Mesogenic 4-(ω-hydroxyalkoxy)-4′-formylazobenzenes

Article abstract of DOI:10.1023/B:RUJO.0000045889.46816.0b

Homologs from a series of 4-(ω-hydroxyalkoxy)-4'-formylbenzenes (C₂,C₃,C₄,C₆ ,C₈) were synthesized. All compounds obtained show properties of thermotropic nematic liquid crystals. The effect of the te

Full text of DOI:10.1023/B:RUJO.0000045889.46816.0b

Russian Journal of Organic Chemistry, Vol. 40, No. 8, 2004, pp. 1113-1116. Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 8, 2004,
pp. 1161-1164.
Original Russian Text Copyright Ó 2004 by Kuvshinova, Zav’yalov, Koifman, Aleksandriiskii, Burmistrov.
Mesogenic 4-(w-Hydroxyalkoxy)-4'-formylazobenzenes
1
1
1
2
2
S.A. Kuvshinova , A.V. Zav’yalov , O.I. Koifman , V.V. Aleksandriiskii , and V.A. Burmistrov
1
Ivanovo State University of Chemical Technology, Ivanovo, Russia
2
Institute of Solutions Chemistry, Russian Academy of Sciences, Ivanovo, 153460 Russia
e-mail: burmistrov@isuct.ru
Received December 20 , 2003
Abstract—Homologs from a series of 4-(w-hydroxyalkoxy)-4'-formylbenzenes (C ,C ,C ,C ,C ) were synthesized.
2
3
4
6
8
All compounds obtained show properties of thermotropic nematic liquid crystals. The effect of the terminal hydroxy
group of the end substituent on the mesomorphous properties of the compounds was analyzed.
Among the known liquid crystals [1] mesogenes with
polar and chemically active substituents hold a special
place. They attract interest due to ability to participate
in strong specific selective interactions that considerably
alter properties of mesophases, and thus these systems
may be regarded as supramolecular liquid crystals [2].
Besides the presence of substituents like OH, COOH,
CHO and others provides a possibility of further chemical
modification of mesogenes aiming at extending the
temperature range of mesophase existence, improving
its thermal stability [3], imparting the liquid crystals
special qualities, in particular, structural selectivity [4].
We formerly prepared a homologous series of 4-alk-
oxy-4'-formylbenzenes [5] possessing nematic and
smectic mesomorphism. A curious feature of the liquid
crystals obtained was the high packing density and
uncommonly strong effect of aliphatic substituents length
on the anisotropy of molecular polarizability of the
mesogenic molecules [6].
In view of the above with the goal to investigate the
effect of the terminal hydroxy group on the
mesomorphous properties of mesogenic aldehydes we
prepared 4-(w-hydroxyalkoxy)-4'-formylbenzenes. The
synthesis was carried out along the scheme:
+
2& + ꢀ1D2+
1+
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1 1
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1
D12 ꢀ+&O
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,,D H
II, n = 2 (a), 3 (b), 4 (c), 6 (d), 8 (e).
1
13
Azobenzenes IIa–e obtained (see table) are orange-
red crystalline compounds that were recrystallized from
ethanol and evacuated till constant temperature of phase
table), IR, H and C NMR spectra. The experimental
1
13
parameters of H and C NMR spectra are fairly
consistent with the characteristics calculated along
programs ACD-Labs.
1
transitions and lack of impurity peaks in their H NMR
spectra.
Temperature of phase transitions of compounds
synthesized was measured by the method of of
polarization thermomicroscopy. The data obtained are
The composition and structure of compounds IIa–e
obtained were confirmed by elemental analysis (see
1070-4280/04/4008-1113Ó2004 MAIK “Nauka/Interperiodica”

1114
KUVSHINOVA et al.
ꢀ
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considerably extending the temperature range of the
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ꢁꢂꢃ
nematic phase. This phenomenon apparently originates
from the strengthening of cohesion interactions both in
the crystal and in the mesophase, and in the latter as seen
from its higher thermostability the additional
intermolecular interactions are essentially anisotropic.
Taking into account the weak orientational ordering of
aliphatic substituents [7] and the orientation of the dipole
moment of the hydroxy group the considerable
contribution of dipole-dipole interactions with its
participation may be excluded. In this case the effect of
the hydroxy substituent may be produced solely by
formation of sufficiently strong intermolecular hydrogen
bonds. Inasmuch as hydroxy and aldehyde groups are
complementary in the H-complexing, as one of arising
supramolecular mesogenic structures may be anticipated
a chain associate A built by the type “head-to-tail”.
However the interaction affording dimers B by the type
,
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,,F ꢀꢁZWꢍ
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Phase diagram of binary mixture of 4-hydroxybutoxy-4'-
formylbenzene (IIc) and 4-hexoxy-4'-formylbenzene (IIId) (I,
N, C are isotropic, nematic, and crystalline phases respectively).
given in the table. The investigation demonstrated that
all azobenzenes IIa–e behave as liquid crystals forming
a nematic phase in sufficiently wide temperature range.
The mesophases were identified by miscibility of
compounds obtained with 4-hexoxy-4'-formylazo-
benzene (IIId) that exhibited nematic liquid crystal
properties in the range 86–95°C [5]. The analysis of phase
diagrams shows that the liquid crystals under study
possess unlimited miscibility in the nematic region (see
figure).
“
tail-to-tail”cannot be disregarded.
Both processes should increase the effective aniso-
tropy of the molecular polarizability and consequently
should result in increased mesophase thermostability. The
disclosure of prevailing mechanism of the specific
intermolecular interaction requires investigation of
anisotropic physical characteristics of liquid crystals
obtained and described in the present article.
The comparison of the properties of compounds
obtained with those of their analogs, 4-alkoxy-4'-
formylazobenzenes [5], (see table) reveals a significant
effect of the terminal hydroxy group on the phase
transition temperature. Therewith the active substituent
stronger increases the temperature of the transition from
the nematic to isotropic state than the melting point thus
EXPERIMENTAL
IR spectra were recorded on Avatar 360FT-IR ESP
instrument from samples pelletized with KBr. NMR
spectra were registered on spectrometer Bruker-AC200
1
at operating frequancies 200.13 ( H) and 50.32 MHz
Yield and characteristics of mesogenic aldehydes HO(CH ) OC H N=NC H CHO (IIa–e)
2
n
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4
6
4
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& + 1 2 ꢁ ꢊꢋꢀꢊꢇꢁ ꢄꢀꢊꢈꢁ
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 8 2004

