Effect of the relative orientation of the two fluoro-substituents on the mesophase behavior of phenylazophenyl benzoates

Article abstract of DOI:10.1080/10426507.2012.660867

Four novel types of laterally di-fluoro substituted 4-(2-(or 3-) fluoro phenylazo)-2- (or 3-) fluoro phenyl-4-alkoxybenzoates were prepared and investigated for their mesophase behavior. In order to deduce the most probable conformation for each of the po

Full text of DOI:10.1080/10426507.2012.660867

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Effect of the Relative Orientation of
the Two Fluoro-Substituents on the
Mesophase Behavior of Phenylazophenyl
Benzoates
M. M. Naoum a , A. A. Fahmi a & H. A. Ahmed a
a Department of Chemistry, Faculty of Science, Cairo University,
Cairo, Egypt
Published online: 30 Jul 2012.
To cite this article: M. M. Naoum , A. A. Fahmi & H. A. Ahmed (2012) Effect of the Relative
Orientation of the Two Fluoro-Substituents on the Mesophase Behavior of Phenylazophenyl Benzoates,
Molecular Crystals and Liquid Crystals, 562:1, 43-65, DOI: 10.1080/10426507.2012.660867
To link to this article: http://dx.doi.org/10.1080/10426507.2012.660867
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Mol. Cryst. Liq. Cryst., Vol. 562: pp. 43–65, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/10426507.2012.660867
Effect of the Relative Orientation of the Two
Fluoro-Substituents on the Mesophase Behavior
of Phenylazophenyl Benzoates
M. M. NAOUM,∗ A. A. FAHMI, AND H. A. AHMED
Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt
Four novel types of laterally di-fluoro substituted 4-(2ꢀ-(or 3ꢀ-) fluoro phenylazo)-2-
(or 3-) fluoro phenyl-4ꢀꢀ-alkoxybenzoates were prepared and investigated for their
mesophase behavior. In order to deduce the most probable conformation for each
of the positional isomers investigated, the dipole moments for each of the four novel
types have been determined experimentally in benzene at 30◦C and compared with those
theoretically calculated using molecular modeling program. Probable conformations
deduced were found to vary according to the relative orientations of the two fluorine
atoms. The results were used to correlate the mesophase behavior, in pure and mixed
derivatives, with the conformation deduced for each series. The study aims to inves-
tigate the effect of the spatial orientation of the two lateral fluoro-substituents on the
mesomorphic properties in their pure and mixed states.
Keywords Binary mixtures; dipole moments; fluorine; liquid crystals
1. Introduction
It is already known that the mesomorphic properties of nematic mesogens are strongly
influenced when a lateral group is appended to a nematic core. The extent of such influence
is dependent on the size, position, and polarity of the lateral substituent. Sterically, a lateral
substituent widens the core and increases the intermolecular separation, which leads to a
reduction in the lateral interactions [1–3] and hence the nematic stability is reduced. Gray
[4] explained that an increase in the breadth of the molecules reduces both the nematic
and smectic mesophase stability. It has been found [5] that the position of the fluoro-
substituents in a given core unit can have marked effects on mesophase formation and
transition temperatures, and melting and clearing points. Generally, fluorine substitution
does not significantly change the overall shape of the core as a fluoro-substituent is only
slightly larger than the hydrogen it replaces. So, such a study involves little change of the
molecular shape, but changes do exist in the polarity and location of dipole moments. For
example, mesophase type and transition temperatures were investigated [6–9] as a function
of fluoro-substitution in 4-hexyloxy-4ꢀ-pentyl terphenyls, where the terminal chains and
the core were the same in each material. Without any lateral fluoro-substituents, the low-
polarity parent compound was found to exhibit smectic A and B phases. Incorporation of
one fluoro-substituent in the central ring results in the introduction of a smectic C phase
∗Address correspondence to M. M. Naoum, Department of Chemistry, Faculty of Science, Cairo
University, Cairo, Egypt. E-mail: magdinaoum@yahoo.co.uk
43

