Supramolecular control over recognition and efficient detection of picric acid

Article abstract of DOI:10.1039/c4cc05888g

Bimetallic Schiff-base Al³⁺ complexes bearing ester functions at the periphery of the ligands are shown to be efficient fluorescent chemosensors for picric acid detection. The prominent role of an association between the chemosensor and the pic

Full text of DOI:10.1039/c4cc05888g

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Supramolecular control over recognition and
efficient detection of picric acid†
Cite this: Chem. Commun., 2014,
50, 12061
Virginie Bereau,*ab Carine Duhayonab and Jean-Pascal Sutter*ab
´
Received 29th July 2014,
Accepted 8th August 2014
DOI: 10.1039/c4cc05888g
www.rsc.org/chemcomm
Bimetallic Schiff-base Al3+ complexes bearing ester functions at the the electron-deficient PA. However, structural evidence of the
periphery of the ligands are shown to be efficient fluorescent association complex formed between picric acid and the fluoro-
chemosensors for picric acid detection. The prominent role of an phore is scarce. A few crystal structures of host–guest adducts
association between the chemosensor and the picric acid in the have been reported for polycyclic aromatic hydrocarbon based
1
detection process is demonstrated. The detection of picric acid in chemosensors6e,8 whereas H NMR evidence has been provided
water is achieved with the sensor deposited on paper.
for imidazolium and calix[4]arene chemosensors.6b,9
Herein we describe a very efficient PA sensor derived from
The detection of explosives has applications in various fields luminescent Schiff-base aluminum complexes. We show that
including military affairs, public security, and pollution concerns supramolecular association plays a pivotal role in the sensing
for humans and ecosystems. Among the molecules referenced as capability, and disclose the crystal structure of the host–guest
explosives is the class of nitroaromatics such as picric acid (PA), adduct resulting from the aggregation of PA and the sensor
identified as a notorious environmental polluting agent. Besides molecule. Remarkably, sensing is achieved in solution and in
several highly sensitive and selective analytical methods for the solid state, the latter allowing the detection of PA in H2O.
the detection of nitroaromatics,1 fluorescent sensors have been
The bimetallic aluminum complexes considered in this study
found to be a challenging and interesting alternative owing to are depicted in Scheme 1. In the following, 1, 2, and 3 abbreviate
their simplicity of operation, their real-time and fast response the cationic molecular moieties of [Al2LPh(H2O)4(NO3)2]Á10H2O,
and their high sensitivity.2 Existing classes of fluorescent sensors [Al2LtBu(H2O)4(NO3)2]Á8H2O, and [Al2L(H2O)4(NO3)2]Á2H2O, respec-
comprise organic or inorganic conjugated polymers3 and metal– tively. Derivatives 1 and 2 possess an electron-rich p-conjugated
organic architectures.4 In parallel, small luminophore molecules backbone functionalized by ester groups. As a matter of comparison,
appear to be a good option for several reasons: their syntheses the complex without the ester moieties, 3, was also investigated.
are straightforward, and their structures and solubility can be Their preparation and characterization are provided in the ESI,† as
chemically tuned to increase the sensing selectivity. In the past well as the molecular crystal structures for 1 and 3 (Table S1, Fig. S1
five years, several attractive fluorescent molecular sensors for PA and S2, ESI†).
detection have been reported. They have been derived from
The photoluminescence emission spectra of 1–3 recorded in
organic dyes,5 aromatic p-conjugated systems6 and some have methanol at 298 K (Table S2, ESI†) were identical in shape and
been elaborated from metal complexes.7 All of those polymeric
and molecular sensors exhibit a turn-off fluorescent response to
PA, likely through a host–guest interaction combined with an
electron transfer process from the electron-rich fluorophore to
a CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne,
31077 Toulouse, France
b
´
Universite de Toulouse, UPS, INPT, LCC, 31077 Toulouse, France.
E-mail: bereau@lcc-toulouse.fr, sutter@lcc-toulouse.fr; Fax: +33 561553003;
Tel: +33 561333157
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of 1, 2, 3 and 1.PA; X-ray diffraction data collection and refinement;
procedures for spectroscopic measurements by emission and NMR; procedures
for theoretical calculations. CCDC 981331–981333. See DOI: 10.1039/c4cc05888g
Scheme 1 Chemical structures of the ester-functionalized complexes 1
and 2, and the parent complex 3 (S stands for solvent molecules).
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Fig. 2 1H NMR spectra for Job’s plot titration of 1 in CD3OD/DMSO-d6
with PA in CD3OD. From bottom to top, the mole fraction r = [1]/([1] + [PA])
are 1; 0.9; 0.8; 0.7; 0.6; 0.5; 0.4; 0.3, 0.1 and 0 (the red dotted line
highlights the PA peak displacement).
