Rotation Restricted Emission and Antenna Effect in Single Metal-Organic Frameworks

Article abstract of DOI:10.1021/jacs.9b06755

Aggregation induced-emission (AIE) and antenna effects are important luminescence behaviors. Thus, investigating their emission mechanisms and revealing their behaviors have become critical but challenging. Here we design and prepare metal-organic framewo

Full text of DOI:10.1021/jacs.9b06755

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Article
Rotation Restricted Emission and Antenna
Effect in Single Metal-Organic Frameworks
Hua-Qing Yin, Xin-Yao Wang, and Xue-Bo Yin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06755 • Publication Date (Web): 06 Sep 2019
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Rotation Restricted Emission and Antenna Effect in Single Metal-
Organic Frameworks
†
†
*,†
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Hua-Qing Yin , Xin-Yao Wang , Xue-Bo Yin
†
State Key Laboratory of Medicinal Chemical Biology and Tianjin Key Laboratory of Biosensing and Molecular
Recognition, College of Chemistry, Nankai University, Tianjin, 300071, China
KEYWORDS: Metal organic frameworks; Rotation restricted emission; Aggregation induced-emission; Coordination
induced emission; Antenna effect
ABSTRACT: Aggregation induced-emission (AIE) and antenna effects are important luminescence behaviors. Thus,
investigating their emission mechanisms and revealing their behaviors have become critical but challenging. Here we design
3+
and prepare metal-organic frameworks (MOFs) with an AIE ligand (i.e., tetrakis(4-carboxyphenyl)pyrazine (L1)) and Ln
3+
3+
3+
ions (including Eu , Tb , and Gd ). The emission from L1 is gradually enhanced during the formation of the MOFs because
coordination restricts the intramolecular rotation. Thus, the emission is called as coordination-induced emission (CIE) with
the same restriction of intramolecular rotation mechanism as AIE. Meanwhile, benzene rings twist to adapt to the MOFs’ rigid
structure, so the emission blueshifts gradually, as an additional evidence of CIE. Both AIE and CIE are “rotation-restricted
3
+
emission (RRE)”. Eu ions exhibit the strongest emission with gradually enhanced intensity during the formation of L1-Eu
3
+
3+
MOF. Combining with emission properties from Tb and Gd ions, the antenna effect is verified. We also validate the
3
+
conditions for the efficient sensitization of Ln ions experimentally and refresh the threshold value of the energy gap between
3
+
-1
triplet state of a ligand and excited state of Ln ions to 3,000 cm . Thus, RRE and antenna effect are revealed and validated
3
+
simultaneously. Because CIE of L1 and antenna effect emission from Eu ions are enhanced simultaneously as strong dual
emissions, ratiometric fluorescence detection is realized with the detection of arginine as a model. Our results incorporate
AIE and CIE into RRE, which provides explicit information for the construction and application of emission systems with AIE
ligands as building blocks. MOFs are also extended to explore the emission mechanism and the energy transfer between
ligands and metal ions.
INTRODUCTION
Antenna effect is often observed from lanthanide
1
2-14
complexes.
Organic ligand acts as “antenna” to absorb
Aggregation-induced emission (AIE) is an attractive
phenomenon as the materials exhibit strong luminescence
in the aggregated or solid states, whereas the emission
photons and produces triplet state through intersystem
1
5
crossing from its singlet state. The triplet state sensitizes
3
+
1
-3
lanthanide (Ln ) ions to ensure a brighter emission than
disappears in their dissolved state. Tang and coworkers
1
6,17
free ions.
The energy gap between singlet and triplet
investigated the phenomenon and mechanism of AIE
2
states of a ligand is critical for efficient emission, whereas
the gap between the triplet state of a ligand and the excited
systematically. Generally, AIE luminogens ( AIEgens ) have
a propeller-shaped molecular structure and have been
3+
18,19
4
5
state of Ln ions is also strictly limited.
Various
developed
with
silole,
tetraphenylethene,
6
7
lanthanide metal-organic frameworks (Ln-MOFs) have
been developed and antenna effect luminescence is well
retained with large Stokes shift, bright visible emission, and
distyrylanthracene, and triphenylethene. In the solution
state, AIEgens undergo intramolecular rotation. Thus, their
excited states suffer from non-radiative annihilation, which
leads to weak emission and even non-luminescence.
