G Model
CCLET 4850 No. of Pages 5
2
Y. Li et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
small organic molecules in the detection. (3) Adjustable chemical
functionality: by designing organic ligands with special functional
groups, it is more likely to interact with host–guest system in
fluorescence sensing, resulting in rapid changes in fluorescence
intensity [25–28].
carboxylic oxygen atoms from three different H2EPA ligands, two
nitrogen-atoms from two different TPT ligands with an average
Co-O distance of 2.048 Å, Co-N distance of 2.163 Å. Each
m2-carboxylate group of the fully deprotonated carboxylic acid
ligands links two Co ions to form binuclear SBU. Furthermore,
ligands and SBU are connected to each other, thus 3D structure
with conjugate structure is formed through the connection
between layers and the porosity is 10.03% (1510 Å out of the
15,061 Å unit cell volume) (Fig. S6 in Supporting information).
MOFs constructed by transition metal ions have relatively fine
luminescence performance [30]. Therefore, the two Co-MOFs were
experimented of solid state fluorescence. As expected, complexes 1
and 2 exhibited different luminescence properties relative to
ligands (Fig. S10 in Supporting information). Complex 1 exhibits
clear emission at lmax =457 nm upon excitation at lex =330 nm
with EXslit =5.0 nm, EMslit =2.5 nm, which has a red-shift of 30 nm
compared to the ligand of H2TEPA at lmax =427 nm. Complex 2
shows the same phenomenon of red shift compared to complex 1.
This change in emission wavelength can be attributed to cobalt
Herein, based on the design of ligands, two different ligands with
alkynyl functional group 5-((triisopropylsilyl)ethynyl)isophthalic
acid (H2TEPA) and 5-ethynylisophthalic acid (H2EPA) were synthe-
sized. Two Co-MOFs with different structures were successfully
synthesized though dual-ligand strategy introducing 2,4,6-tri
(pyridin-4-yl)-1,3,5-triazine (TPT). Complexes 1 and 2 show excel-
lent luminescence properties. Interestingly, as fluorescent sensors,
they exhibit selectivity and sensitivity for Fe3+ and rapidly detect
nitroaromatic compounds in methanol and ethanol, especially for
2,4-NPH and 4-NA and 4-NP through fluorescence quenching with
high quenching efficiency. Therefore, complexes 1 and 2 are
promising sensors for identification and detection to Fe3+ and
nitroaromatic compounds.
The two alkynyl functionalized ligands we designed are shown
in Scheme S1 (Supporting information). Crystal data are shown in
Table S1 (Supporting information). Complex 1 is constructed by
ligand of TPT and H2TEPA (Fig. 1a). By single-crystal X-ray
diffraction analysis, complex 1 crystallizes in the hexagonal crystal
system with a space group R3. The asymmetric coordination unit
consists of a Co2+ ion, two-thirds TPT ligands, one H2TEPA ligand
and a DMF molecule in solvent. Co1 is six-coordinated with four
carboxylic oxygen atoms from three different H2TEPA ligands, two
nitrogen-atoms from two different TPT ligands. All the carboxyl
groups of H2TEPA ligand were deprotonated during the reaction:
one adopts bidentate mode to link a Co2+, one adopts chelating
mode to connect a Co2+ and the groups adopt bridging mode to
connect two Co2+ ions to form binuclear SBUs, and this binuclear
SBUs connected with TPT, H2TEPA ligands form the three-
dimensional (3D) coordination network structure. Meanwhile,
ions in MOFs influences the electron transition containing
p→p*
and n→ * in the original ligand. The fluorescence lifetime and
p
luminescence quantum yield of two complexes are shown in
Table S4 and Fig. S11 (Supporting information). It is precisely
because of the luminescent properties of the two complexes,
luminescence in different solvents was measured, complexes 1 and
2 showed different luminescence intensities (Figs. S12 and S13 in
Supporting information). Complexes 1 and 2 show a stable and
high fluorescence intensity in methanol solution and ethanol
solution as relatively green organic solvents. Therefore, the next
fluorescence sensing experiment was carried out in methanol
solution and ethanol solution, respectively. The measurement
procedures are as follows: 2 mg of solvent-free complex 1 was
dispersed by ultrasonication in 3 mL of methanol solution. During
the fluorescence testing, then 10 mL of methanol solution of M
there are large conjugate system with
p
-
p
effect in this network
(NO3)x (1.0 mmol/L; M = Li+, Ag+, Ba2+, Ca2+, Cd2+, Cu2+, Mg2+, Pb2+
,
structure (Fig. S5 in Supporting information). All Si atoms extend
into the pore from the crystal plane of 010. In addition, by the
SQUEEZE program in the PLATON software calculation [29], the
porosity is 5.03% (757 Å out of the 15,061 Å unit cell volume).
Complex 2 is constructed by ligand of TPT and H2EPA (Fig. 1b).
Single-crystal X-ray diffraction result indicates that complex 2
crystallizes in the monoclinic system with a space group C2/c. The
asymmetric coordination unit consists of a Co2+ ion, one TPT ligand
and one H2EPA ligand. Every Co atom is six-coordinated with four
Hg2+, Cr3+, Al3+ and Fe3+) were added to the solution in turn in each
set of experiments. The corresponding fluorescence intensity were
recorded in fluorescence spectrophotometer. When 150 mL of M
(NO3)x was adding, the corresponding fluorescence intensity is
showed in Fig. 2a. It is obvious that Fe3+ can reduce the
fluorescence intensity of the complex 1 to the greatest extent
relative to the other metal ions (Figs. S18–S20 in Supporting
information), and the decreasing trend of fluorescence intensity is
shown in Fig. 2b.
To further compare the difference between metal ions in the
properties of complexes 1 and 2 constructed by different ligands,
the fluorescence experiment method of complex 2 is the same as
described above. Fe3+ can also reduce the fluorescence intensity of
the complex 2 to the greatest extent, (Fig. 2c and Figs. S21–23 in
Supporting information) and the decreasing trend of fluorescence
intensity was shown in Fig. 2d. In order to further prove the
quenching efficiency of complexes 1 and 2 for Fe3+, we calculated
the quenching constant (Ksv) by the Stern À Volmer (SV) equation,
(I0/I) = 1 + Ksv [A] [31]. Here, I0/I is stand for the initial fluorescence
intensity over the luminescence intensity after addition of the
analyte, [A] is the molar concentration of the analyte. The Ksv value
of complex 1 was calculated to be 1.520 Â104 L/mol towards Fe3+
while 3.543 Â104 L/mol for complex 2, which is higher than the
known MOF-based sensors Eu(acac)3@Zn(C15H12NO2)2 (5 Â103 L/
mol), [32] [Cd(TEB)0.5
]
Á
2DMF
Á
4H2O (6.9 Â103 L/mol) [33], lower
than MIL-53(Al) (9 Â105 L/mol) [34]. To estimate the luminescence
properties of two complexes, the limit of detection (LOD) is
necessary. The detection limit (LOD) is calculated based on the
equation: LOD = 3
test and K value is based on the linear regression, so, the LOD of
complexes 1 and 2 are 0.616 mmol/L and 0.264 mmol/L for Fe3+
s/k, s value is calculated from 11 groups of blank
Fig. 1. (a) The SBU contected with TPT and H2TEPA to form the complex 1 with 3D
frameworks along c axis. (b) The SBU contected with TPT and H2EPA to form the
complex 2 with 3D framworks along a axis.
,
Please cite this article in press as: Y. Li, et al., Two alkynyl functionalized Co(II)-MOFs as fluorescent sensors exhibiting selectivity and
Li, Yue
