Synthesis and liquid crystalline properties of a disc-shaped molecule with azobenzene at the periphery

Article abstract of DOI:10.1016/j.tetlet.2005.02.005

A triphenylene-based novel liquid crystal was synthesised whose peripheral cores are six rod-like azobenzene moieties linked through alkyl chains. A disc-shaped molecule 2,3,6,7,10,11-hexakis-[{4-(4-nitrophenylazo)phenoxy} hexyloxy]triphenylene was prepar

Full text of DOI:10.1016/j.tetlet.2005.02.005

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2303–2306
Synthesis and liquid crystalline properties of a disc-shaped
molecule with azobenzene at the periphery
Md Lutfor Rahman,a,* Carsten Tschierske,b Mashitah Yusoff a and Sidik Silongc
aSchool of Science and Technology, Universiti Malaysia Sabah, Locked Bag 2073, 88999 Kota Kinabalu Sabah, Malaysia
bInstitute of Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes Str. 2, Halle D-06120, Germany
cDepartment of Chemistry, University Putra Malaysia, 43400 Serdang, Malaysia
Received 1 November 2004; revised 25 January 2005; accepted 1 February 2005
Abstract—A triphenylene-based novel liquid crystal was synthesised whose peripheral cores are six rod-like azobenzene moieties
linked through alkyl chains. A disc-shaped molecule 2,3,6,7,10,11-hexakis-[{4-(4-nitrophenylazo)phenoxy}hexyloxy]triphenylene
was prepared by using a ferric chloride oxidative trimerisation of 1,2-bis-[{4-(4-nitrophenylazo)phenoxy}hexyloxy]benzene. Differ-
ential scanning calorimetry and polarising optical microscopy analysis revealed the existence of a nematic mesophase.
Ó 2005 Elsevier Ltd. All rights reserved.
Discotic liquid crystals are usually composed of mole-
cules with a disc-like core and aliphatic tails. Substituted
hexaethers of triphenylene have been widely synthesised
and studied as discotic mesogens. A number of reported
discogens consist of a flat, or nearly flat, rigid core
surrounded by a number of aliphatic sidechains having
binary, trigonal, tetragonal or hexagonal symmetries.
Discotic liquid crystals exhibiting mesophases are used
in many device applications.1,2 The potential uses of
these materials as photoconductors,3 one-dimensional
conductors2,4 and light emitting diodes5 has attracted
considerable attention. Several research groups have
been working on the synthesis of functionalised triphen-
ylene-based discotic liquid crystals and their potential
applications.6–10
incompatible with the molecular topology and induced
the formation of a smectic phase.12 Compatible combi-
nation of discotic and calamitic mesogens results in
the formation of a nematic phase.13
We have reported that when rod-shaped azobenzene
moieties were incorporated into discotic triphenylene
molecules having an acrylate backbone via ester link-
ages, a side group liquid crystal polymer was pro-
duced.14 Recent studied by Shimizu et al.15 on a
triphenylene mesogen with peripheral azobenzene units,
shows kinetically controlled bimesomorphism with
metastable smectic and stable hexagonal columnar
phases created by the rod–disc alternation of the molec-
ular shape, on cooling. This molecule comprised a tri-
phenylene core with six azobenzene units linked via
short (C3) alkyl chains using ester bonds.15
An extensive study on the synthesis of discotic liquid
crystals, based on a triphenylene, showed most of the
compounds to form columnar mesophases.2,6,11 How-
ever, current efforts have been directed towards design-
ing novel molecules, which in some way combine the
features of rod-shaped and disc-like molecules having
photoactive properties. In one example of the combina-
tion of calamitic (rod-shaped) and discotic (disc-shaped)
molecules, the discotic and calamitic moieties were
It is well known that photo-orientation of azobenzene
units is easily induced through trans–cis–trans isomeris-
ation cycles by irradiating liquid crystalline materials
containing chromophores with linearly polarised light.
Thus, azobenzene-containing materials have potential
technological applications such as in high density optical
data storage, optical image processing, dynamic holo-
graphy, optical computing and pattern recognition.16–20
In the work described here we have incorporated azo-
benzene moieties into a triphenylene molecule via alkyl
chains (C6) using etherification, giving, triphenylene-
based discotic liquid crystals having six peripheral
rod-like azobenzene units with terminal nitro groups.
Keywords: Liquid crystals; Nematic phase; Azobenzene; Disc-shaped
molecule.
*
Corresponding author. Tel.: +60883200005742; fax: +6088435324;
e-mail: lutrinl@hotmail.com
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.02.005

