Synthesis, characterization and mesomorphic properties of aromatic acid dimers

Article abstract of DOI:10.1080/15421406.2011.599215

In this study, a new series of acid dimers, N-(4-carboxybenzylidene)-4- alkoxybenzoyloxyaniline [(4-HOOCC₆H₄CHN)-4-C ₆H₄OCOC₆H₄OR, where R = C _mH2_{m + 1} and m = 6, 8, 10, 12, 14, and 16], have been synthesized. The compounds have been characterized by elemental analyses and FTIR, UV-visible, ¹H, and ¹³C NMR spectra. The mesomorphic properties of these acid dimers were studied by differential scanning calorimetry and polarizing optical microscopy. The mesomorphic nature of these compounds depends on hydrogen bonding and on the alkoxy chain length. The compounds with m = 6 and 8 show a characteristic enantiotropic nematic mesophase, whereas those with m = 12 and 14 exhibit monotropic behavior. The compounds with m =10 and 16 exhibit non-mesomorphic behavior.

Full text of DOI:10.1080/15421406.2011.599215

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Synthesis, Characterization and
Mesomorphic Properties of Aromatic
Acid Dimers
Ashwini Pandey a & Bachcha Singh a
a Department of Chemistry (Centre of Advanced Study), Faculty of
Science , Banaras Hindu University , Varanasi , India
Published online: 27 Dec 2011.
To cite this article: Ashwini Pandey & Bachcha Singh (2012) Synthesis, Characterization and
Mesomorphic Properties of Aromatic Acid Dimers, Molecular Crystals and Liquid Crystals, 552:1, 43-52,
DOI: 10.1080/15421406.2011.599215
To link to this article: http://dx.doi.org/10.1080/15421406.2011.599215
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Mol. Cryst. Liq. Cryst., Vol. 552: pp. 43–52, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.599215
Synthesis, Characterization and Mesomorphic
Properties of Aromatic Acid Dimers
ASHWINI PANDEY AND BACHCHA SINGH∗
Department of Chemistry (Centre of Advanced Study), Faculty of Science,
Banaras Hindu University, Varanasi, India
In this study,
a
new series of acid dimers, N-(4ꢀ-carboxybenzylidene)-4-
alkoxybenzoyloxyaniline [(4ꢀ-HOOCC6H4CH N)-4-C6H4OCOC6H4OR, where R =
CmH2m + 1 and m = 6, 8, 10, 12, 14, and 16], have been synthesized. The compounds
have been characterized by elemental analyses and FTIR, UV-visible, 1H, and 13C NMR
spectra. The mesomorphic properties of these acid dimers were studied by differential
scanning calorimetry and polarizing optical microscopy. The mesomorphic nature of
these compounds depends on hydrogen bonding and on the alkoxy chain length. The
compounds with m = 6 and 8 show a characteristic enantiotropic nematic mesophase,
whereas those with m = 12 and 14 exhibit monotropic behavior. The compounds with
m =10 and 16 exhibit non-mesomorphic behavior.
Keywords Differential scanning calorimetry; enantiotropic nematic mesophase;
monotropic nematic mesophase; polarizing optical microscopy; acid dimers
1. Introduction
Supermolecular liquid crystals (LCs) obtained via intermolecular hydrogen bonding have
attracted considerable attention during the last decade [1–21]. Although the first report
of hydrogen bonding in liquid crystals dates back to the 1960s [22,23], much research
took place in the last two decades of the twentieth century [2,6,9,24–28]. New molecular
shapes, self-assembled structures, and unexpected properties have been found in liquid
crystalline materials. A variety of functional groups can be introduced as a part of mesogenic
components in these self-assembled materials. These new exotic liquid crystalline materials
have attracted considerable attention because of their great potential for new dynamic
functions, such as ion, charge, and mass transportation; photo-function; sensing; stimuli-
responsive properties; and bio-function [29,30].
In 1989, Kato and Fre´chet found that 1:1 complexation of benzoic acid and stilbazole
leads to the formation of a well-defined structure of a hydrogen-bonded mesogen by
intermolecular hydrogen bonding between the carboxylic acid and the pyridine [2].
