Study of Y-shaped liquid crystalline materials with polar nitro substituent

Article abstract of DOI:10.1080/15421406.2018.1461183

A new homologous series of Y-shaped liquid crystals namely 4-Nitro-[2′4′bis (4″-n-alkoxybenzoyloxy)] phenyl bisazobenzenes have been synthesized and its thermotropic properties studied on the hot stage of a polarizing microscope. The compounds consist four phenyl rings joined through ester and bisazo linkages with alkoxy and nitro as terminal substituents. The structures of synthesized compounds were confirmed by spectroscopic techniques such as FTIR, ¹HNMR as well as elemental analysis. The compounds were found to exhibit enantiotropic nematic and smectic mesophases. The role of molecular shape, size and polarity of functional groups in the mesophases formation is discussed.

Full text of DOI:10.1080/15421406.2018.1461183

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Study of Y-shaped liquid crystalline materials with
polar nitro substituent
Sandhya Dixit
To cite this article: Sandhya Dixit (2018) Study of Y-shaped liquid crystalline materials
with polar nitro substituent, Molecular Crystals and Liquid Crystals, 664:1, 77-84, DOI:
10.1080/15421406.2018.1461183
To link to this article: https://doi.org/10.1080/15421406.2018.1461183
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Study of Y-shaped liquid crystalline materials with polar
nitro substituent
Sandhya Dixit
Applied Chemistry Department, Faculty of Technology and Engineering, Kalabhavan, The Maharaja Sayajirao
University of Baroda, Vadodara, Gujarat, India
ABSTRACT
KEYWORDS
Nematic; polar nitro
substituent; smectic; Y-
shaped liquid crystals
A new homologous series of Y-shaped liquid crystals namely 4-Nitro-
[2ꢀ4ꢀbis (4ꢀꢀ-n-alkoxybenzoyloxy)] phenyl bisazobenzenes have been
synthesized and its thermotropic properties studied on the hot stage
of a polarizing microscope. The compounds consist four phenyl rings
joined through ester and bisazo linkages with alkoxy and nitro as
terminal substituents. The structures of synthesized compounds were
confirmed by spectroscopic techniques such as FTIR, 1HNMR as well as
elemental analysis. The compounds were found to exhibit enantiotropic
nematic and smectic mesophases. The role of molecular shape, size and
polarity of functional groups in the mesophases formation is discussed.
Introduction
Liquid crystalline materials are fascinating fluids that are widely recognized for their use
in display devices. The molecular structures of liquid crystalline materials are indeed very
varied while at the same time being very similar in principle and style. The generation
of liquid crystal phase is possible in a wide range of structures [1–7]. For new advanced
display technologies and other applications, much more complex combinations of struc-
tural features of liquid crystals are required. This has led to draw attention of researchers
towards designing molecules of different shapes like Banana-shaped, V-shaped, T-shaped
and Y-shaped [8–15]. Such molecules form a group which possesses molecular geometry
different from that of calamitic liquid crystals and discotic liquid crystals. The photo-
responsive properties of azobenzene derivative due to photo-isomerization have made
photochromic materials particular azo compounds promising candidate for optical data
storage.
In this context, this article is concerned with synthesis of novel Y-shaped liquid crystals
with linear rod like building blocks connected via ester and bisazo group and possess termi-
nal nitro and alkoxy group and has a long aromatic chain at the lateral position. The aim of
present work is to investigate the influence of length of main core, length of terminal alkyl
chain and bisazo group on mesogenic behavior of Y-shaped liquid crystals and to establish
structure–property relationship.
CONTACT Sandhya Dixit
snddxt_@yahoo.com
Applied Chemistry Department, Faculty of Technology and Engi-
neering, Kalabhavan, the Maharaja Sayajirao University of Baroda, Vadodara , Gujarat, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/.
©  Taylor & Francis Group, LLC

78
S. DIXIT
Table . Transition temperatures (°C) of Series I.
n -Alkyl CnHn+
group (n) =
Transition temperatures (°C)
Series I
Smectic C
Nematic
Isotropic












