Azomesogens having a lateral hydroxy substituent

Article abstract of DOI:10.1080/15421400802697236

Two homologous series of liquid crystalline azoesters consisting of a lateral hydroxyl substituent on the central benzene ring at 2' positions have been synthesized and thermally characterized. All the 24 members, of both series, exhibit high temperature nematic phases. All the compounds of both series are thermally stable and exhibit a wide mesomorphic range. The homologues have been characterized using Infra Red (IR), Nuclear Magnetic Resonance Spectroscopy (NMR), and Differential Scanning Calorimetry (DSC). Their mesomorphic properties and other characteristics are discussed.

Full text of DOI:10.1080/15421400802697236

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Azomesogens Having a Lateral
Hydroxy Substituent
Sandhya Dixit a & R. A. Vora a
a Department of Applied Chemistry , Faculty of
Technology and Engineering, The M. S. University of
Baroda , Kalabhavan, Vadodara, India
Published online: 18 Mar 2009.
To cite this article: Sandhya Dixit & R. A. Vora (2009) Azomesogens Having a Lateral
Hydroxy Substituent, Molecular Crystals and Liquid Crystals, 501:1, 43-52, DOI:
10.1080/15421400802697236
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Mol. Cryst. Liq. Cryst., Vol. 501, pp. 43–52, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802697236
Azomesogens Having a Lateral Hydroxy Substituent
Sandhya Dixit and R. A. Vora
Department of Applied Chemistry, Faculty of Technology
and Engineering, The M. S. University of Baroda, Kalabhavan,
Vadodara, India
Two homologous series of liquid crystalline azoesters consisting of a lateral
hydroxyl substituent on the central benzene ring at 20 positions have been synthe-
sized and thermally characterized. All the 24 members, of both series, exhibit high
temperature nematic phases. All the compounds of both series are thermally stable
and exhibit a wide mesomorphic range. The homologues have been characterized
using Infra Red (IR), Nuclear Magnetic Resonance Spectroscopy (NMR), and Dif-
ferential Scanning Calorimetry (DSC). Their mesomorphic properties and other
characteristics are discussed.
Keywords: azomesogens; mesomorphism; nematic; smectic
INTRODUCTION
Azo dye liquid crystals have attracted much attention for potential use
in optical storage applications because of their easy availability, rea-
sonable photo-fatigue resistance, and the marked transformation of
their molecular shape up on E to Z photoisomerization [1–7]. Azodyes
have a reasonable dichroic ratio for reflective liquid crystal device
which are of considerable interest owing to their remarkably lower
power consumption [8].
Until early 1970s, it was believed that compounds with terminal
phenolic or amino groups would not be conducive to mesomorphism
due to intermolecular hydrogen bonding which would lead to nonlinear
arrangement of molecules. Schroeder and Schroeder [9] reported a few
terminally hydroxyl substituted mesogens. Vora and Gupta [10] for the
Address correspondence to Sandhya Dixit, Department of Applied Chemistry,
Faculty of Technology and Engineering, The M. S. University of Baroda, P.O. Box 51,
Kalabhavan, Vadodara 390-001, India. E-mail: snddxt_20@yahoo.com
43

44
S. Dixit and R. A. Vora
first time reported mesogenic homologous series with terminal and lat-
eral hydroxyl groups. Subsequently, a few more such homologous ser-
ies with lateral hydroxyl groups have been reported [11–13]. In the
present study, two mesogenic homologous series [series I and II] con-
taining a lateral hydroxyl group have been synthesized.
EXPERIMENTAL
Characterisation
Microanalysis of the compounds were performed on a Coleman
carbon–hydrogen analyzer, and they were in conformity with the
theoretical values. IR spectra were recorded on a Perkin-Elmer spec-
1
trophotometer and Nicolet Impact 400 FTIR. H NMR Spectra were
recorded on a Perkin-Elmer (90 MHz) and (60 MHz) spectrometers.
The liquid crystalline properties were investigated on a Leitz–
Labourlux 12 POL microscope provided with a heating stage. DSC
was investigated on a Metler TA-4000 system.
Synthesis of Series I and II
4-Hydroxy benzoic acid, the appropriate n-alkyl halides, ethyl-p-
aminobenzoate, and paracetamol were used as received. Solvents were
dried and distilled prior to use. The route of synthesis for series I and
II is illustrated in the Scheme 1.
4-n-hexyloxy aniline was synthesized by reported methods [14,15].
4-Carbethoxy-20-40-dihydroxy azobenzene (DA-1) and 4-n-hexyloxy-
2’4’-dihydroxy azobenzene (DA-2) were synthesized using conven-
tional methods of diazotization and coupling [16]. 4-n-Alkoxy benzoic
acids and 4-n-alkoxy benzoyl chlorides were synthesized by the
modified method of Dave and Vora [17]. The two series, namely,
4-carbethoxy-[20-hydroxy 400-n-alkoxy benzoyloxy] azobenzenes and
4-n-hexyloxy-20 hydroxy -40-(400-n-alkoxy benzoyloxy) azobenzenes,
were synthesized by condensing equimolar quantities of 4-n-alkoxy-
benzoyl chlorides with DA-1 and DA-2, respectively, in dry pyridine
[17]. The mixture was allowed to stand overnight at room tempera-
ture. It was acidified with cold 1:1 aqueous hydrochloric acid. The solid
obtained was separated and washed successively with saturated aqu-
eous NaHCO3 and water. The final product was recrystallized several
times from acetic acid until constant transition temperatures were
obtained. They are recorded in Table 1. The elemental analysis of
all the compounds were found to be satisfactory and are recorded in
1
Table 2. IR and H NMR spectral data are given below.

