Supramolecular hydrogen-bonded liquid crystals formed from 4-(4′-pyridylazophenyl)-4″-alkoxy benzoates and 4-substituted benzoic acids

Article abstract of DOI:10.1080/15421400802198631

Four pyridine-based derivatives (I10, I12, I14, and I18) with molecular formula CnH2n+1O-C6H4COOC6H4-N=N-C5H4N, that differ from each other in the number of carbon atoms, n, were prepared and characterized for their mesophase behavior. The number of carbons in the alkoxy chain varies between 10, 12, 14, to 18 atoms, respectively. Liquid crystalline properties were studied for two series of hydrogen-bonded liquid crystal complexes, consisting of the central 4-pyridylazophenyl benzoate and substituted benzoic acids. In the first series of the supramolecular complexes (group A), the lengths of the alkoxy chains on both of the azo pyridine and the benzoic acid components have varied between 10, 12, 14, and 18 carbons. In the second series (group B), the same proton-acceptors, i.e., the pyridyl azo derivatives, were used but with benzoic acids para-substituted with small compact polar groups, X. All of the newly prepared pyridine-based derivatives and their hydrogen-bonded complexes were found to be mesomorphic. The newly prepared pyridine-based derivatives and their supramolecular complexes in both groups were characterized for their mesophase behavior by differential scanning calorimetry, DSC, and polarized light microscopy, PLM. Four 4-alkoxybenzoic acids (CmH2m+1OC6H4COOH, II10, II12, II14, and II18) were used in group A series; the number of carbons (m) of their alkoxy groups also varies between 10, 12, 14, and 18, respectively. Substituents on the other group of acids (IIIa-f) in group B series vary, respectively, between CH3O, CH3, H, Br, CN, and NO2. Smectic C mesophase is induced in most of the binary mixtures investigated.

Full text of DOI:10.1080/15421400802198631

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Supramolecular Hydrogen-
Bonded Liquid Crystals
Formed from 4-(4′-
Pyridylazophenyl)-4″-alkoxy
Benzoates and 4-Substituted
Benzoic Acids
M. M. Naoum a , A. A. Fahmi a & M. A. Alaasar a
a Department of Chemistry, Faculty of Science ,
Cairo University , Cairo , Egypt
Published online: 31 Aug 2012.
To cite this article: M. M. Naoum , A. A. Fahmi & M. A. Alaasar (2008) Supramolecular
Hydrogen-Bonded Liquid Crystals Formed from 4-(4′-Pyridylazophenyl)-4″-alkoxy
Benzoates and 4-Substituted Benzoic Acids, Molecular Crystals and Liquid Crystals,
487:1, 74-91, DOI: 10.1080/15421400802198631
To link to this article: http://dx.doi.org/10.1080/15421400802198631
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Mol. Cryst. Liq. Cryst., Vol. 487, pp. 74–91, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802198631
Supramolecular Hydrogen-Bonded Liquid Crystals
Formed from 4-(40-Pyridylazophenyl)-400-alkoxy Benzoates
and 4-Substituted Benzoic Acids
M. M. Naoum, A. A. Fahmi, and M. A. Alaasar
Department of Chemistry, Faculty of Science, Cairo University,
Cairo, Egypt
Four pyridine-based derivatives (I10, I12, I14, and I18) with molecular formula
=
CnH2nþ1O-C6H4COOC6H4-N N-C5H4N, that differ from each other in the number
of carbon atoms, n, were prepared and characterized for their mesophase behavior.
The number of carbons in the alkoxy chain varies between 10, 12, 14, to 18 atoms,
respectively. Liquid crystalline properties were studied for two series of hydrogen-
bonded liquid crystal complexes, consisting of the central 4-pyridylazophenyl
benzoate and substituted benzoic acids. In the first series of the supramolecular
complexes (group A), the lengths of the alkoxy chains on both of the azo pyridine
and the benzoic acid components have varied between 10, 12, 14, and 18 carbons.
In the second series (group B), the same proton-acceptors, i.e., the pyridyl azo
derivatives, were used but with benzoic acids para-substituted with small compact
polar groups, X. All of the newly prepared pyridine-based derivatives and their
hydrogen-bonded complexes were found to be mesomorphic. The newly prepared
pyridine-based derivatives and their supramolecular complexes in both groups
were characterized for their mesophase behavior by differential scanning
calorimetry, DSC, and polarized light microscopy, PLM. Four 4-alkoxybenzoic
acids (CmH2mþ1OC6H4COOH, II10, II12, II14, and II18) were used in group A
series; the number of carbons (m) of their alkoxy groups also varies between 10,
12, 14, and 18, respectively. Substituents on the other group of acids (IIIa ꢀ f) in
group B series vary, respectively, between CH3O, CH3, H, Br, CN, and NO2.
Smectic C mesophase is induced in most of the binary mixtures investigated.
Keywords: 4-(40-pyridylazophenyl)-400-alkoxy benzoates; 4-substituted benzoic acids;
supramolecular LCs
INTRODUCTION
Binary mixtures of liquid crystalline materials can often lead either
to new liquid crystal phases, not observed in the corresponding pure
Address correspondence to M. M. Naoum, Department of Chemistry, Faculty of
Science, Cairo University, Cairo, Egypt. E-mail: magdinaoum@yahoo.co.uk
74

