Synthesis of novel fluorescent 2-{4-[1-(pyridine-2-yl)-1H-pyrazol-3-yl] phenyl}-2H-naphtho [1,2-d] [1,2,3] triazolyl derivatives and evaluation of their thermal and photophysical properties

Article abstract of DOI:10.1002/jhet.1697

Novel 2-{4-[1-(pyridine-2-yl)-1H-pyrazol-3-yl] phenyl}-2H-naphtho [1,2-d] [1,2,3] triazolyl fluorescent derivatives were synthesized from p-nitrophenylacetic acid and 2-hydrazino pyridine through Vilsmeier-Haack and diazotization reactions. Photophysical properties were evaluated, and results show that compounds have good fluorescence quantum yields. Thermal analysis showed that they are reasonably stable. The structures of the compounds were confirmed by FT-IR, ¹H NMR, ¹³C NMR, and mass spectral and elemental analysis.

Full text of DOI:10.1002/jhet.1697

Month 2013
Synthesis of Novel Fluorescent 2-{4-[1-(Pyridine-2-yl)-1H-pyrazol-3-yl]
phenyl}-2H-naphtho [1,2-d] [1,2,3] triazolyl Derivatives and Evaluation of
Their Thermal and Photophysical Properties
*
Vikas S. Padalkar, Kiran R. Phatangare, and N. Sekar
Tinctorial Chemistry Group, Intermediate and Dyestuff Technology Department, Institute of Chemical Technology, N. P.
Marg, Matunga, Mumbai 400019, Maharashtra, India
*
E-mail: n.sekar@ictmumbai.edu.in
Received February 8, 2012
DOI 10.1002/jhet.1697
Published online in Wiley Online Library (wileyonlinelibrary.com).
Novel 2-{4-[1-(pyridine-2-yl)-1H-pyrazol-3-yl] phenyl}-2H-naphtho [1,2-d] [1,2,3] triazolyl fluorescent
derivatives were synthesized from p-nitrophenylacetic acid and 2-hydrazino pyridine through Vilsmeier–
Haack and diazotization reactions. Photophysical properties were evaluated, and results show that
compounds have good fluorescence quantum yields. Thermal analysis showed that they are reasonably
stable. The structures of the compounds were confirmed by FT-IR, 1H NMR, 13C NMR, and mass spectral
and elemental analysis.
J. Hetercyclic Chem., 00, 00 (2013).
INTRODUCTION
the synthesis and photophysical properties of novel fluores-
cent triazole derivatives.
1,2,3-Triazole and its derivatives are used as fluorescent
brightening agents, UV absorbers to retard the photodegra-
dation of wool [1–3]. Synthesis of heterocyclic systems for
use as fluorescent whiteners has two approaches: the first
one in which the same or different heterocyclic systems are
linked together so as to increase the overall conjugation in
the resulting molecule, and another is incorporation of a
fused system to provide planarity to increase the conjugation
and rigidity in the molecule [4]. Over the last two decades, a
variety of triazole derivatives have been synthesized and
studied for their optical brightening properties [5,6].
Moreover, pyrazole is one of the classes of heterocyclic
compounds and also an important intermediate for the
synthesis of biologically active benzofuran and coumarin
systems. Although various pyrazole derivatives are reported
in literature [7–10], fluorescent derivatives of pyrazole
heterocycle have received little attention. Synthesis and
applications of fluorescent 1,2,3 or 1,3,4-triazole containing
different heterocyclic ring systems are known in literature
as sensors [11–13]. However, there are only few reports
available that describe synthesis and photophysical
properties of triazole with pyrazole heterocycles in a single
moiety [14].
RESULTS AND DISCUSSION
Vilsmeier–Haack reaction for the synthesis of various
heterocyclic ring systems has been exploited. It was envisaged
that the Vilsmeier–Haack reaction on p-nitrophenylacetic
acid would yield malondialdehyde, which would serve as
useful synthon for the construction of various heterocycles.