Azomesogens Having a Lateral Hydroxy Substituent
45
SCHEME 1 Synthetic route to series I and II compounds.

46
S. Dixit and R. A. Vora
TABLE 1 Transition Temperatures (ꢂC) for Series I and II
R ¼ n
Sc
N
I
Series I
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
–
–
–
–
–
–
–
–
101
102
103
96
137
108
116
138
127
101
105
126
125
128
132
130
250
255
245
250
230
235
230
215
197
185
177
170
Decyl
Dodecyl
Tetradecyl
Hexadecyl
Series II
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
–
–
–
–
–
–
–
–
133
105
119
132
117
99
259
256
242
243
225
230
218
220
215
210
200
190
93
108
110
116
120
122
–
93
90
86
FTIR (KBR) Spectra cmꢀ1
=
Series I (nC12H25): 3433 (-OH), 2920, 2800, 1732 (-COO-), 1604 (-N N-),
1469, 1257, 1157, 1100, 1050, 1000, 850, 750, 700.
=
Series II (nC2H5): 3676, 3429 (-OH), 2923, 1728 (-COO-), 1604, (-N N-),
1504, 1469, 1307, 1245, 1153, 1064, 975, 844, 750, 663.
=
Series II (nC12H25): 3568 (-OH), 2920, 2850, 1732 (-COO-), 1605 (-N N-),
1508, 1469, 1399, 1249, 1168, 1006, 975, 884, 759, 690.
Proton NMR Spectra (Solvent CDCl3) 90 MHz, d ppm,
Standard TMs)
Series I (nC10H21): d 0.9–1.8 (m, 22H, 6H of 2 ꢁ CH3 at C-1, C–400 and
16 H of (8 ꢁ CH2) at C–400, 4.0 (t, 2H of -OCH2 at C – 400), 4.4 (q, 2H of

Azomesogens Having a Lateral Hydroxy Substituent
47
TABLE 2 Elemental Analysis of Series I and II Compounds
% Calculated
% Found
R ¼ n-alkyl group
C
H
N
C
H
N
Series I
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
65.71
66.35
66.96
67.53
68.06
68.57
69.04
69.49
70.32
71.08
71.76
72.38
4.76
5.06
5.35
5.62
5.88
6.12
6.34
6.56
6.95
7.31
7.64
7.93
6.66
6.45
6.25
6.06
5.88
5.71
5.55
5.40
5.12
4.87
4.65
4.44
65.34
66.75
66.77
68.01
68.13
69.00
69.06
69.19
69.85
70.78
71.87
71.97
5.06
5.41
5.48
5.23
5.69
6.11
6.08
6.06
7.05
7.41
7.96
8.22
6.86
6.65
6.08
6.45
5.72
5.94
5.71
5.88
4.83
4.53
4.29
3.99
Decyl
Dodecyl
Tetradecyl
Hexadecyl
Series II
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
69.74
70.12
70.58
71.02
71.42
71.81
72.18
72.52
73.17
73.75
74.28
74.77
6.25
6.49
6.72
6.93
7.14
7.33
7.51
7.69
8.01
8.30
8.57
8.81
6.25
6.06
5.88
5.71
5.55
5.40
5.26
5.12
4.87
4.65
4.44
4.25
70.17
70.60
70.35
70.83
71.00
72.37
72.59
72.13
73.51
74.16
74.42
74.32
6.34
6.23
6.39
6.65
6.77
7.49
7.61
7.78
7.80
7.95
8.93
8.96
5.88
6.44
6.05
5.25
6.00
5.35
5.40
4.80
5.26
5.04
4.83
3.89
CH2 of ester at C-1), 6.9 (m, 4H at C-3, C-5, C-300, C -500) 7.7 – 8.15
(m, 7H at C-2, C–6, C-30, C–50, C–60 C–200 and C–600).
Series II (nC5H11): d 0.9 (t, 6H, 2 ꢁ CH3 at C-1 and C–400), 1.2–1.8 (m,
14H, 7 ꢁ CH2 at C–1 and C–400) 4.0 (t, 4H, 2 ꢁ CH2 of O–CH2 at C–1
and C–400), 7.0 (d, 6H at C–2, C–6, C–300, C–500, C–3 and C–5), 7.7–8.3
(m, 5H at C–60, C–30, C–50, C–200 and C–600).
Calorimetry
Calorimetry is a valuable method for the detection of phase transition.
Conclusions may be drawn concerning the nature of the phases that
participate in the transition with the help of calorimetric studies. In
the present study, enthalpies of dodecyl homologues of both the series

