Robust Anionic LnIII-Organic Frameworks: Chemical Fixation of CO2, Tunable Light Emission, and Fluorescence Recognition of Fe3+

Article abstract of DOI:10.1021/acs.inorgchem.0c01782

With the aim of exploring and enriching nanocaged functional platforms of lanthanide-organic frameworks, the subtle combination of [Ln2(CO2)8] secondary building units and [Ln(CO2)4] units by employing the hexacarboxylic acid of 4,4′,4″-(pyridine-2,4,6-triyl)tris(1,3-benzenedicarboxylic acid) (H6PTTBA) successfully realized the self-assembly of highly robust multifunctional {LnIII2}LnIII-organic anionic skeletons of {(Me2NH2)[Ln3(PTTBA)2]·xDMF·yH2O}n (1-Ln), which had remarkable intrinsic nature of high thermal and water stability, large permanent porosity, interconnected nanocaged void volume, and high specific surface area. Here, only the Eu-based framework of 1-Eu was taken as one representative to discuss in detail. Gas-sorption experiments showed that the activated solvent-free 1-Eu framework possessed the outstanding ability to separate the mixed gases of CO2/CH4 (50:50, v/v) with an ideal adsorbed solution theory selectivity of 14. Furthermore, 1-Eu was an efficient and recycled catalyst for the chemical cycloaddition of CO2 and epoxides into their corresponding carbonates, which possessed a better catalytic performance than the documented unique Eu3+-organic framework of [Eu(BTB)(phen)] and could be widely applied in industry because of its simple synthetic conditions and high yield. In the meantime, adjustable emission colors devoted by the efficient Tb3+ → Eu3+ energy transfer confirmed that Eux/Tb1-x-organic framework could be taken as a good substitute for barcode materials by changing the ratio of Eu3+ and Tb3+. Moreover, quantitative luminescence titration experiments exhibited that 1-Eu possessed good selectivity for the identification of Fe3+ in aqueous solution by fluorescence quenching with a low limit of detection value of 6.32 × 10-6 M.

Full text of DOI:10.1021/acs.inorgchem.0c01782

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Robust Anionic LnIII−Organic Frameworks: Chemical Fixation of CO2,
Tunable Light Emission, and Fluorescence Recognition of Fe3+
Hongtai Chen, Liming Fan, Hongxiao Lv, and Xiutang Zhang*
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ABSTRACT: With the aim of exploring and enriching nanocaged
functional platforms of lanthanide−organic frameworks, the subtle
combination of [Ln2(CO2)8] secondary building units and
[Ln(CO2)4] units by employing the hexacarboxylic acid of
4,4′,4″-(pyridine-2,4,6-triyl)tris(1,3-benzenedicarboxylic acid)
(H6PTTBA) successfully realized the self-assembly of highly
robust multifunctional {LnIII2}LnIII−organic anionic skeletons of
{(Me2NH2)[Ln3(PTTBA)2]·xDMF·yH2O}n (1-Ln), which had
remarkable intrinsic nature of high thermal and water stability,
large permanent porosity, interconnected nanocaged void volume,
and high specific surface area. Here, only the Eu-based framework
of 1-Eu was taken as one representative to discuss in detail. Gas-
sorption experiments showed that the activated solvent-free 1-Eu
framework possessed the outstanding ability to separate the mixed gases of CO2/CH4 (50:50, v/v) with an ideal adsorbed solution
theory selectivity of 14. Furthermore, 1-Eu was an efficient and recycled catalyst for the chemical cycloaddition of CO2 and epoxides
into their corresponding carbonates, which possessed a better catalytic performance than the documented unique Eu3+−organic
framework of [Eu(BTB)(phen)] and could be widely applied in industry because of its simple synthetic conditions and high yield. In
the meantime, adjustable emission colors devoted by the efficient Tb3+ → Eu3+ energy transfer confirmed that Eux/Tb1−x−organic
framework could be taken as a good substitute for barcode materials by changing the ratio of Eu3+ and Tb3+. Moreover, quantitative
luminescence titration experiments exhibited that 1-Eu possessed good selectivity for the identification of Fe3+ in aqueous solution
by fluorescence quenching with a low limit of detection value of 6.32 × 10−6 M.
