Design, synthesis and liquid crystalline behavior of ethyl 4-((4-alkoxyphenyl)diazenyl)benzoates

Article abstract of DOI:10.1080/15421406.2018.1442779

A series of ethyl 4-((4-alkoxyphenyl)diazenyl)benzoate is synthesized and the liquid crystalline behavior studied. Synthesized compounds are characterized by FTIR, ¹H-NMR, Mass and single crystal XRD. Differential scanning calorimetry was carri

Full text of DOI:10.1080/15421406.2018.1442779

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Design, synthesis and liquid crystalline behavior of
ethyl 4-((4-alkoxyphenyl)diazenyl)benzoates
Hasnain Mehmood, Zahoor Ahmad, Tashfeen Akhtar, Humaira M. Siddiqi,
Asma Iqbal, Shahid Hameed & Muhammad N. Tahir
To cite this article: Hasnain Mehmood, Zahoor Ahmad, Tashfeen Akhtar, Humaira M. Siddiqi,
Asma Iqbal, Shahid Hameed & Muhammad N. Tahir (2018) Design, synthesis and liquid crystalline
behavior of ethyl 4-((4-alkoxyphenyl)diazenyl)benzoates, Molecular Crystals and Liquid Crystals,
664:1, 14-23, DOI: 10.1080/15421406.2018.1442779
To link to this article: https://doi.org/10.1080/15421406.2018.1442779
Published online: 21 Jun 2018.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Design, synthesis and liquid crystalline behavior of ethyl
-((-alkoxyphenyl)diazenyl)benzoates
Hasnain Mehmooda, Zahoor Ahmada, Tashfeen Akhtara, Humaira M. Siddiqib,
Asma Iqbalb, Shahid Hameedb, and Muhammad N. Tahirc
aDepartment of Chemistry, Mirpur University of Science and Technology (MUST), Mirpur, AJK, Pakistan;
bDepartment of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan; cDepartment of Physics, University of
Sargodha, Punjab, Pakistan
ABSTRACT
KEYWORDS
Azophenyl; OLED; POM;
semestic phases
A series of ethyl 4-((4-alkoxyphenyl)diazenyl)benzoate is synthesized
and the liquid crystalline behavior studied. Synthesized compounds are
characterized by FTIR, 1H-NMR, Mass and single crystal XRD. Differential
scanning calorimetry was carried out to study the phase transitions and
enthalpy changes. Polarizing optical microscopy revealed the meso-
genic properties. These techniques revealed that three compounds
(2a, 2b, 2f) exhibit liquid crystalline properties in the range of 80–104°C.
POM exhibiting focal conical fan like texture which revealed the pres-
ence of smectic phases suggestive of their use in LCD and temperature
sensing devices, besides 2c can be used as green emitter in OLEDs.
Introduction
The discovery of liquid crystal state of matter has had major influence on vast fields of science
and technology. Various research groups have exploited the significance of this novel state
according to its need in various fields such as display systems [1], optoelectronic displays [2]
and structure optimization for minimizing light leakage in liquid crystal displays [3]. The
chemists synthesize new liquid crystals and establish their mesomorphic properties. Several
liquid crystalline materials containing azo, azoxy [4], azoesters, ethers [5] and olefin [6] have
been reported. These moieties keep the core linear, assuring the general structural design of
linear thermotropic liquid crystals. Other construction design of linear thermotropic liquid
crystals include linearly linked ring system, containing rigidity maintaining moieties, flexible
chains, enduring dipoles or polarisable groups [7]. Flexible chains are linked either directly
to the rings or through some hetero atoms such as Oxygen, Nitrogen and Sulphur. The hetero
atoms induce anisotropy in the molecule [8]. Besides maintaining the rigidity of core, Linking
groups are also important to induce versatility of applications. Among these linking groups
azo group is studied for its interesting optical applications for example holography, optical
switching and optical storage devices [9,10], azo dyes [11] and light controlled alignment
– = –
of liquid crystals [12]. Photo isomerization of aromatic azo compounds (Ar N N Ar’) is
=
much easier than geometrical isomerization of C C containing compounds. This unique
CONTACT Tashfeen Akhtar
tashfeenchem@must.edu.pk; tashfeenchem@gmail.com
pur University of Science and Technology (MUST), Mirpur-, AJK, Pakistan.
Department of Chemistry, Mir-
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