As a continuation of work on Vilsmeier–Haack reaction
and heterocyclic compounds synthesis for high-technology
application [21], we develop novel triazoles that contain
pyrazole moiety from phenyl acetonitrile and 2-hydrazino
pyridine.
p-Nitrophenylmalondialdehyde 5 was obtained by
Vilsmeier–Haack formylation of p-nitrophenylacetic acid 3,
which in turn was prepared from phenyl acetonitrile 1 by
nitration followed by hydrolysis reaction. Compound 5
on reaction with 2-hydrazinopyridine 6 yielded the
nitropyazoles 7. The nitro group of 7 was reduced to give
the corresponding amino compound 8, which was diazotized
and coupled with sulfonic acid substituted 2-naphthylamine
to yield the corresponding o-amino azo compounds 11.
These o-amino azo compounds were air oxidized in basic
cupric acetate in dimethyl formamide to give their
corresponding triazoles 12a–12c (Scheme 1), which were
evaluated for their thermal and photophysical properties.
It is our interest to synthesize a triazole, which would
exhibit higher fluorescence quantum yield and also absorp-
tion in the UV region and emission in the visible region of
the spectrum, which is a basic requirement for compounds
that function as fluorescent brightening agent. As a part of
ongoing research on synthesis of fluorescent compounds
for high-technology applications [15–20], here we report
Thermal stability. To examine the thermal stability of
triazoles 12a–12c, thermogravimetric analyses (TGA) were
carried out between 40 and 600ꢀC under nitrogen
© 2013 HeteroCorporation

Vikas S. Padalkar, Kiran R. Phatangare, and N. Sekar
Vol 000
Scheme 1. Synthesis of triazoles 12a–12c.
atmosphere. The TG curves of the compounds are shown in
Figure 1. The TG results indicate that the synthesized
compounds are stable up to 270ꢀC. Above 270ꢀC, the TG
curves of the synthesized compounds show a loss in
weight. TGA curves for 12a–12c are as shown in Figure 1.
Photophysical properties.
An effective fluorescent
fluorophore for biological application has to present a good
fluorescent intensity and high quantum yield with high
photostability. The quantum yields of the fluorescence of
compounds 12a–12c were determined by using anthracene
as a standard. Absorption and emission characteristics of
standard as well as synthesized compounds were measured
at different concentrations of synthesized compounds and
Figure 1. Thermogravimetric analysis overlay graph of compounds 12a–12c.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2013 Synthesis of Novel Fluorescent 2-{4-[1-(Pyridine-2-yl)-1H-pyrazol-3-yl] phenyl}-2H-naphtho
[1,2-d] [1,2,3] triazolyl Derivatives and Evaluation of Their Thermal and Photophysical Properties
Elemental analysis was carried out using FLASH EA 1112 series
instrument of Thermo Finnigan.
standard (2, 4, 6, 8, and 10 ppm levels). Absorbance intensity
values were plotted against emission intensity values. A
linear plot was obtained. Gradients were calculated for
each compound and for standard. All the measurements
were carried out keeping the parameters constant, namely,
solvent and slit width. Relative quantum yield of all
synthesized triazole derivatives 12a–12c were calculated
using a formula reported in literature [22,23].
The fluorescence quantum yields of 12a–12c were
recorded in ethanol at room temperature. It is observed that
the quantum yield of 12a and 12b are much higher than those
recorded for 12c. The higher quantum yields of 12a and 12b
are presumably due to the presence of sulfonic acid groups
on triazole ring of fluorophore 12a and 12b. Sulfonic acid
may enhance the strength of acceptor; as a consequence,
fluorophore acts as good donor-p system–acceptor type of
chromogen, which may enhance the fluorescence properties.
Furthermore, it was observed that the absorption–emission
characteristics of compounds 12a–12c are nearly the same
except that the intensities of absorption and emission showed
some difference. The compound 12c absorbs and emits with
lower intensity as compared with compounds 12a and 12b.
The details of absorption, emission intensity, Stoke’s shift,
and quantum yield are summarized in Table 1.