48
S. Dixit and R. A. Vora
TABLE 3 DSC Data for Series I and II Compounds
Series
I
n
Transition
DH (J gꢀ1
)
DS (Jgꢀ1 Kꢀ1
)
12
Cr–Sm
Sm–N
N–I
Cr–Sm
Sm–N
N–I
36.316
0.1511
0.559
45.671
0.736
0.0972
0.001293
0.001262
0.1267
0.00197
0.00399
II
12
1.919
were measured by differential scanning calorimetry, and the data is
recorded in Table 3.
RESULTS AND DISCUSSION
All the members of series I: 4-Carbethoxy – [20-hydroxy 40 – (400-n-alkoxy
benzoyloxy] azo benzenes and series II: 4-n-Hexyloxy – 20-hydroxy – 40-
(-400-n-alkoxy benzoyloxy)] azo benzenes exhibit enantiotropic nematic
FIGURE 1 The phase behavior for series I.

Azomesogens Having a Lateral Hydroxy Substituent
49
FIGURE 2 The phase behavior for series II.
phases. The higher homologues exhibit smectic mesophases. The
smectic phase persists up to the last member of the series. The plot of
transition temperatures against the number of carbon atoms in the
alkoxy chain (Figs. 1 and 2) shows a steady fall in nematic isotropic
transition temperatures and exhibit odd and even effect.
Table 4 summarizes the average thermal stabilities and the com-
parative geometries of the present series I and II, and the structurally
related series A [18], B [19], and C [20]. Table 4 shows that the
nematic thermal stabilities of series I and II are almost the same. This
indicates that the net effect imparted due to the end-to-end cohesive
forces in both the series viz. series having carbethoxy group as well
as the series having polarisable n-hexyloxy chain, is similar, as all
the other molecular forces are the same.
In Table 4, we have the following:

50
S. Dixit and R. A. Vora
The smectic thermal stability of series I is a little higher (110C)
than that of series II, which can be explained by the fact that the
dipole moment of the terminal –C2H5COO- group in series I, operat-
ing along the long axis of molecules, contribute more to the lateral
attractive forces of series I compared to the polarizable n-hexyloxy
group, resulting in the higher smectic thermal stability of series I.
An early appearance of the smectic phase in series I as compared
to that in series II, can also be attributed to the increased polarity
as explained above.
Table 4 further shows that the molecules of series I and series II are
longer and more polarizable due to the presence of terminal groups at
the both ends, whereas the molecules of series A have a terminal group
at one end only. The length and polarizability are known to enhance
mesogenic properties hence thermal stabilities of series I and series II
should be higher than that of series A, which is the case.
Reference to the geometry of series II and series B show that they
have similar structure except in their terminal substitutions. The
Series II has n-hexyloxy group, whereas the series B has n-butoxy
TABLE 4 Comparative Geometry and Average Thermal Stabilities of Series I
and II, A, B, and C
Transitions
I
II
A
–
B
–
C
Smectic to nematic or Isotropic
130
119.3
144.25
(C12–C16
)
Nematic to isotropic (C1–C8)
Commencement of smectic phase
238.75 236.62 150.8 (C4 – C8) 247.37 155.6
C10 C12 C4
–
–