Fig. 1 Fluorescence quenching of 1 (1 Â 10À6 M in MeOH/DMSO) upon
addition of incremental amounts of PA (1 Â 10À3 M in MeOH) (lexc
=
450 nm). Pictures show the emission of a solution of 1 in MeOH/DMSO
upon addition of solid PA (irradiation under UV lamp at 365 nm).
exhibited a double emission band with maxima at 504 and tetraphenylene (H2) protons were slightly downfield shifted
529 nm for 1 (Fig. 1) and 2 (Fig. S4, ESI†), and were slightly blue whereas the two protons H3 and H4 of the salicylidene ring
shifted compared with those of 3 (518 and 536 nm, Fig. S4, were much more sensitive and were upfield shifted (Fig. 2).
ESI†). This shift is induced by the electron withdrawing group Noteworthy, for PA the signal of the proton at d = 8.98 ppm in a
at position 5 of the salicylidene ring.10 The luminescence pure sample is found at d = 8.65 ppm in the presence of 1
quantum yields found for 1 (F = 0.67) and 2 (F = 0.56) are (Fig. 2). It remains unchanged for a mole fraction of 1 between
among the highest known for this class of materials.7b,10,11 0.9 and 0.5 before being downfield shifted through the molar
They are also significantly higher than that found for 3 (F = 0.20), ratio of 0. These observations suggested that an interaction
as anticipated for the ester-functionalized ligands. The higher occurred in the solution between chemosensor 1 and PA.
efficiency obtained for 1 is ascribed to the aromaticity in the ester A stoichiometry of 1 : PA = 1 : 1 for the predominant species in
group, which has been shown to improve the efficiency of the the solution was determined by an examination of the chemical
molecule to re-emit the absorbed photons.10,11a
shift variations of protons H2 and H3 by the continuous
Upon addition of incremental amounts of picric acid, sub- variation method (Fig. S12, ESI†). The binding constant of
stantial quenching of the emission of 1 and 2 is observed (Fig. 1 chemosensor 1 towards PA was estimated through independent
for 1, Fig. S4, ESI† for 2) whereas there is almost no change in 1H NMR titration from the variation of the chemical shift of H3:
the emission of 3 in the same conditions (decrease of max. 7%, Ka = 728 MÀ1 was obtained (Fig. S13, ESI†). The same experi-
see Fig. S4, ESI†). The influence of the R group of the ester ments were run for chemosensor 2. A smaller association
(i.e. Ph versus tBu) on the quenching efficiency and sensitivity constant value of 273 MÀ1 was measured assuming a 1 : 1
was evaluated by calculating the Stern–Volmer quenching stoichiometry between 2 and PA, as for 1 and PA (Fig. S14,
constant (Ksv) from the fluorescence quenching titration. 1 is ESI†). These values are comparable with those obtained by
more efficient in detecting PA with up to 80% of emission fluorescence measurements with the Benesi–Hildebrand
quenching versus 70% for 2 (Fig. S5, ESI†); Ksv = 3586 MÀ1 for 1 method (Fig. S8 and S9, ESI†): 826 and 576 MÀ1 respectively
and Ksv = 1815 MÀ1 for 2 (Fig. S6, ESI†). Limits of detection for 1 and 2. For 3, no shift of proton signals was observed in the
(LOD) and limits of quantification (LOQ) were also evaluated: presence of PA (Fig. S15, ESI†). These results clearly highlight
LOD were 55 mM (12.6 mg mLÀ1) and 168 mM (38.6 mg mLÀ1), the essential role played by the ester groups at the periphery of
and LOQ were 0.17 mM and 0.51 mM respectively for 1 and 2 the ligand in the adduct formation. NMR experiments show
(Fig. S7, ESI†). The above values are remarkable compared to that the signals of both the PA and the aromatic tetraphenylene
those of other salophen complexes7b and dyes already reported,5b and salicylidene rings of the chemosensors are quite affected,
and are comparable with those of most efficient molecular suggesting close proximity for the two moieties. It is very likely
fluorophores.4i,6c A tentative rationalization for the difference that this supramolecular complex is at the origin of the
between 1 and 2 might be found in the phenyl substituent of fluorescence quenching for 1 and 2 in the presence of PA. As
the ester functions for 1 that could contribute by additional seen for 3, in the absence of such a recognition process the
aromatic interactions with PA.
quenching of the emission is poor.