Synthesis of triazoles 12a–12c
4-Nitrophenyl acetonitrile 2. Concentrated nitric acid
(specific gravity 1.42) (138 mL, 2.15mol) was cooled to 0–10ꢀC
and, phenylacetonitrile 1 (50g, 0.425mol) was added to it at such
a rate that the temperature remained at about 10ꢀC and did not
exceed 20ꢀC during addition. After the addition of phenyl
acetonitrile, the ice-cold bath was removed, and the reaction
mixture warmed at 40ꢀC for 2 h. From the reaction mass poured
on crushed ice (500 g), a yellow solid was separated. The solid
was filtered, washed separately with ice-cold water to remove
trace of acid, and dried to yield 2. Yield: 55 g, 79%; mp 114ꢀC
(115–116ꢀC, lit [22]).
4-Nitrophenylacetic acid 3. Concentrated sulfuric acid
(150 mL, 2.75 mol) was added to water (150 mL), and 4-
nitrophenylacetonitrile 2 (50g, 0.31mol) was added to this mixture.
The reaction mixture was refluxed for 30 min, diluted with 150 mL
of water, and cooled to 0ꢀC when colorless crystalline solid
separated. The solid was filtered off, washed with ice-cold water to
remove trace of acid, and dried to yield 3. Yield: 53 g, 95%;
mp 150ꢀC (151–152ꢀC, lit [22]).
Z-2-(40-Nitrophenyl)-3-N,N-dimethylaminopropenal 4. To a
vigorously stirred solution of DMF (2.9mL) at 0ꢀC, POCl3
(2.8mL) was added dropwise. After 5 min, the 4-nitrophenylacetic
acid 3 (1.81 g, 0.01mol) was carefully added in solution of
DMF (5 mL). The reaction mixture was warmed to 70ꢀC for 12 h
and then poured on ice. After neutralization by K2CO3, a solution
of NaOH 50% (12mL) was added, and a precipitate was obtained
upon cooling at 0ꢀC. The precipitate was dissolved in
dichloromethane and recrystallized from diethyl ether to give
yellow powder 4. Yield: 1.43 g, 65%, mp 131ꢀC (131ꢀC, lit [22]),
EXPERIMENTAL
Materials and methods. All commercial reagents and solvents
were purchased from S.D. Fine Chemicals (India) and were used
without purification. The reaction was monitored by thin layer
chromatography on 0.25 mm silica gel 60 F254 precoated plates
(Merck KGaA, 64271, Darmstadt, Germany), which were
visualized under UV light. The FT-IR spectra were recorded on
1
(recrystallized from diethyl ether). H NMR (DMSO-d6, d, ppm):
9.12 (s, 1H, CHO), 8.19–8.22 (d, 2H, J = 8.7 Hz, Ar–H),
7.35–7.38 (d, 2H, J = 8.7 Hz, Ar–H), 6.97 (s, 1H), 2.87
(s, 6H, HN (CH3)2).
Z-2-(40-Nitrophenyl)-3-hydroxypropenals 5. 33% NaOH
(20 mL) was added to a solution of Z-2-(40-nitrophenyl)-3-N,N-
dimethylaminopropenal 4 (2.2 g, 0.01 mol) in ethanol (20 mL),
and the reaction mixture was stirred at reflux for 3 h. Ethanol
was removed under reduced pressure, and the residue was
cooled down with ice to give a powder, which was filtered and
washed with CH2Cl2, then taken up in water and acidified by
6N HCl to give a precipitate. The solid was dissolved in acetone
and recrystallized from ether to give 5 as a light brown powder.
Yield: 1.37 g, 71%, mp 220ꢀC (223ꢀC, lit [22]), (recrystallized
1
PerkinElmer (USA) 257 spectrometer using KBr discs. H NMR
and 13C NMR spectra were recorded on a VXR 300-MHz and
VXR 75-MHz instrument, respectively, using timethylsilane as an
internal standard (Varian, Palo Alto, CA). Mass spectra were
recorded on Finnigan mass spectrometer (Mediagrafix Art:
Technology, Australia) (EI). The visible absorption spectra of the
compounds were recorded on a Spectronic Genesys 2 UV–vis
spectrometer. TGA measurements were performed on SDT Q 600
v8.2 Build 100 model of TA instruments Waters (India) Pvt. Ltd.
