Synthesis and optical/thermal behavior of new Azo photoisomerizable discotic liquid crystals

Article abstract of DOI:10.1021/ma902460c

In this study, discotic azo compounds were prepared by a reductive coupling of 5-nitroisophthalic acid followed by a convergent synthesis, resulting in five new examples of azo photoisomerizable discotic molecules, which differ from each other by the link

Full text of DOI:10.1021/ma902460c

Macromolecules 2010, 43, 1319–1328 1319
DOI: 10.1021/ma902460c
Synthesis and Optical/Thermal Behavior of New Azo Photoisomerizable
Discotic Liquid Crystals
Eduard Westphal,† Ivan H. Bechtold,‡ and Hugo Gallardo*,†
†
‡
´
´
Departamento de Quımica and Departamento de Fısica, Universidade Federal de Santa Catarina,
ꢀ
Florianopolis, SC 88040-900, Brazil
Received November 6, 2009; Revised Manuscript Received December 21, 2009
ABSTRACT: In this study, discotic azo compounds were prepared by a reductive coupling of 5-nitroi-
sophthalic acid followed by a convergent synthesis, resulting in five new examples of azo photoisomerizable
discotic molecules, which differ from each other by the linking groups (ester, amide or 1,3,4-oxadiazole) and
number of aliphatic chain (4 or 8). The thermal and liquid crystalline properties of the compounds were
completely characterized by differential scanning calorimetry, polarized optical microscopy and X-ray
diffraction analysis. Two products presented a stable columnar hexagonal mesophase with 95 and 134 °C of
range. The results obtained show that the choice of the linking group and number of aliphatic chains has a
significative influence in the materials properties. The photoisomerization capacity was evaluated in solution
by spectroscopic methods and also by 1H NMR, indicating that these molecules are promising candidates for
controlling the conductivity in electro-optical devices.
1. Introduction
possible by breaking the columnar organization of the meso-
phases by melting the compound to its liquid state through a
heating process.3,11 Another way would be to reduce the molec-
ular disk anisometry, a factor that is fundamental to obtaining
this kind of organization. To do this, we can use an azo (NdN)
linkage which is well-known by the reversible trans-cis-trans
photoisomerization,12 resulting in a material with the behavior
schematically presented in Figure 1.
As can be seen, when the molecule is in the trans configuration,
the high discotic anisometry promotes a columnar organization,
in this case, a columnar hexagonal mesophase. After the photo-
isomerization to the cis configuration and consequent lost of
the anisometry, this material becomes an isotropic liquid,
the organization of which can be restored after the reverse
isomerization.13
To this aim, we set out to synthesize five new molecules with
discotic anisometry and containing an azo linkage in their
centers. Besides these requirements, we used different kinds of
functional groups and numbers of aliphatic chains (Figure 2). It is
important to note that, in this study, we aimed to obtain an
unusual type of discotic liquid crystal, where the center of the
structure is not really rigid, being able to rotate through the
binding axis. There are not many examples of this type of discotic
liquid crystals in the literature.14 Also, there are no reports of
discotic liquid crystals containing an azo linkage in the core of the
molecule, only at the periphery.15
In the past few years, considerable attention has been directed
toward creating new types of materials whose microscopic and
macroscopic properties can be controlled by means of an external
stimuli1 such as an electric field, magnetic field, temperature, and
even light.2 The possibility for control at multiple length scales
makes liquid crystals ideal candidates for their use as active
components or scaffolds in self-assembly processes. Discotic
liquid crystal (DLC) molecules consisting of a disk-shaped
aromatic core substituted at its peripheral positions with flexible
alkyl or alkoxy chains exhibit mesomorphism, including colum-
nar, discotic nematic, and chiral discotic phases. These types of
materials have been attracting increasing interest from the
industrial and technological point of view because of their unique
self-assembled structures and their particular anisotropic physical
properties, including electronic conductivity, photoconductivity
and photovoltaic properties,3-5 and also photoluminescence and
electroluminescence,6 which are promising for applications in
electrical and optical molecular devices. Furthermore, this
approach offers an easy means to achieving fine control of the
material properties, possibly increasing the range of applications.
These features are all possible due to an easy and varied form
of functionalization, in which other properties (like luminescence,
ionic charges, photoisomerization, gelification and others)7 can
be incorporated into a material that has a natural and fascinating
self-assembly allied with fluidity.8 Recently, discotic liquid crys-
tals have gained considerable attention as organic semiconduc-
tors, due to their excellent charge carrier mobility.9 As a result of
containing an aromatic and rigid center (conducting core) sur-
rounded by peripheral aliphatic chains (isolating layer), these
compounds are known as molecular nano wires, being capable of
preferential one-dimensional charge migration.10
2. Experimental Section
2.1. Materials and Characterizations. 5-Nitroisophthalic acid
(98%, Acros Organics), hydroquinone (g99%, Merck), 1-bro-
mododecane (98%, Acros Organics), catechol (99þ%, Acros
Organics), 4-hydroxyacetanilide (98%, Aldrich) and 3,4-dihy-
droxybenzonitrile (97%, Aldrich). All other inorganic, organic
and solvents were of the highest purity and purchased from
commercial sources (Merck, Aldrich, Fluka and Acros) and
used as received. Dry pyridine and CH2Cl2 were used when
specified. Pyridine was dried by distillation over KOH while
CH2Cl2 was dried using calcium hydride. TLC was carried out
But the questions of how to obtain control of the intensity of
the conductivity and how to connect and disconnect the nanowire
remain unanswered. In the case of discotic liquid crystals, this is
*Corresponding author. Telephone: þ 55 48 37219544. Fax: þ 55 48
37216850. E-mail: hugo@qmc.ufsc.br.
r 2010 American Chemical Society
Published on Web 01/14/2010
pubs.acs.org/Macromolecules

1320 Macromolecules, Vol. 43, No. 3, 2010
Westphal et al.
4H, -COOH). 13C NMR (DMSO-d6) δ ppm: 127.64, 133.18,
133.47, 152.37, 166.52. Elemental analysis for C16H10N2O8:
Calcd: C, 53.64; H, 2.81; N, 7.82. Found: C, 53.78; H, 2.56; N,
7.73. MS (EI, 70 eV) m/z (%): [Mþ] 358.4 (5.9%).
4-Dodecyloxyphenol (2). This was prepared through a modi-
fied procedure given in the literature.16 First, 44.0 g (400 mmol)
of hydroquinone, 32.0 mL (133 mmol) of 1-bromododecane,
5.92 g (148 mmol) of NaOH, 500 mL of methanol, and 40 mL of
water were mixed and refluxed for 10 h, after which the solvent
was evaporated. The dark gray solid was transferred to a beaker
containing 700 mL of water and the solution was acidified to pH
3 using concentrated hydrochloric acid. The resulting solid was
then macerated in hot water (60 °C) 3 times. A final recrystalli-
zation in hexane gave 37.0 g of the desired material (55%). Mp:
73.7-75.2 °C (lit. 76-77 °C).17 IR (KBr): 3376 (O-H), 2920,
2851, 1515, 1458, 1373, 1240, 1109, 1028, 826. 1H NMR
(CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz, 3H, CH3), 1.25-1.36 (m,
16H, -CH2-), 1.43 (m, 2H, -CH2-), 1.75 (qui, J=6.6 Hz, 2H,
-CH2CH2O-), 3.89 (t, J=6.6 Hz, 2H, -CH2O-), 6.77 (m, 4H,
Ar-H). 13C NMR (CDCl3), δ, ppm: 14.11, 22.68, 26.04, 29.34,
29.36, 29.41, 29.57, 29.59, 29.62, 29.65, 31.91, 68.71, 115.57,
115.94, 149.26, 153.32.Anal. Calcd for C18H30O2: C, 77.65; H,
10.86. Found: C, 78.15; H, 10.06.
Figure 1. Breaking discotic mesophase by photoisomerization.
