Synthesis and liquid crystalline properties of new diols containing azomethine groups

Article abstract of DOI:10.3390/molecules15053260

series of new mesogenic azomethine diols were successfully synthesized by condensation reactions between various chloroalkanols and N, N′-bis(4-hydroxy)-benzylidene-o-toluidine (1). The structures of these compounds were confirmed by CHN, FT-IR, ¹H-NMR, and ¹³C-NMR spectrophotometer. Their thermotropic liquid crystalline behavior was studied using differential scanning calorimetry (DSC) and polarizing optical microscope (POM). 4, 4′-di(4-Hydroxybutoxy)-N- benzylidine-o-tolidine (2a) does not exhibit liquid crystalline properties. A nematic texture was observed for mesogenic diols 2b, and 2d, whereas the diol 2c exhibits a smectic msophase. The increase of terminal alkyl chain in these mesogenic diols leads to a decrease in the transition temperature.

Full text of DOI:10.3390/molecules15053260

Molecules 2010, 15, 3260-3269; doi:10.3390/molecules15053260
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Liquid Crystalline Properties of New Diols
Containing Azomethine Groups
Issam Ahmed Mohammed 1,*, Govindarajan Sankar 1, Melati Khairuddean 2 and
Abu Bakar Mohamad 2
1
School of Industrial Technology, University Sains Malaysia, 11800 Penang, Malaysia
2
School of Chemical Sciences, University Sains Malaysia, 11800 Penang, Malaysia
 To whom correspondence should be addressed; E-Mail: issam@usm.my; Fax: 604-6575854.
Received: 10 March 2010; in revised form: 30 March 2010 / Accepted: 6 April 2010 /
Published: 30 April 2010
Abstract: A series of new mesogenic azomethine diols were successfully synthesized by
condensation reactions between various chloroalkanols and N,N'-bis(4-hydroxy)-
benzylidene-o-toluidine (1). The structures of these compounds were confirmed by CHN,
1
FT-IR, H-NMR, and 13C-NMR spectrophotometer. Their thermotropic liquid crystalline
behavior was studied using differential scanning calorimetry (DSC) and polarizing optical
microscope (POM). 4,4'-di(4-Hydroxybutoxy)-N-benzylidine-o-tolidine (2a) does not
exhibit liquid crystalline properties. A nematic texture was observed for mesogenic diols
2b, and 2d, whereas the diol 2c exhibits a smectic mesophase. The increase of terminal
alkyl chain in these mesogenic diols leads to a decrease in the transition temperature.
Keywords: thermotropic liquid crystal; nematic phase; diols; azomethine
1. Introduction
Organic compounds exhibit thermotropic liquid crystalline (LC) properties when the molecules
possess rod- or disc-like structure. The stabilization and polarizability are two of the most important
factors in liquid crystal devices. Researchers have discovered that small molecules are also capable of
exerting a significant impact on the molecular polarizability as well as the stabilization in the liquid
crystalline compounds. Azomethine (CH=N) linkages present in the backbone provide an attractive
class of high performance material. High thermal stability, excellent mechanical strength, and good

