Synthesis and characterization of [1,2,3]-triazole containing liquid crystals through click reaction

Article abstract of DOI:10.1016/j.molstruc.2009.01.035

Two series of heterocyclic liquid crystalline compounds with [1,2,3]-triazole ring at the terminal position were synthesized and analyzed for mesomorphic properties and its characteristics. Heterocyclic triazole core was introduced in to the target compound through click reaction between aromatic phenolic azide and alkyl propargyl ethers. The structure of the target compounds and intermediates was conformed by the ¹H NMR, ¹³C NMR and IR spectral techniques. All the compounds in both the series were exhibited thermotropic liquid crystalline mesophase. These compounds show enantiotropic Smectic A phase on heating and cooling experiments. This was further confirmed using differential scanning calorimetric investigations. Interestingly, earlier reports on non-polar alkyl group attached to [1,2,3]-triazole heterocyclic ring was not showing any liquid crystalline properties but in the present investigation introduction of polar alkoxy group very near to [1,2,3]-triazole heterocyclic ring displayed liquid crystalline properties.

Full text of DOI:10.1016/j.molstruc.2009.01.035

Journal of Molecular Structure 927 (2009) 7–13
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of [1,2,3]-triazole containing liquid crystals
through click reaction
D. Srividhya a, S. Manjunathan a, S. Thirumaran b, C. Saravanan c, S. Senthil c,
*
a Department of Chemistry, Sona College of Technology, Salem, India
b Department of Chemistry, Annamalai University, Tamilnadu, India
c Department of Chemistry, Anna University, Chennai, Tamilnadu 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2008
Received in revised form 2 January 2009
Accepted 20 January 2009
Available online 28 March 2009
Two series of heterocyclic liquid crystalline compounds with [1,2,3]-triazole ring at the terminal posi-
tion were synthesized and analyzed for mesomorphic properties and its characteristics. Heterocyclic tri-
azole core was introduced in to the target compound through click reaction between aromatic phenolic
azide and alkyl propargyl ethers. The structure of the target compounds and intermediates was con-
formed by the 1H NMR, 13C NMR and IR spectral techniques. All the compounds in both the series were
exhibited thermotropic liquid crystalline mesophase. These compounds show enantiotropic Smectic A
phase on heating and cooling experiments. This was further confirmed using differential scanning calo-
rimetric investigations. Interestingly, earlier reports on non-polar alkyl group attached to [1,2,3]-triazole
heterocyclic ring was not showing any liquid crystalline properties but in the present investigation
introduction of polar alkoxy group very near to [1,2,3]-triazole heterocyclic ring displayed liquid crys-
talline properties.
Keywords:
Triazole
Click chemistry
Liquid crystals
Mesomorphism
Azide–alkyne cycloaddition
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
1,4-phenylene analogues, (b) promote negative dielectric anisot-
ropy and (c) have a tendency to generate a range of different
The preparation of new types of a liquid crystal material
with rings other than six member ring has received less atten-
tion because of its non-linearity and poor mesophase stability.
