Pentacenequinone derivatives for preparation of gold nanoparticles: Facile synthesis and catalytic application

Article abstract of DOI:10.1039/c4ta00397g

We have designed and synthesized pentacenequinone/pentacenequinodimethane derivatives 3, 4 and 6 having nitrile groups using Suzuki-Miyaura coupling and Knoevenagel condensation protocols. Interestingly, these pentacenequinone/ pentacenequinodimethane derivatives form fluorescent aggregates in aqueous media due to their aggregation-induced emission enhancement (AIEE) characteristics. Further these fluorescent aggregates generate gold nanoparticles at room temperature without addition of any external reducing agent, stabilizing agent and shape-directing additives. In addition, these nanomaterials have been found to have high catalytic activity for the reduction of p-nitroaniline to p-phenylenediamine.

Full text of DOI:10.1039/c4ta00397g

Journal of
Materials Chemistry A
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Pentacenequinone derivatives for preparation of
gold nanoparticles: facile synthesis and catalytic
application†
Cite this: J. Mater. Chem. A, 2014, 2,
8369
*
Kamaldeep Sharma nee Kamaldeep, Sandeep Kaur, Vandana Bhalla, Manoj Kumar
and Ankush Gupta
We have designed and synthesized pentacenequinone/pentacenequinodimethane derivatives 3, 4 and 6
having nitrile groups using Suzuki–Miyaura coupling and Knoevenagel condensation protocols.
Interestingly, these pentacenequinone/pentacenequinodimethane derivatives form fluorescent
aggregates in aqueous media due to their aggregation-induced emission enhancement (AIEE)
characteristics. Further these fluorescent aggregates generate gold nanoparticles at room temperature
without addition of any external reducing agent, stabilizing agent and shape-directing additives. In
addition, these nanomaterials have been found to have high catalytic activity for the reduction of p-
nitroaniline to p-phenylenediamine.
Received 22nd January 2014
Accepted 21st March 2014
DOI: 10.1039/c4ta00397g
www.rsc.org/MaterialsA
different morphologies and hence designed and synthesized
novel pentacenequinone/pentacenequinodimethane deriva-
1. Introduction
tives 3, 4 and 6 having nitrile groups. We envisaged that the
target molecules may form uorescent aggregates in aqueous
media and the presence of nitrile groups will facilitate interac-
tion with gold ions.16–18 Further, aer interaction with uores-
cent aggregates, gold ions may be reduced to gold nanoparticles
through the compartmentalization mechanism as in the case of
micellar aggregates.19 We further envisioned that the position of
nitrile groups could inuence the shape and size of gold
nanoparticles. Interestingly, derivatives 3, 4 and 6 exhibit AIEE
characteristics and uorescent aggregates of these derivatives
serve as reactors for preparation of gold nanoparticles. Further,
aggregates of derivative 3 can detect gold ions in the nanomolar
range and reduce gold ions to generate gold nanospheres of size
5–10 nm whereas aggregates of derivatives 4 and 6 generated
prisms on reduction of gold ions. To the best of our knowledge,
this is the rst report where uorescent aggregates of pentace-
Recently, preparation of gold nanoparticles with different
morphologies has attracted a lot of research interest1 due to
their size and shape dependent catalytic properties.2,3 Nano-
particles with high surface to volume ratio act as excellent
catalysts in organic reactions.4 The shape and size controlled
catalysis of nitro compounds compared to the catalytic prop-
erties of gold nanospheres,5 gold nanorods,6 gold nanoprisms7
and gold nanowires8 organized into a network has been widely
investigated and it has been found that gold nanospheres
having small size show maximum catalytic efficiency.4 Thus, it
is very important to control the shape of nanoparticles for
excellent catalytic efficiency. Among various approaches used
for the preparation of gold nanoparticles, the wet chemical
route is an effective approach for the preparation of gold
nanoparticles.9–14 However, this route suffers from the limita-
tions of longer reaction time and high temperature conditions.
