Liquid-crystalline properties of 2,5-bis(4-n-alkoxybenzoyloxy)-3,6- dichloro-1,4-benzoquinones and their copper (II) complexes-synthesis and characterization

Article abstract of DOI:10.1080/15421400903568104

A new series of 2,5-bis(4-n-alkoxybenzoyloxy)-3,6-dichloro-1,4- benzoquinones, C₆Cl₂O₂(OOCC₆H ₄OR)₂ and their copper (II) compounds, C ₆Cl₂O₂(OOCC₆

Full text of DOI:10.1080/15421400903568104

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Liquid-Crystalline Properties of 2,5-
Bis(4-n-Alkoxybenzoyloxy)-3,6-
Dichloro-1,4-Benzoquinones and Their
Copper (II) Complexes—Synthesis and
Characterization
Bachcha Singh a & Ashwini Pandey a
a Department of Chemistry, Faculty of Science , Banaras Hindu
University , Varanasi, India
Published online: 06 Jul 2010.
To cite this article: Bachcha Singh & Ashwini Pandey (2010) Liquid-Crystalline Properties
of 2,5-Bis(4-n-Alkoxybenzoyloxy)-3,6-Dichloro-1,4-Benzoquinones and Their Copper (II)
Complexes—Synthesis and Characterization, Molecular Crystals and Liquid Crystals, 524:1, 3-16, DOI:
10.1080/15421400903568104
To link to this article: http://dx.doi.org/10.1080/15421400903568104
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Mol. Cryst. Liq. Cryst., Vol. 524: pp. 3–16, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903568104
Liquid-Crystalline Properties of
2,5-Bis(4-n-Alkoxybenzoyloxy)-3,6-
Dichloro-1,4-Benzoquinones and Their Copper (II)
Complexes—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)-3,6-dichloro-1,4-benzoquinones,
C6Cl2O2(OOCC6H4OR)2 and their copper (II) compounds, C6Cl2O2(OOC-
=
C6H4OR)2 Cu(CH3COO)2 ꢀ H2O where R CmH2mþ1 and m ¼ 6,7,8,9,10,
11,12,14, and 16 have been synthesized. The compounds have been characterized
by elemental analyses, fourier transform infrared (FTIR) far IR, 1H, and 13C
NMR spectra. The UV-Visible, electron spin resonance (ESR), and magnetic
susceptibility studies of the copper compounds have also been made. Mesomorphic
properties of these compounds were studied by differential scanning calorimetry
and polarizing microscopy. The mesomorphic nature of these compounds depends
on the alkoxy chain length. The compounds (m ¼ 6, 7, 8, 14) exhibit the nematic
mesophase. Compound (m ¼ 9)shows a nematic mesophase in heating cycle and
both a nematic and a smectic A mesophase in the cooling cycle, whereas compound
(m ¼ 10) shows enantiotropic nematic and smectic A mesophases. Compound
(m ¼ 11) shows a monotropic smectic A mesophase and compound (m ¼ 12) shows
an enantiotropic smectic A mesophase, whereas m ¼ 16 nonmesomorphic. The
copper (II) complexes of these compounds were found to be nonmesogenic.
Keywords Chloranil based mesogens; copper (II) compounds; monotropic
mesogens; nematic mesophase; smectic mesophase
Introduction
Liquid-crystalline materials are excellent examples of designed supermolecular
materials and the cooperative physical properties of these materials have led to
numerous technological applications [1]. Liquid crystals are useful materials for
applications in passive block filters, laser-addressed devices, polarizers based on
dichroic effects or thermochromism [2], thermometers (fever indicators, gadgets,
design application, etc.), or warning signals (on heaters) and organic light-emitting
diodes (OLEDS) [3]. Further, active matrix liquid crystals (AMLC) have been
applied to various kinds of displays such as PC monitors, cellular phones and tele-
visions [4,5]. Deformed helix liquid-crystal systems are also potential candidates to
Address correspondence to Bachcha Singh, Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi 221005, India. E-mail: bsinghbhu@rediffmail.com
3

4
B. Singh and A. Pandey
be used as reliable new generation memory devices [6], reflective-type liquid-crystal
display (LCD) spatial light modulators, film reflectors, polarizers, field effect transis-
tors, and sensors [7,8].
The organic liquid-crystalline species whose rigid core is benzoquinone are
interesting systems from the point of view of electron transfer reactions, which play
an important role in biological systems [9–12]. They may have interesting properties
as possible agents for oxidations in the liquid-crystalline state, liquid-crystalline-
charge transfer complexes, and liquid-crystalline conductors [13,14].
Chandrasekhar et al. [15] and Chadrasekhar [16] were the first to report unstable
discotic mesomorph 2,3,5,6-tetrakis(octanoyloxy)-1,4-benzoquinone. Lillya and
Thakur [17] reinvestigated and concluded that both the tetrakis heptanoyloxy- and
octanoyloxy-1,4-benzoquinone A exhibit a columnar mesophase with a narrow tem-
perature range. Matsutani et al. [18] synthesized three series of 2,3,5,6-tetrakis(4-
alkoxybenzoyloxy)-1,4-benzoquinones B, 2,3,5,6-tetraalkoxy-1,4-benzoquinones C,
2,3,5,6-tetrakis(alkanoyl amino)-1,4-benzoquinones
D
and examined their
mesomorphic properties. The benzoates B exhibits a simple crystal-isotropic liquid
transition, suggesting that the aromatic ring of benzoate B was not suitable for
improving coplanarity and rigidity of the 1,4-benzoquinone central core. They also
obtained ethers C that showed two endothermic peaks at 32ꢁC and 55ꢁC, and a
vague texture was reported in the above temperature range. The strengthening of
the intermolecular interaction was achieved by substitution of an ester group of A
by an amide group D, which displayed a dendritic texture after being cooled from
the isotropic liquid. The mesophase was concluded as hexagonal columnar on the
basis of reported optical texture.
Berg et al. (13) reported the 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 exhibited an additional smectic phase (Sc or Sa).
A systematic study on the synthesis, characterization, and mesomorphic proper-
ties of hitherto uninvestigated compounds derived from 2,3,5,6-tetrachloro-1,4-
benzoquinone (chloranil) and their copper (II) complexes have been undertaken
and results are reported here.
Experimental
The Chemicals 4-Hydroxybenzoic Acid (Aldrich, Germany), Chloranil (Himedia,
India), 1-Bromoalkanes (Aldrich, Germany) and Copper (II) Acetate Monohydrate
(Aldrich, Germany), were used as supplied.
Techniques
Elemental analyses were performed on CE-440 Exeter Analytical CHN analyzer
(USA). IR spectra (4000–100 cmꢂ1) were recorded on a Varian 3100 FT-IR Excali-
bur spectrophotometer (USA). 1H and 13C nuclear magnetic resonance (NMR) spec-
tra were obtained by using a JEOL FT NMR AL 300 MHz spectrometer (Japan)
using tetramethylsilane as internal standard. Electronic spectra were recorded on a
UV-1700 Pharma spec Shimadzu UV-Visible spectrophotometer (Japan). Room
temperature magnetic susceptibility measurements were performed on a Cahn-
Faraday balance (USA) using Hg[Co(NSC)4] as standard. The magnetic susceptibility

