Vapor-phase detection of trinitrotoluene by AIEE-active hetero-oligophenylene-based carbazole derivatives

Article abstract of DOI:10.1021/la302309z

New AIEE-active hetero-oligophenylene carbazole derivatives 3 and 4 have been synthesized and serve as fluorescent probes for the selective detection of 2,4,6-trinitrotoluene (TNT) in the vapor phase, the solid phase, and aqueous media. In addition, paper strips prepared by dip-coating a solution of aggregates of derivatives 3 and 4 can provide a simple, portable, sensitive, selective, low-cost method for the detection of TNT on the picogram level.

Full text of DOI:10.1021/la302309z

Article
pubs.acs.org/Langmuir
Vapor-Phase Detection of Trinitrotoluene by AIEE-Active Hetero-
oligophenylene-Based Carbazole Derivatives
Manoj Kumar,* Varun Vij, and Vandana Bhalla*
Department of Chemistry, UGC-Centre for Advanced Studies, Guru Nanak Dev University, Amritsar, Punjab, India 143005
S
* Supporting Information
ABSTRACT: New AIEE-active hetero-oligophenylene carba-
zole derivatives 3 and 4 have been synthesized and serve as
fluorescent probes for the selective detection of 2,4,6-
trinitrotoluene (TNT) in the vapor phase, the solid phase,
and aqueous media. In addition, paper strips prepared by dip-
coating a solution of aggregates of derivatives 3 and 4 can
provide a simple, portable, sensitive, selective, low-cost method
for the detection of TNT on the picogram level.
transporting properties. Interestingly, derivatives 3 and 4
exhibit AIEE characterstics and good selectivity for TNT over
other nitroaromatics such as 2,4-dinitrotoluene (DNT), 1,3-
dinitrobenzene (DNB), and picric acid (PA). Disposable paper
strips prepared by dip-coating solutions of aggregates of
derivatives 3 and 4 can detect TNT on the picogram level,
thus providing a simple, low-cost method for the detection of
TNT in aqueous solution. To the best of our knowledge, AIEE-
active hetero-oligophenyl carbazole derivatives that sense TNT
in the vapor phase, solid phase, and aqueous phase and uphold
good selectivity for TNT over DNT are unprecedented in the
literature. Furthermore, AIEE-active derivatives 3 and 4 exhibit
a more sensitive response toward TNT than do previously
reported chemosensors for TNT.11
INTRODUCTION
■
Trinitroaromatics are well-known primary constituents of many
unexploded land mines worldwide1 and are also considered to
be environmental contaminants because the soil and ground-
water of war zone and military facilities can contain toxic level
of these compounds. Trinitrotoluene (TNT), a widely used
nitroaromatic explosives, is poisonous and carcinogenic and can
adversely affect male fertility.2 TNT can also cause skin
irritation, anemia, and abnormal liver function.2 Therefore, the
development of cost-efficient, selective, sensitive, portable
detection methods for TNT is highly desirable. Although
several strategies using quantum dots,3 gold nanoparticles,4
silver−gold alloy nanostructures,5 and molecular imprinting
with AuNP's6 have been reported, no single strategy
incorporating all of the above-mentioned features has been
reported. Recently, there has been a lot of activity with respect
to the preparation of conjugated polymers7 and fluorescent
nanofibers8 for the efficient detection of nitroaromatic
compounds. However, real-time monitoring of the polymeric
materials is limited because of their multistep conventional
covalent synthesis. Furthermore, most of these reported sensors
exhibit a more sensitive response toward dinitrotoluene (DNT)
than toward to TNT. Thus, the development of highly
selective, sensitive, fluorescent sensor materials for the
detection of TNT at low concentration is still a challenge.9
Recently, AIE(E)-active materials have been used as sensors
for the detection of nitroaromatics.10 AIE(E)-active low-
molecular-weight luminogens are beneficial because they offer
more diffusion channels for the exciton to migrate, allowing
them to be more quickly anihilated by explosive quenchers.10a
Keeping this in mind, we planned to prepare AIEE-active
material for the selective sensing of TNT. Thus, we designed
and synthesized new hetero-oligophenyl-based derivatives 3
and 4 having the carbazole moiety. We have chosen the
carbazole moiety for its good electroluminescence and hole-
RESULTS AND DISCUSSION
■
The reaction of carbazole acetylene derivative 1a12 with 3,4-
bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone 213
in diphenylether furnished compound 3 in 82% yield (Scheme
1). Using the same synthesis procedure as used for 3, we also
prepared compound 4 by heating carbazole phenyl acetylene
derivative 1b14 with 4-bis(4-methoxyphenyl)-2,5 diphenylcy-
clopenta-2,4-dienone 2 in 81% yield (Scheme 1). The
structures of compounds 3 and 4 were confirmed from their
spectroscopic and analytical data (Supporting Information,
Figures S13−S18).