However, the rotation is considerably restricted in solid
2
0-22
long lifetime.
MOFs are the hybrid assembly of metal
2
3,24
ions and polydentate ligands.
Ligands are fixed in the
8,9
frameworks with metal ions as ligand-capturers.
state or poor solvents. The restriction of intramolecular
rotation (RIR) enhances the emission as rotation-restricted
After AIEgens, such as tetraphenylethylene, are modified
by carboxyl or pyridyl groups, they are used as ligands to
1
-3
emission (RRE). AIEgens overcome aggregation caused
quenching (ACQ) of molecular dyes for various
2
5-35
prepare emissive MOFs.
Tetraphenylethylene-based
3
,10,11
25-30
applications.
Real-time monitoring of the RIR
emissive MOFs have been used for detection purpose
2
6,27,31
procedure of AIEgens helps reveal the rotation-restricted
behavior. However, it is challenging and becomes urgent
because any factor that leads to RIR could enhance the
emission from AIEgens, which could be used to design
bright emission systems.
and as white LED materials.
Dincă et al investigated
the strain energy and fluorescence behavior of AIE ligands
3
2-34
in MOFs.
The rigid structure of MOFs restricts the
rotation of benzene rings and achieves higher emission than
3
5
free ligands. However, the above MOFs only show a single
emission from the ligands.
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tetraphenylpyrazine and Thus, the emission is denoted as CIE, different from AIE in
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tetraphenylethylene, with four carboxyl groups, to form 2,
3, 5, 6-tetrakis(4-carboxyphenyl)pyrazine (L1) and
tetrakis(4-carboxyphenyl)ethylene (L2) (Figure 1). During
poor solvent. Moreover, the peak emission shifts from 428
nm before reaction to 414 nm after 2 h for L1-Eu system.
About 14 nm blueshift was observed for L1-Ln systems
(Figure 2b-2d, 3a, and S1). The same UV-Vis absorption
3
+
3+
the formation of MOFs by L1 and Ln ions (including Eu ,
3
+
3+
3+
Tb , and Gd ), the emission from L1 was enhanced and
blueshifted gradually, different from that at fixed
wavelength in solid state or poor solvents. Coordination
enhances the emission from L1, so the emission is denoted
as coordination-induced emission (CIE). CIE is also RRE
spectra are observed from the L1-Ln systems (Figure S2).
3
+
Both L1 and Ln ions show high coordinate valence. The
plane structure of L1 twists to adapt the coordination for
the blue-shifted emission as an additional evidence of CIE.
The emission of solid L1 was observed at 430 nm (Figure
S3), so the blueshift of L1 was induced by coordination but
not simple irregular aggregation in solid state. The
characteristic peak of –COOH at 13 ppm decreased
3
+
with the same RIR mechanism as AIE. Different Ln ions
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show different emission behaviors. Eu ions exhibit strong
emission and the intensity is enhanced gradually during the
formation of L1-Eu MOF. Meanwhile, weak emission is
3+
1
gradually after the addition of Eu ions as revealed from H
NMR spectra (Figure S4), so the coordination occurred in
the system as the hydrogen ions were removed. For the first
time, real-time RIR-induced emission is observed through
the coordination process. We will further discuss the
structure influence of L1 and MOFs on their emission
behaviors.
3+
3+
observed from Tb ions and non-emission from Gd ions.
3
+
Combining with the energy levels of L1, L2, and the Ln
ions, the antenna effect is validated. CIE and antenna effect
emission from L1-Eu MOF are enhanced simultaneously as
strong dual emissions. However, only antenna effect
3
+
emission from Eu ions is exhibited if 1, 4-terephthalic acid
L3) is selected as a ligand. The strong dual emission of L1-
(
Different emission profiles appear at 500-750 nm from
Eu MOF is used for ratiometric fluorescence sensing with
arginine as a model. The color is easily identified with the
naked eyes for visible detection. CIE is a new kind of RRE, so
any interaction that results in RIR could enhance the
emission from AIEgens, which then are used as building
blocks to design and prepare emission systems. MOFs, as a
kind of multifunctional matrix, are also used to explore the
emission mechanism and to reveal the energy interaction
and transfer.
3+
different L1-Ln systems. The characteristic emissions of
3+
Eu ions are well retained with gradually enhanced
intensity along with the reaction (Figure 2b). The emission
3+
from Tb ions is weak and observed after the image is
enlarged (Figure 2c). No emission is observed at 500-750
3+
nm when Gd ions are selected as metal nodes (Figure 2d).
