Liquid crystalline properties of 2,5-bis[4-n-alkoxybenzoyloxy-(4- benzoyloxy)]-3,6-dichloro-1,4-benzoquinones: Synthesis and characterization

Article abstract of DOI:10.1080/15421400903483965

A new series of 2,5-bis[4-n-alkoxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro- 1,4-benzoquinones, C₆Cl₂O₂(OCOC ₆H₄OCOC₆H₄OR)₂, where R=C_mH_2m+1 and m=7, 8, 9, 10, 11, 12, 14, and 16, have been synthesized. These compounds have been characterized by elemental analyses, FT-IR, UV-visible, ¹H, and ¹³C NMR spectra. Mesomorphic properties of these compounds were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The mesomorphic nature of these compounds depends on the alkoxy chain length. The compounds (m=7, 8, 9, 10) exhibit a monotropic nematic mesophase in cooling cycle, while the compounds (m=11, 12, 14, and 16) show no mesogenic behavior in both heating and cooling cycles. The intermolecular interactions play a vital role in the mesogenic nature of these compounds. In higher homologues of the series the intermolecular interactions become more prominent leading to loss in mesogenic nature. Copyright Taylor & Francis Group, LLC.

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Liquid Crystalline Properties of
2,5-Bis[4-n-alkoxybenzoyloxy-(4-
benzoyloxy)]-3,6-dichloro-1,4-
benzoquinones: Synthesis and
Characterization
Bachcha Singh a & Ashwini Pandey a
a Department of Chemistry, Faculty of Science, Banaras Hindu
University, Varanasi, India
Version of record first published: 01 Mar 2010.
To cite this article: Bachcha Singh & Ashwini Pandey (2010): Liquid Crystalline Properties of 2,5-Bis[4-
n-alkoxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinones: Synthesis and Characterization,
Molecular Crystals and Liquid Crystals, 517:1, 138-147
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Mol. Cryst. Liq. Cryst., Vol. 517: pp. 138–147, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903483965
Liquid Crystalline Properties of
2,5-Bis[4-n-alkoxybenzoyloxy-
(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinones:
Synthesis and Characterization
BACHCHA SINGH AND ASHWINI PANDEY
Department of Chemistry, Faculty of Science, Banaras Hindu University,
Varanasi, India
A new series of 2,5-bis[4-n-alkoxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,
4-benzoquinones, C6Cl2O2(OCOC6H4OCOC6H4OR)2, where R ¼ CmH2m þ 1 and
m ¼ 7, 8, 9, 10, 11, 12, 14, and 16, have been synthesized. These compounds have
1
been characterized by elemental analyses, FT-IR, UV-visible, H, and 13C NMR
spectra. Mesomorphic properties of these compounds were studied by differential
scanning calorimetry (DSC) and polarizing optical microscopy (POM). The meso-
morphic nature of these compounds depends on the alkoxy chain length. The com-
pounds (m ¼ 7, 8, 9, 10) exhibit a monotropic nematic mesophase in cooling
cycle, while the compounds (m ¼ 11, 12, 14, and 16) show no mesogenic behavior
in both heating and cooling cycles. The intermolecular interactions play a vital role
in the mesogenic nature of these compounds. In higher homologues of the series the
intermolecular interactions become more prominent leading to loss in mesogenic
nature.
Keywords Differential scanning calorimetry; intermolecular interactions; mono-
tropic nematic mesophase; polarizing optical microscopy
1. Introduction
Liquid crystalline materials may be used in passive block filters, laser addressed
devices, polarizers based on dichroic effects, and devices based on thermochromism
[1]. The organic liquid crystalline materials with benzoquinone rigid core are prone
to electron transfer reactions which may play an important role in biological systems
[2–5]. They may have an important role as oxidants in the liquid crystalline state,
liquid crystalline charge transfer complexes, and liquid crystalline conductors [6,7].
The use of quinone functionality in the linkage unit of laterally bridged oligopor-
phyrin as a switch for controlling electronic coupling between the termini has been
examined [8].