Further gold nanoparticles of bigger size are generated using
this route. Therefore, development of a method for facile and
rapid preparation of gold nanoparticles of different morphol-
ogies and small size is still a challenge. Recently, we reported
aggregates of a pentacenequinone derivative which serve as
reactors for preparation of palladium nanoparticles.15 In
continuation of this work, we are now interested in the devel-
opment of methods for preparation of gold nanoparticles of
nequinone/pentacenequinodimethane
derivatives
having
nitrile groups have been used for facile and rapid preparation of
gold nanoparticles of different shape and size. Previously, small
molecules/polymers having amino groups have been used for
generation of gold nanoparticles,9,10,12,13 however, in compar-
ison to those materials, aggregates of pentacenequinone/pen-
tacenequinodimethane derivatives having nitrile groups
provide a clean, fast and facile method for preparation of gold
nanoparticles of different shape and size (see pS4 in the ESI†).
Further, the lower limit of detection for gold ions is better than
the limit of detection reported previously in the literature (see
pS5 in the ESI†). In addition, these nanomaterials have been
found to catalyze the chemical reduction of p-nitroaniline to p-
phenylenediamine. The reduction of p-nitroaniline is important
Department of Chemistry, UGC Sponsored-Centre for Advanced Studies-I, Guru Nanak
Dev University, Amritsar-143005, Punjab, India. E-mail: vanmanan@yahoo.co.in; Fax:
+91 (0)183 2258820; Tel: +91 (0)183 2258802-9 ext. 3202
† Electronic supplementary information (ESI) available: Optical and spectroscopic
data of derivatives 3–6. See DOI: 10.1039/c4ta00397g
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as it is widely used in industry because its reduction product, the reaction mixture was reuxed overnight. Chlorobenzene
p-phenylenediamine, is used in the preparation of azo dyes, fur was then removed under vacuum, and the residue so obtained
dyes, polymers, rubber antioxidants and photo-developers.20 was treated with water, extracted with 50 ml of dichloro-
The catalytic activity of these nanomaterials is much better than methane, and dried over anhydrous Na2SO4. The organic layer
the catalytic activity of the systems reported in the literature for was evaporated, and the compound was puried by column
this type of reduction (see pS3 in the ESI†).
chromatography using (80 : 20) (chloroform : hexane) as an
eluent to give 0.178 g (30%) of compound 4 as a yellowish
orange solid; m.p. >260 ꢀC; 1H NMR (300 MHz, CDCl3): d ¼ 7.32
[d, 4H, J ¼ 8.1 Hz, ArH], 7.63 [d, 4H, J ¼ 8.7 ArH], 7.78–7.81 [m,
2H, ArH], 8.08–8.10 [m, 2H, ArH], 8.12 [s, 2H, ArH], 8.77 [s, 2H,
ArH]; 8.81 [s, 2H, ArH]; 13C NMR (75.45 MHz, CDCl3): 82.31,
112.00, 113.35, 118.23, 126.17, 127.82, 128.70, 129.18, 129.42,
130.40, 130.65, 131.12, 132.31, 132.76, 133.43, 141.09, 143.72,
158.04, 161.18; TOF MS ES+: 629.14 (M + Na)+; elemental anal-
ysis: calcd for C42H18N6: C 83.16; H 2.99; N 13.85; found: C
83.11; H 2.90; N 13.79%.
2. Experimental section
2.1. Materials
All reagents were purchased from Aldrich and were used
without further purication. THF was dried over sodium and
benzophenone and kept over molecular sieves overnight
before use. Silica gel (60–120 mesh) was used for column
chromatography.
