Synthesis and mesomorphic properties of 2,4-bis(4′-n-pentyloxybenzoyloxy)- benzylidine-4″- n-alkoxyaniline

Article abstract of DOI:10.1080/15421406.2017.1357426

The synthesis and mesomorphic properties of a new series of 2,4-bis(4′-npentyloxybenzoyloxy)- benzylidine-4″ -n-alkoxyaniline (DC5An) are reported. The molecular structure of compounds was confirmed by FTIR, ¹H-NMR, ¹³C-NMR, mass spectroscopy and elemental analysis. The mesomorphic properties were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) measurements. All compounds of the series exhibit nematic (N) and smectic C (SmC) phases. The first four homologues (DC5A1-DC5A4) display a N mesophase, whereas the highest homologues (DC5A5-DC5A10) exhibit an enantiotropic dimorphism N and SmC phases. The mesomorphic properties of the present series are compared and discussed with other structurally related series.

Full text of DOI:10.1080/15421406.2017.1357426

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and mesomorphic properties of 2,4-
bis(4′-n-pentyloxybenzoyloxy)- benzylidine-4″- n-
alkoxyaniline
Wali M. Hamad, Hashim J. Azeez & Ammar H. Al-Dujaili
To cite this article: Wali M. Hamad, Hashim J. Azeez & Ammar H. Al-Dujaili (2017): Synthesis
and mesomorphic properties of 2,4-bis(4′-n-pentyloxybenzoyloxy)- benzylidine-4″- n-alkoxyaniline,
Molecular Crystals and Liquid Crystals, DOI: 10.1080/15421406.2017.1357426
To link to this article: http://dx.doi.org/10.1080/15421406.2017.1357426
Published online: 07 Sep 2017.
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Date: 09 September 2017, At: 03:46

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
https://doi.org/./..
Synthesis and mesomorphic properties of
,-bis(ꢀ-n-pentyloxybenzoyloxy)- benzylidine-ꢀꢀ-
n-alkoxyaniline
Wali M. Hamada, Hashim J. Azeezb, and Ammar H. Al-Dujailic
aDepartment of Chemistry, Koya University, Arbil, Iraq; bDepartment of Chemistry, College of Education,
University of Salahaddine, Arbil, Iraq; cHamdi Mango Center for Scientific Research, University of Jordan, Amman,
Jordan
ABSTRACT
KEYWORDS
Enantiotropy; lateral
substituent; mesomorphism;
mesogen; nematic; smectic
The synthesis and mesomorphic properties of a new series of 2,4-bis(4ꢀ-
npentyloxybenzoyloxy)- benzylidine-4ꢀꢀ -n-alkoxyaniline (DC5An) are
reported. The molecular structure of compounds was confirmed by
1
FTIR, H-NMR, 13C-NMR, mass spectroscopy and elemental analysis.
The mesomorphic properties were studied by differential scanning
calorimetry (DSC) and polarizing optical microscopy (POM) measure-
ments. All compounds of the series exhibit nematic (N) and smectic C
(SmC) phases. The first four homologues (DC5A1-DC5A4) display a N
mesophase, whereas the highest homologues (DC5A5-DC5A10) exhibit
an enantiotropic dimorphism N and SmC phases. The mesomorphic
properties of the present series are compared and discussed with other
structurally related series.
Introduction
For low molecular weight mesomorphic compounds, a major point receiving great attention
is the detailed analysis of the relationship between chemical composition, structure, and
mesomorphic properties [1–3]. Mesomorphic materials have become good candidates for a
potentially wide range of applications, such as in the optical, electrical, and biological medical
fields [4–14].
Aromatic azomethine ester comprising of different polarity of substituents has been
known to either promote or suppress the mesomorphic properties [15, 16]. The aro-
matic esters are known for their thermal stability, easy synthesis, and relative resistance to
hydrolysis [17].
In this work, we describe the synthesis and mesomorphic properties of homologous series
containing three rings in the main core linked through ester and azomethine central linkages
and substituted by a lateral aromatic branch on the central benzene ring, and investigated the
influence of the lateral alkoxy group on mesomorphism.
CONTACT Ammar H. Al-Dujaili
Jordan, Amman,  Jordan.
ah.aldujaili@gmail.com
Hamdi Mango Center for Scientific Research, University of
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