However, it is important to extend the molecular design of li-
quid crystals to involve other ring systems from not only theo-
retical, but also the practical, point of view [1–2]. Particularly,
liquid crystalline compounds containing five-membered hetero-
cyclic rings have been successfully evaluated [3–5]. It was found
that some of the non-linear heterocyclic liquid crystals, such as
pyrazole, thiophene, furane and isoxazole derivatives with a ter-
minally positioned heterocyclic ring form smectic or nematic
phase at low temperature and in wide temperature range. In
addition, the presence of heteroatom has lead to the increase
in molecular dipole and dielectric anisotropy thereby producing
the molecular level polar organisations through generating elec-
trostatic and molecular shape-dependent interactions between
molecules [6]. Compounds containing five-membered hetero-
aromatic rings, such as thiadiazole or oxadiazole, are incorpo-
rated into the principal structure of calamitic mesogens to give
desired properties like (a) have lower melting points than the
liquid crystalline phases [7–14]. The compounds containing five
membered [1,2,3]-triazole ring, which is formed by copper cata-
lysed [3 + 2] dipolar cycloaddition reaction between an organic
azides and terminal alkynes, also known as ‘Click reaction’ has
attracted recent attention in the field of materials chemistry
[15], organic chemistry [16], drug discovery [17], bioconjuca-
tions [18] due to the following advantageous properties: (i) a
high chemical stability (in general, being inert to severe hydro-
lytic, oxidizing, and reducing conditions, even at high tempera-
ture), (ii) a strong dipole moment (5.2–5.6 D), (iii) an aromatic
character, and (iv)
a good hydrogen-bond-accepting ability
[19,20]. In the field of mesogenic materials, very few liquid
crystals are synthesized containing [1,2,3]-triazole ring [21].
Among those compounds, Interestingly [1,2,3]-triazole ring
placed at a terminal position of a calametic mesogen linked
with non-polar alkyl chains were not showing any liquid crys-
talline properties [22,23]. The comparative structures were de-
picted in Scheme 1. In this work we aimed introduce polar
terminal chain next to triazole ring using propargyl alcohol
thereby creating a dipole at the end of mesogen in order to
get liquid crystalline property in the compounds. Thus, we
synthesized two homologous series of non-linear heterocyclic
liquid crystals, 4-(4-alkoxy-1H-[1,2,3]-triazole-1-yl)phenyl-4-
(alkyloxy)benzoate, using Cu(I) catalysed 1,3-dipolar cycloaddi-
tion reaction.
* Corresponding author. Tel.: +91 4422203155; fax: +91 4422200660.
E-mail address: shendils@gmail.com (S. Senthil).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.01.035

8
D. Srividhya et al. / Journal of Molecular Structure 927 (2009) 7–13
The formula of the target molecule is depicted below
times with cold aq. NH4Cl in a small separating funnel. The product
was stored in the refrigerator. The similar procedure was adopted
for the preparation of 3-ethoxy propa-1-yne using diethyl sulphate
instead of dimethyl sulphate. (Yield: 72%). – FTIR (KBr) (cmÀ1):
3289 (CAOAC stretching), 2940 (stretching C„C), 2358 („CAH
stretching), 1105 (CAH aliphatic stretching). 1H NMR (CDCl3, d):
2.51 (s, 1H, „CAH), 4.11 (s, 2H, ACH2OA), 3.24 (s, 3H, AOCH3).
O
N
C
O
CH3
N
n
N
O
O
m
n= 3,4,5,6,7
m= 0,1
2.2.2. 3-Ethoxy prop-1-yne
IR (KBr) (cmÀ1): 2981, (stretching C„C): 2358, („CAH stretch-
ing): 3297, (CAOAC stretching): 1105, (CAH aliphatic stretching).
1H NMR (CDCl3), d ppm): 2.51 (s, 1H, „CAH), 4.11 (s, 2H,
ACH2OA), 3.78 (q, 2H, AOCH2A), 1.11 (t, 1H, CH3).
2. Experimental
2.1. Materials and methods
2.2.3. 4-Azidophenol
4-Aminophenol (41.2 mmol) was dissolved in concentrated
hydrochloric acid (1 ml) and water (20 ml), and the solution was
cooled in an ice bath. A cold solution of sodium nitrite (50.0 mmol)
in water (30 ml) was then added drop wise with stirring. After the
addition was complete, the reaction mixture was stirred for
15 min, and then treated drop wise with sodium azide (53.5 mmol)
in water (40 ml). The resulting solution was left to stir for a further
1.5 h with ice cooling, and then allowed to warm at room temper-
ature in the dark for overnight. Isolation with ether gave a dark
brown solid. The compound decomposed at an appreciable rate
at room temperature, and satisfactory spectral details could not
be obtained, despite several attempts. So this crude sample was ta-
ken forward to the next step without purifications.