Lehn et al. found that diaminopyridines and uracil derivatives form triply hydrogen-
bonded liquid crystalline complexes [31]; they also form columnar liquid crystalline struc-
tures. These early studies by Kato and Fre´chet [1,2] and Lehn et al. [31,32] have shown great
importance of the use of hydrogen bonding not only in the design of liquid crystals but also
∗Address correspondence to Bachcha Singh, Department of Chemistry (Centre of Advanced
Study), Faculty of Science, Banaras Hindu University, Varanasi 221005, India. E-mail: bsinghbhu@
rediffmail.com
43

44
A. Pandey and B. Singh
in the design of condensed states of soft materials, such as polymers, low-molecular-weight
organic molecules, and composites.
Many reports in thecurrent literature involvetheformationof LCs throughthe hydrogen
bonding of aromatic carboxylic acids, as well as from mixtures of unlike molecules capable
of interacting through hydrogen bonding [17,21,24,28–40].
The occurrence of hydrogen bonding in LCs, often referred to as the fifth fundamental
type of interaction, is considered to be the basis for the origin of new stable mesophases,
with a wide thermal stability. It has been well established [21] that the formation of
hydrogen bonding in liquid crystalline materials can be tuned, through the proper selection
of complementary interacting components, to fix the configuration (with respect to the
direction of the LC rigid core) of the hydrogen bonds formed.
Owing to the abovementioned properties of the hydrogen-bonded LCs, it was thought
worthwhile to synthesize a series of hydrogen-bonded compounds having ester and aldimine
linkages with terminal carboxyl group which undergoes dimerization in the presence of
N,N-dimethyl formamide. These series of compounds, namely, N-(4ꢀ-carboxybenzylidene)-
4-alkoxybenzoyloxyaniline, with m = 6∼16, have been synthesized and characterized
on the basis of IR, NMR (1H and 13C) and UV-visible spectroscopy. The mesomorphic
properties have been investigated by polarizing optical microscopy (POM) and differential
scanning calorimetry (DSC). The details of the synthesis and characterization of these
compounds have been described in the present paper.
2. Experimental Details
2.1 Materials
4-Formyl benzoic acid, 4-hydroxybenzoic acid, 1-bromoalkane, 4-aminophenol, and potas-
sium hydroxide purchased from Aldrich Chemicals, USA, were used as received. Reagent-
grade solvents and reactants were purchased commercially and used after purification.
2.2 Techniques
Elemental analyses were performed on a CE-440 Exeter analytical CHN (carbon, hydrogen,
nitrogen) analyzer. IR spectra were recorded on an FTIR Varian 3100 Excalibur series
1
spectrophotometer in KBr pellet in the 4000–100 cm−1 region. H and 13C NMR spectra
were obtained on a JEOL FT-NMR AL 300 MHz spectrometer using tetramethyl silane
as the internal standard. Electronic spectra were recorded on a UV-1700 Pharma Spec
Shimadzu UV-visible spectrophotometer. DSC thermograms were recorded with a Mettler
Toledo TC 15 TA DSC at a rate of 10.0 K min−1 under a nitrogen atmosphere using special
pure grade zinc and indium as standards by taking samples in close-lid aluminum pans.
The transition temperatures from DSC have been determined with an accuracy of 0.1K.
The mesophase type of compounds were identified by visual comparison with known phase
standards using an HT 30.01 NTT 268 Lomo POM fitted with a hot stage with temperature
controlling accuracy of 0.1 K.
2.3 Synthesis of Compounds
4-n-Alkoxybenzoic acids (2), namely 4-hexyloxy-, 4-octyloxy-, 4-decyloxy-, 4-
dodecyloxy-, 4-tetradecyloxy-, and 4-hexadecyloxybenzoic acids, were synthesized by
the method described elsewhere [41].

Mesomorphic Aromatic Acid Dimers
45
2.3.1 4-n-alkoxybenzoyl chlorides (3). These were prepared from the corresponding acids
[42].