Liquid-Crystalline Benzoquinones and Their Copper (II) Complexes
5
was corrected for diamagnetism using Pascal’s constants. Differential scanning
calorimetry (DSC) thermograms were recorded by a Mettler Toledo TC 15 TA dif-
ferential scanning calorimeter (Switzerland) at the rate of 10.0 K minꢂ1 under nitro-
gen atmosphere using spec-pure-grade indium as standard by taking samples in
closed lid aluminum pans. The transition temperatures from DSC have been determ-
ined with accuracy of ꢃ0.1 K. The mesophase type was assigned by visual compari-
son with known phase standards using an HT 30.01 NTT 268 LOMO polarizing
optical microscope (USA) fitted with a hot stage with temperature controlling accu-
racy of 0.1 K. Details of experimental techniques have been described elsewhere
[19,20]. The copper content in the complexes was determined using an atomic
absorption spectrophotometer (ASH Corp., USA).
Synthesis of Compounds
Potassium 4-n-hexyloxybenzoate 2 was prepared by alkylating 4-hydroxybenzoic
acid 1 with 1-bromohexane in ethanol in the presence of KOH as described elsewhere
[21]. All other homologous members of the series were prepared following the same
procedure.
The compound 2,5-bis(4-n-hexyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone
3 was prepared by reaction of potassium 4-n-hexyloxybenzoate 2 with chloranil in
ethanol. The reaction mixture was stirred at 25ꢁC for 8 h and then at 40ꢁC for
30 min. The solution became dark brown and was filtered. The filtrate was evapo-
rated under reduced pressure, resulting in an orange-colored residue. It was washed
with distilled water, dried and recrystallized from chloroform. All other homologues
of the series were prepared in a similar manner. The yield, NMR, IR data, and
elemental analyses for the compounds are summarized as follows.
2,5-Bis(4-n-hexyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C32H34Cl2O8 (3a).
Yield :72%. IR (KBr, cmꢂ1): 2936, 2864 (aliphatic CꢂH), 1680 (C O, quinone),
=
1
=
1601 (Ph), 1578 (C C, quinone), 1296 (OPh), and 773 (CꢂCl). H NMR (CDCl3,
TMS) dH (ppm): 8.06–8.03 (d, 4H, ArH), 6.94–6.91 (d, 4H, ArH), 4.04–4.00 (t, 4H,
ꢂOCH2), 1.85–1.34 (m, 16H, ꢂ[CH2]n), 0.91 (t, 6H, ꢂCH3). 13C NMR dC
(CDCl3): 171.9, 163.6, 132.3, 121.3, 114.1, 77.4, 77.0, 76.5 (CDCl3), 70.8, 68.2,
31.5, 29.0, 25.6, 22.5, 15.7, 13.9 (CH3). UV-Visible (EtOH): kmax ¼ 417, 256 nm.
Elemental analyses: calculated for C32H34Cl2O8(%), C, 62.24; H, 5.54; found, C,
62.21; H, 5.42.
2,5-Bis(4-n-heptyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C34H38Cl2O8 (3b).
Yield: 70%. IR (KBr, cmꢂ1): 2929, 2850 (aliphatic CꢂH), 1678 (C O, quinone), 1604
=
1
=
(Ph), 1576 (C C, quinone), 1302 (OPh) and 773 (CꢂCl). H NMR (CDCl3, TMS) dH
(ppm): 8.05–8.03 (d, 4H, ArH), 6.94–6.91 (d, 4H, ArH), 4.04–4.00 (t, 4H, ꢂOCH2),
1.80–1.25 (m, 20H, ꢂ[CH2]n), 0.89 (t, 6H, ꢂCH3). 13C NMR dC (CDCl3): 172.1,
163.6, 132.3, 121.3, 114.1, 77.4, 77.0, 76.5 (CDCl3), 70.7, 68.2, 31.7, 29.0, 28.9, 25.9,
22.5, 15.7, 14.0 (CH3). UV-Visible (EtOH): kmax ¼ 413, 293, 255 nm. Elemental
analyses: calculated for C34H38Cl2O8 (%), C, 63.25, H, 5.93; found, C, 63.18; H, 5.38.
2,5-Bis(4-n-octyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C36H42Cl2O8 (3c).
Yield: 72%. IR (KBr, cmꢂ1) : 2928, 2852 (aliphatic CꢂH), 1685 (C O, quinone),
=
1
=
1608 (Ph), 1577 (C C, quinone), 1296 (OPh), and 771 (CꢂCl). H NMR (CDCl3,
TMS) dH (ppm): 8.05–8.02 (d, 4H, ArH), 6.93–6.91 (d, 4H, ArH), 4.04–4.00