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
15
characteristic of azo compounds presents more opportunities in photonic applications.
Photo isomerization explains the photo responsive nature of azo compounds including opti-
cal switching [13], photo chemical phase transition in holographic applications [14], photo
imaging [15], photo responsive dopants [16] and polarization holography [17]. In general azo
based liquid crystals have more advantages over the one with ester, ether or olefinic linkages.
Azo compounds are thermally stable so they are strong candidates to study photo induced
effects [18]. Lateral or terminal substitutions in azo compounds induce liquid crystallinity at
low temperature [19]. Considering all these characteristics, present investigation is planned
to synthesize a new homologous series containing azo as linking group in order to under-
stand and establish the relation between liquid crystal and molecular structure by changing
1
flexible part. This study includes synthesis, characterization (IR, UV, H-NMR, 13C-NMR,
Mass spectrometery, Single Crystal XRD) and investigation of mesomorphism through
Differential Scanning Calorimetry (DSC) and Polarizing Optical Microscopy (POM).
Experimental
The following reagents were used as received: ethyl 4-aminobenzoate, hydrochloric acid
(Merck), sodium nitrite (Merck), phenol (Riedel-de Haen AG Seelze-Hannover), sodium
hydroxide (Merck) and alkyl halides of varying chain length (Bromo butane to bromo octade-
cane) (Merck). Acetone (Riedel-de Haen), methanol and n-hexane (Riedel-de Haen) were
used as solvent.
Ultraviolet absorption spectra were recorded on Shimadzu Ultraviolet-1800 spectropho-
tometer. IR spectra were recorded on Shimadzu nicolet iS10 FTIR spectrophotometer.
1H-NMR spectra were recorded on Bruker Avance 300 MHz NMR spectrometer. X-ray
diffractions were recorded on Bruker Kappa APEX II CCD Diffractometer with a graphite
˚
monochromator Mo Kα radiation (λ = 0.71073 A) at room temperature, the structures were
resolved using SHELXL2014/7 (Sheldrick, 2014). Mesomorphic properties were determined
by optical polarizing microscope. The thrmal transition temperatures were taken from the
position of peak maxima. The textures of the mesophases were examined on a Leica DM-
2500p polarising microscope fitted with temperature controller and a hot stage. Transition
temperatures were determined by differential scanning calorimetry using Mettler Toledo
Model DSC 851 e, Switzerland. GCMS spectra were recorded using mass spectrometer Model
6890n, inert mass detector (5870) Agilent Technologies, USA.
Synthesis of ethyl 4-((4-alkoxyphenyl)diazenyl)benzoates (2a-2h)
Ethyl 4-((4-hydroxyphenyl)diazenyl)benzoate (0.001 mol) was dissolved in acetone (20 mL)
and added potassium carbonate (3 eq), to this reaction mixture respective alkyl halide (1.0 eq)
was added and the reaction mixture was subjected to reflux and the completion of the reaction
monitored by TLC. After completion the reaction mixture was poured into cold water and the
precipitated solid was filtered, dried and recrystallzied from ethanol water.
Ethyl 4-((4-butyloxyphenyl)diazenyl)benzoate (2a)
1
λmax = 361 nm; FTIR (ATR): 2958, 1708, 1597, 1473, 1246 cm−1; H-NMR (300 MHz,
acetone-d6, δ ppm): 1.0 (t, 3H, CH3), 1.4 (t, 3H, CH3), 1.5 (Sextet, 2H, CH2), 1.8 (quintet,
2H, CH2), 4.1 (t, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.2 (d, 2H, 2xCH, Ar-H), 7.9- 8.0 (m, 4H,
4xCH, Ar-H) and 8.2 (d, 2H, 2xCH, Ar-H); 13C-NMR (75 MHz, acetone-d6, δ ppm):165.30,