Supramolecular Hydrogen-Bonded Liquid Crystals
75
systems, or to the extension of the temperature range over which
their mesophases are stable [1,2]. The enhancement of the liquid
crystalline region often occurs because of the depression in the melt-
ing point, which reaches a minimum at the eutectic composition.
However, for certain materials, it is possible to observe, instead of
melting point depression, liquid crystal phases that are formed via
tailoring the molecular interaction between the components of the
mixture, thus, increasing the stability of the induced mesophase,
i.e., increasing the mesophase—isotropic transition temperature,
TC [3–7].
Supramolecular liquid crystals are frequently induced by hydrogen
bonding between aromatic carboxylic acids as proton donor and pyri-
dyl fragments as proton acceptors. In such cases, hydrogen bonding
leads to an elongation of the rigid rod segment of the individual com-
ponents, and, in this manner hydrogen-bonded liquid crystal com-
plexes are obtained [8–10].
We have recently [11] reported the preparation and characteriza-
tion of new liquid crystal materials based on supramolecular hydro-
gen-bonded complexes between 4-alkoxybenzoic acids (as the proton
donor) and 4-(40-pyridylazophenyl)-400-substituted benzoates (as the
proton-acceptors). The latter was designed in such a manner as to
combine an azo, ester, and pyridine moieties in one and the same
molecule. Although none of the therein investigated pyridine-based
derivatives was mesomorphic [11], the long tail of the alkoxy group
on the benzoic acid component furnishes complexes with rod-
shaped assembly, and, consequently, the resulting supramolecular
structures were found to be mesomorphic. Furthermore, the binary
phase diagrams reported therein [11], revealed that the electron-
donating substituent (CH3O or CH3), on the pyridine-based moiety,
enhances the stability of the hydrogen-bond, and a complex is
observed (in either the solid or the mesophase), irrespective of
the length of the alkoxy-chain on the acid. These findings have
encouraged us to extend our investigation to prepare, hopefully,
pyridine-based liquid crystals via replacing the polar substituent
(X) by an alkoxy group of varying chain length. This may, on one
hand, render the derivative more or less mesomorphic, and on
the other hand, lead to an increase of the electron-density on the
pyridine nitrogen, due to its powerful electron-donating efficiency,
as the CH3O group did in our previous investigation [11].
The goal of the present study is, first, to prepare a new group of
pyridine based derivatives (I10, I12, I14, and I18) bearing, this time,
an alkoxy group with a number of carbon atoms varying between 10,
12, 14, and 18, respectively, and to investigate their phase behavior

76
M. M. Naoum et al.
via DSC and PLM. The second aim of this investigation is to rule out
their ability towards supramolecular hydrogen-bond formation, indi-
vidually, with two groups of benzoic acids. In the first, II10, II12,
II14, and II18, the substituents are 4-alkoxy group with a number
of carbon atoms varies between 10, 12, 14, and 18 carbons, respect-
ively. In the second group of acids (IIIa–f), the substituents X are of dif-
ferent polarity that changes between CH3O, CH3, H, Br, CN, and NO2,
respectively.
EXPERIMENTAL
Chemicals were purchased from the following Companies: Fluka,
Buchs, Switzerland; MP Biomedicals, Inc., Illkirch, France; BDH,
Poole, England; Aldrich, Wisconsin, USA; and E. Merck, Darmstadt,
Germany.
Preparation of Materials
All benzoic acids (II10–II18) were prepared according to the methods
described previously [12]. Transition temperatures of all acids, as
given in Table 1, agreed with those reported before [10].

