Synthesis and liquid crystalline properties of novel triazene-1-oxide derivatives

Article abstract of DOI:10.1080/15421400802305756

A new group of compounds, C6H5-N(O)=N-NH-C6H4-C(O)-O-C6H4-CH=N-C6H4-OR, (1), [R=n-alkyl group; 1a (R=C7H15 ) to 1h (R=C14H29)], has been designed and synthesized to develop triazene-1-oxide based liquid crystals. All the compounds have been characterized on the basis of their spectral data. Their thermal behavior has been examined and found to exhibit thermotropic liquid crystalline behavior over a wide range of temperature. All members of this family uniformly show nematic phase behavior on melting up to the clearing point during heating as well as on phase entering up to the solidification during cooling. An odd-even effect on transition temperatures has been observed during heating as well as cooling.

Full text of DOI:10.1080/15421400802305756

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Synthesis and Liquid Crystalline
Properties of Novel Triazene-1-
oxide Derivatives
Purak Das a , Achintesh Narayan Biswas a , Pinaki
Bandyopadhyay a & Pradip Kumar Mandal b
a Department of Chemistry , University of North
Bengal , Siliguri, West Bengal, India
b Department of Physics , University of North
Bengal , Siliguri, West Bengal, India
Published online: 22 Sep 2010.
To cite this article: Purak Das , Achintesh Narayan Biswas , Pinaki Bandyopadhyay
& Pradip Kumar Mandal (2008) Synthesis and Liquid Crystalline Properties of Novel
Triazene-1-oxide Derivatives, Molecular Crystals and Liquid Crystals, 490:1, 3-15, DOI:
10.1080/15421400802305756
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Mol. Cryst. Liq. Cryst., Vol. 490, pp. 3–15, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802305756
Synthesis and Liquid Crystalline Properties of Novel
Triazene-1-oxide Derivatives
Purak Das1, Achintesh Narayan Biswas1,
Pinaki Bandyopadhyay1, and Pradip Kumar Mandal2
1Department of Chemistry, University of North Bengal, Siliguri,
West Bengal, India
2Department of Physics, University of North Bengal, Siliguri,
West Bengal, India
=
=
A new group of compounds, C6H5-N(O) N-NH-C6H4-C(O)-O-C6H4-CH N-C6H4-
OR, (1), [R ¼ n-alkyl group; 1a (R ¼ C7H15 ) to 1h (R ¼ C14H29)], has been designed
and synthesized to develop triazene-1-oxide based liquid crystals. All the com-
pounds have been characterized on the basis of their spectral data. Their thermal
behavior has been examined and found to exhibit thermotropic liquid crystalline
behavior over a wide range of temperature. All members of this family uniformly
show nematic phase behavior on melting up to the clearing point during heating
as well as on phase entering up to the solidification during cooling. An odd–even
effect on transition temperatures has been observed during heating as well as
cooling.
Keywords: ester; imine; liquid crystal; nematic; triazene-1-oxide
INTRODUCTION
Triazene-1-oxides and their derivatives [1,2] provide a flexible ligand
frame and are well known for their metal binding properties [3–12].
They offer wide varieties of stereochemistry to different metal ions
[3,4]. Triazene-1-oxide derivatives have also been found to exhibit bio-
logical activities [13–15]. But no report on mesogenic properties of
triazene-1-oxides is available in the literature, despite the fact that
extensive studies on mesogenic behavior of aryldiazene-1-oxides have
been done [16–19]. The flexible frame of triazene-1-oxides prompted us
Address correspondence to Pinaki Bandyopadhyay, Department of Chemistry,
University of North Bengal, Siliguri 734 013, West Bengal, India. E-mail: pbchem@
rediffmail.com
3