44
M. M. Naoum et al.
and a nematic phase, and a massive reduction in the melting point. For the mono-fluoro
terphenyl isomers, it has been shown that the phase sequence varies greatly with the location
on the site of substitution. As the fluoro-substituents increase in number, there is a great
tendency for the compound to exhibit the nematic phase. Melting points are often found to
fall with increasing the degree and location of the fluoro-substitution.
The goal of the present investigation is to measure the dipole moments of the newly
prepared laterally di-fluoro substituted isomers (In–IVn) aiming, firstly, to deduce the
most probable conformation for each of the differently, laterally substituted analogues.
Secondly, it is to relate the mesophase behavior of the compounds, in pure and mixed states,
with the apparent dipole moment (µobs), and consequently the deduced conformations,
their calculated longitudinal (µY) and lateral (µX) dipole moments, as well as the dipolar
anisotropy of the molecule (ꢀµ = µY – µX).
N=N
N=N
N=N
N=N
OCO
OCO
OCO
OCO
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
F
F
(I)
F
F
(II)
F
F
F
(III)
F
(IV)
2. Experimental
Chemicals were of very pure grades and purchased from the following companies: Buchs
(Switzerland), Aldrich (Wisconsin, USA), E. Merck (Darmstadt, Germany), Alfa Aesar
(Karlsruhe, Germany), and Acros (New Jersey, USA).

Fluoro-Substituted Phenylazophenyl Benzoates
2.1. Preparation of Materials
45
Compounds In-IVn were prepared according to the following scheme:
HCl/NaNO2
NH2
N=N
OH
2-(or 3-) F-phenol
F
F
F
(A)
DCC
(A)
+
COOH
CnH2n+1
O
DMAP
CnH2n+1
O
COO
N=N
F
F
(In - IVn)
2.1.1. Preparation of 4-(2ꢀ-(or 3ꢀ-) fluoro phenylazo)-2-(or 3-) fluoro phenol (A). The 2-
(or 3-) fluoro aniline (0.05 mole) was dissolved in dilute hydrochloric acid and cooled in
ice-salt bath to 0◦C. To the resulting solution, a cold aqueous solution (0.05 mole) of sodium
nitrite was added drop-wise with stirring. The cold mixture was then added slowly to a cold
2-(or 3-) fluoro phenol (0.05 mole) in sodium hydroxide solution (1:1). The mixture was
further stirred at room temperature for 1 h, acidified with dilute hydrochloric acid, and the
solid product obtained was filtered and recrystallized twice from glacial acetic acid to give
pure compounds, as indicated by TLC analysis. The melting points, determined by DSC as
sharp peaks, of the four prepared azo dyes A (substituted in positions 2ꢀ,2; 2ꢀ,3; 3ꢀ,2; and
3ꢀ,3) are 109.0◦C, 112.9◦C, 129.3◦C, and 117.1◦C, respectively.
2.1.2. Preparation of 4-(2ꢀ-(or 3ꢀ-) fluoro phenylazo)-2-(or 3-) fluoro phenyl-4ꢀꢀ-
alkoxybenzoates, In - IVn. Molar equivalents of the 4-(2ꢀ(or 3ꢀ-) fluoro phenylazo)-2-(or
3-) fluoro phenol (A) and 4-n-alkoxybenzoic acid (0.05 mole each) were dissolved in 25 mL
dry methylene chloride. To the resulting solution, two-molar equivalents of dicyclohexyl-
carbodiimide (DCC, 0.1 mole) and few crystals of 4-(dimethylamino)pyridine (DMAP),
as catalyst, were added and the solution was stirred for 72 h at room temperature. The
solid was filtered off and the solution was evaporated. The solid product was recrystallized
twice from acetic acid and twice from ethanol to give pure products, as indicated by TLC
analysis, and sharp melting and clearing peaks in their DSC thermograms.
2.2. Physical Characterization
Calorimetric measurements were carried out using a PL-DSC (Polymer Laboratories, Eng-
land). The instrument was calibrated for temperature, heat, and heat flow according to the
method recommended by Cammenga et al. [10]. The measurements were carried out for