Lewis and Brønsted acidic sites, which render porous Ln-
MOFs as an ideal heterogeneous catalyst for various chemical
reactions, such as the synthesis of cyclic carbonates from the
cycloaddition of epoxides and carbon dioxide (CO2).21−24
Moreover, CO2, a huge greenhouse gas devoted to the
consumption of fossil fuels, vehicle emissions, industrial
discharge, and straw combustion, has gradually become a
focus of global concern and could cause devastation with
regard to global pestilence diseases and sea-level rise.25−32
Thereby, the unconquered technology of reducing the amount
of CO2 through consumption poses a challenge to global
scientific researchers. So far, tentative and purposeful
operations, including selective adsorption and storage and
chemical conversion, have been widely launched with the help
of consensus graphene, zeolite sieves, inorganic metal oxide or
INTRODUCTION
■
In recent years, porous metal−organic frameworks (MOFs)
with high specific surface area and ordered passageway
distribution have attracted special attention because of a
wide range of potential applications, including excellent
selective adsorption and storage of volatile gases, fluorescence
recognition of specific molecules, catalytic properties for
specific chemical reactions, drug delivery, and release,
etc.,1−10 whereas, recently, the promising subclass of Ln-
MOFs has been paid great attention because of its special
structural features and good performance.11−16 During the self-
assembly process under different environments, the synergistic
effect of the Ln3+ coordination diversities and applied various
functional organic ligands resulted in massive MOFs with
infinitely structural adjustability, Whereas, as for most Ln-
MOFs, although the Ln3+ cations usually are octa- or
nonacoordinated to form polar covalent bonds by devotion
of nine empty electronic orbitals of 6s, 6p, and 5d, the highest
coordination number could reach 12 with the advantage of 4f
space orbitals when coordinated with smaller molecular
Received: June 16, 2020
ligands, such as NO3 with low charge and repulsion.17−20
−
Thus, octa- or nonacoordinated Ln3+ cations in Ln-MOFs do
not reach the saturated coordination state and could serve as
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clusters, activated MOFs, etc.33−36 At this time, catalytic
cycloaddition of CO2 under wild conditions is the most
economic and convenient treatment method by the trans-
formation of epoxides into cyclic carbonates, which could be
further economically applied in the industrial fields of nontoxic
aprotic solvent, raw material of polymer, pharmaceutical and
pesticide intermediates, battery electrolyte, etc.37−39 Among all
porous MOF materials, Ln-MOFs are one of the most efficient
and sustainable heterogeneous catalysts because Ln3+ cations
own rich empty electronic orbitals and a strong affinity for
large dipole-moment CO2 molecules,40−42 whereas, hitherto,
most of the reported Ln-MOFs were usually self-assembled
from mixed ligands and lack solvent-accessible voids and
structural stability. Therefore, in order to pursue the
applicability of Ln-MOFs in a comparatively complicated
environment, the strict demand for strong skeletons is
necessary. Moreover, with an increase in the length of ligands,
for the class of Ln-MOFs, it is almost inevitable that a decrease
of the skeleton stability or a decrease of the porosity is caused
by interpenetration. Thus, so far, optimization research on the
catalytic platform of Ln-MOFs with respect to the surface area,
porosity, pore size, and adsorption capacity is still relatively
scarce compared to all reported MOFs, which may be
attributed to the laborious crystallization and related structure
characterization due to the highly pluralistic coordination
number and equivocal directionality of Ln3+.43,44
Furthermore, as for Ln-MOFs, the luminescence perform-
ance could be effectively enhanced by the “antenna effect” of
aromatic ligands with energy transfer (ET) of π−π* and n−π*,
by which the disadvantageous of the outer-electron distribu-
tion features of Ln3+ ions could be overcome. In recent years,
with the application of favorably designed π-conjugated
polycarboxyl acids as illuminants, a lot of bimetallic doped
EuxTb1−x MOFs were reported because modulation of the ET
by adjustments to the Tb3+ and Eu3+ contents could offer new
single-molecule white materials and probes for guest molecules
or metal cations baseed on the principle of light switching.
In view of the comparisons of documented MOFs built from
separate organic ligands of 1,3-bis(2,4-dicarboxylphenyl)-
benzene, 2,6-bis(2,4-dicarboxylphenyl)pyridine, 1,3,5-benzene-
tricarboxylic acid, and 1,3,5-(4-carboxylphenyl)benzene, the
large sizes of the organic ligands could increase the MOF
characteristics of surface area, porosity, pore size, and
adsorption capacity, and more carboxyl groups could enhance
the MOF stability.45,46 When the advantages of the above-
mentioned four organic ligands were combined, the
hexacarboxylic acid of 4,4′,4″-(pyridine-2,4,6-triyl)tris(1,3-
benzenedicarboxylic acid) (H6PTTBA) was successfully
synthesized by our research group for the first time. On the
basis of the considerations of the above-mentioned application,
the acidic solvothermal reaction of Ln2O3 and H6PTTBA in
the mixed solvent of N,N-dimethylformamide (DMF)/H2O
generated a series of 3D nanoporous LnIII−organic frame-
works: {(Me2NH2)[Ln3(PTTBA)2]·xDMF·yH2O}n, which
proved that H6PTTBA was an excellent organic linker for
the construction of MOFs with large surface area, high
porosity, and pore size and anticipated adsorption capacity
under convenient hydrothermal conditions. Herein, the Eu-
based framework of {(Me2NH2)[Ln3(PTTBA)2]·4DMF·
3H2O}n (1-Eu) was taken as one representative to discuss in
detail. Structural analysis of 1-Eu confirmed that H6PTTBA
had the following several characteristics: (i) enriched carboxyl
groups could catch enough cations/secondary building units
(SBUs) to form a wall of nanotubes or nanocages; (ii) the
torsion angle between aromatic planes among one H6PTTBA
could range from 0 to 90° and further facilitate the SBU-based
framework construction; (iii) under certain suitable conditions,
the N-heterocycle could serve as a negative charge
compensator to reduce the electronegativity of the MOF by
the protonation of a nitrogen atom or as a Lewis basic site to
prompt the special catalytic reaction.
