Supramolecular liquid crystals induced by hydrogen-bonding interactions between non-mesomorphic compounds. I. 4-(4′-Pyridylazophenyl)-4″- Substituted benzoates and 4-substituted benzoic acids

Article abstract of DOI:10.1080/15421400902841403

Equimolar binary mixtures of the title compounds were prepared to investigate the effect of different polar substituents, either on the pyridine-based derivatives or on the acid component, on the extent and stability of the supramolecular liquid crystal p

Full text of DOI:10.1080/15421400902841403

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Supramolecular Liquid Crystals
Induced by Hydrogen-Bonding
Interactions Between Non-
Mesomorphic Compounds. I.
4-(4′-Pyridylazophenyl)-4″-
Substituted 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 , Gita, Egypt
Published online: 03 Aug 2009.
To cite this article: M. M. Naoum , A. A. Fahmi & M. A. Alaasar (2009) Supramolecular
Liquid Crystals Induced by Hydrogen-Bonding Interactions Between Non-Mesomorphic
Compounds. I. 4-(4′-Pyridylazophenyl)-4″-Substituted Benzoates and 4-Substituted
Benzoic Acids, Molecular Crystals and Liquid Crystals, 506:1, 22-33, DOI:
10.1080/15421400902841403
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Mol. Cryst. Liq. Cryst., Vol. 506, pp. 22–33, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902841403
Supramolecular Liquid Crystals Induced by
Hydrogen-Bonding Interactions Between
Non-Mesomorphic Compounds. I. 4-(40-Pyridylazo
phenyl)-400-Substituted Benzoates and 4-Substituted
Benzoic Acids
M. M. Naoum, A. A. Fahmi, and M. A. Alaasar
Department of Chemistry, Faculty of Science, Cairo University,
Gita, Egypt
Equimolar binary mixtures of the title compounds were prepared to investigate the
effect of different polar substituents, either on the pyridine-based derivatives or on
the acid component, on the extent and stability of the supramolecular liquid
crystal phases induced by intermolecular hydrogen bonding. None of the pyridine-
based derivative or the acid complement is mesomorphic, but the hydrogen-bonded
complexes are. The mixtures prepared were characterized for their mesophase
behavior by differential scanning calorimetry, DSC, and polarized light micro-
scopy, PLM. Five azo pyridine-based derivatives (Ia–e), with molecular formula
X-C6H4COOC6H4-N¼N-C5H4N, were prepared that differ from each other by the
substituent X. The latter varies between CH3O, CH3, H, Br, and NO2. Six 4-substi-
tutedbenzoic acids (Y-C6H4COOH, IIa-f) were used; the substituent Y varies
between CH3O, CH3, H, Br, CN, and NO2. Nematic mesophase is induced in most
of the binary mixtures investigated.
Keywords: 4-(40-pyridylazophenyl)-400-substituted benzoates; 4-substituted benzoic
acids; binary mixtures; supramolecular LCs
1. INTRODUCTION
In recent years, several classes of supramolecular liquid crystals, formed
via hydrogen-bonding interaction of complementary molecules, have
been recognized and characterized for their mesophase behavior [1,2].
A single hydrogen bond between the pyridyl nitrogen and a carboxylic
Address correspondence to M. M. Naoum, Department of Chemistry, Faculty of
Science, Cairo University, P.O. Box 12613, Gita, Egypt. E-mail: magdinaoum@yahoo.
co.uk
22