Supramolecular Hydrogen-Bonded Liquid Crystals
77
TABLE 1 Phase Transition Temperatures (ꢁC) of the acids, IIm
Compound number
m
TS-A
TA-N
TS-I
TN-I
II10
II12
II14
II18
10
12
14
18
101
93
89
122
129
–
–
–
133
126
141
137
–
109
–
–
The pyridine-based azo dyes (In) were prepared according to the fol-
lowing scheme:
Preparation of 4-(40-Pyridylazo)-Phenol (A)
This was prepared according to the method described by Zhang et al.
[13] from 4-aminopyridine and phenol. To a solution of 4.0 g (0.058 mol)
of sodium nitrite in 20 ml water, 5.0 g (0.053 mol) phenol in 45 ml of
10% sodium hydroxide solution (0.1125 mol) was added with stirring
while cooling both solutions to 0ꢁC. The resulting solution was added
to a cooled solution of 4-aminopyridine (6.0 g, 0.064 mol) in 25 ml
(0.25 mol) concentrated hydrochloric acid and 16 ml water with stirring
for 10 minutes below 0ꢁC. Subsequently, a saturated solution of sodium
carbonate was added to adjust the pH of the mixture to about 6. A yel-
low dye is formed which was filtered off and used for subsequent steps
without purification. The yield of the crude product was 75%.
Preparation of 4-(40-Pyridylazophenyl)-400-alkoxy Benzoates
(I10–I18)
The azo A (0.60 g, 0.0027 mol) with its molar equivalent of the acid
(IIm), 1.11 g (0.0054 mol) of DCC, and few crystals of the catalyst,

78
M. M. Naoum et al.
4-(dimethylamino) pyridine (DMAP), were dissolved in 100 ml (1:1)
mixture of CH2Cl2 and THF; the mixture was left to stand, at room
temperature with stirring, for at least 72 hrs. The solid was filtered
off, washed with 20 ml CH2Cl2, and the filtrate allowed to evaporate.
The residue was then soaked in methanol for 2 hrs with stirring and
filtered. The solids (ꢂ50% yield) obtained were found to be TLC pure
and possess sharp transition temperatures as measured by DSC and
are given in Table 2.
Confirmation of Molecular Structure
The molecular formulae of the newly prepared pyridine-based
derivatives (II10–II18) were confirmed via elemental analyses, infra-
red, NMR, and mass spectroscopy. The results agreed, within the
permissible limits, with the proposed structures.
Physical Characterization
Infrared absorption spectra were measured with a Perkin-Elmer B25
1
spectrophotometer, and H-nmr spectra with a Varian EM 350L.
Calorimetric investigations were carried out using a PL-DSC of
Polymer Laboratories, England. The instrument was calibrated for
temperature, heat, and heat flow according to the method recom-
mended by Cammenga et al. [14]. DSC measurements were carried
out on small samples (2–3 mg) placed in sealed aluminum pans. All
of the thermograms have been achieved at a heating rate of 10ꢁC=min
in inert atmosphere of nitrogen gas (10 ml=min).
Transition temperatures were checked and types of mesophases
identified for mixtures prepared and their pure components with a
TABLE 2 Transition Temperatures (ꢁC) and Transition Enthalpies of the
Pyridine-Based Azo Dyes, In
Compound number
n
Tm
DHm (kJ=mol)
TA-I
DHA-I (kJ=mol)
I10
I12
I14
I18
10
12
14
18
96.6
104.2
96.2
26.96
43.72
31.00
29.72
128.8
135.8
–
1.71
4.87
–
99.0
119.5
2.98