The UV−vis spectrum of compound 3 in THF exhibits
absorption bands at 271 and 290 nm. However, in the presence
of 80% H2O in THF, the absorption bands were observed at
240 and 305 nm, which are attributed to the aggregate state of
compound 3. In the visible spectral region of the absorption
Received: June 7, 2012
Revised: July 25, 2012
Published: July 27, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Derivatives 3 and 4
spectrum of 3 (8:2 H2O/THF), a leveling-off tail was observed
(Supporting Information, Figure S2) which is due to the
nanoparticle suspensions, confirming the existence of aggre-
gates of 3 in the solvent mixture (8:2 H2O/THF).15 The
formation of aggregates of derivatives 3 and 4 is supported by
scanning electron microscopy (SEM) images of derivatives 3
and 4 in 8:2 H2O/THF, which show the presence of spherical
aggregates (Figure 1).
fraction is higher than 80%, the solubility of derivatives 3 and 4
in the solvent mixture is relatively low and the number of
emissive molecules per unit volume is decreased. The
fluorescence enhancement due to the restriction-in-rotation
process could not contend with the decreasing trend in the
intensity caused by the smaller number of emitting molecules.
Hence, the fluorescence intensity is decreased.17
We believe that in aqueous media intramolecular rotations
are restricted by the formation of aggregates that block the
nonradiative channels and populate the radiative excitons,
thereby making the molecule emissive in the aggregate state.
We envisioned that if this mechanism is indeed at work then
the solution of compounds 3 and 4 should become more
emissive when the viscosity is increased because the thickening
process is known to hamper intramolecular rotations.18 Thus,
we recorded the fluorescence spectra of compounds 3 and 4 in
highly viscous triethylene glycol (TEG), and the spectra show
much higher emission than that in THF (Supporting
Information, Figure S4). The concentration-dependent emis-
sion studies of compounds 3 and 4 in THF also support the
enhancement in emission upon aggregation that is due to
restricted intramolecular rotations (Supporting Information,
Figures S5 and S6). We carried out the concentration
Figure 1. Scanning electron microscopy (SEM) images of aggregates
of compounds (A) 3 and (B) 4 in 8:2 H2O/THF.
In the fluorescence spectrum, compound 3 exhibits weak
emission (ϕ0 = 0.0018)16 at 363 nm when excited at 290 nm
(Figure 2). However, on addition of an 80% volume fraction of
1
dependent H NMR studies of both compounds 3 and 4
(Supporting Information, Figures S20 and S21). In both cases,
an average downfield chemical shift of 0.08 ppm is observed in
aromatic protons and is attributed to the greater deshielding of
aromatic protons due to the restriction in rotation caused by
intermolecular aggregation.19 Derivative 4 has one additional
rotor in comparison to 3, which sterically hinders the rotation
of rotors around their axes in 4, which is clearly evident from
the smaller quantum yield obtained for derivative 3 as
compared to that obtained for compound 4. Therefore, the
extent of rotation of rotors in compound 4 is already low as
compared to that in compound 3. Therefore, the degree of
restriction in the rotation of the rotors is relatively greater for
compound 3 as compared to that for compound 4. Therefore,
the extent of AIEE is greater in compound 3 as compared to
that in compound 4.