Nakasuji and Yamamoto [9] reported amphoteric redox nature of p-benzoqui-
none with donor and acceptor substituents. Primary, secondary, and tertiary amines
Address correspondence to Bachcha Singh, Department of Chemistry, Faculty of Science,
BHU, Varanasi-221005, India. E-mail: bsinghbhu@rediffmail.com
138

Liquid Crystalline Properties
139
both aliphatic and aromatic are shown to react with p-chloranil in dioxan=
2-propanol [10]. Chandrasekhar et al. [11,12] were the first to report an unstable
discotic mesophase of 2,3,5,6-tetrakis(octanoyloxy)-1,4-benzoquinone. Lillya and
Thakur reinvestigated and concluded columnar mesophase with a narrow tempera-
ture range for both the tetrakis heptanoyloxy and octanoyloxy-1,4-benzoquinone
(1). Matsutani et al. [13] synthesized three series of 2,3,5,6-tetrakis(4-alkoxybenzoy-
loxy)-1,4-benoquinones (2), 2,3,5,6-tetraalkoxy-1,4-benzo-quinones (3), 2,3,5,6-
tetrakis (alkanoylamino)-1,4-benzoquinones (4), and examined their mesomorphic
properties. The benzoates 2 exhibit a simple crystal-isotropic liquid transition, while
the aliphatic esters 1 are mesogenic. Thus, it is concluded that the aromatic ring of
benzoate 2 was not suitable for improving coplanarity and rigidity to the 1,4-benzo-
quinone central core. They also obtained ethers 3 that showed two endothermic
peaks at 32ꢀC and 55ꢀC, and vague texture was reported in the above temperature
range. The strengthening of the intermolecular interaction was achieved by substi-
tution of an ester group of 1 by an amide group to yield 4 which displayed a dendric
texture after being cooled from the isotropic liquid. The hexagonal columnar
mesophase was reported on the basis of optical texture.
Berg et al. [6] reported synthesis of 2,5-dibenzoyloxy-3,6-dihydroxy-1,4-
benzoquinone derivatives, their reduction products, and centrosymmetric tetraesters.
The derivatives with short alkoxy groups (n ¼ 1–5) showed only typical nematic
schlieren textures and droplets at the clearing point. The derivatives with long dode-
cyloxy groups exhibit an additional smectic phase (SC or SA).
In view of the above observations, a systematic study on the synthesis, charac-
terization and the mesomorphic properties of hitherto uninvestigated compounds
derived from 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil) have been under-
taken. The results are communicated in the present article.
2. Experimental
Materials
4-Hydroxybenzoic acid, 1-bromoalkanes, potassium hydroxide, sodium bicarbonate
(all Aldrich), thionyl chloride (Qualigens), and chloranil (Himedia) were used as
obtained. All other reagents and solvents purchased commercially were used after
further purification.
Techniques
Elemental analyses were performed on CE-440 Exeter Analytical CHN analyzer. IR
spectra (4000–100 cmꢁ1) were recorded on Varian 3100 FT-IR Excalibur spectro-
1
photometer. H and 13C NMR spectra were obtained by using a JEOL FT NMR
AL 300 MHz spectrometer using tetramethyl silane as internal standard. Electronic
spectra were recorded on UV-1700 Pharma Spec Shimadzu UV-Visible spectrophot-
ometer. Differential scanning calorimetry (DSC) thermograms were recorded on
Mettler Toledo TC 15 TA differential scanning calorimeter at the rate of
10.0 K minꢁ1 under nitrogen atmosphere using spec pure grade indium as standard
by taking samples in close lid aluminum pans. The transition temperatures from
DSC have been determined with accuracy of ꢂ0.1 K. The mesophase identification
was done by visual comparison with known phase standards using HT-30.01 NTT

140
B. Singh and A. Pandey
268 LOMO polarizing optical microscope (POM) fitted with a hot stage and
temperature controlling accuracy of 0.1 K.