Propargyl alcohol, p-aminophenol, p-hydroxybenzoic acid (Al-
drich, USA) and dimethyl and diethyl sulphate (Merck, Germany)
were used as supplied. Silica gel (MN Kieselgel 100–200 mesh)
was used for column chromatography. The organic solvents diethyl
ether, dichloromethane and other solvents were purified according
to the reported procedure [24].
The chemical structure of intermediates and target materials
were analyzed by infrared spectra with a Bruker IFS 66V Fourier
Transform spectrophotometer using KBr pellets and nuclear mag-
netic resonance spectroscopy using Joel EX-400 FTNMR spectrom-
eter in CDCl3 with TMS as an internal standard. Thin layer
chromatography was performed with TLC sheets coated with sil-
ica; spots were detected by UV irradiation. Transition tempera-
tures and phase transition enthalpies were determined by
Differential Scanning Calorimetry (DSC) using a Perkin-Elmer
DSC7 calorimeter at a heating rate of 5 °C minÀ1. The liquid crystal-
line texture of all the compounds was studied by using a Euromax
Polarizing microscope equipped with a Linkem HFS91 heating
stage and a TP93 temperature programmer. The samples were
made by placing small quantity of the material between two thin
glass cover slips, and the anisotropic behavior observed by heating
and/or cooling at the rates of 5 °C/min. The photographs were ta-
ken with a Nikom FM10 camera and exposed on a Konica film.
2.2.4. 4-[4-(Methoxymethyl)-1H-[1,2,3]-triazol-1-yl] phenol
A solution of 4-azido phenol (0.1 mol) and methyl propargyl
ether (0.1 mol) in DMSO-H2O (4:1) in the presence of 1mole% Cu-
SO4Á5H2O with 10-mol% sodium ascorbate was stirred at room
temperature for 18–28 h. TLC monitored the reaction. The mixture
was poured into cold water and the resulting solution was ex-
tracted with chloroform (3 Â 50ml). The combined organic phase
was dried with sodium sulfate, concentrated, and purified by col-
umn chromatography over silica gel using chloroform as eluent.
The similar procedure was adopted for the preparation of 4-[4-
(ethoxymethyl)-1H-[1,2,3]-triazol-1-yl] phenol.
IR (KBr) (cmÀ1): 3093, 2979, 1600, 1475, 1520, 1282, 1191, 851,
814.
2.2. Synthesis
1H NMR (CDCl3) (d ppm): 3.31 (s, 3H, OCH3), 4.65 (s, 2H,
ACH2OA), 6.97 (d, 1H, Ar–H), 7.47(d, 1H, Ar–H), 7.74 (s, 1H,
NACH).
2.2.1. 3-methoxy prop-1-yne
A solution of 200 g of NaOH in 300 mL of water and (0.3 mol,
16.8 g) propargyl alcohol is placed in the round bottom flask
charged with stirrer. Dimethyl sulphate (0.15 mol, 25.0 g) was
slowly added in 2 h drop wise by means of addition funnel with
the temperature range of 50–55 °C. The propargyl ether is distilled
off as quickly as possible, while the temperature of the heating
bath is gradually raised. The distillation is stopped when the ther-
mometer in the head of the column indicates 95 °C. In order to re-
move some methanol, the content of the receiver are washed three
Mass spectrum(70 eV) m/e 205(M+.), 190(M+.ACH3),
188(M+.AOH).
2.2.5. 4-[4-(Ethoxymethyl)-1H-[1,2,3]-triazol-1-yl] phenol
IR (KBr) (cmÀ1): 3353, 2598, 1600, 1479, 1522, 1240, 1193, 851,
814.