46
M. M. Naoum et al.
small samples (2–3 mg) placed in sealed aluminum pans. All of the thermograms have been
achieved at a heating rate of 10◦C/min in an inert atmosphere of nitrogen gas (10 mL/min).
Transition temperatures were checked and types of mesophases identified for com-
pounds prepared by standard polarized-light microscopy (PLM) of Wild, Germany, attached
to a home-made hot-stage. The temperature was measured by a thermocouple attached to
a temperature controller (Brookfield, England).
Thin layer chromatography was performed with TLC sheets coated with silica gel (E.
Merck); spots were detected by UV irradiation.
Infrared spectra (4000–400 cm−1) were measured with a Perkin Elmer B25 spectropho-
tometer, and 1H NMR-spectra with a Varian EM 350L.
For dipole moment determination, static dielectric constants, εꢀ, were measured with a
WTW Dipolemeter type DM 01, Germany. Refractive indices were measured by an Abbe
Refractometer (made by Bellingham and Stanley Limited, England).
For the construction of binary phase diagrams, the mixtures of the two components were
prepared to cover the whole range of composition. Transition temperatures obtained for all
prepared blends, as measured by both DSC and PLM, agreed within 2◦C–3◦C. In the phase
diagrams, constructed by plotting transition temperatures versus mixture composition, the
symbol “o” denotes solid-mesophase, “ꢀ” mesophase-isotropic transitions, “•” mesophase-
another mesophase, and, “ꢁ” the eutectic temperature.
2.3. Measurement of Dipole Moments
Analytical grade benzene was purified according to the recommended procedure [11]. Since
the dipole moments of all members of a homologous series are the same independent of
the length of the terminal alkoxy-chain (n), the dipole moments of the isomers I12-IV12
were experimentally determined as examples.
Five dilute solutions of the compound in benzene were prepared with weight fractions
ranging from 10−3 to 10−2. The dielectric constants, densities, and refractive index were
measured for each solution at 30◦C, and the dipole moments were calculated as described
earlier [12]. On the basis of the precisions in the measurements of dielectric constants
( 0.0005), densities ( 0.0001), refractive indices ( 0.001), and solution concentrations
( 0.02%), the dipole moment values obtained are believed reliable to 0.05 D.
3. Results and Discussion
3.1. Confirmation of Molecular Structure
Infrared spectra, mass spectra, and elemental analyses for compounds investigated were
1
consistent with the structures assigned. H NMR data showed the expected integrated
aliphatic to aromatic proton ratios in all compounds investigated. Representative infrared,
mass, and 1H NMR spectra are given in Tables 1–3, respectively.
3.2. Mesophase Behavior of Simple Molecules
The phase transition temperatures and enthalpies of the prepared compounds (In–IVn) are
given in Table 4. These data are represented graphically in Fig. 1 as a function of the alkoxy-
chain length. As can be seen from Fig. 1, for the four homologous series investigated there
is no systematic pattern of melting behavior with the increase of the terminal alkoxy-chain
length. With respect to their mesophase behavior, the type of phase or phases observed

