Study of the effect of a heterocyclic ring end group and an azomethane linkage on mesomorphism

Article abstract of DOI:10.1080/15421406.2016.1177879

A novel homologous series RO-C₆H₄-COO-C₆H₄-N = CH-C₅H₄N of liquid crystalline Schiff base esters has been synthesized and studied with a view to correlating molecular structure to the liquid crystal properties on the basis of molecular rigidity and flexibility. All 12 members of series are either monotropic (C₁, C₃, C₅) or enantiotropic (C₂, C₄, C₆ to C₁₆) nematic with the absence of smectogenic character throughout the series. Transition temperatures and LC behavior of the series were determined using an optical polarizing microscope equipped with a heating stage. Transition curves (Cr-N or Cr-I and N-I or I-N) behave in the normal established manner as depicted in a phase diagram. An odd–even effect is exhibited by the N-I transition curve up to the C₈ homologue.

Full text of DOI:10.1080/15421406.2016.1177879

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Study of the effect of a heterocyclic ring end group
and an azomethane linkage on mesomorphism
R. H. Maheta & U. C. Bhoya
To cite this article: R. H. Maheta & U. C. Bhoya (2016) Study of the effect of a heterocyclic ring
end group and an azomethane linkage on mesomorphism, Molecular Crystals and Liquid
Crystals, 633:1, 29-36, DOI: 10.1080/15421406.2016.1177879
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1177879
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Date: 02 September 2016, At: 01:29

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Study of the effect of a heterocyclic ring end group
and an azomethane linkage on mesomorphism
R. H. Maheta and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat (India)
ABSTRACT
KEYWORDS
Liquid crystal; monotropy;
nematic; Schiff base; smectic
A novel homologous series RO-C6H4-COO-C6H4-N = CH-C5H4N of liq-
uid crystalline Schiff base esters has been synthesized and studied with
a view to correlating molecular structure to the liquid crystal proper-
ties on the basis of molecular rigidity and flexibility. All 12 members of
series are either monotropic (C1, C3, C5) or enantiotropic (C2, C4, C6 to C16)
nematic with the absence of smectogenic character throughout the
series. Transition temperatures and LC behavior of the series were deter-
mined using an optical polarizing microscope equipped with a heating
stage. Transition curves (Cr-N or Cr-I and N-I or I-N) behave in the normal
established manner as depicted in a phase diagram. An odd–even effect
is exhibited by the N-I transition curve up to the C8 homologue.
Introduction
Our continuous studies concern the effects of molecular structure on thermotropic liquid
crystal (LC) [1] properties by changing various molecular aspects, such as shape, size, aro-
maticity, terminal, lateral, and linking groups. The LC state is very sensitive and suscepti-
ble to molecular structures [2–7] and such studies can be useful to the manufacture of dis-
play devices [8–13] and many other fields like agricultural productions and pharmaceutical
preparations. The variations in mesomorphic (LC) properties is a direct result of the molec-
ular rigidity and flexibility of the molecule [14–17] whose suitable magnitudes operate the
emergence of an unique state of matter which can flow like liquid and possess optical prop-
erties like crystal. The present investigation is planned to synthesize a novel series of ester
Schiff’s base substances containing heterocyclic ring as terminal and its some (thermometric)
LC properties are compared with the structurally similar azoester series of LC substances, to
derive group efficiency order. Many ester homologous series of LCs have been reported to date
[18–26]
Experimental synthesis
Synthesis
4-Hydroxy benzoic acid was alkylated by suitable alkylating agent by the modified method
of Dave and Vora [27]. 4-((pyridin-2-yl) methyleneamino)phenol (B) was prepared by an
CONTACT U. C. Bhoya
drucbhoya@gmail.com, drucbhoya@sauuni.ernet.in
University, Rajkot -, Gujarat (India).
Department of Chemistry, Saurashtra
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