—
—
—
—
—
—
—















—












—
Materials and methods
4-Hydroxybenzoic acid, n-alkyl halides, thionyl chloride, pyridine, ethanol, potassium
hydroxide, resorcinol, 4-nitroaniline, aniline and sodium nitrite were chemically pure of lab
grade. Solvents were dried and distilled prior to use. The microanalysis of the compound
was performed on a Coleman carbon, hydrogen analyzer. Fourier transform infrared spec-
troscopy (FTIR) spectra were determined on FTIR-8400S (Shimadzu) instrument. 1HNMR
spectra was performed on Bruker AV II 400 MHz spectrometer using tetramethylsilane
(TMS) as internal references substance and CDCl3 as solvent and the chemical shift val-
ues recorded as (ppm units). Thermodynamic quantities enthalpies (ꢀH) and entropies
(ꢀS = ꢀH\T) of transition were determined using differential scanning calorimeter (DSC)
via a DSC-60(Shimadzu) instrument and reported in Jg−1 at a scanning rate of 5°Cmin−1.
UV analysis was carried out on UV-2450(Shimadzu) instrument. Liquid crystalline prop-
erties were determined on a Nikon eclipse Ci POL microscope equipped with a heating
stage.
Synthesis of compounds
4-n-Alkoxy benzoic acids and 4-n-alkoxy benzoyl chlorides were synthesized by the mod-
ified method of Dave and Vora [16]. 4-Nitro (2ꢀ4ꢀ-dihydroxylphenyl) bisazobenzene (Dye-
A) was synthesized by known methods [17]. The series, namely, 4-Nitro-[2ꢀ4ꢀbis((4ꢀꢀ-n-
alkoxybenzoyloxy)] phenyl bisazobenzenes was synthesized by adding dropwise one mole
solution of 4-Nitro (2ꢀ4ꢀ-dihydroxylphenyl) bisazobenzene (Dye-A) in dry pyridine to the
respective two moles solutions of 4-n alkoxybenzoyl chloride at 0°C–5°C. The mixture was
allowed to stand overnight at room temperature. It was acidified with cold 1:1 aqueous
hydrochloric acid and the separated solid was washed successively with dilute sodium
hydroxide solution and water. The final product obtained was recrystallized several times
from glacial acetic acid until constant transition temperatures were obtained which are
recorded in Table 1. The elemental analysis of representative compounds was found to be
satisfactory and recorded in Table 2. The route of synthesis of series I compounds is illustrated
in Scheme 1.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
79
Scheme . The route of synthesis of series I compounds.
Spectral data
FTIR (KBr) spectra (cm−-
)
– =
n-C6H13 homologue: 3028 (C–H str., aromatic), 2900 (C–H str., aliphatic), 1700 ( C O str,
– = –
ester), 1604 ( N N str., azo), 1340, 1270, 1165 (aryl ether), 1050, 850, and 750.
–
–
–
n C16H33 homologue: 3100 (C H str., aromatic), 2925–2850 (C H str., aliphatic), 1741
– =
– = –
( C O str., ester), 1603 ( N N str., azo), 1380, 1285, 1155 (aryl ether), 1050, 880, and 765.
H NMR (CDCl  MHz, δ, ppm, standard TMS)
n-C16H33 homologue: δ = 0.9 (t, 6H, 2 × CH3), 1.29–1.71 (m, 56H, 2 × (CH2)14-), 3.94 (q,
4H, 2 × Ar(OCH2)-), and 6.89 (d,J = 8.8 Hz, 4H, 2 x ArH at C-3ꢀ and C-5ꢀ), 6.91 (s, 1H,
ArH at C-5),7.89 (d, J = 8.6 Hz, 1H, ArH at C-3), 7.91 (d, J = 8.7 Hz,1H, ArH at C-2), 7.99
(d, J = 8.8 Hz, 4H, 2 x ArH at C-2ꢀ and C-6ꢀ), 8.0–8.2 (m, 6H, ArH at C-2ꢀꢀ, C-3ꢀꢀ, C-5ꢀꢀ, C-6ꢀꢀ,
C-3ꢀꢀꢀ and C-5ꢀꢀꢀ), 8.39 (d, J = 8.6 Hz, 2H, ArH at C-2ꢀꢀꢀand C-6ꢀꢀꢀ).
Table . Elemental analysis for butyloxy, hexyloxy, decyloxy and hexadecyloxy derivative.
Molecular formula
C % found
(% calculated)
H % found
(% calculated)
N % found
(% calculated)
CHON
C HO N
CHON
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
C
HON