Fluoro-Substituted Phenylazophenyl Benzoates
47
Table 1. Infrared spectra of some representative examples
Comp.
no.
νCH3
νCH3
νC
νC
νC
νC
Asym.
Sym.
O
N
O
O
I16
II8
III12
IV10
2920
2926
2921
2922
2854
2859
2852
2854
1744
1727
1732
1731
1600
1602
1600
1602
1483
1481
1487
1480
1238
1257
1258
1243
depends mainly upon the location and orientation of the two fluoro-substituents. Thus,
when the two fluorine atoms are located, in both rings, pointing toward the core of the
molecule (i.e., both in position-2, as in homologues In; Fig. 1(a)), the nematic phase is
favored covering the whole series except the highest homologue I16, which is purely
smectogenic possessing the SmC as the only mesophase observed. Conversely, when the
two fluoro-substituents are located in both rings away from the core (i.e., both in position-3,
as in homologues IVn; Fig. 1d), the smectic C phase predominates except for the lower
two homologues IV8 and IV10, which are dimorphic and exhibit a narrow nematic phase
range. In the homologous series IIn, where the fluoro-substituent is located in the terminal
ring pointing towards the core, while the other is located in the central ring away from
the core (i.e., in positions 2 and 3, respectively, Fig. 1b), the lower two homologues (II8
and II10) are purely nematogenic, while the higher three (II12, II14, and II16) are purely
smectogenic, possessing the SmC phase. Finally, in the homologous series IIIn, where the
location of the two fluoro-substituents are the reverse of series IIn (i.e., they are located
in positions 3 and 2, respectively; Fig. 1c), the nematic phase predominates irrespective of
the terminal alkoxy-chain length.
In order to confirm that the nematic phase is the only mesophase exhibited by all the
homologues IIIn, a binary phase diagram was constructed (Fig. 2(a)) between homologue
III8, as an example, and the purely nematogenic 4-hexyloxybenzoic acid, which exhibits
the enantiotropic nematic phase. In a similar way, in order to confirm that the SmC phase
predominates inthehomologues IVn, a binary phasediagramwas constructed(Fig. 2(b)) be-
tween homologue IV16, as an example, and the purely smectogenic 4-hexadecyloxybenzoic
acid, which exhibits the enantiotropic SmC phase. Both diagrams indicated that the nematic
and smectic C phases are the only mesophases observed in the compounds investigated. This
will be further confirmed for the other homologues from the binary mesophase behavior of
mixed isomers.
Table 2. Mass spectra (m/z) of the representative derivatives (In–IVn)
Whole
Compound
n
Structure
CnH2n+1OC6H4CO–
–OC6H4CO–
I16
II8
III12
IV10
16
8
12
10
578
466
522
494
345
233
289
261
121
121
121
121

48

49

50
M. M. Naoum et al.
Figure 1. Effect of the alkoxy-chain length (n) on the mesophase behavior of the derivatives In-IVn:
(a) In, (b) IIn, (c) IIIn, and (d) IVn.
Since the mesophase stability of a liquid crystalline compound is mainly dependent
upon the intermolecular attractions, in which molecular polarity plays a significant role, it
has been shown [13] that in a series of compounds the dipole moment of any compound
is dependent upon the nature and location of the substituent. A change in the extent of
electronic interactions between the substituent and the remainder of the molecule alters
the polarizability and resultant dipole moment of the molecule. It has also been shown
[14,15] that the dipole moments of all members of a homologous series are virtually the
same irrespective of the alkoxy-chain length. This result is in accordance with the fact that
the alkoxy groups are of the same polarity regardless of length and, at the same time, do

Fluoro-Substituted Phenylazophenyl Benzoates
51
Figure 2. Confirmation of the mesophase of selected homologues: (a) III8/hexyloxy benzoic acid,
and (b) IV16/hexadecyloxy benzoic acid.
not affect the extent of conjugative interactions between the alkoxy oxygen and the ester
carbonyl, as is confirmed by infrared spectra.
Polarization and dipole moment data for compounds In–IVn in benzene at 30◦C are
summarized in Table 5. The symbols have the following meanings: DP2 is the molar defor-
mation polarization of the solute obtained by extrapolating the measured molar refraction
for the sodium-D line to infinite wavelength [16], P2∞ is the molar polarization of the
solute at infinite dilution taken as the average of that determined graphically and those
of Hedestrand’s [17] and Palit-Banerjee’s [18] equations, and µ(D) is the dipole moment
determined by the refractivity method form:
ꢀ
µ(D) = 0.01273 [(P2∞ − DP2)T ]
3.3. Deduction of the Most Probable Conformations
Generally, there is a possibility for rotation of any of the three benzene rings around their 1,4-
axes. But, this may take place in cases where the concerned molecule comprises a biphenyl
Table 5. Polarization data for compounds I10–IV10
Hedestrand
Palit-Ban.
Guggenheim
DP2
µobs
(D)
Comp. (Cm3) Graph. P2∞
α
β
µ
γ
µ
ꢀ
α
µ
I10
181.9
516.4
363.9
353.3
3.5
5.5
2.8
5.9
472.0 3.6 0.2 3.7 1.94 2.40 3.2 3.6 3.80 3.35
887.0 12.5 2.3 5.21 0.72 4.95 11.0 12.5 5.43 5.27
485.0 5.6 1.1 3.35 0.47 3.40 5.2 5.6 3.32 3.21
953.0 9.9 0.7 5.51 0.64 5.75 9.2 9.9 5.50 5.63
II10
III10
IV10