Supramolecular Hydrogen-Bonded Liquid Crystals
79
standard polarized light microscope PLM (Wild, Germany) attached to
a home made hot-stage.
Thin-layer chromatography was performed with TLC-sheets coated
with silica gel (E. Merck), whereby spots were detected by a uv-lamp.
Supramolecular complexes in the 1:1 molar ratio of any two compli-
mentary components were prepared by melting the appropriate
amounts of each component, stirring to give an intimate blend, and
then cooling with stirring to room temperature. The transition
temperatures obtained for the prepared complexes, as measured by
both DSC and PLM, agreed within 2–3ꢁC. In the figures, constructed
by plotting transition temperatures versus the alkoxy-chain length
(n or m), the symbol ‘‘ꢁ’’ denotes solid-mesophase, ‘‘&’’ mesophase-iso-
tropic transitions, TC, and ‘‘ꢃ’’ mesophase-another mesophase.
RESULTS & DISCUSSION
Infrared Absorption Spectra of Pyridine-Based Derivatives
Infrared Spectra of 4-(40-Pyridylazophenyl)-400-Substituted
Benzoates (IVa–e
)
Infrared absorption bands of the previously [11] investigated
pyridine based series IVa–e, with their assignments are given in
Table 3. It can be noted from the table that, due to the variation in
the electronic nature of the substituent X, its mesomeric interaction
with the ester C¼O group, via the intervening phenylene ring, will
be different. Thus, the electron withdrawing (Br or NO2) group raises
ꢀ1
=
the C O stretching frequency (1743.5 cm ) in Id or Ie. Conversely, the
electron-releasing character of the CH3O or CH3 groups resulted in
compounds that absorb at lower frequencies (1728 cmꢀ1). Similar con-
clusions were arrived at [12] for the 4-alkoxyphenyl-4-substituted
=
benzoates, where the ester C O absorptions occur at 1742 and
1729 cmꢀ1, for the nitro- and methoxy- substituted derivatives,
TABLE 3 Infrared Absorption Spectra of 4-(40-Pyridylazophenyl)-400-
Substituted Benzoates (IVa–e
)
Compound
X
nC-H aromatic
nC¼O
nN¼N
nC-O ester
nC-O ether
nNO2
IVa
IVb
IVc
IVd
IVe
CH3O
CH3
H
Br
NO2
2920
2920
2920
2916
2931
1728
1747
1728
1743
1697
1585
1593
1585
1582
–
1273
1273
1269
1265
1273
1207
1227
1207
1223
1234
–
–
–
–
1527
1342

80
M. M. Naoum et al.
respectively. That is, electron-withdrawing substituent in the first
compound shifts the absorption to higher frequency; while conjugative
interaction leads to lowering the absorption frequency in the later.
Other absorption frequencies, as given in Table 3 are in consistency
with the structures proposed.
Infrared Spectra of 4-(40-Pyridylazophenyl)-400-alkoxy
Benzoates (In)
The corresponding infrared absorption bands of the series In are
given in Table 4. The nearly identical infrared spectra observed for
the four homologues investigated revealed that the mesomeric shift
of the ester C¼O group attached to the 4-position with respect to the
alkoxy group is not significantly affected by the change in the length
of the alkoxy chain. Fore example, the C-H stretching frequency is cen-
ꢀ1
=
tered, in all compounds of the series, at 2920 cm , and the azo–N N-
band is located at 1585 cmꢀ1. The stretching frequency of the ester C-O
and ether C-O linkages are always found in its proper area, at 1285,
and 1211 cmꢀ1, respectively. Again, other ir-bands are found (Table
4) to confirm the structures proposed.
DSC Analyses
Trends in the mesophase-isotropic liquid transition temperatures (TC)
of liquid crystalline compounds were rationalized in terms of molecu-
lar structure, even for subtle differences as in homologous series. Two
types of terminal substituents may be utilized: the homologous series
and the small compact polar type.
Generally, complementary components in a mixture employed in
the formation of liquid crystals may be either mesomorphic or nonme-
somorphic or both; however, novel liquid crystals can be obtained.
Hydrogen bonding between the two components of the mixture leads
to an elongation of the rigid-rod segment of the individual components
TABLE 4 Infrared Absorption Spectra of 4-(40-Pyridylazophenyl)-40-alkoxy
Benzoates (In)
Compound
nC-H aromatic
nC¼O
nN¼N
nC-O ester
nC-O ether
I10
I12
I14
I18
2920
2920
2920
2916
1724
1728
1728
1736
1581
1585
1581
1585
1285
1285
1285
1288
1211
1211
1211
1215

