 159.60 ppm (C=N).
2
.1.3. Synthesis of Compound 3
Yield 72%. Elemental analysis: found C 75.04, H 8.29, N 5.11; cal-
þ
culated (C H N O ), C 74.97, H 8.14, N 5.14. MS m=z: 544 (M ). IR
3
4
44
2
4
ꢀ1
ꢀ1
ꢀ1
(
KBr), 2958–2858 cm
(CꢀH alkyl), 1617 cm
(C=N), 1267 cm
1
(
1
OꢀCH ). H NMR d (CDCl ), 0.91–0.95 ppm (t, 6H, CH ), 1.36–
2
3
3
.95 ppm (m, 16H, CH ), 3.99 ppm (s, 6H, OꢀCH ), 4.10 ppm (t, 4H,
2
3
OꢀCH ), 6.96 ppm (d, 2H, Ar), 7.30 ppm (m, 6H, Ar), 7.63 ppm (s,
2
1
3
2
H, Ar), 8.43 ppm (s, 2H, CH=N). C NMR d (CDCl ), 14.42 ppm
3
(
1
CH ), 22.98–31.98 ppm (CH ), 56.48–69.46 ppm (OꢀCH ), 109.72–
3
2
2
29.82 ppm (ArꢀC), 150.19–152.08 ppm (ArꢀCꢀO), 159.60 ppm (C=N).
2
.1.4. Synthesis of Compound 4
Yield 75%. Elemental analysis: found C 75.87, H 8.89, N 4.60; cal-
þ
culated (C H N O ) C 75.96, H 8.72, N 4.66. MS m=z: 600 (M ). IR
3
8
52
2
4
ꢀ1
ꢀ1
ꢀ1
(
KBr), 2957–2854 cm
(CꢀH alkyl), 1618 cm
(C=N), 1267 cm
1
(
1
OꢀCH ). H NMR d (CDCl ), 0.88–0.91 ppm (t, 6H, CH ), 1.31–
2
3
3
.95 ppm (m, 24H, CH ), 3.99 ppm (s, 6H, OꢀCH ), 4.10 ppm (t, 4H,
2
3
OꢀCH ), 6.96 ppm (d, 2H, Ar), 7.29 ppm (m, 6H, Ar), 7.63 ppm (s,
2
1
3
2
H, Ar), 8.43 ppm (s, 2H, CH=N). C NMR d (CDCl ), 14.49 ppm
3
(
1
CH ), 23.05–32.20 ppm (CH ), 56.47–69.46 ppm (OꢀCH ), 109.71–
3
2
2
29.83 ppm (ArꢀC), 150.19–152.08 ppm (ArꢀCꢀO), 159.59 ppm (C=N).
2
.1.5. Synthesis of Compound 5
Yield 65%. Elemental analysis: found C 77.02, H 9.43, N 4.24; cal-
þ
culated (C H N O ) C 76.79, H 9.21, N 4.26. MS m=z: 657 (M ). IR
4
2
60
2
4
ꢀ1
ꢀ1
ꢀ1
(
KBr), 2956–2853 cm
(CꢀH alkyl), 1618 cm
(C=N), 1266 cm
1
(
1
OꢀCH ). H NMR d (CDCl ), 0.88–0.92 ppm (t, 6H, CH ), 1.29–
2
3
3
.93 ppm (m, 32H, CH ), 3.99 ppm (s, 6H, OꢀCH ), 4.10 ppm (t, 4H,
2
3
OꢀCH ), 6.96 ppm (d, 2H, Ar), 7.30 ppm (m, 6H, Ar), 7.63 ppm (s,
2
1
3
2
H, Ar), 8.43 ppm (s, 2H, CH=N). C NMR d (CDCl ), 14.51 ppm
3

5
4
G.-Y. Yeap et al.
(
1
CH ), 23.07–32.29 ppm (CH ), 56.47–69.46 ppm (OꢀCH ), 109.71–
3
2
2
29.82 ppm (ArꢀC), 150.19–152.08 ppm (ArꢀCꢀO), 159.60 ppm (C=N).