EXPERIMENTAL SECTION
■
Materials and General Methods. All of the starting reagents and
solvents were commercially available without further purification.
Elemental analyses were performed using a CE instruments (model
EA 1110 elemental analyzer). Powder X-ray diffraction (PXRD) data
were collected on a Panalytical X-Pert Pro diffractometer working in
Bragg−Brentano geometry with Cu Kα radiation. Thermogravimetric
analysis (TGA) was carried out on a PerkinElmer TGA-7
thermogravimetric analyzer under an air atmosphere. Emission and
excitation spectra were carried out on an Edinburgh FLS920
spectrophotometer with an nF900 flash lamp. The luminescence
lifetime was measured on an Edinburgh FLS920 phosphorimeter with
a microsecond lamp (100 mW). The quantum efficiency was
measured using the integrating sphere on a FluoroMax-4 spectropho-
tometer. Single-component gas sorption was tested on an ASAP 2020
analyzer at 273 and 298 K. The catalytic yield was measured and
calculated on a Thermo Fisher Trace ISQ gas chromatography (GC)/
mass spectrometry (MS) instrument. 1H NMR spectra were recorded
on a JEOL-ECX 500 FT instrument in CDCl3 or dimethyl-d6
sulfoxide with n-dodecane as the internal standard. Inductively
coupled plasma (ICP) was analyzed on a IRIS Advantage
spectrometer.
Preparation of 1-Eu. A mixture of Eu2O3 (0.08 mmol, 32.0 mg),
1 mL of 10% HNO3, and H6PTTBA (0.10 mmol, 57.1 mg) was
dissolved in a 8 mL mixed solution of DMF and H2O (7:1, v/v) and
stirred for 30 min to generate a homogeneous solution, which was
further transferred into a 25 mL Teflon-lined autoclave at 110 °C for
2 days. Colorless block crystals were obtained upon slow cooling to
room temperature at a rate of 10 °C h−1, followed by washing with
DMF/acetone. Yield: 95% based on H6PTTBA. Anal. Calcd for 1-Eu
(C72H66N7O31Eu3): C, 43.80; H, 3.34; N, 4.96. Found: C, 43.56; H,
3.58; N, 4.47. IR (KBr pellet, cm−1): 3405 (vs), 1596 (vs), 1394 (vs),
1113 (w), 1017 (w), 783 (s), 705 (w), 516 (w).
Preparation of other 1-Ln Complexes. Other 1-Ln (Ln = Yb,
Ho, Er, Tb, Dy, Lu, and Gd) complexes were prepared under
conditions similar to those of 1-Eu except that Eu2O3 was replaced by
the corresponding lanthanide oxides.
Preparation of 1-EuxTb1−x. The doped 1-EuxTb1−x complexes
were prepared in a manner similar to that of 1-Eu except that Eu2O3
was replaced by equal mixed lanthanide oxides. The Eu3+/Tb3+ molar
ratio for as-synthesized samples of 1-EuxTb1−x was determined by
ICP analysis (Table S4), which confirmed that the ratio of Eu3+/Tb3+
was equal to the one added for the reaction.
X-ray Crystallography. The diffraction data of 1-Ln were
collected on a Bruker Smart-APEX II CCD area detector equipped
with graphite-monochromated Mo Kα radiation (λ = 0.071073 nm)
at 296(2) K. The structures were solved by direct methods and
refined by full matrix least-squares using the SHELXL and SHELXT
packages. All non-hydrogen atoms were refined anisotropic displace-
ment parameters, and hydrogen atoms except those on water
molecules were generated geometrically with fixed isotropic thermal
parameters and refined in the structure factor calculations. The block
of SQUEEZE in PLATON was carried out to remove the disordered
solvent molecules. The solvent content of 1-Eu was concluded from
TGA (Figure S1) and elemental analysis. The crystallographic data
and refinement parameters are shown in Table S1. Selected bond
lengths and angles are listed in Table S2. Furthermore, the crystals
deposited as CCDC 1997911 and 2008436−2008442 could be
downloaded from the Cambridge Crystallographic Data Centre.
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groups offered by five PTTBA6− ligands with Eu(1)−Ocarboxyl
bond distances in the range of 2.319−2.851 Å, which was
shorter than the documented Eu−O distance.47 Moreover, it is
worth mentioning that the separated Eu(1)···Eu(1#) distance
of 4.004 Å was much smaller than those of the reported
dinuclear paddlewheel {Eu2} SBUs,47 which may be attributed
to the effects of rigidity and nonarbitrary torsion among
aromatic cycles in one PTTBA6− ligand. As shown in Figure
1b, the great distortion between three separately branched
phenyl rings and the central N-heterocycle pyridine plane took
place with the respective dihedral angles of 46.91°, 46.98°, and
71.15°, which could be ascribed to the synergistic effect of
[Eu(1)2(CO2)8] SBUs and [Eu(2)(CO2)4] units. Moreover,
although six carboxyl groups of H6PTTBA were thoroughly
deprotonated, a proton was grasped by the N-heterocycle
pyridine to reduce the electronegativity of the host skeleton,
whereas the Eu(2) ion was chelated by four spare carboxyl
groups from four PTTBA6− ligands with μ1-η1:η1 coordination
mode, leaving one dodecahedron geometry (Figure 1a). The
carboxyl groups connected to the Eu(2) ion were called spares
because the host framework could be established from
[Eu(1)2(CO2)8] SBUs and PTTBA6− ligands with neglect of
the Eu(2) ions, which all-in-all played a role in reinforcing the
overall framework by installing the Eu(2) cations in the
negative central area embraced by four carboxyl groups. In
addition, it is worth mentioning that PTTBA6− exhibited an
extremely strong affinity to coordinating with seven metal
cations under acidic solvothermal conditions with three types
of coordination modes of μ1-η1:η1, μ2-η1:η1, and μ2-η1:η1 and
thus easily prompted the formation of one dual-channel 3D
framework enriched with nanovoid cages, as shown in Figure
1e.