Compound 5. To a solution of 1 (0.5 g, 1.07 mmol) and 2a
(0.72 g, 2.25 mmol) in 1,4-dioxane (20 ml) were added K2CO3
(1.18 g, 8.56 mmol), distilled H2O (3 ml), and [Pd(PPh3)4] (0.273 g,
0.236 mmol) under N2, and the reaction mixture was reuxed
overnight. 1,4-Dioxane was then removed under vacuum, and the
residue so obtained was treated with water, extracted with
dichloromethane, and dried over anhydrous Na2SO4. The organic
layer was evaporated, and the compound was puried by column
chromatography using 1 : 1 (chloroform : hexane) as an eluent to
give 0.248 g (35%) of compound 5 as a yellow solid; m.p. >260 ꢀC;
1H NMR (300 MHz, CDCl3): d ¼ 0.92 [t, 6H, J ¼ 6.6 Hz,
OCH2CH2(CH2)3CH3], 1.26 [br, 4H, OCH2CH2CH2CH2CH2CH3],
1.35–1.37 [m, 4H, OCH2CH2CH2CH2CH2CH3], 1.48–1.55 [m, 4H,
2.2. Instruments
UV-vis spectra were recorded on a SHIMADZU UV-2450 spec-
trophotometer, with a quartz cuvette (path length, 1 cm). The
cell holder was thermostatted at 25 ꢀC. Fluorescence spectra
were recorded with a SHIMADZU 5301 PC spectrouorimeter.
Scanning electron microscopy (SEM) images were obtained with
a eld-emission scanning electron microscope (SEM CARL
ZEISS SUPRA 55). Polarized optical microscopy (POM) images
were recorded on a NIKON ECLIPSE LV100 POL. Elemental
analysis (C, H, N) was performed on a Flash EA 1112 CHNS-O
1
analyzer (Thermo Electron Corp.). H NMR spectra were recor-
ded on a JEOL-FT NMR-AL 300 MHz spectrophotometer using
CDCl3 as solvent and tetramethylsilane SiMe4 as internal stan-
dard. UV-vis studies were performed in THF and H2O–THF
mixture. Data are reported as follows: chemical shis in ppm
(d), multiplicity (s ¼ singlet, d ¼ doublet, br ¼ broad singlet, m
¼ multiplet), coupling constant J (Hz), integration, and
interpretation.
OCH2CH2CH2CH2CH2CH3], 1.79 [t, 4H,
J
¼
7.35 Hz,
OCH2CH2(CH2)3CH3], 3.97 [t, 4H, J ¼ 6.6 Hz, OCH2CH2-
(CH2)3CH3], 6.83 [d, 4H, J ¼ 8.7 Hz, ArH], 7.16 [d, 4H, J ¼ 8.7 ArH],
7.69–7.72 [m, 2H, ArH], 8.09 [s, 2H, ArH], 8.11–8.15 (m, 2H, ArH),
8.94 [s, 2H, ArH], 8.95 (s, 2H, ArH); 13C-NMR d (75.45 MHz,
CDCl3): 14.04, 22.62, 25.76, 29.27, 31.62, 68.04, 99.99, 114.22,
129.47, 129.78, 130.12, 130.60, 130.69, 130.95, 131.14, 132.63,
134.40, 135.27, 142.75, 158.55, 182.94; TOF MS ES+: 685.4335 (M
2.3. Synthesis
Compound 3. To a solution of 1 (0.50 g, 1.07 mmol) and 2 + Na + 2)+; elemental analysis: calcd for C46H44O4: C 83.60; H 6.71;
(0.33 g, 2.25 mmol) in 1,4-dioxane (20 ml) were added K2CO3 found: C 83.55; H 6.60%.
(0.89 g, 6.42 mmol), distilled H2O (2.10 ml), and [Pd(PPh3)4]
Compound 6. To a mixture of 5 (0.1 g, 0.151 mmol) in 10 ml
(0.27 g, 0.24 mmol) under N2, and the reaction mixture was dry pyridine were added 0.2 ml of TiCl4 and malononitrile
reuxed overnight. 1,4-Dioxane was then removed under (0.199 g, 3.02 mmol) under N2, and the reaction mixture was
vacuum, and the residue so obtained was treated with water, reuxed overnight. Aer that the reaction mixture was allowed
extracted with dichloromethane, and dried over anhydrous to cool to room temperature. The mixture was then diluted with
Na2SO4. The organic layer was evaporated, and the compound dichloromethane and washed with water in 2 N HCl. The
was puried by column chromatography using 80 : 20 (chlor- organic layer was separated and dried over anhydrous Na2SO4.