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optoelectronic properties are also attributed to the resonance stabilization of the Schiff’s base unit [1].
However, these small molecules have certain drawbacks such as poor solubility that minimize their
practical applications.
Small molecules (in this case, liquid crystal monomers) with geometrical anisotropy and high
polarizability often exhibit one or more liquid crystalline phases besides the crystalline and isotropic
phases [2]. Therefore, a terminal functional group such as a hydroxyl group is employed in order to
enhance these features. One of the typical examples is the study on the effect of lateral hydroxyl
groups on the mesomophism of azobenzene derivatives [3]. To our best knowledge, limited work has
been performed on the compounds containing azomethine linkages and hydroxyls as the end group.
With this background we decided to synthesize a series of novel N,N' bis(4-hydroxyl)benzylidine-o-
tolidine derivatives and to investigate the liquid crystalline properties of the new diols.
2. Result and Discussion
2.1. Synthesis and characterization
Preparation of diols 2(a–d) is explained in Scheme 1. The structures of bisphenol 1 and the diols
2(a–d) were confirmed by elemental analysis (CHN), FT-IR, 1H-NMR and 13C-NMR
spectrophotometry. Figure 1 shows four different IR spectra representing diols 1, 2a, 2b and 2d. In the
obtained FT-IR spectra the disappearance of the NH2 and carbonyl absorption bands at
(3,336–3,472) cm-1and 1,667 cm-1 and the appearance of characteristic absorption bands at 1,620 cm-1
assigned to the presence of a -CH=N bond could be observed. This correlates with the data obtained by
Issam [4] and So et al. [5], where the absorption band for -CH=N group is around 1,619 cm-1. The
sharp absorption band at (3,350–3,414) cm-1 is attributed to the -OH group [6,7]. Absorption bands at
1,480 cm-1 and 1,592 cm-1, which is corresponds to aromatic –C=C–, indicates that an aromatic ring
exists in the synthesized compound. The FT-IR data for the diols 2(a–d) and the reactants are given in
Experimental section.
Scheme 1. Synthesis of mesogenic diols 2 (a–d).
H3C
CH3
2
HO
CHO
H2N
NH2
+
CH3
H3C
N
HO
CH
N
CH
OH
1
OH
Cl
2
-2 HCl
CH3
H3C
OH
O
CH
N
N
CH
O
HO
2 (a - d)
a: n = 4
b: n = 6
c: n = 8
d: n = 10

Molecules 2010, 15
Figure 1. The FT-IR spectra of (A) bisphenol 1 (B) diol 2a, (C) diol 2b and (D) diol 2d.
3262
2965.25
(A)
(B)
1607.67
1480.58
3414.73
1593.09
2929.01
1248.86
834.04
833.25
1605.47
1509.19
1620.82
1162.17
3413.98
%T
1252.71
(C)
(D)
2928.08
1620.82
1509.23
1161.78
1605.12
3426.79
2931.70
1164.29
1620.82
1606.44
3353.44
833.56
800
1252.60
1509.32
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
600
400.0
cm-1
Figures 2 and 3 show typical examples of 1H-NMR and 13C-NMR spectra, corresponding to diol 2b.
1
In the H-NMR spectrum, the methylene protons in (HO-CH2-) and (Ph-O-CH2-) appear at 3.57 ppm
and 4.05 ppm, respectively. This is similar to the data that obtained by Stamatoiu et al. [8]. The singlet
peak at 8.65 ppm is assigned to the azomethine (-CH=N-) group [9]. The azomethine group appeared
at higher field because the nitrogen atom is bonded to an aromatic ring and the nitrogen has a lone pair
of electrons which enables it to delocalize into the aromatic ring. The bands of the -OH protons are not
detected due to overlap with C-H protons. However, we further confirm formation of hydroxyl group
1
by FT-IR which indicates the absorption band at 3,420 cm-1. H-NMR data for the diols 2(a–d) are
given in the Experimental section and the properties are summarized in Table 1.
Figure 2. 1H-NMR spectrum of diol 2b.