Supramolecular Hydrogen-Bonded Liquid Crystals
81
as represented, for our investigated systems, by formula 1:
Transition temperatures of the investigated acids (IIm), as reported
previously [10], are summarized in Table 1. Transition temperatures
and transition enthalpies of pyridine-based derivatives (In) are given
in Table 2. In these tables, the following symbols are used as sub-
scripts to specify the various transition temperatures: solid (S),
nematic (N), smectic A (A), smectic C (C), and isotropic (I). Transitions
of solid to any liquid phase are also denoted as melting point (Tm),
whereas the mesophase-isotropic transition also as clearing tempera-
tures (TC). It can be seen from the table that all of the pyridine based
compounds investigated are of high melting (Tm) points, and all of
them are mesomorphic.
In order to investigate the effect of the alkoxy-chain length, on the
liquid crystalline properties of the supramolecular complexes of the
type as given in Table 5 for group A, the number of carbons in the ter-
minal alkoxy group on the pyridine-based derivatives was once kept
constant at 10, 12, 14, and 18 atoms, while that on the acid comp-
lementary component was alternatively altered between 10 and 18
carbons. Figure 1 correlates the phase transition temperatures of each
pyridine-based homologue with the alkoxy-chain length (m) of the
acid. Alternatively, Fig. 2 reproduces the dependencies, on the chain
length (n) of the pyridine-based component, of the phase transition
temperature of 1:1 complex formed between each homologue of the
acid and the different homologues of the pyridine-based derivatives.
As can be seen from both groups of figures, the Sm C mesophase is
induced together with the Sm A phase in the lower homologues of
either the proton donor (the acid) or proton-acceptor (the base). The
temperature range of the Sm C mesophase increases on the expense
of the Sm A phase so that it appears as the only mesophase for the
highest homologue (m ¼ 18), see Fig. 2d. It can also be noticed
from the figures that the stability of the Sm C phase, i.e., TC-I
,
decreases slightly with increase of the alkoxy-chain length on either

82
M. M. Naoum et al.
TABLE 5 Phase Transition Temperatures (ꢁC) of the Supramolecular
Complexes (Group A) Formed Between the Pyridine-Based Derivatives (In)
and the 4-alkoxy Benzoic Acids (IIm)
Base
Acid
TS-A
TS-C
TA-C
TA-I
TC-I
I10
II10
II12
II14
II18
II10
II12
II14
II18
II10
II12
II14
II18
II10
II12
II14
II18
98.6
96.1
99.0
96.3
96.9
102.0
101.1
99.9
96.0
99.9
103.8
104.5
–
–
–
–
–
–
–
–
–
–
164.8
160.0
156.3
–
163.6
163.1
158.2
158.0
146.5
151.0
156.4
153.1
–
–
–
–
171.2
166.1
163.0
–
158.1
I12
I14
I18
–
–
–
–
–
–
–
–
–
–
–
–
171.9
172.0
168.1
161.9
171.2
170.5
167.8
163.2
166.1
164.6
159.6
158.0
–
–
–
96.4
101.9
105.6
111.6
–
–
–
–
–
–
components. On the other hand, the stability of the Sm A mesophase,
i.e., TA-C, decreases as the alkoxy-chain length of the pyridine-based
constituent is lengthened and disappeared completely at n ¼ 18.
Furthermore, except for the lower homologues, the melting points,
i.e., TS-A or TS-C, are found to be regularly, but slightly, increased with
the length of the alkoxy chain on either component.
Table 6 and Fig. 3 illustrate the dependences on the pyridine-based
alkoxy-chain length (n) of transition temperatures of the supramolecu-
lar complexes (group B) formed between the pyridine-based deriva-
tives (In) and the substituted benzoic acids (IIIa–f).
As can be seen from Fig. 3, the electron-withdrawing substituent
(Br, CN, or NO2), on the acid segment of the supramolecular
complex, induces a stable Sm C mesophase that covers a wide range

Supramolecular Hydrogen-Bonded Liquid Crystals
83
FIGURE 1 Effect of acid-chain length (m) on the transition temperature of
the supra molecular complexes of Group A series, (a) n ¼ 10, (b) n ¼ 12, (c)
n ¼ 14, and (d) n ¼ 18.
of temperatures and is independent of the length of the pyridine-based
alkoxy chain. At the same time, the Sm A mesophase occupies a rela-
tively small temperature range that precedes the Sm C phase.
Conversely, the electron-donating (CH3O or CH3) substituted, as well
as the unsubstituted acids, furnish complexes possessing mainly the
Sm A mesophase with a small Sm C temperature range that disap-
peared completely in the complexes of the higher homologues of the
base (i.e., when n > 14).
It is worthy here to compare the results of Group B with those of the
previously [11] investigated series (group C), where the substituent X
is interchanged with the alkoxy group.