2.3.2 4-[(4ꢀ-carboxy)benzylidene]aminophenol (6). 4-Aminophenol (4) (1.09 g, 10 mmol)
and 4-formylbenzoic acid (5) (1.5 g, 10 mmol) were mixed in ethanol (20 ml), followed
by addition of 2–3 drops of glacial acetic acid, and refluxed for 4 h. The reaction mixture
became dark yellow. The solvent was evaporated under reduced pressure to yield a yellow
residue, which was washed 2–3 times with distilled water, dried, and recrystallized from
chloroform with a 65% yield. FTIR (KBr, cm−1): 3588 (OH phenolic), 3308 (OH acid),
2997, 1687 (C O acid), 1617 (C N), 1583, 1504 (Ph), 1422, 1374, 1289, 1241, 1167 cm−1
.
1H NMR (DMSO, TMS) δ (ppm): 9.63 (s, 1H, OH), 8.70 (s, 1H, CH N), 8.05–6.80
(m, 8H, ArH). 13C NMR δc (DMSO): 167.1, 156.8, 156.0, 142.1, 140.1, 132.6, 129.7,
128.2, 122.8, 115.8, 48.6, 40.3, 39.7, 39.5, 39.2, 38.9, 38.6. UV-visible (DMF): λmax
394, 353, 300, 243 nm. Elemental analyses: calculated for C14H11NO3, (%), C, 69.73; H,
4.59; N, 5.80; Found C, 69.54; H, 4.81; N, 5.49.
=
2.3.3 N-(4ꢀ-carboxybenzylidene)-4-hexyloxybenzoyloxyaniline (7a). To a solution of 4-n-
hexyloxybenzoyl chloride (1.20 g, 5 mmol) in DMF (50 ml) was added 1.20 g (5 mmol) of
4-[(4ꢀ-carboxy)benzylidene]aminophenol (6) along with the addition of 2–3 drops of pyri-
dine, resulting in a yellow solution. The soultion was heated at 80◦C for 9 h when the color
changed from yellow to reddish brown. The reaction mixture was cooled to room tempera-
ture and 50 ml of distilled water was added. The solution was agitated and the product was
extracted with 2 × 50 ml of chloroform. The chloroform extract was dried over magnesium
sulfate (anhydrous), filtered, and recrystallized from chloroform after concentration. The
reddish-brown crystals of N-(4ꢀ-carboxybenzylidene)-4-hexyloxybenzoyloxyaniline were
obtained in a 61% yield. FTIR (KBr, cm−1): 3423, 2931, 2856 (aliphatic C H), 2558 (OH
dimer), 1776 (C O ester), 1688 (C O dimer acid), 1606 (Ph), 1511 (C N), 1466, 1428,
1390 (C O H), 1298 (C O ester), 1166, 1066, 993, 935 (out-of-plane OH bend), 843,769.
1H NMR (DMSO) δH (ppm): 12.56 (s, 1H, OH), 9.82 (s, 1H, CH N), 7.86–6.95 (m,
12H, ArH), 4.02–3.98 (t, 2H, OCH2), 1.72–1.24 (m, 8H, [CH2]n ), 0.92–0.82 (t, 3H,
CH3). 13C NMR (CDCl3) δc (CDCl3): 171.9, 163.6, 132.3, 129.5, 127.1, 121.3, 116.3,
114.1, 77.4, 77.0, 76.5 (CDCl3), 68.2 ( OCH2), 31.5, 29.6, 29.0, 25.6, 22.5, 14.0 (CH3).
UV-visible (CHCl3): λmax = 301, 264 nm. Elemental analyses: calculated for C27H27NO5
(%), C, 72.79; H, 6.10; N, 3.14; Found C, 72.58; H, 5.94; N, 3.11.
All other members of the homologous series, namely N-(4ꢀ-carboxybenzylidene)-
4-octyloxy-, 4-decyloxy-, 4-dodecyloxy-, 4-tetradecyloxy-, and 4-hexadecyloxybenzoylo
xyanilines, were prepared by following the above procedure.
