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

Article abstract of DOI:10.1080/15421400801914996

Phase diagrams of the binary mixtures of the title compounds were constructed to investigate the effect of different polar substituents, on the pyridine-based derivatives, as well as the alkoxy-chain length, on the acids, on the extent and stability of th

Full text of DOI:10.1080/15421400801914996

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Supramolecular Hydrogen-
Bonded Liquid Crystals
Formed from 4-(4′-
Pyridylazophenyl)-4″–Substituted
Benzoates and 4-Alkoxybenzoic
Acids
M. M. Naoum a , A. A. Fahmi a & M. A. Alaasar a
a Department of Chemistry, Faculty of Science, Cairo
University, Giza, Egypt
Version of record first published: 22 Sep 2010.
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″–Substituted Benzoates and 4-Alkoxybenzoic Acids, Molecular
Crystals and Liquid Crystals, 482:1, 57-70
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Mol. Cryst. Liq. Cryst., Vol. 482, pp. 57–70, 2008
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ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400801914996
Supramolecular Hydrogen-Bonded Liquid Crystals
Formed from 4-(40-Pyridylazophenyl)-400–Substituted
Benzoates and 4-Alkoxybenzoic Acids
M. M. Naoum, A. A. Fahmi, and M. A. Alaasar
Department of Chemistry, Faculty of Science, Cairo University,
Giza, Egypt
Phase diagrams of the binary mixtures of the title compounds were constructed to
investigate the effect of different polar substituents, on the pyridine-based deriva-
tives, as well as the alkoxy-chain length, on the acids, on the extent and stability
of the supramolecular liquid crystal phases induced by intermolecular hydrogen
bonding. None of the newly prepared pyridine-based derivatives is mesomorphic,
but the hydrogen-bonded complexes are. The mixtures prepared were characterized
for their mesophase behavior by differential scanning calorimetry (DSC) and polar-
ized light microscopy (PLM). Five pyridine-based derivatives (Ia-e), with molecular
=
formula X-C6H4COOC6H4-N N-C5H4N, were prepared that differ from each
other by the substituent X. The latter varies between CH3O, CH3, H, Br, and NO2.
Four 4-alkoxybenzoic acids (CnH2n þ 1OC6H4COOH, II10–II18) were used; the
number of carbons (n) of their alkoxy groups varies between 10, 12, 14, and 18.
Smectic C mesophase is induced in most of the binary mixtures investigated.
Keywords: 4-alkoxybenzoic acids; 4-(40-pyridylazophenyl)-400-substituted benzoates;
binary mixtures; supramolecular LCs
INTRODUCTION
The role of hydrogen-bonding interaction in molecular recognition and
self-assembly of liquid crystalline phases has been investigated in
recent years and significant work has been conducted [1,2]. Rigid
rod-shaped anisotropic molecules, appropriately functionalized by
recognizable moieties, interact in the melt or in solution and lead
to the formation of supramolecular complexes that may exhibit
Received July 27, 2007; accepted October 3, 2007.
Address correspondence to M. M. Naoum, Department of Chemistry, Faculty of
Science Cairo University, P.O. Box 12613, Giza, Egypt. E-mail: magdinaoum@yahoo.
co.uk
57

58
M. M. Naoum et al.
thermotropic liquid crystalline character. A number of supramolecular
mesogenic materials have been induced by self-assembly through
hydrogen bonding [2]. Following the first and well-established exam-
ples of liquid crystal formation [3] through the dimerization of
aromatic carboxylic acids, several classes of compounds have been
prepared by the interaction of complementary molecules, the liquid
crystalline behavior of which is appreciably dependent on the struc-
ture of the resulting supramolecular system. Kato and Frechet [4]
expanded the latter concept to include mixtures of 4-alkoxybenzoic
acids and pyridine based compounds. A single hydrogen bond between
the carboxyl and pyridine moieties was found [2] to be extremely fruit-
ful for the formation of liquid crystals. Azo dye molecules, on the other
hand, are attracting attention in both academic and application areas
[5–9]. The aim of the present study is to combine the pyridyl and azo
moieties in one single newly prepared pyridine-based azo dye (Ia-e
)
and to investigate possible molecular self-assembly through hydro-
gen-bonding with 4-alkoxy- benzoic acids (II10–II18).
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
previously described [10]. Transition temperatures of all acids, as
given in Table 1, agreed with those reported before [10].