Synthesis of Compounds
Synthesis of 4-Heptyloxy Benzoic Acid (2). To a solution of 4-hydroxybenzoic acid
(1.18 g, 10 mmol) in dry ethanol (30 ml) and of KOH (1.68 g, 30 mmol) in dry ethanol
(60 ml), 1-bromoheptane (1.57 ml, 10 mmol) was added dropwise with constant
stirring. The reaction mixture was refluxed for ꢃ14 h under dry atmosphere and
allowed to cool to room temperature. The solid alkoxy potassium salt thus
obtained was separated out by filtration under suction and treated with dilute
HCl until the pH of the reaction mixture reached to ꢃ2. The crude solid white
product was filtered off, washed thoroughly with water, and recrystallized
successively from glacial acetic acid and toluene. Yield: 72%, IR (KBr, cmꢁ1):
=
3423 (OH), 2929, 2852 (aliphatic CꢁH), 1685 (C O), 1606, 1511 (Ph), 1305, 1256
1
(OPh). H NMR (CDCl3, TMS) dH (ppm): 11.90 (s, 1 H, ꢁOH), 8.06–8.03 (d, 2 H,
ArH), 6.94–6.91 (d, 2 H, ArH), 4.04–4.00 (t, 2 H, ꢁOCH2), 1.83–1.31 (m, 10 H,
ꢁ[CH2]n), 0.89–0.87 (t, 3 H, ꢁCH3).
All the homologous members of the series have been prepared using the above
procedure (Scheme 1).
Synthesis of 2,5-Bis[4-hydroxybenzoyloxy]-3,6-dichloro-1,4-benzoquinone (3). An
aqueous solution (40 ml) of NaHCO3 (0.67 g, 8 mmol) was added to an ethanolic
solution (20 ml) of 4-hydroxybenzoic acid (0.44 g, 8 mmol) till effervescence ceased.
To this solution 2,3,5,6,-tetrachloro-1,4-benzoquinone (Chloranil) (0.98 g, 4 mmol)
was added with stirring. The reaction mixture was stirred further for 6 h with
heating at 40ꢀC. A reddish brown precipitate appeared which was filtered, washed
with water followed by ethanol and ether. The product was washed with toluene
thrice (3 ꢄ 5 ml) to remove chloranil and dried under vacuum. Recrystallization of
the compound from ethyl acetate afforded orange crystals in 72% yield. IR (KBr,
cmꢁ1): 3449 (OH), 1768 (C O ester), 1682 (C O quinone), 1589 (C C quinone),
1502 (Ph), 1294 (OPh), 765 (CꢁCl). 1H NMR (CDCl3, TMS) dH (ppm): 12.94
(s, 2 H, ꢁOH), 7.95 (d, 4 H, ArH), 7.36 (d, 4 H, ArH). 13C NMR (DMSO)
dc (ppm): 173.0, 166.5, 159.1, 150.0, 139.4, 131.4. 130.0, 126.3, 116.0, 40.3, 40.0,
39.7, 39.5, 39.2, 38.9, 38.6. Elemental analyses: calculated (for C20H10Cl2O8), C,
53.47; H, 2.24%, found, C, 53.12; H, 2.01%.
=
=
=
Preparation of 2,5-Bis[4-n-heptyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-
benzoquinone, C48H46Cl2O12 (4a). 4-Heptyloxybenzoic acid (1.88 g, 8 mmol) was
dissolved in dry chloroform (50 ml), and thionyl chloride (0.65 ml, 9 mmol) was
added and refluxed for 7 h. The solvent and excess thionyl chloride were removed
under reduced pressure to give 4-heptyloxy benzoyl chloride as an yellow oil. The
above acid chloride was treated with 2,5-bis[4-hydroxybenzoyloxy]-3,6-dichloro-
1,4-benzoquinone (3) (1.79 g, 4 mmol) in dry dichloromethane (50 ml) containing
2–3 drops of pyridine. The reaction mixture was refluxed for 8 h with constant
stirring. An orange color solution was obtained. The solvent was removed under
reduced pressure to give oil. It was triturated with water (40 ml) and extracted with
CH2Cl2(2 ꢄ 10 ml) and dried. Evaporation of the solvent followed by crystallization
from chloroform yielded orange crystals. Yield: 73%, IR (KBr, cmꢁ1): 2923, 2853

Liquid Crystalline Properties
141
=
=
(aliphatic CꢁH), 1770 (C O ester), 1682 (C O quninone), 1605, 1510 (Ph), 1310,
1
1256 (OPh), 764 (CꢁCl). H NMR (CDCl3, TMS) dH (ppm): 8.08–8.01 (dd, 8 H,
ArH), 6.99–6.90 (dd, 8 H, ArH), 4.06–3.99 (m, 4 H, ꢁOCH2), 1.80–1.27 (m, 20 H,
ꢁ[CH2]n), 0.88–0.86 (t, 6 H, ꢁCH3). 13C NMR (DMSO) dc (ppm): 172.1, 169.4,
166.4, 163.6, 162.8, 140.7, 132.2, 131.4. 131.0, 122.5, 121.7, 121.3, 114.5, 114.3,
113.9, 77.4, 77.0, 76.5 (CDCl3), 68.2 (ꢁOCH2), 68.1, 60.5, 31.7, 29.0, 25.9, 22.5,
14.0 (CH3). UV-visible (EtOH): kmax ¼ 344, 277 nm. Elemental analyses: calculated
(for C48H46Cl2O12), C, 65.08; H, 5.23%, found, C, 64.93; H, 4.81%. All other
homologous members of the series were prepared by the similar procedure. Their
IR, NMR (1H, 13C) UV-visible and elemental data are summarized as follows.