2
.1.6. Synthesis of Compound 6
Yield 67%. Elemental analysis: found C 77.72, H 9.73, N 3.91; cal-
þ
culated (C H N O ) C 77.48, H 9.61, N 3.93. MS m=z: 713 (M ). IR
4
6
68
2
4
ꢀ1
ꢀ1
ꢀ1
(
KBr), 2921–2852 cm
(CꢀH alkyl), 1618 cm
(C=N), 1267 cm
1
(
1
OꢀCH ). H NMR d (CDCl ), 0.88–0.92 ppm (t, 6H, CH ), 1.28–
2
3
3
.93 ppm (m, 40H, CH ), 4.00 ppm (s, 6H, OꢀCH ), 4.10 ppm (t, 4H,
2
3
OꢀCH ), 6.96 ppm (d, 2H, Ar), 7.30 ppm (m, 6H, Ar), 7.64 ppm (s,
2
1
3
2
H, Ar), 8.43 ppm (s, 2H, CH=N). C NMR d (CDCl ), 14.51 ppm
3
(
1
CH ), 23.09–32.32 ppm (CH ), 56.48–69.46 ppm (OꢀCH ), 109.71–
3
2
2
29.82 ppm (ArꢀC), 150.19–152.08 ppm (ArꢀCꢀO), 159.60 ppm (C=N).
2
.1.7. Synthesis of Compound 7
Yield 63%. Elemental analysis: found C 78.31, H 10.07, N 3.60; cal-
þ
culated (C H N O ), C 78.08, H 9.96, N 3.64. MS m=z: 770 (M ). IR
5
0
76
2
4
ꢀ1
ꢀ1
ꢀ1
(
KBr), 2920–2851 cm
(CꢀH alkyl), 1618 cm
(C=N),1268 cm
1
(
1
OꢀCH ). H NMR d (CDCl ), 0.88–0.92 ppm (t, 6H, CH ), 1.28–
2
3
3
.93 ppm (m, 48H, CH ), 3.99 ppm (s, 6H, OꢀCH ), 4.10 ppm (t, 4H,
2
3
OꢀCH ), 6.96 ppm (d, 2H, Ar), 7.30 ppm (m, 6H, Ar), 7.63 ppm (s,
2
1
3
2
H, Ar), 8.43 ppm (s, 2H, CH=N). C NMR d (CDCl ), 14.51 ppm
3
(
1
CH ), 23.08–32.32 ppm (CH ), 56.47–69.46 ppm (OꢀCH ), 109.70–
3
2
2
29.82 ppm (ArꢀC), 150.18–152.07 ppm (ArꢀCꢀO), 159.60 ppm (C=N).
2
.1.8. Synthesis of Compound 8
Yield 60%. Elemental analysis: found C 78.63, H 10.38, N 3.35; cal-
þ
culated (C H N O ) C 78.59, H 10.26, N 3.39. MS m=z: 825 (M ). IR
5
4
84
2
4
ꢀ1
ꢀ1
ꢀ1
(
KBr), 2919–2851 cm
(CꢀH alkyl), 1618 cm
(C=N), 1268 cm
1
(
1
OꢀCH ). H NMR d (CDCl ), 0.90–0.92 ppm (t, 6H, CH ), 1.28–
2
3
3
.90 ppm (m, 56H, CH ), 3.99 ppm (s, 6H, OꢀCH ), 4.10 ppm (t, 4H,
2
3
OꢀCH ), 6.96 ppm (d, 2H, Ar), 7.32 ppm (m, 6H, Ar), 7.63 ppm (s,
2
1
3
2
H, Ar), 8.43 ppm (s, 2H, CH=N). C NMR d (CDCl ), 14.48 ppm
3
(
1
CH ), 23.07–32.31 ppm (CH ), 56.49–69.50 ppm (OꢀCH ), 109.84–
3
2
2
29.87 ppm (ArꢀC), 150.24–152.11 ppm (ArꢀCꢀO), 159.55 ppm (C=N).
2
.1.9. Synthesis of Compound 9
Yield 63%. Elemental analysis: found C 79.20, H 10.78, N 3.15; cal-
þ
culated (C H N O ) C 79.04, H 10.52, N 3.18. MS m=z: 881 (M ). IR
5
8
92
2
4
ꢀ1
ꢀ1
ꢀ1
(
KBr), 2918–2850 cm
(CꢀH alkyl), 1618 cm
(C=N), 1268 cm
1
(
1
OꢀCH ). H NMR d (CDCl ), 0.88–0.92 ppm (t, 6H, CH ), 1.28–
2
3
3
.93 ppm (m, 64H, CH ), 4.00 ppm (s, 6H, OꢀCH ), 4.10 ppm (t, 4H,
2
3
OꢀCH ), 6.96 ppm (d, 2H, Ar), 7.30 ppm (m, 6H, Ar), 7.63 ppm (s, 2H,
2

0
N,N -bis(3-Methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine
55
1
3
Ar), 8.43 ppm (s, 2H, CH=N). C NMR d (CDCl ), 14.52 ppm (CH ),
3
3
2
1
3.09–32.32 ppm (CH2), 56.48–69.47 ppm (OꢀCH ), 109.70–
2
29.82 ppm (ArꢀC), 150.19–152.08 ppm (ArꢀCꢀO), and 159.60 ppm
(
C=N).
