Pentacenequinone derivatives: Aggregation-induced emission enhancement, mechanism and fluorescent aggregates for superamplified detection of nitroaromatic explosives

Article abstract of DOI:10.1039/c4tc01194e

Pentacenequinone derivatives 5-9 have been synthesized by a Suzuki-Miyaura coupling protocol. These derivatives form fluorescent aggregates in mixed aqueous media due to their aggregation-induced emission enhancement (AIEE) attributes. Interestingly, size dependent emission enhancement is observed in the case of derivatives 5-7 and the effect of increase in the number of pyridine rotors on fluorescence emission of pentacenequinone derivatives 6-7 in solution and in the aggregate state confirms that the AIEE phenomenon is at the cost of aggregation driven growth and restriction of intramolecular rotation (RIR). On the other hand, derivatives 8 and 9 having electron rich phenyl groups are donor-accepter-donor type systems which exhibit an intermolecular charge transfer state (ICT) and form fluorescent aggregates in mixed aqueous media. In addition to this, the AIEE characteristics endow pentacenequinone derivatives 5-9 with sensing functionalities such as detection of nitroaromatic compounds. TLC strips of AIEE-active pentacenequinone derivatives 5-9 provide a more convenient and cost-effective approach for the trace detection of nitroaromatic explosives. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc01194e

Journal of
Materials Chemistry C
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PAPER
Pentacenequinone derivatives: aggregation-
induced emission enhancement, mechanism and
fluorescent aggregates for superamplified
detection of nitroaromatic explosives†
Cite this: J. Mater. Chem. C, 2014, 2,
7356
*
Sandeep Kaur,‡ Ankush Gupta,‡ Vandana Bhalla and Manoj Kumar
Pentacenequinone derivatives 5–9 have been synthesized by a Suzuki–Miyaura coupling protocol. These
derivatives form fluorescent aggregates in mixed aqueous media due to their aggregation-induced
emission enhancement (AIEE) attributes. Interestingly, size dependent emission enhancement is
observed in the case of derivatives 5–7 and the effect of increase in the number of pyridine rotors on
fluorescence emission of pentacenequinone derivatives 6–7 in solution and in the aggregate state
confirms that the AIEE phenomenon is at the cost of aggregation driven growth and restriction of
intramolecular rotation (RIR). On the other hand, derivatives 8 and 9 having electron rich phenyl groups
are donor–accepter–donor type systems which exhibit an intermolecular charge transfer state (ICT) and
form fluorescent aggregates in mixed aqueous media. In addition to this, the AIEE characteristics endow
pentacenequinone derivatives 5–9 with sensing functionalities such as detection of nitroaromatic
compounds. TLC strips of AIEE-active pentacenequinone derivatives 5–9 provide a more convenient and
cost-effective approach for the trace detection of nitroaromatic explosives.
Received 6th June 2014
Accepted 1st July 2014
DOI: 10.1039/c4tc01194e
www.rsc.org/MaterialsC
proposed for the increase in emission intensity in the aggre-
gated state.17–21 Thus, more investigation is needed to prove the
Introduction
actual mechanism for the aggregation-induced emission
enhancement (AIEE) phenomenon.
Highly luminescent p-conjugated organic molecules form the
basis of optical and electronic devices such as organic light-
emitting diodes (OLEDs),1 organic light-emitting eld-effect
transistors (OLEFETs)2 and organic uorescent sensors.3
However, most p-conjugated molecules have very high effi-
ciency in dilute solution but relatively weak emission in the
solid state as intra- and intermolecular interactions quench the
emission process.4 Several approaches such as dendritic or
bulky substituent protection,5,6 cross-dipole stacking,7 aggre-
gation-induced emission,8 J-aggregate formation9 and
enhanced intramolecular charge-transfer transition10 have been
used to achieve an intense solid-state emission. A variety of
organic molecules such as siloles,11,12 1-cyano-trans-1,2-bis-
(4-methylbiphenyl)ethylene,13 thienyl-azulene,14 arylethene15
and naphthalimide16 derivatives that emit strongly in their
aggregated or solid state have been reported. Various possible
mechanisms including conformational planarization, J-aggre-
gate formation, twisted intramolecular charge transfer (TICT)
and restriction of intramolecular rotation (RIR) have been
Our research work involves design, synthesis and evaluation
of organic materials having AIEE characteristics for the detec-
tion of various analytes.22–25 In our previous reports, we have
utilized the AIEE phenomena for the preparation of uorescent
aggregates. Further we utilized these aggregates for detection of
nitroaromatic explosives viz. picric acid (PA) and trinitrotoluene
(TNT). However, in none of these reports we have discussed the
effect of nature and the number of rotors on the photophysical
properties of these derivatives in the aggregated state. In addi-
tion, we have not discussed the reason of AIEE phenomena in
these molecules. Since most of these reported derivatives form
spherical aggregates, we could not study the relationship
between morphology of these derivatives with their sensing
sensitivity toward nitroaromatic explosives. In one of the pub-
lished research work, we reported that pentacenequinone
derivative 6 forms uorescent aggregates in aqueous media due
to its AIEE attributes.23 We proposed that RIR26 of the hetero-
aromatic ring at the periphery of the pentacenequinone core in
the aggregated state is the main cause of the AIEE effect. To get
more insight into the mechanism of AIEE, in the present
manuscript, we have designed and synthesized a series of new
AIEE-active molecules based on the pentacenequinone scaffold.
