Femtogram detection of explosive nitroaromatics: Fluoranthene-based fluorescent chemosensors

Article abstract of DOI:10.1002/chem.201201764

Herein we report a novel fluoranthene-based fluorescent fluorophore 7,10-bis(4-bromophenyl)-8,9-bis[4-(hexyloxy)phenyl]fluoranthene (S₃) and its remarkable properties in applications of explosive detection. The sensitivity towards the detection

Full text of DOI:10.1002/chem.201201764

FULL PAPER
DOI: 10.1002/chem.201201764
Femtogram Detection of Explosive Nitroaromatics: Fluoranthene-Based
Fluorescent Chemosensors
N. Venkatramaiah, Shiv Kumar, and Satish Patil*[a]
Abstract: Herein we report a novel flu-
oranthene-based fluorescent fluoro-
(1.15 fgcmꢀ2) detection limit for tri-
ACHTUNGTRNEnNUNG itrotoluene (TNT) and picric acid
Binding energies between model bind-
ing sites of the S3 and analyte com-
pounds reveal that analyte molecules
enter into the cavity created by substi-
tuted phenyl rings of fluoranthene and
are stabilized by strong intermolecular
interactions with alkyl chains. It is an-
ticipated that the sensor S3 could be a
promising material for the construction
of portable optical devices for the de-
tection of onsite explosive nitroaromat-
ics.
phore 7,10-bis(4-bromophenyl)-8,9-
N
(PA), whereas the solution-phase
quenching showed PA detection at the
2–20 ppb level. Fluorescence lifetime
measurements revealed that the
quenching is static in nature and the
quenching process is fully reversible.
A
ACHTUNGTRENNUNG
and its remarkable properties in appli-
cations of explosive detection. The sen-
sitivity towards the detection of nitro-
aromatics (NACs) was evaluated
through fluorescence quenching in sol-
ution, vapor, and contact mode ap-
proaches. The contact mode approach
using thin-layer silica chromatograp-
Keywords: explosives · femtochem-
istry · fluorescence · nitroaromatics ·
sensors
hic plates exhibited
a
femtogram
Introduction
high sensitivity and selectivity.[21,22] Efforts have been made
to employ fluorescent inorganic nanoparticles like silica,
ZnS, and CdSe quantum dots in the detection of explosive
compounds through luminescence quenching.[21,23] Swager
et al. have developed amplifying fluorescent pentiptycene-
Conjugated polycyclic aromatic hydrocarbons (PAHs) like
fluoranthene and fluorene have received considerable atten-
tion for their potential applications in optoelectronics and
fluorescent chemosensors.[1–3] The high thermal stability and
anomalous fluorescence characters of fluoranthene serve as
a basic skeleton for the preparation of a blue-light emitter
for organic light-emitting diodes (OLEDs).[4,5] Fluorescent
conjugated polymers have been extensively used for the de-
tection of highly explosive nitroaromatic compounds
(NACs) for national security, environmental protection, re-
mediation of munitions sites, and forensic investigations.[6–9]
A variety of conjugated polymers such as poly(p-phenylene
vinylenes), polysilanes, polycarbazoles, and polymeric por-
phyrins have been used as sensing materials for the highly
derived poly(p-phenylene vinylene) (PPV) and poly(phen
ACHTUNGTRENNUNGyl-
AHCTUNGTREGeNNNU ne ethynylene) (PPE)-based polymer films for the detec-
tion of TNT and 2,4-dinitrotoluene (DNT) vapors at sub-
part-per-trillion (ppt) levels.[7,11] Recently, Ajayaghosh et al.