3
3
. RESULTS AND DISCUSSION
.1. Thermal Behavior and Texture Observation
The phase-transition temperatures of the target compounds 2–9 are
collated in Table 1. This table that only one transition temperature
was observed for compounds 2 and 3 during heating and cooling
processes. The enthalpies with respect to each transition in 2 (63.5
and 63.6 kJ=mol for heating and cooling cycles, respectively) and 3
(
67.6 and 65.7 kJ=mol for respective heating and cooling processes)
can be associated with the transition from crystal to isotropic liquid
and vice versa. The high transition temperatures observed for com-
pounds 2 and 3 could be due to the presence of a strong intermolecular
attraction among the short-chain molecules leading to restricted ther-
mal motion. As such, the molecules in compounds 2 and 3 are highly
positioned, which caused the anisotropic properties of the molecules
to be reduced to a remarkable extent [6]. When the length of the end
tail increased to C8 as in compound 4, two transitions were observed
ꢁ
during the cooling cycle: first at 124.2 C (isotropic to mesophase) and
followed by the transition from mesophase to crystal phase at
ꢁ
1
04.5 C. The texture observation carried out on compound 4 shows that
it exists as a nematogen (Fig. 1), of which the formation of colorful mar-
ꢁ
ble nematic texture was observed at 114.0 C during the cooling process.
TABLE 1 Transition Temperatures and Relevant Enthalpies (kJ=mol)
of Compounds 2–9
Dimers
Cr ! N
N ! I
I ! N
N ! Cr
2
3
4
5
6
7
8
9
.
.
.
165.6 (63.5)
155.8 (67.6)
132.7 (72.5)
128.0 (53.1)
125.3 (54.3)
122.6 (52.4)
121.8 (127.8)
118.7 (139.0)
.
.
149.7 (63.6)
133.0 (65.7)
104.5 (24.7)
102.1 (39.7)
95.9 (53.3)
93.0 (61.0)
106.8 (76.7)
113.8 (55.2)
124.2 (49.2)
116.4 (49.5)
121.0 (52.6)
115.3 (53.1)
112.8 (53.6)
.
113.7 (35.7)
115.2 (48.9)
117.4 (58.7)
.
.
Notes: Cr, crystal; N, nematic; I, isotropic.
) represents enthalpy (DH ) of the phase transition (kJ=mol).
(
m

5
6
G.-Y. Yeap et al.
ꢁ
FIGURE 1 Photomicrograph of marble texture of compound 4 at 114.0 C.
A further lengthening of the end tail as we move from compound 4
to compounds 5, 6, and 7 has exhibited the enantiotropic behavior, the
two peaks were observed in their thermograms during heating as well
as cooling cycles. A representative DSC trace of compound 5 is
depicted in Fig. 2. Although the CrꢀN and NꢀI phase transitions were
ꢁ
ꢁ
ꢁ
observed at respective temperature of 113.7 C and 128.0 C when they
were heated up, the reverse process occurred at 116.4 C (IꢀN) and
ꢁ
1
02.1 C (NꢀI). The observation on compounds 5–7 under polarized
light shows that the nematic phase appears during heating and cool-
ing cycles. A typical example with regards to the texture among com-
pounds 5–7 is illustrated by compound 6. The nematic droplets in the
isotropic matrix were observed during the cooling process (Fig. 3). As
for compound 8, a similar feature as shown by compound 4 was
observed whereby the nematic phase with fine textures was only
observed during the cooling process. The presence of monotropic beha-
vior in compound 8 could possibly be due to the reduced anisotropy
that destabilizes the mesomorphic properties as reported for the dis-
tinct bent-shape molecule and also the molecule possessing lateral
methoxy groups [15]. This observation indicates that the molecules
with the longer alkyl chain in this series are not linear. The lowering
of linearity gives rise to lower clearing temperature (T ), which is in
c
contrast to the dimers of linear conformation, of which the T were
c
ꢁ
found to be greater than 200 C [16]. This phenomenon has further
been substantiated by the longest flexible tail containing C18 in which

0
N,N -bis(3-Methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine
57
FIGURE 2 DSC trace of compound 5.
