AIEgen modulated per-functionalized flower-like IRMOF-3 frameworks with tunable light emission and excellent sensing properties

Article abstract of DOI:10.1039/d0cc08403d

A series of functional IRMOF-3 frameworks with solid-state luminescence and tuneable light emission (from 490 to 608 nm) have been synthesized by per-functionalizing AIE-active Schiff-bases with zinc. These precursor AIE-active ligands endowed the functional frameworks with boosted fluorescence emission efficiencies (from 0.16% to 1.03%). IRMOF-3-hrevealed a flower-like morphology attributed to the formation of J-aggregates, and could be used as a fluorescent probe for sensitive detection of copper(ii) (135 pM) and thiols (subnanomole).

Full text of DOI:10.1039/d0cc08403d

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AIEgen modulated per-functionalized flower-like
IRMOF-3 frameworks with tunable light emission
and excellent sensing properties†
Cite this: Chem. Commun., 2021,
7, 2392
5
Received 29th December 2020,
Accepted 27th January 2021
a
a
a
b
a
a
Xujie Li,‡ Siqi Xie,‡ Yuanzhuo Hu, Jun Xiang, Lumin Wang, Ruili Li,
ac
b
ab
a
Miao Chen, Fangbin Wang, Qi Liu* and Xiaoqing Chen
*
DOI: 10.1039/d0cc08403d
rsc.li/chemcomm
A series of functional IRMOF-3 frameworks with solid-state lumines- multiple sources of luminescence, including organic ligands
cence and tuneable light emission (from 490 to 608 nm) have been excitation, metal-centered emission, and charge-transfer
7
synthesized by per-functionalizing AIE-active Schiff-bases with zinc. between the metal ions and organic ligands. Besides, the
These precursor AIE-active ligands endowed the functional frameworks intrinsic permanent porosity of MOFs can also enable the
with boosted fluorescence emission efficiencies (from 0.16% to 1.03%). encapsulation or adsorption of guest luminescent species to
8
IRMOF-3-h revealed a flower-like morphology attributed to the for- obtain guest-induced LMOFs. However, regulating the lumi-
mation of J-aggregates, and could be used as a fluorescent probe for nescence properties of LMOF in a controlled and dynamic
sensitive detection of copper(II) (135 pM) and thiols (subnanomole).
manner is still a fundamental and challenging research
goal.
9
Developing functional luminescent materials with spectral
Reported LMOFs with tuneable fluorescence properties
tunability and structural diversity has attracted increasing emphasized the ratio adjustment of the metal ions (lanthanide
research attention in the exploitation of chemosensors, light- ions) and regulation of charge transfer between the metal ions
1
10
emitting diodes (LEDs), biomedicine, and so on.
and organic linkers. Nevertheless, the limited variety of
Benefitting from the significant development of metal– lanthanides severely narrowed their development and applica-
organic frameworks (MOFs) in recent decades, endowing MOFs tions. Inspired by organic dyes, regulating the fluorescence
with luminescence characteristics has been considered as a emission of LMOFs could also be achieved by employing
1
1
new pathway for the fabrication of solid-state luminescent different chromophores or auxochromes as ligands. However,
nanomaterials. In particular, luminescent MOFs (LMOFs) could the notorious aggregation-caused quenching (ACQ) effect
inherit the unparalleled tunability, structural diversity, and makes this strategy encounter a bottleneck. The intriguing
2
chemical and physical properties of MOFs. Moreover, LMOFs aggregation induced emission (AIE) effect pioneered by BenZhong
12
have been considered as a fascinating material class, and Tang has been revealed as a new route of LMOF fabrication. In
3
applied in a wide range of areas, including bioimaging,
contrast to the ACQ effect, the AIE luminogens (AIEgens) display
4
5
chemosensing, intelligent light sensors, and point-of-care less-emission in dilute solution but much higher fluorescence in
testing (POCT). More importantly, the organic–inorganic aggregated states. Previous reports have evidenced that the
hybrid nature of MOF materials can provide the LMOFs with AIEgen based LMOFs can inherit the AIE properties of AIEgens
6
13
14
and numerous merits of MOFs, and show surprisingly boosted
fluorescence emission efficiency due to the restricted intra-
molecular rotations, vibrations, and motions by the MOF
a
College of Chemistry and Chemical Engineering, The Hunan Provincial Key
Laboratory of Water Environment and Agriculture Product Safety,
Central South University, Changsha 410083, Hunan, China.