16
H. MEHMOOD ET AL.
162.63, 155.19, 146.70, 131.74, 130.40, 125.11, 122.28, 114.92, 67.97, 60.86, 31.06, 18.97, 13.71,
13.23; EIMS: m/z 326 (M+), 311, 298, 297, 283, 281, 269, 253, 177, 149, 73.
Ethyl 4-((4-pentyloxyphenyl)diazenyl)benzoate (2b)
1
λmax = 360 nm; FTIR (ATR): 2939, 1708, 1600, 1469, 1260 cm−1; H-NMR (300 MHz,
acetone-d6, δ ppm): 0.9–1.0 (t, 3H, CH3), 1.4–1.6 (m, 7H, CH3, 2x CH2), 1.8–1.9 (quintet, 2H,
CH2), 4.1–4.2 (triplet, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.1–7.2 (d, 2H, 2xCH, Ar-H),7.9-
8.0 (m, 4H, 4xCH, Ar-H) and 8.2 (d, 2H, 2xCH, Ar-H); 13C-NMR (75 MHz, acetone-d6,
δ ppm): 165.30, 162.65, 155.20, 146.70, 131.75, 130.42, 125.11, 122.27, 114.95, 68.25, 60.87,
29.73, 28.19, 22.22, 13.69, 13.42; EIMS: m/z 340 (M+), 325, 311, 283, 267, 191, 163, 177, 73.
Ethyl 4-((4-hexyloxyphenyl)diazenyl)benzoate (2c)
1
λmax = 360 nm; FTIR (ATR): 2939, 1702, 1602, 1471, 1255 cm−1; H-NMR (300 MHz,
acetone-d6, δ ppm): 0.9 (t, 3H, CH3), 1.3–1.6 (m, 9H, CH3, 3x CH2), 1.8–1.9 (quintet, 2H,
CH2), 4.1–4.2 (triplet, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.1–7.2 (d, 2H, 2xCH, Ar-H),7.9-
8.0 (m, 4H, 4xCH, Ar-H) and 8.2 (d, 2H, 2xCH, Ar-H); 13C-NMR (75 MHz, acetone-d6, δ
ppm): 165.30, 162.65, 155.20, 146.69, 131.75, 130.42, 125.12, 122.28, 114.95, 68.27, 60.87,
31.42, 29.74, 25.52, 22.41, 13.70, 13.44; EIMS: m/z 354 (M+), 339, 326, 325, 283, 281, 269,
191, 177, 73.
Ethyl 4-((4-heptyloxyphenyl)diazenyl)benzoate (2d)
1
λmax = 360 nm; FTIR (ATR): 2946, 1714, 1602, 1500, 1265 cm−1; H-NMR (300 MHz,
acetone-d6, δ ppm): 0.9 (t, 3H, CH3), 1.3–1.6 (m, 11H, CH3, 4x CH2), 1.8–1.9 (quintet, 2H,
CH2), 4.1–4.2 (triplet, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.1–7.2 (d, 2H, 2xCH),7.9- 8.0 (m,
4H, 4xCH) and 8.2 (d, 2H, 2xCH); 13C-NMR (75 MHz, acetone-d6, δ ppm): 165.30, 162.65,
155.20, 146.69, 131.75, 130.42, 125.12, 122.28, 114.95, 68.27, 60.87, 31.67, 29.74, 29.48, 25.79,
22.40, 13.70, 13.48; EIMS: m/z 368 (M+), 339, 283, 269, 177, 105, 73.
Ethyl 4-((4-octyloxyphenyl)diazenyl)benzoate (2e)
λmax = 360 nm; FTIR (ATR): 2920, 1712, 1579, 1469, 1260 cm−1. (300 MHz, acetone-d6, δ
ppm): 0.9 (t, 3H, CH3), 1.3–1.6 (m, 13H, CH3, 5x CH2), 1.8–1.9 (quintet, 2H, CH2), 4.1–4.2
(triplet, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.1–7.2 (d, 2H, 2xCH),7.9- 8.0 (m, 4H, 4xCH) and
8.2 (d, 2H, 2xCH); 13C-NMR (75 MHz, acetone-d6, δ ppm):165.30, 162.65, 155.23, 146.73,
131.77, 130.39, 125.09, 122.25, 114.95, 68.28, 60.83, 31.66, 29.70, 29.44, 29.19, 25.82, 22.41,
13.68, 13.45; EIMS: m/z 283, 269, 177, 73.
Ethyl 4-((4-decyloxyphenyl)diazenyl)benzoate (2f)
1
λmax = 361 nm; FTIR (ATR): 2918, 1712, 1600, 1469, 1262 cm−1; H-NMR (300 MHz,
acetone-d6, δ ppm): 0.9 (t, 3H, CH3), 1.2–1.6 (m, 15H, CH3 6xCH2), 1.8–1.9 (quintet, 2H,