Supramolecular Hydrogen-Bonded Liquid Crystals
59
TABLE 1 Phase Transition Temperatures (ꢀC) of the Acids,
II10–II18
Compound number
n
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 (Ia-e) were prepared according to the
following scheme:
Preparation of 4-(40-Pyridylazo)-phenol (A)
This was prepared according to the method described by Zhang et al.
[11] 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 0ꢀC. 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 0ꢀC. Subsequently, a saturated solution
of sodium carbonate was added to adjust the pH of the mixture to about
6. A yellow 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–Substituted
Benzoates (Ia-e
)
The azo A (0.60 g , 0.0027 mol) with its molar equivalent of the acid
(II), 1.11 g (0.0054 mol) of dicyclohexyl carbodi-imide (DCC), and few
crystals of the catalyst dimethylamino pyridine (DMAP), were dis-
solved in 100 ml (1:1) mixture of CH2Cl2 and tetrahydro furan (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,

60
M. M. Naoum et al.
TABLE 2 Melting Points (ꢀC) and Melting Enthalpies of the
Pyridine-Based Azo Dyes, Ia–e
Compound number
X
Tm
DHm (kJ=mol)
Ia
Ib
Ic
Id
Ie
CH3O
CH3
H
Br
NO2
170
162
178
186
205
32.3
31.5
20.1
11.3
65.9
and the filtrate allowed to evaporate. The residue was then soaked in
methanol for 2 hrs with stirring and filtered. The solids (50% yield)
obtained were found to be thin layer chromatography (TLC) pure
and possess sharp melting temperatures as measured by differential
scanning calorimetry (DSC) and are given in Table 2.
Physical Characterization
Calorimetric measurements 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. [12]. DSC measurements were carried
out for small samples (2–3 mg) placed in sealed aluminum pans. All
of the thermograms have been achieved at a heating rate of 10ꢀC=min
in inert atmosphere of nitrogen gas (10 ml=min).
Transition temperatures were checked and types of mesophases
identified for mixtures prepared with a standard polarized light
microscopy PLM (Wild, Germany) attached to a home-made hot-stage.
For phase diagrams, binary mixtures of any two complimentary
components, covering the whole composition range, were prepared
by melting the appropriate amounts of each component, stirring to
give an intimate blend, and then cooling by stirring to room tempera-
ture. The temperatures obtained for the prepared blends, as measured
by both DSC and PLM, agreed within 2–3ꢀC. In the phase diagrams,
constructed by plotting transition temperatures versus mixture
composition, the symbol ‘‘ ’’ denotes solid-mesophase, ‘‘&’’ mesophase-
ꢀ
isotropic transitions, ‘‘ ’’ mesophase-another mesophase, and, ‘‘~’’
eutectic temperature.
.
Conformation of Molecular Structure
The molecular formulae of the newly prepared pyridine-based deriva-
tives (Ia-e) were conducted via elemental analyses, NMR, and mass

Supramolecular Hydrogen-Bonded Liquid Crystals
61
spectroscopy. The results were in agreement, within the permissible
limits, with the proposed structures.
RESULTS AND DISCUSSION
Generally, complementary components in a mixture employed for the
formation of liquid crystals may be either mesomorphic or nonmeso-
morphic 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
as represented, for our investigated systems, by formula 1.
Transition temperatures of the investigated acids (II10–II18), as
reported previously [10], are summarized in Table 1. Melting points
and melting enthalpies of pyridine-based derivatives (Ia-e) are given
in Table 2. In the tables, the following symbols are used as subscripts
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). It can be seen
from the table that all of the pyridine-based compounds investigated
are of high melting (Tm) points, and none of them is mesomorphic.
As can be seen from Table 1, the lower homologs of the acids, II10
and II12, are polymorphic that possess both nematic, N, and smectic
A, Sm A, mesophases, while the higher homologs, II14 and II18,
are purely smectogenic possessing only SmA mesophase. This differ-
ence in behavior of the acids would definitely affect the mesophase
behavior of their mixtures with the pyridine-based derivatives in a
variable manner. The binary phase diagrams of the individual homo-
logs of the acids (II10–II18) with the pyridine derivatives (Ia-e) are
represented graphically in Figures 1–5, respectively.
Binary Mixtures of the Electron-Donating (CH3O or CH3)
Substituted Derivatives (Ia and Ib) with the Four Homologs
of the Acid
In hydrogen-bonded liquid crystals, phase behavior is related to the
strength of the hydrogen bonds involved [13]. In the pyridine=acid