Table 1
Absorption, emission, Stoke’s shift, and quantum yield of compounds 12a–12c.
Quantum yielda
Absorption lmax (nm)
Emission lem
Stoke’s shift
Compound
(Extinction coefficient)
(nm)
(nm)
(Φ)
12a
12b
12c
376 (25766)
364 (24829)
367 (13983)
515
501
521
139
137
154
0.51
0.45
0.24
lmax and lem were measured in nm, and solvent used is DMF,
Samples conc. 1 ꢁ 10ꢂ6 mL
Measurements were carried out at room temperature.
aSolvent used for quantum yield measurement: ethyl alcohol; reference standard used for quantum yields calculation: anthracene.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Vikas S. Padalkar, Kiran R. Phatangare, and N. Sekar
Vol 000
1
1
from diethyl ether). H NMR (DMSO-d6, d, ppm): 14.45 (s, 1H,
966,773, 701, 655cmꢂ1. H NMR (DMSO-d6, d, ppm): 9.25 (d,
1H, J = 8.8, 2.4 Hz), 8.45 (s, 1H), 8.43 (s, 1H), 8.18 (d, 1H,
J = 6.6, 2.4 Hz), 7.94–7.97 (d, 2H, J = 7.7, 2.0 Hz), 7.75–7.78 (d,
1H, J = 8.8, 2.0 Hz), 7.57–7.61 (m, 3H, J = 2.5, 4.0 Hz), 7.42–7.44
(t, 2H, J = 7.4, 6.8,1.8Hz), 7.30 (t, 1H, J = 8.0, 6.2, 1.0Hz),
6.96–6.98 (t, 1H, J = 7.3, 1.4, 0.8Hz), 6.87–6.89 (d, 1H,
J = 8.0, 1.2 Hz), 4.62 (s, 1H). 13C NMR (DMSO-d6, d, ppm):
152.42, 151.02, 140.12, 139.03, 138.43, 135.80, 135.21,
133.67, 132.0, 131.39, 130.99, 130.24, 129.45(s), 127.89,
127.00, 126.98(s), 126.56, 126.45, 126.30, 125.98, 124.67.
Mass: m/z = 469.27 (M+ 1). Anal. Calcd for C24H16N6O3S: C,
61.53; H, 3.44; N, 17.94; S, 6.84. Found: C, 61.62; H, 3.54;
N, 17.87; S, 6. 75.
OH), 8.64 (s, 2H, CHO), 8.18–8.23 (d, 2H, J = 8.8 Hz, Ar–H),
7.82–7.85 (d, 2H, J = 8.8 Hz, Ar–H).
Synthesis of 2-[3-(4-Nitrophenyl)-1H-pyrazol-1-yl] pyridine 7. A
mixture of p-nitrophenylmalondialdehyde 5 (2.0 g, 0.01 mol),
2-hydrazinopyridine 6 (1.10 g, 0.01 mol), and ethanol (40 mL)
was refluxed for 5 h, cooled, and poured into ice water
(100 mL) to yield 7. Yield: 1.56 g, 89%. (Recrystallized from
ethyl alcohol). mp 198–200ꢀC. FT-IR (KBr): 1610, 1578,
1559, 1355, 1340 cmꢂ1 1H NMR (DMSO-d6, d, ppm): 8.45
.
(d, 1H, J = 9.0, 1.8 Hz), 8.29 (s, 1H), 8.27(s, 1H), 8.09
(d, 2H, J = 8.0, 1.8 Hz), 8.01 (d, 2H, J = 8.0, 1.8 Hz), 7.87 (d,
1H, J = 8.6, 2.4 Hz), 7.72 (d, 1H, J = 8.0, 2.4 Hz) 7.70 (d,
1H, J = 8.0, 2.4 Hz). Anal. Calcd for C14H10N4O2: C, 63.15;
H, 3.79; N, 21.04. Found: C, 63.27; H, 3.64; N, 21.00.
Synthesis of 4-[1-(Pyridin-2-yl)-1H-pyrazol-3-yl] aniline 8.