CH2), 4.1–4.2 (triplet, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.1–7.2 (d, 2H, 2xCH, Ar-H),
7.9- 8.0 (m, 4H, 4xCH, Ar-H) and 8.2 (d, 2H, 2xCH, Ar-H); 13C-NMR (75 MHz, acetone-
d6, δ ppm): 165.31, 162.66, 155.23, 131.77, 130.41, 125.10, 114.97, 68.28, 60.86, 31.73, 25.81,
22.43, 13.67, 13.46. EIMS: m/z 283, 269, 177, 73.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
17
Scheme . Synthesis of ethyl -[(-alkoxyphenyl)diazenyl]benzoate.
Ethyl 4-((4-hexadecyloxyphenyl)diazenyl)benzoate (2g)
1
λmax = 359 nm; FTIR (ATR): 2916, 1712, 1583, 1463, 1260 cm−1; H-NMR (300 MHz,
acetone-d6, δ ppm): 0.9 (t, 3H, CH3), 1.2–1.5 (m, 29H, CH3 13xCH2), 1.8–1.9 (quintet, 2H,
CH2), 3.5 (triplet, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.1–7.2 (d, 2H, 2xCH, Ar-H), 8.0 (m, 4H,
4xCH, Ar-H) and 8.2 (d, 2H, 2xCH, Ar-H); 13C-NMR (75 MHz, DMSO-d6, δ ppm): 165.34,
162.75, 131.91, 130.43, 125.12, 122.28, 115.00, 68.39, 60.88, 33.98, 32.74, 31.75, 29.78, 29.52,
28.76, 28.50 28.24 27.91 25.81, 22.44, 13.74, 13.50; EIMS: m/z 283, 269, 177, 73.
Ethyl 4-((4-octadecyloxyphenyl)diazenyl)benzoate (2h)
1
λmax = 358 nm; FTIR (ATR): 2916, 1712, 1585, 1463, 1282 cm−1; H-NMR (300 MHz,
acetone-d6, δ ppm): 0.9 (t, 3H, CH3), 1.2–1.6 (m, 33H, CH3 15xCH2), 1.8–1.9 (quintet, 2H,
CH2), 3.5 (triplet, 2H, CH2), 4.4 (quartet, 2H, CH2), 7.1–7.2 (d, 2H, 2xCH, Ar-H), 8.0 (m, 4H,
4xCH, Ar-H) and 8.2 (d, 2H, 2xCH, Ar-H); 13C-NMR (75 MHz, DMSO-d6, δ ppm): 171.36,
169.85, 169.22, 167.40, 166.68, 130.43, 125.12, 122.27, 115.01, 68.28, 60.83, 34.01, 32.71, 31.72,
29.76, 29.25, 27.88, 25.77, 22.42, 13.47; EIMS: m/z 345, 283, 269, 177, 73.
Results and discussion
The synthesis of the target molecules (2a-h) was achieved by the diazo coupling of ethyl 4-
aminobenzoate and phenol, followed by the condensation with alkyl halides of varying chain
length under basic conditions (Scheme 1).
Where R = CnH2n+1 and n = 4, 5, 6, 7, 8, 10, 16 and 18.
The physical data of the series of compounds in presented in Table 1. The synthesis was
initially studied using FTIR spectra, where the broad absorption of the OH disappeared in
going from the reactant to product. This disappearance of broad OH absorption and appear-
ance of aliphatic CH stretching below 3000 cm−1 was strong indication of the formation of
desired ether linkage.
The synthesized compounds were also characterized by NMR spectroscopy. In 1HNMR
spectrum signals in range 1.0–1.5 ppm were assigned to alkyl chain and a down field triplet
signals at 4.1–4.5 ppm indicated the alkyloxy linkage. The signals of the aromatic protons
exhibited no significant shift from the parent intermediate ethyl 4-(4-hydroxyphenyldiazenyl)
benzoate. The synthesis was varified by the 13C-NMR as well and besides teh signals of aro-
matic carbons new signals in the alipahtic region corresponding to variable alkyl chaing were