80
S. DIXIT
1.412
1.000
0.500
0.000
-0.128
200.00
400.00
600.00
800.00
nm.
Figure . UV-visible spectra of C homologue.
UV study of azo compound
The presence of azo benzene groups leads to the possibility of photo isomerization and pho-
tochromic behavior. Therefore, permeability studies of photo physical properties were carried
out in chloroform solution of concentration C = 2.3 × 10−3 and 2.5 × 10−3 gdm−3 for decyl
homologue and shown in Fig. 1. The compounds displayed primarily 2 absorption with max-
imum absorbance at about 360 nm and 355 nm. The azo compound in the trans form show
a strong band in ultraviolet (UV) region ∼334.40 nm which is attributed to the trans to cis
(π-π∗) transition. The trans form is much more stable than the cis form but each isomer can
be converted into the other by light irradiation of the appropriate wavelength.
DSC studies
As representative cases, the associated enthalpies of phase transition of C10 and C16 homo-
logues were measured by differential scanning calorimetric (DSC). Data are recorded in
Table 3.
Results and discussion
Optical microscopic studies
The mesophase exhibited by compounds of series I were identified by examining a thin film
of a sample sandwiched between a glass slide and cover slip on polarizing optical microscope.
Table . DSC data of C and C homologue.
Peak temperature °C
Homologue
Phase transition
(microscopic reading)
ꢀ (H/Jg−)
)
ꢀ (S/Jg− K−
)
C
Cr-SmC
Sm C-N
N-I
. ()
. ()
. ()
.
.
.
.
.
.
C
Cr- SmC
SmC-I
. ()
. ()
.
.
.
.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
81
Figure . Optical microphotographs of compounds. The Schlieren texture of smectic C mesophase of (a) C
homologue on heating at °C; (b) C homologue on cooling at °C. The threaded texture of nematic
mesophase of (c) C homologue on cooling at °C; (d) C homologue on cooling at °C.
The compounds showed enantiotropic smectic and nematic phases when carefully observed
by POM both in heating and cooling scans.
Optical microphotographs of smectic and nematic phases exhibited by C6, C10 and C16
homologues are shown in Fig. 2 as the representative illustrations. The smectic phase appears
as a Schlieren texture on cooling (a) and (b) while a threaded texture of nematic phase is
observed on cooling (c) and (d).
The phase behavior
In the present study, 12 homologues of the series 4-Nitro-[2ꢀ4ꢀbis (4ꢀꢀ-n-alkoxybenzoyloxy)]
phenyl bisazobenzenes were synthesized and their mesomorphic properties were studied. The
enantiotropic nematic mesophase is exhibited by the very first member i.e. from the methoxy
derivative of the series, and it continues up to n-heptyloy derivative. This is an interesting fea-
ture observed in these compounds. Earlier reports on compounds bearing lateral aromatic
chain shows that lower members is either non-mesogenic or exhibit monotropic smectic
mesophases [18–20]. The n-octyloxy to n-dodecyloxy derivatives exhibit both nematic and
smectic mesophases. The n- tetra and n-hexadecyloxy exhibit only smectic mesophases. The
transition temperatures are plotted versus number of carbon atoms in alkoxy chain and shown
in Table 1. The phase diagram showing phase behavior is depicted in Fig. 3. The N-I transition
temperatures curve shows falling trend as series is ascended. The smectic–nematic transitions
exhibit rising tendency. The nematic temperature range is from 14°C to 70°C and the smectic
temperature range is from 36°C to 49°C.