1,2-Didodecyloxybenzene (3). Prepared through a modified
procedure given in the literature.18 First, 20.00 g (181.6 mmol)
of catechol, 113.0 mL (472 mmol) of 1-bromododecane, 75.0 g
(545 mmol) of K2CO3, 2.0 g (12 mmol) of KI, and 400 mL of
butanone were refluxed under argon atmosphere for 16 h. The
suspension was then filtered and washed 2 times with hot
butanone. The solvent was removed by rotary evaporation
and the solid obtained was dissolved in ethylic ether and washed
with water (2 ꢀ 100 mL). The organic phase was dried with
anhydrous Na2SO4 and the solvent removed. After recrystalli-
zation from ethanol 42.3 g of a white solid (52%) was obtained.
Mp: 45.8-48.1 °C (lit. 47-49 °C).18 IR (KBr): 2953, 2916, 2848,
1594, 1518, 1509, 1466, 1258, 1222, 1122, 731. 1H NMR
(CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz, 6H, CH3), 1.25-1.37 (m,
32H, -CH2-), 1.46 (m, 4H, -CH2-), 1.81 (qui, J=6.6 Hz, 4H,
-CH2CH2O-), 3.99 (t, J=6.6 Hz, 4H, -CH2O-), 6.88 (s, 4H,
Ar-H). 13C NMR (CDCl3), δ, ppm: 14.36, 22.93, 26.29, 29.58,
29.61, 29.69, 29.88, 29.90, 29.95, 32.16, 69.50, 114.31, 121.20,
149.44.
1-Acetyl-3,4-didodecyloxybenzene (4). Under argon atmos-
phere and vigorous stirring, 10.0 g (22.2 mmol) of 1,2-didoce-
cyloxybenzene (3) was dissolved in 50 mL of dry CH2Cl2 and the
solution was cooled to below -5 °C. In small portions, 3.56 g
(26.7 mmol) of AlCl3 were then added, maintaining the solution
temperature below -5 °C throughout. After 10 min, 1.90 mL
(26.7 mmol) of acetyl chloride was added dropwise taking care
not to raise the temperature above 0 °C. After this addition, the
solution was stirred at 0 °C for 30 min, refluxed for 2 h, and
poured into 100 mL of water. To this mixture was carefully
added 20 mL of concentrated hydrochloric acid. The organic
solvent was evaporated, and the solid obtained was filtrated and
recrystallized in ethanol, yielding 8.98 g (83%) of a white solid.
Mp: 63.7-64.9 °C (lit. 65 °C).19 IR (KBr): 2917, 2848, 1665
(CdO), 1582, 1521, 1468, 1425, 1275, 1216, 1153. 1H NMR
(CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz, 6H, CH3), 1.25-1.38 (m,
32H, -CH2-), 1.47 (m, 4H, -CH2-), 1.85 (m, 4H, -CH2-
CH2O-), 2.55 (s, 3H, -COCH3), 4.06 (m, 4H, -CH2O-), 6.86
(d, J=8.4 Hz, 1H, Ar-H), 7.51 (d, J=1.9 Hz, 1H, Ar-H), 7.53
(dd, J=8.4 Hz and 1.9 Hz, 1H, Ar-H). 13C NMR (CDCl3), δ,
ppm: 14.35, 22.93, 26.19, 26.23, 26.46, 29.26, 29.38, 29.59, 29.64,
29.83, 29.85, 29.89, 29.93, 32.16, 69.25, 69.45, 111.73, 112.54,
123.41, 130.44, 149.03, 153.73, 197.19.
Figure 2. Discotic molecules synthesized. The compounds nomencla-
ture following norm: EST for molecules containing ester groups; AMD
for amide groups;OXD for 1,3,4-oxadiazole units; 4 or 8 for thenumber
of alkoxy chains.
using silica-gel Si 60-F254 (Merck). Purifications were carried
out by recrystallization using commercial grade solvents and by
column chromatography on silica-gel 60-200 mesh 60A
(Acros). Infrared spectra were recorded on a Perkin-Elmer
model 283 spectrometer using KBr discs. 1H and 13C NMR
were recorded with a Varian Mercury Plus spectrometer opera-
ting at 400 and 100.6 MHz, respectively. Melting points were
determined with an Olympus BX50 microscope equipped with a
Mettler Toledo FP-82 Hot Stage. Elemental Analysis was
carried out using a Carlo Erba model E-1110 instrument. A
low resolution mass spectrum was recorded on a Shimadzu
model GCMS-QP5050A triple quadrupole mass spectrometer.
(E)-5,50-(Diazene-1,2-diyl)diisophthalic Acid (1). In a beaker,
8.96 g (49.7 mmol) of dextrose and 30 mL of water were added
and heated to 60 °C in a water bath. This solution was added
dropwise to a beaker containing 5.00 g (23.7 mmol) of 5-
nitroisophthalic acid, 5.69 g (142 mmol) of NaOH, and 100
mL of water under a strong magnetic stirring and also heated to
60 °C in a water bath. After the addition, the water bath was kept
under strong stirring for 8 h at 60 °C and 10 h at room
temperature. The suspension then was filtered and the resultant
yellow solid dissolved in water. The solution was acidified with
acetic acid and the precipitate formed was filtered and washed
with a lot of water. The solid obtained was stirred in boiling
water until the color of the suspension turned to a strong orange.