RESULT AND DISCUSSION
Crystal Structure Description. As shown in Figure 1,
single-crystal X-ray diffraction (SCXRD) analysis showed that
■
Figure 1. Porous framework of 1-Eu: (a) coordination mode of a
PTTBA6− ligand; (b) 3D framework of 1-Eu viewed along the [100]
axis; (c) structural detail of interpenetrating channels viewed along
the [010] axis; (d) 3D framework featured by interspersed channels
with dimensions of 5.2 × 4.6 and 10.6 × 9.2 Å2, respectively. All
hydrogen atoms were removed for visual clarity.
Finally, the disassembly analysis of the porous 1-Eu
framework showed that four [Eu(2)(CO2)4] units and eight
[Eu(1)2(CO2)8] SBUs were intermittently linked through
eight PTTBA6− ligands to create one large solvent-residing
cagelike space with an aperture of around 13.8 Å (Figure 1c),
which were further infinitely propagated by edge-sharing the
organic tetracycle carbon skeleton, leaving the 3D interspersed
channels with minimum and maximum open-node windows of
5.2 × 4.6 and 10.6 × 9.2 Å2, respectively, as shown in parts c
and d of Figure 1. In detail, two [Eu(1)2(CO2)8] SBUs and
one [Eu(2)(CO2)4] unit were interlinked via an organic
carbon skeleton to form the small triangular window, whereas
two [Eu(1)2(CO2)8] SBUs and two [Eu(2)(CO2)4] units
formed the slightly larger one. Finally, from the topological
view, [Eu(1)2(CO2)8] SBUs, [Eu(2)(CO2)4] units, and one
PTTBA6− ligand could be simplified as 4-, 4-, and 5-connected
nodes, generating an undocumented 3D TAYZIC topology
1-Eu crystallized in the monoclinic space group C12/c1 and
displayed one nanocaged {[EuIII2]EuIII}−organic 3D host
framework with a void volume of 51.2% derived from
PLATON calculations. In 1-Eu, the EuIII−organic coordination
skeleton of [Eu3(PTTBA)2] was negatively charged and
+
electron-balanced by the H2N(CH3)2 cation dwelling in the
tunnels, which was created from acidic hydrolysis of DMF
1
under solvothermal conditions and confirmed by H NMR
spectroscopy. Moreover, the disordered solvent constituents
were ascertained by the recognized means of SQUEEZE, TGA,
and elemental analysis. For all we know, 1-Eu was the scarcely
reported Ln-MOF model self-assembled from two kinds of
EuIII-centered SBUs bridged by one unique hexacarboxyl
ligand.
As shown in Figure 1b, the asymmetric unit included three
Eu3+ ions [Eu(1) and Eu(2)] as well as two PTTBA6− ligands.
Two μ1-η1:η1 carboxyl groups residing on the 2 position of two
phenyl rings located on both sides of the nitrogen atom named
as the 2 and 6 positions of the N-heterocycle in one PTTBA6−
ligand first chelated the europium center of Eu(1), which was
then connected by one chelated + bridged μ2-η2:η1 carboxyl
group arranged on the 4 position of the phenyl ring residing in
the 4 position of pyridine from another PTTBA6− ligand and
bridged by two 4-position μ2-η1:η1 carboxyl groups of two
phenyl rings residing in the 2 position of pyridine devoted by
two other PTTBA6− ligands. Thus, in brief, the Eu(1) cation
and its commensurable symmetrically generated Eu(1#) were
spanned together to generate one [Eu2(CO2)8] SBU with the
aid of two μ2-η2:η1 and two μ2-η1:η1 carboxylic groups from
four separate PTTBA6− ligands. Thus, the Eu(1) ion was
nonacoordinated by nine oxygen atoms from six carboxyl
4
2
5
5
6
8
̈
with a Schlafli symbol of {4 .6 }{4 .6 }2{4 .6 .8} (Figure 1f).
Thermal and Water Stability. Considering the practical
application, verification for the robustness of the 1-Eu
framework was essential. TGA of 1-Eu was carried out under
an air atmosphere from room temperature to 800 °C, as shown
in Figure S2. Below 400 °C, the weight loss of 18.7% (calcd
19.6%) was in accordance with the loss of aqueous molecules,
DMF, and dimethylamine cations. With rising temperature, the
host framework of 1-Eu began to break down. The final white
products with a residual weight of ca. 34.0% should be Eu2O3,
which was roughly consistent with a calculated value of 31.4%.