30
R. H. MAHETA AND U. C. BHOYA
Scheme . Synthetic route to the novel series.
established method [28]. Coupling of compounds A and B was done by Steglich esterification
to yield 4-(4ꢀ-n-alkoxybenzoyloxy) phenyl amino methylene (2ꢀꢀ-pyridine) [29].
The synthetic route to the novel homologous series of azomethane derivatives is under
mentioned in scheme 1.
Characterization
Representative members of a novel series were characterized by elemental analysis (Table 1),
Infrared spectroscopy, 1HNMR spectroscopy, and mass spectroscopy. Microanalysis was per-
formed on Perkin-Elmer PE 2400 CHN analyzer. IR spectra were recorded on Shimadzu
FTIR-8400, 1HNMR spectra were recorded on Bruker spectrometer using DMSO as solvent
Table . Elemental analysis for ()Ethyloxy()Pentyloxy()Hexadecyloxy derivatives.
Sr. No.
Molecular formula
Elements %Found
Elements %Calculated
CHN
CHN



CHNO
CHNO
CHNO
...
...
...
...
...
...

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
31
and Mass spectra were recorded on Shimadzu GC-MS Model No.QP-2010. The mesogenic
behavior of the homologue derivatives were observed through polarizing microscopy. The
textures of smectogenicmesophase were determined by miscibility method.
Analytical data
H NMR in ppm for hexyloxy derivative
1H NMR (400 MHz, DMSO-d6) δ 0.84 – 0.91 (t, J = 7.6 Hz, 3H), 1.35 – 1.39 (tt, J = 3.3, 5.9 Hz,
4H), 1.42 – 1.52 (m, 2H), 1.77 – 1.83 (p, J = 7.7 Hz, 2H), 4.10 – 4.12 (t, J = 7.4 Hz, 2H), 7.04 –
7.11 (m, 4H), 7.38 – 7.40 (dd, J = 1.1, 4.9, 7.8 Hz, 1H), 7.44 – 7.46 (d, J = 7.5 Hz, 2H), 7.57 –
7.59 (dd, J = 1.1, 8.0 Hz, 1H), 7.63 – 7.66 (td, J = 1.2, 7.9 Hz, 1H), 8.07 – 8.13 (m, 2H), 8.22
(s, 1H), 8.66 – 8.68 (dd, J = 1.2, 5.0 Hz, 1H) ppm.
H NMR in ppm for heptyloxy derivative
1H NMR (400 MHz, DMSO-d6) δ 0.87 – 0.92 (t, J = 8.0 Hz, 3H), 1.20 – 1.35 (m, 8H), 1.38 –
1.48 (m, 2H), 1.71 – 1.77 (p, J = 7.7 Hz, 2H), 4.10 – 4.12 (t, J = 7.4 Hz, 2H), 7.06 – 7.09 (dq,
J = 2.5, 8.5 Hz, 4H), 7.21 – 7.23 (dd, J = 1.0, 8.1 Hz, 1H), 7.36 – 7.46 (m, 3H), 7.54 – 7.58 (td,
J = 1.2, 8.0 Hz, 1H), 8.07 – 8.13 (s, 2H), 8.65 – 8.66 (dd, J = 1.1, 5.0 Hz, 1H) ppm.
H NMR in ppm for tetradecyloxy derivative
1H NMR (400 MHz, DMSO-d6) δ 0.90 – 0.98 (t, J = 7.6 Hz, 3H), 1.20 – 1.26 (m, 20H), 1.67 –
1.75 (d, J = 30.8 Hz, 2H), 3.96 – 3.98 (t, J = 6.5 Hz, 2H), 4.11 – 4.16 (t, J = 7.0 Hz, 2H),
6.89 – 6.91 (d, J = 8.4 Hz, 2H), 7.41 – 7.47 (dd, J = 8.4, 14.8 Hz, 2H), 7.78 – 7.80 (d, J =
8.1 Hz, 3H), 8.07 – 8.10 (d, J = 8.5 Hz, 2H), 8.22 (s, 1H), 8.65 – 8.67 (dd, J = 1.2, 5.1 Hz, 1H)
ppm.
IR in cm− for heptyloxy derivative
IR (KBr): 3070 (Ar str.), 2946, 2852 (-CH3 str.), 1728 (-C = O str.), 1656 (-C = C- Ar str.),
1590 (-CH = N- str.), 1560 (-C = N- str.), 1213, 1074 (-C-O str.), 844, 707 (-CH Ar bend.)
cm−1.
IR in cm− for butyloxy derivative
IR (KBr): 3038(Ar str.), 2930(-CH3 str.), 1750(-C = O str.), 1670 (-C = C- Ar str.), 1588
−1
=
(-CH N- str.), 1560 (-C = N- str.), 1056(-C-O str.), 880, 742 (-CH Ar bend.) cm .
IR in cm− for methyloxy derivative
IR (KBr): 3068 (Ar str.), 2955 (-CH3 str.), 1719, 1728 (-C = O str.), 1690 (-C = C- Ar str.),
1595 (-CH = N- str.), 1560 (-C = N- str.), 1415, 1064 (-C-O str.), 885, 732 (-CH Ar bend.)
cm−1.
Mass of hexyloxy derivative
m/z (rel.int%): 402 (M)+ 149, 121, 93, 65, 44
Mass of octyloxy derivative
m/z (rel.int%): 430 (M)+205, 121, 93, 65, 44
Mass of dodecyloxy derivative
m/z (rel.int%): 486 (M)+ 219, 121, 93, 57, 43