Finally, the solution was filtered while still hot and dried in air to
afford 2.59 g (61%) of an orange solid. Mp: > 350 °C. IR (KBr):
3419 (COO-H), 3077, 2993, 2854, 2664, 2590, 1706 (CdO), 1683,
1650, 1605, 1460, 1420, 1403, 1301, 1261, 1219, 1109, 930, 759,
741, 685. 1H NMR (DMSO-d6) δ ppm: 8.57 (d, J=1.6 Hz, 4H,
4,6-H isoph.), 8.59 (t, J=1.6 Hz, 2H, 2-H isoph.), 13.59 (broad,
1-Acetoxyl-3,4-didodecyloxybenzene (5). Under nitrogen at-
mosphere, 8.78 g (18.0 mmol) of 1-acetyl-3,4-didodecyloxyben-
zene (4) was dissolved in 30 mL of dry CH2Cl2. After complete
solubilization, 5.59 g (32.4 mmol) of MCPBA and a further
10 mL of dry CH2Cl2 were added, the mixture was cooled to

Article
Macromolecules, Vol. 43, No. 3, 2010 1321
0 °C, and 1.80 mL (23.4 mmol) of trifluoroacetic acid was then
added dropwise. After 30 min the solution was heated to room
temperature and vigorously stirred in the dark and under
nitrogen atmosphere for 48 h. After this period, 100 mL of a
saturated solution of NaHSO3 was slowly added and stirred for
a further 30 min. The organic phase was separated and the
aqueous phase extracted with CH2Cl2 (2 ꢀ 40 mL). The
combined organic phases were washed with a NaHCO3 satu-
rated solution (3 ꢀ 50 mL) and brine (1 ꢀ 50 mL) and dried with
anhydrous Na2SO4. The solvent was removed and the solid
obtained was purified by column chromatography using
hexane/ethyl acetate (9:1) affording 6.84 g (75%) of a white
solid. Mp: 53.4-55.1 °C (lit. 54 °C).19 IR (KBr): 2955, 2927,
2873, 2849, 1759 (CdO), 1600, 1517, 1468, 1233, 1206, 1019. 1H
NMR (CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz, 6H, CH3), 1.23-1.36
(m, 32H, -CH2-), 1.45 (m, 4H, -CH2-), 1.78 (m, 4H, -CH2-
CH2O-), 2.27 (s, 3H, -OCOCH3), 3.95 (m, 4H, -CH2O-),
6.59 (dd, J=8.6 Hz and 2.7 Hz, 1H, Ar-H), 6.61 (d, J=2.7 Hz,
1H, Ar-H), 6.84 (d, J=8.6 Hz, 1H, Ar-H). 13C NMR (CDCl3),
δ, ppm: 14.36, 22.93, 26.19, 26.23, 26.46, 29.26, 29.39, 29.60,
29.64, 29.84, 29.86, 29.89, 29.93, 32.16, 69.26, 69.46. 111.74,
112.55, 123.42, 130.45, 149.04, 153.75.
3,4-Didodecyloxyphenol (6). First, 6.45 g (12.8 mmol) of
1-acetoxyl-3,4-didodecyloxybenzene (5) and 100 mL of metha-
nol were heated to reflux, and 5.0 mL of concentrated hydro-
chloric acid was slowly added. The reflux was maintained until
complete conversion of the reagent as verified by TLC (approxi-
mately 7 h). The solvent was then removed and the crude solid
obtained dissolved in chloroform. The organic phase was
washed with distilled water (2 ꢀ 100 mL), dried over anhydrous
Na2SO4 and the solvent evaporated. Recrystallization on
hexane afforded 5.49 g (93%) of a light-gray solid. Mp:
81.2-83.5 °C (lit. 78 °C).19 IR (KBr): 3290 (O-H), 2919,
2849, 1610, 1516, 1464, 1379, 1222, 1167, 1130, 824. 1H NMR
(CDCl3), δ, ppm: 0.88 (t, J=6.6 Hz, 6H, CH3), 1.24-1.35 (m,
32H, -CH2-), 1.44 (m, 4H, -CH2-), 1.77 (m, 4H, -CH2-
CH2O-), 3.92 (m, 4H, -CH2O-), 5.30 (br, 1H, Ar-OH), 6.30
(dd, J=8.6 Hz and 2.3 Hz, 1H, Ar-H), 6.44 (d, J=2.3 Hz, 1H,
Ar-H), 6.75 (d, J=8.6 Hz, 1H, Ar-H). 13C NMR (CDCl3), δ,
ppm: 14.36, 22.93, 26.28, 29.43, 29.61, 29.66, 29.72, 29.76, 29.88,
29.95, 32.16, 69.21, 71.04, 102.47, 106.33, 116.60, 153.18, 150.69,
150.77. Anal. Calcd for C30H54O3: C, 77.87; H, 11.76. Found: C,
77.61; H, 11.88.
1.25-1.35 (m, 16H, -CH2-), 1.43 (m, 2H, -CH2-), 1.74 (qui,
J = 6.6 Hz, 2H, -CH2CH2O-), 3.88 (t, J = 6.6 Hz, 2H,
-CH2O-), 6.64 (d, J=8.8 Hz, 2H, Ar-H), 6.74 (d, J=8.8
Hz, 2H, Ar-H). 13C NMR (CDCl3), δ, ppm: 14.37, 22.93, 26.30,
29.59, 29.67, 29.83, 29.85, 29.88, 29.90, 32.16, 68.94, 115.88,
116.67, 139.96, 152.58. Anal. Calcd for C18H31NO: C, 77.92; H,
11.26; N, 5.05. Found: C, 77.46; H, 11.24; N, 4.85.
3,4-Didodecyloxynitrobenzene (9). This was prepared through
a modified procedure given in the literature.18 First, 4.46 g (10.0
mmol) of 1,2-didodecyloxybenzene (3), 0.097 g (1.4 mmol) of
NaNO2 and 20 mL of dichloromethane were mixed and cooled
to -5 °C, and 2.0 mL of HNO3 (70%) was added dropwise. The
solution was then stirred for 5 min at -5 °C and 1 h at room
temperature and was then poured into 100 mL of cold distilled
water. The organic phase was extracted with CH2Cl2 (4 ꢀ 100
mL). The combined organic phases were washed with NaHCO3
saturated solution (1 ꢀ 100 mL), H2O (2 ꢀ 100 mL), and brine (1
ꢀ 100 mL) and dried over anhydrous Na2SO4. After removal of
the solvent, the solid was recrystallized over ethanol, yielding
3.7 g (75%) of an off-white solid. Mp: 72.5-74.8 °C (lit. 73 -
75 °C).18 IR (KBr): 2955, 2915, 2848, 2363, 1585, 1519, 1497,
1466, 1402, 1350, 1284, 1233, 1140, 1100, 994, 876, 744. 1H
NMR (CDCl3) δ ppm: 0.88 (t, J=6.7 Hz, 6H, CH3), 1.23-1.38
(m, 32H, -CH2-), 1.47 (m, 4H, -CH2-), 1.85 (m, 4H,
-CH2CH2O-), 4.07 (m, 4H, -CH2O-), 6.87 (d, J=8.8 Hz,
1H, Ar-H), 7.73 (d, J=2.7 Hz, 1H, Ar-H), 7.87 (dd, J=8.8 Hz
and 2.7 Hz, 1H, Ar-H). 13C NMR (CDCl3) δ ppm: 14.37, 22.93,
26.14, 26.18, 29.13, 29.17, 29.57, 29.59, 29.61, 29.84, 29.90,
29.92, 32.16, 69.64, 69.68. 108.21, 111.20, 117.90, 141.37,
148.85, 154.90.
3,4-Didodecyloxyaniline (10). In a appropriate hydrogenation
flask, 2.3 g (4.7 mmol) of 3,4-didodecyloxynitrobenzene (9) was
added together with 100 mL of THF and 0.5 g of 10% Pd/C
catalyst. The hydrogenation process was maintained for 20 h.