oform : hexane) as an eluent to give 0.191 g (35%) of compound The solvent was evaporated under reduced pressure to get the
3 as a yellow solid; m.p. >260 ꢀC; 1H NMR (300 MHz, CDCl3): d ¼ crude product. The crude product was puried by column
7.35 [d, 4H, J ¼ 8.1 Hz, ArH], 7.63 [d, 4H, J ¼ 8.1 Hz, ArH], 7.73– chromatography using 1 : 1 (hexane : chloroform) as an eluent
7.76 [m, 2H, ArH], 8.14–8.18 [m, 2H, ArH], 8.19 [s, 2H, ArH], 8.98 to give 0.246 g (43%) compound 6 as a red solid; m.p. >260 ꢀC;
[s, 2H, ArH]; 9.03 [s, 2H, ArH]; TOF MS ES+: 511.0320; elemental 1H NMR (400 MHz, CDCl3): d ¼ 0.85 [t, 6H, J ¼ 6.0 Hz,
analysis: calcd for C36H18N2O2: C 84.69; H 3.55; N 5.49; found: C OCH2CH2(CH2)3CH3], 1.18 [br, 4H, OCH2CH2CH2CH2CH2CH3],
84.32; H 3.49; N 5.39%. Due to the poor solubility of compound 1.28–1.29 [m, 4H, OCH2CH2CH2CH2CH2CH3], 1.40 [br, 4H,
3, its 13C NMR spectrum could not be recorded.
Compound 4. To a solution of 3 (0.50 g, 0.98 mmol) and OCH2CH2(CH2)3CH3], 3.89 [t, 4H,
malononitrile (0.76 g, 19.60 mmol) in 50 ml dry chlorobenzene (CH2)3CH3], 6.75 [d, 4H, J ¼ 8 Hz, ArH], 7.07 [d, 4H, J ¼ 8 Hz,
were added 0.75 ml pyridine and 0.50 ml of TiCl4 under N2, and ArH], 7.68–7.70 [m, 2H, ArH], 7.94 [s, 2H, ArH], 7.98–8.00
OCH2CH2CH2CH2CH2CH3], 1.72 [t, 4H,
J
¼
8
Hz,
J
¼
6 Hz, OCH2CH2-
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[m, 2H, ArH], 8.65 [s, 2H, ArH], 8.68 [s, 2H, ArH]; 13C-NMR d derivative 3 in the solvent mixture of H2O–THF (6/4) conrms
(75.45 MHz, CDCl3): 13.89, 22.48, 25.62, 29.14, 31.51, 68.06, the presence of interconnected rods (Fig. 1A). The rods formed
81.36, 113.76, 114.40, 126.41, 126.67, 128.70, 129.00, 129.41, were visibly transparent and stable at room temperature for
130.43, 130.96, 132.20, 132.58, 133.47, 143.96, 154.62, 158.91, several weeks.
161.83; TOF MS ES+: 778.5134 (M + Na)+; elemental analysis:
calcd for C52H44N4O2: C 82.51; H 5.86; N 7.40; O 4.23; found: C excited at 310 nm. However, a dramatic change in uorescence
The solution of derivative 3 in THF is weakly emissive when
82.46; H 5.80; N 7.36; O 4.20%.
intensity is observed on addition of water to the THF solution of
derivative 3. On addition of 60% volume fraction of water dual
emission at 476 and 562 nm is observed along with enhance-
ment of uorescence intensity (Fig. 1B).