Supramolecular Liquid Crystals
23
proton was found [2] to be fruitful in generating novel liquid crystals.
Generally, hydrogen bonding leads to an elongation of the rigid-rod seg-
ment of the individual components. Complementary components may be
either mesomorphic or non-mesomorphic; in both cases, however, novel
liquid crystals can be obtained. For instance, carboxylic acids, substituted
with a small compact polar group, when interacted with 4-cyano- or
4-nitro-40stilbazole afforded complexes of the general formula 1 [3].
Although none of the two complementary components bears an alkyl
chain, most of the complexes examined proved to exhibit nematic meso-
morphism with high clearing temperatures. The nature of the terminal
substituents (X or Y) was found to have crucial effect upon mesomorph-
ism. These results have encouraged us to widen our previous study [4,5]
to test the possibility of liquid crystal formation between the
non-mesomorphic 4-(40-pyridylazophenyl)-400-substituted benzoates
(Ia–e) and 4-substituted benzoic acids (IIa–f). Using two terminal groups
(X and Y) of variable polarities may affect the stability or the type of the
mesophase to variable extents. The goal of the present study is to inves-
tigate the possibility of mesophase formation between the two non-
mesomorphic components. So, it is planned to prepare supramolecular
complexes (in the 1:1 molar proportions) in order to monitor the effect
of similar and=or different polar substituents on mesophase formation.

24
M. M. Naoum et al.
2. EXPERIMENTAL
Chemicals are of pure grades that 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.
2.1. Preparation of Materials
The pyridine-based azo dyes (Ia–e) were prepared, by the method
described previously [4], according to the following scheme:
The solids obtained were found to be TLC pure and possess sharp
melting temperatures, given in Table 1, that agree with those reported
before [4].
2.2. Physical Characterization
For the investigation of the supramolecular complexes (I=II), binary
mixtures of any two complimentary components, were prepared in a
1:1 molar ratio by melting the appropriate amounts of each compo-
nent, stirring to give an intimate blend, and then cooling with stirring
to room temperature.
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. [6]. 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 10ꢀC=min
in inert atmosphere of nitrogen gas (10 ml=min).
Transition temperatures were checked and types of mesophases
identified for adducts prepared with a standard polarized light micro-
scope PLM (Wild, Germany) attached to a home made hot-stage.
The temperatures obtained for the prepared blends, as measured
by both DSC and PLM, agreed within 2–3ꢀC.

Supramolecular Liquid Crystals
25
TABLE 1 Phase Transition Temperatures (ꢀC), Enthalpies (DH, kJ=mole),
and Entropies (DSc, J=mole=K) of the 1:1 Supramolecular Hydrogen-bonded
Complexes of the Systems I=II
p
System
X
Y
Tm
DHm
TC
DHC
DSC
TC
Ia=IIa
Ia=IIb
Ia=IIc
Ia=IId
Ia=IIe
Ia=IIf
Ib=IIa
Ib=IIb
Ib=IIc
Ib=IId
Ib=IIe
Ib=IIf
Ic=IIa
Ic=IIb
Ic=IIc
Ic=IId
Ic=IIe
Ic=IIf
Id=IIa
Id=IIb
Id=IIc
Id=IId
Id=IIe
Id=IIf
Ie=IIa
Ie=IIb
Ie=IIc
Ie=IId
Ie=IIe
Ie=IIf
CH3O
CH3O
CH3
H
Br
CN
NO2
CH3O
CH3
H
Br
CN
NO2
CH3O
CH3
H
Br
CN
NO2
CH3O
CH3
H
Br
CN
163.7
138.0
170.1
156.5
177.3
170.4
5.34
5.64
4.08
9.83
6.98
4.00
220.5
218.0
193.3
221.4
229.7
241.4
205.6
206.4
181.0
219.5
232.2
238.4
198.7
180.9
161.3
173.4
217.9
214.4
218.4
200.1
174.2
216.0
234.5
241.1
179.4
178.3
190.2
221.2
224.2
–
1.03
0.35
0.10
0.09
0.12
1.74
0.54
0.42
0.31
0.29
2.04
0.23
0.14
0.12
0.09
0.36
0.93
0.92
0.27
0.14
0.08
0.27
0.44
0.46
0.31
0.24
0.11
0.83
0.95
–
2.09 22.20
0.71 22.16
0.47 21.60
0.18 22.45
0.24 22.42
3.38 22.68
1.13 21.87
0.88 21.90
0.68 21.30
0.56 22.20
4.04 22.47
0.45 22.60
0.30 21.71
0.26 21.31
0.21 20.84
0.81 21.13
1.89 22.16
1.89 22.08
0.55 22.16
0.30 21.75
0.18 21.15
0.55 21.12
0.87 22.53
0.89 22.68
0.68 21.27
0.53 21.25
0.23 21.53
1.68 22.23
1.91 22.3
CH3
166.3 12.68
179.2
133.7
179.4
168.9
178.4
7.91
2.63
5.19
4.32
4.35
H
170.1 10.94
163.7
125.8
161.6
162.0
200.6
172.9 11.73
173.9
189.6
3.30
4.57
6.68
8.40
7.07
Br
7.70
7.02
177.9 11.45
181.9
211.3
154.7
141.4
154.8
187.3 18.15
194.9 15.23
155.3 28.42
8.05
6.72
6.40
4.52
6.88
NO2
CH3O
CH3
H
Br
CN
NO2
NO2
–
–
3. RESULTS AND DISCUSSION
Supramolecular hydrogen-bonded liquid crystals are frequently
induced between aromatic carboxylic acids as proton donor and the
pyridine nitrogen, in a pyridine-based derivative, as proton acceptor.
In such cases, neither of the two complementary components should
disturb the alignment of the other. However, each component in the
mixture may be either mesomorphic, or non-mesomorphic, or both;
in each case, novel liquid crystals can be obtained. For our investi-
gated systems, hydrogen bonding leads to an elongation of the indivi-
dual components and represented by formula 2.