After this period the catalyst was removed by filtration, the
solvent evaporated and the crude product purified by column
chromatography using hexane/ethyl acetate (9:1) as the eluent,
obtaining 2.03 g (93%) of a gray solid. Mp: 52.7-53.6 °C (lit. 56
- 58 °C).18 IR (KBr): 3301 and 3208 (N-H prim.), 2917, 2849,
1594, 1523, 1468, 1389, 1279, 1231, 1185, 1121, 1070, 998, 824,
790, 720, 604. 1H NMR (CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz, 6H,
CH3), 1.24-1.34 (m, 32H, -CH2-), 1.44 (m, 4H, -CH2-), 1.76
(m, 4H, -CH2CH2O-), 3.91 (m, 4H, -CH2O-), 6.20 (dd, J=
8.4 Hz and 2.7 Hz, 1H, Ar-H), 6.30 (d, J=2.7 Hz, 1H, Ar-H),
6.73 (d, J=8.4 Hz, 1H, Ar-H). 13C NMR (CDCl3), δ, ppm:
14.36, 22.93, 26.31, 29.56, 29.61, 29.68, 29.74, 29.85, 29.88,
29.91, 29.95, 32.17, 69.18, 71.19, 102.86, 107.05, 117.53,
141.36, 142.19, 150.82. Anal. Calcd for C30H55NO2: C, 78.03;
H, 12.01; N, 3.03. Found: C, 77.72; H, 11.86; N, 2.96.
3,4-Didodecyloxybenzonitrile (11). First, 5.07 g (37.0 mmol)
of 3,4-dihydroxybenzonitrile, 22.0 mL (92.5 mmol) of 1-bromo-
dodecane, 25.19 g (181.2 mmol) of K2CO3, 0.49 g (1.55 mmol) of
TBAB, and 150 mL of butanone were mixed and refluxed for
31 h. After this period, the suspension was filtered and washed
with hot butanone. The solvent was removed and the obtained
solid dissolved in dichloromethane. The organic phase was
washed with NaOH (5%) (1 ꢀ 50 mL), HCl (5%) (1 ꢀ 50 mL),
and H2O (1 ꢀ 50 mL) and dried over anhydrous N2SO4. The solid
obtained after removal of the solvent was recrystallized over
acetonitrile, affording 15.8 g (91%) of a white solid. Mp:
79.8-82.8 °C (lit. 81-83 °C).20 IR (KBr): 2954, 2918, 2872,
2849, 2221 (CtN), 1597, 1581, 1519, 1468, 1422, 1280, 1244,
1139, 992, 812, 722. 1H NMR(CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz,
6H, CH3), 1.25-1.36 m, 32H, -CH2-), 1.46 (m, 4H, -CH2-),
1.83 (m, 4H, -CH2CH2O-), 3.98 (t, J=6.6 Hz, 2H, -CH2O-),
4.02 (t, J = 6.6 Hz, 2H, -CH2O-), 6.86 (d, J = 8.4 Hz, 1H,
Ar-H), 7.07 (d, J=1.9 Hz, 1H, Ar-H), 7.23 (dd, J=8.4 Hz and
1.9 Hz, 1H, Ar-H). 13C NMR (DMSO-d6, -90 °C), δ, ppm:
12.31, 20.59, 24.07, 27.23, 27.27, 27.51, 27.56, 27.61, 29.87, 67.61,
68.05, 107.97, 112.88, 115.84, 117.59, 125.13, 147.72, 151.81.
4-Dodecyloxyacetanilide (7). First, 4.00 g (26.5 mmol) of
4-hydroxyacetanilide, 9.6 mL (40 mmol) of 1-bromododecane,
7.31 g (53 mmol) of K2CO3 and 80 mL of butanone were mixed
and refluxed for 24 h. The suspension was then filtered and
washed 2 times with hot butanone. The solvent was removed by
rotary evaporation and the solid obtained recrystallized from
heptane to afford 8.3 g of a white solid (98%). Mp: 92.1-94.3
°C. IR (KBr): 3312 (CON-H), 2921, 2849, 1661 (CdO), 1608,
1
1548, 1509, 1466, 1406, 1368, 1307, 1237, 1170, 1030, 826. H
NMR (CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz, 3H, CH3), 1.25-1.35
(m, 16H, -CH2-), 1.43 (m, 4H, -CH2-), 1.76 (qui, J=6.6 Hz,
2H, -CH2CH2O-), 2.17 (s, 3H, -COCH3), 3.91 (t, J=6.6 Hz,
2H, -CH2O-), 6.83 (d, J=9.0 Hz, 2H, Ar-H), 7.37 (d, J=9.0
Hz, 2H, Ar-H), 7.47 (br, 1H, Ar-NH-). 13C NMR (CDCl3),
δ, ppm: 14.36, 22.93, 26.26, 29.50, 29.58, 29.63, 29.81, 29.83,
29.87, 29.89, 32.15, 68.52, 114.99, 121.21, 130.78, 156.36, 168.67,
187.46.
4-Dodecyloxyaniline (8). A 5.00 g (15.7 mmol) sample of
4-dodecyloxyacetanilide (7) and 50 mL of H2O were heated to
reflux and 25.0 mL of concentrated hydrochloric acid was
slowly added. The reflux was maintained for 24 h. After this
period, the solution was basified with NaOH aqueous solution
(1M) and the suspension filtered and washed with plenty of
water. The solid obtained was air-dried, affording 4.3 g (99%).
Purification was not necessary. Mp: 56.2-58.3 °C. IR (KBr):
3383 and 3312 (N-H prim.), 2918, 2851, 1610, 1516, 1468, 1390,
1248. 1H NMR (CDCl3), δ, ppm: 0.88 (t, J=6.7 Hz, 3H, CH3),

1322 Macromolecules, Vol. 43, No. 3, 2010
Westphal et al.
5-(3,4-Didodecyloxyphenyl)tetrazole (12). First, 5.00 g (10.6
mmol) of 3,4-didodecyloxybenzonitrile (11), 2.07 g (31.8 mmol)
of NaN3, 1.70 g (31.8 mmol) of NH4Cl, and 30 mL of DMF were
mixed and refluxed for 24 h. After this period, the suspension
was cooled to room temperature and poured into 200 mL of ice/
water and acidified to pH 3 with an HCl aqueous solution
(10%). The solid obtained was filtered, washed with plenty of
water and recrystallized over acetone, affording 5.05 g (93%) of
an off-white solid. Mp: 157.8-159.0 °C. IR (KBr): 2921, 2848,
2744, 2613, 1607, 1512, 1465, 1272, 1239, 1133, 1039, 812, 746.
1H NMR (pyridine-d5), δ, ppm: 0.86 (t, J=6.7 Hz, 6H, CH3),
1.25 (m, 32H, -CH2-), 1.47 (m, 4H, -CH2-), 1.80 (m, 4H,
-CH2CH2O-), 3.96 (t, J = 6.6 Hz, 2H, -CH2O-), 4.05 (t,
J=6.6 Hz, 2H, -CH2O-), 7.16 (s, 1H partially overlapped with
solvent signal, Ar-H), 7.95 (s, 1H, Ar-H), 7.98 (s, 1H, Ar-H).