reported a subfemtogram level of TNT detection with per-
fluoroarene-based gelator coated on disposable paper
strips.[24] However, most of the research work to date has
been directed towards the design of novel fluorescent conju-
gated polymers. The majority of these polymers suffer in-
trinsic interferences from other electron acceptors and re-
quire laborious synthetic work that is time consuming and
inefficient, which leads to difficulties in upscaling, high
throughput screening, and practical applications. Therefore
the development of novel sensory materials with superior
performance and detection methods for traces of nitroaro-
matics are at the heart of current interest. Small-molecule-
based fluorescent chromophores with a rich p-electron-do-
nating character have received greater attention than conju-
gated polymers on account of their better sensitivity. A
series of small-molecule fluorophores such as pyrene, cou-
marins, purpurin, fluorescein, and some other indicator dyes
were employed for the detection of NACs because of their
long-lived excited-states.[24–27] Recently, Zyryanov et al. have
developed 1,4-diarylpentiptycene-based small molecules for
the visual detection of DNT (0.18 ppm) and TNT
(7.7 ppb).[28] Costa et al. have reported substituted p-phenyl-
ene ethynylene trimer (CALIX-PET and TBP-PET)-based
sensitive and selective detection towards 2,4,6-trinitro
ACHTUNGTRENNUNGuene (TNT) and picric acid (PA) in solution and vapor
ACHTUNGTNERtNUNG ol-
phases.[10–14] Various analytical methods such as fluorescence
and gas chromatography coupled with mass spectrometry,
surface-enhanced Raman spectrometry, and electrochemical
methods have been developed for the detection of
NACs.[15–20] In particular, the fluorescence-quenching tech-
ACHTUNGTRENNUNGnique has been widely employed for many polymers with
[a] Dr. N. Venkatramaiah, S. Kumar, Prof. S. Patil
Solid State and Structural Chemistry Unit
Indian Institute of Science, Bangalore 560012 (India)
Tel: (+91)80-22932651
Fax: (+91)80-23601310
E-mail: satish@sscu.iisc.ernet.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201764.
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materials for the detection of NACs.[29] To emphasize the
importance and exploration of fluoranthene as an excellent
donor material for better sensitivity and high detection to-
wards explosive nitroaromatics,[30] herein we report a new p-
conjugated fluoranthene derivative 7,10-bis(4-bromophen-
105.20, g=908; and Z=4. The pertinent crystallographic
data of S3 is provided in the Supporting Information
(Table S1). In the solid state, the two phenyl rings at C7 and
C8 are twisted by torsional angles of 61.16 and 60.258, and
the other two phenyl rings at C15 and C22 are twisted by
6.34 and 11.4 ꢁ, respectively. This twisting of the phenyl
rings leads to a very weak p–p interaction of 6.527 ꢁ, and
the central fluoranthene core has an almost planar structure.
The supramolecular arrangement of S3 along the ab plane
(Figure 1b) shows the formation of Z-shaped structural ar-
rangements in which one of the fluoranthene units enters
into the cavity of two alkyl chains of the other fluoranthene
yl)-8,9-bis[4-ACHTUNGTRENNUNG(hexyloxy)phenyl]fluoranthene (S3) as a sensor
material for the detection of explosive NACs. The presence
of flexible long alkyl chains on fluoranthene forms strong in-
termolecular interactions with analytes, and sterically crowd-
ed phenyl rings on the backbone of S3 hinder aggregation
and excimer formation.
ꢀ
unit and is stabilized by strong C H···p hydrogen-bonding
interactions. We observed the presence of two nonequiva-
Results and Discussion
ꢀ
lent C H···p interactions at the C47 and C52 atoms by 2.372
Compound S3 was synthesized by a three-step process and is
shown in Scheme 1. First, 1,2-bis[4-ACHTUNTRGNEUNG(hexyloxy)phenyl]ethyne
was synthesized by a Sonogashira cross-coupling reaction. In
and 2.781 ꢁ. The formation of different types of packing ar-
rangements in the solid state provoked our interest in their
detection of NACs.
Figure 2 shows the absorption and emission spectra in sol-
ution and thin film on a quartz plate. The absorption spectra
of S3 in chloroform exhibit two major bands with absorption
maxima at 303 and 374 nm, which arise from the p!p*
transitions. The emission spectra show strong blue fluores-
cence at 475 nm with a Stokes shift of 101 nm. The absorp-
tion and emission spectra of S3 in thin film are quite similar
to that of the solution spectra. However, we observed the
split in the emission pattern for thin film at 458 and 486 nm,
which indicates the strong perturbation in the vibronic
Scheme 1. Schematic representation of the synthesis of fluoroanthene de-
rivative S3.
a second step, 7,9-bis(4-bromophenyl)-8H-cyclopenta[a]ace-
naphthylen-8-one (2) was synthesized by heating acenaph-
thenedione and 1,3-bis(4-bromophenyl)propan-2-one to
reflux in ethanol for 15 min, which afforded compound 2 in
95% yield. Finally, a Diels–Alder reaction was carried out
between 1,2-bis[4-ACHTUNGTRENNUNG(hexyloxy)phenyl]ethyne and 7,9-bis-
(4-bromophenyl)-8H-cyclopenta[a]acenaphthylen-8-one in
1,2,4-trichlorobenzene at 2208C in a sealed glass pressure
tube to afford compound S3 in 52% yield.