62
M. M. Naoum et al.
FIGURE 1 Binary phase diagrams for the mixtures of 4-(40-pyridylazo
phenyl)-400-methoxybenzoate, Ia, with the four homologs of acids, (a) II10,
(b) II12, (c) II14, and (d) II18.
complexes, the association is primarily affected by the acidity of the
proton-donor, the acid, and the basicity of the pyridine derivative.
In order to obtain stoichiometric information on the binary mixtures
made from the proton-donor (II10–II18) and the proton acceptor
(Ia&b) which is stabilized by the electron-donating (CH3O and CH3)
groups in the para-position, a series of hydrogen-bonded complexes
was prepared using a varying mol% of Ia or (Ib) as proton acceptor,
and each of the four acid homologs (II10–II18), individually, as proton
donor.

Supramolecular Hydrogen-Bonded Liquid Crystals
63
FIGURE 2 Binary phase diagrams for the mixtures of 4-(40-pyridylazo
phenyl)-400-methylbenzoate, Ib, with the four homologs of acids, (a) II10, (b)
II12, (c) II14, and (d) II18.
Figures 1a–d represents the binary phase diagrams constructed for
the four binary systems prepared from the methoxy-substituted pyri-
dine derivative (Ia) with the four acid homologs, II10–II18, respect-
ively. As can be seen from the figure, each homologs of the acid
induces the SmC phase within the intermediate range of composition,
with varying extents dependent on the length of the alkoxy chain. The
nematic phase persists, in the lower homologs (II10 and II12) up to the
addition of about 20 mol% of the nonmesomorphic component Ia.
Evidence for the formation of liquid crystalline complexes in all four

64
M. M. Naoum et al.
FIGURE 3 Binary phase diagrams for the mixtures of 4-(40-pyridylazo
phenyl) benzoate, Ic, with the four homologs of acids, (a) II10, (b) II12,
(c) II14, and (d) II18.
systems by supramolecular hydrogen bonding in the 50 mol% mixture
(1:1 complex) is provided by the existence of the two eutectic points
before and after the 50 mol%, i.e., at about 30 and 80 mol%. It can also
be noted from Figure 1 that the mesophase characteristics of the 1:1
complex (at 50 mol% composition) varies according to the length of the
alkoxy chain of the acid. Thus, for the lowest homolog (II10, Fig. 1a)
the hydrogen bonded complex Ia=II10 melts at 114ꢀC to SmA, changes
to SmC at 129ꢀC, and isotropizes at 171ꢀC. Increasing the alkoxy-chain

Supramolecular Hydrogen-Bonded Liquid Crystals
65
FIGURE 4 Binary phase diagrams for the mixtures of 4-(40-pyridylazo phe-
nyl)-400-bromobenzoate, Id, with the four homologs of acids, (a) II10, (b) II12,
(c) II14, and (d) II18.
length of the acid to 12 carbons furnishes a complex that melts directly
to a SmC mesophase at 116ꢀC, and to the isotropic liquid at 163ꢀC. The
other two intermediate homologs of the acid, II14 and II18, behave
similarly to the latter; they melt at 112 and 110ꢀC to the SmC phase
and isotropize at 148 and 137ꢀC, respectively. These results revealed
that increasing the alkoxy-chain length of the proton-donor above 10
carbons results in a complex, with the methoxy-substituted proton-
acceptor, Ia, that is purely smectogenic exhibiting the SmC meso-
phase. Furthermore, an increase of the chain length is accompanied