84
M. M. Naoum et al.
FIGURE 2 Effect of Azo-chain length (n) on the transition temperature of
the supramolecular complexes of Group A series, (a) m ¼ 10, (b) m ¼ 12, (c)
m ¼ 14, and (d) m ¼ 18.
The chain-length (m) dependencies of the transition temperatures of
group C complexes are illustrated graphically in Fig. 4. In this
group of complexes, it was observed that polar substitutents, in the
pyridine-based component, are more or less pronounced in promoting
the Sm C mesophase. Thus, each of the electron-withdrawing (Br or
NO2), electron-donating (CH3O or CH3), and the un-substituted
pyridine-based derivatives is found to possess a considerable Sm C

Supramolecular Hydrogen-Bonded Liquid Crystals
85
TABLE 6 Phase Transition Temperatures (ꢁC) of the Supramolecular
Complexes (Group B) Formed Between the Pyridine-Based Derivatives (In)
and the 4-Substituted Benzoic Acids (IIIa–f
)
Acid
X
DaX ꢄ 1024
Base
TS-A
TA-C
TA-I
TC-I
IIIa
CH3O
3.86
I10
I12
I14
I18
I10
I12
I14
I18
I10
I12
I14
I18
I10
I12
I14
I18
I10
I12
I14
I18
I10
I12
I14
I18
83.9
95.6
98.2
96.1
80.4
85.5
84.5
82.4
94.4
86.5
89.6
89.9
81.5
89.2
90.2
93.0
85.2
88.3
89.7
92.8
82.2
87.3
85.2
89.3
152.6
157.8
–
–
163.7
161.1
–
155.8
155.4
–
–
161.9
156.5
–
–
–
–
IIIb
CH3
H
2.12
0.0
164.1
163.1
–
188.3
168.3
–
–
–
155.4
155.7
–
170.2
162.0
–
156.2
159.6
–
–
IIId
IIIe
IIIf
Br
3.11
5.12
4.58
108.0
100.5
111.2
113.0
103.2
112.2
106.5
105.7
108.2
101.7
105.7
106.2
–
–
–
–
–
–
–
–
–
–
–
–
170.1
171.3
175.2
177.9
173.9
162.1
175.8
197.7
185.8
180.4
180.5
193.2
CN
NO2
temperature range with all the investigated homologues of the acid
(IIm). The Sm A phase appeared within a small temperature range
in the case of the complexes formed between the CH3O or CH3-
substituted base and the decyloxy (m ¼ 10) benzoic acid. On the other
hand, the nitro analog showed the Sm A phase with all homologues of
the acid investigated.
Transition Temperature and Polarizability Anisotropy
Generally, the stability of a mesophase is greater, the greater the
lateral adhesion of the rod-shaped molecules, which in turn, would
be augmented by the increase in polarity and=or polarizability of

86
M. M. Naoum et al.
FIGURE 3 Effect of Azo-chain length (n) on the transition temperature of the
supramolecular complexes of Group B series, (a) X ¼ CH3O, (b) X ¼ CH3, (c)
X ¼ H, (d) X ¼ Br, (e) X ¼ CN, and (f) X ¼ NO2.
the central part of the molecule. Of course, the terminal substituent
can affect the polarizability of the aromatic rings to which they are
attached; in addition, they may interact with the lateral portion of

Supramolecular Hydrogen-Bonded Liquid Crystals
87
FIGURE 4 Effect of Acid-chain length (m) on the transition temperature of the
supramolecular complexes of Group C series, (a) X ¼ CH3O, (b) X ¼ CH3, (c)
X ¼ H, (d) X ¼ Br, (e) X ¼ NO2.
an adjacent molecule. As the polarity of the group decreases, the
clearing temperature (TC) seems to be lowered too. This was attribu-
ted [15] to increased intermolecular attractions as the polarity and
polarizability of the substituent are increased. Van der Veen [16]
has deduced an equation that relates the mesophase-isotropic tran-
sition temperatures (TC) to the polarizability anisotropy (Dax) of

88
M. M. Naoum et al.
the bonds CAr-X to small compact substituents. The relation has the
formula
2
TC aðDaM þ DaxÞ ;
ð1Þ
where TC is measured in Kelvin. The term DaM is the polarizability
anisotropy for all the molecular structures except the terminal
substituent, X. Equation (1) can be put in the following form
pffiffiffiffi
TC ¼ a ꢅ DaM þ a ꢅ DaX;
ð2Þ
p
FIGURE 5 Dependence of the TC of the supramolecular complexes of Group
B series on the polarizability anisotropy, DaX, of the polar substituent, X.