52
M. M. Naoum et al.
or terphenyl moiety [5,19]. In the present case, however, the molecules are connected by
ester and azo groups. In fact, the ester group contains no multiple bonds in the chain of
atoms actually linking the benzene rings; however, conjugative interaction within the ester
group and the rings does lead to some double bond character and stiffer structure than
might be expected [20]. In our case, if the central ring is out of plane, the conjugative
interaction between both linking groups and the rings will not be allowed and consequently
the polarizability of the whole molecule will be too much disrupted and destroy any
possible mesophase formation. All members of the four homologous series (In-IVn) are
mesomorphic, hence they should all exist in planar linear conformations. Accordingly, we
are going to limit ourselves to the calculation of the theoretical dipole moments, using
the molecular modeling program [21], only to all possible planar conformations; these are
represented in Scheme 1 by the conformations 1–8. The results of computation for the
Scheme 1. Various possible planar conformations assigned for the homologous systems In–IVn.
eight possible planar conformers, for the four homologous series In–IVn, are collected in
Table 6, together with the experimentally determined apparent dipole moment values, µobs
.
As given in Table 6, the most probable conformation for each homologous series varies
according to the location and orientation of the fluoro-substituent. The dipole moment
components, µX and µY, for each homologous series are calculated by the same program
and the most probable conformations deduced, and the values are collected in Table 7. The
dipole moment component, µY, is that calculated along the long axis of the molecule, and
2
2
µX is accordingly the perpendicular, lateral component in such a way that µobs = µX
+
2
µY
.
In order to investigate the effect of dipole moment on the stability of the mesophase
√
exhibited by compounds In-IVn, values of T C are plotted as function of the observed
Table 6. Dipole moments (D) calculated for conformations (1–8) for compounds of series
In--IVn
Comp.
µobs
1
2
3
4
5
6
7
8
In
3.35
5.27
3.21
5.63
5.28
5.24
5.25
5.67
2.62
2.56
2.56
3.34
2.62
2.56
2.55
3.33
0.94
0.73
0.74
2.27
2.95
2.61
2.62
3.15
1.39
0.24
0.24
1.78
1.39
0.24
0.24
1.78
3.20
2.88
2.89
3.38
IIn
IIIn
IVn

Fluoro-Substituted Phenylazophenyl Benzoates
53
Table 7. Calculated dipole moments (µcalc) of the most probable conformations for the
homologous series, In–IVn, with their longitudinal (µY) and lateral (µX) components, and
anisotropy of dipole moment (ꢀµ)
Series
Conform.
µcalc
µY
µX
ꢀµ
In
8
1
8
1
3.20
5.24
2.89
5.67
3.15
5.23
2.85
5.53
0.17
0.60
0.20
0.26
2.98
4.7
2.65
5.27
IIn
IIIn
IVn
√
dipole moments, µobs in Fig. 3. As can be seen from Fig. 3, the T C linearly, but slightly,
decreases with the total dipole moment of the compound. Contrary to the effect of the
√
terminal substituent, the weak negative T C dependency on the dipole moment may be
attributed first to the negligible steric interaction of the lateral fluoro-substituent, second,
√
Figure 3. Dependence of T C on the observed dipole moment of the homologous series In-IVn:
(a) n = 8, (b) n = 10, (c) n = 12, (d) n = 14, and (e) n = 16.