66
M. M. Naoum et al.
FIGURE 5 Binary phase diagrams for the mixtures of 4-(40-pyridylazo phe-
nyl) 400-nitrobenzoate, Ie, with the four homologs of acids, (a) II10, (b) II12,
(c) II14, and (d) II18.
by a destabilization of the SmC mesophase from 171 to 137ꢀC, while
the melting temperature is not significantly influenced by the length
of the alkoxy chain; it varies irregularly between 110 and 114ꢀC. It
may also be added that the nematic phase observed for the lower acids
(II10 and II12) disappears completely upon the addition of 20 mol% of
the non-mesomorphic base Ia.
Investigation of the binary phase behavior of the methyl-substi-
tuted analog, Ib, with the four homologs of the acid (II10–II18), as

Supramolecular Hydrogen-Bonded Liquid Crystals
67
given in Figure 2, revealed that a supramolecular hydrogen-bonded
complexes are formed between the proton-donors (i.e., the acids) and
the still strong proton-acceptor (i.e., the base Ib). In the present case,
the strength of the proton-acceptor is evidently enhanced by the
electron-releasing character of the 4-methyl group, as in the case of
the 4-methoxy group but to a smaller extent. Complex formation is
again evidenced by the existence of two eutectic points; one preceding
and the other following the 1:1 molar ratio (see Figs. 2a–d). In this
binary system, purely smectogenic (SmC) complexes are observed only
for the complexes between Ib and the two intermediate homologs of the
acid, II12 and II14 (Figs. 2b and c). The complexes of the other two
acids (II10 and II18) both exhibit polymorphic mesophases. Thus,
while the lowest homolog (II10) of the acid forms a 1:1 complex which
melts at 114ꢀC to the SmA phase, changes to the SmC phase at 139ꢀC,
and isotropizes at 169ꢀC, the highest homolog investigated (II18) melts
first to a SmA at 119ꢀC, transforms to the SmC phase at 138ꢀC, and
finally to the isotropic liquid at 156ꢀC. It is worth mentioning that
the nematic phase of the two lowest homologs of the acid (II10 and
II12) persists up to the addition of 20 mol% of Ib.
Binary Mixtures of the Unsubstituted Derivative (Ic)
with the Four Homologs of the Acid
Turning now to the intermediate analog of the proton-acceptors that
bears no substituent, Ic. The binary phase diagrams of Ic with the
homologes of the acid (II10–II18) are represented graphically in
Figures 3a–d, respectively. Examining the binary phase diagrams
revealed that the proton-acceptor strength of the unsubstituted base
is still adequate to be involved in a supramolecular hydrogen-bonded
complex formation. This is again evidenced by the two eutectic points;
the first lies around 30 mol% of the acid, while the second is found
around 80 mol%. Compared with the electron-withdrawing substitu-
ents (CH3O and CH3), the unsubstituted derivative (Ic, X ¼ H)
furnishes the correspondingly less stable SmC mesophase of its
complexes with the homologs of the acid (II10–II18) as is evidenced
by their relatively lower stability (TC-I) and mesomorphic range
(DT ¼ TC-I ꢁ TC-A), see Table 3. Furthermore, in the four diagrams
of this system, the SmC phase is the only mesophase observed for
the 1:1 complexes of all four homolog (see Fig. 3). The SmA phase
persists up to the addition of about 30 mol% of the base Ic, while the
nematic phase in the lower two homologs of the acid disappears
completely by the addition of no more that 10 mol% of Ic.