6
B. Singh and A. Pandey
(t, 4H, ꢂOCH2), 1.85–1.29 (m, 24H, ꢂ[CH2]n), 0.91 (t, 6H, ꢂCH3). 13C NMR dC
(CDCl3): 163.6, 132.3, 114.1, 77.4, 77.0, 76.5 (CDCl3), 68.2, 31.7, 29.3, 29.2, 29.0,
25.9, 22.6, 14.0 (CH3). UV-visible (EtOH): kmax ¼ 415, 296, 260 nm. Elemental
analyses: calculated for C36H42Cl2O8 (%), C, 64.18; H, 6.28; found, C, 64.42; H, 6.60.
2,5-Bis(4-n-nonyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C38H46Cl2O8 (3d).
Yield: 68%. IR (KBr, cmꢂ1): 2919, 2850 (aliphatic CꢂH), 1682 (C O, quinone),
=
1
=
1606 (Ph), 1578 (C C, quinone), 1307 (OPh), and 771 (CꢂCl). H NMR (CDCl3,
TMS) dH (ppm): 8.06–8.03 (d, 4H, ArH), 6.94–6.91 (d, 4H, ArH), 4.04–4.00 (t,
4H, ꢂOCH2), 1.83–1.28 (m, 28H,ꢂ[CH2]n), 0.90 (t, 6H, ꢂCH3). 13C NMR dC
(CDCl3): 170.9, 162.6, 151.9, 137.6, 131.6, 131.3, 120.3, 113.1, 76.4, 76.2, 76.1,
76.0 (CDCl3), 70.3, 70.1, 69.7, 30.8, 28.4, 28.0, 24.9, 21.6, 14.9 (CH3). UV-Visible
(EtOH): kmax ¼ 411, 301, 253 nm. Elemental analyses: calculated for C38H46Cl2O8
(%), C, 65.04; H, 6.60; found, C, 65.32; H, 6.54.
2,5-Bis(4-n-decyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C40H50Cl2O8 (3e).
Yield: 65%. IR (KBr, cmꢂ1): 2926, 2854 (aliphatic CꢂH), 1680 (C O, quinone),
=
1
=
1608 (Ph), 1578 (C C, quinone), 1294 (OPh), and 773 (CꢂCl). H NMR (CDCl3,
TMS) dH (ppm): 8.06–8.03 (d, 4H, ArH), 6.94–6.91 (d, 4H, ArH), 4.04–4.00 (t,
4H, ꢂOCH2), 1.82–1.27 (m, 32H, ꢂ[CH2]n), 0.90 (t, 6H, ꢂCH3). 13C NMR dC
(CDCl3) : 175.2, 171.7, 163.6, 152.9, 132.3, 131.4, 121.3, 114.1, 114.0, 77.4, 77.0,
76.5 (CDCl3), 71.1, 70.7, 68.2, 64.8, 57.4, 31.8, 29.5, 29.3, 29.2, 29.0, 28.7, 26.0,
25.9, 22.6, 15.8, 14.0 (CH3). UV-Visible (EtOH): kmax ¼ 413, 310, 274, 243 nm.
Elemental analyses: calculated for C40H50Cl2O8 (%), C, 65.83; H, 6.90; found, C,
65.28; H, 7.34.
2,5-Bis(4-n-undecyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C42H54Cl2O8 (3f).
Yield: 69%. IR (KBr, cmꢂ1): 2923, 2855 (aliphatic CꢂH), 1680 (C O, quinone), 1606
=
1
=
(Ph), 1576 (C C, quinone), 1301 (OPh), and 776 (CꢂCl). H NMR (CDCl3, TMS) dH
(ppm): 8.04–8.01 (d, 4H, ArH), 6.93–6.90 (d, 4H, ArH), 4.04–4.00 (t, 4H, ꢂOCH2),
1.80–1.25 (m, 36H, ꢂ[CH2]n), 0.88 (t, 6H, ꢂCH3). 13C NMR dC (CDCl3) : 171.7,
163.6, 154.0, 138.6, 132.2, 131.7, 125.7, 121.3, 114.1, 77.4, 77.0, 76.5 (CDCl3), 71.3,
71.3, 71.1, 70.7, 68.2, 31.8, 29.5, 29.2, 25.9, 22.6, 15.8, 15.7, 14.0 (CH3). UV-Visible
(EtOH): kmax ¼ 418, 309, 290, 254, 218 nm. Elemental analyses: calculated for
C42H54Cl2O8 (%), C, 66.57; H, 7.18; found, C, 66.01; H, 6.70.
2,5-Bis(4-n-dodecyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C44H58Cl2O8 (3g).
Yield: 64%. IR (KBr, cmꢂ1): 2918, 2850 (aliphatic CꢂH), 1682 (C O, quinone),
=
1
=
1604 (Ph), 1578 (C C, quinone), 1298 (OPh), and 796 (CꢂCl). H NMR (CDCl3,
TMS) dH (ppm): 8.04–8.01 (d, 4H, ArH), 6.94–6.91 (d, 4H, ArH), 4.04–4.00 (t,
4H, ꢂOCH2), 1.82–1.26 (m, 40H,ꢂ[CH2]n), 0.88 (t, 6H, ꢂCH3). 13C NMR dC
(CDCl3): 171.8, 163.6, 132.3, 131.4, 121.3, 114.1, 114.0, 77.4, 77.0, 76.5 (CDCl3),
71.3, 68.2, 68.1, 31.9, 29.6, 29.5, 25.9, 22.6, 15.8, 14.0 (CH3). UV-Visible (EtOH):
kmax ¼ 415, 298, 277 nm. Elemental analyses: calculated for C44H58Cl2O8 (%), C,
67.25; H, 7.43; found, C, 67.14; H, 7.84.
2,5-Bis(4-n-tetradecyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C48H66Cl2O8 (3h).
Yield: 67%. IR (KBr, cmꢂ1): 2920, 2851 (aliphatic CꢂH), 1677 (C O, quinone),
=
1
=
1608 (Ph), 1577 (C C, quinone), 1299 (OPh), and 800 (CꢂCl). H NMR (CDCl3,
TMS) dH (ppm) : 8.04–8.02 (d, 4H, ArH), 6.94–6.91 (d, 4H, ArH), 4.04–4.00 (t,
4H, ꢂOCH2), 1.82–1.26 (m, 48H, ꢂ[CH2]n), 0.90 (t, 6H, ꢂCH3). 13C NMR dC