15
framework. However, most of the AIEgen functionalized LMOFs
16
are developed based on tetraphenylethylene and its derivates.
E-mail: iliuqi@csu.edu.cn, xqchen@csu.edu.cn
b
Besides, reported AIEgen based LMOFs largely rely on physical
doping or conjugation strategies, in which the intermolecular
forces between organic AIEgens and ligands would be much
Hunan Institute of Food Quality Supervision Inspection and Research,
The Hunan Provincial Key Laboratory of Food Safety Monitoring and Early
Warning, Changsha 410083, Hunan, China
c
College of Life Science, Central South University, Changsha 410083, Hunan, China
Electronic supplementary information (ESI) available: Experimental proce-
17
weaker than that of the chemical bonding strategy. Therefore,
†
1
13
developing new kinds of AIEgen based LMOFs with tuneable
emission, intense solid-state luminescence, and large Stokes
dures, FT-IR spectra, H NMR spectra, C NMR spectra, mass spectra, emission
and adsorption spectra, and MTT. See DOI: 10.1039/d0cc08403d
18
‡
X.-J. L. and S.-Q. X. contributed equally to this work.
shifts is still highly demanded.
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respectively, Fig. S7, ESI†). Nevertheless, type I adsorption
observed for both IRMOF-3 and IRMOF-3-a indicated that the
microporous nature of IRMOF-3-a was maintained. The different
morphology of IRMOF-3-a was also confirmed by field emission
scanning electron microscopy (FESEM), and the results revealed
that IRMOF-3-a exhibited a uniform flower-like spherical mor-
phology with an average diameter of 2 mm (Fig. S8, ESI†) and not
22
Scheme 1 Schematic illustration of LMOF fabrication.
the cube morphology of IRMOF-3. Meanwhile, the results of
powder X-ray diffraction (PXRD) showed a degraded crystal quality
of IRMOF-3-a, which may be attributed to the increased steric
AIE-active Schiff-bases are a newly emerging class of AIEgens hindrance and altered electronic effects after modification (Fig. S9,
2
3
and have been proved to be a useful tool for the development of ESI†).
19
new AIE molecules. More importantly, their independent con-
Besides, the results of thermogravimetric analysis (TGA,
jugated moieties would endow them with simultaneous fluo- Fig. S10A, ESI†) demonstrated the same thermal stability of
20
rescence emission and bonding ability. Thereby, by adopting a IRMOF-3-a and IRMOF-3; a weight loss of about 40% was
pre-synthesis modification strategy, we herein designed and fabri- observed from 450 to 500 1C due to the destruction of the
cated a series of Schiff-base based LMOFs with tuneable emission, IRMOF-3 framework. Nevertheless, a different weight loss of
diverse structure, and detection function. As illustrated in about 25% from 750 to 800 1C was observed for IRMOF-3-a,
Scheme 1, we firstly designed and synthesised the Schiff-bases which might be ascribed to the decomposition of Schiff-bases.
featuring conjugated moieties linked by imine bonds via amine Besides, we also verified the comparable chemical stability of
aldehyde condensation of salicylaldehyde (SAL) and its derivates IRMOF-3-a (Fig. S10B and C, ESI†). X-ray photoelectron spectro-
with 2-aminoterephthalic acid (ATA), and used them as the pre- scopy (XPS) revealed that carbon, oxygen and zinc elements
cursor of the LMOFs. Thereinto, the ATA portion of the precursor were distributed throughout the IRMOF-3-a framework and the
Schiff-bases was designed to react with a zinc ion to form the C 1s spectrum of IRMOF-3-a could be divided into five compo-
IRMOF-3 framework, while the imine bond and the benzaldehyde nents (Fig. S11, ESI†). All of the results demonstrated the
portion would enable the detection of copper ions and fluorescence successful assembly of IRMOF-3-a by adopting the precursor
emission, respectively. By finely varying the substituent groups of Schiff-base 3a. Significantly, the precursor AIE-active ligands
SALs and subsequently the Schiff-bases, the fluorescent properties endowed the IRMOF-3-a with a boosted fluorescence emission
of the resulted LMOFs would be finely tuned, as the fluorescence of efficiency (from 0.16% to 1.03%), which might be attributed to
Schiff-bases with excited state intramolecular proton transfer the restricted intramolecular rotations, vibrations, and motions
(
ESIPT) characteristics is strongly dependent on the substituents by the rigid skeletons and specific topologies of the IRMOF-3
of SALs.
framework.