2.3.4 N-(4ꢀ-carboxybenzylidene)-4-octyloxybenzoyloxyaniline (7b). Yield: 67%. FTIR
(KBr, cm−1): 3421, 2928, 2854 (aliphatic C H), 2554 (OH), 1774 (C O), 1687 (C O
dimer acid), 1605 (Ph), 1511 (C N), 1464, 1428, 1391 (C O H bend), 1298 (C O
1
ester), 1162, 1064, 991, 932 (out-of-plane OH bend), 843, 768. H NMR (CDCl3, TMS)
δH (ppm): 12.56 (s, 1H, CH N), 7.99–6.87 (m, 12H, ArH), 4.37–3.97 (t, 2H, OCH2),
1.81–1.25 (m, 12H, [CH2]n), 0.93–0.88 (t, 3H, CH3). 13C NMR δc (CDCl3): 171.8,
163.6, 132.3, 129.5, 127.1, 121.3, 116.3, 114.0, 77.4, 77.0, 76.5 (CDCl3), 68.2 ( OCH2),
31.5, 29.6, 29.0, 25.6, 22.5, 14.0 (CH3). UV-visible (CHCl3): λmax = 289, 256, 213 nm.
Elemental analyses: calculated for C29H31NO5 (%), C, 73.55; H, 6.59; N, 2.95; Found C,
73.41; H, 6.28; N, 2.56.

46
A. Pandey and B. Singh
2.3.5 N-(4ꢀ-carboxybenzylidene)-4-decyloxybenzoyloxyaniline (7c). Yield: 62%. FTIR
(KBr, cm−1): 3420, 2928, 2854 (aliphatic C H), 2550 (OH), 1773 (C O), 1680 (C O
dimer acid), 1606 (Ph), 1510 (C N), 1464, 1428, 1391 (C O H bend), 1298 (C O
ester), 1162, 1064, 991, 932 (out-of-plane OH bend), 843, 769. 1H NMR (CDCl3, TMS)
δH (ppm) 12.56 (s, 1H, OH), 9.82 (s, 1H, CH N), 7.99–6.87 (m, 12H, ArH), 4.37–3.97
(t, 2H, OCH2), 1.81–1.25 (m, 16H, [CH2]n), 0.93–0.88 (t, 3H, CH3).13C NMR δc
(CDCl3): 171.9, 163.4, 132.2, 129.5, 127.1, 121.2, 116.3, 114.0, 77.4, 77.0, 76.5 (CDCl3),
68.2 ( OCH2), 31.5, 29.6, 29.0, 25.6, 22.5, 14.0 (CH3). UV-visible (CHCl3): λmax = 357,
270, 252, 212 nm. Elemental analyses: calculated for C31H35NO5 (%), C, 74.22; H, 7.03;
N, 2.79; Found C, 73.81; H, 7.11; N, 2.64.
2.3.6 N-(4ꢀ-carboxybenzylidene)-4-dodecyloxybenzoyloxyaniline (7d). Yield: 58%. FTIR
(KBr, cm−1): 3431, 2922, 2852 (aliphatic C H), 2557 (OH), 1774 (C O), 1684 (C O
dimer acid), 1606 (Ph), 1513 (C N), 1464, 1430, 1390 (C O H bend), 1307 (C O ester),
1170, 1061, 991, 935 (out-of-plane OH bend), 845, 770. 1H NMR (CDCl3, TMS) δH (ppm)
12.52 (s, 1H, OH), 9.82 (s, 1H, CH N), 8.05–6.91 (m, 12H, ArH), 4.04–3.97 (t, 2H,
OCH2), 1.82-1.26 (m, 20H, [CH2]n), 0.87 (t, 3H, CH3).13C NMR δc (CDCl3): 171.9,
163.6, 132.3, 129.5, 127.5, 121.2, 116.3, 114.0, 77.4, 77.0, 76.5 (CDCl3), 68.2 ( OCH2),
31.7, 29.5, 25.6, 22.5, 14.0 (CH3). UV-visible (CHCl3): λmax = 279, 264, 253, 228, 213
nm. Elemental analyses: calculated for C33H39NO5 (%), C, 74.83; H, 7.42; N, 2.64; Found
C, 74.56; H, 7.08; N, 2.36.
