Fluorescent tris-imidazolium sensors for picric acid explosive

Article abstract of DOI:10.1021/jo302585a

Two new anthracene-functionalized fluorescent tris-imidazolium salts have been synthesized, characterized, and proven to be selective sensors for picric acid, which is a common constituent of many powerful explosives. Theoretical studies revealed an unusual ground-state electron transfer from picrate anion to the sensor molecules.

Full text of DOI:10.1021/jo302585a

Note
pubs.acs.org/joc
Fluorescent Tris-Imidazolium Sensors for Picric Acid Explosive
Bijan Roy, Arun Kumar Bar, Bappaditya Gole, and Partha Sarathi Mukherjee*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560 012, India
S
* Supporting Information
ABSTRACT: Two new anthracene-functionalized fluorescent
tris-imidazolium salts have been synthesized, characterized,
and proven to be selective sensors for picric acid, which is a
common constituent of many powerful explosives. Theoretical
studies revealed an unusual ground-state electron transfer from
picrate anion to the sensor molecules.
icric acid (PA) is a strong organic acid,1 and like many
Ppolynitrated aromatic compounds, it is a powerful
explosive. Its explosive nature is even stronger than TNT,
and it was widely used as an explosive until World War I.2−5
Moreover, PA has importance in dye industries, pharmaceut-
icals, and chemical laboratories.6,7 Instead of such importance,
PA possesses acute health effects: it is a strong irritant to skin/
eye and can cause potential damage to organs involved in the
respiratory system.8−10 It is to be noted that because of high
solubility of PA in water, it can easily contaminate soil and
groundwater when exposed. Hence, development of efficient
sensors to detect PA at very low concentrations in order to
prevent terrorist threats as well as environmental pollution is a
very appealing field of research.11
Scheme 1. Schematic Presentation of the Synthesis of
Sensors 1 and 2
Various methods have been employed for the detection of
nitroaromatic explosives.12−15 Among these, fluorescence
sensing is widely employed because of its high sensitivity,
quick response, easy sample preparation, etc.16−18 Even though
several π-electron-rich fluorescent organic polymers19−22 and
metal−organic architectures23 have been employed to detect
the presence of electron-deficient nitro-aromatics, development
of suitable soluble and efficient organic chemosensors with high
selectivity for PA is still a very challenging task.24−29
Imidazolium salts have been extensively studied as ionic
liquids and N-heterocyclic carbene (NHC) precursors.30−33
Recently, they have been tested as sensors for anions and
biomolecules.34,35 To the best of our knowledge, they have not
yet been explored as sensors toward the detection of electron-
deficient nitro-aromatics like picric acid. Herein, we report the
syntheses and characterization of two tris-imidazolium salt
based sensors 1 and 2 (Scheme 1), which have been proven to
be potential candidates for picric acid sensing with high
sensitivity and selectivity. The theoretical study revealed an
unusual ground-state electron-transfer phenomenon from
picrate anions to the positively charged sensors molecules.
Treatment of tris(N-imidazolylmethyl)mesitylene (A) with
9-bromomethylanthracene (B) in acetonitrile under refluxing
conditions resulted in the formation of the sensor 1 (Scheme
1) with 65% yield. Tris(p-bromomethylphenyl)methane (D)
was synthesized from triphenylmethane (C) following the
standard bromomethylation reaction (Scheme 1).36 Sensor 2
was synthesized similar to a procedure adopted for 1 upon
treatment of D with 9-(N-imidazolylmethyl)anthracene (E,
Scheme 1) with 57% yield.