We first verified the condensation reaction of ATA with SAL,
The boosted fluorescence and successful construction of
which would afford the original Schiff-base 3a (2-[(2-hydroxy IRMOF-3-a encouraged us to further engineer the luminescence
benzylidene)amino]terephthalic acid, Fig. S1, ESI†). The results properties of IRMOF-3 by varying the precursor Schiff-bases.
1
of high-resolution mass spectra (HRMS), and H NMR and We thus designed and synthesised a series of Schiff-bases 3b–3j
1
3
C NMR spectra (Fig. S2–S4, ESI†) indicated that, via simple via a similar condensation reaction, and their structures were
1
13
and convenient syntheses, the Schiff-base 3a was successfully also confirmed to be correct via FTIR, H NMR, C NMR, and
obtained with a high yield (81%). According to the mechanism HRMS (Fig. S12–S39, ESI†). Thereinto, the electronic effect of
of ESIPT, the Schiff-base 3a could feature the AIE effect, which Schiff-base 3a was modulated by varying the functional groups
was favourably verified by the fact that the fluorescence of on the benzene ring of SAL and retained the basic Schiff-base
Schiff-base 3a was weak in tetrahydrofuran (THF) solution but structure with ESIPT properties. We then constructed a series of
enhanced with the increase of the water fraction (Fig. S5, ESI†). AIE-active LMOFs via analogously co-reacting the resulting
With the successful synthesis of the precursor Schiff-base Schiff-bases with zinc ions (Fig. S40–S43, ESI†). Similarly, the
3a, the AIE-active iso-reticular metal–organic framework-3-a fluorescence quantum yield (QY) and fluorescence lifetime of
(IRMOF-3-a) was constructed via co-incubating Schiff-base 3a IRMOF-3-a were dramatically improved (from 0.16% to 1.03%
with zinc ions, and compared with IRMOF-3 (Fig. S1, ESI†). The and 0.672 to 1.101, respectively). And more importantly, the
FTIR spectra of IRMOF-3 and IRMOF-3-a (Fig. S6, ESI†) show fluorescence emission of the resulting functional IRMOF-3 was
À1
that the peaks around 3500 and 3388 cm , which can be finely tuned from blue (490 nm) to red (608 nm) along with the
ascribed to the amino stretching vibration of IRMOF-3, dis- alteration of the precursor Schiff-base (Fig. 1 and Table S1,
appear for IRMOF-3-a. Besides, like other pre-functionalized ESI†).
2
1
MOFs, due to the modification of the organic block and
We then wanted to decipher the possible photophysical
24
intermolecular hydrogen bonding, the Langmuir surface area mechanism of the resulting AIE-active LMOFs. The molecular
and the pore volume of IRMOF-3-a were all decreased with the geometries in the ground (S ) and excited (S ) states (Tables S2–S4,
0
1
2
À1
3
À1
introduction of SAL (from 1027.38 m g and 0.75 cm g of ESI†) of those precursor Schiff-bases reveal that the Schiff-bases
2
À1
3
À1
IRMOF-3 to 54.10 m
g
and 0.28 cm
g
of IRMOF-3-a with electron-withdrawing groups at position 4 are more coplanar
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Fig. 2 The self-assembly growth process: (A) 1 h, (B) 6 h, (C) 12 h, (D) 24 h,
(
E) low-magnification FESEM image of IRMOF-3-h. (F) Possible mechanism
and DFT calculated model of IRMOF-3-h.
Fig. 1 (A) Tuneable light emission of IRMOF-3-(a–j). (B) The commission
International de L’Eclairage of the fluorescence spectra. (C) Photographs
of IRMOF-3-(a–j) under natural light (bottom) and under 365 nm UV light
illumination (top).
rings positioned above or below the CQN double bond (Fig. 2F),
there was a special offset face-to-face force between individual
25
molecules of the J-aggregate. The coordination interaction
the dihedral angle lose to 1801) than that with electron-donating between zinc ions and ligands and the intermolecular p–p stack-
(
groups (Tables S5–S14, ESI†), and thus benefit the ESIPT. Mean- ing interaction coexisted in the structure of the LMOFs. The lower
while, with the molecule going from the ground state to the the energy of the formed J-aggregate, the easier the morphology
excited state, the distance between the N of the imine and H of the stacking, thus making IRMOF-3-h a special flower-like morphology
hydroxyl is significantly shortened, and the O–H bond length is (Table S15, ESI†).