Liquid-Crystalline Benzoquinones and Their Copper (II) Complexes
7
(CDCl3): 171.1, 163.6, 132.3, 121.2, 114.1, 77.4, 77.0, 76.5 (CDCl3), 71.4, 71.1, 70.7,
68.2, 31.9, 29.6, 29.5, 25.9, 22.6, 15.9, 15.8, 14.1 (CH3). UV-Visible (EtOH):
kmax ¼ 417, 310, 257 nm. Elemental analyses: calculated for C48H66Cl2O8 (%), C,
68.47; H, 7.90; found, C, 68.82; H, 7.54.
2,5-Bis(4-n-hexadecyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, C52H74Cl2O8 (3i).
Yield: 70%. IR (KBr, cmꢂ1): 2922, 2852 (aliphatic CꢂH), 1677 (C O, quinone),
=
1
=
1606 (Ph), 1577 (C C, quinone), 1296 (OPh), and 801 (CꢂCl). H NMR (CDCl3,
TMS) dH (ppm): 8.04–8.01 (d, 4H, ArH), 6.94–6.91 (d, 4H, ArH), 4.04–4.00 (t,
4H, ꢂOCH2), 1.82–1.25 (m, 56H,ꢂ[CH2]n), 0.90 (t, 6H, ꢂCH3). 13C NMR dC
(CDCl3): 171.0, 163.6, 132.2, 114.1, 77.4, 77.2, 77.0, 76.5 (CDCl3), 71.3, 71.1, 70.7,
68.2, 31.9, 29.8, 29.6, 25.9, 22.6, 15.9, 15.8, 14.1 (CH3). UV-Visible (EtOH):
kmax ¼ 416, 293, 254, 217 nm. Elemental analyses: calculated for C52H74Cl2O8 (%),
C, 69.54; H, 8.30; Found, C, 69.92; H, 8.15.
Synthesis of Copper (II) Complex of 2,5-Bis(4-n-Hexyloxybenzoyloxy)-3,6-
Dichloro-1-4-Benzoquinone (4)
A
solution of 2,5-bis(4-n-hexyloxy benzoyloxy)-3,6-dichloro-1,4-benzoquinone
(1.23 g, 2 mmol) in methanol (20 mL) and copper (II) acetate (0.39 g, 2 mmol) in
methanol (20 mL) was mixed. The mixture was stirred at room temperature (25ꢁC)
for 1 h. The resulting light blue precipitate was filtered off and washed with methanol
and dried under vacuum.
The copper (II) complexes of the remaining members of the homologous series
were prepared adopting a similar method. These compounds were characterized as
follows.
2,5-Bis(4-n-hexyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone monoaquacopper(II)
acetate, C36H42Cl2O13Cu (4a). Yield: 82%. IR (KBr, cmꢂ1): 3561 (OH), 2933, 2863
=
(aliphatic CꢂH), 1679 (C O, quinone), 1605 (Ph), 1549 na(COO, acetate), 1409 ns
(COO acetate), 1304 (CꢂO ester) 1249 (OPh), 773 (CꢂCl), 465 (qrCOO or =qCH).
Far IR (nujol): 551, 520, 473, 394, 329, 277, 205, 173, 149, 99 cmꢂ1. UV-Visible
(nujol): kmax ¼ 703, 374, 318, 256, 224 nm. leff: 1.73 BM. Elemental analyses:
calculated for C36H42Cl2O13Cu (%), C, 52.91; H, 5.18; Cu, 7.77; found, C, 52.72;
H, 5.21; Cu, 7.62.
2,5-Bis(4-n-heptyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone monoaquacopper(II)
acetate, C38H46Cl2O13Cu (4b). Yield: 75%. IR (KBr, cmꢂ1): 3427 (OH), 2928,
=
2859 (aliphatic CꢂH), 1679 (C O, quinone), 1606 (Ph), 1554 na(COO acetate),
1405 ns (COO acetate), 1306 (CꢂO ester), 1253 (OPh), 783 (CꢂCl), 465 (qrCOO
or =qCH). Far IR (nujol): 568, 476, 309, 141 cmꢂ1. UV-Visible (nujol): kmax ¼ 690,
334, 309, 217 nm. leff: 1.80 BM. Elemental analyses: calculated for
C38H46Cl2O13Cu (%), C, 53.99; H, 5.48; Cu, 7.51; found, C, 53.82; H, 5.42; Cu, 7.20.
2,5-Bis(4-n-octyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone monoaquacopper(II)
acetate, C40H50Cl2O13Cu (4c). Yield: 72%. IR (KBr, cmꢂ1): 3568 (OH), 2924,
=
2854, (aliphatic CꢂH), 1678 (C O, quinone), 1593 (Ph), 1545 na (COO acetate),
1431 ns(CꢂO acetate), 1299 (CꢂO ester), 1255 (OPh), 771 (CꢂCl), 464 (qrCOO
or =qCH). Far IR (nujol): 553, 503, 477, 390, 357, 324, 282, 231, 204, 156,
101 cmꢂ1. UV-Visible (nujol): kmax ¼ 687, 422, 267 nm. leff: 1.83 BM. Elemental