Azomesogens Having a Lateral Hydroxy Substituent
51
group at that terminus. The nematic thermal stability of the series B is
comparatively higher than that of series II. This may be due to the fact
that in series II, the alkyl chain length is longer by two carbon atoms
as compared to the series B. It is known [21] that increase of methy-
lene spacer in alkoxy group decreases the end-to-end cohesive forces.
This in turn results in lowering of nematic–isotropic transition tem-
peratures. Therefore, series B exhibits higher N–I average thermal
stability compared to series II.
The common aspects between series I, II, and C are that they all
have three phenyl nuclei and an ester central linkage and a lateral
phenolic (-OH) group. The lateral phenolic (-OH) group is on the cen-
tral phenyl ring in series I and II, while it is on the terminal phenyl
ring in series C. The difference between series I, II, and C is in the
central linkage. Series I and II have an azo central linkage, whereas
series C has an azomethine central linkage. Additionally, series I
and II have terminal ester and n-alkoxy substituent, whereas series
C has n-alkoxy substituent at one end only. The terminal group
enhances length and overall polarizabilities of the molecules which
results in higher nematic thermal stabilities of series I and II. This
indicates that the shifting of –OH group on the terminal phenyl ring,
ortho to Schiff base linkage, without n-alkoxy group at that end, low-
ers the nematic–isotropic thermal stabilities of the series C compared
to the series I and II.
The average smectic thermal stability of the series C is higher than
that of the series I and II, even though molecules of the series C are
shorter in length than those of series I and II. The smectic phase also
appears early in the series C. Vora and Gupta [20] have explained this
behavior in the series C based on intramolecular hydrogen bonding
with the schiff base linkage and the close packing of molecules in
layers due to a gap arising from the overall geometry of the molecules.
ACKNOWLEDGMENT
The authors thank Dean Prof. B. S. Parekh, Dean, Faculty of Tech-
nology and Engineering, and Dr. C. N. Murthy, Head, Department
of Applied Chemistry, for providing research facilities.
REFERENCES
[1] Ruslim, C., & Ichimura, K. (1998). Chem. Lett., 789–790.
[2] Kozlovsky, M. V., Shibaev, V. P., Stakhanov, A. I., Weyrauch, T., & Iaase, W. I.
(1998). Liq. Cryst., 24, 759.

52
S. Dixit and R. A. Vora
[3] Shibaev, V. P., Kostomin, S. A., & Ivanov, S. A. (1996). In Polymers as Electro-
Optically and Photo-Optically Active Media, V. P. Shibaev (Ed.), Springer:
Berlin, 37.
[4] Ikeda, T., & Tsutsumi, O. (1995). Science, 268, 1873.
[5] Anderle, K., & Wendorff, J. (1994). Mol. Cryst. Liq. Cryst., 243, 51.
[6] Hermann, D. S., Rudquist, P., Ichimura, K., Kudo, K., Komitov, L., & Lagerwall,
S. T. (1997). Phys. Rev. E., 55, 2857.
[7] Asaki, T., Ikeda, T., & Ichimura, K. (1994). J. Am. Chem. Soc., 116, 625.
[8] Bauman, D. (1988). Mol. Cryst. Liq. Cryst., 159, 197.
[9] Schroeder, D. C., & Schroeder, J. P. (1974). J. Am. Chem. Soc., 96, 4347; Schroeder,
D. C., & Schroeder, J. P. (1976). J. Org. Chem., 41, 2566.
[10] Vora, R. A., & Gupta, R. (1979). Mol. Cryst., Liq. Cryst. Lett., 56, 31; Vora, R. A., &
Gupta, R. (1980). In Liquid Crystals, Chandrasekhar, S. (Ed.), Heyden Verlag: 589.
[11] Bayle, J. P., Berdague, P., Ho, M. S., & Fung, B. M. (1993). Liq. Cryst., 14, 667.
[12] Bayle, J. P., Perez, F., Berdague, P., Judeinstein, P., Allouchi, H., Chasseav, D.,
Cotrait, M., & Lapontaine, E. (1995). Liq. Cryst., 19, 345.
[13] Bayle, J. P., Perez, & Judeinstein, P. (1995). New. J. Chem., 19, 1015.
[14] Vyas, G. M., & Shah, N. N. (1963). Org. Synth. Cl. (Revised edition of annual
Vol. 30–39) John Wiley and sons, Inc.: New York, Vol. VI, p. 836.
[15] Giswell, T. R., Klandermann, B. H., & Batesey, B. C. Mol. Cryst. Liq. Cryst., 22,
211.
[16] Vogel, A. I. (1989). Aromatic compounds: Aromatic amines, formation of dia
azonium salts and their coupling reactions. In: Text Book of Practical Organic
Chemistry, 5th Edn., ELBS, and Longmann Group, Ltd., p. 946.
[17] Dave, J. S., & Vora, R. A. (1970). In Liquid Crystals and Ordered Fluids,
Johnson, J. F. & Porter, R. S. (Eds.) (Plenum Press), p. 477.
[18] Nandedkar, H. (1999). Ph.D. Thesis, Applied Chemistry Dept., M. S. University of
Baroda.
[19] Kevat, J. B. (1998). Ph.D. Thesis, Applied Chemistry Dept., M. S. University of
Baroda, Baroda.
[20] Vora, R. A., & Gupta, R. (1979). Mol. Cryst. Liq. Cryst., 56(7), 31.
[21] Gray, G. W. (1962). Molecular Structure and Properties of Liquid Crystal, Academic
Press: London.