4
P. Das et al.
to design a new group of triazene-1-oxides with the potential to exhibit
liquid crystalline properties.
Herein we wish to report the synthesis, characterization, and
mesogenic behavior of a new group of triazene-1-oxide (1) having
two additional phenyl rings as core groups linked via ester and imine
functional groups.
EXPERIMENTAL SECTION
Measurements
Infrared spectra were recorded by JASCO 5300 FT-IR spectrometer.
Electronic spectra were obtained using Perkin Elmer Lambda 2S spec-
1
trophotometer. H NMR spectra were recorded on Bruker DPX 300
spectrometer. Microanalyses (C, H, N) were performed using an Ele-
mentan Vario EL III Carlo Erban 1108 elemental analyzer at CDRI,
Lucknow, India. Optical microscopic study at different temperatures
was done using a polarizing microscope (Manufacturer: Labex K. K.
International, India; Model: Co ax 10B(POL, 150X). The probe tem-
perature was controlled using Metler FP 80 control system and
FP82 hot stage.
Synthesis
n-Alkyl bromides, nitro benzene, 4-hydroxyaniline, 4-hydroxybenzal-
dehyde, 4-aminobenzoic acid, sodium nitrite, ammonium chloride,
sodium acetate and zinc powder were purchased from E. Merck Ltd.,
India and were used without further purification.
Synthesis of 3-(40-Carboxylatophenyl)-1-phenyltriazene-1-
oxide
3-(40-Carboxylatophenyl)-1-phenyltriazene-1-oxide
has
been
synthesized by coupling 4-carboxylatophenyldiazoniumchloride with
b-phenylhydroxylamine following a procedure reported by Sogani
and Bhattacharya [2]. The product was finally recrystallized from a
mixture of ethanol and acetone (1:1 v=v). Yield: 48%.

Triazene-1-oxide Liquid Crystals
5
Synthesis of 3-(40-(400-Formylphenyl)carboxylatophenyl)-1-
phenyl-triazene-1-oxide
The compound has been prepared by esterification of 3-(40-Carboxy-
latophenyl)-1-phenyltriazene-1-oxide with p-hydroxybenzaldehyde.
The esterification was made following a reported method [20]. A
mixture of 3-(40-carboxylatophenyl)-1-phenyl-triazene-1-oxide (0.51 g,
2 mmol), dicyclohexyl carbodiimide (DCC) (0.41 g, 2 mmol), p-hydroxy-
benzaldehyde (0.24 g; 2 mmol), and dimethylaminopyridine (DMAP)
(0.02 g, 0.2 mmol) in the mole ratio 1:1:1:0.1 were taken in dichloro-
methane (25 mL), and the mixture was stirred at room temperature
for 24 hrs. The reaction mixture was filtered and the filtrate was col-
lected. 3-(40-(400-Formylphenyl)carboxylatophenyl)-1-phenyltriazene-
1-oxide was isolated by recrystallization from aqueous ethanol. Yield:
0.65 g, 89%.
Synthesis of 4-(n-Alkyloxy)nitrobenzene
4-Alkyloxynitrobenzenes were prepared following
a reported
method [21]. The 4-(n-alkyloxy)nitrobenzenes were collected either
as pale yellow liquid or pale yellow solid (C10–C14) after the complete
removal of the solvent. Yield: 85%.
Synthesis of 4-(n-Alkyloxy)aniline
4-Alkyloxyanilines were prepared following a general method [20].
The 4-(n-alkyloxy)anilines were collected as a whitish solid after
recrystallization from ethanol.
Synthesis of 3-(40-(400-Imino(4000-(n-alkyloxy)phenyl)-
phenyl)carboxylatophenyl)-1-phenyltriazene-1-oxide
All the compounds have been prepared by condensation between
3-(40-(400-formyl-phenyl)carboxylatophenyl)-1-phenyltriazene-1-oxide
and 4-(n-alkyloxy)aniline. The condensation was made following
reported method00[022] for the preparation of imines, the synthesis of
3-(40-(400-imino(4 -(n-octyloxy)phenyl)phenyl)carboxylatophenyl)-1-
phenyl-triazene-1-oxide has been given as a representative case.
A mixture of 3-(40-(400-formylphenyl)carboxylatophenyl)-1-phenyl-
triazene-1-oxide (0.72 g, 2 mmol) and 4-(n-octyloxy)aniline (0.47 g,
2 mmol) was taken in 50 mL dry alcohol along with few drops of acetic
acid and was refluxed for 5 hrs. The reaction mixture was then cooled to
room temperature and kept overnight at room temperature. The solid
deposited by this time was filtered off. The solid was repeatedly washed
by ethanol (5 ꢀ 10 mL). The 3-(40-(400-imino(4000-(n-octyloxy)phenyl)-
phenyl)carboxylatophenyl)-1-phenyltriazene-1-oxide was then recrys-
tallized from a mixture of dichloromethane and ethanol (1:5 v=v).