18
H. MEHMOOD ET AL.
Table . Physical data of the Ethyl -[(-alkoxyphenyl)diazenyl]benzoate.
Sr. no
Appearance
Rf value∗
Yield %
Melting point
a
b
c
d
e
f
Orange red solid
Orange red solid
Orange red solid
Orange red solid
Orange solid
Orange solid
Orange solid
.
.
.
.
.
.
.
.
%
%
%
%
%
%
%
%
–°C
–°C
–°C
–°C
–°C
–°C
–°C
–°C
g
h
Orange solid
∗(n-hexane: Acetone :)
Figure . Structure of ethyl -((-butyloxyphenyl)diazenyl)benzoate (a) in crystal, thermal ellipsoid drawn
at % probability level.
observed. The important signals are the appearance of two alkoxy carbon at around 60 and
68 ppm, respectivley for the ester and ethre linkages. Although the middle carbons of the
chaign exhibitting less number of signals as compared to the total number of carbons and
this is because of the overlappign of the signals due to negligible effect of the electronegative
group as the distance increases. However, the signals of two methyl groups one for the CH of
the ester and the other for the terminal CH3 of the alkyl chaing were found at the most upfield
of the spectrum, which confirmed the formation of proposed ether linkage. The synthesis
was further confirmed by the mass spectromety using EI-MS. The presence of molecular ion
and the fragments at different mass to charge ratio corresponidng to characteristic parts of
the structure confirmed the desired synthesis. The mass fragmentation pattern exhibiting
the α-cleavage of ester leading to loss of 73 amu and exhibitin a fragment at m/z 196+R.
the mass spectra also exhibit the fragment of alkoxyphenyl at m/z 92+R and a fragment
at M – 92 +R, this fragments results by the fragmentation from of the azo linkage, other
importatn fragments for shorter chain lengths are exhibited in the scheme below (Scheme 2).
However with the increase in the chain length it looks from the spectral data that the stability
is decreased or there is possible decompositon during the vaporization and ionization and
few fragments are seen in the specra.
The single crystal analysis of 2a and 2b further supported the proposed structures and syn-
thetic design. The ORTEP diagram of 2a and 2b is shown in the Figs. 1 and 2, respectively.
Figure . Structureofethyl-((-pentyloxyphenyl)diazenyl)benzoate(b)incrystal, thermalellipsoiddrawn
at % probability level.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
19
Scheme . Sugested mass fragmentation pattern of ethyl -[(-alkoxyphenyl)diazenyl]benzoate.
Torsional angle of 180° between N1, N2, C7, C10 and N1, N2, C1, C12 in 2a and 2b, respec-
=
tively, confirmed that molecule is in its stable trans form across the N N bond. The crystal
and refinement data in presented in Table 2 and selected bond lengths and bond angles are
shown in Table 3. The crystallographic data can be obtained for the compounds with CCDC
1541387 (2a) and CCDC 1541388 (2b) from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Mesomorphic properties of all the synthesized compounds were investigated by employing
DSC and POM techniques. DSC therms indicating phase transitions corresponding to heat-
ing/cooling cycle at the rate of 10°C/min. All synthesized compounds except (2a), (2b) and
(2f) displayed sharp melting points indicating non-mesogenic behavior. Figure 2 shows the
transition temperatures versus number of carbon atoms in alkyl chain. DSC thermogram of 2a
was recorded by taking three scans two heating and one cooling scan at the rate of 10°C/min.
The DSC result revealed the presence of more than one transition for 2a, 2b and 2f which was
further studied under POM.
Table . Single crystal XRD data of compounds 2a and 2b.
a
b
Chemical formula
Mr
CHNO
.
Monoclinic, P/c
CHNO
.
Monoclinic, C/c