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M. R. Lutfor et al. / Tetrahedron Letters 46 (2005) 2303–2306
pound 8 was used to dialkylate catechol, in the presence
+ -
i
O2N
O2N
NH2
N2Cl
of potassium carbonate, to give 1,2-bis-[4-(4-nitrophen-
ylazo)phenoxyhexyloxy]benzene 9. Finally, a disc-like
molecule, 2,3,6,7,10,11-hexakis-[4-(4-nitrophenylazo)-
phenoxyhexyloxy]triphenylene 10 was prepared by oxi-
dative cyclotrimerisation of 9 with ferric chloride as
the oxidant (Scheme 1).
6
ii
iii
iv
O2N
N
N
OH
7
The structures of the intermediates and the final
compound were confirmed by spectroscopic analysis.
The 13C NMR spectrum of 10 (Fig. 1), had signals
corresponding to all the carbon atoms of 10 (only one
azobenzene unit is shown in detail in Figure 1, that is,
R = O(CH2)6OC6H4–4-N2–C6H4–4-NO2). There are 18
carbon atoms in the triphenylene core but only three dis-
tinct positions (x, y, z). The x, y and z positions corre-
spond to three signals and all other signals correspond
to the carbon atoms of the six rod-shaped azobenzene
moieties and the alkyl chains (Fig. 1).
O2N
N
N
O(CH2)6Br
8
OR
RO
OR
v
OR
9
OR
RO
RO
OR
10
Mesomorphic properties: The phase transition tempera-
tures as well as the phase transition enthalpy changes
were determined using differential scanning calorimetry
(DSC). The DSC thermograms of 10 showed two endo-
thermic peaks at 133.9 and 146.1 °C and the enthalpy
R : (CH2)6O
N
N
NO2
Scheme 1. Reagents and conditions: (i) NaNO2, 3 equiv HCl, 2 °C;
(ii) NaOH, C6H5OH, 2 °C, 64%; (iii) K2CO3, KI, Br(CH2)6 Br, reflux,
68%; (iv) catechol, K2CO3, KI, reflux, 46%; (v) FeCl3, CH2Cl2, H2SO4,
rt, 26%.
changes of these transitions were 49.4 and 1.56 JgÀ1
,
respectively (Fig. 2). On cooling, two exothermic peaks
were also observed at 141.3 and 102.7 °C and the enthal-
py changes of these transitions were 2.25 and 46.4 JgÀ1
respectively.
,
Scheme 1 outlines our synthetic approach. The periphe-
ral units of the mesogenic part were prepared by diazoti-
sation of 4-nitroaniline and then coupling of diazonium
salt 6 with phenol yielding 4-(4-nitrophenylazo)phenol
7. The flexible spacer was introduced by alkylation of
phenol 7 with a 10-fold excess of 1,6-dibromohexane
in the presence of potassium carbonate as base to give
1-bromo-4-(4-nitrophenylazo)phenoxyhexane 8. Com-
The phase structures were determined by polarising
optical microscopy. The polarised optical micrograph
of 10, observed in the liquid crystalline phase upon cool-
ing, is shown in Figure 3. On cooling the isotropic
phase, a schieleren texture was observed under the pola-
rising microscope at 136.5 °C (Fig. 3). This schieleren
texture is of high fluidity and the optical texture is of a
Figure 1. 13C NMR spectrum of 10. Only one azobenzene unit with an alkyl chain is shown in the chemical structure, that is, R = O(CH2)6OC6H4–
4-N2–C6H4–4-NO2.