We have chosen the pentacenequinone scaffold as it is rigid and
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 5–9. See DOI: 10.1039/c4tc01194e
‡ These authors equally contributed to this work.
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is a planar molecule and has tendency to form ordered thin aggregates33 (Fig. S1 in ESI†). Similar results were obtained in
lms which make it a good candidate for the preparation of the case of pentacenequinone derivatives 6 and 7 under the
organic electronic devices.27 We envisioned that varying the same set of conditions as used for derivative 5 (Fig. S2–S3 in
number of hetero-aromatic rotors on the pentacenequinone ESI†).
core would inuence photophysical properties of the molecules
In the uorescence spectrum, the dilute solution of deriva-
34
which will be helpful in understanding the mechanism of AIEE tive 5 in DMSO exhibits a weak emission band (FF ¼ 0.030) at
and hence will provide the molecular designs for the prepara- 460 nm when excited at 310 nm (Fig. 1). However, a dramatic
tion of uorescent aggregates.28,29 Thus, with this hypothesis in change in uorescence emission intensity is observed in the
mind, we designed and synthesized mono, di and tetra H2O–DMSO mixture. On addition of 10% volume fraction of
substituted pentacenequinone derivatives 5–7. We observed water an emission band appears at 468 nm. Addition of 30%
that in the aggregated state emission intensity of these deriva- volume fraction of water leads to enhancement of emission
tives increased with increase in the number of pyridine rotors along with the red shi in the emission band from 468 nm to
on the pentacenequinone core. Further, comparison of uo- 481 nm (Dl ¼ 13 nm). Further addition of water fractions up to
rescence studies of derivatives 6–7 suggests that the AIEE 50% leads to enhancement in emission intensity which is
phenomenon is due to aggregation driven-growth and restric- clearly visible to the naked eye under the illumination of 365 nm
tion of intramolecular rotation. In addition to this, we synthe- (inset, Fig. 1). The scanning electron microscopy (SEM) analysis
sized the pentacenequinone based derivatives 8 and 9 and of derivative 5 in the H2O–DMSO (1 : 1, v/v) mixture showed the
found that these derivatives are donor–accepter–donor type presence of irregular shaped aggregates (Fig. S4A in ESI†). The
systems which exhibit an intermolecular charge transfer state dynamic light scattering (DLS) studies clearly showed an
(ICT) and form uorescent aggregates in mixed aqueous media. average size around 660 nm, 440 nm and 220 nm in 10%, 30%,
The presence of alkyl chains on pentacenequinone derivative 8 and 50% H2O–DMSO solvent mixtures of derivative 5 respec-
enhances the emission efficiency in the aggregated state as tively (Fig. S4B in ESI†). We assume that as the particles tend to
compared to derivative 9. Further, we utilized TLC strips coated shrink in size with increasing water content in the solvent
with derivatives 5–9 for instant detection of nitroaromatic mixture,35 a more coplanar conformer imposed by the con-
explosives.
gested environment in the shrunk particles may be responsible
for this bathochromic shi.
However, addition of more than 50% water fraction to DMSO
solution of derivative 5 results in decrease in uorescence
emission intensity (Fig. S5 in ESI†). This phenomenon is oen
observed in derivatives with AIEE properties36 as aer the
aggregation only the molecules on the surface of aggregates
emit light and contribute to the uorescent intensity upon
excitation and this leads to a decrease in emission intensity.36
We also carried out uorescence studies of pentacenequinone
derivatives 6 (ref. 23) and 7 under same set of conditions as used
for derivative 5 (Fig. S5 and S6 in ESI†). Addition of water
fractions up to 50% to the DMSO solution of derivative 7 leads
to red shiing (Dl ¼ 28 nm) of the emission band from 464 to
492 nm along with enhancement of the emission signal. The
Results and discussion
Pentacenequinone derivatives 5–7 were synthesized by Suzuki–
Miyaura coupling of boronic ester 4a30 with respective penta-
cenequinone derivatives 1–3 (ref. 31 and 32) (Scheme 1). The
structures of derivatives 5, 6 (ref. 23) and 7 were conrmed from
their spectroscopy and analytical data (Fig. S44–S47 in ESI†).
We evaluated the photophysical properties of pentacene-
quinone derivatives 5–7 by UV-vis and uorescence spectros-
copy. The UV-vis spectrum of derivative 5 in DMSO exhibits a
characteristic absorption band at 310 nm corresponding to the
p–p* transition of the pentacenequinone moiety (Fig. S1 in
ESI†). On addition of water fractions (up to 50%) to the DMSO
solution of derivative 5, a level-off long wavelength tail appears
which is attributed to the Mie scattering due to the formation of
Fig.
1 Fluorescence spectra of derivative 5 (10 mM) showing
the variation of fluorescence intensity in H2O–DMSO mixtures.
lex ¼ 310 nm. The inset photograph shows the fluorescence intensity
at (a) 0% and (b) 50% water in DMSO.
Scheme 1 Pentacenequinone based derivatives 5–7.
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DLS studies of derivative 7 in 10%, 30%, and 50% H2O–DMSO enhancement observed in the case of derivative 6, 5.6 fold
solvent mixtures showed aggregates of an average size around enhancement is observed in the case of 5. This may be attrib-
305 nm, 550 nm and 768 nm, respectively (Fig. S7A in ESI†). The uted to a greater degree of the restriction in rotation of a higher
enhancement and red shiing of the emission signal are related number of rotors.
to the aggregation driven growth of the aggregates14 of deriva-
We investigated the effect of temperature on the uores-
tive 7. A linear relationship between emission enhancement cence spectra of derivative 5 in H2O–DMSO (1 : 1, v/v) solvent
and size of aggregates of derivative 7 is observed (Fig. S8A in mixture and found that uorescence intensity decreases with
ESI†). Similar emission behaviour is observed in the case of increase in the temperature up to 75 ꢀC which clearly shows
derivative 6 (Fig. S7B and S8B in ESI†). The SEM image of conversion of the aggregated state to a monomer like state at
derivative 6 showed irregular shaped aggregates whereas ake high temperature (Fig. S12 in ESI†). At high temperature pyri-
like morphology was observed in the case of derivative 7 dine rotors of pentacenequinone derivatives 5–7 rotate fast
(Fig. S9A and S9B in ESI†).
which lead to a non-radiative decay process and hence make
7 in them less emissive. Similar results were obtained in the case of
The quantum yield of aggregates of derivative
H2O–DMSO (1 : 1, v/v) increased to 0.43 which is 16 times derivatives 6 and 7 (Fig. S13 in ESI†). The decrease in uores-
higher than that in DMSO solution. In the case of derivative 6 cence intensity of derivative 7 with increase in solution
and 5, an increase of 14 and 7 times respectively is observed to temperature was up to 82%. On the other hand, decrease of 74%
that of their DMSO solutions.
and 48% is observed in the case of derivatives 6 and 5 respec-
To get insight into the mechanism of AIEE, we investigated tively (Table 1). The viscosity and temperature dependent
the effect of variations in the viscosity and temperature on studies suggest that RIR plays a crucial role in emission
uorescence behaviour of derivatives 5–7. For this, the uo- enhancement in the case of derivatives 5–7.
rescence spectra of derivative 5 in the viscous mixture of DMSO
We carried out time resolved uorescence studies to deter-
and glycerol with different glycerol fractions were recorded mine the life times of these derivatives in the aggregated state.
(Fig. 2) and it was found that upon addition of 90% volume The uorescence life time data (Table 1) for derivatives 5–7 in
fraction of glycerol, the uorescence intensity increases. This is the aggregated state were obtained by tting the time resolved
attributed to high viscosity that hampers intramolecular rota- curves based on the double-exponential function. Although
tion, leading to closure of the non-radiative decay channel, there is a small difference between uorescence radiative rate
hence, making the molecule emissive. In a 50% DMSO–glycerol constants37 (Kf) of derivative 5 (0.733 ꢁ 109 sꢂ1), 6 (0.251 ꢁ 109
solvent mixture, increase in uorescence emission is purely due sꢂ1) and 7 (0.233 ꢁ 109 sꢂ1), the non-radiative decay constant
to the viscosity effect as the derivative is soluble in this mixture. (Knr) of derivative 5 (2.6 ꢁ 109 sꢂ1) was larger than those of
Abrupt increase in uorescence intensity is observed in the derivatives 6 (0.36 ꢁ 109 sꢂ1) and 7 (0.30 ꢁ 109 sꢂ1). These
presence of more than 50% volume fraction of glycerol in studies show that increase in the number of rotors accelerate
DMSO. This is due to the combined effect of viscosity and the decrease in the non-emissive rate constant.37,38 In the case of
aggregation as molecules are less soluble in these mixtures. derivatives 6 and 7, in the aggregated state major fraction of
Similar results were obtained in the case of pentacenequinone molecules undergoes radiative decay through the slow pathway.
derivatives 6 and 7 (Fig. S10 in ESI†).
On the other hand, in the case of derivative 5, 72% of molecules
To compare the increase in uorescence intensity of these undergo radiative decay through the fast pathway. However, the
derivatives, we plotted the change in the peak intensity vs. decay time of derivative 5 in 50% water fraction (0.30 ns) is
glycerol content (Fig. S11 in ESI†). Interestingly, uorescence longer than that of 5 in 0% water fraction (0.081 ns) which
intensity of derivative 7 increased by 11.8 times to that in pure implies formation of ordered aggregates.39 We also carried out
DMSO solution (Table 1) compared to the 9.56 fold the uorescence anisotropy studies40–42 of derivative 7 which
show an increase in the average anisotropy value of derivative 7
in DMSO from 0.05 to 0.32 in H2O–DMSO (1 : 1, v/v) (Fig. S14
and S15 in ESI†). This result suggests decreased molecular
motion in the case of derivative 7 on aggregation.
In the next part of our investigation, we were interested in
studying the effect of the presence of electron rich phenyl rotors
instead of electron decient rotors on the AIEE characteristics
of pentacenequinone derivatives. In the view of this, we
synthesized pentacenequinone derivatives 8 and 9 with or
without an alkyl chain respectively by Suzuki–Miyaura coupling
of boronic esters 4b43 and 4c44 with pentacenequinone deriva-
tive 2 (Scheme 2). The structures of these derivatives were
conrmed from their spectroscopy and analytical data
(Fig. S48–S53 in ESI†).
We evaluated the aggregation behaviour of derivatives 8 and
Fig. 2 Fluorescence spectra of derivative 5 (5 mM) showing the vari-
9 by UV-vis and uorescence spectroscopy. The absorption
spectrum of dilute solution of derivative 8 in THF shows a
ation of fluorescence intensity in DMSO–glycerol mixtures with
different glycerol fractions.
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Table 1 Comparative photophysical properties of derivative 5–7
i
Quantum yield
Quantum yield
Emission
(I ꢂ I0)/
Kfh
Knr
Derivative lmax (FF)b in solution (FF)c in aggregates enhancement
I/I0
I0 ꢁ 100e A1/A2 sF1g (ns) sF2 (ns) (109 sꢂ1
)
(109 sꢂ1
)
a
d
f
g
b
5
6
7
481
481
492
0.030
0.029
0.027
0.22
0.41
0.43
7.3 (FFc/FF
)
5.6
9.56 74
11.8 82
48
72/28 0.30
36/64 0.078
35/65 0.13
2.02
1.63
1.84
0.733
0.251
0.233
2.6
0.36
0.30
14.13 (FFc/FF
15.92 (FFc/FF
)
)
b
b
a
b
c
d
Emission maximum (nm). Solution in DMSO. Aggregates in H2O–DMSO with 50 vol% of water. Increase in uorescence intensity in 90%
e
volume fraction of glycerol in DMSO. Decrease in % age uorescence intensity with increase in temperature upto 75 ꢀC in 50 vol% of water in
f
g
DMSO. A1, A2: fractional amount of molecules in each environment. sF1 and sF2: biexponential life time of aggregates in 50 vol% of water in
DMSO. h Radiative rate constant (Kf ¼ FF/sF).37 Non-radiative rate constant (Knr ¼ (1 ꢂ FF)/sF).37
i
Scheme 2 Pentacenequinone based derivatives 8 and 9.
Fig. 3 Fluorescence spectra of 8 (10 mM) showing the variation of
fluorescence intensity in H2O–THF mixtures with different water
characteristic absorption band at 300 nm corresponding to
p–p* transition of the pentacenequinone moiety (Fig. S16A in
ESI†). However, in the presence of water as co-solvent, a new
absorption band at 420 nm is observed along with appearance
of a level-off long wavelength tail (Fig. S16A in ESI†). This new
band may be assigned to the intermolecular charge-transfer
(ICT) state.4,45 Similar results were obtained in the case of
derivative 9 under the same set of conditions as used for
derivative 8 (Fig. S16B in ESI†). We carried out SEM analysis of
derivatives 8 and 9 which shows spherical aggregates in the
H2O–THF mixture (Fig. S17A and S17B in ESI†).
Further, solution of derivative 8 in THF is weakly emissive
when excited at 300 nm (FF ¼ 0.0039). A dramatic change in
uorescence behaviour of derivative 8 is observed when water is
added to the THF solution of derivative 8 (Fig. 3). The addition
of the 60% volume fraction of water to the THF solution of
derivative 8 shows dual emission along with enhancement of
emission intensity (Fig. 3). The two emission maxima at 475 and
555 nm resulted from the locally excited state and the inter-
molecular charge-transfer state,45 respectively. However, the
abrupt increase in uorescence intensity of the emission band
at 555 nm is observed (FF ¼ 0.24) on increasing the water
content to 90%. This change is clearly visible to naked eye under
the illumination of 365 nm light (inset, Fig. 3). We believe that
formation of the ICT state and conformational xation upon
aggregate formation is the reason for emission enhancement.46
Similar results were obtained in the case of derivative 9
under the same set of conditions as used for derivative 8
(Fig. S18 in ESI†). We carried out the uorescence life time
studies of derivatives 8 and 9 to get insight into the nature of the
excited state (Table S1 pS14 in ESI†). The time resolved
fractions. Inset photographs (under 365 nm UV-light) (a) in pure THF
(b) with the addition of 90% water in THF.
uorescence studies showed increase in life time of derivative 8
from 0.95 ns in THF solution to 3.53 ns in the aggregated state
and a large increase in uorescence rate constant from 4.07 ꢁ
106 sꢂ1 in solution to 6.7 ꢁ 107 sꢂ1 in aggregated state (Table S2
pS14 in ESI†). Further, we monitored the uorescence decay of
derivative 8 in H2O–THF (7/3) at two different emission
maxima45 (Table S1 pS14 in ESI†). The sF values (2.82 and 3.07
ns) observed at 555 nm were correspondingly longer than those
observed (0.28 and 1.96 ns) at 480 nm. Further, the decay times
of 3.07 and 6.54 ns were measured when the H2O–THF ratios
were 7/3 and 9/1, respectively. We believe that longer sF values
originate from the ICT state as the charge transfer state of
organic molecules generally exhibits a longer decay time as
compared to the locally excited state. Interestingly, aggregates
of derivative 9 showed higher life time (9.48 ns) than that of
aggregates of derivative 8. This may be attributed to effective
p-stacking of phenyl groups in the case of derivative 9 as the
presence of alkyl side chains in derivative 8 prevents the inter-
chain p-stacking.47 These studies show that derivatives 8 and 9
are donor–acceptor–donor type systems exhibiting the ICT state
and form uorescent aggregates in mixed aqueous media.45,48
On the other hand, ICT phenomena is not observed in the case
of derivatives 5–7 having electron decient pyridine groups
where aggregation driven-growth and RIR are the reasons of
AIEE in these derivatives. Thus, the above results provide
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another approach to develop materials having high uores-
cence efficiency in the aggregated state.
Detection of nitroaromatic explosives
Recently, detection of nitroaromatic explosives has become an
important issue as they are considered to be environmental
contaminants and toxic to the living organisms.49 Thus, the
detection of trace amounts of explosives is very important in
combating terrorism, for maintaining the national security and
providing environmental safety.50–53 The large scale use of
explosives by terrorist groups has prompted the scientic
community to develop novel sensing materials for their detec-
tion. Various methods are available for the detection of nitro-
aromatics, such as GC-MS, ion-mobility spectroscopy (IMS),
surface enhanced Raman spectroscopy and various other
spectroscopic techniques.54–59 However, these methods cannot
be used in the eld due to their high cost, lack of selectivity and
sensitivity.60 Fluorescence spectroscopy on the other hand is a
highly sensitive technique hence one of the rst choices for the
detection of nitroaromatics.52 In recent years, chemical sensors
with high sensitivity for the detection of derivatives such as
trinitrotoluene (TNT), dinitrotoluene (DNT), RDX and picric
acid (PA), have been reported.61 However, most of the reported
chemosensors work well in organic solvents.62–66 Thus, the
development of sensitive chemosensors for the detection of
nitro explosives which work well in the aqueous environment is
still a challenge.
In view of this, we utilized the uorescent aggregates of
pentacenequinone derivatives 5–9 for the detection of nitro-
aromatic explosives in mixed aqueous media. For this, we
carried out the uorescence titrations of derivatives 5–7 in
H2O–DMSO (1 : 1) and 8/9 in H2O–THF (9 : 1) mixtures toward
various nitroderivatives such as PA, TNT, DNT, 1,4-dinitroben-
zene (DNB), 1,4-dinitrobenzoic acid (DNBA), 1,4-benzoquinone
(BQ), nitromethane (NM), and 2,3-dimethyl-2,3-dinitrobutane
(DMDNB).
All the pentacenequinone derivatives 5–9 exhibit similar
behaviour to that of nitroaromatic explosives and since penta-
cenequinone derivative 7 shows a higher quantum yield in its
aggregate state and thus, we have focused our discussion on the
nitroaromatic explosives sensing property of aggregates of
derivative 7 in the following discussion. The addition of
58 equiv. of PA to the solution of 7 leads to signicant
quenching in uorescence emission (Fig. 4) which is clearly
visible to the naked eye under the illumination of UV light of
365 nm (inset, Fig. 4). Similar results were obtained with other
pentacenequinone derivatives 5–9 upon addition of PA
(Fig. S19–S23 in ESI†) and are summarized in Table 2.
Fig. 4 Change in fluorescence spectra of compound 7 (10 mM) with
the addition of PA in the H2O–DMSO (1 : 1) mixture; the inset showing
the fluorescence change (a) before and (b) after the addition of PA.
indicating a superamplied quenching effect.68,69 These results
suggest that the observed uorescence quenching of the uo-
rescent aggregates is attributed to the electron transfer and/or
energy transfer quenching mechanism.53,70 We measured uo-
rescence life time of aggregates of derivatives 5–9 in the absence
and presence of PA (Fig. S24–S28 in ESI†). The uorescence
lifetimes of aggregates of 5–9 were found to be invariant at
different concentrations of PA thus, indicating that the aggre-
gates of 5–9 decay mainly through the static pathway.71,72
Further, the spectral overlap between the absorption spec-
trum of PA and the emission spectrum of aggregates in the
wavelength region of 420–480 nm (Fig. S29–S33 in ESI†) allows
the energy transfer from the excited state of aggregates of
derivatives 5–9 to the ground state to PA, further increasing the
uorescence quenching efficiency.53,73,74 The Stern–Volmer
constant of derivative 7 at a lower concentration of PA is found
to be 6.52 ꢁ 103 Mꢂ1. The detection limit of derivative 7 for PA is
calculated to be 600 nM. The better detection level in the case of
derivative 7 as compared to derivatives 5 and 6 is due to the
presence of a ake like structure in derivative 7 which is formed
by entangled piling of the aggregates (Fig. S9B in ESI†).75 The
ake like structure provides the increase in porosity as well as a
large surface area which leads to good contact with the explo-
sives and enhances the sensing sensitivity.75,76 However, in the
case of derivatives 5 and 6 irregular aggregates are observed
(Fig. S4A and S9A in ESI†) which prefer to accumulate in a
compact manner which leads to less porosity, hence, lower
sensitivity was observed.75 The Stern–Volmer constants and
detection limits for PA with other pentacenequinone derivatives
are summarized in Table 2 and a lower detection limit in the
Table 2 Stern–Volmer constant and detection limit of derivatives 5–9
Stern–Volmer
Detection limit
(in nM)
Further, we studied the uorescence emission response of
aggregates of pentacenequinone derivatives 5–9 to PA by using a
Stern–Volmer relationship. We observed a linear Stern–Volmer
plot at a lower concentration of PA (Fig. S19–S23 in ESI†) which
indicate that uorescence quenching of aggregates of pentace-
nequinone derivatives 5–9 involves a static quenching through
the excited state interaction.67 However at a higher concentra-
tion of PA, the plot bents upwards (Fig. S19–S23 in ESI†), thus,
Derivative
PA (equiv.)
constant (in Mꢂ1
)
5a
6a
7a
8b
9b
62
60
58
40
40
3.40 ꢁ 103
4.36 ꢁ 103
6.52 ꢁ 103
8.44 ꢁ 103
3.61 ꢁ 103
720
650
600
250
290
a
b
In H2O–DMSO (1 : 1). In H2O–THF (9 : 1).
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intramolecular rotation are the main causes of the AIEE
phenomenon. On the other hand, derivatives 8–9 having phenyl
groups are donor–accepter–donor type systems which exhibit
an intermolecular charge transfer state and form uorescent
aggregates due to conformational xation upon aggregate
formation in mixed aqueous media. In addition to this, aggre-
gates of derivatives 5–9 can detect nitroaromatic explosives in
mixed aqueous media. Pentacenequinone derivative 8 having
an alkyl chain showed a better detection limit as compared to
derivative 9, thus, providing another molecular design for
increasing the sensitivity of the molecule. We also utilized TLC
strips coated with derivatives 5–9 for instant detection of
nitroaromatic explosives which provide a simple, portable and
low cost method for the detection of nitroaromatic explosives.
Fig. 5 Photographs of compound 7 on test strips (a) before and (b)
after dipping into aqueous solutions of PA.
Fig.
6 Photographs (under 365 nm UV light) of fluorescence
quenching of aggregates of derivative 7 on test strips for the visual
detection of a small amount of PA (A) test strip; PA of different
concentrations: (b) 10ꢂ4 M, (c) 10ꢂ6 M, (d) 10ꢂ8 M.
Experimental section
General experimental procedure: please see pS3 in ESI.†
General procedure for the synthesis of derivatives 5, 7–9:
(compound 6 was synthesised by reported method23)
case of derivative 8 indicates that the presence of alkyl chains
provides big cavities which allow PA molecules to diffuse more
quickly.47,77 Thus, derivative 8 shows good sensitivity to PA. The
quenching in uorescence emission of derivatives 5–9 was also
observed with TNT, DNT, DNB, DNBA, BQ, NM, and DMDNB.
The results of uorescence studies of derivatives 5–9 with PA,
TNT, DNT, DNB, DNBA, BQ, NM, and DMDNB are summarized
in Fig. S34 and S35 in the ESI.† From these data we may
conclude that uorescent aggregates of derivatives 5–9 are
sensitive for the detection of nitroaromatic explosives.
Further, for practical applications we have utilized the uo-
rescent aggregates for the detection of nitroaromatic explosives
in the solid state. For this we prepared test strips by dip-coating
solution of pentacenequinone derivatives 5–9 on TLC strips
followed by drying the strips under vacuum. TLC strips of AIEE-
active pentacenequinone derivatives show strong emission
which becomes non-emissive when dipped into aqueous solu-
tion of PA which can be observed by naked eye (Fig. 5 and S36–
S39 in ESI†). For detection of very small amounts of PA, we
prepared the aqueous solutions of PA of different concentra-
tions and 3 mL of each solution were placed on 5–9 test strips
(Fig. 6 and S40–S43 in ESI†).
To a solution of 1/3/2/2 (0.5/0.5/0.5/0.5 g, 1.3/0.80/1.07/2.45
mmol) and 4a/4a/4b/4c (0.293/0.699/0.72/0.457 g, 1.43/3.36/
2.25/0.98 mmol) in 20 mL of 1,4-dioxane were added K2CO3
(0.359/0.885/1.18/1.08 g, 2.6/6.41/8.56/7.84 mmol), distilled H2O
(1.3/3.2/3/3 mL) and Pd(Cl)2(PPh3)2(0.2/0.123/0.273/0.249 g,
0.286/0.176/0.236/0.215 mmol) under N2, and the reaction
mixture was then reuxed overnight. Aer completion of the
reaction (TLC), the mixture was cooled to room temperature.
1,4-Dioxane was then removed under vacuum, and the residue
so obtained was treated with water, extracted with CH2Cl2 and
dried over anhydrous Na2SO4. The organic layer was evaporated
and the compound was puried by column chromatography to
give compound 5/7/8/9.
Derivative 5: chloroform–methanol (97 : 3); 60%yield; M.Pt.
>260 ꢀC; 1H NMR (300 MHz, CDCl3, ppm): d ¼ 7.46–7.50 (m, 1H,
ArH), 7.70–7.73 (m, 2H, ArH), 7.91–7.95 (m, 1H, ArH), 8.03–8.07
(m, 1H, ArH), 8.11–8.15 (m, 2H, ArH), 8.22–8.30 (m, 2H, ArH),
8.69 (d, 1H, J ¼ 3.3 Hz, ArH), 8.95 (s, 2H, ArH), 8.97 (s, 1H, ArH),
9.01 (s, 1H, ArH), 9.02 (s, 1H, ArH); TOF MS ES+: 386.2; due to
poor solubility of compound 5, its 13C NMR spectrum could not
be recorded. Elemental analysis: calcd for C27H15NO2: C,
84.14%; H, 3.92%; N, 3.63%; found: C, 84.05%; H, 3.62%; N,
3.60%.
Dark spots of different strengths were formed, which shows
the practical applicability of test strips by varying the concen-
tration of PA even up to a 10ꢂ8 M level. Thus, these results show
that derivatives 5–9 are excellent chemosensors for the instant
visualization of the trace amount of PA in solution as well as in
the solid state.
Derivative 7: chloroform–methanol (97 : 3); 30% yield; M.Pt.
>260 ꢀC; 1H NMR (300 MHz, CDCl3, ppm): d ¼ 7.28 (s, 4H, ArH),
7.49–7.52 (m, 4H, ArH), 8.24 (s, 4H, ArH), 8.59 (s, 8H, ArH), 9.06
(s, 4H, ArH); TOF MS ES+: 617.3 (M + 1)+; due to poor solubility
of compound 7, its 13C NMR spectrum could not be recorded.
Elemental analysis: calcd for C42H24N4O2: C, 81.80%; H, 3.92%;
N, 9.09%; found: C, 81.75%; H, 3.62%; N, 9.00%.
Conclusions
Derivative 8: chloroform–hexane (1 : 1); 35% yield; M.Pt.
In conclusion, we synthesized pentacenequinone derivatives >260 ꢀC; 1H NMR (300 MHz, CDCl3): d ¼ 0.92 [t, 6H, J ¼ 6.6 Hz,
5–9 by a Suzuki–Miyaura coupling strategy. Emission intensity OCH2CH2(CH2)3CH3], 1.26 [br, 4H, OCH2CH2CH2CH2CH2CH3],
of derivatives 5–7 increased in the aggregated state with 1.35–1.37 [m, 4H, OCH2CH2CH2CH2CH2CH3], 1.45–1.55 [m, 4H,
increase in the number of pyridine rotors on the pentacene- OCH2CH2CH2CH2CH2CH3], 1.79 [t, 4H,
J
¼
7.35 Hz,
quinone core. Further, uorescence studies of derivatives 6–7 OCH2CH2(CH2)3CH3], 3.97 [t, 4H, J ¼ 6.6, OCH2CH2(CH2)3CH3],
suggest that aggregation driven-growth along with restriction of 6.83 [d, 4H, J ¼ 8.7 Hz, ArH], 7.16 [d, 4H, J ¼ 8.7 ArH], 7.69–7.72
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[m, 2H, ArH], 8.09 [s, 2H, ArH], 8.11–8.15 (m, 2H, ArH), 8.94 [s, 13 B. K. An, D. S. Lee, Y. S. Park, H. S. Song and S. Y. Park, J. Am.
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Acknowledgements
VB is highly thankful to DST (Ref. no. SR/S1/OC-63/2010) and
CSIR (Ref. no. 02(0083)/12/EMR-II) for nancial support and 22 V. Bhalla, A. Gupta and M. Kumar, Org. Lett., 2012, 12, 3112–
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AG is thankful to CSIR (New Delhi) for senior research fellow- 23 V. Bhalla, A. Gupta and M. Kumar, Chem. Commun., 2012, 48,
ship, and SK is thankful to DST for INSPIRE fellowship.
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24 M. Kumar, V. Vij and V. Bhalla, Langmuir, 2012, 28, 12417–
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