2,5-Bis[4-n-octyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone,
C50H50Cl2O12 (4b). Yield: 71%. IR (KBr, cmꢁ1): 2928, 2853 (aliphatic CꢁH), 1775
=
=
(C O ester), 1681 (C O quinone), 1604, 1511 (Ph), 1306, 1256 (OPh), 766 (CꢁCl).
1H NMR (CDCl3, TMS) dH (ppm): 7.90–7.85 (dd, 8 H, ArH), 7.03–6.97 (dd, 8 H,
ArH), 4.04–4.00 (m, 4 H, ꢁOCH2), 1.74–1.27 (m, 24 H, ꢁ[CH2]n), 0.86 (t, 6 H,
ꢁCH3). 13C NMR (CDCl3) dC (ppm): 171.9, 166.4, 163.6, 162.8, 132.4, 132.2,
131.4. 122.5, 121.3, 116.3, 114.1, 113.9, 77.4, 76.9, 76.5 (CDCl3), 68.2 (ꢁOCH2),
68.1, 60.5, 31.7, 29.2, 29.1, 29.0, 25.9, 22.6, 14.0 (CH3). UV-visible (EtOH):
kmax ¼ 276, 251 nm. Elemental analyses: calculated (for C50H50Cl2O12), C, 65.71;
H, 5.51%, found, C, 65.42; H, 5.93%.
2,5-Bis[4-n-nonyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone,
C52H54Cl2O12 (4c). Yield: 72%. IR (KBr, cmꢁ1): 2926, 2854 (aliphatic CꢁH), 1773
=
(C O ester), 1682 (C ¼ O quinone), 1605, 1512 (Ph), 1307, 1255 (OPh), 766(CꢁCl).
1H NMR (CDCl3, TMS) dH (ppm): 8.08–8.04 (dd, 8 H, ArH), 6.95–6.87 (dd, 8 H,
ArH), 4.06–4.00 (m, 4 H, ꢁOCH2), 1.83–1.28 (m, 28 H, ꢁ[CH2]n), 0.88–0.86 (t, 6
H, ꢁCH3). UV-visible (EtOH): kmax ¼ 349, 284, 268 nm. Elemental analyses:
calculated (for C52H54Cl2O12), C, 66.31; H, 5.77%, found, C, 66.22; H, 5.34%.
2,5-Bis[4-n-decyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone,
C54H58Cl2O12 (4d). Yield: 76%. IR (KBr, cmꢁ1): 2923, 2852 (aliphatic CꢁH), 1775
(C ¼ O ester), 1681 (C ¼ O quinone), 1604, 1511 (Ph), 1306, 1256 (OPh), 770 (CꢁCl).
1H NMR (CDCl3, TMS) dH (ppm): 8.06–8.03 (dd, 8 H, ArH), 6.94–6.91 (dd, 8 H,
ArH), 4.04–4.00 (m, 4 H, ꢁOCH2), 1.83–1.27 (m, 32 H, ꢁ[CH2]n), 0.90–0.86 (t,
6 H, ꢁCH3). UV-visible (EtOH): kmax ¼ 273, 245, 217 nm. Elemental analyses:
calculated (for C54H58Cl2O12), C, 66.86; H, 6.02%, found, C, 66.58; H, 6.31%.