Moreover, the PXRD patterns of the fresh 1-Eu samples
immersed in water for 2 weeks or even boiling water for 72 h
proved that the original framework maintained good integrity
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(Figure S3). This unusual heat- and water-resisting behavior
could be attributed to the strong linking mode between
binuclear SBUs and single polycarboxylic ligands, which was
further reinforced by [Eu2(COO)4] units.
Catalytic Performance for the Cycloaddition of CO2
with Epoxide. In our preliminary study on the catalytic
reaction of CO2 with epoxides, the optimal reaction conditions
were explored by taking styrene oxide as the model substrate,
and the obtained data along with the varied conditions are
listed in Table 1. First of all, referring to the traditional dosage
Gas-Adsorption Studies. In view of the superior nano-
channel environment and appropriate pore size (7.2−10.4 Å)
in 1-Eu, which were elucidated from the SCXRD analysis, the
sorption studies for CO2 and CH4 were fulfilled. Prior to the
sorption experiment, the crystalline sample of 1-Eu was
activated by the solvent exchange with acetone for 3 days and
evacuation at 150 °C under vacuum for 12 h, and further
checked by the PXRD pattern (Figure S9). As shown in Figure
S4, the N2 adsorption−desorption results tested at 77 K
manifested that the Brunauer−Emmett−Teller specific surface
area and pore volume were 1735 m2 g−1 and 0.49 cm3 g−1,
respectively, which was in accordance with the structural data.
Furthermore, the adsorption capacity of fully activated 1-Eu
samples for CO2 could reach 81.1 and 54.8 cm3 g−1 at 273 and
298 K under 1 bar (Figure 2). The impressive enhanced uptake
Table 1. Cycloaddition Reaction of CO2 with Styrene Oxide
a
under Various Conditions
b
entry
1-Eu (mg)
TBAB (mol %)
T (°C)
yield (%)
1
2
3
4
5
6
7
40
40
40
40
40
0
2.5
2.5
2.5
5
5
5
60
70
80
60
70
80
80
39
60
72
75
99
38
5
40
0
a
Reaction conditions: 20 mmol of styrene oxide, 40 mg of 1-Eu
b
1
catalyst, solvent-free, CO2 (1 atm), 24 h. Checked by H NMR and
GC−MS with n-dodecane as the internal standard.
with slight modifications, solvent-free phenylethylene carbo-
nate was collected with a yield of 39% from the starting
reactant of styrene oxide under the conditions of 1 atm of CO2,
40 mg (1.26 mol %) of activated 1-Eu catalyst, and a 2.5 mol %
(16 mg) cocatalyst of tetrabutylammonium bromide (TBAB)
at 333 K (60 °C) (entry 1, Table 1). Then, with an increase of
the temperature, the yield ascended gradually (entries 2 and 3)
and reached 72% at 353 K (80 °C). Therefore, in order to
avoid excessive energy consumption at high temperature, the
quantity of the cocatalyst TBAB was doubled in the following
reaction. As exhibited in entries 4 and 6, the transformation of
styrene oxide to phenylethylene carbonate could rise to 75% at
333 K and be nearly stopped with a yield of 99% at 353 K
(entry 5). For comparison, other reaction conditions were the
same as that of entry 6 except for the absence of catalyst 1-Eu,
and the yield was only 38%, indicating that 1-Eu had an
essential catalytic role in the cycloaddition reaction of CO2 and
epoxides. Therefore, the optimal dosages of 1-Eu and TBAB
were set at 40 mg (1.26 mol %) and 5 mol % (32 mg),
respectively, along with the conditions of 1 atm of CO2, 353 K
(70 °C), and 24 h. It is worth mentioning that cycloaddition
reactions catalyzed by Ln-MOFs have been comparatively
scarcely covered so far.58
To test the catalytic universality of 1-Eu, a series of
industrialized cycloaddition reactions from the typical epoxide
substrates were carried out by employing the concluded
optimal conditions. As shown in Table 2, the catalytic
generality of the activated 1-Eu framework was confirmed by
the high yields of the cycloaddition transformation for six
selected epoxide regents. Moreover, it is worth discussing that
the high polarity of the substituents had a greatly adverse
influence on the reaction rate and yield, which was confirmed
by two sets of experimental comparisons including entries 1
and 2 and entries 3 and 4. Meanwhile, the larger substituents
were not conducive to the reaction, which could be verified by
entries 1 and 3 and entries 5 and 6. In short, the catalytic
Figure 2. CO2and CH4 sorption isotherm of 1-Eu at 273 and 298 K.