6
P. Das et al.
Analytically pure compound was obtained by repeated recrystalliza-
tion. Yield: 0.77 g, 66%.
The other compounds of this series were obtained using same
procedure as that of 3-(40-(400-imino(4000-(n-octyloxy)phenyl)phenyl)car-
boxylatophenyl)-1-phenyltriazene-1-oxide. The characterization data
for 1a to 1h of this homologous series are summarized as follows.
3-(40-(400-Imino(4000-(n-heptyloxy)phenyl)phenyl)-
carboxylatophenyl)-1-phenyltriazene-1-oxide, [1a]
Yield: 63%. Anal. Calcd for C33H34N4O4: C, 72.00; H, 6.18; N, 10.18;
found: C, 71.98; H, 6.17; N, 10.21; mp: 68.53ꢁC; IR (KBr cmꢂ1) 3213 m,
1740 s, 1605 s, 1504 s, 1469 s, 1423 m, 1307 m, 837 s, 813 w; UV-VIS nm
(e dm3 molꢂ1 cmꢂ1
)
273(46000), 356(87400); 1H NMR (300 MHz,
CDCl3): 0.89 (t, 3H), 1.32 (m, 8H), 1.85 (m, 2H), 3.98 (t, 2H), 6.95
(d, 2H), 7.26 (d, 4H), 8.50 (s, 1H), 7.53 (d, 2H), 7.26 (d, 2H), 8.01
(d, 2H), 7.52 (m, 3H), 10.93 (s, 1H), 8.23 (d, 2H), 8.11 (m, 3H).
3-(40-(400-Imino(4000-(n-octyloxy)phenyl)phenyl)-
carboxylatophenyl)-1-phenyltriazene-1-oxide, [1b]
Yield: 66%. Anal. Calcd for C34H36N4O4: C, 72.34; H, 6.38; N, 9.93;
found: C, 72.36; H, 6.37; N, 9.91; mp: 70.34ꢁC; IR (KBr cmꢂ1) 3247 m,
1736 s, 1605 s, 1504 s, 1474 s, 1419 m, 1319 m, 837 s, 813 w; UV-VIS
nm (e dm3 molꢂ1 cmꢂ1) 273(37300), 356(50800); 1H NMR (300 MHz,
CDCl3): 0.89 (t, 3H), 1.30 (m, 10H), 1.84 (m, 2H), 3.98 (t, 2H), 6.95
(d, 2H), 7.26 (d, 4H), 8.51 (s, 1H), 7.52 (d, 2H), 7.13 (d, 2H), 7.98
(d, 2H), 7.52 (m, 3H), 10.93 (s, 1H), 8.24 (d, 2H), 8.11 (m, 3H).
3-(40-(400-Imino(4000-(n-nonyloxy)phenyl)phenyl)-
carboxylatophenyl)-1-phenyltriazene-1-oxide, [1c]
Yield: 68%. Anal. Calcd for C35H38N4O4: C, 72.66; H, 6.57; N, 9.69;
found: C, 72.61; H, 6.52; N, 9.67; mp: 68.53ꢁC; IR (KBr cmꢂ1) 3247 m,
1738 s, 1612 s, 1504 s, 1469 s, 1423 m, 1307 m, 848 s, 813 w; UV-VIS
nm (e dm3 molꢂ1 cmꢂ1) 274(44300), 357(28100); 1H NMR (300 MHz,
CDCl3): 0.89 (t, 3H), 1.29 (m, 12H), 1.82 (m, 2H), 3.98 (t, 2H), 6.95
(d, 2H), 7.26 (d, 4H), 8.51 (s, 1H), 7.52 (d, 2H), 7.13 (d, 2H), 7.98
(d, 2H), 7.52 (m, 3H), 10.93 (s, 1H), 8.24 (d, 2H), 8.12 (m, 3H).
3-(40-(400-Imino(4000-(n-decyloxy)phenyl)phenyl)-
carboxylatophenyl)-1-phenyltriazene-1-oxide, [1d]
Yield: 73%. Anal. Calcd for C36H40N4O4: C, 72.97; H, 6.76; N, 9.46;
found: C, 72.93; H, 6.72; N, 9.49; mp: 79.04ꢁC; IR (KBr cmꢂ1) 3247 m,
1740 s, 1605 s, 1504 s, 1473 s, 1423 m, 1315 m, 837 s, 813 w; UV-VIS
nm (e dm3 molꢂ1 cmꢂ1) 273(92500), 355(14500); 1H NMR (300 MHz,