Palladium-carbon catalyst (10%, 0.05 g) was added portionwise
for 5–10 min to a hot solution of 7 (2.0 g, 0.007 mol) in ethanol
(mL) containing hydrazine hydrate (2.61 g, 0.052 mol). The
mixture was refluxed for 1 h. The hot solution was filtered
2-{4-[1-(Pyridin-2-yl)-1H-pyrazol-4-yl]phenyl}-2H-naphtho
[1,2-d][1,2,3]triazole-7-sulfonic acid (12b). Purified yield:
79%. Brown solid, mp >300ꢀC (crystallized from ethanol).
FT-IR (KBr): 3558, 3121, 1453, 1181, 1031, 765, 717c cmꢂ1
.
1H NMR (DMSO-d6, d, ppm): 9.28 (d, 1H J = 9.0, 2.4 Hz),
8.46–8.45 (d, 1H, J = 8.0, 4.0 Hz), 8.20–8.21 (d, 1H, J = 6.0,
2.0 Hz), 7.98–8.00 (d, 3H, J = 7.7, 1.8 Hz), 7.81 (d, 1H,
J = 8.8, 2.4 Hz), 7.64–7.70 (t, 3H, J = 8.0, 6.8, 2.4 Hz), 7.41–
7.44 (t, 2H, J = 11.0, 7.3, 1.4 Hz), 7.30 (t, 1H, J = 8.0, 6.2,
1.0 Hz), 6.97–6.99 (t, 2H, J = 8.0, 6.8, 1.4Hz), 6.58 (s, 1H). 13C
NMR (DMSO-d6, d, ppm): 152.21, 150.67, 139.12, 139.00,
137.12, 135.23, 134.09, 133.10, 132.67, 130.43, 129.46, 128.67
(s), 128.12, 127.00, 126.46 (s), 125.98, 124.45, 123.70, 122.93,
122.47, 121.67. Mass: m/z = 469.23 (M+ 1). Anal. Calcd for
C24H16N6O3S: C, 61.53; H, 3.44; N, 17.94; S, 6.84. Found: C,
61.59; H, 3.49; N, 17.86; S, 6.84.
through
a Whatman paper (Swastik Scientific Company,
Mumbai,India) to remove Pd, and further the filtrate was
filtered through silica gel (10.0 g), and the solvent was
evaporated. A pure product was obtained, which was further
used without purification. Yield: 1.62 g, 89%, mp 174ꢀC. FT-
IR (KBr): 3540, 3230, 1605 cmꢂ1 1H NMR (DMSO-d6, d,
.
ppm): 8.56 (s, 1H), 8.42 (s, 1H), 7.97 (d, 1H, J = 11.0, 2.8 Hz),
7.80 (d, 1H, J = 8.2, 2.2 Hz), 7.41(d, 1H, J = 8.6, 2.8 Hz), 7.18
(d, 1H, J = 8.6, 2.6 Hz), 6.75 (d, 2H, J = 5.5, 2.4 Hz), 6.68 (d,
2H, J = 5.5, 2.4 Hz ), 5.88 (s, 2H, –NH2). Anal. Calcd for
C14H12N4: C, 71.17; H, 5.12; N, 23.71. Found: C, 71.21;
H, 5.08; N, 23.68.
2-{4-[1-(Pyridin-2-yl)-1H-pyrazol-4-yl]phenyl}-2H-naphtho
[1,2-d][1,2,3]triazole (12c). Purified yield: 69%. Yellowish
brown solid, mp >300ꢀC (crystallized from ethanol). FT-IR
(KBr): 3493, 3155, 1612, 1488, 1181, 1028, 699 cmꢂ1 1H
.
General procedure for preparation of o-amino azo
compounds 11a–11c. 4-[1-(Pyridin-2-yl)-1H-pyrazol-3-yl] aniline
8 (2 g, 0.008 mol) was dissolved in 5N HCl (4 mL) with stirring and
was cooled to 0–5ꢀC, and sodium nitrite (0.69 g, 0.010 mol) was
added in portions. 2-Aminonaphthyl substituted sulfonic acid
10a–10c (1.78 g, 0.008 mol) was neutralized with 10% aqueous
sodium carbonate, and the diazo compound 9 was added to it
while carefully maintaining the pH of the solution at pH 6 by
adding sodium acetate. The solution was stirred at 10ꢀC for 2h
and then at 30ꢀC for 1 h. It was finally heated at 70ꢀC for 3h.