value at 298 K was comparable to some famous MOF-based
adsorbents, such as BUT-11 (53.5 cm3/g),48 MIL-100(Fe)
(50.2 cm3 g−1),49 etc. Moreover, the CO2 isosteric heat of
adsorption (Qst) fitted by using the virial equation (Figure S5)
for the CO2 isotherms measured at 273 and 293 K was 26.75
kJ mol−1 at zero loading and equivalent to the values of MAF-
25 (26.3 kJ mol−1),50 InOF-1 (29 kJ mol−1),51 etc. It should
be noted that the strong charge-density cations of EuIII in 1-Eu
serving as the Lewis acidic sites will play an important role in
host−guest interaction, as discussed by Zhao et al.52
Meanwhile, there was evidence that the exposed coordinated
oxygen atoms as unorthodox Lewis basic sites also could form
stronger dipole−dipole interactions with the CO2 quadru-
pole.53−57 However, there was an indispensable fact that the
comparatively low amount of CH4 also could be adsorbed with
the values of 17.3 and 9.8 cm3 g−1 at 273 and 298 K,
respectively. In order to check the adsorption selectivity
between CO2 and CH4, the ideal adsorbed solution theory
(IAST) calculation by the dual-site Langmuir−Freundlich
equation for fitting the single-component adsorption isotherm
parameters determined that the selectivity coefficient of CO2/
CH4 (50:50, v/v) was 14 in at 100 kPa under 298 K, which
proved that the separation of CO2 from natural gas was quite
straightforward and feasible (Figure S6).
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ate.59,60 Thus, along with the successive liberation of the cyclic
carbonate, the catalysts of 1-Eu and TBAB returned to the
original state.
Table 2. Cycloaddition Reaction of CO2 and Various
Epoxides with 1-Eu as the Catalyst
a
It is worth mentioning that the activated 1-Eu displayed a
better catalytic performance than the documented Eu3+−
organic framework of [Eu(BTB)(phen)] (phen = 1,10-
phenanthroline), which was constructed from the single
SBUs of [Eu2(phen)(CO2)6] and 1,3,5-benzenetricarboxylic
acid.52 We speculated that this may be attributed to the
adverse effect of steric hindrance from phen, which in the
meantime reduced the polarization ability of Eu3+ as the
Brønsted acid to CO2 molecule.61 Furthermore, the content of
Eu3+ in the activated 1-Eu sample was 28.8% and much higher
than that in [Eu(BTB)(phen)] (15.3%). Furthermore, the
catalytic performance displayed by 1-Eu was better than that of
some of the known porous MOFs self-assembled from 3d
series metal ions, as listed in Table S3. Thus, an excellent
catalytic platform should not only have high porosity and large
specific surface area but also possess smaller steric hindrance
around active open metal sites and high metal content. So, we
sincerely hope that keeping these factors in mind can make us
more effective at constructing porous catalytic MOFs in the
future.
Fluorescence Characteristics. Ln-MOFs endowed with
partial filling of the 4f shells of Ln3+ were proven to own
excellent luminescent properties including large Stokes shifts,
long luminescence lifetimes, and sharp characteristic emissions,
especially for Eu/Tb-MOFs. Therefore, the solid-state
luminescence spectra of 1-Eu and 1-Tb were investigated at
room temperature. The emission spectra of 1-Eu excited at
320 nm exhibited four distinct characteristic peaks of 591, 613,
651, and 700 nm (Figure S10b), which should be assigned to
a
Reaction conditions: substrates (20 mmol), NBu4Br (5 mol %), 1-
b
5
7
5
7
5
7
5
7
Eu catalyst (40 mg), CO2 (1 atm), 70 °C, 24 h. Determined by GC−
MS with n-dodecane as the internal standard.
the D0 → F1, D0 → F2, D0 → F3, and D0 → F4
transitions of the Eu3+ ion. The high (5D0 → 7F2)/(5D0 → 7F1)
luminescence intensity ratio confirmed that Eu3+ cations were
located at nonconversion coordination centers, consistent with
the SCXRD data. However, for 1-Tb, the emission spectra of
1-Tb excited at 320 nm displayed four completely different
characteristic peaks of 489, 546, 585, and 621 nm (Figure
activities of activated porous MOFs for the transformation of
epoxides were affected by the electron-withdrawing altitude of
the polar substituents, steric hindrance of the substrates, and
reaction conditions.
5
7
S10c), which caustically corresponded to D4 → FJ (J = 6, 5,
4, and 3) diagnostic transitions of Tb3+ cations. The strongest
emission peak at 617 nm induced from the 5D0 →7F2
electronic transition enabled 1-Tb to emit bright-green
luminescence upon UV-light irradiation. In order to further
explore the photophysical behavior of 1-Eu and 1-Tb, the
luminescence lifetimes (τ) were estimated from the lumines-
cent decay curve. As shown in Figure S11, the decay curves of
both 1-Eu and 1-Tb could be well fitted by the single-
exponential function as I(t) = I0 exp(−t/τ), with lifetimes of
854 and 135 μs (Ex = 300 nm), respectively, which were
comparable to those of the documented EuIII/TbIII-MOFs.
Furthermore, the absolute quantum yields (Φ) were calculated
as 82.32% and 18.95%, respectively. As far as we know, 1-Eu
exhibited the highest quantum yield in the solid state
compared to all documented EuIII-MOF materials,62−64
which could partly confirm the highly effective ET from the
PTTBA6− ligand to Eu3+ ions in the nanocaged architecture of
1-Eu. Meanwhile, the missing characteristic emission peak of
free H6PTTBA (450 nm; Figure S10a) in 1-Eu and 1-Tb
proved that the effective ET process increased from organic
ligands to Ln3+ cations rather than intraligand n−π or π−π*
electron transitions. The triplet (T1) energy for the H6PTTBA
ligand derived from the phosphorescence spectra (Figure S12)
In addition, as one negative heterogeneous catalytic
platform, 1-Eu could be easily recycled by the processing
operation of centrifugal precipitation, filtration, DMF/MeOH
cleansing, and drying. As displayed in Figure S7, there was no
obvious decrease in the catalytic efficiency for the cyclo-
addition reaction of styrene oxide and CO2 after 1-Eu was
reused many times. The characteristics peaks of PXRD for the
purified 1-Eu sample were nearly in accordance with the
simulation results from the single-crystal structure data (Figure
S9), confirming that the catalyst of 1-Eu possessed a renewable
performance without damage to the integrity of the framework.