Furthermore, to investigate the properties of 3 and 4 as
AIEE-active materials for the recognition of nitroaromatics, we
studied the fluorescence behavior of 3 and 4 toward different
nitro compounds, viz., 2,4,6-trinitrotoluene (TNT), 2,4-
dinitrotoluene (DNT), picric acid (PA), 1,4-dintrobenzene
(DNB), p-nitrotoluene (PNT), p-nitrobenzene (PNB), p-
nitrophenol (PNP), 2,3-dinitro-2,3-dimethyl butane
(DNDMB)}, and two reference aromatic compounds (benzoic
acid (BA) and benzoquinone (BQ)).
Figure 2. Fluorescence emission spectra of 3 (1 μM) for different
H2O/THF ratios. λex = 290 nm. The inset shows the difference in
fluorescence for 3 in (i) THF and (ii) H2O/THF.
water, the emission band at 363 nm shows a maximum
enhancement (ϕAIEE = 0.59). The increase in the fluorescence
of compound 3 in 80% H2O in THF with respect to that in
pure THF can also be seen by the naked eye (inset of Figure 2).
Under the same set of conditions, similar absorption and
emission behavior was observed for 4 (ϕ0 = 0.006, ϕAIEE = 0.43;
Supporting Information, Figures S1 and S3). When the water
It was observed that on addition of 10 μM TNT to the
solution of 3 (Figure 3) and 13 μM TNT to the solution of 4
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behavior was observed for compound 4 (Supporting
Information, Figures S14 and S15). The charge-transfer
mechanism was further confirmed by cyclic voltammetry. The
higher energy of the LUMO (lowest unoccupied molecular
orbital, −3.56 eV) of 3 allows the electron to jump to the
lower-energy LUMO of TNT (−3.7 eV)10a (Supporting
Information, Figure S16).
To verify the charge-transfer mechanism, we also recorded
1
the H NMR of derivative 3 after adding 2 equiv of TNT
(Supporting Information, Figure S19). The average downfield
shift of 0.08 ppm in all signals was observed, which validated
the charge transfer between compound 3 and TNT.
The quenching of fluorescence emission with different
compounds tested follows the order TNT > DNT > PA >
DNB > PNT > NB > PNP (Supporting Information, Figures
S8 and S9). No change in fluorescence emission was observed
on addition of BA and BQ, which demonstrates that
recognition behavior is observed with respect to nitro
compounds only (Figure 4). It is evident from the results
that the most electron-deficient aromatic substrate engendered
the greatest quenching. This finding is consistent with the
proposed mechanism in which a nitroaromatic analyte acts as a
fluorescence quencher as a result of a charge-transfer event.
The charge-transfer mechanism is more assisted in
quenching derivatives 3 and 4 with TNT because the lower-
energy LUMO (lowest unoccupied molecular orbitals) of TNT
(−3.7 eV) is much lower than that of other nitroaromatics (e.g.,
2,4-dinitrotoluene (DNT, −3.5 eV), nitrobenzene (DNB,
−3.35 eV), etc.22). Thus, the higher energy of the LUMO
(−3.56 eV) of 3 allows the electron to jump to the lower-
energy LUMO of TNT more easily than to the lower-energy
LUMO of other nitroaromatics.
Figure 3. Change in the fluorescence of 3 (1 μM in 8:2 H2O/THF)
on adding 10 μM TNT. The inset shows the difference in the
fluorescence of 3 (i) before and (ii) after adding TNT to H2O/THF.
(Supporting Information, Figure S7), 94 and 92% quenching of
fluorescence emission was observed, respectively (Figure 4).
Figure 4. Selectivity graph of 3 and 4 toward TNT for 10 and 13 μM
analyte, respectively. The inset shows the Stern−Volmer plot of %
quenching vs TNT concentration.