2
W. M. HAMAD ET AL.
Experimental
Materials
4-Hydroxyacetanilide, 2,4-dihydroxybenzaldehyde, tetrahydrofuran (THF), triethylamine
(Et3N), benzene, absolute ethanol, thionyl chloride, potassium hydroxide, 4-hydroxybenzoic
acid, hydrochloric acid, sodium carbonate, and glacial acetic acid were of reagent grade
obtained from Sigma-Aldrich Chemicals and used as received.
Instrumentations
Elemental analyses (C, H, N) were carried out using a Perkin-Elmer model 2400 instru-
1
ment. H-NMR and 13C-NMR spectra were obtained with Avance III Bruker 500 MHz
instrument using tetramethylsilane as the internal standard. Infrared spectra were recorded
on a Shimadzu 8000 FTIR spectrophotometer in the wave number range 4000–400 cm−1
using KBr discs or films. Mass spectra were recorded on IEOL JMS-7 high resolution
instrument.
The optical behavior observations were made using Olympus BX40 microscope equipped
with a Leitz Laborlux 12 Pols hot stage and PR600 controller. The textures shown by the
compounds were observed using polarized light with crossed polarizer (POM) with the sam-
ple in a thin film sandwiched between a glass slide and cover slip. Photographs of texture
were obtained using a camera Olympus PM-10AD model. The transition temperatures and
enthalpies were investigated using differential scanning calorimetry DSC (TA instruments
Q1000 DSC), ramp rate: 10°C per min under nitrogen atmosphere. Temperature and heat
flow calibrated with standard indium of purity <99.99%.
Synthesis
4-n-pentyloxybenzoic acid, 4-n-pentyloxybenzoyl chloride, 4-n-alkyoxyacetanilides, and
4-n-alkoxyanilines were prepared using conventional methodologies [18, 19]. 2,4-Bis(4ꢀ-n-
heptyloxybenzoyloxy) benzaldehyde (DC5) was synthesized by the method of Pugh and
Schrock and Zhou et al. [20, 21].
Synthesis of series
2,4-bis(4ꢀ-n-heptyloxybenzoyloxy)-benzylidine-4ꢀꢀ-n-alkyloxyaniline (DC5An)
2,4-Bis(4ꢀ-n-pentyloxybenzoyloxy) benzaldehyde (0.31 g, 0.5 mmol) and 4-n-
alkyloxyanilines (0.5 mmol) and a few drops of glacial acetic acid as catalyst in (10 mL)
of an absolute ethanol were refluxed for 6–7 hr. The yellow precipitate formed during the
cooling of mixture was filtered off, washed two or three times with cold ethanol. Finally,
the solid that was obtained (DC5An) was recrystallized from an absolute ethanol until the
transition temperature remained constant. Yield (69%–72%).
FTIR Spectra for AC5A2
FTIR (KBr, cm−1): 2960–2850 (νC–H, aliphatic); 1740–1725 (νC=O, ester); 1605–1625 (νC=N
azomethine); 1105–1250 (νC−O−C, aryl-ether); 619–889 (νC–H, aromatic).
,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
1H-NMR spectra for AC5A2
1
–
= –
H-NMR (DMSO-d6), ppm: δ = 8.61 (s, 1H, CH N ); 6.87–8.13 (dd, 15H, aromatic rings);
–
–
–
–
– – –
3.99–4.09 (t, 6H, CH2O ); 1.40–3.35 (m, 15H, 6 CH2 and O C CH3) and 0.89–0.92
(t, 6H, CH3).
–
13
–
C NMR spectra for AC5A2
13
–
–
–
– = –
C NMR (DMSO-d6), ppm: δ = 163.91–164.55 ( COO ); 162.08 ( C N ); 158.3, 154.45,
152.79, 144.21, 133.78, 132.64, 126.75, 123.11, 188.16, 115.33, 110.57 for aromatic carbons;
68.5, 63.61 ( CH2O–); 40.62, 40.01, 39.57, 35.55, 28.6, 28.07, 22.34 for methylene carbons;
15.11, 14.36 [ CH3].
–
–
Results and discussion
Characterizations
The synthetic route used for the preparation of 2,4-bis(4ꢀ-n-pentnyloxybenzoyloxy)
benzylidine-4ꢀꢀ-n-alkyloxyaniline (DC5An) is shown in Scheme 1. All compounds were char-
1
13
–
acterized by elemental analysis, FTIR, H-NMR, and C NMR spectroscopy. The percentage
yields and analytical data of each compound in the series are listed in Table 1. The percentages
of C, H, and N from the elemental analysis were in good agreement with the calculated values
for compounds DC5An.
The FTIR spectrum of compounds of series CD5An showed bands at 2960–2850 (νC–H
aliphatic); 1740–1725 (νC=O, ester); 1605–1625 (νC=N, azomethine); 1105–1250 (νC−O−C
aryl-ether); 619–889 (νC–H, aromatic).
,
,
The 1H-NMR spectrum of series CD5An showed the following characteristic absorptions:
–
= –
δ = 8.61–8.72 singlet for CH N group; 6.55–8.33 doublet of doublet for aromatic rings;
–
–
3.99–4.09 triplet for CH2O in heptyloxy terminal groups; 3.35–3.53 triplet or quartet for
CH2O group of alkoxy terminal group attached to the aniline ring; 1.76–1.80 triplet for
–
–
–
–
–
CH3 of alkoxy terminal groups attached to the aniline ring; 1.31–1.52 multiplet for CH2
units in alkoxy chain; 0.89–0.92 triplet for methyl group of pentyloxy terminal chain.
13
–
The C NMR spectrum of compounds of series CD5An showed the following signals:
13
–
–
–
– = –
C NMR (DMSO-d6), ppm: δ = 163.91–164.55 ( COO ); 162.08 ( C N ); 158.3– 110.57
–
–
for aromatic carbons; 68.5–63.61 ( CH2O ); 40.62– 22.34 for methylene carbons; 15.11–
1
13
–
–
14.36 [ CH3]. The data of FTIR, H-NMR, and C NMR spectrum along with elemental
analyses data confirmed the structural formula of the synthesized compounds.
Mesomorphic properties
The phase transition temperatures and corresponding enthalpy changes of series DC5An
were determined using a DSC. The mesophases of all the compounds were observed under
a polarizing optical microscopy (POM) during heating and cooling cycles. The thermal
behavior obtained by DSC is consistent with the data observed by POM, where it was pos-
sible to identify the mesomorphic character of the mesophases. The data obtained from the
DSC and POM analyses are summarized in Table 2. On heating, the first endothermic peak
accompanied with high enthalpy indicates a transition from solid crystalline state to a liquid
crystalline mesophase. The second and third endothermic transitions at higher temperature