Sensors 1 and 2 were purified by washing with acetonitrile
several times followed by drying under vacuum and were fully
characterized by NMR, solution-state ESI-MS, and elemental
analyses. X-ray diffraction quality single crystals of 1 were
obtained by slow evaporation of a solution of 1 in a MeOH−
H2O mixture. X-ray diffraction analysis revealed that 1
crystallized in the monoclinic crystal system with a C2/c
space group. The crystallographic parameters of 1 are provided
in Table S1 (see the Supporting Information). Among several
possibilities, sensor 1 adopts cis,cis,cis-conformation and exhibits
a bowl-shaped structure in the solid state (Figure 1). Several
attempts to crystallize sensor 2 failed, but the DFT-optimized
Received: December 10, 2012
Published: January 10, 2013
© 2013 American Chemical Society
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Note
structure indicates a similar cis,cis,cis- conformational arrange-
ment in the gas phase.
for fluorescence titration experiments. The fluorescence
titration for sensor 1 and 2 with PA revealed that fluorescence
emission intensity rapidly died down upon addition of
increasing amounts of PA solution (Figure 3). The appearance
of intense yellow color upon addition of colorless DMSO
solution of sensor 1 to a pale yellow solution of PA in
chloroform indicated the formation of [1⊃PA] complex
(Figure 3). A similar observation was also noticed in the
progress of the formation of [2⊃PA] (Figure S16, Supporting
Information). The selectivity of the sensors was investigated by
using several aromatic/nonaromatic compounds as analytes
upon addition of 200 μL of 1 × 10−3 M analytes (100 equiv)
solution in chloroform into 2 mL of 1 × 10−6 M solution of
sensor (1 or 2) in DMSO. Interestingly, the quenching
efficiencies of sensor 1 and 2 toward PA were found to be 92%
and 91%, respectively, indicates their effectiveness to detect PA
over other aromatic compounds (Figure 4).
Figure 1. X-ray crystal structure of 1 (left) and energy-optimized
structure of 2 (right). H atoms and Br− ions are omitted for clarity.
Color codes: C = gray and N = blue.
Both 1 and 2 exhibited almost identical spectral natures
(UV−vis and fluorescence). The UV−vis absorption spectra in
DMSO displayed absorption bands characteristic of anthra-
cene37 with absorption maxima (λmax) at 352, 371, and 391 nm,
while their emission spectra exhibited strong bands with
emission maxima (λmax) at 399, 422, and 446 nm upon
excitation at 371 nm (Figure 2).
Figure 4. Fluorescence quenching efficiencies of sensors 1 and 2
toward different analytes. PA = picric acid; DDQ = 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone; 4-NP = 4-nitrophenol; TNT = trinitroto-
luene; TNB = trinitrobenzene; 4-NT = 4-nitrotoluene; 3,4-DNT =
3,4-dinitrotoluene; 2,4-DNT = 2,4-dinitrotoluene; NB = nitrobenzene;
NM = nitromethane; BA = benzoic acid.
Figure 2. Absorption (violet) and emission (green) spectra of 1 in
DMSO.
To judge the sensing ability of 1 and 2 toward electron-
deficient nitro-aromatics, several analytes were used in CHCl3
Figure 3. (Left) Fluorescence intensity vs analyte concentration plot for sensor 1 titrated upon gradual addition of PA. Inset: visual color change due
to the formation [1⊃PA] complex. (Right) Stern−Volmer plots for sensors 1 and 2 using PA as quencher. Inset: Stern−Volmer plots at lower
concentration region of PA.
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This fact is again supported by high Stern−Volmer constants
KSV = 3.8 × 104 M−1 and 3.3 × 104 M−1 for sensors 1 and 2,
respectively. The Stern−Volmer plots showed two distinct
regions: linear variation at a lower concentration of PA is
mainly due to static quenching, whereas a steep curve at higher
concentration of PA is presumably due to dynamic quenching.
The static quenching can be attributed due to the ground-state
charge-transfer complex formation, which is confirmed by UV−
vis (Figure S17, Supporting Information) and NMR experi-
ments as well as single-crystal X-ray diffraction. However, the
dynamic quenching mechanism is confirmed by the fluo-
rescence lifetime measurement (Figure S18, Supporting
Information).
(LUMO) of 1 is −0.312 eV (Figure 6), while the energies of
HOMO and LUMO of picrate anion are −0.125 eV and +0.005
Significant UV−vis spectral changes with the appearance of a
new band at 430 nm in comparison to the UV−vis spectra of
free sensors and PA indicated their complex formation [1/
2⊃PA] (Figure S17, Supporting Information). The [1⊃PA]
and [2⊃PA] were isolated as yellow precipitates from a
methanolic solution of 1/2 and PA.