26
M. M. Naoum et al.
For the formation of liquid crystalline materials through hydrogen
bonding interactions, the nature of the interacting components
coupled with the directionability of hydrogen-bonds are the main
(but not the sole) factors contributing to the exhibition of liquid crys-
tallinity. The appropriate shape and stability of the hydrogen bonded
supramolecular complexes are also required. All possible 1:1 supra-
molecular complexes, made from each of the five pyridine-based deri-
vatives (Ia–e) with each of the substituted benzoic acids (IIa–f) were
investigated for their mesophase behavior by DSC and PLM, and
the results are collected in Table 1.
Since the mesophase is not shown by either of the two interacting
components, the mesophase stabilization of the 1:1 complex is attribu-
ted to the formation of the new and elongated mesogen, obtained
through a single intermolecular hydrogen bond which allows the com-
plex to behave as a single liquid crystalline compound. Complex
formation was supported by FTIR measurements, according to which
the initial dimeric acid is replaced by the heterodimeric complex [7].
Certainly, the relative acid (or basic) strength of the two complemen-
tary components of the complex has a crucial effect upon mesomor-
phism. This in turn, is influenced by the nature of the terminal
substituents on both the pyridine-based component (X) and=or on the
acid complement (Y) of the supramolecular complex. Thus, electron-
donating substitution on the pyridine side is expected to increase
electron-density on the nitrogen of the pyridine ring that leads to
strengthening the resulting hydrogen-bond. On the other hand,
electron-withdrawing substituent on the acid side will enhance acidity
of the acid-proton, and consequently adds extra stability to the
hydrogen-bond.
That is, the methoxy (and less effectively the methyl) substitution
on the pyridine side, together with the nitro or cyano (and less effec-
tively the bromo) groups on the acid partner of the complex, will