2,5-Bis[4-n-undecyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone,
C56H62Cl2O12 (4e). Yield: 75%. IR (KBr, cmꢁ1): 2931, 2855 (aliphatic CꢁH), 1773
(C ¼ O ester), 1684 (C ¼ O quinone), 1605, 1511 (Ph), 1307, 1256 (OPh), 770 (CꢁCl).
1H NMR (CDCl3, TMS) dH (ppm): 8.05–7.96 (dd, 8 H, ArH), 6.93–6.87 (dd, 8 H,
ArH), 4.04–3.97 (m, 4 H, ꢁOCH2), 1.80–1.27 (m, 36 H, ꢁ[CH2]n), 0.91–0.87 (t,
6 H, ꢁCH3). UV-visible (EtOH): kmax ¼ 342, 274 nm. Elemental analyses: calculated
(for C56H62Cl2O12), C, 67.39; H, 6.26%, found, C, 67.54; H, 6.42%.
2,5-Bis[4-n-dodecyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone,
C58H66Cl2O12 (4f). Yield: 68%. IR (KBr, cmꢁ1): 2925, 2851 (aliphatic CꢁH), 1775
=
=
(C O ester), 1681 (C O quinone), 1605, 1510 (Ph), 1307, 1257 (OPh), 770 (CꢁCl).
1H NMR (CDCl3, TMS) dH (ppm): 8.05–7.96 (dd, 8 H, ArH), 6.93–6.87 (dd, 8 H,
ArH), 4.04–3.97 (m, 4 H, OCH2), 1.80–1.31 (m, 40 H, ꢁ[CH2]n), 0.91–0.87 (t, 6 H,

142
B. Singh and A. Pandey
ꢀCH3). UV-visible (EtOH): kmax ¼ 293, 271 nm. Elemental analyses: calculated (for
C58H66Cl2O12), C, 67.89; H, 6.48%, found, C, 67.62; H, 6.27%.
2,5-Bis[4-n-tetradecyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone,
C62H74Cl2O12 (4g). Yield: 74%. IR (KBr, cmꢀ1): 2924, 2854 (aliphatic CꢀH), 1781
=
=
(C O ester), 1683 (C O quinone), 1605, 1510 (Ph), 1310, 1258 (OPh), 765 (CꢀCl).
1H NMR (CDCl3, TMS) dH (ppm): 8.06–7.82 (dd, 8 H, ArH), 7.10–6.95 (dd, 8 H,
ArH), 4.04–3.97 (m, 4 H, ꢀOCH2), 1.70–1.21 (m, 48 H, ꢀ[CH2]n), 0.91–0.82 (t,
6 H, ꢀCH3). UV-visible (EtOH): kmax ¼ 302, 268 nm. Elemental analyses: calculated
(for C62H74Cl2O12), C, 68.81; H, 6.89%, found, C, 68.76; H, 6.82%.
2,5-Bis[4-n-hexadecyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone,
C66H82Cl2O12 (4h). Yield: 70%. IR (KBr, cmꢀ1): 2919, 2852 (aliphatic CꢀH), 1775
=
=
(C O ester), 1682 (C O quinone), 1605, 1511 (Ph), 1307, 1283 (OPh), 766 (CꢀCl).
1H NMR (CDCl3, TMS) dH (ppm): 8.11–7.96 (dd, 8 H, ArH), 6.90–6.87 (dd, 8 H,
ArH), 4.04–3.97 (m, 4 H, ꢀOCH2), 1.80–1.25 (m, 56 H, ꢀ[CH2]n), 0.90–0.85 (t,
6 H, ꢀCH3). UV-visible (EtOH): kmax ¼ 341, 277 nm. Elemental analyses:
calculated (for C66H82Cl2O12), C, 69.64; H, 7.26%, found, C, 69.52; H, 7.12%.
