Direct synthesis of functionalized PCN-333 via linker design for Fe3+ detection in aqueous media

Article abstract of DOI:10.1039/c8dt01508b

A new luminescent mesoporous metal-organic framework (PCN-604) was successfully synthesized from trimetric clusters, Al₃(O)(CH₃COO)₆, and pyridine-based tritopic linkers, 4,4′,4′′-(pyridine-2,4,6-triyl)tribenzoate (PTB), v

Full text of DOI:10.1039/c8dt01508b

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Direct Synthesis of Functionalized PCN‐333 via Linker Design for
Fe3+ Detection in Aqueous Media
Yingmu Zhang, Xinyu Yang, Hong‐Cai Zhou*
Received 00th January 20xx,
Accepted 00th January 20xx
A new luminescent mesoporous metal‐organic framework (PCN‐604) was successfully synthesized from trimetric clusters,
Al3(O)(CH3COO)6 and pyridine‐based tritopic linkers, 4,4',4''‐(pyridine‐2,4,6‐triyl)tribenzoicate (PTB) via a solvothermal
reaction. The MOF with MTN topology possesses remarkable aqueous and thermal stability. Due to the efficient binding
sites rendered by the pyridyl‐based linkers, PCN‐604 shows fast fluorescence response towards trace amount of Fe3+ ions in
water in a fluorescence‐quenching mode. The Stern‐Volmer quenching constant is calculated to be 8.53 x 103 M‐1 and the
limit of detection is 6.2 µM, which convincingly demonstrate the capability of the MOF for Fe3+ detection.
DOI: 10.1039/x0xx00000x
www.rsc.org/
spectrophotometry (AAS).12‐14 Among them, luminescent
sensing stands out because of its portability, simplicity, fast and
visible response. Accordingly, various luminescent materials
have been designed and synthesized in the pursuit of sensing
the metal ion with high sensitivity and selectivity.15‐18
Introduction
Metal‐Organic Frameworks (MOFs), an emergent class of
porous materials, have gathered intense attention in the past
two decades. The judiciously selected metal clusters and
organic linkers impart MOFs with unique properties, namely,
intrinsic crystallinity, porosity, structural or functional diversity,
and tunability, which make them promising candidates in many
applications such as gas storage, separation, catalysis, drug
delivery and sensing.1‐5 Among various MOFs, mesoporous
MOFs with the ability to encapsulate large molecules like
organometallic catalyst, nanoparticles and enzymes, have been
extensively utilized in chemo/bio applications.6, 7 However, in
spite of the benefits brought by such encapsulation, the
accommodation of functional large molecules sacrifices partial
pore volume, which decreases the diffusion efficiency and
accessibility of the subsequent‐coming species. Hence, it would
be ideal to directly synthesize mesoporous MOFs with
functionalized building blocks, especially organic linkers.8
Luminescent
MOFs
(LMOFs)
with
their
unique
photoluminescence properties have recently bloomed out as
potential chemical sensors for Fe3+ recognition.3, 19 However,
most of luminescent MOFs exhibit moderate aqueous stability
and rely on ion exchange mechanism for Fe3+ detection, which
is time‐consuming and sometimes leads to the collapse of MOF
structures. Therefore, luminescent MOFs that display good
aqueous stability as well as fast response towards Fe3+ ions in
water system are pressingly needed.
MIL‐100 series are promising candidates for Fe3+ detection
due to their superb stability in water due to the inert bonds
formed between the tritopic linkers and trimeric metal
clusters.20 The expanded version of MIL‐100, known as PCN‐
333(M) (M= Fe, Al, Sc), can also survive such harsh conditions.21,
22
Instead of large channels, PCN‐333 contains large cavities
Iron is one of the most ubiquitous elements in life and
environment. It plays indispensable role in many biological
processes such as oxygen metabolism, electron transfer and
synthesis of DNA and RNA.9 Permissible limit of iron in drinking
water given by the World Health Organization (WHO) is 0.3
mg/mL.10 Both the deficiency and excess of Fe3+ from the
normal permissible limit will break cellular homeostasis and
cause serious biological disorders like microcytic hypochromic
anemia and Alzheimer’s disease.11 To dates, many methods
have been put forward for Fe3+ detection including liquid
chromatography, inductively coupled plasma atomic mass
with a diameter of 5.5 nm, which will not only prompt the pre‐
concentration of the analytes but also accelerate the diffusion
of incoming species. However, the original PCN‐333 shows no
fluorescent response to Fe3+ ions due to the lack of efficient
binding sites. Herein, in this work, through judicious selection of
organic building blocks, we directly built a luminescent MOF of
PCN‐333, named as PCN‐604, from aluminium trimetric clusters
and pyridine‐based 4,4',4'' ‐(pyridine‐2,4,6‐triyl)tribenzoic acid
(H3PTB) ligands. Like PCN‐333, the MOF exhibits excellent
thermal and chemical stability and the ultrapores can be
activated directly through regular degas process for further
usage. Working as a luminescent senor, PCN‐604 shows intense
luminescence in water and has promising sensitivity and
selectivity towards trace amount of Fe3+ ions demonstrated by
the high quenching constant of 2.85 x 104 M‐1 and low detection
limit of 6.2 µM.