Supramolecular Liquid Crystals
27
furnish the most mesophasically stable adducts. Recalling Table 1,
ꢀ
this was found to be true. The clearing temperature (TC ¼ 241.4 C)
of the complex containing the substituents (X¼CH3O and Y¼NO2) is
ꢀ
considerably higher than that of the complex (TC¼179.4 C) bearing
opposite substitution, i.e., X¼NO2 and Y¼CH3O. Also, the entropy
change associated with the clearing transition (DSC) showed an analo-
gous behaviour (DSC ¼ 3.38 and 0.68 J=mol=K, respectively). That is
the mesophase stability, TC, of the adducts formed from the methoxy
or the methyl-substituted pyridine-based derivatives (Ia or Ib) with
the variously substituted acids (IIa–f) decreases in the order:
NO2 > CN > Br > CH3O > CH3 > H;
whereas, TC of the adducts, formed from the cyano- (or nitro-) substi-
tuted acids (IIe or IIf) with the variously substituted pyridine-based
compounds, decreases in the order:
CH3O > Br > CH3 > CH > NO2:
It can be also concluded that, for the systems containing proton-
acceptor bearing an electron-releasing group, hydrogen-bonding
complexation with acids substituted with electron-withdrawing group,
not only enhances liquid crystallinity, but also increases the degree of
molecular ordering.
3.1. Thermal Mesophase Stability and Polarizability
Anisotropy of Car-X and/or Car-Y Bonds
As we concluded above, in hydrogen-bonded liquid crystal complexes,
phase behavior is related to the strength of the hydrogen bonds
involved [8]. In the pyridine=acid complexes, the association is primar-
ily affected by the acidity of the proton-donor, the acid, as well as the
basicity of the pyridine derivative. The relationship between the stabi-
lity of the mesophase, expressed as the clearing temperature, TC, and
the anisotropy of polarizability (DaX) of bonds to the small compact
terminal substituent (Car-X), was studied by van der Veen [9]. The
relationship has the form
2
TCaðDaM þ DaXÞ ;
ð1Þ
where TC is measured in Kelvin. Equation (1) can be put in the
form [10]
T1C=2aðDaM þ DaXÞ ¼ a ꢁ DaM þ a ꢁ DaX:
ð2Þ

28
M. M. Naoum et al.
The term DaM is the polarizability anisotropy of all the molecular
structure except the terminal substituent, X, and ‘‘a’’ is the proportion-
ality constant. Thus, if, TC1=2 of any of our investigated hydrogen-
bonded series bearing one and the same acid substituent ‘‘Y’’ but of
FIGURE 1 Dependence of the mesophase stability (TC) of the investigated
1:1 adducts on the polarizability anisotropy (DaX) of the pyridine-based
substituent (X).

Supramolecular Liquid Crystals
29
varying base substitution ‘‘X’’, is plotted against the polarizability
anisotropy to the bond X, DaX, a straight line is expected with a slope
‘‘a(X)’’, and intercept equals ‘‘a(X) ꢁ DaM(Y)’’, where ‘‘a(X)’’ is the slope of
the of (TC1=2 ꢂ DaX) dependency, and ‘‘DaM(Y)’’ is the polarizability
anisotropy of the remainder of the molecular structure except the sub-
stituent (X), but still including the substituent (Y). Figure 1 illustrates
the correlations between the TC1=2 values and DaX of the bonds to X in
the pyridine-based component of the supramolecular complex,
individually, with the variously substituted acids (IIa – IIf).
FIGURE 2 Dependence of the mesophase stability (TC) of the investigated 1:1
adducts on the polarizability anisotropy (DaY) of the acid substituent (Y).

30
M. M. Naoum et al.
Alternatively, if TC1=2 values of any series bearing the same
pyridine-based substituent (X), whereas the acid component bears dif-
ferent substituent (Y), is plotted against the polarizability anisotropy
(DaY) of the substituent (Y), a straight line is again expected, but in this
case the slope is ‘‘a(Y)’’ and the intercept is ‘‘a(Y) ꢁ DaM(X)’’, where ‘‘DaM(X)’’
is the polarizability anisotropy of the remainder of the molecular struc-
ture comprising the substituent (X). The TC1=2 values are again plotted,
individually, for each series bearing one and the same substituent X, as
a function of DaY and depicted in Fig. 2. As can be seen from Figs. 1 and
2, fairly linear dependencies were observed in nearly all series investi-
gated, except for the system where X¼NO2 (Fig. 2). This is expected,
since electron-withdrawing substitution on the base-side of the
complex decreases electron-density on the pyridine-nitrogen and, con-
sequently, destabilizes the complex formed resulting in irregularity of
data. It would be worthy mentioning here that the bromo-substituted
derivatives behave, more or less, as the unsubstituted analogues,
where its electron-withdrawing inductive power is counterbalanced
by its mesomeric character, resulting in an increased mesophase stabi-
lity of the resulting complex as evidenced by their relatively high TC
values (see Table 1, where X¼Br while Y is varied).
Figures 1 and 2 were used to calculate the slope and intercept of
each of their regression lines from which DaM(X) and DaM(Y) are calcu-
lated. The results of the computation are given in Table 2. Table 2
shows that the polarizability anisotropy DaM(Y) of the complex except
the substituent X decreases, according to Y, in the order
NO2 > CH3O ꢃ CH3 > H ꢃ Br:
TABLE 2 Regression Analyses Data for the van der Veen Correlations
Slope ꢄ
DaM(X)
ꢄ
DaM(Y)
ꢄ
System
X
Y
1023 Cm3
Intercept
1024 Cm3
1024 Cm3
I=IIa
I=IIb
I=IIc
I=IId
I=IIf
Ia=II
Ib=II
Ic=II
Id=II
Ie=II
CH3O
CH3
H
Br
NO2
1.89
1.91
1.94
2.09
1.62
2.11
2.16
2.57
2.16
1.26
21.68
21.34
20.72
21.85
22.14
21.64
21.35
20.72
20.97
21.37
–
–
–
–
–
11.50
11.17
10.52
10.45
13.67
–
–
–
–
–
CH3O
CH3
H
Br
NO2
10.12
9.88
8.06
9.71
16.96