54
M. M. Naoum et al.
√
Figure 4. Dependence of T C on the lateral dipole moment component, µX, calculated for the
homologous series In–IVn: (a) n = 8, (b) n = 10, (c) n = 12, (d) n = 14, and (e) n = 16.
to the weak conjugative interaction with the mesogenic portion of the molecule, and third,
to the weak dipole–dipole repulsion as a consequence of the parallel alignment of the
rod-shaped molecules. In order to decide which component of the dipole moment is more
pronounced on the lateral interaction between neighboring molecules that leads to stabi-
√
lization of the mesophase, values of T C are once plotted as function of the lateral, µX, and
another against the longitudinal, µY, components of the dipole moments in Figs 4 and 5,
respectively. Comparison between figures with Fig. 3 reveals that the dependency on either
of the dipole moment components, µX or µY, parallels that of the total dipole moment.
Such a destabilization effect (the negative slope) varies irregularly and slightly with the
increase of the terminal alkoxy-chain length. Since the anisotropy of molecular structure
is generally reflected into a dipolar anisotropy (ꢀµ = µY − µX), it would be worthy
to investigate the effect of the latter value on the mesophase behavior of the compounds
(Fig. 6). Again, as can be seen from Fig. 6, the mesophase stability decreases with ꢀµ. A
common feature observed from Figs 3–6 is that the mesophase stability of the homologues
IVn is always much above the linear dependency. This may be attributed to a face-to-face

Fluoro-Substituted Phenylazophenyl Benzoates
55
√
Figure 5. Dependence of T C on the longitudinal dipole moment component, µY, calculated for
the homologous series In–IVn: (a) n = 8, (b) n = 10, (c) n = 12, (d) n = 14, and (e) n = 16.
alignment of the rod-shaped molecules existing in conformation 1 that leads to increased
lateral adhesion and consequently greater nematic stability, in the lower homologues (n =
8 and 10), and the appearance of the SmC phase in all homologues of group IV.
3.4. Orientation of Lateral Fluorine and Entropy Change of Transition
The entropies of the nematic–isotropic (◦), smectic–isotropic (•), or smectic C–nematic (ꢀ)
transitions were calculated for all members of the four series I–IV and the results are shown
in Table 4, and represented graphically as a function of the alkoxy-chain length in Fig. 7.
Since the homologues In exist in the conformation 8 in which the two fluorine atoms are sit-
uated together on one side of the conformer but opposite to the ester C=O group, the lateral
dipole moment component (ꢀµX = 0.17 D) is small and the apparent dipole moment (3.2
D) is relatively small. Hence, the entropy change (ꢀSN-I, Fig. 7(a)) increases slightly with n
up to n = 12 carbons, and due to the increased lateral adhesion with increase of alkoxy-chain
length to n = 16, the entropy change jumps to 4.9 J/mol/K as a result of the formation of the
more ordered SmC phase. With respect to the homologues IIn, they exist in conformation 1

56
M. M. Naoum et al.
√
Figure 6. Dependence of T C on the dipolar anisotropy, ꢀµ, calculated for the homologous series
In–IVn: (a) n = 8, (b) n = 10, (c) n = 12, (d) n = 14, and (e) n = 16.
with the lateral fluorine atoms attached to one and the same side that carries the ester C=O
group of the conformer. Such an arrangement has led to the increase of the lateral dipole
moment component (ꢀµX) to 0.6 D and an apparent dipole moment value of 5.24 D. These
enhanced dipole moment values facilitate the generation of the SmC phase starting from
the homologue II12. Similar to homologues In, the homologues IIIn exist in the conformer
8 with lower dipole moment values (ꢀµX = 0.2 and µobs = 2.89 D). The mesophase ob-
served in all homologues investigated were of the SmC type and the entropy changes during
phase transition are relatively small. Alternatively, the homologues IVn resemble those of
IIn in which they exhibit in the conformation 1 and have apparent dipole moment values
of 5.67 D. Accordingly, the lower homologues, IV8 and IV10, are dimorphic possessing
both SmC and N mesophases with relatively high entropy of transitions. Higher homo-
logues are purely smectogenic with high entropy change (e.g., ꢀSC–I = 28.0 and 21.8 J/
mol/K for the homologue IV8 and IV12, respectively). Alternatively, when comparison
is made between isomers of the four series, i.e., bearing similar alkoxy groups, ꢀSN–I
decreases in an order that differs according to the relative orientation of the two lateral
fluorine substituents. Thus,