spectrometry
(ICP‐MS)
and
atomic
absorption
Department of Chemistry, Texas A&M University, College Station,
Texas 77840‐3012, USA. E‐mail: zhou@chem.tamu.edu
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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mL/50 mL). The reaction mixture was heated up to 100 °C and
32 g KMnO4 was added in portions with 8 g each time. After 24
hours, the dark mixture was cooled down and filtered to give a
clear filtrate. The organic solvent was vacuumed up and the
aqueous phase was acidified with 6M hydrochloric acid (until pH
< 3). The obtained pale‐yellow solid was then filtered, washed
with water and acetone several times and dried to give the pure
product (1.89 g, 90%). 1H NMR (300 MHz, dmso): δ 8.51 (d, J =
8.5 Hz, 4H), 8.44 (s, 2H), 8.25 (d, J = 8.5 Hz, 2H), 8.13 (d, J = 8.5
Hz, 6H).
Experimental
Materials and Instrumentation
p‐toluadehyde, 4‐methylacetophenone, ammonium acetate,
potassium permanganate (KMnO4), aluminum chloride
hexahydrate (AlCl3∙6H2O), N, N‐diethylformamide (DEF),
tetrafluoroacetic acid (TFA) were purchased from either VWR or
Sigma Aldrich. Unless otherwise noted, all commercial
chemicals were used without further purification. Synchrotron
powder X‐ray diffraction (PXRD) was carried out with Bruker D8‐
Discover diffractometer equipped with a Mo sealed tube (λ =
0.72768 Å) on the beamline 17‐BM at the Advanced Photon
Source, Argonne National Laboratory. In‐house powder X‐ray
diffraction (PXRD) was carried out on a Bruker D8‐Focus Bragg‐
Brentano X‐ray powder diffractometer equipped with a Cu
sealed tube (λ = 0.72768 Å) at 40 KV and 40 mA. UV‐Vis spectra
were recorded on Shimadzu UV‐2450 spectrophotometer.
Fluorescence spectra were collected on Horiba Fluorolog
spectrofluorometer. Thermogravimetric analyses (TGA) were
carried out on a Shimadzu TGA‐50 thermal analyzer from
Synthesis of PCN‐604
In a 4 mL Pyrex vial, AlCl3∙6H2O (30 mg, 124.2 mmol), H3PTC
(10mg, 22.8 mmol) and 270
in 2 mL N, N‐diethylformamide (DEF). The mixture was then
heated in a 135 C oven for 2 days to afford white crystalline
L TFA were ultrasonically dissolved

powder. Yield: 4 mg.
Activation of PCN‐604
50 mg as‐synthesized PCN‐604 samples were firstly washed
~
temperature to 600 C at a ramp rate of 5 C /min in a flowing
with DMF (3 mL x 3) and acetone (3 mL x 3). The samples were
again immersed in acetone to undergo solvent‐exchange
process with fresh acetone three times within 12 hours.
Afterwards, the acetone was decanted and the mixture samples
were dried under vacuum for 10 min, transferred into BET tube
and then activated by the ‘outgas’ function of Micrometritics
air atmosphere. Nuclear magnetic resonance (NMR) data were
collected on a Mercury 300 spectrometer. Low pressure gas
adsorption measurements were performed on an ASAP 2020
with the extra‐pure quality gases.
Synthesis of H3PTB
ASAP 2020 system for one hour at 150
sorption measurement by the instrument.
