G Model
CCLET-6204; No. of Pages 4
Z.-Y. Li, Z.-Q. Yao, R. Feng et al.
Chinese Chemical Letters xxx (xxxx) xxx–xxx
asymmetric unit contains two thirds crystallographic independent
Tb atoms and one ligand. Both Tb1 and Tb2 are 9-coordinated by O
atoms, and the neighbouring Tb1 or Tb2 atoms are connected by
three bridging m2-O atoms from carboxylate groups to form rod-
like [Tb(COO)3] SBUs (Fig.1a). The free program SHAPE 2.0 suggests
that the proximate ideal geometry of the nine-coordinated Tb1 and
indicating excellent resistance ability to multiple solvents. The
stabilities in aqueous solution with different pH values were also
performed, and NKU-130 was proved to maintain its crystallinity
between pH 3–12 even after 300 days (Fig. 2). Benefit from its
thermal and pH stabilities, NKU-130 can be a candidate material
for fluorescent detection in the liquid phase under harsh
conditions.
Tb2 is spherical tricapped trigonal prism (STTP), and their
t values
have been listed in Table S3 (Supporting information). The rod
SBUs of Tb1 and Tb2 are alternatively arranged and these atoms
exhibit different spiral directions. The connecting m-phthalate
groups between rod SBUs exhibit a two-fold disorder respectively
towards inner and outer orientations along the c-axis (Fig. 1b). The
pending electron-rich pyrazolyl groups of ligands are towards the
inside of the aperture, which leads to a pore shape alteration from
original hexagonal to triangular and an aperture reduction to 4.0 Å
(Fig.1c). The space-filling model of the trigonal channel is shown in
Fig. 1d. From the topological point of view, when simplifying the
ligand as a 4-connected node, the single 3D framework of NKU-130
can then be viewed as a 4,6-connected stp net with the point
symbol of {4.62}3{46.63.86} (Fig. S7 in Supporting information).
Considering the requirements for thermal stability of MOFs in
fluorescent detection, thermogravimetric analysis (TGA) was
carried out. The bulk crystals of NKU-130 were soaked into DMF
and EtOH separately for three days, dried in vacuum at 100 ꢀC and
TGA was performed subsequently under Ar atmosphere. The TGA
curve (Fig. S10 in Supporting information) indicates a slight weight
loss under 150 ꢀC, which demonstrated the removal of guest
solvents in minuscule channels. Meanwhile, the curve shows a
platform before 540 ꢀC, manifesting a remarkably high thermal
stability. Variable-temperature powder XRD measurements dem-
onstrated that the framework of NKU-130 could remain stable up
to 400 ꢀC (Fig. S11 in Supporting information).
The photoluminescence (PL) spectra and fluorescence quantum
yield of NKU-130 in the solid state were measured at room
temperature (Fig. S13 in Supporting information). The sharp peaks
in PL spectra observed at 489, 545, 585 and 624 nm can be assigned
to the 5D0→7F1, 5D0→7F2, 5D0→7F3 and 5D0→7F4 transitions,
respectively, which are attributed to the antenna effect and
characteristic f-f emission bands [33–35]. Fluorescence quantum
yield of NKU-130 in the solid state was calculated to be 21.0%. In
addition, the PL spectra of NKU-130 dispersed in H2O and DMF also
possess strong emission at 545 nm when excited at 327 nm.
To characterize the trace-quantity sensing ability towards nitro
explosives, fluorescence titrations were performed with the
incremental addition of analytes to NKU-130 dispersed in water
(Fig. 3). After incremental addition of PA solution (1 Â10À3 mol/L),
rapid and high fluorescence quenching was observed with the
naked eyes. As shown in Fig. 3a, the bright green emission of NKU-
130 sharply weakens with gradually adding PA solution. For
illustrating the selectivity of detecting PA, fluorescence quenching
efficiencies were also performed with other nitro aromatics, such
as 1,4-dinitrobenzene (1,4-DNB), 2,6-dinitrotoluene (2,6-DNT),
1,3-dinitrobenzene (1,3-DNB) and nitrobenzene (NB). As shown in
Fig. S14 (Supporting information), with the dropping of diverse
nitro explosives at the concentration of 2.5 Â10À3 mol/L, the
fluorescence of NKU-130 could still be observed. The remaining
nitro explosives have almost no quenching effect, indicating that
NKU-130 has a high PA selectivity (Fig. S15 in Supporting
information).
Affected by the selective quenching effect, we also performed
luminescence quenching titration with low concentration PA,
thus obtaining the detection limit of PA. The fluorescence
quenching efficiency was analyzed by the Stern-Volmer (SV)
equation, (I0/I) = Ksv[A] + 1, where I0 and I are the initial
fluorescence intensity before and after the analyte is added,
respectively. [A] is the molar concentration of the analyte, and Ksv
is the quenching constant (L/mol) [36–38]. At low concentrations,
the SV plot of PA is almost linear. The quenching constant for PA
was calculated to be 2.012 Â 104 L/mol from the direct fitting of the
plot, which demonstrates the high quenching capability of NKU-
130 toward PA. The calculated detection limit is 1.3 Â 10À5 mol/L
(3 ppm). Besides, we also carried out the titration experiments to
The solvent and pH stability of NKU-130 was verified by PXRD
measurements. As shown in Fig. S12 (Supporting information), the
as-synthesized crystals were immersed in several common
solvents (including DMF, DMAc, ethyl acetate, acetone, dichloro-
methane, methanol, tetrahydrofuran and water) for 7 days. The
PXRD patterns are consistent well with the simulated one,
Fig. 1. (a) The asymmetric unit of NKU-130, A = -x + y, y, 1/2+z; B = 1-x + y, y, -1/2+z.
The numbers after underline in atom labels represent for different sets of disorder
structures. Yellow: Tb, red: O, blue: N, grey: C. (b) The rod SBU of [Tb(COO)3] and
ligand. (c) View of the 3D framework structure of NKU-130 along the c-axis. (d)
Space-filling model of NKU-130.
Fig. 2. PXRD patterns for NKU-130 immersed in aqueous solutions (pH 2–13) and
exposed in air.
2
Li, Zi-Ying
