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tensity decreased upon the addition of 5 ppm of NB and was
nearly completely quenched at a concentration of 100 ppm,
with a high quenching efficiency of 92.5% which is higher
than or comparable to other MOF sensors for NB.[7b,13d,25]
complex in UV light vanished upon the addition of the solu-
tion of TNP, which quenched nearly 78% of the initial fluores-
cence intensity. The fluorescence quenching by TNP could be
easily discerned at low concentration (4 mm). Fluorescence
quenching titrations were also performed with nitroaromatic
compounds, such as 2,4,6-trinitrotoluene (TNT), 2,4-DNT, 2,6-
DNT, m-DNB, NB, and nitroaliphatic compounds, such as
DMNB, NM, and 1,3,5-trinitro-1,3,5-triazacyclohexane. All other
nitro compounds showed little effect on the fluorescence in-
tensity. These results demonstrate that such a cadmium(II)
compound has a high selectivity for TNP relative to other nitro
compounds.
Among the five nitro compounds (Supporting Information,
Figure S1), complex 1 is more sensitive to p-DNB relative to
other four nitro compounds at room temperature. Complex
1 exhibits extremely high detection sensitivity toward p-DNB
explosives, with a high quenching efficiency of 99.998%. Also,
the quenching phenomenon of 1 in the solid state is consis-
tent with that realized in the liquid-sensing process, thus indi-
cating that the quenching mechanism should be based on the
nature of the complex rather than the testing environments.
Up to now, several MOF-based fluorescence sensors have been
developed for the detection of nitroaromatic explosives. For
example, Li and co-workers reported the highly luminescent
MOFs[13a,27a] [Zn2(oba)2(bpy)]·3DMA (H2oba=4,4’-oxybis(benzoic
acid), bpy=4,4’-bipyridine, DMA= dimethylacetamide) and
Among the tested aromatic organic molecules (toluene, ben-
zene, and nitro compounds), the emission of 1 can only be
quenched by nitro-compound explosives (Figure 7). Therefore,
the fluorescence response of 1 to nitro-compound explosives
was attributed to the electron-transfer quenching mechanism;
that is, in the presence of nitro compounds, the excited elec-
tron of complex 1 undergoes a transfer to the nitro compound
instead of relaxation to the ground state with fluorescence
emission. Because the photoluminescence of 1 originated from
the ligand, the sensitive response of 1 to nitro-compound ex-
plosives could be attributed to the electron-rich property of
the ligand, which facilitates the excited-state electron-transfer
process. Though there are cavities in the framework of 1, the
absence of an accessible path excludes the possibility of ana-
lyte encapsulation during the sensing process. Therefore, the
sensing mechanism of 1 should not be based on guest-in-
duced quenching,[13] in which analyte molecules are included
in the pores as a guest and interact directly with the fluoro-
phore. The particles of 1 could be dispersed well in solution
with DMF, which enables the nitroaromatic explosive mole-
cules to be closely adsorbed on the surface of the particles
and facilitates possible electron transfer.[28] A similar solvent-de-
pendent fluorescence quenching of MOFs has also been re-
ported by Chen, Qian, Sun, and Mukherjee.[5a,b,7b,29] Therefore,
luminescence quenching behavior observed in 1 is not only
the result of the electron-deficient nature of nitroaromatic
compounds and the highly electron-rich conjugated frame-
work structure, but also the highly dispersible nature of the
MOF particles.
[Zn2(bpdc)2(bpee)]·2DMF
(bpdc=4,4’-biphenyldicarboxylate,
bpee=1,2-bipyridylethene), which exhibit unique selectivity in
the detection of nitroaromatic compounds with different
groups and high explosives. The excellent fluorescence
quenching response to 2,3-dimethyl-2,3-dinitrobutane (DMNB,
a taggant required by law in all commercial plastic explosives)
can be further attributed to the pore confinement of the ana-
lyte inside the molecule-sized cavities of such a zinc(II) com-
plex, thus facilitating stronger interactions between the DMNB
molecule and the host framework, as reflected by the relatively
small difference in the quenching percentages for NB, which
exhibits only 10% higher sensitivity (94% quenching at 10 s)
relative to DMNB. However, NB in [Zn2(oba)2(bpy)]·3DMA
quenches the emission by as much as 84%, and the order of
quenching efficiency for the selected nitroaromatic com-
pounds is NB>m-DNB>NTꢁp-DNB>dinitrotoluene (DNT).
Notably, this order is not fully in accordance with the trend of
electron-withdrawing groups, but it is fully consistent when
the vapor pressure of each analyte is also taken into consider-
ation.
The fact that NB exhibits the strongest quenching effect can
be attributed to two factors: high vapor pressure and the
strongly electron-withdrawing NO2 group. Although the vapor
pressure of NT is comparable to that of NB, the quenching effi-
ciency (29%) is significantly less because of the presence of
the electron-donating CH3 group. Similarly, although m- and p-
DNB have two strongly electron-withdrawing NO2 groups,
both have very low vapor pressures at room temperature. In-
terestingly, Ghosh and co-workers demonstrated the lumines-
cent 3D MOF [Cd(NDC)0.5(PCA)]·Gx (G=guest molecules, NDC=
2,6-napthalenedicarboxylic acid, PCA=4-pyridinecaboxylic
acid) for the highly selective detection of 2,4,6-trinitrophenol
(TNP).[27b] To explore the ability of such a cadmium(II) complex
to sense a trace quantity of nitro explosives, fluorescence-
quenching titrations were performed with the incremental ad-
dition of analytes to such a cadmium(II) complex dispersed in
MeCN. Fast and high fluorescence quenching was observed
upon incremental addition of a solution of TNP in MeCN
(1 mm). The visible bright-blue emission of such a cadmium(II)
Simultaneously, various anions were selected to carry out
the anion-sensing function in view of the cationic framework
and porosity of 1. Different aqueous solutions of anions (that
2À
is, FÀ, ClÀ, BrÀ, IÀ, SO42À, ClO4À, CO32À, BF4À, NO3À, Cr2O7
,
NO2À, and OAcÀ) were used to soak complex 1. The lumines-
cent measurements illustrate that the different anions have
a great influence on the luminescent intensity of 1. Remarka-
2À
bly, the Cr2O7 ion has the largest quenching effect on the lu-
2À
minescent emission (Figure 9). The toxic Cr2O7 ion is very
harmful to human health and the environment and can be ac-
cumulated in living organisms, thus leading to serious diseas-
es.[30] Although there are several reports on the exchange of
anions based on MOFs, it is still rare to explore the exchange
and capture of pollutant anions by using a cadmium(II) lumi-
nescent cationic framework.[31] Therefore, the exchange and
2À
capture capacity of 1 regarding the Cr2O7 ion was further
Chem. Eur. J. 2015, 21, 14171 – 14178
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