13C NMR (DMSO-d6, -90 °C), δ, ppm: 14.35, 22.64, 26.22,
29.29, 29.40, 29.63, 31.92, 69.72, 70.04, 114.02, 115.44, 117.76,
121.28, 150.05, 152.22. Anal. Calcd for C31H54N4O2: C, 72.33;
H, 10.57; N, 10.88. Found: C, 72.61; H, 10.39; N, 10.43.
isoph.). Anal. Calcd for C88H126N6O8: C, 75.71; H, 9.10; N,
6.02. Found: C, 75.14; H, 9.75; N, 6.08.
(E)-Tetrakis(3,4-didodecyloxyphenyl) 5,50-(Diazene-1,2-diyl)-
diisophthalamide (AMD8). The product was recrystallized three
times over butanone and cold filtrated, affording 1.01 g (68%).
Mp: (liquid crystal) Cr-126.5 °C-Colh-221.5 °C-I. IR (KBr):
3275 (CON-H), 2956, 2923, 2870, 2853, 1652 (CdO), 1607,
1539, 1514, 1467, 1427, 1407, 1389, 1289, 1258, 1230, 1134, 1021,
1
803, 720. H NMR (CDCl3, -50 °C), δ, ppm: 0.89 (m, 24H,
CH3), 1.19-1.46 (br, 144H, -CH2-), 1.75 (br, 16H,
-CH2CH2O-), 3.89 (br, 16H, -CH2O-), 6.72 (br, 4H, Ar-H),
7.19 (br, 4H, Ar-H), 7.52 (br, 4H, Ar-H), 8.29 (br, 4H, 4,6-H
isoph.), 8.54 (br, 2H, 2-H isoph.), 9.25 (br, 4H, -CONH-Ar).
Anal. Calcd for C136H222N6O12: C, 76.57; H, 10.49; N, 3.94.
Found: C, 76.40; H, 10.21; N, 4.05.
(E)-1,2-Bis{3,5-bis[5-(3,4-didodecyloxyphenyl)-1,3,4-oxadiazol-
2-yl]phenyl}diazene (OXD8). The product was macerated over
hot ethanol and recrystallized twice over toluene/acetonitrile,
affording 1.31 g (84%). Mp: (liquid crystal) Cr-102.0 °C-
Colh-236.6 °C-I. IR (KBr): 2956, 2921, 2871, 2851, 1604, 1554,
1497, 1466, 1424, 1391, 1271, 1219, 1139, 735. 1H NMR
(CDCl3), δ, ppm: 0.87 (t, J = 6.7 Hz, 24H, CH3), 1.23-1.44
(m, 128H, -CH2-), 1.52 (m, 16H, -CH2-), 1.88 (m, 16H,
-CH2CH2O-), 4.12 (m, 16H, -CH2O-), 7.02 (d, J=8.6 Hz,
4H, Ar-H), 7.73 (s, 4H, Ar-H), 7.78 (d, J=8.6 Hz, 4H, Ar-H),
8.89 (s, 4H, 4,6-H isoph.), 9.00 (s, 2H, 2-H isoph.). 13C NMR
(CDCl3), δ, ppm: 14.40, 22.98, 26.33, 29.41, 29.53, 29.67, 29.75,
29.96, 32.22, 69.41, 69.77, 111.86, 113.06, 115.90, 121.12, 123.98,
126.76, 149.71, 152.95, 162.95, 165,84. Anal. Calcd for
General Procedure for the Synthesis of the Target Products. In
a round-bottomed flask equipped with a condenser and a drying
tube was inserted 0.25 g (0.70 mmol) of the (E)-5,50-(diazene-1,2-
diyl)diisophthalic acid (1), 5 mL of SOCl2 and 1 drop of DMF
were placed inside, and the mixture was refluxed for 18 h. The
remaining SOCl2 was removed by vacuum distillation. After
this, 4.2 equiv of the desired intermediate (2, 6, 8, 10, or 12) were
added together with 20 mL of dry pyridine and the mixture
refluxed for a further 48 h. The solution was then cooled to room
temperature and poured into 300 mL of ice/water. The precipi-
tate was filtered, washed with plenty of water, and purified.
C140H218N10O12: C, 75.29; H, 9.84; N, 6.27. Found: C, 75.53;
H, 9.58; N, 6.13.
(E)-Tetrakis(4-dodecyloxyphenyl)
5,50-(Diazene-1,2-diyl)-
2.2. Thermal Analysis. Thermal transitions and enthalpies
were determined by DSC measurements carried out using
Shimadzu equipment with a DSC-50 module using a heating/
diisophthalate (EST4). The product was macerated over hot
ethanol and recrystallized over toluene, affording 0.88 g (90%).
Mp: 166.9-167.8 °C. IR (KBr): 2953, 2918, 2872, 2850, 1752
cooling rate of 10 °C min-1 and a nitrogen flow of 50 mL min-1
.
1
and 1740 (CdO), 1509, 1468, 1243, 1156, 830, 744. H NMR
TGA measurements were carried out using Shimadzu equip-
ment with a TGA-50 module using a heating rate of 10 °C min-1
and nitrogen flow of 50 mL min-1. Mesomorphic textures were
determined using an Olympus BX50 microscope equipped with
a Mettler Toledo FP-82 hot stage and a PM-30 exposure control
unit.
(C6D6, -60 °C), δ, ppm: 0.92 (t, J = 6.7 Hz, 12H, CH3),
1.23-1.35 (m, 64H, -CH2-), 1.41 (m, 8H, -CH2-), 1.67
(qui, J=6.6 Hz, 8H, -CH2CH2O-), 3.69 (t, J=6.6 Hz, 8H,
-CH2O-), 6.83 (d, J=9.2 Hz, 8H, Ar-H), 7.14 (d, J=9.2 Hz,
8H, Ar-H), 9.12 (d, J=1.6 Hz, 4H, 4,6-H isoph.), 9.36 (t, J=1.6
Hz, 2H, 2-H isoph.). 13C NMR (CDCl3), δ, ppm: 14.37, 22.93,
26.28, 29.50, 29.59, 29.63, 29.62, 29.84, 29.88, 29.90, 32.16,
68.69, 115.42, 122.32, 122.46, 129.21, 132.03, 144.14, 152.71,
157.41, 164.21. Anal. Calcd for C88H122N2O12: C, 75.50; H,
8.78; N, 2.00. Found: C, 75.52; H, 9.05; N, 2.10.