3+
The Eu emission enhancement is about 1.3 times higher
3+
than that of Tb ions as showed in Figure S5. The emission
enhancement of L1 in L1-Gd system is 1.2 times and 1.6
times higher than that of L1-Tb and L1-Eu systems,
RESUILTS AND DISCUSSION
3+
respectively. In L1-Eu system, the Eu ions enhancement is
.7 times higher than L1 enhancement (Figure S6). The
stronger emission from L1 is related to the weaker emission
Dynamical Monitor of CIE and Antenna Effect
Emissions. The fluorescence property of L1 was first
investigated in the water/glycerol system. L1 emission was
enhanced gradually at a constant 412 nm with increased
glycerol content (Figure 2a). Thus, the introduction of
hydrophilic carboxyl groups did not disturb obviously the
AIE effect of L1. Fluorescence profiles of L1 were further
1
3
+
of the corresponding Ln ions in L1-Ln systems (Figure 2b-
3
+
2d and 3b). The emission behaviors of L1 and Ln ions
motivates us to analyze their energy relationship and
transfer.
3
+
3+
3+
recorded in the presence of Eu , Tb , and Gd ions in N, N’-
dimethylformamide (DMF). The emission from L1 was
enhanced gradually even if DMF is a good solvent (Figure
Energy Variety during Formation of MOFs. Figure 3c
3
+
illustrates the energy relationship between L1 and Eu ,
3
+
3+
Tb or Gd ions that induces antenna effect. L1 absorbs
photons and is excited to its triplet state (T ) through
intersystem crossing from its singlet state (S ). Figure 1b
and 1c illustrate its energy level of HOMO and LUMO. The
2
b-2d). The rotatable phenyl rings in L1 are fixed with the
1
coordination procedure to enhance the emission.
1
a
b
3
+
excited state emits its emission directly or sensitizes Ln
L1
L2
L3
1
6
ions. According to Reinhold’s empirical rule, intersystem
crossing is effective when energy gap (ΔE1) between S and
of a ligand is larger than 5,000 cm . Based on the UV-Vis
absorption of L1 (Figure S7a) and phosphorescence of L1-
1
of L1 are 29,412 cm (340
nm) and 24,096 cm (415 nm), respectively, with ΔE1 of
5,316 cm . 14 nm blueshift of emission from L1 is observed
after the reaction. Correspondingly, the ΔE1 increases to
6,048 cm , so we observe variable ΔE1 in single system
during the formation of L1-Ln MOFs. ΔE1 of 5,316-6,048
cm is larger than 5,000 cm , so intersystem crossing of L1
from S to T is efficient for antenna effect procedure.
L1
1
−1
T
1
c
−1
Gd MOF (Figure S7b), S
1
and T
L2
L3
−
1
−
1
−
1
Figure 1 (a) Molecular structures of 2,3,5,6-tetrakis(4-
carboxyphenyl)pyrazine (L1), tetrakis(4-carboxyphenyl)
ethylene (L2), and 1,4-terephthalic acid (L3). (b) Energy
levels of HOMO and LUMO with their difference (ΔE) of
different ligands. (c) Molecular orbital diagrams of LUMO
and HOMO of L1.
−1
−1
1
1
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Figure 2. (a) Fluorescence spectra of L1 in in water/glycerol mixture with different water contents. Fluorescence profiles of
3+
(
5
b) L1-Eu system, (c) L1-Tb system (inset: the enlarged part reveals the characteristic emissions from Tb ions at 493, 549,
3+
89, and 625 nm), and (d) L1-Gd system for the reaction between L1 and the Ln ions at different time. (The fluorescence
intensity of the three L1-Ln systems are measured at 1, 2, 3, 4, 5, 6, 7, 15, 30, 60, and 120 min.)
Figure 3. (a) Wavelength blueshift and (b) intensity variation of the emission from L1 in L1-Ln systems along with the
3
+
3+
3+
reaction. (c) Schematic representation of the energy levels of L1, Eu , Tb , and Gd ions that induce the antenna effect.