FIGURE 3 Photomicrograph of phase transition from isotropic to nematic
ꢁ
droplets of compound 6 at 121.0 C.

5
8
G.-Y. Yeap et al.
ꢁ
the thermogram shows only one peak during heating (118.7 C) and
ꢁ
cooling (113.8 C) processes. This observation can be associated with
the direct isotropization and crystallization, suggesting that the mol-
ecule is distorted from linearity, which led to inefficient packing and
lowering of the thermal stability [17].
The nematogenic behaviour observed among compounds 4–8 sug-
gests that the presence of the methoxy group in the meta position of
the aromatic system has weakened the lateral interactions between
molecules, which is important for promoting higher mesophase stab-
ility, and therefore it restricts only the existence of the nematic phase
[
17,18]. As the series ascends from compounds 2 to 9, it is important to
note that during the heating process, the clearing points decrease
gradually with increasing alkyl chain length (Fig. 4). This observation
0
complies with previously reported homologous series of p,p -bipheny-
lene esters of p-alkoxy and p-carbalkoxybenzoic acids in which the
longest chain homologue has the lowest thermal stability [10]. It
implies that as the alkyl chain length increases, the rigidity of the long
molecular axis is further weakened and therefore the linearity of the
FIGURE 4 Trend of the clearing points during the heating process of com-
pounds 2–9: —&— - clearing point (during heating), ꢃ - crystal to isotropic
(
not mesogenic), and ꢃꢃ - crystal to isotropic (monotropic).

0
N,N -bis(3-Methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine
59
molecule affected. Consequently, the molecules do not fit readily into
the parallel molecular arrangement of the nematic phase due to the
more significant bending conformation. As the result, the phase stab-
ility is decreased and lower transition temperatures are observed in
compound 9.
3.2. Physical Characterization
To elucidate the molecular structure of the desired compounds 2–9,
1
13
the spectroscopic techniques of FT-IR, H NMR, and C NMR were
carried out at room temperature.
From the FT-IR spectroscopy, it was observed that the diagnostic
bands assignable to the alkyl group were present within the frequen-
ꢀ1
cies of 2850–2919 cm with relative intensities ranging from weakest
absorption for compound 2 to the strongest for compound 9. This
characteristic is in accordance with the length of terminal alkyl groups
attached to the central core in which compound 2 is the shortest mem-
ber with terminal butyl groups. The bands with medium intensities
ꢀ1
were seen at 1616–1620 cm , which can be attributed to the azo-
methine (CH=N) group in intermediate compound 1 as well as com-
pound 2–9. On the other hand, the ether group (ArꢀOꢀCH )
2
ꢀ1
absorbed strongly in the frequency range of 1266–1268 cm .
The effort to further substantiate the molecular structure determi-
1
nation was supported by the employment of high resolution H and
13
C NMR spectroscopic techniques. From the NMR spectra, all com-
pounds exhibited almost similar trends in terms of splitting and
chemical shifts. Hence, the shortest member of the series of compound
2
was chosen for the following discussion.
From the H NMR data of compound 2, a triplet at d ¼ 0.99–
1
1
.04 ppm was assigned to the methyl protons of the alkyl chain. The
subsequent two sets of methylene protons at each terminal gave rise
to two separate sets of multiplets within chemical shift range of
d ¼ 1.50–1.94 ppm. For the longer homologues (3–9), the presence of
indistinguishable overlapping multiplets was observed for the methyl-
ene protons. At d ¼ 3.99 ppm, a singlet was observed, and this can be
assigned to the methoxy protons at the lateral position of the outer
aromatic rings. On the other hand, the absorption of another two sets
of methoxy protons which are also the methylene protons of the alkyl
groups, was detected as a triplet at d ¼ 4.11 ppm. The following set of
doublet, multiplet, and a singlet within the range of d ¼ 6.96–7.63 ppm
were assigned to the four different types of aromatic protons. At
low field, a characteristic singlet was observed at d ¼ 8.43 ppm,
which can be ascribed to the two azomethine protons, in which the

6
0
G.-Y. Yeap et al.
deshielding effect was caused by the inductive effective of N atom
which reduced the electron density on adjacent C atom.