(E)-Tetrakis(3,4-didodecyloxyphenyl) 5,50-(Diazene-1,2-diyl)-
diisophthalate (EST8). The product was recrystallized twice over
2-propanol, affording 1.10 g (73%). Mp: 153.8-154.9 °C. IR
(KBr): 2955, 2923, 2870, 2850, 1739 (CdO), 1605, 1509, 1468,
1427, 1391, 1261, 1215, 1183, 1125. 1H NMR (CDCl3), δ, ppm:
0.87 (t, J=6.7 Hz, 24H, CH3), 1.20-1.35 (m, 128H, -CH2-),
1.47 (m, 16H, -CH2-), 1.83 (m, 16H, -CH2CH2O-), 4.00
(m, 16H, -CH2O-), 6.80 (d, J = 8.4 Hz, 4H, Ar-H), 6.82
(s, 4H, Ar-H), 6.93 (d, J=8.4 Hz, 4H, Ar-H), 9.01 (s, 4H,
4,6-H isoph.), 9.13 (s, 2H, 2-H isoph.). 13C NMR (CDCl3), δ,
ppm: 14.35, 22.92, 22.93, 26.26, 26.29, 29.40, 29.59, 29.62,
29.65, 29.69, 29.87, 29.89, 29.94, 29.95, 32.16, 32.27,
69.54, 70.07, 107.73, 113.10, 114.38, 129.19, 132.03, 134.16,
144.57, 147.45, 150.09, 152.73, 164.14. Anal. Calcd for
2.3. XRD Analysis. X-ray diffraction experiments were
carried out with the X’PERT-PRO (PANalytical) diffracto-
˚
meter using Cu KR radiation (λ=1.5418 A) with an applied
power of 1.2 kVA. The scans were performed in continuous
mode from 2° to 30° (2θ angle) and the diffracted radiation
collected with an X’Celerator detector. The samples were pre-
pared by heating a certain amount of the desired compound on a
glass plate until the compound melted to the liquid state,
followed by cooling to room temperature, resulting in a film
approximately 1 mm thick. The films were then placed in the
diffractometer chamber on the TCU2000 temperature control
unit (Anton Paar), which allows a precise control of the sample
temperature during the measurement. The films were first
heated until the isotropic phase and the diffraction patterns
collected after cooling back to the mesophase.
2.4. UV-Vis Spectroscopic Measurements and Photoisomeri-
zation. The UV-vis absorption spectra were recorded using an
HP UV-vis model 8453 spectrophotometer. The samples were
dissolved in spectroscopic chloroform and maintained in the
dark for 48 h before the spectra were recorded. For the photo-
isomerization analysis a UVP 4W lamp model UVGL-25 was
used. The excitation band used has a maximum at 356 nm and a
width at half-maximum of 32 nm. All the optical analyses were
performed at room temperature (25 °C).
C136H218N2O16: C, 76.43; H, 10.28; N, 1.31. Found: C, 76.12;
H, 10.35; N, 1.26.
(E)-Tetrakis(4-dodecyloxyphenyl)
5,50-(Diazene-1,2-diyl)-
diisophthalamide (AMD4). The product was recrystallized once
over butanone and once over methylisobutilketone, affording
0.76 g (78%). Mp: 241.0 °C. IR (KBr): 3288 (CON-H), 2953,
2922, 2871, 2853, 1650 (CdO), 1611, 1535, 1513, 1468, 1413,
1242, 1171, 825. 1H NMR (CDCl3), δ, ppm: 0.88 (s, 12H, CH3),
1.19-1.49 (br, 64H, -CH2-), 1.75 (br, 8H, -CH2CH2O-),
3.98 (br, 8H, -CH2O-), 7.12 (br, 8H, Ar-H), 7.39 (br, 4H,
-CONH-Ar), 7.70 (br, 8H, Ar-H), 8.23-8.54 (br, 6H, 2,4,6-H
3. Results and Discussion
3.1. Synthesis. As can be seen in Figure 2, all the desired
discotic compounds have a common center, which is a

Article
Macromolecules, Vol. 43, No. 3, 2010 1323
diphenyldiazene unit. To take advantage of this structural
similarity, we opted for a convergent type of synthesis.
Initially, all the intermediates were prepared and fully char-
acterized, and in the final step, they were reacted to obtain
the target molecules. The diphenyldiazene-derived com-
pound (1) was prepared by a reductive coupling of 5-nitroi-
sophthalic acid (Scheme 1). For this purpose, we used a mild
reductive agent, such as dextrose in a strongly alkaline water
solution.
The synthesis of the 5-arm types used for the preparation
of the different target compounds is outlined in Scheme 2.
The first type (2) was prepared by a simple monoalkylation
of hydroquinone. The second, 3,4-didodecyloxyphenol (6),
was synthesized by a more elaborated route, starting with an
alkylation of catechol followed by a Friedel-Crafts acyla-
tion using aluminum trichloride. In the next step, 4 was
converted to compound 5 by a Baeyer-Villiger oxidation
using 3-chloroperbenzoic acid (MCPBA). A final acid
hydrolysis afforded the desired phenol 6.
(10) was prepared by a mononitration of 3, via two-phase
nitration, followed by reduction of the nitro group by
catalytic hydrogenation using a 10% Pd/C catalyst. Finally,
the fifth and final intermediate was obtained by initial
alkylation of 3,4-dihydroxybenzonitrile using tetrabutyl-
ammonium bromide (TBAB) as the phase-transfer catalyst,
followed by a Huisgen 1,3-dipolar cycloaddition over the
nitrile group,21 converting it to the tetrazole heterocycle.
Scheme 3 outlines the method used to obtain the desired
discotic molecules. Initially the (E)-5,50-(diazene-1,2-diyl)-
diisophthalic acid (1) was converted to the respective acid
chloride using SOCl2 and a catalytic amount of DMF. The
acid chloride obtained was then refluxed in pyridine with the
appropriate intermediate (2, 6, 8, 10, or 12), resulting in the
desired final products with good yields (68 to 90%).
The structures of all the compounds were characterized by
IR and 1H and 13C NMR spectra and elemental analysis, and
in the case of compound 1 also the mass spectrum.
3.2. Thermal and Mesomorphic Behavior. The thermal
behavior of the compounds EST4, EST8, AMD4, AMD8,
and OXD8 was investigated by DSC, POM and TGA, and
the results are summarized in Table 1. Two of the five
molecules synthesized showed liquid crystal properties. The
DSC analysis showed a typical columnar phase transition,
crystal-to-columnar-to-isotropic (Cr f Col f I). Com-
pounds AMD8 and OXD8 exhibited an enantiotropic beha-
vior. AMD8 melts to a mesophase at 126.5 °C and to
isotropic liquid at 221.5 °C. In a similar way, OXD8 also
presents a very stable mesophase, ranging from 102.0 to
236.6 °C. In both cases the mesophase was characterized by
POM as a discotic hexagonal columnar phase (Colh). The
textures observed after cooling from isotropic liquid can be
observed in Figure 3.
The first intermediate containing the amino group (8) was
obtained by an O-alkylation of 4-hydroxyacetanilide fol-
lowed by an acid hydrolysis. The two-chain aniline derivative
Scheme 1. Synthesis of Compound 1a
a Conditions (yields): (a) (i) Dextrose, NaOH, H2O, 60 °C; (ii) H3Oþ
(61%).
Scheme 2. Synthesis of the Intermediates 2, 6, 8, 10, and 12a
a Conditions (yields): (a) (i) 1-bromododecane, NaOH, MeOH/H2O, reflux; (ii) H3Oþ (55%); (b) 1-bromododecane, K2CO3, KI, butanone, reflux
(52%); (c) AlCl3, acetyl chloride, dry CH2Cl2, -5 °C (83%); (d) MCPBA, CF3COOH, dry CH2Cl2, 0 °C (75%); (e) HClconc., MeOH, reflux (93%); (f )
1-bromododecane, K2CO3, butanone, reflux (98%); (g) (i) HClconc, H2O, reflux; (ii) NaOH (99%); (h) NaNO2, HNO3 70%, CH2Cl2, -5 °C (75%); (i)
H2, Pd/C 10%, THF, rt (93%); (j) 1-bromododecane, K2CO3, TBAB, butanone, reflux (91%); (k) NaN3, NH4Cl, DMF, reflux (93%).