In view of the structure characteristics of 1-Eu and related
works documented so far, the catalytic mechanism for the
cycloaddition of epoxides with CO2 was speculated, as
exhibited in Figure S8. First, CO2 and epoxides could easily
get into the 1D channels and concentrate the cagelike void
volume of 1-Eu. Second, epoxide molecules were slightly
constrained by the bare electron-free orbitals of the Eu3+
cations by way of van der Waals while the polarized epoxy
ring was activated. Finally, the nucleophilic anion of Br−
offered by the cocatalyst of TBAB assaulted the less-hindered
carbon atom to result in the intermediate alkylcarbonate anion,
which preferably attacked the closest carbon atom of the
polarized CO2 molecule, generating the salts of alkylcarbon-
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of 1-Gd at 77 K was calculated to be 20858 cm−1 (480 nm),
whereas for 1-Eu, the energy gap ΔE(T1−5D0) of 3558 cm−1
was in the range of 2500−4000 cm−1, which confirmed that 1-
Eu possessed the ideal limit value according to Latva’s
empirical rule. Furthermore, the energy difference of 1-Tb
[ΔE (T1−5D4)] isolated from the ideal limit area was
calculated as 358 cm−1, which showed that the H6PTTBA
ligand could sensitize the ion of Eu3+ far more sufficiently than
the ion of Tb3+. Thus, the higher quantum yield of 1-Eu
relative to that of 1-Tb should be attributed to the more
effective ET process.64
Tuning of the Luminescent Color for Bimetallic
Doped 1-EuxTb1−x. Isomorphic features of 1-Eu and 1-Tb
prompted us to investigate the tunable luminous characteristics
of 1-EuxTb1−x. Samples of 1-EuxTb1−x synthesized from the
incremental ratio of Eu2O3/Tb2O3 under similar hydrothermal
reaction conditions were determined by the ICP measure-
ments, as listed in Table S4. Furthermore, the PXRD patterns
confirmed that the obtained crystalline samples of doped 1-
EuxTb1−x were well consistent with 1-Eu and 1-Tb (Figure
S13), indicating that the coordination behavior transmitted
smoothly from a monometallic structure to a doped bimetallic
structure. A gradient color change with the increasing ratio of
Eu3+/Tb3+ irradiated at 268 nm from light green to orange to
red occurred, as shown in Figure S14. As shown in Figure 3,
Luminescence Sensing. In order to explore the
fluorescence applications of the high robust platform of 1-
Eu, the sensitive response to a series of some guest cations was
fulfilled and the detailed manipulations were carried out as
follows: 2 mg of the 1-Eu sample was uniformly dispersed in 3
mL of a M(NO3)x aqueous solution (M = Li+, K+, Na+, Ca2+,
Mg2+, Mn2+, Ba2+, Cd2+, Zn2+, Hg2+, Cu2+, Co2+, Ni2+, Cr3+,
Al3+, and Fe3+) and ultrasonicated for 30 min. Under the
interfere of metal cations, the recorded luminescence
intensities of the above resultant M@1-Eu suspensions showed
that only the dominant emission peak (613 nm) was observed,
as shown in Figure 4. With the addition of alkali-metal cations
Figure 4. Luminescence intensity of 1-Eu for different metal ions.
of Li+, Na+, and K+, it was obvious that only negligible changes
of the luminescence intensity could be traced. As for the
alkaline-earth cations (Mg2+ and Ca2+), transition-metal ions
(Cd2+, Mn2+, Co2+, Zn2+, Ni2+, Hg2+, and Cu2+), and trivalent
ions (Cr3+ and Al3+), a comparatively slight fluorescence
quenching rate could be observed. Comparatively speaking, the
cations of Fe3+ caused almost complete luminescence
quenching to the red emission peak of 613 nm devoted by
1-Eu. Furthermore, competitive experiments in equal propor-
tion for the above-mentioned cations to Fe3+ ions were
performed and showed that other metal ions had no significant
effect on the Fe3+-induced luminescence response, which
revealed that 1-Eu could efficiently sense Fe3+ ions in a water
solution without interference (Figure S15).
Figure 3. Luminescence spectra of doped 1-EuxTb1−x (x = ratio of
Eu3+; λex = 320 nm).