3. Results and discussion
The emission maxima at 476 and 562 nm result from the
locally excited state and the intermolecular charge-transfer
state, respectively. We believe that head to tail alignment of the
molecules (J-aggregation) is responsible for the appearance of
an emission band at longer wavelength (Fig. S2 in the ESI†).26–29
Further, the restriction of intramolecular rotation due to
formation of J-aggregates rigidies the molecule and induces
planarization of the molecule, thus, making it more emissive.29
However, a further increase of water fraction resulted in a
decrease in emission intensity (Fig. S3A in the ESI†). This
phenomenon is oen observed in compounds with AIEE
properties as aer the aggregation only the molecules on the
surface of the aggregate emit light and contribute to the uo-
rescence intensity upon excitation, and this leads to a decrease
in emission intensity.30–33 We obtained SEM images of derivative
3 in 60%, 80% and 90% H2O–THF mixtures and in all cases rods
were formed with no signicant difference in size (Fig. S3B in
Pentacenequinone derivative 3 was synthesized by Suzuki–
Miyaura coupling of boronic acid 221 with 2,3-dibromopenta-
cenequinone 1,22 in 35% yield (Scheme 1). The structure of
derivative 3 was conrmed from its spectroscopic and analytical
data. The 1H NMR spectrum of derivative 3 showed three
singlets at 8.19, 8.99 and 9.04 ppm, two doublets at 7.35 and
7.63 ppm and two multiplets from 7.73–7.76 and 8.14–8.18
ppm, corresponding to the aromatic protons (Fig. S36 in ESI†).
The mass spectrum of derivative 3 showed a parent ion peak at
m/z 511.0320 (M + 1)+ (Fig. S37 in the ESI†). These spectroscopic
data corroborate structure 3 for this compound.
The UV-vis spectrum of derivative 3 in THF exhibits an
absorption band at 310 nm. On addition of water (#60%
volume fractions) to the THF solution of derivative 3, a new
band is observed at 423 nm along with the appearance of a level-
off long wavelength tail (Fig. S1 in ESI†). This new band may be
due to the intermolecular charge transfer (ICT) state.23,24 This
interaction originates from the overlapping of the pentacene-
quinone moiety with the nitrile groups of the neighbouring
molecule which increases the molecular dipole and the ICT
state becomes prominent, thus, resulting in the formation of a
new absorption band (Fig. S1 in the ESI†).24 Further, Mie scat-
tering in the visible region is attributed to the formation of
aggregates.25 The scanning electron microscopy (SEM) image of
Fig. 1 (A) SEM image of derivative 3 showing the rods in a solvent
mixture of H2O–THF (6/4). Scale bar 20 mm. (B) Fluorescence spectra
of 3 (10 mM) showing the variation of fluorescence intensity in various
Scheme
1 Synthesis of pentacenequinone/pentacenequinodi- H2O–THF mixtures. Inset photographs (under 365 nm UV light): 0%
methane derivatives 3, 4, 5 and 6.
and 60% water in THF.
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the ESI†). This shows that the size of the particles does not
There was signicant quenching in uorescence emission
change on increasing the water fraction, thus, the observed upon addition of Au3+ ions (0–50 equiv.) to the solution of
change in the emission behavior may be attributed to the derivative 3 (H2O–THF, 6/4) (Fig. 2B). The changes in absorption
decrease in the number of emitting molecules.
and emission spectra suggest interaction of gold ions with the
The presence of nitrile groups at the periphery of the pen- nitrile groups of the rods of derivative 3, and their subsequent
tacenequinone core prompted us to investigate its sensing reduction to zero valent gold atoms. The calculated detection
behaviour toward different metal ions by UV-vis and uores- limit is found to be 100 ꢁ 10ꢂ9 M for Au3+ ions and the calcu-
cence spectroscopy. The UV-vis studies of aggregates of deriva- lated Stern–Volmer constant of aggregates of derivative 3 for
tive 3 (10 mM) in H2O–THF (6/4) were carried out toward Au3+ ions is 5.53 ꢁ 103 Mꢂ1 (Fig. S10 in the ESI†). Under the
different metal ions such as Cd2+, Ba2+, Hg2+, Fe3+, Ni2+, Zn2+, same conditions as used for Au3+ ions, we tested the behaviour
Cu2+, Pb2+, Ca2+, Co2+, Ag+, Au3+, Na+, K+, Li+, Pd2+, Cr3+, and Al3+ of aggregates of derivative 3 toward other metal ions such as
ions as their perchlorate/chloride/or both perchlorate and Cd2+, Ba2+, Hg2+, Fe3+, Ni2+, Zn2+, Cu2+, Pb2+, Ca2+, Co2+, Ag+,
chloride salts (Fig. S5–S6 in the ESI†).