4
W. M. HAMAD ET AL.
Scheme . Synthetic route for compounds DCAn, reagents, and conditions: (i) CHBr, KOH, Ethanol/HO,
Reflux,  hr; (ii) SOCl, CH, Reflux,  hr; (iii) THF, EtN, °C,  hr; (iv) CnHn+Br, KOH, Ethanol, Reflux,  hr;
(v) KOH, Ethanol, Reflux,  hr; and (vi) Ethanol, Glacial acetic acid, Reflux,  hr.
presented lower associated enthalpy, which indicates transitions between distinct mesophases
and to the isotropic state.
All the members of series DC5An exhibit enantiotropic mesophases. The CD5A1 to
CD5A4 homologues are purely enantiotropic nematogenic. With a further increase in the
length of the alkoxy chain from DC5A5 to DC5A10 the enantiotropic smectic C (SmC) phase
was observed, which accompanied the nematic (N) phase. The N phase starts from n = 1 and
persists up to the last member of the series. The persisting of N phase indicates that the net
effect imparted due to the end-to-end cohesive forces in this series is similar as all the other
molecular forces are the same.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
Table . Elemental analysis data for the series DCAn.
Analysis % found (calcd.)
Compound
Molecular formula
CHON
Molecular weight (g mol−
)
C
H
N
(DCA)
(DCA)
(DCA)
(DCA)
(DCA)
(DCA)
(DCA)
(DCA)
(DCA)
(DCA)