Abbreviations: A = absorption; F = fluorescence; P = phosphorescence; L = luminescence from L1-Ln MOFs. ISC = intersystem
crossing; ET = energy transfer; S = singlet; and T = triplet; ΔE1 = energy gap between S and T of L1; ΔE2 = energy gap
of L1 and excited state of Ln ions] (d) Enlargement of pink part in (c) for energy level of L1 along with the
[
1
1
3
+
between T
reaction.
1
-
1
The energy gap (∆E2) between T
excited state of Ln ions also influences the sensitization
efficiency. In L1-Eu system, ∆E2 ranges from 5,919 to 6,596
1
of a ligand and the
cm (Table S1). Previous work illustrated that ∆E2 must be
3
+
-1
higher than 3,500 cm to achieve irreversible energy
transfer to sensitize Ln ions. Thus, Eu ions are
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+
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L1-Gd MOF framework forms with an augmented scu
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efficiently sensitized by L1. Correspondingly, CIE of L1 and
antenna effect emission from Eu ions are dynamically
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topology. Each L1 is connected by eight different Gd ions.
3+
3
+
monitored, simultaneously (Figure 2b). The characteristic
Each Gd node is connected by six oxygen atoms, among
which four atoms are from carboxyl oxygen of L1 and two
atoms are free oxygen. The adjacent Gd nodes are
connected by free oxygen to form a zigzag metal chain
structure. A diamond-shaped channel is observed with the
sizes of 12.1 and 9.3 Å in the frameworks (Figure 4c). The
connection between Gd clusters and L1 constructs a
highly porous framework. L1 molecules twist to adapt the
MOFs structure. Thus, the dihedral angle between pyrazine
core and benzene arms becomes 80.4° in L1-Gd MOF from
0.1° of free L1 (Figure 4d). After coordination of L1 to Ln
ions, the fluorescence is enhanced as CIE. Meanwhile, the
twisted structure of L1 in MOFs achieves the blueshift of 14
nm.
3
+
emissions from Tb ions are observed but lower than that
3
+
3+
from Eu ions (Figure 2c). ΔE2 between T
1
of L1 and
3
+
-1
excited state of Tb ranges from 2,864 to 3,596 cm during
the reaction (Table S1). Energy transfer efficiency is
improved with the increased ΔE2. The emission from Tb
3
+
3
+
ions is enhanced gradually, although the emission is very
weak. After the formation of rigid topology in L1-Eu and L1-
Tb systems, the rotation of benzene rings is considerably
restricted.
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To further validate the threshold value of ∆E2, the
3
+
emission behavior of L2 was tested in the presence of Ln
ions. The structure and energy level of HOMO/LUMO of L2
are illustrated in Figure 1. Emission from L2 is enhanced
and blueshifted gradually as CIE (Figure S8a). S1 of L2 is
The Fourier transform infrared spectra of L1 and three
kinds of MOFs are recorded (Figure S11). The absorption
bonds in MOFs at approximately 1,710 cm illustrate that
L1 is partly deprotonated. The absorption bonds indicate
-1
26
2
8,571 cm (Figure 1), while its T
1
3+
is 20,499 and 21,459
-1
−1
cm before and after reacting to Eu ions (Figure S8). ∆E1
of L2 during the formation of L2-Eu MOF is 7,112-8,072
cm , so its intersystem crossing from S
−
1
3+
1
to T
1
is efficient.
the formation of MOFs by coordinating of −COOH and Ln
3
+
∆E2 between L2 and Eu ions ranges from 2,949 to 3,959
ions. Thermogravimetric analysis reveals that the MOFs are
stable until 400 °C. Initially 10% of the residual solvents in
channels are lost. Then, the MOFs collapse beyond 400°C
with the loss of L1 ligand to remain the metal oxides (Figure
S12). The Brunauer-Emmett-Teller (BET) surface area of
L1-Eu MOF is 480.42 m²/g and the pore size is
approximately 11.2 Å, which is consistent with the
simulated crystal structure (Figure S13). After soaking in
water, and pH 11 or 3 solutions for 24 h, the PXRD
characteristic peaks of L1-Eu MOF are still stable (Figure
S14), indicating that it is stable enough for real application.
cm (Table S1). The weak emission from Eu3+ ions is
attributed to the limited sensitization, although the
emission is enhanced gradually (Figure S8d). Both
examples of L1-Tb and L2-Eu illustrate that, when ∆E2 is
-1
-
1
3+
larger than 3000 cm , the Ln ions are sensitized for
3
+
3+
antenna effect emission effectively. Tb and Gd ions with
higher energy level cannot be sensitized by L2 so that L2-
Tb and L2-Gd systems only show single emission from L2
with increased intensity as CIE (Figure S8). Thus, we
3+
validate the conditions for the efficient sensitization of Ln
ions experimentally and refresh the threshold value of ∆E2
to 3,000 cm by the dynamical monitoring method. Ln3+
ions, combined with AIE ligands, are used to validate the
mechanisms of RRE and antenna effect emission.