2.3.7 N-(4ꢀ-carboxybenzylidene)-4-tetradecyloxybenzoyloxyaniline (7e). Yield: 55%.
FTIR (KBr, cm−1): 3450, 2918, 2851 (aliphatic C H), 2562 (OH), 1778 (C O), 1684
(C O dimer acid), 1607 (Ph), 1513 (C N), 1469, 1429, 1392 (C O H bend), 1304
1
(C O ester), 1171, 1061, 992, 941 (out-of-plane OH bend), 846, 718. H NMR (CDCl3,
TMS) δH (ppm) 12.56 (s, 1H, OH), 9.82 (s, 1H, CH N), 7.86–6.95 (m, 12H, ArH),
4.02–3.98 (t, 2H, OCH2), 1.72–1.24 (m, 24H, -[CH2]n), 0.92–0.82 (t, 3H, CH3). 13C
NMR δc (CDCl3): 171.9, 163.6, 132.3, 129.4, 127.5, 121.1, 116.3, 114.1, 77.4, 77.0, 76.5
(CDCl3), 68.2 ( OCH2), 31.7, 29.5, 29.0, 25.6, 22.5, 14.0 (CH3). UV-visible (CHCl3):
λmax = 279, 227, 213 nm. Elemental analyses: calculated for C35H43NO5 (%), C, 75.37; H,
7.77; N, 2.51; Found C, 75.12; H, 7.54; N, 2.38.
2.3.8 N-(4ꢀ-carboxybenzylidene)-4-hexadecyloxybenzoyloxyaniline (7f). Yield: 58%.
FTIR (KBr, cm−1): 3452, 2922, 2855 (aliphatic C H), 2559 (OH), 1775 (C O), 1688
(C O dimer acid), 1605 (Ph), 1515 (C N), 1469, 1431, 1390 (C O H bend), 1305
1
(C O ester), 1172, 1058, 991, 925 (out-of-plane OH bend), 841, 714. H NMR (CDCl3,
TMS) δH (ppm): 12.52 (s, 1H, OH), 9.82 (s, 1H, CH N), 7.86–6.95 (m, 12H, ArH),
4.02–3.98 (t, 2H, OCH2), 1.72–1.24 (m, 28H, [CH2]n), 0.92-0.82 (t, 3H, CH3). 13
C
NMR δc (CDCl3): 171.9, 163.6, 132.2, 129.3, 127.5, 121.1, 116.4, 114.1, 77.4, 77.0, 76.5
(CDCl3), 68.2 ( OCH2), 31.7, 29.5, 29.0, 25.6, 22.5, 14.0 (CH3). UV-visible (CHCl3):
λmax = 280, 230, 210 nm. Elemental analyses: calculated for C37H47NO5 (%), C, 75.86;
H, 8.08; N, 2.39; Found C, 75.52; H, 7.92; N, 2.14.
3. Results and Discussion
The synthetic route for the compounds 4-[(4ꢀ-carboxy)benzylidene] aminophenol (6) and
its ester derivative N-(4ꢀ-carboxybenzylidene)-4-alkoxybenzoyloxyaniline (7) is outlined
in Scheme 1. The elemental analyses, FTIR, and NMR spectra are fully consistent with

Mesomorphic Aromatic Acid Dimers
47
O
O
RBr, EtOH
H2m+1CmO
HO
O-K+
KOH, reflux, 14 h
OH
1
Conc. HCl
O
H2m+1CmO
OH
2
SOCl2 / CHCl3
reflux 7h
O
H2m+1CmO
Cl
3
O
O
NH2
HO
+
OH
4
AcOH /
EtOH
5
O
HO
N
CH
OH
6
C5H5N / DMF
O
N
CH
H2m+1CmO
C
O
OH
O
7
Scheme 1. Route for the synthesis of N-(4ꢀ-carboxybenzylidene)-4-alkoxybenzyoyloxyaniline.
the structure. The IR spectrum of 4-[(4ꢀ-carboxy)benzylidene]aminophenol (6) exhibits
major bands at 3588 , 3308, 2997, 1687, 1617, 1583, and 1504 cm−1, which are charac-
teristic of ν(OH phenolic), ν(OH acid), aromatic C H, ν(C N), and ring C C stretch,
respectively. The compound undergoes esterification with 4-n-hexyloxybenzoyl chloride in
the presence of pyridine to form N-(4ꢀ-carboxybenzylidene)-4-hexyloxybenzoyloxyaniline

48
A. Pandey and B. Singh
(7). This compound exhibits IR bands at 3423, 2931, 2856, 2558, 1776, 1688, 1606, 1511,
1390, and 1298 cm−1, which are attributed to ν(OH) acid, ν(aliphatic C H), ν(OH) dimer
acid, ν(COO ester), ν(C O dimer acid), ν(Ph), ν(C N), ν(C O H bending mode), and
ν(C O ester). Further, a characteristic band for the dimeric carboxylic acid resulting from
the out-of-plane bending of the bonded O H appears near 935 cm−1
.