32
R. H. MAHETA AND U. C. BHOYA
Table . Transition temperatures in °C.
Compound No.
n-alkyl chain CnHn+ (n)
Sm
Nm
Isotropic
























—
—
—
—
—
—
—
—
—
—
—
—
()

()

()



















Sm = Smectic, Nm = Nematic, () indicate monotropy
Results and discussion
4-((Pyridin-2-yl) methyleneamino) phenol (m.p-108°C and yield- 88%) is a non LC compo-
nent, which on linking with dimeric 4-n-alkoxy benzoic acid yielded Schiff’s bases of LC sub-
stances, whose transition temperatures are relatively lower than the corresponding n-alkoxy
acids. Transition and melting temperatures as determined from POM (Table 2) are plotted
against the number carbon atoms present in n-alky chain ‘R’ of–OR end group. Transition
curves Cr-N or Cr-I and N-I or I-N are obtained by linking like or related points showing
phase behaviors of novel series as shown in a phase diagram (Fig. 1) Cr-N/I transition curves
follows a zigzag path of rising and falling tendency in usual manner with overall descend-
ing tendency. N-I or I-N transition curve smoothly decends and rises to C10 homologue and
again descended after passing through maxima at C10 derivative. Odd-even effect is observed
for N-I (or vice versa) transition curve upto C8 homologue. Then, odd-even effect disappears
for higher homologues of longer n-alkyl chain from and beyond C8 derivatives. Thus, N-I or
Figure . Phase behavior of series.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
33
I-N transition curve behaved in normal manner. The textures of nematic phase are threaded
or Schlieren. Analytical, spectral and thermal data supported molecular structures of homo-
logue. Thermal stability for nematic is 116.0°C and mesophase length is relatively shorter.
The LC properties from homologue to homologue undergo variation with changing number
of carbon atom or methylene units in n-alkyl chain ‘R’ of–OR end group.
The lowering of transition temperatures as compared to the corresponding n-alkoxy ben-
zoic acids is attributed to the breaking of hydrogen bonding between two molecules of ben-
zoic acids by esterification process. The exhibition of LC property from very first member of
series in either monotropic or enantiotropic manner is due to the disalignment of molecules
at angle less than ninety degree centigrade under the influence of exposed thermal vibra-
tions. The suitable magnitudes of anisotropic forces of intermolecular end to end attractions
and closeness as a consequence of favorable molecular rigidity and flexibility leads to arrange
and organize molecular floating on the surface maintaining statistically parallel orientation
order, depending upon an angle of disalignment below ninety degree, which resists or with-
stands the effect of exposed thermal vibrations for some degree of temperature difference.
Hence, nematogenicmesophase formation appears from C1 to C16 members of a series. All
the homologue molecules from and beyond their isotropic temperature randomly oriented in
all possible directions with high order of disorder or randomness or high entropy ꢀS = ꢀH/T
in uncontrolled manner; but, on carefully cooling the same, the C1, C3 and C5 homologues
exhibited nematogenicmesophase formation in irreversible manner below their isotopic tem-
perature which is termed as monotropic transition temperature and corresponding homo-
logues are called monotropic mesomorphs. Rest of the homologues of the present novel series
were similarly (C2, C4,C8 and C16) and careful cooling form and below isotropic temperature
gave rise to reappear nematicmesophase formation exactly at a temperature at which nemat-
icmesophase was appear on heating the same. Thus, the nematogenic homologues formed and
showed nematic phase in reversible manner i.e. on heating and cooling mesophase appeared
at the same temperature. Such transitions are called enantiotropic transitions and correspond-
ing homologues area called as enantiotropic homologues. None of the nematogenic exhibited
monotropicsmectic mesophase formation due to insufficient intermolecular forces of cohe-
sion and closeness to attain sliding layered molecular arrangement in floating condition. The
exhibition odd-even effect by N-I transition curve is due to the sequential addition of methy-
lene unit at the N-alkyl chain ‘R’ of-OR group. The N-I curves for odd and even homologues
merged into each other at C8 homologues and then odd-even effect disappeared for the higher
homologue because, longer n-alkyl chain ‘R’ of-OR may coil or bend or flex or couple to lie
with major axis of core structure of a molecule and acquire unusual and unexpected status
of n-alkyl chain disturbing intermolecular parameters related to cohesive forces and close-
ness. The changing trend in LC properties form homologue to homologue in the same series
is attributed to the actual status of n-alkyl chain ‘R’ of-OR group related to effective molecu-
lar length, length to breadth ratio, ratio of the molecular polarity to polarizibility permanent
dipole moment across the long molecular axis, magnitudes of dispersion forces dipole-dipole
interactions, suitable magnitudes of molecular rigidity and flexibility etc. to induce mesophase
of proper degree of mesomorphism. Some thermometric properties of presently investigated
novel series-1 are compared with the other structurally similar analogous series-X [30] as
under in Fig. 2.
Homologous series-1 of present investigation and a homologous series-X selected for
comparative study are identical with respect to two phenyl rings and one central bridge-
COO-as well as–N = nitrogen atom bonded to middle phenyl ring. Moreover left n-alkoxy
terminal end group (-OR) is commonly present for the same homologue from series to series.