Fluoro-Substituted Phenylazophenyl Benzoates
57
for n = 8, ꢀSN–I decreases in the order:
IV > III > I > II;
for n = 10, ꢀSN–I decreases in the order:
III > IV > II > I;
for n = 12, ꢀSN–I decreases in the order:
IV > III, while ꢀSC–I for I > II; and
for n = 16, ꢀSC–I decreases in the order:
IV > I > II.
Figure 7. Entropy change of transition (ꢀS) as a function of alkoxy-chain length (n).
3.5. Mesophase Behavior of Binary Mixtures
Turning now to the investigation of the mesophase behavior of mixed systems, we have
to limit such investigation to binary mixtures of isomeric components bearing the same
alkoxy-chain but differing in the position of the fluoro-substituents. This still leads to a great
number of phase diagrams. So, it seems sufficient to investigate, as examples, mixtures with
isomeric components bearing extreme alkoxy-chain length, namely, n = 8 and 16 carbons.

58
M. M. Naoum et al.
3.5.1. Binary Mixtures of Isomers, In /IIn. The binary phase diagrams of the two binary
systems, each made from the two isomers from series In and IIn, are illustrated for n =
8 and 16 in Fig. 8. As can be seen from the figure, in the binary mixture of the lower
homologues I8/II8, since both components are nematogenic, their binary mixture forms
an ideal solution in which linear TN–I composition dependence is observed. In the higher
homologous system (I16/II16), the SmC predominates since both components are smecto-
genic (SmC) and, consequently, ideal mesophase behavior takes place as evidenced by the
linear TC–I composition dependence. Such an ideal mesophase behavior may be attributed
to a back-to-face arrangement of molecules existing between the conformation 8 (for the
homologues In) and conformation 1 (for the homologues IIn; see Fig. 8). With respect to
Figure 8. Binary phase diagrams of the systems (In/IIn): (a) n = 8, (b) n = 16.

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59
their solid mixtures, the first system (I8/II8) forms a continuous solid composition, while
the second (I16/II16) showed eutectic composition. This behavior indicates that the crystal
lattices of the two components are not dependent on molecular structure.
3.5.2. Binary Mixtures of Isomers, In /IIIn. The binary phase diagrams of the two binary
systems, I8 /III8 and I16 /III16, are illustrated in Fig. 9. In the lower homologous system
I8 /III8, the two components form a continuous solid composition with an ideal nematic
behavior since both components are nematogenic. This behavior may have led to a back-to-
face alignment of the conformation 8 for both components (I8 and II8). The reverse holds
true for the higher homologous system, I16 /III16, where their mixtures exhibit eutectic
composition while in the melt, mesophase separation takes place. This may be attributed
to the difference in the mesomorphic nature of the two components of the mixture where
the first component I16 is nematogenic while the other III16 is smectogenic.
Figure 9. Binary phase diagrams of the systems (In/IIIn): (a) n = 8, (b) n = 16.

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M. M. Naoum et al.
3.5.3. Binary Mixtures of Isomers, In /IVn. The corresponding binary phase diagrams of
the two binary systems, I8/IV8 and I16/IV16, are illustrated in Fig. 10. In their binary
mixtures, the components of the lower homologous system are of different mesomorphic
nature in which I8 is nematogenic while I16 is dimorphic (N and SmC). Accordingly,
their mixtures exhibit ideal nematic dependence, evidenced by the linear TN–I composition
dependence, while the SmC phase of IV8 completely disappeared upon the addition of ≈40
mol% of I8. Their solid mixtures possess a eutectic composition at about ≈40 mol% of I8.
On the other hand, since the two components of the higher system I16/IV16 exhibit only
Figure 10. Binary phase diagrams of the systems (In/IVn): (a) n = 8, (b) n = 16.