The dimer state of this compound formed as a result of intermolecular hydrogen
bonding depicts major hydrogen bonding peaks centered at 2856, 1688, and 935 cm−1
assigned to ν(OH dimer acid) and ν(C O) dimer acid, and an exclusive band for OH out-
of-plane bending mode characteristic of the dimeric acid appears at 935 cm−1. Comparison
of the dimeric hydrogen-bonded peaks with the non-hydrogen-bonded peaks for the same
compound depicts a major or a significant shift in the C O for the non-hydrogen-bonded
acid appearing at 1714 cm−1 to 1688 cm−1 in the hydrogen-bonded compound along with
the appearance of a new band for out-of-plane OH bending at 935 cm−1, which is absent in
non-hydrogen-bonded compound.
1
The H NMR spectrum of 4-[(4ꢀ-carboxy)benzylidene]aminophenol (6) exhibits sig-
nals at δ 9.63 (s), 8.70 (s), and 8.05–6.80 (m), which are attributed to OH, CH N,
and aromatic ring protons, respectively. The major signals in the 13C NMR spectrum of the
compound (6) are observed at δ 167.1 for CH N, 156.8 for C OH, 142.1 for carbon of
aromatic ring bonded to N of CH N, and the one centered at 132.6 is assigned for the aro-
matic ring carbon. The signals appearing for 1H and 13 C NMR are fully consistent with the
structure. Furthermore, the proton NMR of the compound of N-(4ꢀ-carboxybenzylidene)-
4-hexyloxybenzoyloxyaniline (7a) exhibits sharp signals at δ 12.56 (s), 9.82 (s), 7.99–6.87
(m), 4.37–3.97 (t), 1.81–1.25 (m), and 0.93–0.88 are attributed to OH, CH N, aro-
matic ring, OCH2, [CH2]n, and CH3 protons, respectively. The 13C NMR signals for
the major functional groups for this compound are observed at δ 171.9 for COOH attached
to aromatic ring, 163.6 for CH N, 132.3, 129.5 for carbons of aromatic ring, 77.4, 77.0,
77.5 for CDCl3, and the ones centered at 68.2, 25.6, and 14.0 are attributed for OCH2,
OCH2CH2, and CH3 carbon signals, suggesting the consistency of the signals with the
molecular structure.
The electronic spectrum of the compound N-(4ꢀ-carboxybenzylidene)-4-
hexyloxybenzoyloxyaniline (7a) display UV bands at 289, 256, and 213 nm, which
are assigned for n-π∗, B-band of benzene, and π–π∗ transitions.
All other members of the homologous series with intermolecular hydrogen-bonding-
generating acid dimers exhibit similar IR, NMR, and UV-visible bands, which are already
explained and their structures have been elucidated on the basis of the abovementioned
analytical techniques.
4. Optical Properties
The liquid crystalline properties of N-(4ꢀ-carboxybenzylidene)-4-n-alkoxybenzoyl
oxyaniline 7a–7f were investigated by POM using a heating and cooling stage. The phase-
transition temperatures and enthalpies were measured by carrying out DSC thermal analysis.
The Cr, N, and I denote crystalline, nematic, and isotropic phases, respectively. The com-
pound 7a (Table 1) exhibited endothermic peaks with calculated enthalpy and entropy at
68.4◦C (ꢀH = 0.3 kJ mol–1) (ꢀS = 0.9 JK−1 mol−1), 98.5◦C (ꢀH = 14.2 kJ mol−1) (ꢀS =
38.2 JK−1 mol−1), 107.4◦C (ꢀH = 0.2 kJ mol−1) (ꢀS = 2.3 JK−1 mol−1), 134.8◦C (ꢀH =
2.2 kJ mol−1) (ꢀS = 5.4 JK−1 mol−1), 146.5◦C (ꢀH = 0.1 kJ mol−1) (ꢀS = 0.3 JK−1 mol−1
)
and 186.7◦C, which defines crystal-to-crystal polymorphic phases, and the peaks centered
at 146.5◦C and 186.7◦C display nematic and isotropic phase transitions, respectively. The

Mesomorphic Aromatic Acid Dimers
49
Table 1. Transition temperatures (T) of the compound N-(4ꢀ-carboxybenzylidene)-4-n-
alkoxybenzoyloxyaniline.