.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
.(.)
C
HON
C ON
CHON
HON
CHON
H
C
C
C
C
C
HON
HON
HON
HON


The increasing of carbon chain (n > 4) in the studied series aids the formation of smectic
mesophase due to enhanced lateral attraction as compared with terminal attractions, which
facilitates lamellar packing required for the formation of the smectic mesophase [22]. In con-
trast, the short terminal alkoxy group in the first four compounds enables the significant forces
of attraction which serve to stabilize parallel orientation by end-to-end molecular attractions
required for the formation of N mesophase only. The appearance of smectic A (SmA) phase
for compound DC5A9 was understandable, but could be due to the fact that the three alkoxy
terminal groups with 5, 5, and 9 carbon atoms give rise to more planar and polarizable geo-
metrical structure.
The plot of transition temperatures against the number of carbon atoms in the alkoxy chain
(Fig. 1) shows a steady fall in N→I transition temperatures and exhibits a marked odd–even
effect up to the n-octyloxy derivative and then falls as series is ascended. The SmC→N transi-
tions curve rises up to the DC5A8 derivative and then falls smoothly for the last higher mem-
bers. The Cr→SmC transition curve rises, passes through maxima at n-octyloxy derivative,
and descends in a usual expected manner.
The mesophases exhibited by compounds of series DC5An were identified according to
their optical textures, which were observed by POM, using the classification systems reported
by Demus, and Richter [23] and Dierking [24].
All of the compounds in the series DC5An display N mesophase. The optical photomi-
crographs of DC5A4 are depicted in Fig. 2 as a representative illustration. Upon cooling the
isotropic liquid of each compound, N droplets appeared and coalesced to form the typical
schlieren texture with characteristic two- and four-brush singularities.
Table . Transition temperatures and associated enthalpy changes of series CDAn.
Compound
Transition temperatures (°C)(ꢀH, kJ mol−
)
DCA
DCA
DCA
DCA
DCA
DCA
DCA
DCA
DCA
DCA
Cr  (.) N  (.) I
Cr  (.) N  (.) I
Cr  (.) N  (.) I
Cr  (.) N  (.) I
Cr  (.) SmC  (.) N  (.) I
Cr  (.) SmC  (.) N  (.) I
Cr  (.) SmC  (.) N  (.) I
Cr  (.) SmC  (.) N  (.) I
Cr  (.) SmC  (.) SmA  (.) N  (.) I
Cr  (.) SmC  (.) N  (.)
=
Cr = Crystal; N = Nematic; SmA = smectic A; SmC smectic C; I = Isotropic Transition.