3. Results and Discussion
The compound 2,5-bis[4-hydroxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-
=
benzoquinone 3 exhibits IR absorption bands at 3449 (OH), 1768 (C O ester), 1682
ꢀ1
=
=
(C O quinone), 1589 (C O quinone), 1502 (Ph), 1294 (OPh), and 765 (CꢀCl) cm
.
The compound 2,5-bis[4-n-heptyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-
benzoquinone 4a displays IR spectral bands at 2923, 2853 (aliphatic CꢀH), 1770
=
=
(C O ester), 1682 (C O quinone), 1605, 1510 (Ph), 1310, 1256 (OPh), and 764
(CꢀCl) cmꢀ1. The disappearance of v(OH) band and appearance of a band centered
at 1770 cmꢀ1 due to phenyl ester moiety in the compound 4a confirms its structure.
Proton NMR spectrum of the compound 2,5-bis[4-hydroxybenzoyloxy]-3,6-
dichloro-1,4-benzoquinone exhibits signals at d 12.94 (s, 2H, OH) and 7.95 (d, 4H,
ArH), 7.36 (d, 4H, ArH) ppm. The compound 2,5-bis[4-n-heptyloxy benzoyloxy-(4-
benzoyloxy)]-3,6-dichloro-1,4-benzoquinone 4a exhibits NMR signals at d 8.08–8.01
(dd, 8 H, ArH), 6.99–6.90 (dd, 8 H, ArH), 4.06–3.99 (m, 4 H, ꢀOCH2), 1.80–1.27
(m, 20 H, ꢀ[CH2]n), and 0.88–0.86 (t, 6 H, ꢀCH3) ppm. Thus the disappearance of
d OH and the appearance of additional signal of aromatic ring in the compound 4a
confirms the formation of this compound as inferred from IR spectra also. The chain
length has no significant effect on the position of the signals due to ꢀOCH2, ꢀ[CH2]n,
and CH3 protons, respectively. The UV-visible spectrum of compound 2,5-bis[4-n-
heptyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone 4a exhibits UV
bands at 344 and 277 nm which are assigned as n ! pꢁ and=or p ! pꢁ transitions.
4. Optical Properties
The liquid crystalline properties of 2,5-bis[4-n-alkoxybenzoyloxy-(4-benzoyloxy)]-
3,6-dichloro-1,4-benzoquinones 4a–i were investigated by POM using a hot stage.
The phase transition temperatures and enthalpies were measured by carrying out
DSC thermal analysis. The phase transition temperatures along with the enthalpy
values for the compounds are summarized in Table 1. The symbols K, N, and I

Liquid Crystalline Properties
143
Table 1. Thermal transition temperatures, associated enthalpy (DH), and entropy
(DS) changes of 2,5-bis[4-alkoxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-
benzoquinones
Compound
Transitions
T=ꢀC
DH=kJmolÀ1
DS=JKÀ1molÀ1
4a
K1–I
I–N
86.4
80.5
64.1
47.5
64.2
71.4
84.3
104.6
220.0
150.5
109.7
83.3
81.94
23.99
34.32
8.22
0.22
17.04
8.29
0.08
0.06
0.11
0.15
16.04
0.12
16.82
0.10
0.17
0.07
0.25
0.24
5.11
19.99
0.73
18.25
8.26
0.42
0.45
0.41
1.13
4.01
4.19
18.05
15.79
55.98
24.95
0.15
2.67
5.56
0.06
0.16
0.42
1.47
3.57
3.84
227.99
67.86
101.80
25.64
0.65
49.47
23.20
0.21
0.12
0.25
0.39
45.01
0.35
N–K1
K1–K
K1–K2
K2–K3
K3–K4
K4–K5
K5–I
4b
I–N
N–K5
K5–K
K1–K2
K2–K3
K3–K4
K4–K5
K5–I
4c
64.4
79.3
47.74
0.26
0.40
0.15
0.59
104.6
145.4
186.9
150.7
109.7
55.2
39.5
64.8
84.4
90.2
104.9
145.8
187.1
150.5
109.5
102
87.7
63.6
91.3
69.0
64.6
77.4
84.0
192.0
150.5
109.4
98.1
81.1
67.1
I–N
N–K5
K5–K4
K4–K
K1–K2
K2–K3
K3–K4
K4–K5
K5–K6
K6–I
0.62
15.56
63.96
2.16
51.06
22.74
1.11
1.07
0.89
2.66
10.48
11.17
50.04
46.91
153.66
72.95
0.44
4d
I–N
N–K6
K6–K5
K5–K4
K4–K
K–I
4e
4f
I–K
K1–K2
K2–K3
K3–K4
K4–I
7.61
15.57
0.12
0.37
1.09
3.96
I–K4
K4–K3
K3–K2
K2–K1
K1–K
10.08
11.29
(Continued)

144
B. Singh and A. Pandey
Table 1. Continued
Compound
4g
Transitions
K–I
T=ꢀC
DH=kJmolꢁ1
DS=JKꢁ1molꢁ1
67.6
62.6
44.6
73.1
192.2
150.7
109.6
60.8
20.14
17.74
12.38
71.25
0.04
0.11
0.20
0.67
67.44
59.13
52.86
38.97
205.86
0.08
0.25
0.52
