A colorimetric and fluorescent chemosensor for the detection of an explosive-2,4,6-trinitrophenol (TNP)

Article abstract of DOI:10.1039/c1cc10400d

A fluorescent and colorimetric receptor (L) for specific recognition of TNP was described. The origins of this remarkable affinity was disclosed by the crystal structure of corresponding host-guest complex L·TNP.

Full text of DOI:10.1039/c1cc10400d

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COMMUNICATION
A colorimetric and fluorescent chemosensor for the detection
of an explosive—2,4,6-trinitrophenol (TNP)w
Yu Peng,* Ai-Jiang Zhang, Ming Dong and Ya-Wen Wang*
Received 20th January 2011, Accepted 18th February 2011
DOI: 10.1039/c1cc10400d
A fluorescent and colorimetric receptor (L) for specific recognition
of TNP was described. The origins of this remarkable affinity
was disclosed by the crystal structure of corresponding
host–guest complex LÁTNP.
Scheme 1 Synthesis of receptor L.
Supramolecular chemistry based on host–guest interaction and
Stimulated by some known pyridyl anthracene sensors,13
self-organization first harnessed preorganization for the design of
tailor-made molecular receptors effecting molecular recognition
from metal ions to anions and chiral molecular substrates.1
Especially, recognition of neutral guests has attracted much
attention owing to their great biological and environmental
relevance.2 However, relatively few examples that rely on color
change3 have been reported so far, although they represent a
convenient visual detection method in classical chemical analysis.
In particular, rapid detection of explosives such as trinitro-
toluene (TNT) is important owing to their broad applications
including tactical and humanitarian demining, and forensic
and criminal investigations etc.4 Homeland security applications
are attracting increased research, because terrorists frequently
employ bombs.5 Most of available sensors are composed of
polymers, nanomaterials etc.6 and fewer are organic small
molecules.7 Surprisingly, less attention has been paid to picric
acid (2,4,6-trinitrophenol, TNP),8 although its explosive
power is somewhat superior to that of TNT and it is also
widely used in the manufacture of rocket fuels, fireworks,
matches and so on.9 Also TNP has been recognized as an
environmental contaminant and is harmful to wildlife and
humans.10 So the development of fast, convenient and specific
analytical methods for TNP is highly desirable.11 In connection
with our continuing research of sensors for biologically and
environmentally important guest species,12 herein, we present
the first colorimetric and fluorescent receptor (L) with multiple
hydrogen-bonds and p–p interactions for specific recognition
of TNP.
we designed and synthesized N-acylhydrazone L14 starting
from isonicotinohydrazide and anthracene-9-carbaldehyde
(Scheme 1, see ESIw for details).
Anthracene is a polycyclic aromatic hydrocarbon (PAH),
which can emit strong fluorescence at about 420 nm
for monomer formation and at above 470 nm for excimer
formation.15 When interacting with a compound containing
strong electron-withdrawing groups, fluorescence quenching
of anthracene might occur due to the formation of a possible
non-fluorescent complex. Guided by this rational analysis, we
surmised that the differences of fluorescent properties between
compound L and its complex LÁTNP might enable it to be
used as a sensor to detect the presence of TNP in solution. As
shown in Fig. 1, L only displays an excimer emission band at
483 nm when excited at 419 nm in DMF solution (low
solubility in other solutions). Upon the addition of TNP, a
prominent quenching and color change of fluorescence were
observed. The total fluorescence intensity of L decreased to
3% when 3.0 equiv. of TNP was present. These results
indicated that the formation of complex LÁTNP destroyed
the p–p interactions between anthracene rings of L in
solution.16 The corresponding quantitative analytical data
State Key Laboratory of Applied Organic Chemistry, Key Laboratory
of Nonferrous Metals Chemistry and Resources Utilization of Gansu
Province and College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China.
E-mail: pengyu@lzu.edu.cn, ywwang@lzu.edu.cn;
Fax: +86-931-8912582; Tel: +86-931-3902316
w Electronic supplementary information (ESI) available: Details of
synthesis, powder XRD pattern, UV-vis titrations. CCDC 809217.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc10400d
Fig. 1 Fluorescence quenching of L (50.0 mM) upon the addition of
TNP (0 to 3.0 equiv.) in DMF (lex = 419 nm). Inset: Fluorescence
change of L. Left to right: L only, L + TNP (1.0 equiv.), L + TNP
(2.0 equiv.), L + TNP (3.0 equiv.).
c
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of L were downfield shifted obviously upon the addition of TNP
(1.0 equiv.) (Dd = 0.33, 0.31, 0.54 ppm, respectively), which
indicated that the hydrazone group and the pyridine group of L
with TNP formed strong hydrogen bonds.7c No further changes
occurred upon addition of further TNP, indicating the 1 : 1
(host–guest) binding mode was formed in solution.
Next, the interactions of L with the nitroaromatics were also
investigated in the solid state. When 1.5 mL of TNP (3.0 Â
10À3 M) solution was added to 1.5 mL of L (9.0 Â 10À4 M)
solution in ethanol (0.5% DMF, v/v), a vermilion precipitate
was observed. Upon increase of the concentration of TNP,
futher precipitate were generated (Fig. S3, ESIw). However, when