68
M. M. Naoum et al.
TABLE 3 Phase Transition Temperatures (ꢀC) of the 1:1 Supramolecular
Hydrogen-Bonded Complexes of the Systems I=II
System
X
n
TS-A
TS-C
TA-N
TA-C
TC-I
DT
Ia=II10
Ia=II12
Ia=II14
Ia=II18
Ib=II10
Ib=II12
Ib=II14
Ib=II18
Ic=II10
Ic=II12
Ic=II14
Ic=II18
Id=II10
Id=II12
Id=II14
Id=II18
Ie=II10
Ie=II12
Ie=II14
Ie=II18
CH3O
10
12
14
18
10
12
14
18
10
12
14
18
10
12
14
18
10
12
14
18
114
–
–
–
114
–
–
119
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
129
–
–
–
139
–
–
138
–
–
–
171
163
148
137
169
162
152
156
148
145
156
156
166
165
162
158
155
159
163
155
42
46
36
27
29
46
37
18
16
7
14
16
42
35
30
26
30
38
39
38
116
112
110
–
116
115
–
132
138
142
130
124
130
132
132
–
CH3
H
–
–
–
–
–
–
–
–
Br
–
–
NO2
87
94
98
92
125
121
124
117
–
–
–
Binary Mixtures of the Electron-Withdrawing (Br and NO2)
Substituted Derivative (Id and Ic) with the Four Homologs
of the Acid
As it is well known, the remaining two substituents on the proton-
acceptor, I, interact electronically, in different manners, with the rest
of the molecule. Thus, while the nitro group, in Ie, induces an electro-
meric shift of electrons towards the nitro group, the bromine substitu-
ent interacts electronically with the remainder of the molecule in two
opposing manners. In the first, it attracts the electrons by its inductive
effect due to its relatively high electronegativity. While in the second,
it induces an electromeric shift of the electrons towards the pyridine-
nitrogen. These two opposing effects may render the basicity of the
molecule of Id, more or less, comparable with the unsubstituted
analog, Ic. Thus examining the binary phase diagrams of the bromo-
substituted pyridine-based azo dye, Id, with the four homologs of the
acid (II10–II18), as represented graphically in Figure 4, revealed that
the basicity of Id is still adequate to form supramolecular hydrogen-
bonded complexes at the 1:1 molar ratio. In each of the four phase dia-
grams of this system, Id=II two eutectic points were observed before

Supramolecular Hydrogen-Bonded Liquid Crystals
69
and after the 50 mol% composition. Such a complex is purely SmC
independent of the length of the alkoxy chain of the acid. The stability
of the phase and its temperature range are relatively high compared
with those of the unsubstituted analog. This findings revealed that
the electromeric donating character of the bromine atom with the rest
of the molecule, overcomes its electron-withdrawing inductive effect,
thus, rendering the molecule more basic compared with the unsubsti-
tuted analog. Accordingly, its hydrogen-bonded complexes with the
acid homologs are more stable.
As expected for the nitro-substituted analog, its strong electron-
withdrawing character will diminish the electron density on the pyri-
dine-nitrogen and, consequently, its basicity and proton-acceptor
capability. Hence, the possibility of hydrogen-bonded complex forma-
tion is accordingly reduced. This was found to be the case. As can be
seen from Figure 5, which reproduces the binary phase diagrams of
the system Ie=II revealed that no complex formation was detected with
all acid homologs, since only one eutectic point is observed in each dia-
gram. The higher Tc (TC-I) and DT (¼TC-I ꢁ TC-A) values for the
50 mol% composition (see Table 3) may be attributed to the possible
anti-parallel alignment of the dipolar molecules that enhances the
overall molecular length, and consequently its phase stability.
CONCLUSIONS
Five pyridine-based azo dye derivatives of the type 4-(40-pyridylazo
phenyl)-400-substituted benzoates, Ia-e, in which the substituent varies
between CH3O, CH3, H, Br, and NO2 groups, respectively. Their
molecular structures were checked via elemental analyses, NMR,
and mass spectroscopy. The effect of the polarity of the substituent
(electron donating or electron withdrawing) on supramolecular hydro-
gen-bonded complexing with four homologs of 4-alkoxybenzoic acids,
II10–II18, was investigated by constructing their corresponding
binary phase diagrams. The study revealed the following
1. All mixtures, in their intermediate composition ranges, showed an
SmC mesophase.
2. Except for the nitro-substituted, the other analogs proved to form
supramolecular hydrogen-bonded complexes in 1:1 molar ratio
with all the investigated homologs of the acid.
3. Although the nitro-substituted analog did not show such a com-
plex, its mixtures with the acids possessed stable SmC mesophases
that could be attributed to an antiparallel alignment of the
molecules.

70
M. M. Naoum et al.
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