Triazene-1-oxide Liquid Crystals
7
CDCl3): 0.87 (t, 3H), 1.28 (m, 14H), 1.85 (m, 2H), 3.98 (t, 2H), 6.95 (d,
2H), 7.26 (d, 4H), 8.51 (s, 1H), 7.53 (d, 2H), 7.26 (d, 2H), 7.99 (d, 2H),
7.51 (m, 3H), 10.93 (s, 1H), 8.23 (d, 2H), 8.12 (m, 3H).
3-(40-(400-Imino(4000-(n-undecyloxy)phenyl)-
phenyl)carboxylatophenyl)-1-phenyltriazene-1-oxide, [1e]
Yield: 82%. Anal. Calcd for C37H42N4O4: C, 73.27; H, 6.93; N, 9.24;
found: C, 73.34; H, 6.97; N, 9.27; mp: 85.07ꢁC; IR (KBr cmꢂ1) 3247 m,
1740 s, 1605 s, 1504 s, 1473 s, 1423 m, 1315 m, 837 s, 813 w; UV-VIS
nm (e dm3 molꢂ1 cmꢂ1) 272(58100), 355(38700); 1H NMR (300 MHz,
CDCl3): 0.87 (t, 3H), 1.27 (m, 16H), 1.82 (m, 2H), 3.92 (t, 2H), 7.10
(d, 2H), 7.26 (d, 4H), 8.50 (s, 1H), 7.51 (d, 2H), 7.13 (d, 2H), 7.98
(d, 2H), 7.52 (m, 3H), 10.93 (s, 1H), 8.23 (d, 2H), 8.12 (m, 3H).
3-(40-(400-Imino(4000-(n-dodecyloxy)phenyl)-
phenyl)carboxylatophenyl)-1-phenyltriazene-1-oxide, [1f]
Yield: 65%. Anal. Calcd for C38H44N4O4: C, 72.55; H, 7.09; N, 9.03;
found: C, 72.55; H, 7.06; N, 9.07; mp: 85.79ꢁC; IR (KBr cmꢂ1) 3201 m,
1732 s, 1605 s, 1504 s, 1474 s, 1423 m, 1307 m, 837 s, 813 w; UV-VIS
nm (e dm3 molꢂ1 cmꢂ1) 272(23000), 356(24000); 1H NMR (300 MHz,
CDCl3): 0.87 (t, 3H), 1.27 (m, 18H), 1.83 (m, 2H), 3.98 (t, 2H), 6.96
(d, 2H), 7.26 (d, 4H), 8.51 (s, 1H), 7.52 (d, 2H), 7.13 (d, 2H), 7.98
(d, 2H), 7.52 (m, 3H), 10.93 (s, 1H), 8.23 (d, 2H), 8.12 (m, 3H).
3-(40-(400-Imino(4000-(n-tridecyloxy)phenyl)-
phenyl)carboxylatophenyl)-1-phenyltriazene-1-oxide, [1g]
Yield: 71%. Anal. Calcd for C39H46N4O4: C, 73.82; H, 7.25; N, 8.83;
found: C, 73.85; H, 7.23; N, 8.81; mp: 95.35ꢁC; IR (KBr cmꢂ1) 3247 m,
1740 s, 1612 s, 1504 s, 1477 s, 1423 m, 1307 m, 837 s, 813 w; UV-VIS
nm (e dm3 molꢂ1 cmꢂ1) 275(28500), 357(43600); 1H NMR (300 MHz,
CDCl3): 0.87 (t, 3H), 1.27 (m, 20H), 1.80 (m, 2H), 3.98 (t, 2H), 6.94
(d, 2H), 7.26 (d, 4H), 8.50 (s, 1H), 7.53 (d, 2H), 7.24 (d, 2H), 7.98
(d, 2H), 7.52 (m, 3H), 10.93 (s, 1H), 8.24 (d, 2H), 8.12 (m, 3H).
3-(40-(400-Imino(4000-(n-tetradecyloxy)phenyl)-
phenyl)carboxylatophenyl)-1-phenyltriazene-1-oxide, [1h]
Yield: 68%. Anal. Calcd for C40H48N4O4: C, 74.07; H, 7.41; N, 8.64;
found: C, 74.13; H, 7.43; N, 8.67; mp: 91.82ꢁC; IR (KBr cmꢂ1) 3247 m,
1739 s, 1612 s, 1504 s, 1477 s, 1423 m, 1319 m, 844 s, 813 w; UV-VIS nm
(e dm3 molꢂ1 cmꢂ1) 275(16300), 356(35500); 1H NMR (300 MHz,
CDCl3): 0.87 (t, 3H), 1.27 (m, 22H), 1.80 (m, 2H), 3.98 (t, 2H), 6.94
(d, 2H), 7.26 (d, 4H), 8.51 (s, 1H), 7.52 (d, 2H), 7.14 (d, 2H), 7.98 (d,
2H), 7.52 (m, 3H), 10.93 (s, 1H), 8.24 (d, 2H), 8.10 (m, 3H).