Compounds
Transition temperatures (oC)
7a
Cr1 68.4 (0.3)a (0.9)b, Cr2 98.5 (14.2) (38.2), Cr3 107.4 (0.2)
(2.3), Cr4 134.8 (2.2) (5.4) Cr5 146.5 (0.1) (0.3), N 186.7 (0.2)
(0.5), I 184.3 (0.1) (0.3), N 144.3 (0.2)(0.5), Cr5 125.8 (2.4)
(6.0), Cr4 104.6 (0.4) (1.1), Cr379.9 (13.8) (39.2), Cr
Cr 134, N 153, I 146, N 121, Cr
Cr 174, I 150, Cr
Cr 120, I 118, N 98, Cr
Cr 110, I 87, N 83, Cr
Cr 240, I 99, Cr
7b
7c
7d
7e
7f
b
aThe value within the first set of parentheses denotes enthalpy in kJ mol−1; the value within
the second set of parentheses denotes entropy in JK−1 mol−1; Cr signifies crystal phase, N signifies
nematic phase, and I signifies isotropic liquid.
exothermic peaks appear at 184.3◦C (ꢀH = 0.1 kJ mol−1) (ꢀS = 0.3 JK−1 mol−1), 144.3◦C
(ꢀH = 0.2 kJ mol−1) (ꢀS = 0.5 JK−1 mol−1), 125.8◦C (ꢀH = 2.4 kJ mol−1) (ꢀS =
6.0 JK−1 mol−1), 104.6◦C (ꢀH = 0.4 kJ mol−1) (ꢀS = 1.1 JK−1 mol−1), and 79.9◦C
(ꢀH = 13.8 kJ mol−1) (ꢀS = 39.2 JK−1 mol−1), respectively, which defines isotropic-to-
nematic phase transition followed by subsequent crystal phases. This compound displays
enantiotropic nematic phase with the appearance of Schlieren texture as seen under op-
tical microscopy. During the heating cycle at 146.5◦C, the crystal phase transforms into
radial bubbles, which sharply diminishes at the isotropic point. During the cooling cycle
at 184.3◦C, again the appearance of numerous radial bubbles or nematic droplets are seen,
which enlarge and distribute as small patches showing striations embedded within the
patches, giving the appearance of a very similar grandjean texture which is observed in the
chiral nematic liquid crystals. The small patches showing striations or grooves and ridges
embedded within the patches show a strong similarity with the “dermal plicae,” which
appears in the impression of the human fingerprint. However, it may be best compared with
the grandjean texture of the chiral nematic mesophase.
The compound 7b under optical microscopy displays nematic mesophase at 134◦C
duringtheheatingcyclewiththeappearanceof concentricradial nematicdroplets occupying
the full length of the vision area as seen under the optical microscope. The nematic droplets
very slowly are seen to perish at the isotropization temperature of 153◦C. During the cooling
cycle at 146◦C, nematic droplets with Schlieren texture are seen to appear. Within a larger
droplet, numerous encircling bubbles with black stripes enclosed within the bubble cavity
are seen, providing the evidence of the nematic mesophase. As the temperature further goes
down to 121◦C during the cooling cycle, a sharp transition toward the crystal phase is seen
for this compound. Thus, this compound also displays enantiotropic mesophase with the
appearance of nematic phase during both the heating and the cooling cycle.