X-ray-quality single crystals were obtained by a slow sol-
vent evaporation method in a dichloromethane/ethanol mix-
ture at ambient temperature. The single-crystal X-ray struc-
ture of S3 (Figure 1a) revealed a monoclinic space group,
P21/c, with a=12.976, b=22.970, c=14.783 ꢁ; a=90, b=
Figure 2. The optical absorption and fluorescence spectra of S3 in chloro-
form and in thin film.
Figure 1. a) Single-crystal X-ray structure of S3. The hydrogen atoms are omitted for clarity. b) The supramolecular arrangement of S3 along the ab plane.
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Detection of Explosive Nitroaromatics
FULL PAPER
states due to the heavy-atom effect in the excited state. The
optical bandgaps were calculated to be 3.084 and 3.017 eV
in solution and thin film, respectively. Based on the spectral
pattern of the absorption and fluorescence, we can infer that
S3 exhibits weak p–p stacking interactions, and that the ar-
rangement of the molecules in the thin film is quite similar
to the solid-state packing. The fluorescence lifetime of S3 in
chloroform was found to be approximately 11.37 ns by the
time-correlated single-photon counting (TCSPC) method.
The quantum yield (F) of S3 is 25% in chloroform and 44%
in hexane.
To explore the potential applications of S3 as a sensory
material for the detection of NACs, such as PA, TNT, DNT,
2,4-dinitrophenol (DNP), and common interfering agents
like 1,4-dichlorobenzene (DCB), benzoquinone (BQ), and
other analytes, fluorescence titration experiments were car-
ried out in ethanol. Figure 3a shows the fluorescence
quenching behavior of S3 upon gradual addition of a 0.1 mm
solution of PA. The fluorescence intensity was found to de-
crease significantly as a function of concentration, and 80%
quenching was observed after the addition of picric acid
(10 mL, 0.1 mm). Further, fluorescence quenching was ana-
lyzed by the Stern–Volmer (SV) equation I0/I=1+Ksv[PA],
in which I0 and I are the fluorescence intensities of S3 in the
presence and absence of PA, respectively, and Ksv is the
Stern–Volmer constant.
On the basis of quenching, a linear SV plot was obtained
with a rate constant of 1.24ꢂ105 mꢀ1, which was found to be
much higher than those of other fluorescent chemosensors
toward PA.[36–38] On the other hand, we have not observed
any significant changes in the fluorescence quenching behav-
ior for other NACs at this concentration level. However, flu-
orescence was efficiently quenched when S3 was treated
with gradual addition of NACs (10 mL) at a concentration of
5ꢂ10ꢀ3 m (Figure S12 in the Supporting Information). On
the basis of the SV plots (Figure 3b), Ksv was found to be
1828mꢀ1 for TNT, 1772mꢀ1 for DNT, and 178mꢀ1 for BQ.
This suggests that sensor S3 is highly sensitive towards the
detection of PA in ethanol. The appearance of a linear SV
plot for all analytes is due to the formation of static or dy-
namic quenching processes. However, static quenching in-
Figure 3. a) Fluorescence quenching of S3 (1ꢂ10ꢀ6 m) upon addition of
different concentrations of PA (1ꢂ10ꢀ4 m) in ethanol (lex =370 nm). The
inset shows the Stern–Volmer plot for different concentrations of PA.