1
3
From the C NMR data of compound 2, the peak attributable to the
methyl carbons was observed at d ¼ 14.26 ppm. The following two
peaks at d ¼ 19.60–31.50 ppm were assigned to the two different types
of methylene carbons of the alkyl groups. The methoxy carbons
absorbed at d ¼ 56.49–69.14 ppm. The aromatic proton and quartern-
ary carbons gave rise to different peaks at d ¼ 109.73–152.09 ppm.
The appearance of another peak at d ¼ 159.60 ppm was subsequently
assigned to the azomethine carbon.
4. CONCLUSION
0
A series of eight novel homologous symmetrical dimers N,N -bis[(3-
methoxy-4-alkoxy phenyl)methylene]benzene-1,4-diamine (2–9) with
different terminal alkyl groups of even parity ranging from C4
(
C H ) to C18 (C H ) were synthesized and characterized. All com-
4 9 18 37
pounds were mesogenic, exhibiting nematic phase, except compounds
, 3, and 9. Although compounds 5–7 are enantiotropic, compounds 4
2
and 8 are monotropic in nature. A comparison made revealed that the
nematogenic characteristics of this series can be affected by the pres-
ence of methoxy group at the meta position of the aromatic rings in
which the mesophase and thermal stability were reduced, due to less
lateral interaction among the molecules.
ACKNOWLEDGMENTS
G.-Y. Yeap thanks Universiti Sains Malaysia and the Malaysian
government, especially the Ministry of Science, Technology and Inno-
vation, for providing the facilities and Research Grant No.
3
05=PKIMIA=612923.
REFERENCES
[
[
[
1] Vorlander, D. (1927). Z. Phys. Chem., 126, 449.
2] Rault, J., Liebert, L., & Strzelecki, L. (1975). Bull Soc. Chem. Fr., 5, 1175.
3] Marcelis, A. T. M., Koudijs, A., Karczmarzyk, Z., & Sudh o¨ lter, E. J. R. (2003). Liq.
Cryst., 30(11), 1357.
[4] Imrie, C. T. & Luckhurst, G. R. (1998). In: Handbook of Liquid Crystals, Demus, D.,
Goodby, J., Gray, G. W., Spiess, H. W., & Vill, V. (Eds.), Weinheim: Wiley-VCH, Vol.
2
B, 801.
[
[
5] Imrie, C. T. (1999). Struct. Bonding, 95, 150.
6] Gray, G. W. (1974). In: Liquid Crystals and Plastic Crystals, Gray, G. W. & Winsov,
P. A. (Eds.), Ellis, Horwood Ltd.: Chichester, U.K.

0
N,N -bis(3-Methoxy-4-alkoxybenzylidene)-1,4-phenylenediamine
61
[
[
[
7] Hirata, H., Waxman, S. N., Teucher, I., & Labes, M. M. (1973). Mol. Cryst. Liq.
Cryst., 20, 343.
8] Vora, R. A. & Gupta, R. (1979). Mol. Cryst. Liq. Cryst. Lett., 56, 31; (1980). In:
Liquid Crystals, Chandrasekhar, S. (Ed.), Heyden Verlag, London, 589.
9] Sakagami, S., Nonaka, K., Koga, T., & Takase, A. (1998). Mol. Cryst. Liq. Cryst.,
3
12, 23.
[
[
[
[
[
[
10] Dewar, M. J. S. & Schroeder, J. P. (1965). J. Org. Chem., 30, 2296.
11] Gray, G. W., Jones, B., & Marson, F. (1956). J. Chem. Soc., 1417.
12] Berdague, P., Bayle, J. P., Ho, M. S., & Fung, B. M. (1993). Liq. Cryst., 14(3), 667.
13] Eidenschink, R. (1979). Kontakte (Merck), 1, 15.
14] Cerrada, P., Marcos, M., & Serrano, J. L. (1989). Mol. Cryst. Liq. Cryst., 170, 79.
15] Rao, N. V. S., Singha, D., Das, M., & Paul, M. K. (2002). Mol. Cryst. Liq. Cryst., 373,
1
05.
[
[
[
16] Yelamaggad, C. V., Mathews, M., Hiremath, U. S., Nair, G. G., Rao, D. S. S., &
Prasad, S. K. (2003). Liq. Cryst., 30(8), 899.
17] Perez, F., Judeinstein, P., Bayle, J. P., Roussel, F., & Fung, B. M. (1997). Liq.
Cryst., 22(6), 711.
18] Date, R. W., Imrie, C. T., Luckhurst, G. R., & Seddon, J. M. (1992). Liq. Cryst.,
1
2(2), 203.