34
R. H. MAHETA AND U. C. BHOYA
Figure . Structurally similar series.
However, they differ from each other with respect to units of tailed parts i.e. = CH-C5H4N
and = N-C6H5 of series-1 and a series-X, respectively. i.e. = CH-bonded to heterocyclic
ring containing nitrogen atom i.e. series-1 and = N-bonded to homocyclic phenyl ring in
series-X. Thus, variation in LC properties and the degree of mesomorphism will depend
upon the magnitudes of differing features of series-1 and X as mentioned above. Following
Table 3 represents some thermometric properties of series-1 and X in comparative manner.
From Table 3 it is clear that,
• Presently investigated series-1 is nematogenic only but, series X under comparison is
smectogenic plus nematogenic.
• Smectogenicmesophase formation commences from C10 homologue in series-X, but it
does not commence till the last homologue of series-1. The nematicmesophase forma-
tion commences from very first member of series-1 and X under comparative study.
• Smectogenicmesophase does not stabilize at all in series-1, but it stabilizes in case of
series-X whose smectic thermal stability in 114.0
• Nematogenic thermal stability is in increasing order from series-1 to X.
• Upper and the lower mesophase lengths are in increasing order from series-1 to series-X.
However, the difference between upper and lower mesophase lengths are 21.0 and 22.0
for series-1 and X, respectively.
The exhibition of nematic, mesophase formation by both the series under comparison is
by usual established environmental condition of suitable magnitudes of anisotropic forces
of intermolecular end to end attractions, cohesions and closeness as consequence of favor-
able molecular rigidity and flexibility as required to exhibit nematogenic character with more
or less mesophase length according to thermal resistivity of individual molecules of each
homologue or for the same homologue form series to series. However, the exhibition of smec-
togenic character by series-X is due to trans -N = N- configuration which maintains linearity,
Table . Thermal stability in °C.