8
B. Singh and A. Pandey
analyses: calculated for C40H50Cl2O13Cu (%), C, 55.01; H, 5.77; Cu, 7.27; found, C,
55.12; H, 5.72; Cu, 7.12.
2,5-Bis(4-n-nonyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone monoaquacopper(II)
acetate, C42H54Cl2O13Cu (4d). Yield: 81%. IR (KBr, cmꢂ1): 3568 (OH), 2925,
=
2858, (aliphatic CꢂH), 1678 (C O, quinone), 1601 (Ph), 1546 na (COO acetate),
1427 ns (COO acetate), 1302 (CꢂO ester), 1254 (OPh), 776 (CꢂCl), 466 (qrCOO
or =qCH). Far IR (nujol): 552, 501, 473, 393, 360, 332, 294, 257, 205, 170,
110 cmꢂ1. UV-Visible (nujol): kmax ¼ 666, 476, 457, 244 nm. leff: 1.85 BM.
Elemental analyses: calculated for C42H54Cl2O13Cu (%), C, 55.96; H, 6.03; Cu
7.05; found C, 55.58; H, 6.21; Cu 7.50.
2,5-Bis(4-n-decyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone monoaquacopper(II)
acetate, C44H58Cl2O13Cu (4e). Yield: 65%. IR (KBr, cmꢂ1): 3565 (OH), 2930, 2852
=
(aliphatic CꢂH), 1678 (C O, quinone), 1605 (Ph), 1547 na (COO acetate), 1430 ns
(COO acetate), 1301 (CꢂO ester), 1252 (OPh), 773 (CꢂCl), 468 (qrCOO or =qCH).
Far IR (nujol): 468, 295, 188 cmꢂ1. UV-Visible (nujol): kmax ¼ 668, 301, 281 nm.
leff: 1.87 BM. Elemental analyses: calculated for C44H58Cl2O13Cu (%), C, 56.86;
H, 6.29; Cu, 6.83; found, C, 56.67; H, 6.52; Cu, 6.54.
2,5-Bis(4-n-undecyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone monoaquacopper(II)
acetate, C46H62Cl2O13Cu (4f). Yield: 70%. IR (KBr, cmꢂ1): 3568 (OH), 2923, 2855
=
(aliphatic CꢂH), 1679 (C O, quinone), 1608 (Ph), 1547 na (COO acetate), 1433 ns
(COO acetate), 1304 (CꢂO ester), 1254 (OPh), 798 (CꢂCl), 467 (qrCOO or =qCH).
Far IR (nujol): 555, 517, 474, 396, 327, 276, 205, 154 cmꢂ1. UV-Visible (nujol) :
kmax ¼ 670, 424, 300 nm. leff: 1.92 BM. Elemental analyses: calculated for
C46H62Cl2O13Cu (%), C, 57.70; H, 6.52; Cu, 6.63; found, C, 57.36; H, 6.25; Cu, 6.30.
2,5-Bis(4-n-dodecyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone monoaquacopper(II)
acetate, C48H64Cl2O12Cu (4g). Yield: 72%. IR (KBr, cmꢂ1): 3568 (OH), 2923, 2854
=
(aliphatic CꢂH), 1677 (C O, quinone), 1602 (Ph), 1548 na (COO acetate), 1435 ns
(COO acetate), 1301 (CꢂO ester), 1256 (OPh), 773 (CꢂCl), 468 (qrCOO or =qCH).
Far IR (nujol): 475, 386, 352, 323, 261, 205, 155 cmꢂ1. UV-Visible (nujol):
kmax ¼ 675, 423, 380, 263, 220 nm. leff: 2.01 BM. Elemental analyses: calculated
for C48H66Cl2O13Cu (%), C, 58.50; H, 6.75; Cu, 6.44; found, C, 58.27; H, 6.74;
Cu, 6.18.
2,5-Bis(4-n-hexadecyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone
monoaqua-
copper(II) acetate, C56H82Cl2O13Cu (4i). Yield: 82%. IR (KBr, cmꢂ1): 3572 (OH),
=
2921, 2852, (aliphatic CꢂH), 1679 (C O, quinone), 1601 (Ph), 1543 na (COO
acetate), 1437 ns (COO acetate), 1300 (CꢂO ester), 1255 (OPh), 771 (CꢂCl), 470
(qrCOO or =qCH). Far IR (nujol): 477, 393, 355, 269, 227, 164, 94 cmꢂ1. UV-Visible
(nujol): kmax ¼ 691, 413, 293 nm. leff: 2.11 BM. Elemental analyses: calculated for
C56H82Cl2O13Cu (%), C, 61.27; H, 7.52; Cu, 5.78; found, C, 61.56; H, 7.58; Cu, 5.46.
Results and Discussion
The compounds 2,5-bis(4-n-alkoxybenzoyloxy)-3,6-dichloro-1,4-benzoquinones 3a-i
and their copper (II) complexes 4a-i have been prepared as shown in Scheme 1. The
elemental data are in agreement with the compounds formed. The IR and NMR
spectral data are fully consistent with the structure. In the IR spectrum of 3a, the

Liquid-Crystalline Benzoquinones and Their Copper (II) Complexes
9
Scheme 1. Synthetic route for the compounds and their corresponding copper (II) complexes.
bands appearing at 2936, 2864, 1680, 1601 1578, 1296, and 773 cmꢂ1 are attributed
=
=
to n(aliphatic CꢂH), n(C O) quinone, ring, n(C C) quinone n(OPh), and n(CꢂCl)
modes, respectively. Its 1H NMR spectrum displays peaks at d 8.06–8.03, 6.94–6.91,
4.02–4.00, 1.85–1.34, and 0.91, which are attributed to rings (first two peaks),
ꢂOCH2, ꢂ[CH2]n and –CH3 protons, respectively. In the IR spectrum of the copper
complex of 2,5-bis(4-n-hexyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone 4a split-
ting of (quinone) band occurs. One band is observed at lower frequency and an other
one of low intensity is observed at almost similar frequency (1679 cmꢂ1). This sug-
gests that only one quinone oxygen is bonded to the metal ion. The carboxylate
ion may be bonded to a metal ion in the following three modes.