1324 Macromolecules, Vol. 43, No. 3, 2010
Scheme 3. Synthesis of Final Products (EST4, EST8, AMD4, AMD8, and OXD8)a
Westphal et al.
a Conditions (yields): (a) SOCl2, DMFcat., reflux (not isolated); (b) intermediate 2, dry pyridine, reflux (90%); (c) intermediate 6, dry pyridine, reflux
(73%); (d) intermediate 8, dry pyridine, reflux (78%); (e) intermediate 10, dry pyridine, reflux (68%); (f) intermediate 12, dry pyridine, reflux (84%).
Table 1. Phase Transitions, Enthalpies, and Thermal Stability of
Target Compounds (Temperatures Presented in °C and ΔH in
Compounds AMD8 and OXD8 displayed a pseudo focal-
conic optical texture with linear birefringent defects
(Figure 3), suggesting hexagonal columnar structures. A
large area of homeotropic domains was observed, suggesting
a Colh phase with a preferentially uniaxial character. A
variable temperature XRD experiment confirmed the Colh
phase.
kJ mol-1
)
T (ΔH)a
heating
T (-ΔH)a
cooling
b
compound
transitions
Tdec.
EST4
Cr-Cr0
Cr0-I
132.5 (64.5)
167.2 (69.5)
-
159.2 (70.8)
353
EST4 and AMD4 have a thermal property of an ordinary
solid, melting directly to liquid state. This difference of
behavior in comparison with AMD8 and OXD8 can be
explained by the number of aliphatic chains present at the
molecule periphery. In this regard, 4 aliphatic chains seems
to be a low number in terms of achieving a good packing and
stabilization of a columnar mesophase. On the other hand,
EST8 has the same number of aliphatic chains as AMD8 and
OXD8 but, as in the case of EST4, it does not have liquid
crystal properties. This could be the result of a lower degree
of planarity due to the presence of ester groups. This lack of
planarity difficult the molecular packing, preventing the
assembling in a columnar mesophase. This is confirmed by
comparing the liquid state transition temperatures, where
EST8 has a lower transition temperature than AMD8 and
OXD8 (in a similar way, EST4 has a lower melting point than
AMD4). Also, the lack of planarity hinders the solidification
EST8
Cr-Cr000
Cr000-Cr
Cr -Cr000
Cr000-I
51.2 (3.2)c
67.0 (32.8)c
108.4 (4.4)
154.4 (0.9)
-
-
-
-
341
AMD4
AMD8
OXD8
Cr-Cr0
Cr0-I
195.1 (36.8)
241.0 (42.8)
171.8 (16.2)
230.6 (34.8)
360
388
384
Cr-Colh
Colh-I
126.5 (11.1)
221.5 (25.0)
-
196.2 (3.9)
Cr-Cr0
Cr0-Colh
Colh-I
83.9 (39.0)c
102.0 (11.9)c
236.6 (5.4)
-
-
223.8 (2.8)
a Determined by DSC using a heating/cooling rate of 10 °C min-1
.
b Determined by TGA using a heating rate of 10 °C min-1. Onset of
decomposition under nitrogen. c Estimated values by deconvolution of
overlapped bands.

Article
Macromolecules, Vol. 43, No. 3, 2010 1325
Figure 3. Optical textures observed under crossed polarizers. Photo-
micrographs showing hexagonal columnar mesophase textures. Parts a
and b show compound AMD8 at 205.3 and 209.5 °C, respectively
(200ꢀ). Parts c, d, e and f show compound OXD8 at 221.0, 227.9, 225.2,
and 215.3 °C, respectively (100ꢀ).
of the material, which is noted by the absence of a defined
exothermic peak on cooling in the DSC. Furthermore,
AMD8 contains four amide groups, which enhance the
attractive interactions between the discotic mesogens by
intermolecular hydrogen bonding. Amide groups have al-
ready been used by several research groups to obtain stabi-
lized columnar phases.22
Figure 4. X-ray diffraction patterns of hexagonal columnar mesophase
of OXD8 at 150 °C (upper) and AMD8 at 200 °C (lower).
Table 2. XRD data for OXD8 and AMD8 at Mesophase Temperature
˚
dobs(A)
˚
dcal (A)
˚
cell const (A)
comp (temp.)
(hkl)
OXD8
(150 °C)
34.6
17.5
≈4.6
3.4
(100)
(200)
broad
(001)
34.8
17.4
R = 40.2a
XRD experiments were performed to confirm the Colh
mesophases of compounds AMD8 and OXD8 observed in
the POM and to obtain their structural parameters. In the
spectra obtained, as shown in Figure 4, there is a well-defined
peak (100) at the low angle region for both compounds.
Typical XR diffractograms of well organized Colh meso-
phases show the following order peaks related to the Miller
Indices (100), (110) and (200), whose ratios obey the relation
1:(3)1/2:2 according to the first peak.9,23 Here, compound
OXD8 presented only the (200) reflection, while compound
AMD8 presented neither the (110) nor (200) reflections,
indicating a weak correlation order. However, as the disk-
like structure of the molecules favors columnar packing
resulting in columnar phases due to their long-range stacking
order, the existence of only one fundamental Bragg reflection
at the low angle region suggests that the only possibility is the
hexagonal symmetry. In such case, 2D rectangular and
oblique lattices are excluded.24
AMD8
(200 °C)
30.8
≈4.6
(100)
broad
30.8
R = 35.6b
√
a R = (2/n 3)(d100 þ 2d200), where n = 2. b Calculated without d200
and n = 1.
The mesophase conformation parameters obtained from
the XRD data are given in Table 2. It can also be noted that
the molecular diameter for both mesogens is between the
minimum and maximum values estimated for these com-
pounds25 (23.6 A/53.7 A and 21.9 A/51.3 A for OXD8 and
AMD8, respectively). This indicates that the aliphatic chains
are not in the most extended form and/or that there is an
interdigitation of the aliphatic chains of the molecules in
different columns. In both compounds, even after cooling to
ambient temperature, the diffractograms are very similar
and also the absence of new peaks supports the lack of
˚
˚
˚
˚
Figure 5. Optical absorption spectra of compounds (0) EST4; (O)
EST8; (g) AMD4; (4) AMD8 and ()) OXD8) in CHCl3 at room
temperature. The concentrations of the solutions were 5.0 ꢀ 10-5 mol
L
-1, except for OXD8 (5.0 ꢀ 10-6 mol L-1).

1326 Macromolecules, Vol. 43, No. 3, 2010
Westphal et al.
crystallinity of these materials (see Supporting Information).
The easy rotation around the ester group, difficult a colum-
nar packing of the mesogens, resulting in the absence of
liquid crystalline behavior for EST8 and EST4.