1
The H NMR spectral analyses of [1⊃PA] and [2⊃PA]
showed the presence of PA with significant shifts of the sensors
peak positions (Figure 5) and also confirmed the formation of
Figure 6. Pictorial representation of the charge transfer phenomenon
in the ground state which occurs from the HOMO of picrate anion to
the LUMO of sensor 1 and also compared with TNT.
eV, respectively. Hence, the ground-state electronic charge
transfer takes place from the HOMO of picrate anion to the
LUMO of sensor 1.
1
Figure 5. H NMR spectra of sensor 1 (red) and its PA complex
The sensors 1 and 2 are highly sensitive and can detect PA
down to 467 and 354 ppb, respectively (Supporting
Information). To check the efficiency of the sensors in aqueous
media, DMSO solutions of 1 and 2 were titrated with aqueous
solution of PA. The similar large value of quenching efficiencies
in aqueous media indicated the potential applicability of 1 and
2 for the detection of PA in groundwater (Supporting
Information). Though PA and TNT have similar reduction
(blue) in DMSO-d6.
complexes between the sensors (1 or 2) with PA in 1:2 molar
ratios (Figure S12, Supporting Information). Job’s plot analysis
experiment for the determination of stoichiometry was
inconclusive because of the overlapping absorption wavelength
regions for sensors and PA (for PA, λmax= 356 nm in water, 336
nm in CH2Cl2 and 378 nm in DMSO).
−
potentials (RNO2/RNO2 = −0.39 V for PA and −0.30 V for
Finally, X-ray diffraction study on single crystals of [1⊃PA]
as obtained from slow vapor diffusion of ethyl acetate into
acetonitrile solution of [1⊃PA] unambiguously confirmed the
formation of the [1⊃2PA] complex. Although the quality of the
diffraction data of [1⊃2PA] was poor, the present data set
clearly indicated the presence of two picrate (P‑) ions per
molecule upon replacing two bromide counteranions from 1
and their locations (Figure S3, Supporting Information). The
[1⊃2P−] crystallized in monoclinic crystal system with Pc space
group. The solid-state packing diagram revealed that picrate
moieties formed columnar arrangement intervened between
anthracene moieties of the sensor molecules via mainly π−π
stacking interactions (Figure S3, Supporting Information).
To our surprise, the ground-state charge transfer occurs from
picrate anions to the positively charged sensor molecules in the
gas phase as evident from the DFT calculations. The DFT
studies revealed that the energy of the highest occupied
molecular orbital (HOMO) of the free sensor 1 is −0.406 eV
and the energy of the lowest occupied molecular orbital
TNT),38 considerably high quenching efficiencies observed for
only PA (91% for 1 and 92% for 2) in comparison to TNT
(17% for 1 and 16% for 2) (Figure 4) can be attributed to
deprotonation of the strongly acidic phenolic −OH group
followed by anion exchange with the sensor molecules.
Moreover, the theoretical calculations also revealed the lower
quenching efficiency of TNT because its HOMO (HOMO and
LUMO energies are −0.325 and −0.144 eV, respectively) has
lower energy than the LUMO of sensor (Figure 6).
In conclusion, we have reported two tris-imidazolium salts (1
and 2) that selectively sense PA at the ppb level in both organic
and aqueous media. To the best of our knowledge, compounds
1 and 2 represent the first examples of imidazolium salts
potential for the detection of trace amount of powerful
explosive picric acid. The crystal structures of sensor 1 and its
PA complex [1⊃2P−] were successfully established by X-ray
crystallography. In the complex [1⊃2P−], picric acid exists as
picrate ion upon deprotonation, which leads to quenching of
fluorescence of 1.
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Note
8.20 (d), 7.64 (m), 7.22 (d), 7.08 (d), 6.48 (s), 5.58 (s), 5.28 (s). 13
C
EXPERIMENTAL SECTION
■
NMR (DMSO-d6, 100 MHz): δ 161.2, 144.0, 142.1, 136.1, 133.3,
131.4, 130.9, 130.5, 129.7, 128.6, 128.0, 125.9, 125.5, 124.5, 123.5,
123.3, 122.9, 68.8, 51.8, 45.4. Anal. Calcd (vacuum-dried sample) for
C88H65BrN12O14: C, 66.29; H, 4.11; N, 10.54. Found: C, 65.93; H,
4.32; N, 10.23.