82
S. DIXIT
Figure . The plot of transition temperatures versus number of carbon atom in alkoxy chain.
Mesogenic properties and molecular constitution
There is a close relationship between mesomorphism and molecular constitution of organic
compounds. Hence, thermal stability, a measure of mesomorphism, can be correlated with
the molecular constitution of the compounds. Figure 4 shows the transition temperatures and
molecular structures of the n-octyloxy derivative of the present series I and the structurally
related compounds A [21] and B [11] reported in literature.
The comparison of n-octyloxy derivative (compound 8) of the present series with the
structurally related compound A shows that compound 8 exhibit enantiotropic smectic and
nematic phases while in compound A only smectic mesophase is observed. The different
liquid crystalline behavior of these compounds can be explained due to different molecu-
lar structures possessed by these compounds. As can be seen from the Fig. 4 that compound 8
possess four phenyl rings joined through ester and bisazo linkages and have alkoxy and nitro
terminal groups, while compound A composed of three phenyl rings joined with ester and azo
linkages with alkoxy and nitro terminal groups. Hence compound 8 differs with compound A
in the presence of one additional phenyl ring and azo linkage, as all others structural features
are the same. Exhibition of nematic phase of compound 8 as compared to none in material
A may be attributed to presence of one extra phenyl ring and azo linkage which enhances
length and polarizability of molecules of compound 8. This factor results in higher length
to breadth ratio which in turn enhances stronger terminal attractions favoring formation of
nematic phase in compound 8. Increased length and polarizability enhances both mesogenic
properties and thermal stabilities.
The width of smectic phase of present compound 8 is 27°C while that of compound A
is 45°C. Gray [22] has explained that material which required more thermal energy to dis-
organize the molecular arrangement of the smectic mesophase, is thermally more stable. As
can be seen from Fig. 4, more thermal energy has to supplied to disorganize the molecular
arrangement of smectic melt of compound 8, evidenced by the fact that the SmC-N transition
temperature is higher by 19°C than that for Sm A- Iso transition temperature of compound A.
The range and thermal stability of the mesophase is a more important factor in relating meso-
morphic behavior to chemical constitution since the chemical grouping gives rise to inter-
molecular attractions, which, in turn, determine the mesophase range and thermal stability.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
Figure . Comparative molecular structures of compound , A and B.
Comparison of compound B with compound 8 reveals that width of nematic phase of com-
pound 8 is higher by 27°C but smectic phase length is lower by 7°C than that of compound B.
Now as can be seen from their molecular structures in Fig. 4 that molecules of compound 8
possess higher length due to presence of one additional benzene ring in comparison to com-
pound B. Exhibition of higher nematic phase length of compound 8 is due to higher length
as explained above. The higher smectic phase length of compound B can be explained on the
basis of presence of polar lateral bromo group which increases polarizability of the molecules
of compound B and favors lateral intermolecular cohesive forces to become stronger resulting
in wider smectic phase than that of compound 8.

84
S. DIXIT
Conclusion
A new Y-shaped liquid crystal molecule based on bisazobenzene core connected to an alkoxy
and nitro group at the terminal with a long aromatic chain at the lateral position has been
prepared. The materials were found to exhibit enantiotropic nematic and smectic phases in
the broad temperature ranges. The extended core length of the compounds with bisazoben-
zene group was found to induce enantiotropic nematic phase at the beginning in series and
also to increase their thermal and mesophase stability. Our result indicated that main rela-
tionship between molecular structures and mesomorphic behavior well known for calamitic
liquid crystals are valid for Y-shaped liquid crystals also. The azobenzene units showed trans
-cis–trans isomerization; therefore, these mesogenic compounds bearing bisazo group can be
a good candidate for display technology and various photonic application including optical
storage.
Funding
The authors are sincerely thankful to the Office of Research and Consultancy Cell, The Maharaja Saya-
jirao University of Baroda, Vadodara for the award of Research Project (RCC/Dir/2015/217/13) for
financial support.
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