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Figure 3. 13C-NMR spectrum of diol 2b.
Table 1. Physical properties and CHN data for diols 2 (a–d).
CHN
Yield
(%)
Melting Point
Mesogenic diol
Color
(ºC)
Calculated (%)
C:80.0, H: 5.70,
N: 6.66.
Found (%)
N,N'-bis(4-hydroxyl)
C: 79.7, H: 5.60,
N: 6.56.
Yellow
88
58
70
68
81
281-282
benzylidine-o-tolidine (1)
4-4-di(4-hydroxybutoxy)-N-
benzylidene-o-tolidine (2a)
4-4-di(6-hydroxyhexoxy)-N-
benzylidene-o-tolidine (2b)
4-4-di(8-hydroxyoctoxy)-N-
benzylidene-o-tolidine (2c)
C:76.59, H: 7.09,
N: 4.96.
C: 76.43, H: 7.14,
N: 5.06.
Yellow
Brown
Yellow
Yellow
242-244
160-162
119-120
108-109
C: 77.14, H: 7.74,
N: 4.51.
C: 77.30, H: 7.79,
N: 4.50.
C:78.1, H: 8.28,
N: 4.14
C:78.28, H: 8.11,
N: 4.20.
4-4-di(10-hydroxydecoxy)-N- C:78.68, H: 8.74,
benzylidene-o-tolidine (2d) N: 3.82.
C:78.60, H: 8.90,
N: 3.80.
2.2. Thermal and optical analysis
The mesophases of the diols 2(a–d) were studied by polarizing optical microscope (POM) and
differential scanning calorimetry (DSC). Table 2 shows the mesophase confirmation and the transition
temperatures for diols 2(b–d). DSC analysis was carried out for all diols obtained and Figure 4 shows
the observed transition temperature for diol 2b. The crystal phase to nematic phase transition
temperature (Cr-N) and nematic phase to isotropic phase transition (N-I) temperature is observed
under POM. When the diols are observed under POM at 5 ºC/min (to correlate with the DSC rate), the
results revealed that the diols 2b, and 2d exhibit a nematic liquid crystal phase, diol 2c exhibits a
smectic liquid crystal phase, whereas diol 2a do not show any liquid crystalline behavior, but simply
changes from a solid to an isotropic fluid. Compounds that exhibit liquid crystalline behavior are due
to their high aspect ratio (length to breadth ratio). The POM images for all obtained compounds are
shown in Figure 5. It is worth mentioning that similar studies have been done on liquid crystals with

Molecules 2010, 15
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four aromatic rings in the rigid core and when a three and four carbon alkyl chain is added to its
terminal end they did not exhibit liquid crystal phases [4]. The addition of the flexible chain length
decreased the melting point of these diols. This observation is supported by the previous research [10–
12]. The lateral substituents introduced into mesogen molecules depress the thermal stability of the
mesophases, it is due to the overall anisotropic broadening of the molecule will influence cooperative
packing needed in the structures that form mesophase [13,14].
Figure 4. DSC thermogram of diol 2b.
Figure 5. Polarized optical micrographs of (a) diol 2a, (b) diol 2b, (c) diol 2c, and (d) diol 2d.
The effect of a flexible substituent in depressing the mesophase stability is greater in more ordered
phases such as smectics than nematics [15]. The alkylation of the flexible chain gives a larger nematic
phase range when it is added at the terminal end (located as far as possible from the rigid part of the
molecule) [16]. However, the effect of dilution by adding the length of flexible chain has a limit, and
further addition of the carbon chain length will increase the transition temperatures [17]. Therefore, it
could be predicted that further addition of flexible chain length might increase the transition
temperature point. Nevertheless, there are cases that the transition temperature will increase with
increase in the alkyl chain length as reported in the works of Dave et al. [18]. Chemical crosslinking