Crystal system, space group
Temperature (K)

˚
a, b, c (A)
. (), . (), . ()
. (), . (), . ()
β (°)
V (A )
Z
. ()
. ()

. ()
. ()


˚
Radiation type
Mo Kα
.
Mo Kα
.
μ (mm−
)
Crystal size (mm)
. × . × .
Data collection
No. of measured, independent and
observed [I > σ(I)] reflections
Rint
, , 
, , 
.
.
.
.
−
˚
(sin θ/λ)max (A
)
Refinement
R[F > σ(F)], wR(F), S
No. of reflections
No. of parameters
No. of restraints
., ., .



., ., .


H-atom treatment
H-atom parameters constrained
H-atom parameters constrained
(ꢀ/σ)max
ꢀꢀmax, ꢀꢀmin (e A
.
., −.
−
˚
)
., −.

20
H. MEHMOOD ET AL.
Table . Selected bond lengths and bond angles of ethyl -((-butyloxyphenyl)diazenyl)benzoate.
˚
Bond length (A)
˚
Bond type
a
Bond length (A)
b
Bond length
–
–
CH CH
CA CA
CA O
C O
C O
C C
C C
C O
N N
.
.
.
.
.
.
.
.
C C
C O
C O
C O
C C
C C
C O
.
.
.
.
.
.
.
.
–
– –
CH O CO
COO
O
C-Ar
=
C
=
Ar C
O CH
C
–
– = –
N
N
N N
Table Selected bond angles of 2a and 2b.
a
Bonds
b
Bonds
Angle (deg)
Angle (deg)
O-CA-CA
O-C-O
O-C-C
C-C-C
O-C-C
C-C-C
.
.
.
.
.
.
O-C-C
O-C-O
O-C-C
C-C-C
O -C-C
.
.
.
.
.
.
C-C-C
The heating and cooling scans of 2a indicated enantiotropic transitions, a small transition
at 73°C and the crystallization peak at 87°C. However when 2a observed under polarizing
optical microscope, liquid crystal phase started appearing at 104°C with broken focal conic
texture, which on further cooling developed a conical fan like texture at 95°C, characteristic
of the smectic C mesophase were observed which are presented in Fig. 3. This mesogenic
behavior lasts upto 80°C and disappeared at further cooling and converted to crystal phase.
The compound 2b, with just an increment of a methylene unit exhibited a single broad
endothermic peak at 91°C whereas two exothermic peaks appeared at 83°C and 55°C, respec-
tively, in cooling scan of DSC. When studied under POM, at 94°C clear circular focal conic
structures started appearing and this texture was stable up to 60°C showing a range of almost
30°C as compared to the lower homologue. This behavior is attributable to the bent core struc-
ture and a pentyloxy chain in 2b which breaks the symmetry of the layered structures resulting
in development of smectic phase (Fig. 4).
Figure . (a) Smectic phase in a at °C (b) smectic phase in 2f at °C.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
21
Figure . b at (a) °C and (b) °C.
It was thought that by appending different alkoxy chain lengths a regular change in
mesogenic properties would be observed but no regular trend in mesogenic properties was
observed. The compounds with 6,7,8,9,16 and 18 spacer lengths exhibited only a single large
peak corresponding to the melting transition in their DSC thermograms. Whereas the heat-
ing and cooling scans of 2f also showed enantiotropic behavior and two peaks indicating the
presence of mesophase were observed. Further investigation under POM revealed clear focal
conical fan like texture which appeared at 103°C till 91°C with range of 12°C, further cooling
converted to the crystalline phase at 90°C.
The inappropriate magnitude of anisotropic forces of end-to-end intermolecular interac-
tions, inauspicious molecular rigidity and unfavorable flexibility to maintain the parallel ori-
entation of molecules in floating conditions is responsible for the presence of smectic phase.
– = –
However, favorable molecular rigidity maintained by the N N linking the phenyl rings, n-
alkoxy group connected to the terminal rings and overall satisfactory molecular polarizability
favor the lamellar packing of 2a, 2b and 2f in their crystal lattice. Thus, these homologues
having suitable magnitudes of intermolecular anisotropic forces preferably maintain layered
arrangement in crystal lattices. These layers show sliding layered arrangement of molecules
when exposed to definite range of temperature under exposed thermal vibrations. As a result
of this thermal exposition the forces which are holding the layers in crystal are weakened than
the force of attraction between molecules at mesomorphic transition. Thus, layers slide over
each other and result into the formation of smectic mesophase. Further heating leads to the
weakening of intermolecular forces to such extent that do not allow the molecules to slide
Figure . UV-Vis absorption of compounds a–h.

22
H. MEHMOOD ET AL.
Figure . Photoluminescence spectra of butyloxy (2a), pentyloxy (2b) and hexyloxy (2c).
over each other in layers, hence parallel orientation is disrupted and the sample is converted
to isotropic liquid. On the other hand, same smectic phases appear when cooled the isotropic
liquid. The results established that all the mesogenic members of this synthesized series of
compounds displayed almost similar smectic phases with conical fan like textures due to pres-
ence of identical azo and ester groups. Whereas by increasing the methylene units in terminal
or tail decreased the liquid crystal transition range increasing the melting temperatures.
In order to study the photoluminescent properties UV-Vis and PL spectra were recorded
in solution of acetone. The emission spectra were recorded by the excitation wavelength taken
from the UV-Vis spectra. Figures 5 and 6 shows UV-Vis absorption and photoluminescence
spectra of compounds, respectively. The compounds 2c after excitation at 351 nm exhibit
emission at 524 nm hence can be used as green emitter.
Conclusion
– = –
Current investigation was designed to contain two phenyl rings bonded through N N to
constitute the rigid core. Members of the investigated series are entirely smectogenic phase.
Only three members C4, C5 and C10 showed liquid crystalline behavior.
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