C before the gas
1,3‐di‐p‐tolyl‐propenone
In a 250 mL round‐bottom flask, p‐toluadehyde (6.0 g, 50
mmol), 4‐methylacetophenone (6.8 g, 51 mmol) and 3 %
aqueous sodium hydroxide (150 mL) were mixed and rigorously
stirred at room temperature for 30 min. The mixture was then
heated up to 60 °C and allowed to react for another 12 hours.
After cooling to the room temperature, the resulted light‐yellow
precipitate was filtered, washed thoroughly with water and air‐
dried for next step without further purification.
Fluorescence quenching and titration experiments
To study the sensing capability of PCN‐604 towards metal ions,
fifteen different metal ions, Na+, K+, Ag+, Mg2+, Ca2+, Mn2+, Fe2+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, Cr3+, Fe3+ were chosen and
tested. ~15 mg fully‐grounded powder samples were soaked in
30 mL deionized water and ultrasonicately treated more than
30 min to prepare well‐dispersed MOF suspension. Afterwards,
1 mL of 1mM aqueous solutions of the metal ions were added
respectively to the 2 mL of as‐prepared MOF suspension in
cuvettes. In titration experiments, Fe3+ aqueous solution (1mM)
was added to 2 mL of MOF suspension in an incremental
manner from 50 µL to 1000 µL. All the corresponding
fluorescence spectra were recorded at room temperature. The
quenching percentage was calculated with the formula: Q = (1 ‐
I/I0) x 100%, where I0 and I are the fluorescence intensity of the
suspension before and after addition of analytes.
2,4,6‐tri‐p‐tolyl‐pyridine
4‐methylacetophenone (4.0 g, 30 mmol), 1,3‐di‐p‐tolyl‐
propenone (7.0 g, 30 mmol) and sodium hydroxide (4.8 g, 120
mmol) were crashed together with pestle and mortar for about
2 hours until the mixture turned to yellow powder. The solid
was then transferred into a round‐bottom flask which contains
an excess of ammonium acetate (20 g) dissolved in 200 mL
ethanol. The reaction mixture was heated under reflux
overnight. Upon cooling to the room temperature, the
precipitate was filtered, washed with water three times and
dried. The crude product was further purified by column
chromatography (eluent: ethyl acetate : hexane = 10 : 1) to yield
1
white solid (4.2 g, 40 %). H NMR (300 MHz, CDCl3): δ 8.07 (d, J
= 8.3 Hz, 4H), 7.79 (s, 2H), 7.61 (d, J = 8.4 Hz, 2H), 7.28 (d, J = 8.4
Hz, 6H), 2.42, (s, 9H).
4,4',4'' ‐(pyridine‐2,4,6‐triyl)tribenzoic acid (H3PTB)
In a 500 mL three‐necked flask, 2,4,6‐tri‐p‐tolyl‐pyridine (1.62 g,
4.7 mmol) was added to a mixing solvent (pyridine/water=200
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water treatment. According to the N2 isotherm, the crystallinity
of PCN‐604 slightly decreased after acid or base treatment, as
the N2 uptake reduced to 1698 cm3/g (Fig. 1b). Thermal stability
of PCN‐604 is determined by thermogravimetric analysis (TGA)
(Fig. S4). It shows that the decomposition temperature of PCN‐
604 is 540 °C and the three weight losses are corresponded to
Results and discussion
In order to achieve the direct synthesis of functionalized PCN‐
333 structure, 4,4',4'' ‐(pyridine‐2,4,6‐triyl)tribenzoic acid
(H3PTB) was chosen and synthesized with a modified procedure
based on previously reported literature (Scheme S1).23 Unlike
other tritopic linkers functionalized with hanging substituents
such as 1,3,5‐benzentribenzoic acid (H3BTB) and its derivatives,
the pyridine‐based linker is only decorated with one nitrogen
atom on the central benzene, which not only serves as an
additional binding site, but also, more importantly, releases the
rotating steric hindrance when building MOFs, allowing the six
oxygen atoms on the linker to stay in the same plane to adopt
approximate D3h symmetry, an indispensable prerequisite of
24
forming MTN topology.22,
The synthesis of PCN‐604 was
guided by the kinetically tuned dimensional augmentation
(KTDA) method with AlCl3 and H3PTB as starting materials.25 As
confirmed by the powder X‐ray diffraction (PXRD) patterns,
PCN‐604 is isostructural to PCN‐333 and shares the same cubic
MTN topology (Fig. S1).21 The corresponding structural model
of PCN‐604 was simulated based on the reported PCN‐333
structure by Material Studio 6.0.26 The space group, ꢀꢁ3ꢃ, was
ꢂ
used to describe the MOF structure. PCN‐604 is composed of
hybrid supertetrahedra building units connected in a vertex
sharing manner, leading to a mesoporous structure. The
supertetrahedra consist of aluminum octahedral trimers at the
four vertexes linked by the organic linkers occupied at the four
faces. Like PCN‐333, PCN‐604 also exhibits two types of
mesoporous cages. The smaller cage is built of 20
supertetrahedra sharing vertex with a pentagonal window of
25.9Å in diameter. The larger cage is hexacaidecahedral
(hexagonal‐truncated trapezohedral) formed by 24
supertetrahedra, displaying both pentagonal windows and
hexagonal windows of 30 Å in diameter. The large cage lies in a
honeycomb arrangement in the [111] projection. The inner
diameter is 11 Å for the super tetrahedral cage, 34 Å for the
dodecahedral cage and 55Å for the hexacaidecahedral cage.