6
W. M. HAMAD ET AL.
Figure . Dependence of transition temperatures on the increasing number of carbon atoms (n) in the ter-
minal alkoxy chains for the CDAn series compounds.
The homologues with longest alkoxy chain (n = 5–10) exhibit an enantiotropic dimor-
phism N and SmC except for the homologue with n = 9 additional SmA phase is observed.
As indicated above, upon further cooling of the N phase, the emergence of the SmC phase was
observed in its schlieren texture (Figs. 3 and 4). Figure 5 displays the SmA mesophase texture
which was identified by the appearance of a broken focal-conic texture.
It was found that there is a close relationship between mesomorphism and molecular con-
stitution of mesomorphic compounds [25–31]. This relationship could be correlated with the
molecular constitution of these compounds by the thermal stability, which is a measure of
mesomorphism (the mean value of the transition temperature from the mesomorphic state
to the isotropic liquid state).
Table 3 shows the comparison of transition temperatures, mesomorphic properties, and
the thermal stability, of the present series (DC5An) and the structurally related compounds I
[14], II [13], and DC9A2 [32] reported in literature. Compounds DC9A2 and DC5A2 show
a very similar mesomorphic behavior (in terms of transition temperatures and mesophase
type), displaying N mesomorphism. The members in both series with short alkoxy chain (n
= 2) show a N mesophase; a lateral interaction to give rise to a layered smectic order is not
favored because of the difference of the volume occupied by the flexible melted chains (one
Figure . Mesophase optical texture of the N phase obtained on cooling. A schlieren texture and the typical
N droplets (× magnification) for the compound DCA at °C.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
Figure . Mesophase optical texture of the transition stripes at N (thread-like texture) to SmC (schlieren
texture) phase transition during the cooling (× magnification) for the compound DCA at °C.
Figure . Mesophase optical textures exhibited by (DCA) on heating from crystal phase, the typical
thread-like (schlieren) texture of (SmC) phase at °C (× magnification).
side is fixed in C9 and C5, respectively), and only a short range positional order is achieved.
As the length of both alkoxy chains becomes longer, the smectic order is favored.
The Schiff’s bases (series DC5An and DC9An) and the azo compounds (series I and II)
have the same central rigid core and the same alkoxy chain. The difference between these
Figure . Mesophase optical texture of the SmA phase obtained on cooling. A typical focal-conic fan texture
(× magnification) for the compound DCA at °C.

8
W. M. HAMAD ET AL.
Table . Comparison between the transition temperatures, mesomorphism, and average thermal stabilities
for the series I, II, DCA, and DCA.
Serie
Transition temperatures°C−
Thermal stability°C−
I
II
Cr  N  I
Cr  SmC ?I
Cr  N  I
Cr  N  I
. SmC→I (C–C)
. SmC→I (C, C). N→I (C, C, C, C)
. SmC→I (C–C) . N→I (C–C)
. SmC→I (C–C) . N→I (C–C)
DCA
DCA
series is in the azomethine and azo linkage group. The results show that the azomethine
compounds have a higher thermal stability than azo compounds. It seems likely that, in
comparison, the increased mesogenic character of Schiff base compounds is due to the
azomethine group which is coplanar with the aromatic rigid core; these compounds therefore
have a higher conjugated character and a major polarizability, both of which favor mesophase
formation and, in particular, smectic phases.
Conclusions
A new series of 2,4-bis(4ꢀ-n-pentyloxybenzoyloxy)benzylidine-4ꢀꢀ-n-alkoxyaniline (DC5An)
has been prepared by varying terminal lateral alkoxy chain length (n = 1–10). The chem-
1
ical structures of these compounds were examined by elemental analysis, FTIR, H-NMR,
13
–
C NMR, and mass spectroscopy. The results are in agreement with the considered molec-
ular structure. The mesomorphic properties and optical textures of the resultant compounds
were characterized by DSC and POM. The change in terminal alkoxy chain length has

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
9
pronounced effect on the mesomorphic behavior. The first four homologues, i.e., (DC5A1–
DC5A4) exhibited N mesophase and that DC5A5–DC5A10 displayed dimesomorphism N
and SmC phases. The mesomorphic properties and thermal stabilities of the present series
are compared with those of structurally related homologous series.
References
[1] Demus, D., Goodby, J., Gray, G. W., Spiess, H. W., & Vill, V. (1999). Physical Properties of Liquid
Crystals, Wiley-VCH: New York.
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