X
Series→
Smectic-IsotropicOrSmectic-
NematicCommencement of
smectic phase
Nematic-IsotropicCommencement
of nematic phase
Total, upper and lower mesophase
lengths in °C, t to t
—
.(C–C)C
.(C–C)C
. to .CC
.(C–C)C
. to .CC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
comparatively more than its comparative group -N = CH- under identical position. More-
over the bonding of -N = N- with homocyclic phenyl ring ( = N-phenyl) being more polar-
izable and polar as compared to = CH-C = N- heterocyclic ring unit of series-1. Therefore,
probability of forming lamellar packing of molecules in the crystal lattices of series-X being
more favorable than in case of series-1. Thus, the layered molecular arrangement in rigid
crystalline state and subsequently the sliding layered molecular arrangement of molecules
from C10 homologue commence and continued up to C16 last homologue for different range
of temperature persisted to show smectogenic character in addition to nematic property in
case of series-X, but does not persist for smectic phase formation in case of series-1 contain-
ing heterocyclic ring in place of homocyclic phenyl ring. Early commencement of smectic
phase in series-X as compared to series-1 is attributed to the extent of molecular planarity
maintained as required for smectogenic character from C10 homologue favorably in series-X,
but the extent of molecular noncoplanarity fails to adopt smectogenic molecular organiza-
tion possibility till the last C16 homologue of series-1. The potential ability as emerging from
molecular structures of both the series-1 and X are sufficient enough to maintain statistically
parallel orientation order of molecules which are floating on the surface of by suitable mag-
nitudes of anisotropic forces of end to end attractions from very first member of both the
series under comparison in monotropic or enatiotropic manner, respectively. The increasing
order of thermal stabilities for mesophase as well as upper and lower mesophase lengths from
series-1 to X is attributed to thermodynamic quantity enthalpy (ꢀH and ꢀHx) difference of
series-1 and X whose values are basically depended on molecular structure and constitutional
difference. Thus, when a quantum of heat energy exposed upon a homologue substance, the
magnitudes of resistivity towards exposed thermal vibration vary from homologue to homo-
logue in the same series and series to series for the same homologue, which, causes differ-
ences in transition temperature, thermal stabilization, magnitudes of end to end and lateral
cohesion and closeness, dispersion forces, mesophase stabilization and the upper and lower
degree of mesomorphism, thermal resistivity etc. Thus, present series is nematogenic, whose
mesophase lengths are shorter and of middle ordered melting type.
Conclusions
• Homocyclic phenyl ring bonded to –N=N- central bridge is potentially more capable of
inducing smectic and nematic phase of higher thermal stabilities and longer mesophase
lengths as compared to heterocyclic ring bonded to-N=CH- centered bridge.
• The group efficiently order derived for smectic and nematic on the base of (a) thermal
stability(b) early commencement of mesophase and (c) mesophase length, are as under
(a) Smectic:
Series X > Series 1
Nematic:
Series X > Series 1
(b) Smectic:
Series X > Series 1
Nematic:
Series X > Series 1
(c) Total mesophase length (Sm+N)
Upper and lower
Series X > Series 1