Porosity and stability characterization
As PTB shares almost the same size with 4,4′,4′′‐s‐triazine‐2,4,6‐
triyl‐tribenzoic acid (TATB), the porosity of PCN‐604 is close to
that of PCN‐333(Al). Instead of activation with supercritical CO2,
which is usually used to degas mesoporous MOFs, the as‐
synthesized PCN‐604 can be activated in common way.21, 27 N2
adsorption was performed at 77 K to determine the porosity of
PCN‐604. The MOF shows 1952 cm3/g (STP) total N2 uptake. The
Brunauer–Emmett–Teller surface area and Langmuir surface
area are 2633 m2/g and 3561 m2/g respectively. Two steep
increases at P/P0 = 0.35 and 0.5 on the N2 adsorption isotherm
correspond to the two types of mesoporous cages. The
experimental pore volume is 3.2 cm3/g, slightly less than that of
PCN‐333 (3.85 cm3/g) (Table S1). Aqueous stability tests are also
performed via water treatment. The as‐activated samples are
immersed into aqueous solutions with various pH values and
are allowed to stay overnight. PXRD patterns indicate that PCN‐
604 can survive in pure water and aqueous solutions with pH
from 3 to 8 (Fig. 1a). The N2 isotherms are also tested after
Fig. 1. (a) PXRD patterns of simulation PCN‐604 (red), pristine PCN‐604 (black), samples
treated with water, pH = 8 aqueous solution (purple) and pH = 3 aqueous solution (green).
(a) N2 sorption isotherms of pristine PCN‐604 (black), samples treated with water (blue),
pH = 3 aqueous solution (purple) and pH = 8 aqueous solution (red‐pink) at 77k.
the departure of free solvent (up to 100 °C), coordinated solvent
(230 °C) and combustion of the organic ligand (490 °C). The
desired aqueous stability as well as the excellent thermal
stability can be ascribed to the strong electrostatic interaction
between the trivalent aluminium clusters and carboxylate
linkers.
Luminescence properties
Considering that PCN‐604 is built of Al3+ metal clusters with
complete electron subshells and electron‐rich π‐conjugated
ligands, the obtained network is supposed to be photoactive for
further applications.15 The photoluminescence properties of
both PCN‐604 and its acid ligand H3PTB in solid state were firstly
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explored at room temperature. According to the solid‐state increasing of solvent polarity. When it comes to water, a
fluorescence spectra, the free acid ligand shows strong emission broader emission occurs with blue‐shift of 10nm,
a
centered at 475 nm and 640 nm when excited at 424 nm, demonstrating the existence of the hydrogen bond interactions
32
assigned to π → π* and n → π* transiꢀons respecꢀvely between the H2O molecules and organic ligands.31,
(Fig.S10); while PCN‐604 cannot be fully excited at the same Fluorescence lifetime of the MOF suspension was also
excitation wavelength, even though it displays similar emission measured, showing a multi‐exponential decay. The amplitude‐
profile compared to its acid ligand (Fig.S8). PCN‐604 exhibits weighted average lifetime (τav) is calculated to be 5.33 ns
intense fluorescence emission upon 382 nm excitation with the (Fig.S12‐13).