8
P. Das et al.
RESULTS AND DISCUSSION
Synthesis
The 3-(40-carboxylatophenyl)-1-phenyltriazene-1-oxide has been
synthesized by coupling 4-carboxylatophenyldiazonium chloride with
b-phenylhydroxylamine following a procedure reported by Sogani
and Bhattacharya [2].
The carboxylic acid function of 3-(40-carboxylatophenyl)-1-phenyl-
triazene-1-oxide was esterified with p-hydroxybenzaldehyde using
dicyclohexylcarbodiimide (DCC) and catalytic amount of N,N-dimethy-
lamino-pyridine (DMAP) following a reported method [20]. The final
step involves the condensation [22] between 3-(40-(400-formylphenyl)-
carboxylatophenyl)-1-phenyltriazene-1-oxide and p-(n-alkyloxy)aniline
to yield the desired product (1) as shown in Scheme 1.
SCHEME 1 Synthesis of 3-(40(400-imino(4000-(n-alkyloxy)phenyl)phenyl)carbox-
ylatophenyl)-1-phenyltriazene-1-oxide.

Triazene-1-oxide Liquid Crystals
9
Spectral Properties
The IR spectra of all the compounds show absorptions in the range of
3212–3247 cmꢂ1 which are assigned to the N-H stretching vibrations.
The lowering in stretching frequency compared to the normal nNꢂH
stretching vibration indicates the presence of intramolecular hydrogen
bonding [23]. Strong absorptions in the region of 1731–1740 cmꢂ1 and
1604–1612 cmꢂ1 for all the compounds are observed due to the pres-
ence of ester [24] and imine [20,24,25] groups, respectively. The
nꢂN NꢂNHꢂ (symmetric triazene) has been found as a multiple band
B
in the range of 1423–1504 cmꢂ1 [26]. The nN!O stretch is observed as a
medium band in the region of 1307–1319 cmꢂ1 [27]. The dN!O (bend-
ing vibration) has been observed as split bands around 837 cmꢂ1 and
813 cmꢂ1 [5]. The IR spectrum of representative compound has been
shown in Fig. 1.
The electronic spectra of all the compounds in dichloromethane
were measured in the region 250–500 nm. All the compounds show
two absorption bands, one near 274 nm and the other near 356 nm.
The absorption near 356 nm is believed to be n!pꢃ transition based
on the triazene group [2,28]. The electronic spectrum of representative
compound has been shown in Fig. 2.
1
The low field H NMR signal ꢄ10.9 d corresponds to the intensity
representing the NꢂH proton [29]. It is observed that the chemical
shift of NꢂH proton remains unaffected by dilution, which indicates
the existence of intramolecular hydrogen bonding of the molecules in
solution. The signal for aldimine proton uniformly appears at ꢄ8.5
FIGURE 1 Infrared spectrum of 1f in KBr disc.