1H NMR (CDCl3, d ppm): 3.60 (q, 2H, AOCH2A), 1.21 (t, 3H,
ACH3), 4.65 (s. 2H. ACH2OA) 6.94 (d, 1H, Ar–H), 7.2(d, 1H, Ar–
H), 7.83 (s, 1H, N–CH).
Mass spectrum(70 eV) m/e 219(M+.), 190(M+.AC2H5),
202(M+.AOH).
2.2.6. 4-Alkyloxybenzoic acid
p-Hydroxy benzoic acid (0.1 mol) was dissolved in ethanol
(100 ml). potassium hydroxide (0.3 mol) was dissolved in water
(10 ml) and potassium iodide (100 mg) was added to it. The mix-
ture was heated to reflux and 1-bromoalkane (0.12 mol) was added
drop wise to the refluxing mixture. It was continued for 15 h. The
solution was concentrated by distilling out the excess ethanol un-
der reduced pressure. The remaining solution was poured in to the
ice-cold dilute HCl with stirring. A precipitate formed was recrys-
Scheme 1. Comparative structural features of synthesized and published triazole
containing liquid crystal.

D. Srividhya et al. / Journal of Molecular Structure 927 (2009) 7–13
9
tallized from ethanol. (Yield 60–75%). The representative spectral
2.2.7. 4-[4-(Methoxymethyl)-1H-[1,2,3]-traizole-1-yl]phenyl 4-
hexyloxybenzoate
DCC (0.004 mol) and DMAP (1% w/w) was dissolved in the
dichloromethane under nitrogen atmosphere and then to this
solution 4-[4-(methyloxymethyl)-1H-[1,2,3]-triazole-1-yl)phenol
data of 4-hexyloxybenzoic acid was shown in below:
IR (KBr) (cm-1): 2933, 1775, 3058, 1465, 2851, 1256.
1H NMR (CDCl3, d ppm): 0.96 (t, 3H, CH3), 3.53 (t, 2H, OCH2),
1.33–1.48 (m, 8H, CH2), 8.01 (d, 1H, Ar–H), 7.27(d, 1H, Ar–H).
OH
HO
NH2
Dimethylsulphate(m=0)
NaOH
90oC Diethylsulphate(m=1)
NaN3
NaNO2/HCl
CH3
O
m
+ Na2SO4
+
2H2O
HO
N3
+
+
HOOC
OH Br
CH3
CuSO4 .5H 2
O
n
IPA/H2O
Sodiumascorbate
KOH/KI
Ethanol
N
N
HOOC
O
CH3
N
OH
+
n
O
m
DCC/DMAP
RT
CH2Cl2
24hr
O
N
C
O
CH3
N
n
N
O
O
m
n
3
n= 3,4,5,6,7
5
6
7
4
m
m=0,1
0
M10
E10
M6 M7 M8 M9
E6 E7 E8 E9
1
Scheme 2. Scheme of synthesis of triazole containing liquid crystals.
Fig. 1. 1H NMR Spectrum of E10
.

10
D. Srividhya et al. / Journal of Molecular Structure 927 (2009) 7–13
Fig. 2. 13C NMR Spectrum of M7.
(0.002 mol) and alkoxybenzoic acid (0.0021 mol) were added and
the resulting mixture stirred at room temperature for one day. Pre-
cipitated urea was removed by filtration, and the solution was
washed with 5% acetic acid solution and water to remove unre-
acted DCC. The organic phase was dried over anhydrous magne-
sium sulphate and filtered. The solvent in the filtrate was
removed under reduced pressure. The product was purified by col-
umn chromatography using dichloromethane as eluent, and
recrystallised from ethanol giving white solid. The similar proce-
dure was adopted for synthesis of other compound in this
homologues.
3. Results and discussion
3.1. Synthesis
The nonlinear liquid crystalline compounds and their corre-
sponding precursors were prepared by the synthetic route de-
IR (KBr) (cmÀ1): 3138, 2928, 2858, 1712, 1609, 1475, 1214, 831.