For the detection of TNT vapors, we exposed a solution of 3
and 4 in 8:2 H2O/THF to vapors of TNT by inserting the vial
containing the solution into a sealed vial containing solid
trinitrotoluene at room temperature. The 25 and 22%
quenching of 3 (Figure 5) and 4 (Supporting Information,
The quenching of fluorescence in compound 3 (8:2 H2O/
THF) upon the addition of TNT can also be observed by the
naked eye (inset of Figure 3). The detection limits of 3 and 4 as
fluorescent sensors for TNT are found to be 30 × 10−9 and 40
× 10−9 mol L−1, respectively (Supporting Information, Figures
S10 and S11), which are low enough for the detection of
submillimolar concentrations of TNT. We also investigated the
emission behavior of 3 in pure THF in the presence of 10 μM
TNT, where only a 40% quenching of fluorescence emission
was observed (Supporting Information, Figure S17). This
finding demonstrates the utility of aggregates in the detection
of TNT.
The Stern−Volmer plots of aggregates 3 and 4 are linear
(inset of Figure 4) and give quenching constants (KSV)20 of
13.3 × 105 and 10.0 × 105 M−1, respectively. These Stern−
Volmer constants are considerably larger than those in previous
reports.11,21 We propose that the quenching of fluorescence of
aggregates of 3 on addition of nitroaromatics is ascribed to the
static quenching. The normalized overlays of the absorption
spectrum of TNT and emission spectra of 3 and 4 do not show
any significant overlaps (Supporting Information, Figures S12
and S13), which indicates that the main quenching mechanism
for TNT is charge transfer between the higher-energy state of
the host and the lower-energy state of the TNT.
Figure 5. Change in fluorescence of 3 (1 μM in 8:2 H2O/THF) on
exposing to the vapors of solid 2,4,6-trinitrotoluene after 0, 1, 5, 10, 15,
20, 25, and 30 min at λex = 290 nm.
Figure S18) solution in 8:2 H2O/THF is observed, respectively,
within 1 min, which went up to 85 and 80% quenching of the
emission intensity within 30 min, respectively, at room
temperature (Figure 6). These results reveal that aggregates
of 3 and 4 are responsive to TNT in solution and the vapor
phase.
A tail was found in the UV−vis spectra of compound 3 on
addition of 20 μM TNT. The formation of this tail is probably
due to the interaction between aggregates of 3 and TNT, which
can facilitate the charge transfer between them.10a Similar
During the preparation and packaging of explosive devices,
nitroaromatics can contaminate the human body, clothing, and
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to these solid films resulted in abrupt quenching in otherwise
visible fluorescence under the excitation of the 365 nm UV
lamp (Figure 7D). These results show the practical applicability
of derivatives 3 and 4 toward the instant visualization of traces
of TNT.
EXPERIMENTAL SECTION
■
General Information. All of the UV−vis spectra were recorded on
a Shimadzu UV-2450 spectrophotometer. All fluorescence spectra
were recorded on a Shimadzu RF 5301 PC spectrofluorometer. 1H and
13C NMR spectra were recorded on a JEOL-FT NMR-AL 300 MHz
spectrophotometer using CDCl3 as the solvent and TMS as the
internal standard. The data are reported as follows: chemical shifts in
ppm (δ), multiplicity (s = singlet, d = doublet, br = broad singlet, m =
multiplet, dd = double doublet, and dt = double triplet), coupling
constants (Hz), integration, and interpretation. Silica gel 60 (60−120
mesh) was used for column chromatography.
Figure 6. Plot of % quenching of fluorescence of compounds 3 and 4
vs exposure time (min) of TNT vapors.
other materials in the surroundings.23 In this context, we
prepared test strips by dip-coating solutions of aggregates of 3
and 4 in 8:2 H2O/THF on Whatman filter paper followed by
drying the strips under vacuum to test the residual
contamination in contact mode.