Au3+, Na+, K+, Li+, Pd2+, Cr3+ and Al3+ ions (Fig. S11 and S12 in
Among the various metal ions tested aggregates of derivative the ESI†) but no signicant change in uorescence intensity was
3 show selectivity toward Au3+ ions (Fig. S7 in the ESI†). Upon observed except in the case of Cu2+ and Fe3+ ions where 12%
addition of Au3+ ions (50 equiv.) to the solution of derivative 3 in and 39% quenching of uorescence emission was observed,
mixed aqueous media (H2O–THF, 6/4), the absorption band at respectively (Fig. S7A in the ESI†). Further the aggregates of
305 is red-shied to 318 nm which suggests interaction between derivative 3 serve as a more sensitive probe for Au3+ ions in
aggregates of derivative 3 and Au3+ ions (Fig. 2A). A new comparison to other Au3+ ion receptors reported in the litera-
absorption band is observed at 562 nm which indicates ture (see pS5 in the ESI†). To get insight into the mechanism of
formation of gold nanoparticles (inset of Fig. 2A and S8 in the formation of gold nanoparticles, we slowly evaporated the
ESI†).34 The intensity of this band increases with time and the solution of aggregates of derivative 3 containing gold nano-
rate constant35 for the formation of gold nanoparticles was particles. Aer several days, precipitates were observed. The
found to be 1.43 ꢁ 10ꢂ3 sꢂ1 (see pS13 in the ESI†).
precipitates were ltered and washed with THF. The 1H NMR of
the residue obtained aer evaporation of THF solution showed
the upeld shi of aromatic signals (Fig. S13 in the ESI†) and its
IR spectrum showed a new band at 1725 cmꢂ1 attributable to
the carbonyl stretching vibrations. Further, the intensity of the
band corresponding to the –CN group was reduced with a slight
shi (from 2229 to 2227 cmꢂ1) towards lower frequency
(Fig. S14 and S15 in the ESI†). Though we are not able to fully
explain the mechanism of reduction yet on the basis of the
above results we suggest that upon addition of Au3+ ions to the
solution of aggregates of derivative 3, the gold ions enter the
network of interconnected channels, interact with nitrile
moieties16–18 and get reduced. Thus, aggregates of derivative 3
function as a reducing agent as well as a stabilizing agent for the
preparation of gold nanoparticles.11,36 We also carried out
UV-vis studies of derivative 3 in THF towards Au3+ ions.
However, the absorption spectrum does not indicate formation
of nanoparticles (Fig. S16 in the ESI†). These studies suggest the
importance of water in the preparation of gold nanoparticles.
The polarized optical microscopic (POM) image of the
sample containing gold nanoparticles of derivative 3 shows
birefringence at room temperature, thus indicating an ordered
morphology (Fig. S17 in the ESI†). The powder X-ray diffraction
(XRD) analysis of the sample containing gold nanoparticles of
derivative 3 (Fig. S18 in the ESI†) showed the characteristic
pattern of face-centered cubic (fcc) gold (0).37
The TEM image of aggregates of derivative 3 in the presence
of gold ions shows the presence of nanospheres (Fig. 3). We also
tested generation of gold nanoparticles using 4-bromobenzo-
nitrile. Upon addition of Au3+ ions (50 equiv.) to the solution of
4-bromobenzonitrile in mixed aqueous media (H2O–THF, 6/4),
no absorption band corresponding to gold nanoparticles was
observed in the visible region. However, aer allowing the
solution to stand for one hour a broad absorption band with
Fig. 2 (A) UV-vis spectra of derivative 3 (10 mM) upon addition of Au3+
ions (50 equiv.) and with time in H2O–THF (6/4). Inset: enlarged UV-vis
spectra in the range 450–700 nm showing a band at 562 nm. (B)
Fluorescence spectra of derivative 3 (10 mM) upon addition of Au3+ ions
in H2O–THF (6/4); buffered with HEPES, pH ¼ 7.0.