1.5 mL of other nitro-compounds (10.0 Â 10À3 M) solution was
added to 1.5 mL of L (9.0 Â 10À4 M) solution in ethanol (0.5%
DMF, v/v), neither precipitation nor color change occurred.
Then we studied the precipitate generated from the L–TNP
system in order to understand the interactions in the solid
state. The precipitate can be isolated by filtration and is
soluble in DMF. After a few days, vermilion block crystals
were obtained by slow evaporation of the solvent at room
temperature (Fig. 5a). A single-crystal X-ray diffraction analysis
reveals that a 1 : 1 host–guest complex LÁTNP18 was formed
(Fig. 5a) by L and TNP as in solution. The crystal structure
shows that the nitrogen of the pyridine group of L is proto-
nated to form a cation while picric acid is deprotonated to
form the anion,19 where the pyridine group is changed from a
hydrogen-bond acceptor to the hydrogen-bond donor
(Fig. 5a). Molecules of L adopt the E form exclusively in the
complex. The L cations and picrate anions are self-assembled
Fig. 2 Fluorescence spectra of L (50.0 mM) with various nitro-
compounds (1.0 equiv.) in DMF (lex = 419 nm).
(linear concentration range, RSD, LOD, LOQ)17 were carried
out and calculated (Fig. S1, ESIw).
Other nitroaromatics such as TNT (2,4,6-trinitrotoluene),
DNT (2,6-dinitrotoluene), NB (nitrobenzene), NP (4-nitrophenol),
DNB (1,3-dinitrobenzene) and NBA (4-nitrobenzoic acid) and
nitroamines (RDX, HMX) were used to investigate the selec-
tivity of L by fluorescence spectra (Fig. 2). Upon the addition
of these compounds to a DMF solution containing compound
L, only TNP caused the remarkable quenching of the fluores-
cence emission band, while other nitro-compounds quenched
the emission intensity only to a small extent. These results
implied that there are strong interactions between L and TNP.
Moreover, a color change from pale yellow to deep yellow was
observed by naked-eye upon the addition of TNP (Fig. 3). In
the profile of the UV-vis spectra generated from the addition
of TNP to L, the increase of the absorption peak at about 390 nm
is in good agreement with the color change (Fig. S2, ESIw).
To explore the interactions between L and TNP in solution
1
further, H NMR titration experiments were also carried out.
As shown in Fig. 4, the signals of H2A, H1 and H5, H2 and H4
Fig. 3 Color change of L ([L] = 0.45 mM, [various nitroaromatics]
= 1.0 mM). Left to right: L only, TNP, TNT, DNT, NB, NP, DNB
and NBA.
Fig. 5 (a) Photos of L and LÁTNP and the X-ray structure of the
host–guest complex LÁTNP; (b) The hydrogen-bond interactions around
TNP in the complex; (c) The hydrogen-bond interactions and p–p
interactions around L in the complex (the hydrogen-bond interactions
and p–p interactions are indicated with dashed turquoise and bright
green lines, respectively. Some hydrogen atoms are omitted for clarity).
Fig. 4 Partial 1H NMR spectra (400 MHz) of L (5.0 mM) with TNP
in d6-DMSO: (a) L only; (b) L + TNP (0.2 equiv.); (c) L + TNP (0.4
equiv.); (d) L + TNP (0.6 equiv.); (e) L + TNP (0.8 equiv.); (f) L +
TNP (1.0 equiv.).
c
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to form a supramolecular structure via multiple hydrogen-bond
and p–p interactions. As shown in Fig. 5b and c, each picrate
acts as a hydrogen-bond acceptor to connect six ligands via 9
hydrogen-bonds, and each L acts as hydrogen-bond donor
and acceptor to connect six picrates and two other ligands via
13 hydrogen-bonds in which the D–A distances vary from
2.713 to 3.470 A and the corresponding D–HÁ Á ÁA angles are in
the range 114.8–169.91 (Table S2, ESIw), while relatively weak
C–HÁ Á ÁO interactions can not be ignored.20 It is noteworthy
that the two hydrogen-bonds between N1 of the pyridine
group and O1 of the hydrazone group and N2 of the hydra-
zone group and O8 of the picrate group are very strong21
owing to their short distance. These results implied that the
pyridine group and the hydrazone group of L were the key
sites for specific interaction with TNP, which was in good
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1
agreement with the results of H NMR titration experiments
in solution. Three intermolecular face-to-face p–p stacking
interactions are also evident (Fig. 5c). The first one, between
two picrate rings, presents a pcentroid–pcentroid distance of 3.569 A
and a dihedral angle of 0.001. The second and the third
interactions, between the anthracene ring and pyridine ring
from different L, are the same and both have a pcentroid–pcentroid
distance of 3.640 A and a dihedral angle of 5.441. No p–p
stacking interactions between anthracene rings were observed
which indicated the origin of the prominent fluorescence
quenching. In addition, a powder X-ray diffraction study of
the precipitate was also conducted (Fig. S4, ESIw) and the
resulting experimental powder XRD pattern is in good agree-
ment with the corresponding simulated one of the crystal LÁ
TNP. The above results showed that there is an organized
self-assembly between L and TNP during the sensing process.
In summary, we report a specific, colorimetric and fluorescent
receptor (L) for TNP. As a general design strategy, structural
modifications of L may allow us to create further fluorescent
sensor candidates for TNP in the future.
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=
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Chem. Commun., 2011, 47, 4505–4507 4507