10
B. Singh and A. Pandey
The magnitude of difference Dn(COO) between the na(COO) and ns(COO) fre-
quency is used for distinction among the above three modes of bonding [22]. The
asymmetric and symmetric stretching bands of carboxylate appear at 1549 and
1409 cmꢂ1. The value of Dn(COO) lying in the range 106–149 cmꢂ1 for the chelating
bidentate carboxylate suggests bonding of acetate in bidentate chelating manner.
The far-IR spectrum of copper acetate monohydrate exhibits bands at 525, 480,
374, 330, 277, 233, 178, and 127 cmꢂ1. The compound 2,5-bis (4-n-hexyloxy
benzoyloxy)-3,6-dichloro-1,4-benzoquinone 3a shows bands at 322, 272, 210, and
129 cmꢂ1, respectively. The comparison of the far-IR spectra of the ligand and
complex reveals the appearance of a new bands in the copper complex of 2,5-bis(4-n-
hexyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone 4a at 551, 473, and 394 cmꢂ1
,
which are attributed to (Cu-O) of quinone, (Cu-O) of acetate moiety, and (Cu-O)
of H2O moiety, respectively, giving direct evidence of bonding between copper (II)
and quinone. The electronic spectrum of chloranil exhibits bands at 408, 387, 337,
288, and 244 nm are due to nꢂpꢄ and pꢂpꢄ transitions. The spectrum of 2,5-bis(4-n-
hexyloxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone 3a shows bands at 417 and
256 nm, which are assigned to nꢂpꢄ and pꢂpꢄ transitions, respectively. The copper
complex 4a reveals bands at 703, 374, 318, 256, and 224 nm. A new broad band at
703 nm in the spectrum of the copper (II) complex may be assigned to an envelope
of the dzx, dxz ! dx2 ꢂ y2, dxy ! dx2 ꢂ y2, and dz2 ! dx2 ꢂ y2 transitions resulting
from Jahn-Teller distortion. The remaining bands are assigned to p–pꢄ and n–pꢄ
transitions of the ligand moiety. All the copper compounds exhibit magnetic
moments in the range 1.73–2.11 BM, indicationg the presence of one unpaired
electron, thereby suggesting that copper (II) acetate is monomeric.
The IR, NMR and electronic spectral data of the remaining ligands and their
copper (II) complexes are explained and similar conclusions are drawn regarding
the bonding of the ligands to the copper (II) and geometry of the resulting
complexes. The copper (II) complexes were found to be insoluble in many solvents;
therefore, crystals could not be developed and hence a single crystal study could not
be pursued. The ESR spectrum of copper (II) complex of 2,5-bis(4-n-hexyloxyben-
zoyloxy)-3,6-dichloro-1,4-benzoquinone 4a is not resolved. It gives lines at 2940 G
in parallel and at 3100 G in the perpendicular regions. These signals yield g
(2.207) and g (2.093). The higher value of g indicates distortion from octahedral
k
?
k
geometry around Cu(II) and the presence of unpaired electron in dx2 ꢂ y2 orbital
of the metal ion [23,24].
Optical Properties
The liquid-crystalline properties of 2,5-bis(4n-alkoxybenzoyloxy)-3,6-dichloro-1,4-
benzoquinones 3a-i and their copper (II) complexes 4 were investigated by polarizing
optical microscope (POM) using a hot stage. The phase transition temperatures and
enthalpies were measured by carrying out DSC thermal analysis. The phase tran-
sition temperatures and the corresponding enthalpy values for the compounds 3a-i
and their copper (II) complexes 4a-i are summarized in Tables 1 and 2, respectively.
The symbols K, SmA, N, and I are used to denote crystalline, smectic A, nematic,
and isotropic phases, respectively. Compound 3b exhibits endothermic peaks in
heating cycle at 71.1ꢁC, 87.3ꢁC, and 104.9ꢁC with enthalpies 5.14, 42.80, and
0.40 kJmolꢂ1, which define crystal (K1) to crystal (K2) phase, crystal (K2) to
mesophase, and mesophase to isotropic phase transitions, respectively. The