To signify the vapor-mode test, we also performed a paper
strip test in vapor mode by placing a fluorescent paper strip
over the top of the glass vial containing solid TNT for 5 min at
room temprature. The circular area that was exposed to the
TNT vapors was quenched (Figure 7A), which implies the
practical sensitivity and applicability of both 3 and 4 toward
vapors of TNT.
We performed the paper strip test on both derivatives in
contact mode in which fluorescence quenching was observed
upon dipping the test strips into a solution of TNT (10−3 M)
(Figure 7B). We also checked the effect of various
concentrations of TNT solution on the fluorescent paper
strip by applying small spots of different concentrations of
TNT to test strips. Dark spots of different strengths were
formed, which shows that the regulation of the quenching
behavior of TNT is also practically applicable by varying the
concentration of TNT even up to 10−12 M (Figure 7C). In
addition to this, to realize the applications of these carbazole-
based derivatives in the solid state, we also studied the response
of TNT on the thin solid films of both derivatives on glass
plates. For this, we put a drop of the aggregate solution (10−3
M in 8:2 H2O/THF) of both derivatives on a glass plate and
evaporated the solvent to make thin solid films that are
fluorescent in nature. Adding a drop of TNT (10−5 M in THF)
Synthesis of Compound 1-[3-(9-Hexyl)carbazoyl]-3,4-bis(p-
methoxyphneyl)-2,5,6-triphenylbenzene (3). A solution of 1a
(0.5 g, 1.8 mmol) and 2 (0.816 g, 1.83 mmol) in 1 mL of
diphenylether was refluxed for 18 h under an inert atmosphere. Cold
methanol (10 mL) was added to the reaction mixture. The methanol
layer was decanted off, and the insoluble dark oil was subjected to
column chromatography using 3:7 CHCl3/hexane as the eluent. The
compound was recrystallized from methanol to give 3 as a beige solid.
1
Yield 82%. H NMR (300 MHz, CDCl3): δ 0.79 (t, 3H, J = 4 Hz,
CH3), 1.18−1.22 (m, 8H, CH2), 3.57 (s, 3H, OCH3), 3.60 (s, 3H,
OCH3), 4.12 (t, 2H, J = 4 Hz, NCH2), 6.37 (d, 2H, J = 4 Hz, ArH),
6.41 (d, 2H, J = 4 Hz, ArH), 6.62 (d, 2H, J = 4 Hz, ArH), 6.68 (d, 2H,
J = 4 Hz, ArH), 6.81−6.82 (m, 6H, ArH), 6.98 (d, 1H, J = 4 Hz, ArH),
7.02−7.12 (m, 5H, ArH), 7.23−7.34 (m, 3H, ArH), 7.61 (s, 1H, ArH),
7.87 (d, 1H, J = 4 Hz, ArH), 7.88 (s, 1H, ArH). 13C NMR (75 MHz,
CDCl3): δ 13.94 (CH3), 22.50 (CH2), 26.91 (CH2), 28.89 (CH2),
31.51 (CH2), 42.51 (NCH2), 54.88 (OCH3), 107.57 (ArH), 112.21
(ArH), 112.50 (ArH), 118.65 (ArH), 118.85 (ArH), 120.15 (ArH),
121.70 (ArH), 122.38 (ArH), 122.96 (ArH), 125.38 (ArH), 126.02
(ArH), 126.92 (ArH), 127.59 (ArH), 127.99 (ArH), 129.68 (ArH),
129.99 (ArH), 131.71 (ArH), 132.52 (ArH), 132.61 (ArH), 132.64
(ArH), 138.64 (ArH), 139.06 (ArH), 139.72 (ArH), 140.58 (ArH),
140.62 (ArH), 140.91 (ArH), 141.53 (ArH), 142.21 (ArH), 157.04
(ArH), 157.29 (ArH). MS m/z 692: [M + 1]+ calcd for C50H45NO2:
C, 86.80%; H, 6.56%; N, 2.02%; found: C, 86.60%; H, 6.52%; N,
2.00%.
