Carboxyl and nitrite functionalized graphene quantum dots as a highly active reagent and catalyst for rapid diazotization reaction and synthesis of azo-dyes under solvent-free conditions

Article abstract of DOI:10.1016/j.dyepig.2014.09.023

Carboxyl and nitrite functionalized graphene quantum dots was prepared from carboxyl and hydroxyl functionalized graphene quantum dots using NaNO₂ in the absence of mineral acids. This functional group conversion was confirmed by FT-IR spectroscopy, photoluminescence and X-ray diffraction. The carboxyl and nitrite functionalized graphene quantum dots was used as an effective nitrosonium ion source and reusable catalyst for the efficient diazotization of a variety of arylamines without using any additional acid. Subsequent azo-coupling of these freshly prepared diazonium salts with a range of active aromatic compounds led to the requisite azo-dyes in excellent yields in very short reaction times with a simple experimental procedure.

Full text of DOI:10.1016/j.dyepig.2014.09.023

Dyes and Pigments 113 (2015) 522e528
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Dyes and Pigments
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Carboxyl and nitrite functionalized graphene quantum dots as a highly
active reagent and catalyst for rapid diazotization reaction and
synthesis of azo-dyes under solvent-free conditions
,
Hassan Valizadeh a *, Ashkan shomali a, Saideh Nourshargh a, Rahim Mohammad-Rezaei b
a Organic Chemistry Research Lab., Department of Chemistry, Faculty of Sciences, Azarbaijan Shahid Madani University, P.O. Box: 53714-161, Tabriz, Iran
b Analytical Chemistry Research Lab., Faculty of Sciences, Azarbaijan Shahid Madani University, P.O. Box: 53714-161, Tabriz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Carboxyl and nitrite functionalized graphene quantum dots was prepared from carboxyl and hydroxyl
functionalized graphene quantum dots using NaNO2 in the absence of mineral acids. This functional
group conversion was confirmed by FT-IR spectroscopy, photoluminescence and X-ray diffraction. The
carboxyl and nitrite functionalized graphene quantum dots was used as an effective nitrosonium ion
source and reusable catalyst for the efficient diazotization of a variety of arylamines without using any
additional acid. Subsequent azo-coupling of these freshly prepared diazonium salts with a range of active
aromatic compounds led to the requisite azo-dyes in excellent yields in very short reaction times with a
simple experimental procedure.
Received 26 June 2014
Received in revised form
10 September 2014
Accepted 13 September 2014
Available online 20 September 2014
Keywords:
Azo-dyes
Carboxyl-functionalized GQDs
Nitrite-functionalized GODs
Solvent free azo-coupling
Diazotization
© 2014 Elsevier Ltd. All rights reserved.
Graphene catalyst
1. Introduction
these compounds [11e13]. Although each of these methods have
specific merits, they suffer from various drawbacks including long
Due to their diverse application in various fields of organic dye
chemistry, azo-dyes and pigments are among the most important
classes of organic colorants [1,2]. About three thousand azo dyes are
known and are currently in use worldwide. Most of these com-
pounds are monoazo compounds, which have the common struc-
ture unit of eN]Ne, linking two aromatic systems. Despite the
toxicity of some azo dyes, dozens of additional monoazo dyes are
applied in drugs and cosmetics [3]. Compounds containing an
arylazo group have been known as pharmaceutically important
materials in the literature [4,5]. Azo-dyes are active in biological
systems via their oxidationereduction behavior [6], and many in-
vestigations have been reported on the synthesis, spectroscopic
properties and dyeing performance of these compounds [7e10].
The synthesis of azo-dyes is usually carried out at low temperature
in two steps: diazotization of the arylamine with sodium nitrite in
hydrochloric or sulfuric acid followed by reaction with active aro-
matic compounds under alkaline conditions. Recently, new ver-
sions of this chemistry have been employed for the preparation of
reaction times and the use of concentrated acids such as hydro-
chloric or sulfuric acid. There is therefore considerable interest in
developing simple methods that require low manufacturing cost
while minimizing environmental pollution. Aromatic diazonium
salts are important intermediates for the synthesis of azo dyes
which are prepared via diazotization of arylamines using nitrite
salts and strong protic acids as a nitrosonium ion source instead of
nitrous acid. Organic nitrite esters have also been used as alterna-
tive sources of nitrosonium ion in the presence of protic solvents in
organic solvents [14,15]. Recently, we reported the synthesis of azo-
dyes using nitrite ionic liquid (IL-ONO) and its immobilized form on
silica as an affective nitrosonium sources [16,17]. Zarei et al. re-
ported the synthesis of diazonium salts using supported reagents at
room temperature [18]. Synthesis of 1-naphthol derived azo-dyes
in the presence of SiO2/HIO4 under solvent-free conditions has
also been reported [19].