The inset also shows changes in the fluorescence intensity after addition
of 10 mm of PA to S3. b) The Stern–Volmer plot for different concentra-
tions of various nitroaromatics with S3. c) Fluorescence lifetime measure-
ments (lem =470 nm) for different concentrations of PA with S3 in etha-
nol.
volves the formation of
a ground-state nonfluorescent
charge-transfer (CT) complex, whereas dynamic quenching
involves the excited-state electron transfer from fluorophore
to oxidative quencher. To understand this phenomenon, flu-
orescence lifetime studies were carried out at different con-
centrations of PA. The fluorescence lifetime of S3 was found
to be invariant with concentration of PA (Figure 3c), which
suggests that quenching follows static phenomena through
ground-state charge-transfer complex formation. This was
bound charge-transfer complex. The formation of a charge-
transfer complex was further confirmed by electronic ab-
sorption spectra. The spectrophotometric titration of S3 (5ꢂ
10ꢀ6 m) with different concentrations of picric acid (PA) in
ethanol is shown in Figure S14 in the Supporting Informa-
tion. Upon gradual addition of PA to S3, there was a slight
increase in the absorption maxima at 303 nm along with the
formation of a new broad absorption peak at 424 nm. The
intensity of the new absorption peak increased gradually as
1
further confirmed by H NMR spectroscopy and electronic
absorption titration of S3 with different concentrations of
picric acid (Figure S13 in the Supporting Information).
1
Upon addition of PA to S3, the H NMR spectra shows a
gradual upfield shift in the doublet proton at d=7.75 to
7.72 ppm for a 1:1 molar ratio and reaches equilibrium. This
indicates that the gradual upfield shift is attributed to a
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S. Patil et al.
the concentration of PA increased. This indicated the forma-
tion of a donor–acceptor complex between the p-electron-
rich S3 fluorophore and picric acid. Similar results were ob-
served upon treatment of S3 with other electron-deficient ni-
troaromatics.
The molecular interaction with other electron-deficient
compounds such as 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ), 1,2-acenaphthylenedione (ANQ), 1-chloroan-
thraquinone (1-ClAQ), and 9,10-phenanthrenequinone
(PAQ) was also investigated. We observed the quenching of
fluorescence intensity at 475 nm with the appearance of a
new emission peak at 390 nm. The intensity of the new emis-
sion peak increased and showed nonlinear behavior in the
SV plot, which suggests that S3 follows both static as well as
dynamic quenching processes (Figure S15 in the Supporting
Information). Figure 4 shows the histogram of quenching ef-
Figure 5. Electron-density profiles of S3 and S3–PA complexes with the
B3LYP/6-31g* basis set.
Figure 4. The fluorescence-quenching efficiency of S3 upon addition of
different analyte solutions at 258C.
ficiency for different nitroaromatics and other electron-defi-
cient interferants. Common analytes such as dichloroben-
zene (DCB), pyridine (Py), and 4-bromobenzoic acid (BBA)
have little effect on the fluorescence intensity. On the other
hand, electron-deficient nitroaromatics brought about the
greatest quenching depending upon the number of nitro
substituents present on the toluene and phenol moiety. PA
exhibits the highest quenching response at parts-per-billion
levels (2–20 ppb), whereas DNT and TNT show a level of
detection of approximately 1.1 ppm in solution.
To further understand the mode of interaction and unusu-
al sensitivity of PA with S3, density functional calculations
(DFT) were carried out at the B3LYP/6-31g* level using the
Gaussian 03 package.[35] From the optimized geometry, it
was observed that the HOMO orbital has large overlap of
electron density localized on the phenyl rings of the alkyl
chains and the fluoranthene backbone, whereas the electron
density in the LUMO is largely localized on the fluoran-
thene unit (Figure 5). The LUMO energy of S3 (ꢀ1.76 eV)
lies higher in energy than PA (ꢀ3.89 eV). With a view to ob-
taining strong evidence for the fluorescence-quenching re-
sponse and the type of interaction, we optimized the geome-
tries of the fluorophore–analyte complexes. On the basis of
the single-crystal analysis, the analytes form two types of flu-
orophore–analyte complexes in which analyte molecules
enter the cavity created by substituted phenyl rings of fluo-
ranthene and are stabilized by strong intermolecular interac-
tions (type 1). The other possibility is to form a cofacial
stacked geometry and strong intermolecular p–p interac-
tions (type 2). A representative example of S3–PA com-
plexes is shown Figure 5.
The total energy of type 1 S3–PA complex is found to be
less than type 2 by 14.82 kcalmolꢀ1. This indicates that the
alkyl chains of S3 have a strong energetic affinity toward
picric acid due to the proximity of the electron density and
two of the nitro groups of picric acid oriented towards the
alkyl chains by 2.57 and 2.55 ꢁ, whereas the other nitro and
alcohol groups are oriented out of the cavity. The calculated
binding energy for the type 1 S3–PA complex is
32123.29 kcalmolꢀ1, which is energetically higher than
type 2 (31228.79 kcalmolꢀ1). Figure 5 shows the frontier mo-
lecular orbital pictures of S3 and S3–PA complexes. For
type 1, the HOMO orbitals are localized on phenyl and fluo-
ranthene units similar to S3 and the LUMO orbitals are lo-
calized on picric acid. This facilitates a strong electron trans-
fer from the excited state of S3 to PA in the fluorescence-
quenching process in type 1 rather than in type 2. Similar re-
sults were observed for S3–TNT complexes and are shown in
the Supporting Information (Figure S19).
To demonstrate the potential utility of the material to-
wards the detection of nitroaromatics in the vapor phase, a
thin film of S3 was fabricated by spin-coating techniques on
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Detection of Explosive Nitroaromatics
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quartz substrate at a rate of 2000 rpm and a concentration
of 1 mg per 0.2 mL in chloroform. The film was annealed at
508C for 2 h. The fluorescence response of the films to the
vapors of NACs was ascertained by inserting the film into
sealed vials that contained solid explosives at room temper-
ature. Figure 6 shows the time-dependent fluorescence
due to differences in partial pressure, redox potential of the
analytes, electron-accepting capability of S3, and adsorptive
affinity of film toward the NACs. The trapping of PA vapors
into the cavities created by alkoxy chains of phenyl rings
and the formation of strong intermolecular interactions with
alkyl chains of S3 results in high quenching performance.
Figure 6b shows the reversibility of the S3 film towards the
detection of PA. The film was exposed to saturated vapors
of PA at room temperature for 120 s and the emission spec-
trum was recorded. The film was washed with methanol and
dried at 458C under vacuum, the emission spectrum was re-
corded, and the whole process was repeated. The results
show that the initial fluorescence intensity was significantly
retained after several washings, thereby indicating a high
photostability of the film. A quenching efficiency of 88%
was observed even after 6 cycles of exposure to PA vapors.
It is well known that the vapor pressures of nitroaromatics
such as PA and TNT are extremely high when sensing the
proximity of explosives. Therefore, we carried out a surface
sensing approach to test the viability by coating S3 on thin-
layer silica chromatographic plates. The visual change in the
fluorescence intensity makes it applicable to onsite instant
detection of explosives.
To test this possibility, we prepared different concentra-
tions of various NACs. Each solution (5 mL) was spotted
onto the S3-coated thin-layer silica plate to give a spot area
of approximately 0.2 cm2. The visual fluorescence response
to different analytes at different concentration levels is
shown in Figure 7a. The fluorescence intensity was found to
decrease as a function of concentration, and the minimum
amount of TNT and PA detectable by the naked eye was as
low as 5 mL of 1ꢂ10ꢀ13 m solution with a detection limit of
approximately 1.15 fgcmꢀ2. Figure 7b shows the fluores-
cence-quenching pattern of S3-coated TLC plates for differ-
ent concentrations of TNT solution. We observed 12 and
17% quenching for 1ꢂ10ꢀ13 m solutions of TNT and PA, re-
spectively. A comparison of the fluorescence-quenching effi-
ciency for different nitroaromatic analytes is shown in Fig-
ure 7c. The quenching effect was found to be higher for
TNT and PA than the other nitroaromatics.
Figure 6. a) Time-dependent emission spectra of S3 thin film upon expo-
sure to the saturated vapors PA at 60 s time intervals. The inset shows
the fluorescence-quenching efficiency for different saturated vapors of ni-
troaromatics. b) The reversibility of S3 thin film to the saturated vapors
of PA after 120 s of exposure time.