Molecules 2010, 15
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imposes additional constraints on the motion of chain segments which reduced the free volume that
caused an increased in transition temperature [14]. Nevertheless, the clearing temperatures decrease
with increasing spacer lengths too [11]. The linker group used in our present work is CH=N (an
azomethine group). This linker group generally will have a higher transition temperature when
compared to the ester linker group (COO). In the latter the oxygen atom of the central carbonyl group
will be bumped into the non-bonded sides of the adjacent hydrogens in the aromatic ring, thereby
causing considerable strain on the molecules [18]. This will cause some twist around the C-O bond and
force the benzene ring with ester linkage out of the plane in the molecules. Thus reduces the
coplanarity of the molecule and the broadening of rigid core. All diols used are of an even number of
carbon alkyl chain length (n = 4, 6, 8, and 10) hence the odd-even effect is not shown in the melting
points trend. The odd-even effect is exhibited when a subsequent increase in flexible chain length (one
by one carbon atom) will result in an inconsistent increase or decrease in transition temperature. The
methyl group that is bonded to each aromatic ring also plays an important role in decreasing the
transition states. In this research, one of the starting materials (o-tolidine) possesses two methyl groups
where each one is bonded to the aromatic ring in the meta position will increase molecular broadening
of the rigid core, such that intermolecular interactions are responsible for mesomorphism decreased as
the intermolecular separation increases. This would in turn increase the entropy (ΔS) of the nematic to
isotropic phase transition. As explained earlier, 4-4'-di(6-hydroxyhexoxy)-N-benzylidene-o-tolidine
(diol 2b), 4-4'-di(8-hydroxyoctoxy)-N-benzylidene-o-tolidine (2c) and 4-4'-di(10-hydroxydecoxy)-
N,N'-benzylidene-o-tolidine (2d), only show marbled texture when heated above its melting point. The
isotropic points for diols 2(b–d) are 275.2 ºC, 231.0 ºC and 251.0 ºC, respectively. Diols 2b and 2d
exhibit typical nematic texture and no trace of smectic structure was seen in these diols. Diol 2a do not
show liquid crystalline behavior, but only melts at 242.11 ºC. The reason that this compound does not
exhibit any liquid crystal phase because is presumably because the carbon chain is not long enough.
Table 2. Transition temperatures of diols 2 (a–d).
Transition temperatures, DSC (ºC)
N-I
(ºC)*
Mesogenic diol
Cr-N (ºC)*
∆T/ º C
Tm
Ti
4-4-di(4-hydroxybutoxy)-N-
benzylidene-o-tolidine (2a)
4-4-di(6-hydroxyhexoxy)-N-
benzylidene-o-tolidine (2b)
4-4-di(8-hydroxyoctoxy)-N-
benzylidene-o-tolidine (2c)
4-4-di(10-hydroxydecoxy)-N-
benzylidene-o-tolidine (2d)
__
__
__
242.11
__
160.1
126.0
103.0
275.2
231.0
251.0
115.1
105.0
148.0
160.0
130.0
105.0
285.10
228.0
257.0
Cr = crystal; N = Nematic phase; I = isotropic; * = as observed under POM.
3. Experimental
3.1. Materials
Unless otherwise stated, all chemicals were commercially obtained and used without further
purification or distillation: o-tolidine (Fluka, Germany), 4-hydroxybenzaldehyde (MERCK, Germany),
4-chloro-1-butanol (Fluka, Germany), 6-chloro-1-hexanol (Fluka, Germany), 8-chloro-1-octanol

Molecules 2010, 15
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(Sigma Aldrich, Germany), 10-chloro-1-decanol (Sigma Aldrich, Germany), potassium carbonate
(Fisher Chemicals, UK), absolute ethanol 99.5% (Systerm®, Malaysia), 1-butanol 99.5% (R&M
Chemicals, Malaysia), N,N-dimethylformamide (Systerm®, Malaysia)
3.2. Instrumentation
FT-IR spectra were measured using a Perkin-Elmer 2000 FTIR equipped with a potassium bromide
(KBr) beam splitter. Thirty-two (32) scans were collected between 4000 cm-1 and 400 cm-1 with a
resolution of 2.0 cm-1. The 1H-NMR and 13C-NMR spectra were obtained using a Bruker 400 MHz and
a 300 MHz spectrometer. The samples were prepared at 100 mg/mL in CDCl3 with tetramethylsilane
(TMS) as the internal reference. CHN microanalyses were performed using a Perkin Elmer 2400 LS
Series CHNS=O analyzer. Differential scanning calorimetry (DSC) studies were carried out with a
Perkin-Elmer DSC7 series at heating rate of 5 ºC/min in nitrogen, Melting points of LC monomers
were obtained based on the peak of the melting temperatures from DSC analysis. Polarizing optical
microscope (POM), a Carl Zeiss Axioskop equipped with 40 Linkam LTS350 hot stage, Linkam
TMS94 temperature controller and Linkam LNP cooling system (pump) was used to determine the
liquid crystalline mesophase. Thermal stability was studied using Perkin-Elmer TGA-7 under N2
atmosphere. The heating rate was programmed to 5 ºC/min and it was performed at the temperature
range of 30–900 ºC.
3.3. Preparation of Bisphenol N,N'-bis(4-hydroxy)-benzylidene-o-tolidine (1).
p-Hydroxybenzaldehyde (12.21 g, 0.10 mol) was added dropwise into a solution of
o-tolidine (10.61 g, 0.05 mol) in absolute ethanol (60 mL) in a 500 mL reaction flask. The mixture was
refluxed for 6 h with magnetic stirring. The product was then filtered, washed several times with
diethyl ether and dried in the vacuum oven at 70 ºC. Final purification was carried out by re-
crystallization using 1-butanol to give yellow crystals. Yield was 74% with a melting point of
281–282 ºC. FT-IR (KBr disc): 3,414 cm-1 (O-H stretch), 1,608 cm-1 (C=N stretch), 1,516 cm-1 (C=C
1
stretch). H-NMR (400 MHz): δ 8.56 (s, 2H, H-C=N), 6.05 (2H, -OH), 7.79–6.85 (dd, 8H, aromatic
protons), 7.35 (d, 2H, aromatic protons), 7.27 (d, 2H, aromatic protons), 7.76 (s, 2H, aromatic
protons), 2.34 (s, 6H, Ph-CH3) ppm. 13C-NMR (CDCl3): δ 18.64, 116.21, 122.68, 126.34, 129.12,
130.62, 131.08, 132.14, 141.24, 151.36, 160.58, 161.23 ppm.
3.4. Preparation of mesogenic diols 2(a–d)
Chloroalkanol (0.40 mol) and sodium carbonate (0.45 mol) were dissolved in DMF (400 mL).
Bisphenol 1 (0.20 mol) in DMF (60 mL) was added in the above mixture in a 500 mL reaction flask.
The mixture was refluxed at 130 ºC for 12 h. The product was precipitated by pouring the obtained
solution into cold distilled water (2 L). The precipitate was then filtered, washed several times with
diethyl ether and dried in a vacuum oven at 90 ºC. Final purification was carried out by re-
crystallization using the mixture solvent of DMF-butanol (1:1).
4,4'-di(4-Hydroxybutoxy)-N-benzylidine-o-tolidine (2a). Colour: yellow. Yield: 58%. Melting point:
242–244 ºC. FT-IR (KBr disc): 3,414 cm-1 (O-H stretch), 1,607 cm-1 (C=N stretch), 1,509 cm-1 (C=C