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UV-visible and uorescence studies of derivatives 4 and 6
showed that these derivatives form uorescent aggregates in the
H2O–THF mixture due to their AIEE-attributes (Fig. S21–S24 in
the ESI†). We obtained the SEM images of derivatives 4 and 6 in
mixed H2O–THF solvent mixtures (Fig. S4A and S4B in the ESI†).
The SEM image of aggregates of derivative 4 shows the presence
of spherical aggregates in the 60% H2O–THF mixture (6/4)
(Fig. S4A(a) in the ESI†), however, a rod like morphology was
observed on increasing the water fraction from 60% to 90%
(Fig. S4A(b) in the ESI†). No signicant change in size was
observed in the case of spherical aggregates of derivative 6 on
increasing the water fraction from 60% to 90% (Fig. S4B in the
ESI†). The absorption and uorescence studies of aggregates of
Fig. 3 TEM image of gold nanoparticles of derivative 3; scale bar
20 nm.
weak intensity is observed at 540 nm (inset of Fig. S19 in the derivatives 4 and 6 (10 mM) were carried out in H2O–THF toward
ESI†).38 The SEM image of the solution of 4-bromobenzonitrile Au3+ ions (Fig. S25–S28 in the ESI†) and it was found that
in the presence of gold ions shows the presence of aggregated aggregates of derivatives 4 and 6 serve as nanoreactors for
spherical gold nanoparticles (Fig. S20 in the ESI†). However, preparation of gold nanoparticles. The formation of gold nano-
non-aggregated spherical gold nanoparticles are obtained using particles is supported by powder XRD and TEM studies (Fig. S29–
aggregates of derivative 3 in the presence of gold ions (Fig. 3). S31 in the ESI†). The TEM image of gold nanoparticles generated
This study highlights the importance of aggregates of derivative by aggregates of derivative 4 shows the presence of prisms
3 for preparation of non-aggregated gold nanoparticles. To get (Fig. 4A). On the other hand, nanoprisms as well as nanospheres
insight into the role of the position of nitrile groups, we also were observed in the TEM image of gold nanoparticles generated
prepared pentacenequinodimethane derivatives
4
and
6
by aggregates of derivative 6 (Fig. 4B). On the basis of these
(Scheme 1). The Knoevenagel condensation of derivative 3 with results, we may conclude that the aggregates of derivative 3
malononitrile yielded compound 4 in 30% yield. Further, having nitrile groups at the periphery form a uniform layer
Suzuki-Miyaura coupling of boronic acid 2a39 with 2,3-dibro- around the metal ions and bind strongly with gold ions, hence,
mopentacenequinone 1 yielded compound 5 in 35% yield and the rate of reduction is faster (see pS27 in the ESI†).40 This results
its Knoevenagel condensation with malononitrile furnished in greater availability of reduced gold nanoparticles, therefore the
compound 6 in 43% yield.