The compounds 7d and 7e display a monotropic nematic phase as seen under optical
microscope with the appearance of nematic phase at 118◦C and 87◦C during the cooling
cycle only. During the heating cycle, both the compounds reach their isotropic temperature
at 120◦C and 110◦C with no appearance of mesophase during the heating cycle. Thus, a
sharp transition from crystal to isotropic phase was recorded for both the compound with

50
A. Pandey and B. Singh
prominent monotropic nematic mesophase with the appearance of characteristic textural
pattern during the cooling cycle only. For both the compounds 7d and 7e, as seen individ-
ually under the optical microscope, the nematic droplets appear during the cooling cycle
and get covered by the crystal phases at 98◦C and 83◦C, respectively.
The compounds 7c and 7f exhibited a sudden crystal-to-isotropic phase transition
at 174◦C and 240◦C, respectively. During the cooling cycle, again isotropic-to-crystal
phase transition are encountered at 150◦C and 99◦C, respectively, with no exhibition of
mesophase during both the heating and the cooling cycle. Both these compounds were
non-liquid crystalline, exhibiting a high isotropization temperature at 174◦C and 240◦C,
respectively.
Thus, a structural–functional correlation is observed for the series of mentioned com-
pounds 7a-7f being analyzed for liquid crystallinity. The forces governing the liquid crys-
tallinity in the series of mentioned compounds 7a-7f are intermolecular interactions arising
due to hydrogen bonding. These systems possess favorable intermolecular interactions via
hydrogen bonding, which is responsible for generating the mesogenic behavior in this series
of compounds.
The dimer state of the compound generated as a result of hydrogen bonding possesses
the favorable intermolecular interaction responsible for generating the mesomorphic be-
havior. These intermolecular interactions have been tuned by the incorporation of aromatic
rings and long alkoxy chains, which stabilize the mesomorphic properties. The compounds
7a, 7b, 7d, and 7e exhibited nematic mesophase, which is stabilized by the anisotropic van
der Waal forces generated as a result of increasing alkoxy chain length. These anisotropic
forces assist the hydrogen bonding forces, and as a result, the compounds display enan-
tiotropic and monotropic nematic phase for the homolog with lower number of alkoxy
chain length. But in higher members of the series with m = 16, the van der Waal forces
overcome the hydrogen bond interaction, resulting in packing of the molecules and leading
to the loss of mesomorphic behavior. Because the magnitudes of the dipole–dipole and
dispersion forces that hold the thermotropic phases are critical, when they are too weak or
too strong, the liquid crystalline character is lost. These forces being critical, (m = 10) (too
low), do not bring the mesogenic nature in this compound. Being too low, these forces do
not assist the intermolecular interaction generated as a result of hydrogen bonding.
Thus π–π dispersion forces from aromatic ring and intermolecular interactions as a
result of hydrogen bonding favor the mesomorphic behavior, which is further assisted by
anisotropic forces generated from the long alkoxy chain. In higher alkoxy chain length
with m = 16, these forces are overcome by strong van der Waal forces resulting from the
anisotropic alkoxy chain, which results in the loss of mesomorphic behavior.
5. Conclusions
A
new series of mesomorphic acid dimers, N-(4ꢀ-carboxybenzylidene)-4-n-
alkoxybenzoyloxyaniline, formed as a result of intermolecular hydrogen bonding having
terminal alkoxy chains (m = 6∼16) have been synthesized. The chemical structures of
the final products were investigated by FTIR, UV-visible, 1H, and 13C NMR spectra. The
existence of nematic phase was confirmed by optical microscopy. It was found that forces
leading to the favorable molecular stacking as a result of hydrogen bonding results in acid
dimers which exhibit mesomorphic properties. The prominent hydrogen bonding stabiliz-
ing the nematic mesophase is further suppressed by strong van der Waal forces originating
from long alkoxy chain length with m = 16, which results in the loss of mesomorphic
behavior in higher members of the series.

Mesomorphic Aromatic Acid Dimers
51
Acknowledgments
The authors thank the Head of the Department of Chemistry, Banaras Hindu University
(BHU), Varanasi, India, for providing laboratory facilities. One of the authors (Ashwini
Pandey) thanks the Council of Scientific and Industrial Research (CSIR, New Delhi, India)
for financial support in the form of senior research fellowship at BHU. Thanks are also due
to Mr. Satish Tiwari for CHN analyses, Mr. Shishir, Mr. Nagendra, and Mr. Amit for 1H,
13C NMR, and IR and electronic spectroscopy.
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