The synthesis of tris(N-imidazolylmethyl)mesitylene (A) from
trisbromomethylmesitylene was carried out by following the standard
literature procedure.39 9-(Bromomethyl)anthracene (B) was synthe-
sized in two steps involving reduction of 9-anthracenecarboxaldehyde
to the corresponding alcohol followed by substitution of −OH group
by bromine using reported methodologies.40 9-(N-Imidazolylmethyl)-
anthracene (E) was also synthesized by a known literature
procedure.41
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR of all new compounds, ESI-MS, UV−vis, and
fluorescence spectra, X-ray crystal data, thermal ellipsoid plots
of 1 and [1⊃2P−], and tables for optimized coordinates. This
material is available free of charge via the Internet at http://
pubs.acs.org. .
Caution! The nitroaromatic compounds used in this study, specially
TNT and picric acid, are very powerful explosives. They must be
handled with care and also in very small quantities.
Synthesis of Tris(p-bromomethylphenyl)methane (D). Tri-
phenylmethane (5 g, 20.46 mmol), paraformaldehyde (5 g), and 33%
HBr in acetic acid (50 mL) were taken in a 250 mL round-bottom
flask and heated at 120 °C with stirring for 24 h. The solution was then
poured into 500 mL of ice-cold water to obtain a dark yellow solid
which was collected by filtration and subsequently washed with plenty
of water and dried in an oven at 60 °C overnight. The compound was
purified by column chromatography by eluting with a 5% DCM/
hexane mixture. Isolated yield = 6.3 g (59%). Mp = 101 °C. 1H NMR
(CDCl3, 400 MHz): δ 7.31 (d, 6H), 7.06 (d, 6H), 5.50 (s, 1H), 4.48
(s, 6H). 13C NMR (CDCl3, 100 MHz): δ 143.9, 136.5, 130.2, 129.6,
56.4, 33.7. Anal. Calcd (vacuum-dried sample) for C22H19Br3: C,
50.51; H, 3.66. Found: C, 50.77; H, 3.38;
Synthesis of Sensor 1. Tris(N-imidazolylmethyl)mesitylene (A)
(300 mg, 0.83 mmol) and 9-bromomethylanthracene (B) (676.7 mg,
2.5 mmol) were dissolved in 50 mL of acetonitrile, and the solution
was refluxed for 72 h. The light brown solid formed was isolated by
filtration followed by washing with acetonitrile. Isolated yield = 635
mg (65%). Mp = 228 °C. 1H NMR (DMSO-d6, 400 MHz): δ 9.38 (s,
3H), 8.85 (s, 3H), 8.50 (d, 6H), 8.21 (d, 6H), 7.60 (m, 12H), 7.52 (s,
3H), 7.40 (s, 3H), 6.57 (s, 6H), 5.52 (s, 6H), 2.21 (s, 9H). 13C NMR
(methanol-d4, 100 MHz): δ 142.3, 136.1, 132.0, 131.4, 130.8, 129.8,
129.7, 128.1, 125.6, 123.0, 122.9, 122.7, 122.3, 45.8, 15.9. MS (ESI) =
m/z 1093.15 [1 − Br−]+, 506.11 [1 − 2Br−]2+. Anal. Calcd (vacuum-
dried sample) for C66H57Br3N6: C, 67.53; H, 4.89; N, 7.16. Found: C,
67.79; H, 4.70; N, 7.38.
AUTHOR INFORMATION
Corresponding Author
*Tel: 91-80-22933352. Fax: 91-80-23601552. E-mail: psm@ipc.
iisc.ernet.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the CSIR-India is gratefully acknowl-
edged. We are also grateful to STC-IISc for financial support.
■
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23.7 mg (95%). Mp =166 °C. H NMR (acetone-d6, 400 MHz): δ
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