1H NMR titration experiments were carried out to highlight
During the emission and NMR titration experiments, high
possible interactions between PA and the chemosensors. concentrations of PA into solutions of 1 afforded the appearance
Salient variations during titration of 1 in CD3OD/DMSO-d6 with of an orange solid. Single crystals have been obtained by slow
PA in CD3OD were as follows: the signals of the imine (H1) and inter-diffusion of MeOH solutions of 1 and PA in large excess
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Fig. 5 Photographs (under 365 nm UV light) of the fluorescence response
of 1 on test strips. (a) Blank paper showing blue emission under UV light, (b)
paper impregnated with 1 (real colour is greenish yellow) and (c) colour
spots created by depositing (A) 5 mL of H2O, (B) 5 mL of 1 Â 10À5 M PA in
H2O, (C) 5 mL of 1 Â 10À3 M PA in H2O, (D) 5 mL of 1 Â 10À1 M PA in H2O.
Fig. 3 Crystal structure of the host–guest adduct 1.PA showing the
H-bond interactions between PA and the sensor (in dashed lines). H atoms
of coordinated MeOH have been omitted for clarity.
those energy levels reflect the ease of transferring an electron from
the sensor in its excited state to the analyte in its fundamental state
(LUMO). Luminophores 1, 2 and 3 have their LUMO higher in
energy than that of PA but lower than those of the other aromatic
compounds considered here. Consequently, only PA is anticipated
to lead to a charge-transfer process and a subsequent fluorescence
quenching. This was indeed confirmed experimentally (Fig. S10,
ESI†); except for PA, none of the considered molecules altered the
luminescence of the sensors, thus confirming their good selectivity
in their response to PA. However, while experimental results with
PA for 1 and 2 support the anticipated behaviour, there is a deep
discordance for 3, for which luminescence is hardly modified.
Obviously, favourable positioning of the frontier orbitals is not the
unique driving force of charge transfer and the subsequent
luminescence quenching. This confirms the importance of the
adduct formation in the sensing process.
Finally, we show in Fig. 5 a visual demonstration of the use
of solid 1 as a sensor for PA. Whatmans filter paper has been
impregnated with a solution of 1 (5 Â 10À4 M in MeOH/DMSO).
After drying, the paper was exposed to spots of the aqueous PA
solution at various concentrations. The visual detection
response was observed under 365 nm UV light and showed
the fluorescence quenching with PA concentrations down to
10 ppm. This test also demonstrates that the supported sensor 1
is able to efficiently detect picric acid in water.
In summary, we have identified a novel, highly fluorescent
molecule capable of detecting picric acid efficiently, even in
aqueous solutions. More importantly, the gathered results
show that molecular association and recognition have a pivotal
effect on the sensing efficiency. Our observations suggested
that the molecular associations detected either in solution or in
solid state are conditioned by the presence of ester groups at
the periphery of the Schiff base. These associations contribute
to the emission quenching of the reported fluorophores by a
charge transfer process. It is the combination of the two
phenomena, supramolecular association and charge transfer,
that is at the origin of the remarkably efficient detection of PA.
(ESI†). The crystal structure (Fig. 3 and Fig. S3, ESI†) revealed a
1 : 2 host–guest adduct, 1.PA, in which two picrate molecules are
trapped within the pockets formed by two facing phenyl-ester-
functionalized salicylidene moieties of 1. Hydrogen bonding
between the host and the guests is established between the O
atoms of the phenolates and the H atoms of the tetraiminophe-
nyl ring with distances of 2.375(3) and 2.396(3) Å for O26Á Á ÁH101
and O19Á Á ÁH131, respectively. It can be noticed that the phenyl
ester rings lay almost parallel to the picrate planes suggesting
also weak p–p and p–H interactions between these moieties.
Such weak supramolecular interactions are likely to favor and
stabilize the adduct 1.PA.
A 1H NMR spectrum of 1.PA crystals (in DMSO-d6 for solubility
reasons) again reveals a slight shift of the signals of the three
protons, H1, H2 and H3, with respect to 1 (Fig. 4), thus confirming
that a degree of interaction is still in place between the two ionic
parts of the 1.PA salt even in DMSO. In addition, the orange solid
previously obtained during the NMR titration experiments for high
1
concentrations of PA was isolated; its H NMR spectrum perfectly
matches that of 1.PA (Fig. 4). The emissive characteristic of a
solution of 1.PA was also investigated and revealed weak fluores-
cence (Fig. S11, ESI†). This observation confirms that the vanishing
of the fluorescence for the ester-functionalized luminophore is
related to the establishment of an association between 1 and PA.
DFT calculations were undertaken to estimate the relative
positions of the molecular frontier orbitals of 1–3 and PA, as
well as for a series of chemically related molecules: 4-nitro-
phenol, phenol, 4-nitrobenzene, benzene and toluene (Fig. S16
and Table S3, ESI†). In the fluorescence quenching process,
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The blue dotted lines highlight the peaks that are shifted by the presence
of PA).
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