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61
the SmC phase, their mixture showed an ideal mesophase behavior. Eutectic composition is
also shown by this system. The ideal mesophase composition behavior may be rationalized
through a back-to-face alignment of the conformer 8 (for In) and 1 (for IVn). As mentioned
above, the fluoro-substituent is not very different in size to the hydrogen it replaces, so it
will not cause a significant steric effect in the alignment of the two isomeric components.
3.5.4. Binary Mixtures of Isomers, IIn /IIIn. The binary phase diagrams of the two binary
systems, II8/III8 and II16/III16, are illustrated in Fig. 11. For the lower system II8 /III8
both components are nematogenic, hence their mixture exhibits ideal mesophase behavior
Figure 11. Binary phase diagrams of the systems (IIn/IIIn): (a) n = 8, (b) n = 16.

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M. M. Naoum et al.
with eutectic solid composition. A back-to-face arrangement is probable. Conversely, the
mesophase of the higher homologous mixture suffers mesophase separation since the
component II16 is smectogenic while III16 is nematogenic.
3.5.5. Binary Mixtures of Isomers, IIn /IVn. The binary phase diagrams of the two bi-
nary systems, II8/IV8 and II16/IV16, are illustrated in Fig. 12. In the lower homologous
Figure 12. Binary phase diagrams of the systems (IIn/IVn): (a) n = 8, (b) n = 16.

Fluoro-Substituted Phenylazophenyl Benzoates
63
mixture II8/IV8, the component II8 is nematogenic while the other component IV8 is
dimorphic (N and SmC). Accordingly, their mixture shows an ideal nematic behavior with
the disappearance of the SmC phase of IV8 upon addition of about 40 mol% of II8. In the
other system, II16/IV16, both components exhibit the SmC phase, and as a result their mix-
ture exhibits an ideal SmC composition dependence. A back-to-face arrangement is quite
suitable to explain such ideal behavior. In both systems, eutectic composition was detected.
3.5.6. Binary Mixtures of Isomers, IIIn /IVn. The binary phase diagrams of the two binary
systems, III8/IV8 and III16/IV16, are illustrated in Fig. 13. As with the previous system,
Figure 13. Binary phase diagrams of the systems (IIIn/IVn): (a) n = 8, (b) n = 16.

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M. M. Naoum et al.
the lower homologous mixture showed ideal nematic-composition dependence, while the
SmC phase of IV8 disappears upon addition of about 60 mol% of III8. Also, the difference
in the mesophase characteristics of the two components of the higher homologous system
has caused mesophase separation. Both systems showed eutectic composition.
4. Summary and Conclusions
Four homologous series of the laterally di-fluoro substituted derivatives, 4-(2ꢀ-(or 3ꢀ-) fluoro
phenylazo)-2-(or 3-) fluoro phenyl-4ꢀꢀ-alkoxybenzoates, were prepared. These compounds
were investigated for their mesophase behavior in pure and mixed states.
The second lateral fluorine atom introduced into the central benzene ring increases
further the lateral dipole moment as well as disrupting the symmetry of the molecules thus
generating molecular tilting to various extents. Combining the added properties of the two
lateral fluoro-substituents, significant changes were observed in the mesomorphic behavior
of the compounds and their binary mixtures. Varying extents of mesomeric interactions
between the central fluoro-substituent and the ester C O group, depending on its position,
are reflected on the resultant dipole moments, and consequently on their mesomorphic
properties.
Various conformations were deduced for the four homologous series via measurement
of the dipole moment and comparing it with those theoretically calculated for the various
linear planar conformations. The conformations deduced were found to vary according to
the relative position of the two fluorine atoms attached to the central and terminal rings.
Binary phase diagrams were constructed for binary mixtures of two isomers (with
n = 8 and 16 carbons) of the same skeletal structure except for the position of two lateral
fluoro-substituents.
Except for the two systems I8/II8 and I8/III8, all binary mixtures investigated were
found to exhibit eutectic compositions with relatively low melting points that vary between
60◦C and 80◦C dependent on the relative position of the two fluoro-substituents on each
of the two components of the mixture. Investigation of the binary mesophase behavior
revealed that the two molecules of most of the binary mixtures investigated are arranged in
a back-to-face pattern.
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