Molecules 2010, 15
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1
stretch). H-NMR (400 MHz): δ 8.37 (s, 2H, -CH=N), 7.87–6.98 (dd, 8H, aromatic protons), 7.50 (d,
2H, aromatic protons), 7.45 (d, 2H, aromatic protons), 7.85 (s, 2H, aromatic protons), 4.03 (t, 4H, -O-
CH2-), 3.65 (m, 4H, HO-CH2-), 2.41 (s, 6H, Ph-CH3), 1.89–1.49 (m, 8H, -CH2- protons) ppm. 13C-
NMR (CDCl3): δ 18.5 (Ph-CH3), 25.57–32.31 (-CH2-), 62.00 (-O-CH2-), 69.54 (-CH2-OH),
114.0–160.74 (aromatic carbons), 162.24 (-CH=N-) ppm. Elemental analysis: found: C, 76.43; H,
7.14; N, 5.06, C36H40N2O4.Calcd: C, 76.59; H, 7.09; N, 4.96.
4,4'-di(6-Hydroxyhexoxy)-N-benzylidine-o-tolidine (2b). Colour: Brown. Yield: 70%. Melting point:
160–162 ºC. FT-IR (KBr disc): 3,419 cm-1 (O-H stretch), 1,621 cm-1 (C=N stretch), 1,509 cm-1 (C=C
1
stretch). H-NMR (400 MHz): δ 8.35 (s, 2H, -CH=N), 7.90–7.02 (dd, 8H, aromatic protons), 7.51 (d,
2H, aromatic protons), 7.49 (d, 2H, aromatic protons), 7.87 (s, 2H, aromatic protons), 4.05 (t, 4H, -O-
CH2-), 3.65 (m, 4H, HO-CH2-), 2.41 (s, 6H, Ph-CH3), 1.90-1.30 (m, 16H, -CH2- protons) ppm.13C-
NMR (CDCl3): δ 18.52 (Ph-CH3), 25.57-32.31 (-CH2-), 62.65 (-O-CH2-), 68.64 (-CH2-OH),
114.43–158.74 (aromatic carbons), 161.24 (-CH=N-) ppm. Elemental analysis: found: C, 77.30; H,
7.79; N, 4.50, C40H48N2O4.Calcd: C, 77.14; H, 7.74; N, 4.51.
4,4'-di(8-Hydroxyoctoxy)-N-benzylidine-o-tolidine (2c). Colour: yellow. Yield: 68%. Melting point:
119–120 ºC. FT-IR (KBr disc): 3,419 cm-1 (O-H stretch), 1,621 cm-1 (C=N stretch), 1,509 cm-1 (C=C
1
stretch). H-NMR (400 MHz, CDCl3): δ 8.35 (s, 2H, -CH=N), 7.90–6.96 (dd, 8H, aromatic protons),
7.49 (d, 2H, aromatic protons), 7.46 (d, 2H, aromatic protons), 7.88 (s, 2H, aromatic protons), 4.02 (t,
4H, -O-CH2-), 3.66 (m, 4H, HO-CH2-), 2.42 (s, 6H, Ph-CH3), 1.86–1.29 (m, 24H, -CH2- protons) ppm.
13C-NMR (CDCl3,): δ 18.5 (Ph-CH3), 25.57–32.31 (-CH2-), 62.00 (-O-CH2-), 69.54 (-CH2-OH),
114.50–160.45 (aromatic carbons), 162.40 (-CH=N-) ppm. Elemental analysis: found: C, 78.28; H,
8.11; N, 4.20, C44H56N2O4.Calcd: C, 78.1; H, 8.28; N, 4.14.
4,4'-di(10-Hydroxydecoxy)-N-benzylidine-o-tolidine (2d). Colour: yellow. Yield: 81%. Melting point:
108–109 ºC. FT-IR (KBr disc): 3,353 cm-1 (O-H stretch), 2931 cm-1, 2,862 cm-1, 1,620 cm-1 (C=N
stretch), 1509 cm-1 (C=C stretch). 1H-NMR (400 MHz, CDCl3): δ 8.39 (s, 2H, -CH=N), 7.92–6.98 (dd,
8H, aromatic protons), 7.51 (d, 2H, aromatic protons), 7.49 (d, 2H, aromatic protons), 7.88 (s, 2H,
aromatic protons), 4.07 (t, 4H, -O-CH2-), 3.68 (m, 4H, HO-CH2-), 2.46 (s, 6H, Ph-CH3), 1.89–1.25 (m,
32H, -CH2- protons) ppm. 13C-NMR (CDCl3) δ 18.5 (Ph-CH3), 25.57–32.31 (-CH2-), 62.00 (-O-CH2-),
69.54 (-CH2-OH), 114.0–160.74 (aromatic carbons), 162.24 (-CH=N-) ppm. Elemental analysis:
found: C, 78.60; H, 8.90; N, 3.80, C48H64N2O4.Calcd: C, 78.68; H, 8.74; N, 3.82.
4. Conclusions
A series of N,N'-bis(4-hydroxyl)benzylidine-o-tolidine compounds with various alkyl chain lengths
(4, 6, 8, and 10), was successfully synthesized. The diols were found to demonstrate enantiotropic
nematic phase based on the POM and DSC observations. Diols 2b and 2d exhibit enantiotropic
nematic liquid crystal phase (marble texture and shredded texture respectively) but diol 2a did not
exhibit any liquid crystalline phase, while the diol 2c exhibits a smectic mesophase. DSC analyses
showed that the increase in alkyl chain length in diols will result in a the decrease in the transition
temperature for this series of compounds.

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Acknowledgements
The authors would like to thank Universiti Sains Malaysia for RU research grant
No.PTEKIND/33399/11900
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Sample Availability: Not available.
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