Supramolecular Liquid Crystals
31
The high anisotropy of polarizability of the nitro-substituted com-
plex may be ascribed to the high molecular ordering observed in the
pure solid component (DS ¼ 65.9 ꢄ 103=(205 þ273) ¼ 137.8 J=mol=K).
On the other hand, as can also be deduced from Table 2, DaM(X) of the
complex, except the substituent Y decreases, according to X, in the order
NO2 > CH3O > CH3 > Br ꢃ H:
This nearly similar order, again ensures the similarity between the
unsubstituted and bromo derivatives, either in the acid or pyridine
sides of the complex.
FIGURE 3 (a) Dependence of the polarizability anisotropy (DaM(Y)) of the
investigated 1:1 adducts, except the substituent (X), on the polarizability
anisotropy (DaY). (b) Dependence of the polarizability anisotropy (DaM(X)) of
the investigated 1:1 adducts, except the substituent (Y), on the polarizability
anisotropy (DaX).

32
M. M. Naoum et al.
In order to investigate the relative effectiveness of the substituents
(X with Y) on the mesophase stability, the polarizability anisotropy of
the whole molecular structure except either of the substituents X or Y
(i.e., DaM(X) and DaM(Y)) are plotted as a function of DaX and DaY,
respectively, in Figs. 3a, b. Figure 3 showed acceptable linear depen-
dencies of the polarizability of anisotropy DaM(X) or DaM(Y) on DaX or
DaY, respectively, except for the nitro substituent. This is again in
accordance with the irregular results obtained before.
CONCLUSIONS
Five pyridine-based azo derivatives of the type 4-(40-pyridylazo
phenyl)-400-substituted benzoates, Ia–e, were prepared in which the
substituent varies between CH3O, CH3, H, Br, and NO2 groups,
respectively. The effect of the electron donating or electron withdraw-
ing character of the substituent on both sides of the supramolecular
hydrogen-bonded complexes with six differently substituted benzoic
acids, IIa–f, on the possibility of formation, and stability of the
mesophase of the resulting adducts was investigated via differential
scanning calorimetry and polarized-optical microscopy. The study
revealed that:
1. Most of the mixtures, in their 1:1 molar ratio, proved to exhibit a
nematic mesophase.
2. The stability of the mesophase, expressed as the clearing tempera-
ture, is found to be greatly influenced by the nature of the substitu-
ent on either side of the new elongated mesogen.
3. Electron-donating substituents on the pyridine side, together with
electron-withdrawing substitution on the acid side of the complex,
were found to be most effective on mesophase stability.
p
4. Acceptable linear correlations, between TC and the polarizability
anisotropy of bonds to substituent on either side of the complexes,
were obtained.
5. Except for the nitro substituent, the polarizability anisotropy of the
whole molecular structure except one of the two substituents was
almost linearly related to those of the bond with the substituent
remained, indicating a strong dependency of the stability of the
mesophase upon substitution on either side of the complex.
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