2.00
209.31
I–K
4h
K1–K2
K2–K3
K3–I
I–K3
K3–K2
K2–K1
K1–K
49.2
are used to denote crystalline, nematic, and isotropic phases, respectively. The com-
pound 2,5-bis[4-n-heptyloxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-benzoquinone
4a exhibits an endothermic peak at 86.4ꢀC (DH ¼ 81.94 kJmolꢁ1) due to the crystal-
to-isotropic transition. Three exothermic peaks are observed at 80.5ꢀC (DH ¼
23.99 kJmolꢁ1), 64.1ꢀC (DH ¼ 34.32 kJmolꢁ1), and 47.5ꢀC (DH ¼ 8.22 kJmolꢁ1
)
showing transitions from the isotropic-to-mesophase, mesophase-to-crystal, fol-
lowed by another crystal phase. The texture of the mesophase observed under polar-
izing microscope is characterized by the appearance of numerous dense nematic
bubbles (droplets) which possess disclination (defects) invariably within them. These
disclinations seen within the radial structure are hedgehog defects giving rise to
schlieren texture with characteristic black brushes 2–4 in numbers. The disclination
within the nematic droplets observed in cooling cycle are arising as the elastic energy
density of a nematic molecule is sufficiently large, and the orientation of the nematic
director becomes indistinct. These disclinations are high energy structures and the
nematic ordering adopts a configuration which minimizes both the number and
strength of these defects. The changes in the nematic alignment (due to applied field,
changes in temperature, etc.) can force a disclination of one type to transform into
another form but these studies have not been performed in our case. A closer look
reveals that the brushes come together in a singular point and are four-fold. The sin-
gularities are topological defects which are assigned certain strength. The molecule
shows (þve) defects with the rotation of four-fold brushes in the same direction.
The defect-free sample area is also seen indicating that the defects are of same
strengths and can annihilate.
The compound 4b exhꢁib1its endothermic peaks at 64.ꢁ21ꢀC (DH ¼ 0.22 kJmolꢁ1),
71.4ꢀC (DH ¼ 17.04 kJmol ), 84.3ꢀC (DH ¼ 8.29 kJmol ), and 104.6ꢀC (DH ¼
0.08 kJmolꢁ1), which define the subsequent crystal-to-crystal phase transition (poly-
morphism), and the highest temperature peak 220ꢀC is due to the crystal-to-isotropic
liquid phase transition. The exothermic peaks of this compound are observed at
150.5ꢀC (DH ¼ 0.11 kJmolꢁ1), 109.7ꢀC (DH ¼ 0.15 kJmolꢁ1), and 83.3ꢀC (DH ¼
16.04 kJmolꢁ1), which define isotropic-to-mesophase and mesophase-to-subsequent
crystal phase transitions, respectively. Thus, this compound shows significant poly-
morphic changes in the heating cycle only. The texture of the mesophase observed
under polarizing microscope is characteristic of monotropic radial nematic bubbles
(droplets) appearing in cooling cycle with disclination within the droplets showing
schlieren texture. The defect inside the radial structure is of hedgehog type. The

Liquid Crystalline Properties
145
radial curvature in liquid crystalline system costs energy. The elastic free energy of
the nematic mesophase scales with the curvature per unit length. At a high enough
curvature, the elastic free energy density of the nematic phase can exceed free energy
for melting the nematic into the isotropic phase. In these regions, the nematic direc-
tor becomes indeterminant and a defect or disclination is formed. The similar phase
transition temperature and textural pattern is observed for compounds 4c and 4d,
except that the latter shows some more polymorphic changes in the heating cycle.