Both diffractograms shown in Figure 4 have a broad
˚
diffuse band at 4.6 A, which is due to a liquid-like order
between the aliphatic chains.24,26 The absence of the (001)
Bragg’s reflection for AMD8, when compared with OXD8
˚
(around 3.4 A), indicates a lesser periodicity in the columnar
packing of the former, which can also be noted through a
small difference in the texture of the mesophase of the two
compounds.
3.3. Optical Behavior and Photoisomerization. The optical
absorbance of the target compounds in CHCl3 solution are
shown in Figure 5 and summarized in Table 3. The concen-
trations of the solutions are given in the figure and also in the
table. The respective solutions utilized were maintained in
the dark for 48 h before the analysis. Thus, we can infer that
the compounds were exclusively in the trans configuration.
According to the data presented, we observe that the
addition of alkoxy chains promotes two different effects,
Figure 6. Photoisomerization of compound EST8 in chloroform
solution (5.0 ꢀ 10-5 mol L-1) at room temperature. The upper right
corner time represents the irradiation time at 356 nm. The inset
illustrates the small increase of absorption in 430 nm and also the
isosbestic point.
Figure 7. Evidence of photoisomerization (trans-cis) and thermal isomerization (cis-trans) of compound EST4 observed through 1H NMR using
C6D6 has solvent. (a) NMR obtained at room temperature. (b) Same sample after 10 min of irradiation at 356 nm at room temperature. (c) NMR
recorded at 60 °C for the same sample.

Article
Macromolecules, Vol. 43, No. 3, 2010 1327
Table 3. UV-Vis Absorption Spectroscopic Data at Room Tem-
perature (Solvents Used: CHCl3)
alkoxy chain (-CH2O-). After this, the sample (kept inside
the NMR tube) was maintained under irradiation for 10 min,
at 356 nm, and the spectrum was recorded at room tempera-
ture (Figure 7b). Now, we observe a duplication of the
aromatic and aliphatic hydrogens. The formation of the cis
isomer is the result of the photoisomerization reaction
provoked by light irradiation at 356 nm. When the same
sample was heated and its NMR spectrum recorded at 60 °C,
the duplicated peaks disappeared (Figure 7c). The heating
reverses the photoisomerization, promoting an equilibrium
shift toward the stable trans configuration, which is the
thermodynamically more stable form. In general, not all
peaks were duplicated, only the hydrogens closer to the N=
N bond. The closer the azo bond, the bigger the peak shift.
Clearly, the hydrogens at the end of the aliphatic chains do
not show a significant difference in the chemical shift.28
a
compd
λmax (nm)
ꢀ
εmax
concnb
EST4
282
309
2.4 ꢀ 104
5.0 ꢀ 10-5
2.4 ꢀ 104
EST8
292
312c
3.7 ꢀ 104
5.0 ꢀ 10-5
AMD4
AMD8
OXD8
301
293
4.0 ꢀ 104
5.6 ꢀ 104
5.0 ꢀ 10-5
5.0 ꢀ 10-5
5.0 ꢀ 10-6
323
253
8.9 ꢀ 104
6.8 ꢀ 104
a Units = L mol-1 cm-1 b Units = mol L-1 c Shoulder.
. .
but both attributable to the increase of the electronic density
in the benzene ring to which they are bonded. The first effect
is a shift of the absorption bands, being more pronounced in
the aromatic ring than in the azo band. In the EST group, the
addition of alkoxy chains causes a bathochromic shift,
promoting the superposition of these absorption bands in
the case of the EST8 compound, as observed in Figure 5.
This superposition, as already mentioned, is derived from the
bathochromic shift in the benzene absorption band from the
EST4 (282 nm) to EST8 (292 nm), while the azo absorption
band stays practically unaltered (from 309 to 312 nm). On
another hand, the same addition promotes a small hypso-
chromic shift on the absorption bands of the AMD group.
The second effect is a hyperchromic one, increasing the ε
value. In a similar way, the higher degree of planarity and
consequent conjugation effectiveness of the amide functional
group and 1,3,4-oxadiazole heterocycle also promote a
hyperchromic effect over the benzene absorption band.
Although molecules containing the 1,3,4-oxadiazole het-
erocycle are known to exhibit strong blue luminescence,27
OXD8 does not show luminescence in the visible region.
EST4, EST8, AMD4, and AMD8 also did not show lumi-
nescence in visible region.
4. Conclusions
We have designed and synthesized new examples of azo
photoisomerizable discotic molecules. This opens up ways not
only to understand the detailed physical properties of discotic
liquid crystals, but also to design novel electro-optical devices.
Their structural characterization was achieved by FTIR, NMR,
and elemental analysis. All of the compounds had good thermal
stability with decomposition temperatures higher than 341 °C.
The mesomorphic behavior of these compounds was studied by
DSC and POM, indicating that two of the five molecules (AMD8
and OXD8) present liquid crystal mesophases, which were
identified through POM and established via XRD studies as
columnar hexagonal. All the compounds showed trans to cis
photoisomerization under visible light and cis to trans thermal
isomerization. In the case of those with a columnar mesophase,
this property seems to be promising in terms of controlling the
conductivity through the columns, as graphically illustrated
herein.
Acknowledgment. This work was supported by CAPES,
CNPq, FAPESC and INCT-Catalise. The X-ray diffraction
ꢀ
experiments were performed at the Laboratorio de Difrac-ao de
~
Raios-X (LDRX-DF/UFSC).
The photostationary equilibrium capacity of the target
compounds was also analyzed. With this purpose, the same
solution used to obtain the optical absorbance spectra was
used. The solution was irradiated at a wavelength of 356 nm
and the absorbance spectra were measured from time to
time, during 1 h of irradiation. All the compounds showed
trans to cis photoisomerization under light, which is exem-
plified in Figure 6 for compound EST8 (the photoisomeriza-
tion for the other compounds are listed in the Supporting
Information). This isomerization process is confirmed by the
reduction of the absorbance band around 300 nm, attributed
to the trans configuration, and the small increase of the band
at around 430 nm relating to the cis form. We can also
observe that in the first few minutes, the photoisomerization
is fast, with the speed decreasing with time. But even after
half an hour this process continues, and the photostationary
point was not reached after 1 h.
The photoisomerization process was also qualitatively
observed during the 1H NMR analysis of compound
EST4. Initially, a sample of EST4 was prepared in C6D6,
heated to certify that all compound was in the trans config-
uration and the spectrum was recorded at room temperature
(Figure 7a). We observe two peaks around 9.0-9.5 ppm,
attributed to the aromatic hydrogens from the benzene ring
that is bonded to the NdN group, and a AA0XX0 system for
p-phenylene group at 6.8 and 7.2 ppm. Also, we observe a
triplet at 3.6 ppm that is attributed to the hydrogens of the
Supporting Information Available: Figures showing 1H
NMR and 13C NMR spectra of the compounds prepared and
an MS spectrum of 1, DSC, TGA, and the photoisomerization
process of EST4, EST8, AMD4, AMD8, and OXD8. XRD at
room temperature for AMD8 and OXD8. This material is
available free of charge via the Internet at http://pubs.acs.org.
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