-1
To illustrate the merits of Ln3+ and AIE ligand for the
preparation of emissive MOFs, the fluorescence profile of
3
+
terephthalic acid (L3) is recorded in the presence of Eu ,
3
+
3+
3+
Tb , and Gd ions. The characteristic emissions of Eu and
3
+
Tb ions are well observed with antenna effect procedure
Figure S9a and S9b). However, the emission from L3
(
decreases gradually with low intensity. Without CIE
property, L3 shows a lower emission than L1 and L2 in L1-
3
+
Gd and L2-Gd systems. Thus, Ln ions, combined with AIE
ligands with suitable energy level, could be used to prepare
MOFs with strong dual emissions from both CIE and
antenna effect.
Preparation and Characterization of L1-Ln MOFs. The
colorless MOFs crystals are obtained through the
3+
3+
solvothermal reaction (Figure S10). L1 and Eu , Tb , or
3+
Gd are mixed in DMF/ethanol/water system with the help
of formic acid after heated at 100 °C for 48 h to obtain their
crystals (the detail is listed in Supporting Information). The
three MOFs show the PXRD patterns that are similar to each
other, illustrating their same topological structures (Figure
4a). Thus, L1-Gd MOF is tested to reveal the structure with
single-crystal X-ray diffraction analysis. L1-Gd MOF
crystallizes in an orthorhombic space group. The
framework contains 8-c rectangular organic linker and 6-c
Figure 4. (a) Stimulated L1-Ln MOFs and experimental
PXRD patterns of L1-Eu MOF, L1-Tb MOF, and L1-Gd MOF.
(
6
b) 8-connected L1 (top) and GdO cluster (bottom). (c)
Pore sizes and 1D channel in L1-Gd MOF. (d) Structure of
free L1 with twist angle of 20.1° (top) and L1 in L1-Gd MOF
with twist angle of 80.4° (bottom) between pyrazine core
and benzene arms.
6
tetranuclear GdO cluster (Figure 4b). The (8, 6)-connected
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Optical Property of L1-Eu MOF. L1 shows a broad
Notably, temperature influences energy transfer
efficiency. Thus, temperature-dependent fluorescence is
used to verify the AIE property of L1-Eu MOF. The emission
from nanosheets increases with the decrease in
temperature (Figure S22). At low temperature, the benzene
rings’ rotation was restricted, the thermal relaxation
decreased partly to improve the emission from L1. L1 is
already fixed partly in MOFs, so the fluorescence
enhancement (1.7-fold) is lower than that of free L1 (more
1
2
3
4
5
6
7
8
9
absorption between 290 nm and 390 nm in the UV-Vis
absorption spectra. However, the absorption shifts to 265-
330 nm in L1-Eu MOF (Figure S15). The emission from L1
is observed at 429 nm, but blueshifts to 415 nm after the
formation of L1-Eu MOF because of the twisted structure of
L1 in MOFs. Moreover, the excitation spectra are the same
at approximately 250-400 nm (Figure S16 and S17). Strong
emission is observed at 630 nm from Eu3+ ions in L1-Eu
MOF with the excitation ranged from 250 nm to 400 nm.
3
7
3+
than 5-fold). The emission from Eu ions also shows a
twofold enhancement. Intermolecular rotation-induced
thermal dissipation decreases at low temperature, so
3
+
Thus, the emission from Eu ions is related to L1. That is,
1
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+
L1 is excited and then sensitizes Eu ions with the antenna
3+
effect procedure.
energy transfer is improved from L1 to Eu ions, whose
emission is enhanced. Thus, structure-dependent AIE
property is observed.
To determine the optimal excitation and quantum yield of
L1-Eu MOF, different excitation wavelengths from 240 nm
to 320 nm is tested. 280 nm excitation provides the
strongest fluorescence for L1-Eu MOF (Figure S18a). The
Application of L1-Eu MOF for Ratiometric Detection.