MESOGENIC 4-(w-HYDROXYALKOXY)-4'-FORMYLAZOBENZENES
1115
+
2
2
1
2
+
+ 2
1
2
1
1
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+
2
2
+
2
+
1
1
2
1
2
1
2
+
%
(13
respect to TMS and were measured from internal
C). Chemical shifts were presented in d-scale with
lized from 80% acetic acid. Yield of bright-orange
crystalline compound I 29.9 g (87%), mp 185°C.
1
13
references TMS ( H) and cyclohexane, 27.6 ppm ( C–
4
-(2-Hydroxyethoxy)-4'-formylazobenzene (IIa). A
mixture of 2.3 g (10 mmol) of compound I, 0.81 g
10 mmol) of ethylene chlorohydrin, and 1.66 g
1
{
H}). The error in recalculation of the chemical shifts
1
3
in the C NMR spectra did not exceed ±0.01 ppm.
(
The phase transition temperature was measured and
textures of compounds obtained were investigated with
the use of polarization microscope Polam P-211 equipped
with a heating block. The phase transition temperatures
were measured with an accuracy ±0.2°C.
(12 mmol) of potassium carbonate in 150 ml of DMF
was heated at reflux at vigorous stirring for 4 h. The hot
reaction mixture was poured into 400 ml of ice water,
the precipitate was filtered off and dried in air. Then it
was passed through a column packed with Al O (eluent
2
3
chloroform) and recrystallized from ethanol. Yield of
orange crystalline compound IIa 2.5 g (92%), mp
4
-Hydroxy-4'-formylazobenzene (I). A suspension
of 18.3 g (150 mmol) of 4-aminobenzaldehyde was
prepared at 0°C in 100 ml of water. At the same
temperature was separately dissolved 9.9 g (160 mmol)
of sodium nitrite in 50 ml of water. To the suspension of
–
1
1
(
(
7
31.7°C. IR spectrum, n, cm : 3415 (OH), 2950, 2869
1
CH), 1698 (C=O). H NMR spectrum, d, ppm: 4.02 t
2H, HOCH ), 4.16 t (2H, CH O), 7.07 d (2H, Ar–H ),
.93 d (2H, Ar–H ), 8.00 s (4H, Ar–H ), 10.08 s (1H,
CH O). C NMR spectrum, d, ppm: 61.25 (HOCH ),
9.53 (CH OAr), 114.85 (C ), 123.04 (C ), 125.34 (C ),
30.67 (C ), 136.93 (C ), 147.17 (C ), 156.04 (C ),
61.85 (C ), 191.69 (C ). Found, %: C 67.31; H 5.84;
4
2
2
5
6,7
4
-aminobenzaldehyde was simultaneously added at
8
13
2
vigorous stirring the solution of sodium nitrite and 50
ml (330 mmol) of 24% hydrochloric acid solution
maintaining the pH of the reaction mixture at 7. The
completion of diazotization was checked by a starch-
iodide paper strip test. The solution of diazo compound
was adjusted to pH 7 by adding 4% NaOH solution. To
the neutral solution of diazo compound at cooling and
stirring was added a cooled to 0°C solution of 14.2 g
4
5
6
6
1
1
2
7
12
10
11
1
3
8
N 9.87. C H N O Calculated, %: C 66.67; H 5.19;
1
5
14
2
3.
N 10.37.
Likewise were prepared compounds IIb–e.
The study was carried out under financial support of
the Russian Foundation of Basic Research (grant 02-03-
32463a) and of program of the Division of Chemistry,
Physics, and Mathematics of the Russian Academy of
Sciences ¹ 10002-251/OkhNM-02/129-134/280703-
1139.
(
150 mmol) of phenol in 50 ml (150 mmol) of 6% NaOH
solution. The end of the azo coupling was checked by a
drop test with sodium phenolate. The reaction product
was precipitated by adding to the reaction mixture 60 g
of NaCl, the precipitate was filtered off, and recrystal-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 8 2004

1116
KUVSHINOVA et al.
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 8 2004