maximum emission peak at 433 nm (Fig.S9). The notable
difference in fluorescence properties might be induced by
ligand‐to‐metal charge transfer (LMCT) in the coordination
Sensing measurements
material.28 The fluorescence properties of PCN‐604 dispersed in
Considering the desired chemical stability, fluorescence
different solvents are also investigated (Fig.S11). The
performance as well as the efficient pyridyl binding site of PCN‐
fluorescence spectra indicate trivial solvent effect of the MOF
604, we investigated the sensing capability of the as‐
suspension in most organic solvents like acetone, DMF, THF,
synthesized MOF towards various metal ions in water. 2 mg/mL
aqueous suspensions of MOF powder were ultrasonicately
prepared and 1 mM aqueous solutions of fifteen different
nitrate salts of metal cations, Na+, K+, Ag+, Mg2+, Ca2+, Mn2+, Fe2+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, Cr3+, Fe3+, were added to the
suspensions respectively. Fluorescence intensities of these
suspensions before and after the additions of metal ions were
all recorded at room temperature upon excitation at 350 nm
and the quenching efficiencies were calculated and compared
for qualitative studies of sensitivity. According to the
fluorescence spectra, these metal ions exert different
quenching effects on the fluorescence of PCN‐604. Among
them, Fe3+ causes the highest fluorescence quenching in respect
of emission intensities with the quenching efficiency of 74%;
while others display either moderate or negligible quenching
efficiencies (from 10% to 30%) as shown in Fig.2b. It is also
worthy to note that the fluorescence quenching occurs within 1
min, a rapid response which might be due to intrinsic
mesoporous structure of PCN‐604 that allows the analyte to
diffuse and interact the host network rapidly. The selectivity of
PCN‐604 for Fe3+ detection was also explored based on
competitive experiments in the coexistence of other metal ions.
1 mL aqueous solution that contains various metal ions except
for Fe3+ (1 mM for each) was introduced to 1 mL MOF
suspension, leading to a decrease in fluorescence intensity by
18.6 %. The change is similar to that caused by the individual
addition of these metal ions, indicating no accumulative
quenching effects. Sequential addition of 1 mL aqueous solution
of Fe3+ (1mM) to the suspension resulted in a dramatic
fluorescent quenching by 64 %, which demonstrates that the
sensing process is resistant to the interference of other metals.
As a control experiment, the fluorescent intensities of the
Fig. 2. (a) Fluorescence spectra of PCN‐604 suspension in response to different
mixture of MOF suspension and deionized water were also
tested and the presence of deionized water slightly lowered the
intensity by 17.4 % due to the dilution of the original suspension
(Fig.S16).
concentrations of Fe3+. Fe3+ aqueous solution (1 mM) was added gradually from 0 µL to
1000 µL (100 µL each time). (b) Fluorescence quenching of PCN‐604 dispersed in
aqueous solutions of fifteen different metal cations (1 mM). Excitation at 350 nm, room
temperature.
To further validate the high sensitivity for Fe3+ detection,
titration experiments were carried out with the concentration
of Fe3+ ranging from 0 to 1000 µM. With the gradual addition of
Fe3+ ion, the fluorescence intensity of the suspension was
steadily quenched to the lowest point. The quenching constant
(Ksv) was further calculated based on the Sterm‐Volmer (SV)
CH3CN, MeOH, EtOH, CH2Cl2 displaying maximum emission
peaks centred at 379 nm upon excitation of 350 nm. It should
be pointed out that, compared with the spectra of other
solvents, there is an additional shoulder peak at 423 nm in
weak‐polar solvent CH2Cl2, which might be caused by the pyridyl
30
group.29
The shoulder emission disappears with the
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equation: (I0/I) = 1 + Ksv[M], where I0 is the initial intensity, I is
the intensities of the corresponding MOF suspensions varying
with different concentrations of Fe3+ ([M]), and Ksv is the
quenching constant (M‐1), which is an important indicator of
sensing ability of a fluorescent sensor.29 It is shown that the SV‐
plot presents quiet a linear shape in the low concentration
range and slightly upper bends at high concentration level
(Fig.2a). ksv value is calculated to be 8.53 x 103 M‐1 based on the
linear part and which is better, if not, comparable to previously
reported MOF‐based Fe3+ sensors (Table S2). The limit of
detection (LOD) turns to be 6.2 µM calculated from the Ksv value
and standard deviation Sb of three repeated fluorescence
measurements of blank solutions (3Sb/Ksv). The large Ksv value
and small LOD demonstrate the great feasibility of PCN‐604 to
serve as a fluorescent sensor for Fe3+ detection in aqueous
system.