Consequently, quantitative luminescence titration experi-
ments were fulfilled to explore the sensing sensitivity of 1-Eu
toward Fe3+ ions in an aqueous solution. As illustrated in
Figure S16, the fluorescent intensity of 1-Eu decreased
gradually with an increased concentration of Fe3+ ions in an
aqueous solution. Fluorescent intensity changes were recorded
with a gradual increase in the emission intensity of the Eu3+ ion
5
7
at 617 nm determined by the transition of D0 → F2, the
Tb3+-dominated D4 → F5 peak at 547 nm consequently
receded, which proved that the systematically modulated
characteristic emission could be realized by adjusting different
ratios of the Eu3+ and Tb3+ contents within a range of 7−37%.
On the basis of documented mixed Eu3+/Tb3+−organic
platforms, the disproportionate and rapid change of the
dominated emission peak with the enhanced ratio of Eu3+
was ascribed to ET from Tb3+ to Eu3+, whose ET efficiency
5
7
along with the 5D0
→
7F2 emission variations, and the
fluorescence quenching efficiency was quantitatively analyzed
by using the Stern−Volmer equation. As shown in Figure S17,
the global relationship between I0/I (I0 and I were the
luminescence intensities before and after the addition of an
analyte, respectively) and the concentration of the Fe3+ ion
([Fe3+]) could not match the Stern−Volmer equation, as
explained by the coexistence of a static and dynamic quenching
process, but was well fitted with the following equation: I0/I =
0.871 exp([Fe3+]/0.039) + 0.412. Meanwhile, a good linear
correlation was observed for the plot of I0/I = 1 + KSV [Fe3+] in
the range of 0−0.4 mM, with the KSV value of 1-EU for Fe3+
ions being 4.75 × 104 M−1, which should be attributed to self-
absorption or an ET process, as shown in Figure S18. On the
basis of the Ksv value and standard deviation (δ), the limit of
(ηET) was well in accordance with the classical equation ηET
=
1 − τ/τ0, where τ and τ0 were the 5D4 lifetime of Tb3+ ions in
the presence and absence of doped Eu3+ ions. Moreover, as
shown in Table S4, there is a linear correlation between the ET
efficiency (ηET) increase and the contents of the Eu3+ ions with
the molar ratio in the range of 15.56−45.93%, which
confirmed that doped Eu3+/Tb3+ frameworks with adjustable
emission colors could be taken as one good substitute for
barcode materials.
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Article
detection for Fe3+ ions was estimated to be 6.32 × 10−6 M by
the ratio of 3δ/KSV and much lower than most of the reported
EuIII-MOFs for Fe3+ detection.65−67 In the meantime, PXRD
analysis of the samples soaking in a Fe3+ aqueous solution
confirmed that the fluorescence quenching could not be
attributed to the framework collapse of 1-Eu, as shown in
Figure S9. Meanwhile, elemental analysis showed that there
was no obvious change for the proportion of the EuIII content,
which confirmed that the fluorescent quenching was not
caused by ion exchange. On the basis of the documented
devotions, the fluorescent quenching mechanism should be
attributed to the fact that the strong competitive UV−vis
absorption of FeIII ions in the range of 320−400 nm led to the
luminescence quenching effect (Figure S19).68−70 By the way,
Ru3+ ion with an outer electronic configuration similar to that
of Fe3+ ion in a aqueous solution also played an adverse
influence on the luminescence intensity of 1-Eu but
considerably lower than that of Fe3+ ion, as shown in Figure
S20.
of China; orcid.org/0000-0002-5654-7510;
Email: xiutangzhang@163.com
Authors
Hongtai Chen − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s Republic
of China
Liming Fan − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s Republic
of China
Hongxiao Lv − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s Republic
of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01782
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
CONCLUSIONS
■
This work was financially supported by the Natural Science
Foundation of China (Grants 21101097 and 21801230) and
the Opening Foundation of Key Laboratory of Laser &
Infrared System of Shandong University (Grant 2019-LISKFJJ-
005).
In conclusion, the acidic solvothermal self-assembly of Ln2O3
and 4,4′,4″-(pyridine-2,4,6-triyl)tris(1,3-benzenedicarboxylic
acid) (H6PTTBA) in a mixed solvent of DMF and H2O
generated a highly robust multifunctional {LnIII2}LnIII−organic
framework, which featured the intrinsic characteristics of
interconnected nanocaged void volume, large permanent
porosity, high specific surface area, and high thermal and
water stability. Performance analyses exhibited that the
representative of Eu-based 1-Eu could efficiently separate the
mixed gases of CO2/CH4 (50:50, v/v) and catalyze the
chemical transformation of epoxides and CO2 into the
corresponding carbonates. In the meantime, adjustable
emission colors devoted by the efficient Tb3+ → Eu3+ ET
confirmed that the Eux/Tb1−x−organic framework could be
taken as a good substitute for barcode materials by changing
the ratio of Eu3+ and Tb3+. Moreover, 1-Eu could be taken as
one fluorescent probe for Fe3+ in an aqueous solution by
fluorescence quenching.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01782.
■
sı
Crystallographic data and refinement parameters,
comparison of catalytic cycloadditions of CO2, TGA
curves, PXRD patterns, N2 absorption/desorption
isotherms, recyclability study, catalytic mechanism,
luminescence emissions and decay curves, emission
intensity and spectra, and UV−vis absorption (PDF)
Accession Codes
CCDC 1997911 and 2008436−2008442 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
Xiutang Zhang − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s Republic
■
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