10
P. Das et al.
FIGURE 2 Electronic spectrum of 1g in dichloromethane.
d [24] for all the members. The specific assignments of the signals for
aliphatic and aromatic hydrogen atoms have been made and the
chemical shift values (d, ppm) given in the experimental section, which
are in good agreement with the reported chemical shift values of
hydrogen atoms in similar environment in the other compounds
1
[30,31]. The H NMR spectrum of representative compound has been
shown in Fig. 3.
FIGURE 3 1H NMR spectrum of 1g in CDCl3.

Triazene-1-oxide Liquid Crystals
11
TABLE 1 Transition Temperatures (ꢁC) Obtained from Optical Microscopic
Study for 1a to 1h
ꢁ
ꢁ
Heating ꢁC
Heating ꢁC
Comp.
1a
R
Cr
Cooling C
N
Cooling C
I
C7H15
C8H17
C9H19
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
68.5
58.4
70.3
52.4
78.3
54.6
79.0
47.3
85.1
58.6
85.8
59.7
95.4
76.4
91.8
69.8
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
161.0
156.7
175.7
160.4
143.9
142.1
187.5
175.9
177.9
174.7
176.3
176.2
163.5
162.3
174.3
173.3
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
1b
1c
1d
C
10H21
1e
C11H23
1f
C12H25
1g
C13H27
C14H29
1h
Thermal Behavior
The transition temperatures for compounds 1a to 1h obtained from the
optical microscopic studies have been given in the Table 1. All the
members show mesomorphic behavior with nematic phase [32] during
FIGURE 4 Optical texture of compound 1a at (a) 132ꢁC and (b) 65ꢁC during
cooling.

12
P. Das et al.
FIGURE 5 Optical texture of compound 1h at (a) 125ꢁC and (b) 85ꢁC during
cooling.
heating as well as during cooling. Representative optical textures have
been shown in Figs. 4 and 5.
The variation of melting point and nematic-isotropic trantransition
temperature with the number of carbon atoms (n) in the alkoxy chains
of compound (1) is shown in Fig. 6. An odd–even effect on transition
temperature has been observed for the transition temperatures during
FIGURE 6 Variation of transition temperatures with the number of carbon
atoms present in the alkoxy chain during heating. [
nematic–isotropic transitions.]
solid–nematic,

Triazene-1-oxide Liquid Crystals
13
FIGURE 7 Variation of transition temperatures with the number of carbon
atoms present in the alkoxy chain during cooling. [
nematic–solid transitions].
isotropic-nematic,
heating as well as cooling process. The melting point for any odd
member is higher than any even neighbours. The reverse is observed
for the clearing temperature Fig. 6.
During cooling the phase entering temperature for any even
member is higher than the odd neighbours and the solidification tem-
perature for the odd members are greater than the even neighbours
Fig. 7. On cooling the members show the similar pattern of variation
of phase transition temperature with the number of carbon atoms
(n) of alkoxy chains as that of heating Figs. 6 and 7.
CONCLUSIONS
In our endeavor to incorporate mesogenic behaviour in 1-phenyl-
3-phenyltriazene-1-oxide systems,
a new series of compounds,
3-(40-(400-imino(4000-(n-alkoxy)phenyl)phenyl)carboxylatophenyl)-1-phe-
nyltriazene-1-oxide (1) has been designed and successfully synthe-
sized. All the compounds exhibit thermotropic liquid crystalline
behaviour over wide range of temperature. All the compounds exhibit
nematic phase on melting up to the clearing point during heating as
well as on phase entering up to the solidification temperature during
cooling. The thermal behavior of metal complexes with this new group
of triazene-1-oxides is under investigation.

14
P. Das et al.
ACKNOWLEDGMENTS
The financial support (SR=S1=IC-08=2007) from DST, Government of
India, is gratefully acknowledged. We thank CSIR (India) for the
award of a fellowship (PD) and UGC (New Delhi) for Special Assist-
ance Programme to our Department.
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