Conclusions
To conclude, we successfully synthesized a novel mesoporous
MOF (PCN‐604) from predesigned functional organic linkers
through direct synthesis method. PCN‐604 is constructed from
trimetric aluminium clusters and 4,4',4''‐(pyridine‐2,4,6‐
triyl)tribenzoic acid (H3PTB). The crystallinity and porosity of the
MOF are confirmed by the powder X‐ray diffraction combined
with N2 isotherms. The stability of PCN‐604 is evaluated through
water treatments and thermogravimetric analysis (TGA),
showing that the material can sustain its crystallinity in the
aqueous solutions with pH in the range of 3‐8 and will not
decompose below 540 °C. Fluorescence properties of the MOF
in both solid state and solutions are explored based on the
excitation and emission spectra as well as fluorescent lifetime
showing the material displays good emission in water. Notably,
the fluorescence of PCN‐604 can be quenched rapidly within
seconds by the aqueous solution of Fe3+. The rapid quenching
process can be ascribed to the mesoporous cavities of the
network that can pre‐concentrate the analytes and hinder the
diffusion constrains. The quenching efficiency of Fe3+ is 74%,
the highest one compared to those of other metal cations.
Moreover, verified by the quenching testes and titration
experiments, PCN‐604 possesses satisfactory sensitivity and
selectivity indicated by the large quenching constant and
detection limit, 8.53 x 103 M‐1 and 6.2 µM, respectively. To have
a deep look, the good sensing performance is driven by the
host‐guest interactions involving fluorescence resonance energy
transfer (FRET) or/ and photoinduced electron transfer (PET) and the
intrinsic electron‐donating pyridine groups on the linker also
plays an essential rule during the sensing process.
Mechanism
Several possible mechanisms have been well documented to
explain the fluorescence quenching by Fe3+ ions including the
collapse of the framework structure, ion exchange, host‐and‐
guest interactions via fluorescence resonance energy transfer
(FRET) or/ and photoinduced electron transfer (PET).33‐35 The
maintained crystallinity of PCN‐604 after Fe3+ treatment is
demonstrated by the PXRD patterns, which excludes the
possibility of structural collapse. In addition, the rapid
quenching effect (with several seconds) and the strong covalent
bonds formed between the high valent Al3+ clusters and
carboxylate groups can, to some extent, rule out the ion
exchange between the externally induced Fe3+ and the
framework, which is time‐consumed and kinetic‐driven. Hence,
one can propose that the main mechanism for the fluorescence
quenching effect is the host‐guest interaction. With the
unsaturated 3d orbital (3d5 4s0), the highly charged Fe3+ is a
better electron acceptor compared to other cations;
meanwhile, the organic linkers with conjugated moieties turn
out to be good electron donors. Herein, the quenching effect is
likely to originate from the photoinduced electron
transfer(PET). Upon excitation, the excited electrons are
proposed to be transferred to the lowest unoccupied molecular
orbital (LUMO) of Fe3+. The process leads to nonradiative
pathway and is competitive to radiative LMTC within the
framework, impeding the fluorescence of the MOF. Moreover,
the N atoms on the pyridine groups of the organic linkers can
donate their lone‐pair electrons to the metal ion, enhancing the
interactions between the targeted analytes and the host
material. As a result, the fluorescence quenching is triggered. It
is also reasonable that the sensing mechanism underlies the
distance‐dependent fluorescence resonance energy transfer
(FRET). In order to prove it, the adsorption spectra of various
metal ions were collected. The absorption of Fe3+ has the largest
degree of overlapping with the emission of PCN‐604, making
the energy transfer feasible. The overlap for other metal ions is
negligible and is in good agreement with the trend of quenching
efficiency (Fig.S17).
Conflicts of Interest
There are no conflicts of interest to declare.
Acknowledgements
This work was supported by the Center for Gas Separations
Relevant to Clean Energy Technologies, an Energy Frontier
Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences (DE‐
SC0001015), U.S. Department of Energy, Office of Fossil Energy,
National Energy Technology Laboratory (DE‐FE0026472), and
Robert A. Welch Foundation through a Welch Endowed Chair to
HJZ (A‐0030).
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