Nematic phases of disc-and rod-shaped molecules

Article abstract of DOI:10.1080/714965607

Substantial effort has been made to prepare mixtures of disc-shaped and rod-shaped mesogens, since these mixtures provide one of the requirements in one of the routes towards biaxiality in nematic mesophases. We present the synthesis and mesomorphic prope

Full text of DOI:10.1080/714965607

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Novel Anthraquinone Based
Discotic Metallomesogens
Jaishri J. Naidu a & Sandeep Kumar a
a Centre for Liquid Crystal Research , P.O. Box 1329,
Jalahalli, Bangalore, 560013, India
Published online: 18 Oct 2010.
To cite this article: Jaishri J. Naidu & Sandeep Kumar (2003) Novel Anthraquinone
Based Discotic Metallomesogens, Molecular Crystals and Liquid Crystals, 397:1, 17-24,
DOI: 10.1080/714965607
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Mol. Cryst. Liq. Cryst., Vol. 397, pp. 17/[317]–24/[324], 2003
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390213618
NOVEL ANTHRAQUINONE BASED DISCOTIC
METALLOMESOGENS
Jaishri J. Naidu and Sandeep Kumar
Centre for Liquid Crystal Research, P.O. Box 1329, Jalahalli,
Bangalore-560013, India
A number of copper and palladium complexes of 1-hydroxy-2,3,5,6,7-penta-
alkoxy-9,10-anthraquinones were prepared. Lower homologues of both the
series are not liquid crystalline while higher homologues were found to be
liquid crystalline forming columnar mesophases. The columnar phase can be
induced in non liquid crystalline homologues by doping with TNF. While
copper complexes were found to be thermally stable, all the palladium com-
plexes were unstable.
Keywords: discotic liquid crystals; columnar mesophases; anthraquinone; metallomesogens
INTRODUCTION
Twenty five years ago, in 1977, Chandrasekhar and his colleagues reported
that not only rod-like molecules, but also compounds with disc-like mole-
cular shape are able to form mesophases [1]. They prepared a number of
benzene-hexa-n-alkanoates and from thermodynamic, optical and X-ray
studies, it was established that these materials form a new class of liquid
crystals in which molecules are stacked one on top of the other in columns
that constitute a hexagonal arrangement.
The introduction of metal into mesogenic molecules has brought about a
considerable expansion in the field of discotic liquid crystals, because the
coordination geometry of many metals (particularly square-planar) causes
the molecule to adopt an approximately planar structure. The application
of transition metal coordination chemistry in the design of liquid crystals
has led to the discovery of a large number of novel compounds capable of
exhibiting non-calamitic mesomorphism [2].
Discotic metallomesogens have recently become increasingly important
due to their electronic and optoelectronic properties. The quasi-one-
We are grateful to Professor S. Chandrasekhar for many helpful discussions.
17/[317]

18/[318]
J. J. Naidu and S. Kumar
dimensional order of closely packed disks in columnar mesophase of
metallomesogens makes these materials very promising candidates for
charge and energy migration studies [3–10]. A number of disc-like mole-
cules such as phthalocyanines, porphyrins, b-diketones, dithiolene, ben-
zalimines, incorporating a variety of metals such as Ni, Cu, Pd, VO, Lu, Eu,
displaying various mesophases such as columnar, discotic nematic, discotic
lamellar, etc. have been synthesized and characterized [11–26].
b-Diketonate Complexes
b-Diketonates were the first disc-like complexes reported to exhibit
mesomorphism in the pure state. They are amongst the most widely syn-
thesized and studied metallomesogens [2,11–14]. They exhibit either
calamitic or discotic mesomorphism depending on subtle differences in the
molecular structure. A number of b-diketone derivatives are known to show
nematic, smectic, columnar and lamellar mesophases [2,11–14]. b-Diketone
complexes having both calamitic and discotic features are also known
[27,28].
Giroud and Billard described a series of b-diketone complexes con-
taining four aromatic rings symmetrically or unsymmetrically with various
alkyl chains [15,29]. Copper complexes of the series were mesomorphic
while the nickel complexes were non-liquid crystalline. On the bases of
X-ray studies by Levelut, it was proposed the mesophase of Cu complexes
was lamellar [30]. In the past two decades a number of b-diketone based
discotic liquid crystals were prepared and studied extensively by Ohta and
others [17,22,31–39].
Comparison between b-Diketone and 1-hydroxyanthraquinone
Systems
Anthraquinone derivatives are one of the earliest systems reported to
form columnar mesophases. A number of alkoxy and alkanoyloxy deri-
vatives are known to show different columnar mesophases. If we compare
the structure of a 1-hydroxyanthraquinone derivative with that of a b-
diketone molecule, it can be clearly seen that there is a similarity
between the two systems. Therefore, the rich chemistry that has been
developed for the b-diketone system can also be applied to the anthra-
quinone molecule and thus, a variety of novel metallomesogens can be
prepared [40–41].

Anthraquinone Based Metallomesogens
19/[319]
Synthesis and Characterization of Anthraquinone based
Metallomesogens
We have previously reported the synthesis of various 1-hydroxy-2,3,5,6,7
pentaalkoxyanthraquinones [42]. These ligands were easily converted to
metal complexes by refluxing with metal acetate in acetonitrile-pyridine as
shown in Scheme 1. Two series of complexes; one with copper and other
with Pd were prepared in this way. All the compounds were purified by
microfiltering the dichloromethane solution followed by crystallization in
dichloromethane-ethanol mixture.
SCHEME 1 Synthesis of anthraquinone metal complexes.

20/[320]
J. J. Naidu and S. Kumar
1
All the Pd complexes 2f-l were characterized from their H NMR, 13C
NMR, IR, UV-Vis and elemental analysis while the Cu complexes 2a-e were
characterized from their elemental analysis (the paramagnetic nature of
copper(II) hampered their characterization by NMR spectroscopy).
CONCLUSION
Several novel discotic liquid crystalline anthraquinone based metal com-
plexes in which a Cu or Pd metal was co-ordinated into the mono-
functionalized anthraquinone ligand were prepared and characterized.
While the Pd complexes of 1-hydroxy-2,3,5,6,7-pentaalkoxyanthra-9,10-
quinone were found to be thermally unstable at higher temperature, the Cu
complexes of the same ligand were stable. The synthesis of new anthra-
quinone ligand and the incorporation of metal in it opens many exciting
possibilities. A variety of novel symmetrical and unsymmetrical (e.g. a
combination of anthraquinone and b-diketone ligands) metallomesogens
incorporating various metals such as, Cu, Ni, Pd, VO, Tl, etc., can be rea-
lized. Metal bridged novel symmetrical and unsymmetrical trimers and
polymers can also be prepared.
EXPERIMENTAL
General Information
Chemicals and solvents (AR quality) were used without any purification.
All reactions were monitored by employing TLC technique using appro-
priate solvent system for development. All solvent extracts were washed
successively with water, brine and dried over anhydrous Na2SO4 and
concentrated at reduced pressure on a Buchi E1 rotary evaporator.
Chromatographic separations were performed on silica gel (230–
400 mesh) and neutral aluminum oxide. Thin-layer chromatography (TLC
was performed on aluminium sheets precoated with silica gel (merck,
Kieselgel 60, F254). Visualisation of the spots on TLC plates was achieved
by exposure to UV light. Thermal behaviour was checked using a Mettler
FP82HT hot stage and central processor in conjuction with a Leitz
DMRXP polarizing microscope as well as by differential scanning colori-
metry (DSC7 perkin-Elmer). Ultraviolet Spectra recorded on UV-Vis
spectrometer (Perkin-Elmer Lambda 20). IR Spectra were recorded on
Perkin-Elmer FT-IR spectrometer Spectrum 1000 NMR Spectra were
recorded on a 200 or 400 MHz machine (Bruker). All chemical Shifts were
recorded in d units downfield from Me4Si, and J values are given in Hz.

Anthraquinone Based Metallomesogens
21/[321]
13C NMR Spectra were recorded on Bruker AC 200 (50 MHz) or 400
(100 MHz) Spectrometer. Elemental analysis were performed on Carlo
Erba Strum Entazione Elemental Analyzer-MOD. 1106 or on an Euro
Vector Elemental Analyzer.
Preparation of Metal Complexes of 1-hydroxy-2,3,5,6,7-
pentaalkoxy-9,10-Anthraquinones: General Procedure
A
solution of 1-hydroxy-2,3,5,6,7-pentaalkoxy-9,10-anthraquinone
(0.25 mmol), copper(II) acetate or palladium(II) acetate (0.25 mmol) in
acetonitrile-pyridine (ꢀ15 ml) was refluxed for about 24 hours in nitrogen
atmosphere. The solid obtained was filtered and washed with methanol.
The crude product was purified by microfiltering the dichloromethane
solution followed by crystallization in dichloromethane-ethanol mixture. All
the metal complexes were obtained as brown solids in about 50% yield.
2a-e (Copper Complexes)
IR data: Similar spectrum is obtained for all the compounds 2a-e: nmax
/
cmꢁ1 2955, 2921, 2852, 1660, 1630, 1582, 1510, 1467, 1349, 1284, 1140.
UV-Vis data: all samples were measured in dichloromethane and similar
spectrum is observed for all the compounds. lmax/nm 500 (sh), 420 (sh),
336, 289.
Elemental analysis: All the compounds give satisfactory elemental
analysis.
2f-l (Palladium Complexes)
1H NMR (200 MHz, CDCl3): All the compounds give similar spectra
differing in only the number of alkyl chain CH2 protons. d 7.7 (s, 2H), 7.4 (s,
2H), 4.1 (m, 20H), 1.8–1.3 (m, alkyl chain CH2), 1.0–0.8 (m, 30H). 13C NMR
(100 MHz, CDCL3): All the compounds give similar spectra differing in only
the number of alkyl chain C signals. d 180.7, 178.1, 163, 157, 156, 147, 142,
131, 130, 120, 114, 108.8, 74.6, 69.7, 32.3–14.4 (alkyl chain carbons). IR
data: Similar spectrum is obtained for all the compounds 2f-l: vmax/cmꢁ1
2956, 2856, 2363, 1660, 1630, 1582, 1561, 1511, 1461, 1352, 1320, 1283,
1262, 1147. UV-vis data: all samples were measured in dichloromethane
and similar spectrum is observed for all the compounds. lmax/nm 550 (sh),
420 (sh), 365. Elemental analysis: All the compounds give satisfactory
elemental analysis.
Thermal Behaviour
The mesophase behaviour of all the complexes was checked by polarized
optical microscopy as well as by differential scanning calorimetry (DSC).
In both the series, lower homologues were found to be non-liquid

22/[322]
J. J. Naidu and S. Kumar
crystalline but all the higher homologues were mesogenic. The crystalline
copper complexes 2a-c, melt at 184, 175, 170ꢂC respectively. The ther-
mograms recorded for copper complex 2d on heating showed three
endothermic peaks. The first one, located at 151.7ꢂC, corresponds to an
enthalpy of 12.1 J/g, related to the transition from the crystal into a
columnar mesophase. The second weak peak at 156ꢂC corresponds to an
enthalpy of only 0.43 J/g, is associated with a columnar phase to columnar
phase transition. The third peak at 165.8ꢂC with enthalpy of 6.3 J/g is for
the columnar phase to isotropic phase transition. The thermal behaviour
was found to be reversible. Upon cooling from the isotropic liquid, the
thermograms showed three exothermic peaks at 164ꢂC, 155ꢂC and 148ꢂC
with roughly the same enthalpy values. A Leitz DMRXP polarizing
microscope with a Mettler FP82HT heating stage was used to observe the
optical textures of the phases. On cooling from the isotropic liquid, the
copper complex 2d shows usual texture of columnar phase. Fingerprints
texture appeared at about 155ꢂC. The texture is very similar to the
texture reported for the Colrd mesophase of hexaalkanoyloxy tripheny-
lenes [43,45]. The compound crystallizes at about 148ꢂC. Compound 2e
shows similar behaviour [41]. All the palladium complexes start decom-
posing on heating and therefore, their thermal behaviour is not repro-
duceable. As mentioned above, lower homologues of both the series are
not liquid crystalline but the columnar phase can be induced by doping
them with electron acceptor such as trinitrofluorenone (TNF) [40].
Mesophase induction and stabilization due to charge-transfer interac-
tions in metallomesogens as well as in classical liquid crystals is well
documented [46].
TABLE 1 Phase Transition Temperatures (Peak Temp.) and Enthalpies of New
Anthraquinone Copper Complexes. Cr ¼ Crystal, Col ¼ Columnar Liquid Crystalline
phase, I ¼ isotropic
Thermal transitions/ꢂC and enthalpy changes (J/g in parentheses)
Compound
Heating scan
Cooling scan
2a
2b
2c
2d
Cr 184 I
Cr 175 I
Cr 170 I
I 155 Cr
I 161 Cr
I 150 Cr
Cr 151.7 (12.1) Col 156 (0.43)
Col 165.8 (6.3) I
Cr 71.9 (42) Col 121 (1.5) Col
146.2 (4.7) I
I 164 (3) Col 155 (0.3) Col 148
(12) Cr
I 144.2 Col 118.0 Col 67.1 Cr
2e

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23/[323]
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