Ratiometric fluorescence detection improves the signal-to-
noise ratio and decreases the effect of environmental
3
D emission profile of L1-Eu MOF illustrates the same result
3 8 , 3 9
40
that 280 nm excitation is optimal for efficient emission
(Figure S18b). The absolute fluorescence quantum yield (Φ)
of solid L1 is 12.9% under ambient temperature, but
increases to 60.8% observed from L1-Eu MOF. Thus,
fluorescence is obviously improved for both CIE and
antenna effect in MOFs as strong dual emissions. The
lifetime of the emission at 620 nm is 134 µs (Figure S19).
Given that L1-Eu MOF shows strong dual emissions under
single excitation, we explore its fluorescence property for
ratiometric fluorescence sensing.
factors.
Amino acids are essential to the organism.
Sensing of amino acids play an important role in disease
4
1
42
diagnosis, such as Alzheimers, liver cirrhosis, and
4
3
pancreatitis. Arginine is associated with a variety of
diseases such as cardiovascular and cerebrovascular
diseases, hepatopathy, and hyperammonemia.
4
4,45
Thus, a
sensitive and selective method for the detection of arginine
needs be developed.
L1-Eu MOF nanosheets exhibit blue and red dual
emissions under 280 nm excitation. The pyrazine core of L1
would interact with the guanidinium group on arginine.
Thus, L1-Eu MOF nanosheets were selected as ratiometric
fluorescence probe for the detection of arginine. After
soaked in arginine solution for 24 h, L1-Eu MOF retains its
characteristic PXRD peaks, indicating that it is stable
enough for the detection of arginine (Figure S23). After the
addition of arginine, the fluorescence of L1-Eu MOF
nanosheets become stable after 10 minutes (Figure S24).
The blue emission from L1-Eu MOF is enhanced, but red
emission slightly decreases (Figure 6a). A good linear
response is observed between the ratio of I415/I620 and the
concentration of arginine (Figure S25). L1-Eu MOF is pink,
but the color shifts to blue with the increase of arginine
content and is easily identified by the naked eyes (Figure 6b
and 6c). L1-Eu MOF shows obvious turn-on response
without background interference. The detection of limit got
To analyze the structural effect of MOFs on their optical
behaviors, we prepared L1-Eu MOF with different
morphologies (Figure 5a-5f) by tuning the ratio of DMF or
N, N’-dimethylacetamide (DMA)/ethanol/water (detail in
Table S2). Their PXRD patterns are coincident to each other
(
Figure 5g). Thus, all of the L1-Eu MOF possess the same
topology. The fluorescence profiles are recorded for
different L1-Eu MOFs at the same concentration (Figure
5h). The emission from nanosheets MOF is the strongest.
Meanwhile rod-like MOF shows higher emission than bulk
and spherical MOFs. Then, we explore the AIE property of
L1-Eu MOF by comparing the emission from nanospheres
and nanosheets in DMF with different water contents
(
Figure S20). Blue emission from nanosheets is enhanced
obviously compared with that from nanospheres with
increased water contents. There are uncoordinated L1
exposed on the surface of the nanosheets. With water
content increased, the flexible and soft L1-Eu MOF
nanosheets aggregated to restrict the intramolecular
rotation of L1, so the emission was enhanced. The
structure-dependent AIE property is demonstrated because
more L1 is exposed on the surface of nanosheets, which
have higher AIE activity than nanospheres (Figure S21).
with LOD=kS
recommended in spectral analysis, and S
b
/a, where LOD is the detection of limit; k=3 as
is the standard
b
deviation of the blank signal and a is the sensitivity of the
46
sensor. The detection limit of 15 nM is lower than or the
47-49
same as that of other fluorescence methods.
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Figure 5. High-resolution transmission electron microscopy (HRTEM) images of L1-Eu MOF with different morphologies: (a)
sphere, (b) nanosphere, (c) bulk, (d) rod, (e) nanorod, and (f) nanosheet. (Scale bar: a, d: 500 nm; b, e: 100 nm; c: 1 μm, and f:
2
00 nm). (g) PXRD patterns and (h) fluorescence spectra of different morphology L1-Eu MOF. Inset in (h): photographs of
different L1-Eu MOF under 302 nm excitation, from left to right: a-f.