Supramolecular Hydrogen-Bonded Liquid Crystals
89
where ‘‘a’’ is the proportionality constant. The polarizability
anisotropy, DaX, of the various CAr-X bonds was calculated using
the data of LeFevre and coworkers [17–19] and the values are
p
included in Table 6. Thus, if TC is plotted against DaX for the
two series of liquid crystalline complexes (groups B and C), a
straight line is expected, the slope of which equals ‘‘a’’ and the inter-
p
cept equals ‘‘a ꢅ DaM.’’ The
TC values for group B series were
plotted as a function of DaX and the plots are depicted in Fig. 5.
p
FIGURE 6 Dependence of the TC of the supramolecular complexes of Group
C series on the polarizability anisotropy, DaX, of the polar substituent, X.

90
M. M. Naoum et al.
TABLE 7 Van der Veen Data for the Two Series B and C
Series
n or m
Slope ‘‘a’’ ꢄ 10ꢀ22
Intercept (aDaM)
Group B
10
12
14
18
10
12
14
18
0.80
2.68
9.28
16.69
6.20
8.21
1.45
ꢂ 0
21.14
20.90
20.69
20.59
20.68
20.55
20.68
20.73
Group C
For the sake of comparison, the corresponding data for the supra-
molecular hydrogen-bonded liquid crystals previously investigated
(group C) are similarly plotted in Fig. 6.
Applying the method of least squares using Eq. (2) on all the
regression data in Figs. 5 and 6, the corresponding ‘‘a and DaM’’ for
each group of data and the results are collected in False Table 7. These
results revealed that the slopes ‘‘a’’ in both series are generally small,
indicating that substitution, whether in the pyridine-based constitu-
ent or in the acid partner, is poor in promoting stability to the meso-
phase induced by the hydrogen-bond within the complex.
Nevertheless, comparing the values of any two corresponding data
in the two series (B and C) revealed that the slopes are somewhat
greater when substituent X is in the pyridine-based portion and the
alkoxy chain in the acid is short (n or m ¼ 10 and 12). The reverse
holds good for the higher two homologues (n or m ¼ 14 and 18). It is
worth remembering here that any two corresponding members in
the two series (B and C) the first is produced by the exchange of the
two terminal substituents, X- and -OR (compare series B and C).
CONCLUSIONS
Four pyridine-based azo dye derivatives of the type 4-(40-pyridylazo
phenyl)-400-alkoxy benzoates, I10–I18, in which the alkoxy chain
length varies between 10 and 18 carbons, respectively, were prepared
and investigated for their mesophase behavior. Three groups of supra-
molecular complexes, having in common, the 4-pyridylazo phenyl
benzoate=benzoic acid core, were investigated via differential scanning
calorimetry (DSC) and polarized-light microscopy (PLM). In the first
(group A), each of the pyridine-based and benzoic acid portions of
the complexes carries an alkoxy group of varying chain length. In

Supramolecular Hydrogen-Bonded Liquid Crystals
91
the second (group B), a polar substituent (X) is introduced to the acid
portion while an alkoxy group, again of varying chain length, is alter-
natively attached to the pyridine-based moiety. A reversed substi-
tution, to that in group B was realized in group C.
The study revealed that:
1. The Sm C mesophase is induced in all cases, irrespective of the
polarity of the substituent X or the length of the alkoxy group.
2. Increasing the alkoxy chain length, either in the pyridine-based or
benzoic acid moieties, enhances the Sm C mesophase to the extent
that it stands as the only mesophase when m or n ¼ 18 carbons.
3. The effect of the polar substituent X on the stability of the Sm C
phase is more pronounced when it is on the pyridine-based moiety
and the chain on the acid is short (m ¼ 10 or 12). Conversely, for
longer alkoxy chain, the Sm C stability is enhanced when the
substituent X is on the acid partner.
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