The red o-amino azo compounds 11a–11c were filtered and
washed with dilute sodium carbonate solution. Wet cake of o-amino
azo was directly used for the next step, that is, formation of
triazoles 12a–12c.
General procedure for the formation of triazole 12a–12c.
The o-amino azo compounds 11a–11c (0.01 mol) were dissolved
in DMF (5 mL), and basic cupric acetate (0.012mol) solution was
added to it. The reaction mixture was heated at 120ꢀC, and air
was rapidly bubbled through it. After 15–30 min, the red color
disappeared, and the mixture was poured in ice-cold water
(10mL). The solid that separated was filtered, washed well
with water, and dried in oven at 60ꢀC for 12h to yield the
crude triazolyl compounds, which were further recrystallized
from DMF.
NMR (DMSO-d6, d, ppm): 9.21 (d, 1H, J = 8.6, 2.2 Hz), 9.10
(d, 2H, J = 6.6, 2.5 Hz), 8.45–8.47 (d, 1H, J = 8.0, 2.2 Hz),
8.24–8.27 (d, 3H, J = 8.0, 2.2 Hz), 8.00–8.02 (d, 2H, J = 7.7,
4.0 Hz), 7.71 (d, 1H, J = 7.3, 2.4 Hz), 7.05 (d, 2H, J = 8.0,
1.0 Hz), 6.84–6.99 (t, 3H, J = 7.3, 6.8,1.4 Hz), 6.78 (d, 1H,
J = 8.0,1.4 Hz). 13C NMR (DMSO-d6, d, ppm): 149.67,
149.00, 138.29, 138.12, 136.76, 134.93, 134.34, 133.67,
132.54, 130.07, 128.99, 128.23, 127.92 (s), 126.56, 126.02,
125.45(s), 123.95, 123.21, 122.19, 120.43, 119.54. Mass:
m/z = 389.20 (M + 1). Anal. Calcd for C24H16N6: C, 74.21;
H, 4.15; N, 21.64. Found: C, 74.26; H, 4.12; N, 21.62.
CONCLUSION
In summary, three novel fluorescent 2-{4-[1-(pyridine-2-yl)-
1H-pyrazol-3-yl] phenyl}-2H-naphtho [1, 2-d] [1,2,3] triazo-
lyl derivatives were successfully synthesized from phenyl
acetonitrile, 2-hydrazinopyridine, and sulfonic acid substi-
tuted 2-naphthylamine. The position effect of the sulfonic
group on absorption and emission properties of compounds
was investigated. Photophysical properties study reveals that
synthesized triazoles have good quantum yields, absorption
ability in UV region, and emission in visible region of the
spectrum. The three triazoles are thermally stable up to
270ꢀC, and they can be good candidates as fluorescent
brightening agents.
Spectral data
2-{4-[1-(Pyridin-2-yl)-1H-pyrazol-4-yl]phenyl}-2H-naphtho
[1,2-d][1,2,3]triazole-6-sulfonic acid (12a). Purified yield:
73%. Yellowish brown solid, mp >300ꢀC (crystallized from
ethanol). FT-IR (KBr): 3643, 3085, 1612, 1487,1317,1180,1028,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2013 Synthesis of Novel Fluorescent 2-{4-[1-(Pyridine-2-yl)-1H-pyrazol-3-yl] phenyl}-2H-naphtho
[1,2-d] [1,2,3] triazolyl Derivatives and Evaluation of Their Thermal and Photophysical Properties
Acknowledgment. The authors are thankful to the Indian Institute
of Technology, Mumbai, for providing analytical support.
Khlebnikov, V.; Kim, J. Y.; Romero, K.; Wu, X. J Bioorg Med Chem Lett
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet