Fluorescence and visual sensing of nitroaromatic explosives using electron rich discrete fluorophores

Article abstract of DOI:10.1039/c1jm10406c

Several π-electron rich fluorescent aromatic compounds containing trimethylsilylethynyl functionality have been synthesized by employing Sonogashira coupling reaction and they were characterized fully by NMR ( ¹H, ¹³C)/IR spectroscop

Full text of DOI:10.1039/c1jm10406c

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PAPER
Fluorescence and visual sensing of nitroaromatic explosives using electron rich
discrete fluorophores†
Sankarasekaran Shanmugaraju,* Sachin A. Joshi and Partha Sarathi Mukherjee*
Received 26th January 2011, Accepted 20th April 2011
DOI: 10.1039/c1jm10406c
Several p-electron rich fluorescent aromatic compounds containing trimethylsilylethynyl functionality
have been synthesized by employing Sonogashira coupling reaction and they were characterized fully
1
13
by NMR ( H, C)/IR spectroscopy. Incorporation of bulky trimethylsilylethynyl groups on the
peripheral of the fluorophores prevents self-quenching of the initial intensity through p–p interaction
and thereby maintains the spectroscopic stability in solution. These compounds showed fluorescence
behavior in chloroform solution and were used as selective fluorescence sensors for the detection of
electron deficient nitroaromatics. All these fluorophores showed the largest quenching response with
high selectivity for nitroaromatics among the various electron deficient aromatic compounds tested.
Quantitative analysis of the fluorescence titration profile of 9,10-bis(trimethylsilylethynyl)anthracene
with picric acid provided evidence that this particular fluorophore detects picric acid even at ppb level.
A sharp visual detection of 2,4,6-trinitrotoluene was observed upon subjecting 1,3,6,8-tetrakis
(
trimethylsilylethynyl)pyrene fluorophore to increasing quantities of 2,4,6-trinitrotoluene in
chloroform. Furthermore, thin film of the fluorophores was made by spin coating of a solution of 1.0 ꢀ
ꢁ3
1
0
M in chloroform or dichloromethane on a quartz plate and was used for the detection of vapors of
nitroaromatics at room temperature. The vapor-phase sensing experiments suggested that the sensing
process is reproducible and quite selective for nitroaromatic compounds. Selective fluorescence
quenching response including a sharp visual color change for nitroaromatics makes these fluorophores
as promising fluorescence sensory materials for nitroaromatic compounds (NAC) with a detection limit
of even ppb level as judged with picric acid.
4
irritation. Moreover, excess TNT may result in liver, eye and
1
. Introduction
3,4
neurological damage. A selective and highly sensitive chemo-
sensor for the detection and elimination of unexploded nitro-
aromatic explosives in buried land mines is an urgent need for the
protection of living organisms, environmental remediation and
Design of suitable chemical sensors for the detection of nitro-
aromatic compounds (NAC) has attracted much attention in the
last decade in an effort to better combat terrorism and control
1
environmental pollution. Among various nitroaromatic
5
national security. Various methods are available for sensing
compounds trinitrotoluene (TNT), dinitrotoluene (DNT) and
picric acid (PA) are the primary constituents of many unex-
2
ploded land mines worldwide. In addition to their explosive
nitroaromatic explosives including metal detectors, which are
inefficient for explosive screening in an airport due to plastic
casing. Instrumental techniques available for the purpose of
nitroaromatic explosive detection include GC-MS, ion mobility
spectroscopy (IMS), surface-enhanced Raman spectroscopy, and
nature, nitroaromatics are also considered as environmental
3
contaminants and are toxic to living organisms. For example,
continuous exposure to one of the widely used secondary nitro-
aromatic explosives TNT can cause headaches, anemia and skin
6
various other spectroscopic techniques. These instrumental
techniques are selective, and some are expensive and not easy to
operate in the field. Moreover, many simple techniques are
inefficient for explosive detection in the field due to high cost and
Department of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore, 560 012, India. E-mail: shanmugam@ipc.iisc.ernet.
in; Fax: +91-80-2360-1552; Tel: +91-80-2293-3352; psm@ipc.iisc.ernet.in
7
a lack of selectivity. Thus there is an urgent need to find an
efficient and more reliable detection technique which is cheap
and very easy to use. Among various transduction methods used
for explosive detection, the fluorescence quenching, in particular,
is considered to be the most effective tool for sensing nitro-
aromatic explosives in recent years owing to its high sensitivity,
1
†
Electronic supplementary information (ESI) available: NMR ( H and
C) spectra of the fluorophores 2 and 4. The fluorescence quenching
1
3
titration profile of the fluorophores 4 and 5 with picric acid along with
their corresponding Stern–Volmer plot and Cartesian coordinates of
the complexes for the DFT study are available. See DOI:
8
10.1039/c1jm10406c
easy visualization, and short response time for detection.
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Identifying and distinguishing related (chemically or structur-
ally) compounds by optical detection methods are challenging
9
tasks for chemical sensing based detection. One of the best
characterization of a series of trimethylsilylethynyl (TMS)
substituted small molecule-based fluorophores (1–5). Fluo-
rophores 1–5 (Chart 1) showed fluorescence behavior in chlo-
roform solution and were used as selective and sensitive
fluorescence quenching based sensors for the detection of nitro-
aromatic explosives. Fluorescence quenching titration revealed
that the nitroaromatic compounds (NAC) showed the largest
quenching response with high selectivity among the various other
electron deficient aromatic compounds. We also report that
detection strategies to distinguish structurally related nitro-
aromatics is to make a series of fluorophores which would
eventually provide differential fluorescence quenching
10
11
responses. Recently, a family of Zn(salicylaldimine) based
sensors showed differential optical response to closely related
nitroaromatics. Here, we report a series of discrete p-electron-
rich fluorophores (Chart 1) which exhibit differential quenching
responses for the structurally related nitroaromatics compounds.
In addition to the available fluorophores such as conjugated
1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene
(5)
indicates
a detectable visible color change upon subjecting to increased
concentration of trinitrotoluene (TNT) in chloroform solution.
In the case of fluorophore 9, 10-bis(trimethylsilylethynyl)
anthracene (4) and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene
(5), a thin film prepared by spin coating of a chloroform/
12
organic polymers by Swager et al., inorganic conjugated poly-
13
mers by Trogler et al. and Zn(salicylaldimine) based small
14
molecule by Knapp et al., we have recently shown that finite 3D
15
ꢁ3
supramolecular cages, synthesized via a directional bonding
approach, can sense nitroaromatics in solution. Despite the
intensive and extensive efforts made during the past decade, the
development of p-conjugated fluorophores for sensitive and
selective detection of nitroaromatic explosives is still required.
Two important characteristics need to be considered in designing
a new small molecule-based fluorophore for sensing nitro-
aromatics. Firstly, the fluorophore should be p-electron rich and
planar in nature. Secondly, a bulky substituent must be attached
on the peripheral of the chromophore to prevent the intermo-
lecular excimer formation through p–p interaction. Our strate-
gies to design a new fluorophore for sensing nitroaromatics
dichloromethane solution (1.0 ꢀ 10 M) over quartz plate
showed significant fluorescent quenching responses upon expo-
sure to saturated vapors of nitroaromatic explosives. The vapor
phase sensing performance of these thin films showed highly
selective and reproducible quenching efficiency towards nitro-
aromatic compounds.
2. Results and discussion
2
.1 Synthesis of fluorophores (1–5)
p-Electron rich aromatic compounds like pyrene and anthracene
have been used extensively as fluorescence probes in several
applications such as pyrene-labeled oligonucleotides as probes to
(
NAC) are very simple, i.e., the introduction of trimethylsilyle-
thynyl (TMS) substituent on the peripheral of the fluorophores.
This strategy produces a significant increase in fluorescence
emission intensity of the fluorophores and is expected to prevent
the intermolecular stacking that decreases the life of the fluo-
rophores. It has been reported that the substitution of a bulky
trimethylsilylethynyl (TMS) group on the fluorophore results in
a large bathochromic shift of absorption and emission bands
17,18
study DNA hybridization.
In spite of extensive use as fluo-
rescence sensors, electron rich aromatic compounds have two
significant drawbacks as fluorescence probe. Most of the
aromatic monomers show absorption and emission characteris-
tics in the UV region and form excimers at higher concentra-
19
tion. As our aim is to sense nitroaromatic explosives by
fluorescence quenching of electron-rich aromatic fluorophores, it
is desirable to have fluorophores that absorb and emit in the
visible region in order to minimize the spectral overlap of the
16
than that of unsubstituted fluorophores. This behavior is
explained in terms of the s–p interaction between the C–Si s and
acetylene p bonds. Here, we wish to report the synthesis and
Chart 1
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intrinsic fluorescence of nitroaromatics that occurs in the UV
region. The well known method to bathochromically shift the
emission peak is to increase the conjugation of the fluorophores
by substituting unsaturated functional groups. We selected the
trimethylsilylethynyl-group as a suitable substituent to increase
the conjugation of fluorophores. Interestingly, the trimethylsi-
lylethynyl functional group plays a dual role: its incorporation to
pyrene/anthracene fluorophores increases the conjugation
through s–p interaction between the C–Si s and ethynyl p
detection. The mechanism of fluorescence quenching involves the
formation of charge-transfer (CT) complexes between the elec-
tron rich fluorophore and electron deficient quencher. In this
study several trimethylsilylethynyl (TMS) substituted fluo-
rophores (1–5) have been selected to sense nitroaromatic
compounds (NAC) based on the proposed quenching
mechanism.
To demonstrate the ability of the selected fluorophores for the
detection of nitroaromatic explosives, we first performed fluo-
rescence quenching titration experiments using the fluorophores
4 and 5 with nitroaromatics such as 2,4,6-trinitrotoluene (TNT)
and picric acid (PA) in solution. Upon gradual addition of
nitroaromatics to a chloroform solution of the fluorophores 4
and 5, the initial fluorescence intensity was quenched efficiently.
A linear Stern–Volmer plot was obtained from the fluorescence
quenching titration and the calculated Stern–Volmer binding
16
bonds and it prevents the intermolecular excimer formation
by p–p stacking) to maintain the spectroscopic stability of the
fluorophores in solution.
(
The trimethylsilylethynyl (TMS) substituted fluorophores (1–5)
were synthesized by employing palladium-catalyzed Sonogashira
coupling of corresponding aryl halides and trimethylsilylacetylene
under a stream of nitrogen atmosphere (Scheme 1).
4
ꢁ1
5
ꢁ1
constants (Ksv) were 1.97 ꢀ 10 M for TNT, 2.15 ꢀ 10 M for
PA (4); and 1.4 ꢀ 10 M for TNT, 1.1 ꢀ 10 M for PA (5)
Fig. 2, 3, S5 and S6 in the ESI†). The obtained quenching
3
ꢁ1
4
ꢁ1
2
.2 Photophysical study of the fluorophores
(
Absorption maxima (labs) along with their molar absorptivity
coefficients (3), and fluorescence emission maxima (lem) of the
fluorophores (1–5) including solution state fluorescence quantum
yield with respect to the anthracene standard in aerated chloro-
form are given in Table 1. The absorption and fluorescence
emission of the trimethylsilylethynyl substituted fluorophores
constants for TNT and PA are comparable with previously
reported values for other small molecules like fluorescein, mal-
21
achite green, phenol red, etc. The linear Stern–Volmer plots
suggest that fluorescence quenching involves a static-quenching
mechanism at lower concentration of nitroaromatics.
To explore the selectivity, we have also investigated the effect
of other electron deficient aromatic compounds on the initial
fluorescence intensity of the fluorophores (1–5). The selected
fluorophores showed differential fluorescence quenching
responses among various nitroaromatics compounds, which
allowed discriminating of closely related nitroaromatic explo-
sives. The obtained results are shown in Fig. 4, which indicates
that almost all the fluorophores show greater quenching response
for electron deficient nitroaromatic compounds than other elec-
tron deficient non-nitroaromatics such as benzoquinone,
4-methoxybenzoic acid and benzoic acid. The tested non-nitro-
aromatic quenchers are almost non-polar in nature compared to
the nitro-substituted aromatic quenchers. Since the reduction
potential of non-nitroaromatic quenchers are not significantly
(
1–5) shifted to longer wavelength (bathochromic shift) in
comparison to their starting aryl halides (Fig. 1). Moreover,
absorption coefficients of the fluorophores increased with
increased number of trimethylsilylethynyl substituents. The
induced longer wavelength shift of the fluorophores after
attaching the trimethylsilylethynyl group might be due to the s–
p interaction between C–Si s and acetylene p bonds, which
destabilizes the highest occupied molecular orbital (HOMO) and
stabilizes the lowest unoccupied molecular orbital (LUMO) of
20
the fluorophores.
2
.3 Fluorescence quenching study in solution
2
b
As mentioned above, one of the most promising approaches for
sensing nitroaromatic explosives is fluorescence quenching based
different from the nitro-substituted aromatic quenchers, we
believe that the observed higher fluorescence quenching
Scheme 1 Schematic representation of the synthesis of fluorophores (1–5) from the corresponding aryl halides.
132 | J. Mater. Chem., 2011, 21, 9130–9138 This journal is ª The Royal Society of Chemistry 2011
9

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Table 1 Photophysical data of the fluorophores (1–5) taken in aerated chloroform
Molar absorptivity co-
3
ꢁ1
ꢁ1
efficient 3 ꢀ 10 /M cm
Fluorescence emission
maxima lmax/nm
a
Quantum yield (F)
Fluorophores
Absorption maxima lmax/nm
(lmax/nm)
1
2
3
4
5
294, 279
304
248, 283
269, 400, 417, 440
440, 413, 316, 304, 259, 251
34 (294)
48 (304)
42 (248)
80 (269)
76 (440)
316
337, 355
375
444, 471
442, 468
0.003
0.04
0.078
0.105
0.164
a
Using anthracene (F
absorption (labs) and emission (lem) maxima, respectively.
f
¼ 0.27) as the fluorescence standard in ethanol at room temperature. Values in bold in columns II and III represent the highest
result of the formation of charge-transfer (CT) complex between
them.
On the basis of purely electronic consideration, p-electron
rich 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (5) might show
maximum fluorescence quenching response to the electron
deficient nitroaromatic compounds. However, it was observed
that electron rich pyrene fluorophore shows poor quenching
response towards nitroaromatics than other fluorophores tested
(
Fig. 4). We ascribe that this poor quenching response may be
Fig. 1 Electronic absorption (left) and fluorescence emission (right)
spectra of the fluorophores (1–5) in aerated chloroform solution.
due to the steric effect of bulky trimethylsilyl groups. The
substitution of four bulky trimethylsilyl groups on the periph-
eral of the pyrene chromophore may be too sterically
demanding to permit an optimal interaction between the fluo-
rophore and the electron deficient nitroaromatics which in turn
results in poor fluorescence quenching response. On the other
0
hand, due to the non-planar nature, 4,4 -bis(trimethylsilyle-
thynyl)biphenyl (2) fluorophore is expected to have weak
interaction with electron deficient nitroaromatics, and in turn,
shows poor fluorescence quenching response. Indeed, experi-
mental observation showed that fluorophore
2 exhibits
moderate quenching response towards nitroaromatics. But the
fluorescence quenching of non-planar fluorophore 2 is much
higher in comparison to the p-electron rich planar fluorophore
Fig. 2 Fluorescence quenching (left) of the fluorophore 4 (10 mM) with
TNT in chloroform solution (lex ¼ 270 nm, excitation and emission slit
width is 1 nm) and Stern–Volmer plot (right).
5. The reason for this observed anomaly in quenching response
is attributed to the less-steric hindrance of trimethylsilyl
substituents on 2 and in turn, it increases the effective binding of
nitroaromatic quenchers with fluorophore 2 compared to fluo-
rophore 5. In order to understand the nature of interaction of
fluorophore with analytes and to provide further support to the
observed fluorescence quenching response, density functional
theory (DFT) calculation was carried out using the Gaussian 03
program. We used the MPWB1K hybrid meta density func-
22
tional developed by Zhao and Truhlar and 6-31G* basis set to
optimize the structure of fluorophore–analyte complexes. The
electron deficient picric acid (PA) was optimized with 1,8-bis
(
trimethylsilylethynyl)anthracene (4) and 1,3,6,8-tetrakis(trime-
Fig. 3 Fluorescence quenching (left) of the fluorophore 5 (10 mM) with
TNT in chloroform solution (lex ¼ 316 nm, excitation and emission slit
width is 1 nm) and obtained Stern–Volmer plot (right).
thylsilylethynyl)pyrene (5) in view to obtain the firm evidence
for the observed fluorescence quenching response. The opti-
mized structure of the complexes 4$PA and 5$PA are given in
Fig. 5. As can be seen from Fig. 5 both the fluorophores (4 and
5) show strong intermolecular p–p interaction and maintained
a co-facial stacked geometry with picric acid (PA). The calcu-
lated intermolecular distances between the fluorophore molec-
responses of nitro-compounds is probably due to the strong
dipolar or electrostatic interaction between the electron-deficient
analytes with p-electron rich aromatic fluorophores. This
observation is fully consistent with the proposed quenching
mechanism in which the electron-deficient nitroaromatics act as
a fluorescence quencher to electron-rich fluorophores as the
ꢀ
ular plane and that of picric acid are almost the same (3.17 A
ꢀ
for 4$PA and 3.30 A for 5$PA).
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Fig. 4 Changes in fluorescence intensity of the fluorophores (1–5) (plotted as quenching efficiency) observed upon the addition of various analytes.
BQ ¼ benzoquinone, 4-MeOBA ¼ 4-methoxybenzoic acid, BA ¼ benzoic acid, NB ¼ nitrobenzene, NT ¼ nitrotoluene, DNP ¼ dinitrophenol,
TNT ¼ 2,4,6-trinitrotoluene and PA ¼ picric acid.
from the p-electron rich discrete fluorophore 5 to the electron-
deficient picric acid and therefore shows poor fluorescence
quenching response towards nitroaromatics in solution.
2
.4 Sensitivity
To find the sensitivity of the fluorophores towards nitro-
aromatics, fluorescence quenching titration was performed using
fluorophore 4 with picric acid (PA) at parts per billion (ppb)
concentrations. The quantitative analysis of the fluorescence
titration profile (Fig. 6) shows that this particular fluorescence
sensor can respond to the presence of picric acid as low as ppb
level in solution. This level of sensitivity comes under the
Fig. 5 Optimized geometry of complex 4$PA (top) and complex 5$PA
(
bottom) (color codes: red ¼ oxygen, blue ¼ nitrogen, grey ¼ carbon,
white ¼ hydrogen and light green ¼ silicon).
Moreover, the estimated interaction energy for the complex
ꢁ
1
4
$PA is 16.32 kcal mol , which is higher than that of complex
ꢁ
1
5
$PA (14.09 kcal mol ); this reflects the fact that fluorophore 5
forms a weak excited charge transfer complex with picric acid.
The possible reason for the observed weak complexation of 5
with picric acid could be attributed to the propinquity of elec-
tron-density from the methyl groups on TMS to the plane of
picric acid and that decreases the efficiency of electron transfer
Fig. 6 Changes in initial fluorescence intensity of the fluorophore 4
(
3
0.005 mM in CHCl ) upon gradual addition of PA (0–8 ppb in meth-
anol). (The excitation wavelength lex ¼ 270 nm, excitation and emission
slit width ¼ 5 nm.)
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134 | J. Mater. Chem., 2011, 21, 9130–9138
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permissible limit of nitroaromatic explosives such as TNT in
23
drinking water established by the US EPA.
2
.5 Visual detection of nitroaromatics
Design of visual sensors for the detection of explosives has been
a popular field of research in last two decades owing to their ease
of interpretation, simple material handling, rapid detection and
more suitable method to practice in the field. The formation of
24
a colored complex creates a new chromophore which shows
totally different photo-physical characteristics from the starting
explosive residue and sensor molecules including color changes.
The sharp color change is the clear identification for the presence
of particular chemical explosives. In this study, we have observed
a marked color change upon subjecting 5 with trinitrotoluene
Fig. 8 Absorption spectral changes (left) of fluorophore 5 in chloroform
ꢁ
5
ꢁ4
(
5.0 ꢀ 10 M) upon titrating with TNT in methanol (0–5 ꢀ 10 M) at
ꢃ
2
5
C and its corresponding stoichiometry plot (right) monitored at
4
12 nm.
resonance corresponding to both the fluorophore 5 and picric
acid was observed and subsequent progressive dilution of the
NMR sample solution resulted in a gradual downfield shifting of
proton resonances (Fig. S7, ESI†). The downfield shifts of
resonances attributed to the shifting of equilibrium position from
the bound charge transfer state to unbound state of analytes.
(
TNT) in different concentration in chloroform solution (Fig. 7).
The formation of a charge transfer (CT) complex between
p-electron rich fluorophore 5 and electron deficient nitro-
aromatic TNT was further supported by electronic absorption
spectroscopic study. Slight change in the initial absorption
intensity of 5 was noticed upon the gradual addition of trini-
1
ꢁ3
Based on the electronic absorption and H NMR titration
trotoluene (TNT) in methanol (1.0 ꢀ 10 M) to a chloroform
ꢁ6
experiments, we think that the marked visual color change of
fluorophore upon mixing with nitroaromatic quencher is due to
the charge transfer complex formation between p-electron donor
fluorophores and electron-acceptor nitroaromatic analytes in
solution.
solution (1.0 ꢀ 10 M) of fluorophore 5 at room temperature.
The strong absorption band of 5 at 303 nm, 316 nm, 389 nm and
4
12 nm decreases in intensity, while a new broad band at
ꢂ342 nm gradually rises upon the addition of increasing amounts
of TNT (Fig. 8). In addition to the perturbation observed in the
electronic absorption spectra, the progressive appearance of two
well defined isosbestic points at 321 nm and at 367 nm indicated
the formation of a charge-transfer or donor–acceptor complex
between p-electron rich donor 5 and electron acceptor TNT. The
binding ratio of fluorophore 5 with the quencher (TNT) was
calculated by stoichiometry plot analysis and it was found to be
2
.6 Solid-state fluorescence quenching study
Although, sensors in solution are simple and more selective
towards nitroaromatic explosives, from the viewpoint of prac-
tical use, thin-film sensors are more suitable for field test.
Advantages of film sensors over solution sensors are reusability
and easy possibility of making into devices. Therefore, the design
and preparation of fluorescent film for nitroaromatic explosives
1
: 2 (Fig. 8). Thus, one molecule of 5 complexes with two
quenchers (TNT) in solution.
The formation of charge-transfer complex was further verified
1
1
25
by H NMR spectroscopic analysis. In a typical H NMR titra-
tion experiment, a pale yellow solution of picric acid was mixed
detection have become the main focus of material scientists.
The solid state fluorescence quenching experiment was per-
formed for the vapor of 4-nitrotoluene (NT) with thin film of the
fluorophores 4 and 5. The thin film of the fluorophore 4 showed
strong fluorescence and upon exposure to saturated vapor of
with fluorophore 5 in 2 : 1 molar ratio in 500 mL of CDCl . Upon
3
complexation, a significant upfield shifting of the proton
4
-nitrotoluene, the initial fluorescence intensity of the film
quenched dramatically (Fig. 9). Fig. 9 clearly indicates that the
thin film of 4 is sensitive to the vapor of 4-nitrotoluene (NT).
Exposure of the film to the saturated vapor of 4-nitrotoluene for
3
00 s causes nearly 18% quenching of the fluorescence and the
Fig. 7 Visual color changes observed upon subjecting 17 mM solution of
5
in chloroform with increasing quantity of TNT in chloroform (from left
Fig. 9 Fluorescence spectra (left) of the thin film of 4 upon exposure to
the saturated vapor of 4-nitrotoluene (NT) at different time and
quenching efficiency (right) against time.
to right TNT, fluorophore, 2, 4, 6, 8 and 10 equivalents of TNT) in room
light (top) and under hand hold UV light (below).
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efficiency of fluorescence quenching increased further with
increasing exposure time. Nearly 40% quenching efficiency was
observed after 7 min exposure of the film to NT vapor. On the
other hand, the thin film of fluorophore 5 exhibits nearly 40%
reduction in the initial fluorescence intensity for 360 s exposure of
film to a saturated vapor of 4-nitrotoluene (Fig. 10). Hence,
fluorophore 5 looks to be more efficient and faster in response
towards nitroaromatics compared to fluorophore 4 in solid state.
Conversely, the fluorophore 4 exhibits more quenching efficiency
towards nitroaromatics than fluorophore 5 in solution phase and
the observed higher quenching performance of 5 in solid phase
may be attributed to the rapid vapor diffusion of analytes into
the film, which can increase the efficiency of binding while this
may be more difficult in solution-phase due to the steric
hindrance of methyl groups on TMS. The reduction in fluores-
cence intensity of the thin film of fluorophores upon exposure to
the nitroaromatic vapor is ascribed to the charge transfer
complex formation between electron-rich fluorophore and elec-
tron-deficient nitroaromatics. It is noteworthy that the film of
fluorophore 5 is not only responsive to the vapor of nitrotoluene
Fig. 12 Reproducibility of sensing ability of thin film 4 to the saturated
vapor of picric acid (PA). The film was excited at l ¼ 380 nm and the
corresponding emission was monitored from l ¼ 430 nm. The excitation
and emission slit width for all the measurements is 3 nm.
efficiency of the film of fluorophore 5 follows the order of NB >
NP > NT (Fig. 11). The reason for the observed differences in the
quenching efficiency of the film to different nitroaromatic
compounds might be due to their difference in vapor pressure
and redox potential. Thus, in the present case, the significantly
faster quenching response of nitrobenzene (NB) than nitro-
phenol (NP) and nitrotoluene (NT) could be attributed to the
(
NT), but also shows the differential quenching responses to the
vapor of other nitroaromatic compounds like nitrobenzene (NB)
and nitrophenol (NP). On comparison, the quenching efficiency
of the film is more sensitive to the nitrobenzene vapor than that
of nitrotoluene and nitrophenol (Fig. 11). 120 s exposure of the
film of 5 to the saturated vapor of nitrobenzene results in nearly
ꢃ
higher vapor pressure (1.0 mm of Hg at 44.4 C) of NB. The poor
5
0% reduction in the initial emission intensity.
quenching response of nitrotoluene might be due to its lower
redox potential (ꢁ1.2 V) compared to other nitroaromatic
compounds.
Whereas 120 s exposure of the same film to the saturated
vapors of nitrotoluene and nitrophenol exhibits nearly 25% and
6% quenching, respectively (Fig. 11). Overall, the quenching
4
Interestingly, the exposure of the film of 5 to the saturated
vapors of other electron deficient aromatic compounds like
benzoic acid (BA), benzoquinone (BQ) and anthraquinone (AQ)
results in no reduction in intensity rather than a little enhance-
ment of initial fluorescence intensity. Hence, all these sensing
experiments revealed that the thin film of fluorophore 5 exhibits
highly selective quenching response towards nitroaromatics.
2
.7 Recyclability of sensing process
As far as a real-time security application is concerned, the sensing
process of a sensor molecule must be a highly recyclable one. The
reproducibility of the sensing process was examined using a thin
Fig. 10 Fluorescence spectra (left) of thin film of 5 upon exposure to the
saturated vapor of 4-nitrotoluene (NT) at different time interval and
quenching efficiency (right) against time.
ꢃ
film of the fluorophore 4 with picric acid (PA) at 25 C. The film
was first exposed to a saturated PA vapor at room temperature
for 300 s of exposure and then its emission spectrum was recor-
ded immediately. After the measurement, the film was washed
several times with distilled water and dried in hot air supplied by
a heat gun. The emission spectrum of the film was recorded again
and the whole process was repeated for three times. After three
cycles the film regained 97% of the initial fluorescence intensity
and thus the film can be used for significant number of sensing
cycles. The obtained results are given in Fig. 12, which clearly
suggests that the sensing response of the film to the PA vapor is
basically reproducible.
3
. Conclusion
Fig. 11 The percentage quenching efficiency of the thin film of fluo-
rophore 5 by different analytes at room temperature with the exposure
time of 120 s.
Conjugated polymeric sensors have been widely studied for
explosives sensing as several excitons can be quenched by one
9
136 | J. Mater. Chem., 2011, 21, 9130–9138
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molecule of analyte. But there are a few drawbacks of polymers:
time consuming synthesis, poor solubility in many cases and
difficult to control the structure. We report here a series of tri-
methylsilylethynyl substituted p-electron rich aromatic fluo-
rophores which can act as selective and sensitive fluorescence
sensors for nitroaromatics explosives. Strong quenching of the
initial fluorescence intensity of the fluorophores (1–5) was
observed upon the gradual addition of the solution of electron
deficient nitroaromatics. The observed fluorescence quenching is
ascribed to the charge transfer between fluorophore and nitro-
aromatics. Moreover, the differential fluorescence quenching
response of the fluorophores to closely related nitroaromatics
allows the selective discrimination of the structurally related
explosive species. Quantitative analysis of the fluorescence
quenching profile shows that these molecules are sensitive and
able to show the marked quenching response for nitroaromatics
as low as parts per billion level concentrations in solution. Vapor
phase quenching study of the thin film of fluorophore 4 or 5
revealed that the sensing process is basically recyclable and
selective only for nitroaromatic compounds. Sensitive and
selective fluorescence response including sharp visual color
changes for nitroaromatic compounds makes these fluorophores
as promising fluorescence sensory materials for nitroaromatics
chromatography (silica gel) using hexane as eluent to obtain the
expected 9,10-bis[trimethylsilylethynyl]anthracene
4 as an
intense red solid in 60% yield. Anal. calcd for C H Si : C,
2
4
26
2
1
77.77; H, 7.07. Found: C, 77.85; H, 7.18%. H NMR (CDCl
MHz): d 8.58 (d, 4H, J ¼ 10.0 Hz, anthracene-H), 7.62 (d, 4H,
J ¼ 10.0 Hz, anthracene-H), 0.42 (s, 18H, trimethylsilyl).
NMR (CDCl ): d 132.7, 127.6, 127.3, 122.8, 118.9, 108.6, 101.9,
0.69 ppm. HRMS: calcd. for C24 370.63; found 371.16
3
, 400
13
C
3
H
1
26Si
2
(M + H). IR (KBr): n ¼ 2124 cmꢁ for ethynyl group.
4.3 Fluorescence quenching titrations in solution
A 2 mL stock solution (10 ꢀ 10ꢁ M) of the fluorophores (1–5)
6
was placed in a quartz cell of 1 cm width and analytes solution
3
(
10 ꢀ 10ꢁ M) was added in an incremental fashion. Their cor-
responding fluorescence emission spectra were recorded at
2
98 K. Each titration was repeated at least three times to get
concordant value. For all measurements excitation wavelength
ex) 280 nm for 1, 305 nm for 2, 249 nm for 3, 270 nm for 4,
16 nm for 5 and the corresponding emission wavelength (lem
was monitored at 290 nm for 1, 315 nm for 2, 260 nm for 3,
80 nm for 4, and 326 nm for 5. Both excitation and emission slit
(
l
3
)
2
widths were 1 nm for 1–5 except for 3 were at 5 nm. Relative
fluorescence intensities were measured for the solution of fluo-
rophores 1–5 in chloroform and the various analytes were used as
quencher in methanol. There was no change in shape of the
emission spectra except gradual quenching of the initial fluo-
rescence emission intensity upon titration with electron deficient
nitroaromatic quenchers. Analysis of the normalized fluores-
(
NAC). Thus, the present results demonstrate that small mole-
cule fluorophores can also be viewed as a suitable class of
compounds for further development of efficient sensors by
introducing various substituents that can enhance the binding
interaction between the sensors and explosives constituents.
cence emission intensity (I
quenchers concentration ([Q]) was well described by the Stern–
Volmer equation I
/I ¼ 1 + KSV[G]. The Stern–Volmer binding
constant was calculated from the slope of the Stern–Volmer plot.
0
/I) as a function of increasing
4
. Experimental section
0
4
.1 Synthesis of 4,4 -bis[trimethylsilylethynyl]biphenyl (2)
0
In a flame dried 100 mL round bottom Schlenk flask were dis-
0
solved 4,4 -dibromobiphenyl (3.0 g, 9.6 mmol) and 4 mL trime-
4
.4 Preparation of thin films
thylsilylacetylene in freshly distilled 100 mL dry triethylamine. A
The stock solution (1.0 ꢀ 10ꢁ M) of the fluorophores 4 or 5 in
chloroform or dichloromethane was added drop wise over
a clean surface of quartz plate and then the plate was kept under
rotation at 60 rounds per second speed for 60 s. This process was
repeated at least thrice to get a better film and the film was dried
in air before the fluorescence quenching study.
3
3 2 2
mixture of 150 mg of [(PPh ) PdCl ] (0.21 mmol), 75 mg of tri-
phenylphosphine (0.29 mmol) and 75 mg of CuI (0.4 mmol) was
added, and the suspension was refluxed with stirring under
nitrogen atmosphere for 12 h. The volatile solvent was removed
under vacuum and the resulting residue was purified by column
chromatography (silica gel) using hexane as eluent to obtain the
0
expected 4,4 -bis[trimethylsilylethynyl]biphenyl 2 as an off-white
solid in 80% yield. Anal. calcd for C22
1
Found: C, 76.29; H, 7.61%. H NMR (CDCl
H
26Si
2
: C, 76.23; H, 7.56.
4.5 Solid-state fluorescence quenching study
3
, 400 MHz): d 7.52
13
The fluorescence quenching of the fluorophores 4 and 5 with
(
s, 8H, biphenyl-H), 0.26 (s, 18H, trimethylsilyl). C NMR
CDCl ): d 140.6, 132.9, 127.2, 122.8, 105.3, 95.7, 0.47 ppm.
346.61; found 347.6 (M + H). IR
4
-nitrotoluene in solid state was performed by inserting the
(
3
freshly prepared thin film into the beaker containing solid
4-nitrotoluene and cotton gauze, which prevents the direct
contact of the film with 4-nitrotoluene and maintains the vapor
pressure of the quencher during measurements. After a period of
exposure, film was placed in the sample holder immediately and
checked for the fluorescence quenching of the initial film inten-
sity. For all measurements the excitation wavelength
lex ¼ 380 nm and corresponding emission was monitored at
430 nm in the case of fluorophore 4. For fluorophore 5, the film
was excited at l ¼ 440 nm and corresponding emission was
monitored at l ¼ 450 nm. The excitation and emission slit width
were at 5 nm for 4 and 2 nm for 5. The observed fluorescence
quenching was calculated using the following equation
HRMS: calcd. for C22H26Si
2
KBr): n ¼ 2154 cmꢁ for ethynyl group.
1
(
4
.2 Synthesis of 9,10-bis[trimethylsilylethynyl]anthracene (4)
In a flame dried 100 mL round bottom Schlenk flask were dis-
solved 9,10-dibromoanthracene (3.0 g, 8.9 mmol) and 4 mL tri-
methylsilylacetylene in freshly distilled 50 mL dry triethylamine.
A mixture of 150 mg of [(PPh ) PdCl ] (0.21 mmol), 75 mg of
3
2
2
triphenylphosphine (0.29 mmol) and 75 mg of CuI (0.4 mmol)
was added, and the suspension was refluxed with stirring under
nitrogen atmosphere for 12 h. The volatile solvent was removed
under vacuum and the resulting residue was purified by column
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 9130–9138 | 9137

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h ¼ (I
0
ꢁ I/I
0
) ꢀ 100%
114, 3418; (f) J. Hu, L. Ronger, J. H. Yip, Y. K. Wong, D. L. Ma
and J. J. Vittal, Organometallics, 2007, 26, 6533; (g) V. W.-W. Yam,
R. P. L. Tang, K. M. C. Wong and K. K. Cheung, Organometallics,
Where, I is the initial fluorescence intensity of the fluorophore
0
2001, 20, 4476; (h) S. Das and P. K. Bharadwaj, Inorg. Chem., 2006,
5, 5257; (i) S. Das and P. K. Bharadwaj, Org. Lett., 2005, 7, 1573;
and I is the fluorescence intensity after a period of exposure time.
4
(
(
j) S. Das and P. K. Bharadwaj, Cryst. Growth Des., 2007, 7, 1192;
k) W. Ming, V. Vajpayee, S. Shanmugaraju, Y. P. Zheng, Z. Zhao,
Acknowledgements
H. Kim, P. S. Mukherjee, K.-W. Chi and P. J. Stang, Inorg. Chem.,
011, 50, 1506; (l) S. Shanmugaraju, A. K. Bar, S. J. Joshi,
Y. P. Patil and P. S. Mukherjee, Organometallics, ASAP, 2011, DOI:
0.1021/om2000019.
2
S.S. gratefully acknowledges the Council of Scientific and Industrial
Research, New Delhi, India for the award of research fellowship. P.
S.M. and S.A.J thank the Department of Science and Technology
1
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S. E. Stitzel, T. P. Vaid and D. R. Walt, Chem. Rev., 2000, 100,
(DST), India, for the financial support. Authors appreciate very
fruitful and constructive comments from the referees.
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