Liquid-Crystalline Benzoquinones and Their Copper (II) Complexes
11
Table 1. Phase transition temperatures, enthalpies (DH), and entropies (DS) of
2,5-bis(4-n-alkoxybenzoyloxy)-3,6-dichloro-1,4-benzoquinones
Compound
3a
Transitions
T (ꢁC)
DH=kJ molꢂ1
DS=JKꢂ1 molꢂ1
K1–K2
K2–K3
K3–I
65.0
72.2
100.8
142.5
102.0
87.8
71.1
87.3
104.9
142.1
101.6
78.8
65.9
72.5
94.0
87.3
41.4
57.6
85.8
100.3
71.4
42.8
64.7
71.9
105.2
101.4
64.8
59.1
38.6
49.6
64.3
69.7
77.5
100.8
68.5
66.6
72.1
146.6
64.1
58.6
45.2
60.5
92.4
0.72
16.33
33.25
0.71
0.50
36.37
5.14
42.80
0.40
0.37
0.64
19.67
29.02
35.76
5.94
18.28
14.24
7.83
7.10
0.08
11.0
32.91
0.39
1.93
1.07
1.19
5.85
0.92
0.64
2.40
0.11
4.45
5.91
0.04
22.40
1.75
3.28
1.04
3.75
0.74
14.76
22.23
36.42
2.13
47.30
88.95
1.70
I–N
N–K3
K3–K
K1–K2
K2–N
N–I
1.33
100.80
14.93
118.78
1.05
0.89
1.70
55.89
85.61
103.49
16.18
50.73
45.28
23.68
19.78
0.21
3b
I–N
N–K2
K2–K1
K1–K
K–N
3c
N–I
I–N
N–K
K–N
N–I
I–N
N–SmA
SmA–K
K–SmA
SmA–N
N–I
3d
31.93
104.21
1.15
5.59
2.82
3e
3f
I–N
3.17
N–SmA
SmA-K
K1–K2
K2–K3
K3–K4
K4–K5
K5–I
I–SmA
SmA–K
K1–K2
K2–SmA
SmA–I
I–SmA
SmA–K2
K2–K1
K1–K2
K2–I
17.31
2.77
2.05
7.43
0.32
12.98
16.86
0.10
65.57
5.15
9.50
2.47
11.12
2.22
46.38
66.65
99.66
3g
3h
(Continued)

12
B. Singh and A. Pandey
Table 1. Continued
Compound
Transitions
T (ꢁC)
DH=kJ molꢂ1
DS=JKꢂ1 molꢂ1
I–N
79.6
74.3
61.8
48.5
73.2
82.2
100.1
74.8
5.46
4.82
12.50
2.86
5.87
11.90
0.56
37.97
15.48
13.87
37.32
8.7
16.7
30.2
N–K2
K2–K1
K1–K2
K2–K3
K3–I
3i
I–K3
K3–K2
1.50
109.14
exothermic peaks in cooling cycle of this compound appear at 142.1ꢁC, 101.6ꢁC,
78.8ꢁC, and 65.9ꢁC having enthalpies 0.37, 0.64, 19.67, and 29.02 and entropies
0.89, 1.70, 55.89, and 85.61 J Kꢂ1 molꢂ1, respectively, which define isotopic to meso-
phase and mesophase to crystal phase transitions, respectively. The crystal to meso-
phase transition involves more enthalpy compared to the mesophase to isotropic
phase transition. A similar trend is also observed in the entropy. The texture of
the mesophase observed for this compound under polarizing microscope is charac-
teristic of the nematic mesophase. The nematic droplets observed in the heating
and cooling cycles consist of the characteristic black stripes or black brushes emanat-
ing from the central nuclei. These stripes 2 and 4 in number formed by disclination
lines are generally arising due to defects in the molecular orientational order. The
nematic droplets at 87.3ꢁC in the heating cycle with prominent Schlieren texture
merge to form a film in which the radiating stripes are not so conspicuous
(Fig. 1(a)) compared to that in the cooling cycle at 142.1ꢁC. The joining of radiating
arms of one nucleus (molecule) with the other nuclei in the nematic texture is clearly
seen (Fig. 1(b)). Similar mesomorphic properties are observed for the compound 3c.
Compounds 3a and 3h exhibit similar mesomorphism exhibiting a nematic meso-
phase in the coolinng cycle only.
Compound 3e shows endothermic peaks at 64.7ꢁC, 71.9ꢁC, and 105.2ꢁC, which
are due to transitions of crystal to SmA, SmA to nematic phase, and nematic to iso-
tropic phase, respectively. The smectic A phase (Fig. 2) with a focal conic texture is
transient and changes to a nematic phase with characteristic black brushes and then
to an isotropic phase at 105.2ꢁC. In the cooling cycle, the isotropic phase changes to
a nematic phase at 101.4ꢁC, which further transits to a stratified structure with
well-defined interlayer spacing SmA mesophase with ordered hexagonal structure
at 64.8ꢁC and finally transitions to solid crystalline phase at 59.1ꢁC. Thus, this com-
pound exhibits an enantiotropic smectic A to nematic mesophase transition.
Compound 3d shows crystal to nematic and nematic to isotropic transitions in
the heating cycle. However, in the cooling cycle it shows a nematic to SmA to crystal
phase transition. The phase transition temperatures and accompanying enthalpies
and entropies are given in Table 1. Compound 3g exhibits endothermic peaks at
66.6ꢁC, 72.1ꢁC, and 146.6ꢁC, which define crystal to crystal, crystal to SmA, and
SmA to isotropic phase transitions, respectively. The exothermic peaks appear at
64.1ꢁC, 58.6ꢁC, and 45.2ꢁC and show a reversal of the phase sequence I ! SmA !
K2 ! K1. Compound 3f exhibits numerous endothermic peaks indicating crystal to