36
R. H. MAHETA AND U. C. BHOYA
• Mesomorphism is very sensitive and susceptible to molecular structure; as consequence
of molecular rigidity and flexibility.
• Present investigation supports and raises the credibility to the conclusions drawn earlier.
Acknowledgments
We thank the Department of Chemistry (DST-FIST Funded & UGC-SAP Sponsored), Saurashtra Uni-
versity, Rajkot, for research work. Special thanks to “National Facility for Drug Discovery through New
Chemical Entities (NCE’s) for sample analysis. We also thankful to Dr. A. V. Doshi Ex. Principal M.V.M.
Science and Home Science College Rajkot, for his valuable co-operation and suggestions during present
investigation as and when needed.
References
[1] Reinitzer, F. (1888). Monatsh Chem., 9, 421–441.
[2] Gray, G. W., & Windsor, P. A. (1974). Liq. Cryst. Plastic Cryst., Ellis Horwood: Chichester, U.K.,
1(4), 103–153.
[3] Gray, G. W. (1962). Molecular Structure and the Properties of Liquid Crystal, Academic Press: Lon-
don, VII +314 pp. illus. 63s.
[4] Imrie, C. T. (1999). Structure Bond., 95, 149–192.
[5] Gray, G. W., & Jones, B. (1954). Journal of Chemical Society, 2556–2562.
[6] Henderson, P. A., Niemeyer, O., & Imrie, C. T. (2001). Liq. Cryst., 28, 463–472.
[7] Imrie, C. T., Luckhrust, G. R., Demus, D., Goodby, J. W., Graw, G. W., Spiess, H., & Vill, V. (1998).
Eds. Wiley-VCH, Weinheim, 801–833.
[8] Kim, W. S., Elston, S. J., & Raynes, F. P. (2008). Display, 29, 458–463.
[9] Hertz, E., Lavored, B., & Faucher, O. (2011). Nature photon., 5, 78–93.
[10] Talwee, I., Shah, S., Ramteke, V., & Syed, I. (2012). IJPRAS, 1(2), 06–11.
[11] Narmura, S. (2012). Displays, 22(1), 1.
[12] Omray, L. K. (2013). Current Trends in Technology and Science, 2(4), 347–352.
[13] Jain, U. K., & Sehar, I. (2014). Tropical Journal of Pharmaceutical Research, 13(1), 73–80.
[14] Hird, M., Toyne, K. J., & Gray, G. W. (1993). Liq. Cryst.,14, 741.
[15] Hird, M., Toyne, K. J., Gray, G. W., Day, S. E., & McDonnell, D. G. (1993). Liq. Cryst., 15, 123.
[16] Collings, P.J. & Hird, M, (1998). Introduction to Liquid Crystal Chemistry and Physics, Taylor and
Francis, U.K.
[17] Marcos, M., Omenat, A., Serrano, J. L., & Ezcurra, A. (1992). Adv. Mater., 4, 285.
[18] Bhoya, U. C., Vyas, N. N., & Doshi, A. V. (2012). Mol. Cryst. Liq. Cryst., 552, 104–110.
[19] Demus, D. (1988). Mol. Cryst. Liq. Cryst., 165, 45–84.
[20] Demus, D. (1989). Liq. Cryst., 5, 75–110.
[21] (i) Suthar, D. M., & Doshi, A. V. (2013). Mol. Cryst. Liq. Cryst., 575, 76–83. (ii) Chuhan, H. N., &
Doshi, A. V. (2013). Mol. Cryst. Liq. Cryst., 570 (1), 92–110. (iii) Chaudhari, R. P., Chauhan, M.
L., & Doshi, A. V. (2013). Mol. Cryst. Liq. Cryst., 575, 88–95. (iv) Bhoya, U. C., Odedara, D. A., &
Doshi, A. V. (2011). Der Pharma Chemica, 3(3), 207–212.
[22] Cseh, L., Csunderlik, C., Costisor, O. (2005). Chem. Bull., 50(64), 1–2.
[23] Patel, B. H., & Doshi, A. V. (2005). Mol. Cryst. Liq. Cryst., 605, 61–69.
[24] Doshi, A. V., & Makwana, N. G. (2011). Der Pharma Chemica, 3(2), 433–439.
[25] Patel, V. R., & Doshi, A. V. (2010). Der Pharma Chemica, 2(6), 429.
[26] Doshi, A.V., Joshi, C. G. , & Vyas, N. N. (2011). Der Pharma Chemica, 3(2), 433–439.
[27] Dave, J. S., & Vora, R. A. (1970). Liquid Crystal and Ordered Fluids, Johnson, J. F., and Porter, R.
S., eds. Plenum Press: New York, 477.
[28] Gowri, M., Jayabalakrishnan, C., Srinivasan, T., & Velmurugan, D. (2012). Mol. Cryst. Liq. Cryst.,
569, 151–161.
[29] (a) Greene, T. W., & Wuts, P. G. M. (1991). Protective Groups in Organic Synthesis, 2nd Ed.
John Wiley and Sons: New York. (b) Kocienski, P. J. (1994). Protecting Groups, Georg Thieme:
Stuttgart.
[30] Ganatra, K. J., & Doshi , A. V. (2000) Ph. D. Thesis submitted to Saurashtra University, Rajkot.