The compound 4f shows endothermiꢁc1 peaks and enthalpy at 64.6ꢀC (DH ¼
0.15 kJmolꢁ1), 77.4ꢀC (DH ¼ 2.67 kJmol ), and 84.0ꢀC (DH ¼ 5.56 kJmolꢁ1), show-
ing subsequent crystal-to-crystal phase transitions (polymorphism), and one at
192.0ꢀC exhibits a transition from crystal to isotropic liquid phasꢁe.1 The exothermic
peaks at 150.5ꢀC (DH ¼ 0.16 kJmolꢁ1), 109.4ꢀC (DH ¼ 0.42 kJmol ), 98.1ꢀC (DH ¼
1.47 kJmolꢁ1), 81.1ꢀC (DH ¼ 3.57 kJmolꢁ1), and 67.1ꢀC (DH ¼ 3.84 kJmolꢁ1) depict
Scheme 1. Synthetic route for 2,5-bis[4-n-alkoxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-
1,4-benzoquinones.

146
B. Singh and A. Pandey
the subsequent crystal to crystal phase transitions. The investigation of the texture
under optical microscope reveals no mesomorphic property for this compound both
in heating and cooling cycles. Thus, this compound exhibited a rich polymorphism in
both the cycles. The similar phase transitions and non-mesomorphic behaviour is
observed for compound 4h. The compounds 4e and 4g exhibit sharp endothermic
and exothermic peaks corresponding to a sudden phase transition from crystal to
isotropic phases at 91.3ꢀC and 67.6ꢀC ꢁa1nd subsequent isotropic to crystal phase
transition at 69.0ꢀC (DH ¼ 24.95 kJmol ) and 62.6ꢀC (DH ¼ 17.74 kJmolꢁ1), res-
pectively. The optical analysis revealed no texture for these compounds, and there-
fore, the compounds, were found to be non-mesogenic in nature.
The structure–mesophase correlation of these compounds reveals that the
homologous members of this series are calamitic in nature. The lower members with
m ¼ 7, 8, 9, 10 show monotropic nematic phase, and the higher homologs with
m ¼ 11, 12, 14, and 16 do not display mesomorphic behavior. The presence of ester
bridging group increases flexibility and mobility of particular part of liquid crystal
molecule stabilizing the nematic mesophase [14], but as the alkoxy chain length
increases, the non-polar ends dominate over the polarity, and molecules are pre-
vented to slide over each other, thereby increasing the intermolecular forces and
resulting in the loss of mesogenic property.
Conclusions
A new series of 2,5-bis[4-n-alkoxybenzoyloxy-(4-benzoyloxy)]-3,6-dichloro-1,4-
benzoquinones have been synthesized having terminal alkoxy chains (m ¼ 7ꢃ16).
The chemical structures of the products were investigated by IR, NMR, UV-visible,
1H, and 13C NMR spectra. The mesomorphic properties and optical textures of the
products were characterized by DSC and POM. The existence of nematic (schlieren
texture) was confirmed by POM. It was found that the mesogenic nature of these
compounds is dependent on the alkoxy chain length. The lower homologues with
(m ¼ 7, 8, 9, 10) exhibit monotropic nematic mesophase, whereas the higher homo-
logues were non-mesogenic in nature.
Acknowledgment
The authors thank the Head, Department of Chemistry, BHU, Varanasi, for provid-
ing laboratory facilities. One of the authors (A.P.) thanks UGC for financial support
as Project Fellow and BHU for research scholarship. Thanks are also due to
Mr. Satish Tiwari for CHN and DSC analyses, Mr. R. C. P. Bipin for NMR, and
Mr. V. N. Pandey for IR and electronic spectral data.
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