1H NMR (CDCl3, d ppm): 1.25–1.87 (m, 8H, CH2), 3.55, (s, 3H,
OCH3), 4.65 (s, 2H, ACH2OA), 6.96 (d, 1H, Ar–H), 7.48 (d, 1H, Ar–
H), 7.74 (d, 1H, Ar–H), 8.04 (d, 1H, Ar–H), 8.1 (s, 1H, NACH).
13C NMR: 14, 29, 64, 66, 68, 114, 121, 123, 146, 151, 162.
2.2.8. 4-[4-Ethoxymethyl)-1H-[1,2,3]-traizole-1-yl] phenyl 4-hexyloxy
benzoate
DCC (0.004 mol) and DMAP (1% w/w) was dissolved in the
dichloromthane under nitrogen atmosphere and then to this solu-
tion (0.002 mol) 4-(4-(ethyloxy)-1H-[1,2,3]-triazole-1-yl)phenol
and (0.0021 mol) alkoxybenzoic acid were added and the resulting
mixture stirred at room temperature for one day. Precipitated
material was removed by filtration, and the solvent removed under
reduced pressure. The product was purified by column chromatog-
raphy using dichloromethane as eluent, and recrystallised from
ethanol giving white solid. A similar procedure was adopted for
the preparation of other compounds in the series by reaction of
the corresponding acids.
IR (KBr) (cmÀ1): 2920, 1712, 3080, 1417, 2959, 1265.
1H NMR (CDCl3) (d ppm): 0.89 (t, 3H, ACH3), 1.25–1.92 (m, 8H,
CH2), 3.59 (q, 2H, OCH2), 4.73 (s, 2H, ACH2OA), 4.00 (t, 2H,
AOCH2A) 7.02 (d, 1H, Ar–H), 7.87 (d, 1H, Ar–H), 7.92 (d, 1H, Ar–
H), 8.00 (d, 1H, Ar–H), 8.05 (s, 1H, NACH).
13C NMR: 14, 15, 29, 63, 66, 68, 114, 121, 123, 134, 151, 154,
164.
Fig. 3. The representative HOPM photograph of the M10 and E8.

D. Srividhya et al. / Journal of Molecular Structure 927 (2009) 7–13
11
Table 1
Transition temperatures of the compounds observed in DSC.
Compounds
Yield (%)
Transition temperaturesa (°C)
Ti–Tm (DT)
Symbol
m
n
Cr
Tm
SmA
Ti
I
R10 (Ref. [22])
Chiral aliphatic chain
74
85
73
72
65
63
73
73
68
70
79
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
137
119
115
114
113
112
109
108
106
105
104
–
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
–
–
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
–
M6
M7
M8
M9
M10
E6
E7
E8
E9
E10
0
0
0
0
0
1
1
1
1
1
3
143
135
136
138
141
121
123
124
127
131
24
20
22
25
29
12
15
18
22
27
4
5
6
7
3
4
5
6
7
a
All the transition temperature were observed in DSC during cooling cycle.
Table 2
Transition temperatures of the compounds observed in HOPM.
Compounds
Yield (%)
Transition temperaturesa (°C)
Ti–Tm (DT)
Symbol
m
n
Cr
Tm
SmA
Ti
I
R10 (Ref. [22])
Chiral aliphatic chain
74
85
73
72
65
63
73
73
68
70
79
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
137
–
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
–
–
–
M6
M7
M8
M9
M10
E6
E7
E8
E9
E10
0
0
0
0
0
1
1
1
1
1
3
119.2
114.9
114.0
113.3
112.1
108.8
107.9
106.2
105.4
104.0
133.1
135.2
135.8
137.9
141.3
120.6
123.0
124.1
127.2
131.0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
13.9
20.3
21.8
24.6
29.2
11.8
15.1
17.9
21.8
27.0
4
5
6
7
3
4
5
6
7
a
All the transition temperature were observed in hot stage optical polarizing microscope(HOPM) during cooling cycle.