Liquid-Crystalline Benzoquinones and Their Copper (II) Complexes
13
Table 2. Phase transition temperatures, transition enthalpies (DH), and entropies
(DS) of 2,5-bis(4-n-alkoxybenzoyloxy)-3,6-dichloro-1,4-benzoquinone, monoaqua
copper (II) acetate complexes
Compound
4a
Transitions
T (ꢁC)
DH=kJ molꢂ1
DS=JKꢂ1 molꢂ1
K1–K2
K2–K3
K3–K4
K4–I
64.9
105.2
146.4
187.6
183.5
142.2
101.4
59.0
2.72
2.73
2.11
2.41
1.67
2.34
3.36
3.35
0.25
7.77
1.59
1.16
0.61
16.80
1.31
1.28
5.17
4.95
2.77
2.72
2.84
2.62
2.25
2.51
2.07
2.93
3.99
1.97
1.99
1.44
1.42
1.43
1.45
1.09
1.06
0.95
1.89
1.52
1.31
1.73
2.36
2.23
8.04
7.21
4.58
5.23
3.65
5.63
8.97
10.08
0.74
22.12
4.19
2.76
1.32
35.78
2.87
3.09
13.80
13.34
8.19
7.18
6.77
5.68
4.47
5.03
4.53
7.05
10.65
5.82
6.45
3.71
4.20
3.78
3.34
2.36
2.10
1.74
3.48
3.04
2.86
4.16
6.30
6.71
I–K4
K4–K3
K3–K2
K2–K1
K1–K2
K2–K3
K3–K4
K4–K5
K5–K6
K6–I
4b
64.2
78.2
105.9
146.7
186.5
196.4
182.6
141.0
101.4
97.9
I–K6
K6–K5
K5–K4
K4–K
K1–K2
K2–K3
K3–K4
K4–K5
K5–I
4c
64.9
105.4
146.3
187.8
229.9
225.4
183.5
142.2
101.4
65.2
I–K5
K5–K4
K4–K3
K3–K2
K2–K1
K1–K2
K2–I
4d
4e
35.1
114.5
64.9
K1–K2
K2–K3
K3–K4
K4–K5
K5–K6
K6–I
105.2
146.1
187.5
229.6
272.5
268.6
225.7
183.5
142.4
101.4
59.09
I–K6
K6–K5
K5–K4
K4–K3
K3–K2
K2–K1
(Continued)

14
B. Singh and A. Pandey
Table 2. Continued
Compound
4f
Transitions
T (ꢁC)
DH=kJ molꢂ1
DS=JKꢂ1 molꢂ1
K1–K2
K2–K3
K3–K4
K4–K5
K5–I
65.5
92.3
0.61
6.36
1.72
90.52
6.81
0.14
0.47
11.48
0.62
2.77
3.08
2.56
2.75
2.15
2.61
2.21
2.94
3.61
3.92
16.57
9.09
17.08
24.65
1.80
17.40
4.53
193.72
13.94
0.30
106.0
194.5
215.2
181.9
140.8
109.1
99.8
I–K5
K5–K4
K4–K3
K3–K
K1–K2
K2–K3
K3–K4
K4–K5
K5–I
1.13
30.04
1.66
8.19
8.13
6.10
5.96
4.27
5.23
4.84
4g
64.9
105.4
146.3
187.8
229.9
225.4
183.5
142.2
101.4
58.9
117.3
131.3
113.6
90.8
I–K5
K5–K4
K4–K3
K3–K2
K2–K1
K1–K2
K2–I
7.08
9.64
11.80
42.44
22.45
44.16
67.81
4i
I–K2
K2–K1
crystal polymorphism and changes to an isotropic phase at 77.5ꢁC. However, on
cooling it goes from isotropic to SmA followed by crystallization. Thus, this com-
pound has features of monotropic liquid crystallinity with the appearance of an
SmA phase only in the cooling cycle. The origin of the smectic phase is the inter-
layer attractions, which are weak compared to the lateral forces between the
molecules, and the layers are able to slide over one another relatively easily. This
gives rise to fluid property to the system having higher viscosity than the nematic
mesophase. The flexibility of layers leads to distortion accounting for the focal
conic pattern of smectic A mesophase. Compound 3i exhibits no mesomorphic beha-
vior and only a crystal to isotropic phase transition is observed. The endothermic
peaks appearing at 48.5ꢁC, 73.2ꢁC, and 82.2ꢁC define the K1 ! K2, K2 ! K3,
K3 ! I phase transitions respectively. A mosaic pattern is observed at 73.2ꢁC but
it does not define a mesophase. It has a tendency toward nematic phase. The
exothermic peaks appearing at 100.1ꢁC and 74.8ꢁC define I ! K3 and K3 ! K2
transitions, respectively.
The enantiotropic nematic mesophase for the compounds with m ¼ 6, 7 and 8,
enantiotropic nematic and Smetic A mesophases for m ¼ 9 and 10, monotropic smec-
tic A mesophase for m ¼ 11, enantiotropic Smectic A mesophase for m ¼ 12, and
monotropic nematic mesophase for m ¼ 6, 14 are inferred. The compound with
m ¼ 16 is nonmesogenic in nature.

Liquid-Crystalline Benzoquinones and Their Copper (II) Complexes
15
Figure 1. (a) Photomicrograph of the nematic Schlieren texture at 87.3ꢁC of compound 3b in
the heating cycle. (b) Photomicrograph of the nematic Schlieren texture at 142.1ꢁC of com-
pound 3b in the cooling cycle.
Figure 2. Photomicrograph of the smectic A phase at 64.7ꢁC of compound 3e in the heating
cycle.

16
B. Singh and A. Pandey
Conclusions
The following structure-mesophase relationship for this series of the compounds is
inferred. As the alkoxy chain length increases, the nematic phase is first be stabilized
(Mair Saupe theory) [25] until the smectic phase appears due to further association
promoted by mutual interaction of the chains. The copper (II) complexes exhibit
crystal to crystal and crystal to isotropic phase transitions in the heating cycle and
subsequent crystal to crystal transitions in the cooling cycle with no mesomorphism.
The nonmesogenic nature of the copper complexes may be due to strong hydrogen
bonding between coordinated water molecule and the oxygen, which is sufficient to
pack the molecules giving only solid-state properties.
Acknowledgments
The authors thank the head of the Department of Chemistry, BHU, Varanasi, for
providing laboratory facilities. One of the authors (AP) thanks UGC for financial
support as Project Fellow and BHU for research scholarship. Thanks are also due
to Satish Tiwari for CHN analyses, R. C. P. Bipin for NMR, and V. N. Pandey
for IR and electronic spectral data. We are thankful to SAIF, IIT Bombay, for
recording ESR spectra.
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