Conclusion
quenching of S3 film upon exposure to saturated vapors of
PA. As expected, the fluorescence intensity was found to de-
crease as the time of exposure elapsed. The initial emission
intensity was significantly reduced by 33% after 60 s of ex-
posure and then by 66% after 180 s. Upon continuous expo-
sure, 92% quenching was observed after 420 s of exposure
time and ultimately reached its equilibrium. This indicates
that S3 film exhibits a fast response for the detection of PA
at ppb levels. It is interesting to note that the film of S3 also
shows response to the vapors of DNT, DNP, NP, and NT
(Figure S17 in the Supporting Information). However, the
extent of quenching efficiency was found to be smaller than
the saturated vapors of PA and is shown in the inset of Fig-
ure 6a. The variations in the quenching behavior might be
In conclusion, we have developed a novel fluoranthene-
based small-molecule fluorescent chemosensor for femto-
gram detection of TNT and picric acid. The unique supra-
molecular arrangement of the fluoranthene molecules dem-
onstrates that nitroaromatics enter into the cavity created
by the substituted phenyl rings and are stabilized by strong
intermolecular interaction with the alkyl chains, thereby re-
sulting in efficient fluorescence quenching. The sensor S3 is
highly sensitive and is able to detect picric acid at the ppb
level, whereas it shows ppm-level sensitivity toward TNT in
ethanol. The vapor-phase fluorescence-quenching study re-
vealed that the fluorescence of S3 film is efficiently
quenched by DNT and PA vapors. The quenching process is
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creasing the electron-donating strength for better sensitivity
are ongoing in our laboratory.
Experimental Section
All the nitroaromatics such as PA, TNT, DNP, DNT, NP, NT, and NB
were obtained from Sigma Aldrich and were used without further purifi-
cation.
Caution: PA, TNT, and other NACs used in the present study are highly
explosive and should be handled only in small quantities.
1H and 13C NMR spectra were recorded using Bruker 400 and 100 MHz
NMR spectrometers and were calibrated by using TMS as an internal ref-
erence. Chemical shifts are reported in parts per million (ppm). The UV/
Vis spectra were recorded using a Perkin–Elmer (Lambda 35) UV/Vis
spectrometer. All the absorption spectra in solution were recorded in
CHCl3 (concentration of 1ꢂ10ꢀ5 molLꢀ1) and the spectra in the solid
state were recorded from films of compounds spin-coated on quartz sub-
strate. Steady-state fluorescence emission studies were carried out using
a Spex FluoroLog-3 spectrofluorometer (Jobin–Yvon Inc.). The fluores-
cence-quenching experiments were carried out in a quartz cell in ethanol
medium. The solid-state fluorescence-quenching performance in the
vapor phase was tested under saturated vapors of NACs and data was
collected from the front face of the film. Time-resolved fluorescence
measurements were carried out with time-correlated single-photon count-
ing (TCSPC) with nanosecond LED (295 nm) at a repetition rate of
1 MHz used to excite the samples. Single-crystal X-ray diffraction data-
sets were collected using an Oxford Xcalibur (Mova) diffractometer
equipped with a EOS CCD detector that used MoKa radiation (l=
0.71073 ꢁ).[31] The crystal was maintained at the desired temperature
during data collection using the Oxford Instruments Cryojet-HT control-
ler.[32] All structures were solved by direct methods using SHELXS-97
and refined against F2 using SHELXL-97. Hydrogen atoms were fixed
geometrically and refined isotropically. The WinGX package was used
for refinement and production of data tables and ORTEP-3 for structure
visualization and making molecular representations that showed the ellip-
soids at 30% probability level.[33] Analysis of the hydrogen-bonding and
p–p interactions was carried out using PLATON, and packing diagrams
were generated by using MERCURY.[34] All quantum chemical calcula-
tions were carried out in Gaussian 03 by using the B3LYP hybrid func-
tion with 6-31g* basis set.[35]
Synthesis procedure: Compound 1,2-bis[4-ACTHNUTRGNEU(GN hexyloxy)phenyl]ethyne was
synthesized by a Sonogashira cross-coupling reaction and the experimen-
tal details are provided in the Supporting Information (Scheme S1).
Synthesis of 1,3-bis(4-bromophenyl)propan-2-one (1): A solution of 4-
bromophenylacetic acid (5.0 g, 23 mmol) in dichloromethane was added
dropwise to a mechanically stirred solution of dicyclohexylcarbodiimde
(DCC; 2.4 g, 12 mmol) and N,N-dimethylaminopyridine (DMAP; 0.71 g,
0.58 mmol) in dichloromethane (40 mL). The reaction was allowed to
proceed overnight under an argon atmosphere at room temperature. The
reaction mixture was filtered to remove dicyclohexylurea, and the filtrate
was subjected to rotary evaporation. The product was purified by column
chromatography with 10% ethyl acetate/hexane as eluent (53% yield).