scribed in Scheme 2. [1,2,3]-Triazole containing phenolic com-
pound was synthesized through the Cu(I)-catalyzed ‘Click Reac-
tion’ of 4-azidophenol with 3-methoxy propa-1-yne and/or
3-ethoxy propa-1-yne. The aimed nonlinear liquid crystalline com-
pounds M6–M10 and E6–E10 have been synthesized by the esterifi-
cation reaction of various spacer substituted 4-alkoxy benzoic acid
with corresponding phenolic compounds containing triazole ring
using DCC and DMAP as the catalyst at room temperature. The
chemical structure of the targeted compounds was confirmed by
1H and 13C NMR spectral techniques. Representative spectra were
in Figs. 1 and 2.
Phase details and transition temperatures of all the compounds
are summarized in Tables 1 and 2. Transition temperature depicted
in R10 was taken from Ref. [22]. The representative DSC thermo-
gram of M8 and E10 was shown in Fig. 4, respectively. The meso-
phase was stable from 18 to 32 °C for all the compounds in
which M series (methyl group at the polar end) show superior
mesophase stability than E series (ethyl group at the polar end)
due to addition of one carbon reduces the packing efficiencies. It
can be seen from the Table 1 that the chiral non-polar substituent’s
present at the terminal position of compound R10 did not showing
any mesomorphic properties. The reason could be shorter core
(three rings) may not be enough to maintain the aspect ratio to im-
part the liquid crystalline phase in which [1,2,3]-triazole heterocy-
cle causes a bending of the core (c.148.9u) and consequently a
linearity deviation that seems detrimental to the mesomorphic
packing. But their counterparts synthesized here are showing good
mesomorphic properties with appreciable phase stability. Though
it was a non-linear, the polar ether group generates enough dipole
in addition to triazole unit which help to impart liquid crystalline
properties. In addition the melting point of synthesized compound
was decreased around 25 °C from that of the compound R10 [22]
though polar chain end possess only two carbons.
3.2. Mesophase and thermal properties
The two series of nonlinear liquid crystalline compounds differ-
ing in their alkyl chain present in the terminal position of triazole
ring were synthesized for the study of mesomorphic properties and
its characteristics. A detailed synthetic route is shown in Scheme 2.
Liquid crystalline behavior of the compounds was investigated
using hot stage polarizing microscopy. All the compounds from
both the series show smectic A (SmA) mesophase. On cooling from
the isotropic liquid, tiny batonnets developed at the edge of melted
substance, which grown up to the focal-conic fan texture, shown in
Fig. 3, which is the characteristic of SmA texture. For further con-
firmation, all the compounds were subjected to differential scan-
ning calorimetry investigation, which shows two sharp
endothermic peaks, corresponding to the melting temperature
(Tm) (high enthalpy) and isotropic temperature (Ti) (low enthalpy)
respectively, confirming the formation of liquid crystalline phase.
4. Conclusion
Two series heterocyclic liquid crystalline compounds with the
heterocyclic ring at the terminal position were synthesized and
their mesomorphic properties were analyzed with respect to phase
transition temperature and chemical structure. The structure of all

12
D. Srividhya et al. / Journal of Molecular Structure 927 (2009) 7–13
Fig. 4. DSC thermogram of M8 and E10
.
the target compounds was conformed by the 1H NMR, 13C NMR and IR
spectroscopic methods. DSC measurements and HOPM investigation
confirm the liquid crystalline nature of the synthesized com-
pounds. Introduction of polar oxygen in near to the heterocyclic
ring produce liquid crystalline property, whereas it was absent in
the case of non-polar alkyl chains attached directly to heterocyclic
ring in these type of three ring compounds. The reason could be di-
pole-dipole interaction of polar alkoxy group increases the attrac-
tion and leads to arrangement of molecule in the smectic layer
structures.
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