To explore the selectivity of L1-Eu MOF to arginine,
competition experiments are conducted for other amino
acids. As shown in Figure S26, the emission responding to
arginine is unaffected, except for tryptophan and lysine
because of their natural fluorescence at 390 and 430 nm,
respectively. However, their natural fluorescence ensures
that they can be easily distinguished (Figure S27).
To better understand the detection mechanism of L1-Eu
MOF to arginine, L1, TCPB-Eu MOF prepared with 1, 2, 4, 5-
3+
tetrakis(4-carboxyphenyl)benzene, and Eu are used to
record their fluorescence responses to arginine with L2-Eu
MOF as a reference. The difference between TCPB and L1 is
that the pyrazine core is replaced by benzene ring.
However, TCPB-Eu MOF exhibits a weak response and L2-
Eu shows no response to arginine (Figure S28 and S29),
whereas free L1 shows a good linear response to arginine
Figure S30). That is, L1 responds to arginine in L1-Eu MOF.
The pKa of arginine is 12.48, so it is positive and acts as
electrophile under acidic, neutral, and alkaline conditions.
Multiple hydrogen bonds form between lone electron pair
of nitrogen on L1 and guanidyl of arginine. No obvious
difference was observed between pure L1 and L1+arginine
from their UV-Vis spectra (Figure S31). Only alkyl hydrogen
(
1
but no guanidyl hydrogen were observed from H NMR
Figure 6. (a) Fluorescence profiles of L1-Eu MOF
responding to arginine with different concentrations. (b)
Commission International de L’Eclairage of fluorescence
spectra of (a). (c) Photographs of L1-Eu MOF in the
presence of arginine with different concentrations under
285 nm excitation. (d) Arginine blocks into the pores of L1-
Eu MOF and interacts with nitrogen through hydrogen
bonds. (e) Enlarged image of the part enclosed by the green
circle in (d) that reveals the intermolecular interaction
between guanidinium group and nitrogen atoms.
spectra of arginine. However, after intermolecular
interaction between L1 and arginine, some peaks of
arginine red shifted, but that of L1 blue shifted as observed
1
from the H NMR spectra in Figure S32. Arginine is
approximately 6.5 Å in length and 2.5 Å in width, so it enters
the channel of L1-Eu MOF easily. Combining with the
hydrogen bond interaction, arginine restricts the rotation of
benzene rings in L1 to expand the conjugate range (Figure
6d and 6e) and to enhance the blue emission. The
interaction between L1 and arginine does not disturb the
3
+
energy transfer from L1 to Eu ions. Thus, the red emission
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is stable and acts as a reference for ratiometric fluorescence
sensing and visible detection.
Orthogonal Metal Coordination and Host–Guest Interactions. J. Am.
Chem. Soc. 2018, 140, 7674-7680.
. Chen, M.; Li, L.; Nie, H.; Tong, J.; Yan, L.; Xu, B.; Sun, J. Z.; Tian, W.;
Zhao, Z.; Qin, A.; Tang, B. Z. Tetraphenylpyrazine-based AIEgens:
facile preparation and tunable light emission. Chem. Sci. 2015, 6,
1
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CONCLUSION
1
932-1937.
7. Xie, M. H.; Cai, W.; Chen, X.; Guan, R. F.; Wang, L. M.; Hou, G. H.; Xi,
X. G.; Zhang, Q. F.; Yang, X. L.; Shao, R. Novel CO Fluorescence Turn-
In summary, for the first time, we designed a novel strategy
to monitor the emissions from CIE and antenna effect
dynamically during the formation of single MOFs. The
emission from L1 was enhanced and blueshifted because of
the coordination. Correspondingly, the enhanced emission
from the antenna effect was revealed in real-time. CIE was
observed with the same RIR mechanism as AIE. In
combination with the energy variation during the formation
of MOFs, we validated and renewed the threshold value of
the energy gap between triplet state of the ligands and
2
On Quantification Based on a Dynamic AIE-Active Metal-Organic
Framework. ACS Appl. Mater. Interfaces 2018, 10, 2868-2873.
8. Kokado, K.; Machida, T.; Iwasa, T.; Taketsugu, T.; Sada, K. Twist
of C-C Bond Plays a Crucial Role in the Quenching of AIE-Active
Tetraphenylethene Derivatives in Solution. J. Phys. Chem. C 2017,
122, 245-251.