Figure 7. a) Photograph of S3 coated on the thin-layer silica chromato-
graphic plates before and after the addition of different concentrations of
various nitroaromatics by the contact mode method with a spot area of
approximately 0.2 cm2 upon exposure to the UV lamp at 365 nm.
1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.43 (d, 3J
4H), 7.00 (d, 3J(H,H)=6.8 Hz, 4H), 3.67 ppm (s, 4H); 13C NMR
ACHTUNGTREN(NUNG H,H)=6.8 Hz,
AHCTUNGTRENNUNG
(100 MHz, CDCl3, 258C, TMS): d=204.20, 132.58, 131.83, 131.21, 121.24,
48.43 ppm.
b) Change in the emission spectral pattern of S3-coated TLC plate (lex
=
370 nm) against different concentrations of TNT (1ꢂ10ꢀ13 to 1ꢂ10ꢀ3 m).
c) Quenching efficiency of various NACs toward the fluorescence emis-
sion of S3-coated TLC plates.
Synthesis of 7,9-bis(4-bromophenyl)-8H-cyclopenta[a]acenaphthylen-8-
one (2): Compound 2 was synthesized by double Knoevenagel condensa-
tion reaction between 1,3-bis(4-bromophenyl)propan-2-one (1; 1.87 g,
5.08 mmol) and acenaphthylene-1,2-dione (0.92 g, 5.08 mmol) in ethanol
(50 mL). A solution of KOH (0.095 g) in ethanol (2 mL) was added drop-
wise, and the reaction mixture was heated to reflux for 15 min under vig-
orous stirring. The reaction mixture was cooled to 08C, and the dark
green solid was filtered, washed with ethanol, and dried (95% yield).
The obtained compound was insoluble in common solvents.
highly reversible and static in nature. Femtogram detection
levels for PA and TNT were achieved by the contact-mode
approach on thin-layer silica chromatographic plates. Efforts
towards the structural modification of fluoranthene by in-
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Detection of Explosive Nitroaromatics
FULL PAPER
Synthesis of 7,10-bis(4-bromophenyl)-8,9-bis[4-
thene (S3): A 15 mL pressure glass tube was charged with 7,10-bis(4-bro-
mophenyl)-8H-cyclopenta[a]acenaphthylen-8-one (0.102 g, 0.2 mmol) and
N
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´
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A
E
7.6 Hz, 2H), 7.17 (d, 3J
4H), 6.65 (d, 3J
(t, J
G
1.44–1.36 (m, 4H), 1.31 (s, 8H), 0.89 ppm (t, 3J
13C NMR (100 MHz, CDCl3, 258C, TMS): d=156.76, 140.69, 138.98,
136.38, 136.24, 136.12, 133.12, 132.06, 131.81, 131.66, 131.41, 129.66,
127.66, 126.69, 123.03, 120.96, 113.04, 67.70, 31.59, 29.18, 25.64, 22.53,
13.98 ppm; elemental analysis calcd (%) for C52H48Br2O2: C 72.22, H
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Acknowledgements
The authors would like to acknowledge the NMR Research Centre, IISc
for NMR spectra. We would also like to thank Mr. K. Durga Prasad and
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from DST and DIT is gratefully acknowledged.
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Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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ÞÞ
These are not the final page numbers!

S. Patil et al.
Explosive discoveries: We report a
Chemosensors
novel fluoranthene-based fluorescent
fluorophore 7,10-bis(4-bromophenyl)-
8,9-bis[4-ACTHNUTRGNE(NUG hexyloxy)phenyl]fluoran-
N. Venkatramaiah, S. Kumar,
S. Patil* ........................ &&&& — &&&&
thene (S3) and its remarkable proper-
ties in applications of explosive detec-
tion (see figure). It is anticipated that
sensor S3 could be a promising mate-
rial in the construction of portable
optical devices for onsite detection of
explosive nitroaromatics.
Femtogram Detection of Explosive
Nitroaromatics: Fluoranthene-Based
Fluorescent Chemosensors
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!