Synthesis of Compound 1-[3-(9-Hexyl)carbazoyl]-3,4-bis(p-
methoxyphneyl)-2,5-diphenylbenzene (4). A solution of com-
pound 1b (0.35 g, 1.0 mmol) and compound 2 (0.45 g, 1.05 mmol) in
1 mL of diphenylether was refluxed for 18 h under an inert
Figure 7. Paper strip test. (A) Vapor-mode detection of TNT (a, c) before and (b, d) after placing the test strips of 3 and 4 over the glass vial
containing TNT for 5 min. (B) (a, c) Before and (b, d) after dipping the test strips of 3 and 4, respectively, in the TNT solution (10−3 M in THF).
(C) Application of small spots of different concentrations of TNT ((i) 10−3, (ii) 10−5, (iii) 10−7, (iv) 10−9, and (v) 10−12 M) on test strips of (I) 3
and (II) 4. (D) Change in the fluorescence of 3 and 4 in the solid state in the presence of TNT. (a, c) Thin films of 3 and 4, respectively, (b, d) After
adding 1 drop of TNT solution (10−4 M in THF) to the thin film of 3. All images were taken under 365 nm UV illumination.
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atmosphere. Cold methanol (10 mL) was added to the reaction
mixture. The methanol layer was decanted off, and the insoluble dark
oil was subjected to column chromatography using 3:7 CHCl3/hexane
as the eluent. The compound was recrystallized from methanol to give
4 as a beige solid. Yield 81%. 1H NMR (300 MHz, CDCl3): δ 0.84 (t,
3H, J = 4 Hz, CH3), 1.19−1.25 (m, 8H, CH2), 3.62 (s, 6H, OCH3),
4.09 (t, 2H, J = 4 Hz, NCH2), 6.42 (d, 4H, J = 4 Hz, ArH), 6.70−6.89
(m, 22H, ArH), 7.30−7.36 (m, 2H, ArH), 7.51 (s, 1H, ArH), 7.76 (d,
1H, J = 8 Hz, ArH). 13C NMR (75 MHz, CDCl3): 13.96 (CH3), 22.49
(CH2), 26.77 (CH2), 28.71 (CH2), 31.46 (CH2), 42.84 (NCH2),
54.86 (OCH3), 106.69 (ArH), 108.42 (ArH), 112.11 (ArH), 118.17
(ArH), 119.92 (ArH), 121.42 (ArH), 123.51 (ArH), 124.86 (ArH),
126.45 (ArH), 126.54 (ArH), 129.42 (ArH), 131.46 (ArH), 131.58
(ArH), 132.44 (ArH), 133.39 (ArH), 140.20 (ArH), 140.72 (ArH),
141.10 (ArH), 156.83 (ArH). MS m/z 768: [M +1]+ calcd for
C56H49NO2; C, 87.58%; H, 6.43%; N, 1.82%; found: C, 87.42%; H,
6.40%; N, 1.80%.
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CONCLUSIONS
■
We designed and synthesized AIEE-active derivatives 3 and 4.
Both derivatives form fluorescent organic aggregates in mixed
aqueous media and the aggregates work as efficient, selective
fluorescent sensors for the nanomolar detection of TNT in
solution, solid, and vapor, which enhances the scope of
materialization of the TNT probes.
ASSOCIATED CONTENT
* Supporting Information
■
S
Fluorescence, UV−vis, NMR, and mass spectra of compounds
3 and 4. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*(M.K.) E-mail: mksharmaa@yahoo.co.in. (V.B.) E-mail:
vanmanan@yahoo.co.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the DST for financial support (ref. nos. SR/
S1/OC-47/2007 and SR/S1/OC-63/2010) and DRDO, India
for providing 2,4,6-trinitrotoluene. We are also grateful to Dr.
Abhimanew Dhir for fruitful discussions.
ABBREVIATIONS
AIEE = aggregation-induced emission enhancement
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(22) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem.
Soc. 2003, 125, 3821.
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