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. Mei, J.; Hong, Y.; Lam, J. W.; Qin, A.; Tang, Y.; Tang, B. Z.
Aggregation-induced emission: the whole is more brilliant than the
parts. Adv. Mater. 2014, 26, 5429-5479.
3
+
-1
excited state of the Ln ions to 3,000 cm for efficient
1
0. Liang, J.; Tang, B. Z.; Liu, B. Specific light-up bioprobes based on
3
+
antenna effect. L1 as ligand and Eu ions as metal nodes
were used to prepare MOFs with strong dual emission
under single excitation. The strong dual emission achieved
ratiometric fluorescence sensing and visible detection with
arginine as a model. We anticipate that systems with flexible
parts, the same as AIEgens, achieve enhanced fluorescence
after being fixed in rigid MOF structure for the development
of bright systems. Our results indicate that MOFs can be
used as the matrix to explore the fluorescence behavior and
mechanism.
AIEgen conjugates. Chem. Soc. Rev. 2015, 44(10), 2798-2811.
11. Feng, G.; Liu, B. Aggregation-Induced Emission (AIE) Dots:
Emerging Theranostic Nanolights. Acc. Chem. Res. 2018, 51, 1404-
1414.
1
2. Kuroda, Y.; Sugou, K.; Sasaki, K. Nonameric Porphyrin
Assembly:ꢀ Antenna Effect on Energy Transfer. J. Am. Chem. Soc.
2000, 122, 7833-7834.
1
3. Yang, C.; Liu, S.; Xu, J.; Li, Y.; Shang, M.; Lei, L.; Wang, G.; He, J.;
Wang, X.; Lu, M. Efficient red emission from poly(vinyl butyral)
films doped with a novel europium complex based terpyridyl as
ancillary ligand: synthesis, structural elucidation by Sparkle/RM1
calculation, and photophysical properties. Polym. Chem. 2016, 7,
1147-1157.
ASSOCIATED CONTENT
1
4. Su, Y.; Zhang, D.; Jia, P.; Gao, W.; Li, Y.; He, J.; Wang, C.; Zheng, X.;
Supporting Information. Reagents, instruments, and
methods. SEM&TEM images, BET, TGA, and PXRD curves of
MOFs. The fluorescence and UV-Vis spectra of MOFs. Detection
details of arginine. This material is available free of charge via
the Internet at http://pubs.acs.org.
Yang, Q.; Yang, C. Bonded-luminescent foam based on europium
complexes as a reversible copper (II) ions sensor in pure water.
Eur. Polym. J. 2019, 112, 461-465.
1
5. Wang, Y. M.; Tian, X. T.; Zhang, H.; Yang, Z. R.; Yin, X. B.
Anticounterfeiting Quick Response Code with Emission Color of
Invisible Metal-Organic Frameworks as Encoding Information. ACS
Appl. Mater. Interfaces 2018, 10, 22445-22452.
AUTHOR INFORMATION
1
6. Wu, S.; Lin, Y.; Liu, J.; Shi, W.; Yang, G.; Cheng, P. Rapid Detection
Corresponding Author
of the Biomarkers for Carcinoid Tumors by a Water Stable
Luminescent Lanthanide Metal-Organic Framework Sensor. Adv.
Funct. Mater. 2018, 28, 1707169.
*E-mail: xbyin@nankai.edu.cn; Fax: (+86) 022-23503034
Notes
1
7. Su, Y.; Zhang, D.; Jia, P.; Gao, W.; Li, Y.; Bai, Z.; Liu, X.; Deng, Q.;
The authors declare no competing financial interest.
Xu, J.; Yang, C. Highly selective and sensitive long fluorescence
lifetime polyurethane foam sensor based on Tb-complex as
ACKNOWLEDGMENT
This work was supported by National Natural Science
Foundation of China (Grants No. 21874074, 21675090, and
-
chromophore for the detection of H
PO
2 4
in water. Spectrochim.
Acta. A Mol. Biomol. Spectrosc. 2019, 217, 86-92.
18. Heine, J.; Muller-Buschbaum, K. Engineering metal-based
luminescence in coordination polymers and metal-organic
frameworks. Chem. Soc. Rev. 2013, 42, 9232-9242.
2
1435001) and China Scholarship Council (20183101).
1
9. Meyer, L. V.; Schonfeld, F.; Muller-Buschbaum, K. Lanthanide
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