Graphene has received much attention in physics, chemistry,
and in material science due to its unique properties, including high
surface area, electronic conductivity, and mechanical stability [20].
There is strong current interest in graphene quantum dots (GQDs)
which are graphene sheets smaller than 100 nm [21]. QDs have
been modified chemically to modulate their chemical and optical
* Corresponding author. Tel.: þ98 4133856447; fax: þ98 4134327541.
E-mail
addresses:
h-valizadeh@azaruniv.edu,
hvalizadeh2@yahoo.com
(H. Valizadeh).
http://dx.doi.org/10.1016/j.dyepig.2014.09.023
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

H. Valizadeh et al. / Dyes and Pigments 113 (2015) 522e528
523
Elmer FT-IR spectrometer.1H and 13C NMR spectrawere recorded on a
Bruker Avance AC-400 MHz using CDCl3 as solvent and TMS as an
internalstandard. Melting pointsare uncorrectedand were measured
in open glass-capillaries using Stuart melting point apparatus.
CNGQDs structure was characterized by X-ray diffractometry (XRD;
BrukerAXSmodelD8Advance). TEMimagewasrecordedwithPhilips
CM-10 (Eindhoven, The Netherlands). Fluorescence spectra and in-
tensity measurements were carried out using an FP-6200 spectro-
fluorometer (JASCO Corporation, Tokyo, Japan) with a wavelength
range of 220e730 nm (with 1 nm intervals).
2.2. Synthesis of carboxyl and nitrite functionalized graphene
quantum dots (CNGQDs)
Freshly prepared GQDs (25 g) was dissolved in water (100 mL).
Then a concentrated solution of sodium nitrite in water (18 mL) was
added dropwise to the GQDs solution at 0 ꢀC in 30 min. While cold,
NaCl saturated aqueous solution (25 mL) was added to the reaction
mixture and the modified GQDs were extracted with ethyl acetate
(3 ꢁ 25 mL). The solvent was evaporated and the residue was dried
under vacuum at room temperature for 4 h to produce carboxyl-
and nitrite-functionalized graphene quantum dots (CNGQDs;
19.5 g) (Scheme 2).
Scheme 1. GQDs-ONO as an effective nitrosonium source and highly active catalyst for
the synthesis of azo-dyes.
properties [22]. Zhang et al. have successfully prepared water-
soluble, uniform sized GQDs with strong yellow emission by
electro-chemical exfoliation of graphite followed by room tem-
perature reduction with hydrazine [23]. Carboxyl- and hydroxyl-
functionalized graphene quantum dots (CHGQDs) were synthe-
sized by Pan et al. [24]. In this report we prepared carboxyl- and
nitrite-functionalized graphene quantum dots (CNGQDs) by con-
version of HO-functionalized group of CHGQDs to eONO by using
NaNO2 in the absence of any additional acid. This functional group
conversion was confirmed by comparison of the FT-IR and photo-
luminescence of CHGQDs with CNGQDs.
Our interest in employing ionic liquids (ILs), water, or solvent-
free systems as green methods for organic transformations
[25e27] provided the impetus to employ CNGODs as an effective
nitrosonium source and active catalyst for facile diazotization of
anilines and subsequent synthesis of azo-dyes by reaction with
reactive aromatic compounds under solvent-free conditions at
room temperature (Scheme 1).