unchanged (Tables S5–S14, ESI†). In addition, HOMO electrons of
In view of the enhanced fluorescence intensities of the
the ground state are mainly located in the benzene ring with an obtained LMOFs, we considered the potential application of
2
+
amino group and with a few in the imine moiety. While in the the resulting LMOFs in Cu detection, since a copper ion is an
excited state, the electrons of the LUMO are mainly distributed in essential element for plants and animals, and abnormal copper
the phenol ring and imine moiety (Fig. S44 for Schiff-base 3a and concentrations are associated with the death of bacteria and
Fig. S45–S53 for Schiff-bases 3b–3j, ESI†). The apparent distance algae, dysfunction of the brain and gastrointestinal tract, and
between N and H changes and the electron migration from the so on. Therefore, the detection of copper ions is of great
ground state to the excited state also provides the prerequisite for significance in environmental protection and human health.
ESIPT properties of the Schiff-bases. Besides, the attachment of an As a proof-of-concept, we employed IRMOF-3-h as the fluores-
electron-withdrawing group substituent enables the decrease of cent probe in the developed sensor, and the specificity and
the energy gap between the HOMO and LOMO of the keto-form selectivity of the fluorescence sensor were verified satisfactorily
tautomer (Tables S5–S14, ESI†). The gap is negatively correlated by challenging the detection with similar ions and interferents
with its electron-withdrawing ability, which would red-shift the (Fig. 3A). And under the optimized conditions (Fig. 3B), the
emission wavelength, and vice versa. All these results of the sensor also exhibited an excellent performance with a low limit
theoretical calculations are consistent with those of the of detection (LOD) of 135 pM, which was much lower than the
2
+
experiments.
allowed concentration of Cu (32 mM) in drinking water
Besides, the images of FESEM also revealed different morphol- permitted by the World Health Organization (WHO) and most
ogies and structures for those AIE-active LMOFs (Fig. S54, ESI†).
Thereinto, the LMOFs with precursor Schiff-bases with electron
withdrawing groups including IRMOF-3-b, IRMOF-3-c, IRMOF-3-
d, IRMOF-3-g, and IRMOF-3-h all demonstrated a flower-like
morphology with different sizes. The self-assembly growth process
of flower-like AIE-active LMOFs was also investigated via imaging
the same sample after 1, 6, 12, and 24 hours of aging time, in
which IRMOF-3-h was taken as the proof-of-concept (Fig. 2A–E).
The results indicated that every single particle was composed of
nanosheets and grew by stacking monomers one by one on a
2
+
Fig. 3 (A) Selectivity of the IRMOF-3-h based sensor for Cu detection.
The concentrations of the targets were 100 nM. (B) Fluorescence emission
spectra of the IRMOF-3-h based sensor exposed to various concentrations
spherical particle core, and finally formed stacked flowers. The
unique flower-like morphology of these AIE-active LMOFs may be
attributed to the fact that a more stable structure J-aggregate than
monomers was formed for Schiff-base based IRMOF-3. Thereinto,
2+
of Cu . Fluorescence emission spectra of the IRMOF-3-h based sensor
exposed to various concentrations of Cu : 0, 1, 2, 4, 6, 8, 10, 15, 20, 30,
2+
due to the intermolecular p–p stacking interaction of the benzene 50, 100 nM from top to bottom.
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of the reported references (Table S16, ESI†). Besides, when Laboratory of Food Safety Monitoring and Early Warning
we generalized the fluorescent sensor to real water samples (2020KFJJ06), and the Key R&D Project in the industrial field
(from the Yudai river in Hunan), the detection method showed funded by Hunan Provincial Science & Technology Department,
high accuracy (in comparison with the results of ICP-MS) China (2019GK2131).
and satisfactory recoveries ranging from 98.5% to 102.2% (Table
S17, ESI†). These results exemplified that the functionalized
LMOF fluorescent probes can be applied for environmental water
Conflicts of interest
samples.
Since biothiols can coordinate with copper ions and thus the
IRMOF-3-h would be recovered, we then used the IRMOF-3-h/Cu
There are no conflicts to declare.
26
complex as a biothiol fluorescent probe. And as expected, the
2+
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(
(
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