growth of nanoparticles takes place in all the directions leading
The structures of derivatives 4, 5 and 6 were conrmed from to formation of nanospheres in the case of derivative 3.40
their spectroscopic and analytical data. The 1H NMR spectrum
of derivative 4 showed three singlets at 8.12, 8.77 and 8.81 ppm,
two doublets at 7.32 and 7.63 ppm and two multiplets from
7.78–7.81 and 8.08–8.10 ppm, corresponding to the aromatic
protons (Fig. S38 in the ESI†). The mass spectrum of derivative 4
showed a parent ion peak at m/z 629.1482 (M + Na)+ (Fig. S40 in
the ESI†). The 1H NMR spectrum of derivative 5 showed one
broad signal at 1.26 ppm corresponding to CH2 groups of the
hexyl chain, three singlets at 8.09, 8.94 and 8.95 ppm, two
doublets at 6.83 and 7.16 ppm corresponding to the aromatic
protons, three triplets at 0.92, 1.79 and 3.97 ppm and four
multiplets from 1.35–1.37, 1.45–1.55, 7.69–7.72 and 8.11–8.15
ppm due to the hexyl chain and aromatic protons, respectively
(Fig. S41 in the ESI†). The mass spectrum of derivative 5 showed
a parent ion peak at m/z 685.4335 (M + Na + 2)+ (Fig. S43 in the
ESI†). Further, the 1H NMR spectrum of derivative 6 showed two
broad signals at 1.18 and 1.40 ppm corresponding to CH2
groups of the hexyl chain, three singlets at 7.94, 8.65 and 8.68
ppm, two doublets at 6.75 and 7.07 ppm, corresponding to the
aromatic protons, three triplets at 0.85, 1.72 and 3.89 ppm and
three multiplets from 1.28–1.29, 7.68–7.70, and 7.98–8.00 ppm
due to the hexyl chain and aromatic protons, respectively
Fig. 4 TEM images of gold nanoparticles of (A) derivative 4 (B)
(Fig. S44 in the ESI†). The mass spectrum of derivative 6 showed
derivative 6; scale bar (A) 200 nm and (B) 20 nm, (C) UV-vis spectra for
a parent ion peak at m/z 778.5134 (M + Na)+ (Fig. S46 in the
the reduction of p-nitroaniline to p-phenylenediamine using gold
ESI†). These spectroscopic data corroborate structures 4, 5 and
6 for these compounds.
nanoparticles of derivative 3 as a catalyst. Photographs show two
different color solutions: (a) yellowish color p-nitroaniline; (b) the
reduced colorless product p-phenylenediamine.
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However, in the case of derivatives 4 and 6, nonspherical
shape gold nanoparticles were obtained which may be
attributed to the non-uniform growth of nanoparticles. We
believe that aggregates of pentacenequinodimethane deriva-
tives 4 and 6 form non-uniform layers around the metal ions,
thus, resulting in slow reduction of gold ions which is
responsible for the formation of gold nanoparticles of non-
uniform size.41
Acknowledgements
We are thankful to DST (ref. no. SR/S1/OC-63/2010), CSIR (ref.
no. 02(0083)/12/EMR-II) and DST-PURSE for nancial support.
We are also thankful to UGC for awarding the university with
the Potential for Excellence (UPE) project. S.K. is thankful to
DST for the INSPIRE fellowship.
To check the shape and size dependent catalytic activity of
gold nanoparticles, the reduction of p-nitroaniline to p-phenyl-
enediamine using NaBH4 was investigated (see pS7 in the ESI†).
The completion of the reaction was conrmed from the color of
the solution as well as from UV-vis spectroscopy. Upon addition
of the catalytic amount of gold nanoparticle solution 5 mL (2.5
nmol) generated by aggregates of derivative 3 to the solution of
p-nitroaniline and sodium borohydride, a progressive decrease
in the absorbance band at 378 nm and a progressive increase
with blue shi in the absorption band at 234 nm accompanied
by the appearance of a new absorption band at 303 nm corre-
sponding to p-phenylenediamine42 were observed in ve
minutes (Fig. 4C). On the other hand, the reduction reaction of
p-nitroaniline to p-phenylenediamine using NaBH4 in the
presence of gold nanoparticles generated by derivatives 4 and 6
took 24 minutes and 18 minutes, respectively (Fig. S33 and S34
in the ESI†). The calculated rate constants35 for the catalytic
reduction of p-nitroaniline by gold nanoparticles of derivatives
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