2.3. General procedure for the synthesis of azo dyes 6aeq
The mixture of aniline derivative (30 mmol) and CNGQDs (0.3 gr)
was thoroughly ground at room temperature for 15 min. The dia-
zonium salt product was assayed by well-known azo-coupling re-
action with either a phenol or an aniline derivative. For the coupling
reaction with phenolic compounds or anilines, the mixture was
continuously added to the phenol or aniline (30 mmol) which had
been moistened with water moistened with water (0.5 mL). Then
water (30 mL) was added to the mixture and stirred to dissolve the
GQDs. After that the participated dyes were filtered and washed
three times with cold water to afford the crude azo-dyes which
were purified by re-crystallization from EtOH/water. Saturated so-
lution of sodium chloride was added to the filtrate containing GQDs
while stirring, then the GQDs was extracted with ethyl acetate
3 ꢁ 15 mL and evaporation of ethyl acetate led to the GQDs.
2. Experimental
2.4. Selected spectroscopic data (see Table 2 for physical data and
comparison with literature data)
2.1. Chemicals
2.4.1. 1-(4-Nitrophenylazo)-2-naphthol (6a)
All reagents were purchased from Merck and used without further
C
16H11N3O3: FT-IR (KBr, cmꢂ1): 3401 (bs), 1660, 1619. 1H NMR
purification. Infrared spectra were recorded with KBr on a Perkin
(400 MHz, CDCl3)
d
(ppm): 12.06 (broad, OH); 8.48 (1H, dd, J ¼ 8.12
Scheme 2. Preparation of GQD-ONO from GQD-OH by using NaNO2.

524
H. Valizadeh et al. / Dyes and Pigments 113 (2015) 522e528
Fig. 1. A) TEM image of the synthesized GQDs; B) XRD patterns of CHGQDs and CNGQDs.
and 2.04 Hz); 7.87 (2H, d, J ¼ 7.98 Hz); 7.61 (2H, d, J ¼ 7.98); 7.57
(1H, d, J ¼ 7.25 Hz); 7.49 (1H, d, J ¼ 7.25 Hz); 7.48 (1H, dd, J ¼ 7.96
and 2.02 Hz); 7.44 (1H, dt, J ¼ 7.65 and 1.98 Hz); 7.42 (1H, dt, J ¼ 7.75
2.4.4. 2-(2-Carboxylphenylazo)-1H-indole-3-carbaldehyde (6g)
16H11N3O3: FT-IR (KBr, cmꢂ1): 3402, 3099, 1660, 1527, 1340; 1H
NMR (400 MHz, CDCl3) (ppm): 11.93 (1H, s, NH); 11.15 (1H, s,
C
d
and 1.96 Hz); 13C NMR (100 MHz, CDCl3)
d
(ppm): 142.33, 137.38,
COOH); 9.85 (1H, s, CHO); 8.17 (2H, d, J ¼ 7.61 Hz); 7.91 (2H, d,
134.08, 130.25, 128.57, 127.28, 124.82, 121.83, 120.28, 119.45, 119.18,
117.11, 115.61, 112.38.
J ¼ 7.46 Hz); 7.79 (2H, m); 7.45 (2H, m); 13C NMR (100 MHz, CDCl3)
d
(ppm): 163.15, 160.13, 134.42, 131.28, 130.93, 128.87, 127.94,
126.73, 125.64, 124.59, 124.16, 123.39, 122.52, 120.11, 117.89, 115.22.
2.4.2. 1-Phenylazo-2-naphthol (6b)
2.4.5. 2-Phenylazo-1H-indole-3-carbaldehyde (6i)
C
16H12N2O: FT-IR (KBr, cmꢂ1): 3410 (bs), 1668, 1615. 1H NMR
C
15H11N3O1: FT-IR (KBr, cmꢂ1): 1782, 1614, 1543, 1132; 1H NMR
(400 MHz, CDCl3)
d
(ppm): 12.08 (broad, OH); 8.05 (1H, dd, J ¼ 7.37
(400 MHz, CDCl3) d (ppm): 11.81 (1H, s, NH); 9.92 (1H, s, CHO); 8.12
and 2.13 Hz); 7.68 (2H, m); 7.55 (1H, d, J ¼ 7.82 Hz); 7.48 (1H, dd,
J ¼ 7.22 and 2.11 Hz); 7.46 (1H, dt, J ¼ 7.74 and 2.04 Hz); 7.45 (1H, d,
J ¼ 7.82 Hz); 7.42 (1H, dt, J ¼ 7.55 and 1.89 Hz); 7.17 (3H, m); 13C
(1H, dd, J ¼ 7.56 and 2.11 Hz); 7.41 (1H, dd, J ¼ 7.49 and 2.04 Hz);
7.85 (2H, m); 7.41 (2H, m); 7.23 (3H, m); 13C NMR (100 MHz, CDCl3)
d
(ppm): 163.54, 131.43, 130.92, 125.76, 124.55, 123.11, 122.74,
NMR (100 MHz, CDCl3)
d (ppm): 140.00, 138.25, 135.45, 128.86,
122.33, 121.55, 121.12, 118.42, 116.15, 113.84.
128.41, 127.58, 124.57, 121.41, 120.38, 120.16, 119.22, 117.27, 116.19,
110.21.
2.4.6. 4-(4-Nitrophenylazo) resorcinol (6j)
C
12H9N3O4: FT-IR (KBr, cmꢂ1): 3450 (bs), 3385, 1624, 1616. 1H
NMR (400 MHz, CDCl3) d (ppm): 12.03 (s, OH); 8.92 (s, OH); 7.85 (d,
2.4.3. 1-(4-Hydroxyphenylazo)-2-naphthol (6c)
C
16H12N2O2: FT-IR (KBr, cmꢂ1): 3412, 3390, 1661, 1615. 1H NMR
2H, J ¼ 8.20 Hz); 7.83 (2H, d, J ¼ 8.20 Hz); 7.74 (1H, d, J ¼ 8.15 Hz);
6.62 (1H, dd, J ¼ 8.15 and 2.41 Hz); 6.32 (1H, d, J ¼ 2.41 Hz). 13C NMR
(400 MHz, CDCl3)
d
(ppm): 12.11 (2H, OH); 7.69 (1H, d, J ¼ 7.58 Hz);
(100 MHz, CDCl3)
d (ppm): 156.24, 149.73, 148.14, 146.16, 130.91,
7.63 (1H, d, J ¼ 7.58 Hz); 7.55 (1H, dd, J ¼ 7.45 and 2.13 Hz); 7.49 (1H,
dt, J ¼ 7.61 and 2.08 Hz); 7.43 (1H, dt, J ¼ 7.59 and 2.14 Hz); 8.26 (1H,
dd, J ¼ 7.54 and 2.20 Hz); 7.59 (2H, d, J ¼ 7.51); 7.51 (2H, d, J ¼ 7.51);
128.35, 127.33, 120.91, 110.64, 105.88.
13C NMR (100 MHz, CDCl3)
d
(ppm): 159.55, 138.95, 135.44, 129.33,
2.4.7. 4-(4-Nitrophenylazo) N,N-dimethylaniline (6n)
128.56, 127.18, 123.53, 122.31, 122.30, 121.59, 120.42, 116.89, 115.91,
111.33.
C
14H14N4O2: FT-IR (KBr, cmꢂ1): 1613, 1512, 1479, 1536, 1335. 1H
NMR (400 MHz, CDCl3)
d
(ppm): 7.74 (2H, d, J ¼ 8.02 Hz); 7.71 (2H,
Fig. 2. A) FT-IR spectrum of CHGQDs and CNGQDs; B) Fluorescence microscopy spectra of CHGQDs and CNGQDs.

H. Valizadeh et al. / Dyes and Pigments 113 (2015) 522e528
525
Table 1
Diazotization of anilines using CNGQDs and synthesis of azo dyes.
Entry
Amine
Amine number
Coupling reagent
Product
Product number
OH
NO2
N
1
3a
6a
O2N
NH2
N
OH
OH
N
2
3b
6b
NH2
N
OH
OH
N
3
3c
6c
HO
NH2
N
OH
Cl
Cl
CHO
NO2
N
4
3d
6d
N
HO
O2N
NH2
OH
CHO
CHO
CHO
OH
N
5
6
7
3e
3f
6e
6f
HO
NH2
N
N
N
H
H
CHO
CHO
Br
N
Br
NH2
N
N
H
N
H
CHO
COOH
CHO
N
3g
6g
N
NH2
N
H
N
H
COOH
NO2
N
NH2
8
9
3h
6h
O2N
OH
N
HO
CHO
CHO
N
3i
6i
NH2
N
N
H
N
H
(continued on next page)

526
H. Valizadeh et al. / Dyes and Pigments 113 (2015) 522e528
Table 1 (continued )
Entry
Amine
Amine number
Coupling reagent
Product
Product number
OH
OH
NH2
NO2
10
3j
N
6j
O2N
N
HO
OH
CHO
CHO
Me
N
11
12
3k
3l
6k
6l
Me
NH2
N
N
N
H
H
NO2
N
NH2
H2N
O2N
N
H2N
N
13
14
15
3m
3n
3o
6m
6n
6o
NH2
Me2N
Me2N
Et2N
N
Me2N
Me2N
Et2N
NO2
N
NH2
O2N
N
NO2
N
NH2
O2N
N
CHO
NO2
N
N
HO
NH2
16
17
3p
3q
6p
6q
O2N
OH
CHO
OH
NO2
N
N
HO
NH2
O2N
OH
HO
d, J ¼ 8.02); 7.66 (2H, d, J ¼ 7.95 Hz); 6.65 (2H, d, J ¼ 7.95 Hz); 3.61
(6H, s, NMe2); 13C NMR (100 MHz, CDCl3)
(ppm): 135.52, 128.83,
126.53, 125.54, 123.33, 120.17, 117.84, 115.11, 35.12.
3. Result and discussion
3.1. Preparation and structural features of CHGQDs
d
Carboxyl- and hydroxyl-functionalized graphene quantum dots
(CHGQDs) was prepared according to the literature procedure [25].
Fig.1A shows the TEM image of the synthesized GQDs [28]. It can be
seen that nano-size sheets of CHGQDs are dispersed in the solution.
CHGQDs were converted to carboxyl- and nitrite-functionalized
graphene quantum dots (CNGQDs). The typical XRD patterns of
CHGQDs and CNGQDs are shown in Fig. 1B. The wide-angle peaks
belong to the (002) planes of GQDs according to previous reports
2.4.8. 2-Hydroxy-5-(4-nitrophenylazo) benzaldehyde (6p)
C13H9N3O4: FT-IR (KBr, cmꢂ1): 3412, 3113, 1648, 1606, 1522,
1479, 1332, 1285. 1H NMR (400 MHz, CDCl3)
d (ppm): 11.48 (1H,
s); 10.06 (1H, s); 8.37 (2H, d, J ¼ 7.26 Hz); 8.28 (1H, d,
J ¼ 2.21 Hz); 8.25 (1H, dd, J ¼ 7.21, 2.21 Hz); 8.07 (2H, d, 7.26 Hz);
7.19 (1H, d, J ¼ 7.21 Hz). 13C NMR (100 MHz, CDCl3)
d (ppm):
160.19, 135.83, 134.18, 130.21, 129.34, 128.62, 121.51, 117.67, 112.16,
110.99.

H. Valizadeh et al. / Dyes and Pigments 113 (2015) 522e528
527
[24,29]. The (002) interlayer spacing of CHGQDs and CNGQDs are
3.82 Å and 3.77 Å respectively (Fig. 1B), which are larger than that
of bulk graphite (3.34 Å). During the modification process in which
eOH groups were converted to eONO groups on the GQDs, the
interlayer spacing did not changed noticeably.
The FT-IR and photoluminescence of two CHGQDs and CNGQDs
are shown in Fig. 2. The data are consistent with conversion of
CHGQDs to CNGQDs. As it can be seen from Fig. 2A the signal at
1375 cmꢂ1 in FT-IR spectrum of CNGQDs is related to N]O bond.
Fig. 2B shows the photoluminescence of CHGQDs and CNGQDs on
excitation at 388 nm. The strong photoluminescence peaks at
503 nm in the case of CHGQDs and CNGQDs originate from free
zigzag sites with a carbene-like triplet ground state [30]. The main-
tenance and heightening of the photoluminescence of the CNGQDs
shows the success of GQDs modificationprocess, and the potential of
CNGQDs in fluorescence sensing and imaging applications.
substrates examined, the reactions were completed within
15e25 min under solvent-free conditions. Moreover, faster re-
actions and generally higher yields of azo-dyes were observed as
compared to our previously reported reaction using task-specific
nitrite ionic liquids [11,37,38]. We examined this procedure on a
50 mmol scale and found that they underwent a smooth trans-
formation to the related derivative of azo-dyes in good yields,
suggesting that the present procedure is amenable to scale-up.
Upon completion, the CHGQDs was extracted with ethyl acetate
and the extracted CHGQDs was converted into the nitrite func-
tionalized CNGQDs. CHGQDs could be recovered with high recovery
>96% in each cycle. In representative cases the azo-coupling reac-
tion was repeated 3 times by using recovered CHGQDs and the
corresponding azo-dye was isolated in 89e90% yield, showing that
CHGQDs can be recycled with minimal loss of activity.
In summary, we have introduced carboxyl- and nitrite-
functionalized graphene quantum dots, CNGQDs as an effective
nitrosonium source and active catalyst for solvent and acid-free
diazotization reaction and synthesis of azo-dyes in excellent
yields and with short reaction times. The present acid and base free
protocol is applicable to the synthesis of a variety of azo dyes.
3.2. Reactions
The reaction of p-nitroaniline (1 mmol) with NaNO2 in aqueous
HOAc at 0e5 ꢀC and subsequent azo-coupling with sodium
b-
naphthoxide was used as model. This gave product 6a in 25% yield
after 2 h (based on p-nitroaniline). By comparison, the reaction rate
improved noticeably when carboxyl-functionalized CHGQDs was
used instead of HOAc. The model reaction proceeded smoothly to
give a 95% yield of 6a in only 15 min at 0e5 ꢀC under these con-
ditions. We then transformed the hydroxyl groups of CHGQDs to
nitrite in order to examine its dual role as catalyst and nirosonium
source. A very fast reaction was observed at 0 ꢀC with almost
complete conversion in less than 15 min in the absence of any
added acid when the CNGQDs were employed. We also examined
Acknowledgment
We are most grateful to Prof. Kenneth Laali (University of North
Florida, Department of Chemistry) for reviewing the manuscript
before submission and for many helpful remarks and suggestions
for improvement.
The partial financial assistance from the Research Vice Chan-
cellor of Azarbaijan Shahid Madani University is gratefully
acknowledged.
the reaction by addition of
b-naphthol instead of its anion and
found that the reaction was completed in less than 20 min and the
expected azo dye 6b was isolated in 96% yield (entry 2). Solvent-
free conditions at room temperature were also evaluated for this
procedure and no difference was observed in comparison with the
reaction in water. The generality of this promising reaction was
evaluated by employing a variety of aniline derivatives, and
different coupling reagents (Table 1).
References
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Table 2
Melting points of the products and comparison of their yields with the reported
values.
Entry Product Time (min)
number
M.P (ꢀC)
Yielda (%)
Diazonium Azo Found
Reported
[Ref.]
Found Reporetd
[Ref.]
salts
dyes
1
2
3
4
5
6
7
8
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
6o
6p
6q
5
10
10
5
10
10
5
5
5
5
5
5
5
5
5
5
5
10
10
10
15
15
15
10
10
15
10
15
10
15
10
10
15
10
249e250 248e252 [31] 98%
130e131 131e132 [32] 96%
191e193 192e193 [33] 94%
125e126 127e128 [34] 89%
194e197 194e197 [18] 89%
160e163 162e163 [18] 93%
192e195 190e195 [18] 94%
205e207 207 [17]
191e194 190e195 [18] 91%
183e185 185 [33] 95%
131e135 130e135 [18] 89%
201e202 199e202 [35] 91%
110e112 108e112 [35] 90%
223e226 225e228 [19] 94%
143e145 145 [17]
183e185 184e186 [36] 93%
188e190 188e190 [35] 95%
92% [18]
87% [17]
88% [18]
78% [34]
81% [18]
82% [18]
84% [18]
64% [36]
77% [18]
90% [18]
74% [18]
90% [17]
81% [19]
74% [35]
85% [17]
85% [34]
88% [17]
96%
9
10
11
12
13
14
15
16
17
95%
[16] Vogel A. Textbook of practical organic chemistry including qualitative organic
analysis. 5th ed. New York: Longman; 1978.
a
Isolated yield.

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H. Valizadeh et al. / Dyes and Pigments 113 (2015) 522e528
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