M. R. Lutfor et al. / Tetrahedron Letters 46 (2005) 2303–2306
2305
mer effect) rather than as a disc-like unit, which would
lead to a columnar organisation.
4-(4-Nitrophenylazo)phenol 7: 4-Nitroaniline (10 g,
0.0724 mol) was dissolved in methanol (200 ml) and
water (50 ml) with 25 ml of concd hydrochloric acid
and the mixture was cooled to 2 °C. Sodium nitrite
(5 g, 0.0924 mol) in water (20 ml) was added dropwise
and the mixture stirred for 1 h. After this, phenol
(6.8 g, 0.0724 mol) in acetone (100 ml) and water
(50 ml) was added and the mixture was maintained at
pH 8–9 using sodium hydroxide solution, and stirred
for 2h. Then, dilute hydrochloric acid (ca. 80 ml, 10%)
and water (300 ml) were added and the resulting precip-
itate was collected by filtration. The product was crystal-
lised twice from methanol and ethanol, respectively.
Yield: 11.2g (64%) and mp 207 °C. 1H NMR
(600 MHz, acetone-d6) d: 8.33 (2H, d, J = 8.6 Hz, Ar-
H), 7.94 (4H, dd, J = 8.5 Hz, J = 5.8 Hz, Ar-H), 7.00
(2H, d, J = 5.7 Hz, Ar-H), 5.88 (1H, s, Ar-OH).
Figure 2. DSC heating and cooling traces of compound 10 (10 °C
minÀ1).
1-Bromo-4-(4-nitrophenylazo)phenoxyhexane 8: Com-
pound 7 (2.40 g, 9.87 mmol) was dissolved in dry
acetone (130 ml) and potassium carbonate (10 g,
74 mmol), potassium iodide (50 mg) and a 10-fold excess
of 1,6-dibromohexane (24 g, 98.7 mmol) were added and
the mixture was refluxed for 24 h under argon. The reac-
tion mixture was filtered hot and the acetone was
removed under reduced pressure. Then, hexane was
added to remove excess 1,6-dibromohexane, the product
was insoluble in hexane. The product was collected by
filtration and was dissolved in dichloromethane and
water (1:1). The organic phase was washed with dilute
hydrochloric acid (ca. 50 ml, 10%), sodium carbonate
solution (ca. 50 ml, saturated) and water successively.
The organic fraction was dried with sodium sulfate
and the solvent was removed under reduced pressure.
The product was recrystallised from ethanol. Yield:
2.62 g (68%) and mp 128 °C. 1H NMR (600 MHz,
CDCl3) d: 8.33 (2H, d, J = 8.4 Hz, Ar-H), 7.94 (4H,
dd, J = 8.6 Hz, J = 5.9 Hz, Ar-H), 7.02H(2, d,
J = 8.6 Hz, Ar-H), 4.05 (2H, t, J = 6.6 Hz, OCH2),
Figure 3. Optical micrograph of 10 showing nematic phase at 136 °C.
typical nematic phase. There was no further phase tran-
sition on continued cooling, except for crystallisation.
3.42(H2 , t, J = 5.5 Hz, CH2Br), 1.86–1.82(4H, m,
CH2), 1.55–1.48 (4H, m, CH2).
Shimizu et al.15 described the unusual behaviour in
mesophase formation for a disc-shaped molecule. The
triphenylene core and peripheral azobenzenes are
commonly linked by flexible chains, which allowed the
conformational change in mesophase formation. Thus,
the rod-shaped parts can easily recognise each other
and form a smectic layered structure but with less molec-
ular recognition along the central core. Therefore, fluid
nematic and layered smectic mesophases occur in disc-
shaped molecules.
1,2-Bis-[4-(4-nitrophenylazo)phenoxyhexyloxy]benzene 9:
Compound 8 (2.03 g, 5 mmol) was dissolved to dry
acetone (120 ml) and catechol (0.275 g, 2.5 mmol),
potassium carbonate (0.675 g, 5 mmol) and potassium
iodide (50 mg) were added and the mixture was refluxed
for 40 h under argon. The mixture was poured into ice-
cold water and acidified with dilute hydrochloric acid
(ca. 10 ml, 10%). The resulting precipitate was filtered
off and partitioned between dichloromethane and water.
The organic phase was washed with sodium carbonate
solution and water, dried with sodium sulfate and the
solvent was evaporated under reduced pressure. The
product was recrystallised from ethanol. Yield: 1.67 g
In conclusion, a disc-shaped molecule was prepared
whose molecular architecture was composed of a tri-
phenylene central core and six rod-like azobenzenes as
the peripheral units linked through alkyl chain spacers.
The combination of calamitic and discotic units led to
a nematic mesophase although the molecule is disc-
shaped. It seems that the triphenylene unit acts only as
a linking unit, interconnecting the rod-like units (oligo-
1
(46%) and mp 137 °C. H NMR (600 MHz, CDCl3) d:
8.33 (4H, d, J = 8.4 Hz, Ar-H), 7.93 (8H, dd,
J = 7.5 Hz, J = 6.1 Hz, Ar-H), 6.97 (4H, d, J = 8.6 Hz,
Ar-H), 6.88–6.82(4H, m, Ar-H), 4.03 (8H, m, OCH ),
2
1.86–1.82(8H, m, OCH CH2), 1.55–1.49 (8H, m, CH2).
2

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M. R. Lutfor et al. / Tetrahedron Letters 46 (2005) 2303–2306
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2,3,6,7,10,11-Hexakis-[4-(4-nitrophenylazo)phenoxyhex-
yloxy]triphenylene 10: Compound 9 (0.55 g, 0.723
mmol) was added to a stirred suspension of iron(III)
chloride (0.325 g, 2 mmol) in dichloromethane (25 ml)
containing 2drops of concd sulfuric acid. The mix-
ture was stirred for 2h and then excess iron(III) chloride
was removed by filtration. About 60 ml of methanol
was added to quench the reaction and the resulting
precipitate was washed several times with methanol.
The product was purified by column chromatography
on silica gel with chloroform as eluant. The solid was
recrystallised from ethanol: chloroform (10:1); yield
0.14 g (26%). 1H NMR (600 MHz, CDCl3) d: 8.32
(12H, d, J = 8.4 Hz, Ar-H), 7.92(24H, dd, J = 8.6 Hz,
J = 5.9 Hz, Ar-H), 6.98 (12H, d, J = 8.9 Hz, Ar-H),
6.88 (6H, s, Ar-H), 4.03 (24H, tt, J = 6.5 Hz,
J = 5.7 Hz, OCH2), 1.85–1.82(42H, m, OCH 2CH2),
1.55–1.48 (24H, m, CH2). 13C NMR (125 MHz, CDCl3)
d: 26.01, 26.02, 29.25, 29.41, 68.41, 69.13, 109.99,
114.85, 121.14, 123.0, 124.61, 125.53, 146.72, 148.14,
149.10, 155.91, 162.72. Elemental analysis calcd for
C126H126N18O24 (2276.45): C, 66.48; H, 5.58; N, 11.08.
Found: C, 66.25; H, 5.32; N, 11.32%.
14. Silong, S.; Lutfor, M. R.; Wan Yunus, W. M. Z.;
Rahman, M. Z.; Ahmad, M.; Haron, J. Mater. Res. Soc.
Symp. Proc. 2002, 709, 105–109.
Acknowledgements
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This research was supported by IRPA grant (No: 01-02-
10-0035EA0033) from the Ministry of Science, Technol-
ogy and Innovation, Malaysia. Sincere thanks goes to
the German Academy Exchange Service for providing
a short term fellowship [code: A/03/19615].
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