Flexible metal-organic frameworks for the wavelength-based luminescence sensing of aqueous pH

Article abstract of DOI:10.1039/c8tc03036g

Luminescence sensing is commonly based on the change of emission intensity, lifetime, and/or quantum yield of luminescent materials. Wavelength-based luminescence sensing has been comparatively rarely documented. Flexible metal-organic frameworks (MOFs) with rich fluorophores are promising sensory materials due to their unique stimuli-responsive luminescence. However, it is still challenging to construct such materials with high hydrolytic stability, which would allow them to be utilized in water systems. Here, we present two isoreticular flexible MOFs synthesized by carbazole-derived ligands, which show high porosity and excellent stability in aqueous solutions with a wide range of pH values. Interestingly, both MOFs show a structural breathing behavior with variation of the guest molecules inside their pores due to the topologically allowed flexibility. With their frameworks breathing, sensitive fluorescence changes in the wavelength of the maximum emission have been observed. Remarkably, the wavelengths of the maximum emission are well dependent on the pH values (1 to 8) of the aqueous solution in a linear relationship, suggesting a new means of accomplishing pH sensing based on emission wavelength instead of intensity. Wavelength-based fluorescence sensing thus offers an alternative way of achieving pH indication, free from some of the drawbacks associated with intensity-based fluorescence sensing, which should be fundamentally important for the development of new pH sensors in environmental and life sciences.

Full text of DOI:10.1039/c8tc03036g

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Wang, M. Zhao, Y. Cui and J. J. Li, J. Mater. Chem. C, 2018, DOI: 10.1039/C8TC03036G.
Volume
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Number
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January 2016 Pages 1–224
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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DOI: 10.1039/C8TC03036G
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Flexible Metal-Organic Frameworks for Wavelength-Based
Luminescence Sensing of Aqueous pH
a
a
a
a
b
a,*
Xiu-Liang Lv, Lin-Hua Xie, Bin Wang, Minjian Zhao, Yuanjing Cui and Jian-Rong Li
Received 00th January 20xx,
Accepted 00th January 20xx
Luminescence sensing is commonly based on the change of emission intensity, lifetime, and/or quantum yield of
luminescent materials. Wavelength-based luminescence sensing was rarely documented, comparatively. Flexible metal-
organic frameworks (MOFs) with rich fluorophores are promising sensory materials due to their unique stimuli-responsive
luminescence. However, it is still challenging to construct such materials with high hydrolytic stability, which can be
utilized in water systems. Here we present two isoreticular flexible MOFs synthesized by carbazole derived ligands, which
show high porosity and excellent stability in aqueous solutions with a wide range of pH values. Interestingly, both MOFs
show structural breathing behavior upon the variation of guest molecules inside their pores due to the topologically
allowed flexibility. With their frameworks breathing, sensitive fluorescence changes in wavelength of the maximum
emission have been observed. Remarkably, the wavelengths of maximum emission are well dependent on the pH values (1
to 8) of aqueous solutions in a linear relationship, suggesting a new means of pH sensing based on emission wavelength
instead of intensity. The wavelength-based fluorescence sensing thus offers an alternative way for pH indication free of
some drawbacks in intensity-based fluorescence sensing, which should be fundamentally important for the development
of new pH sensors in environmental and life science.
DOI: 10.1039/x0xx00000x
www.rsc.org/
well separated emissions (Scheme 1c). Although analyses
based on the two methods are free of above mentioned
interferences, associated measurements or syntheses of
1
. Introduction
Luminescence sensing are essential tools in many fields of
science and technology for the sensing and detection of
different substances, such as gaseous molecules (O , CO , CO,
sensory materials are much more difficult and complicated.
New advanced luminescent materials are thus highly desirable
for chemical sensing and detection in facile but reliable
manners.
2
2
VOCs, etc.), heavy metal ions, explosives, and biological targets
intracellular pH, oligonucleotides, glucose, cancer biomarkers,
(
1
etc.). Most luminescence sensors are based on the changes
-4
Metal-organic frameworks (MOFs) are a class of newly
in the emission intensity of one transition (Scheme 1a). developed porous solid materials, constructed from organic
However, such intensity-based luminescence sensing often linkers and metal-containing nodes through coordination
7
suffers from interferences from many factors, including drift of bonds. These materials have attracted intense attention over
optoelectronic system (e.g. lamp for excitation intensity and the last two decades due to their structural variety and great
detector for emission collection), probe concentration, and potential in many fields, such as gas storage, separation and
5
,6
8-10
other substances coexisted in the systems.
There are catalysis.
generally two ways to overcome these drawbacks. One is the luminescent MOFs are promising materials applicable in
Many works have also demonstrated that
1
1-13
measurement of luminescence lifetime or quantum yield molecule sensing and detection.
Scheme 1b), and the other is the ratiometric sensing method conventional porous materials, such as zeolites and activated
Compared with
(
which is based on analyte dependent intensity ratio of two carbon, one distinct characteristic of MOFs is their structural
flexibility, which, in some cases, is sensitive and responsive
1
4-16
toward external stimuli, such as guest accommodation.
The flexible but crystalline porous structures of MOFs with
a.
Beijing Key Laboratory for Green Catalysis and Separation and Department of
Chemistry and Chemical Engineering, College of Environmental and Energy
Engineering, Beijing University of Technology, Beijing 100124, P. R. China. E-mail: highly tailorable conjugated organic components thus make
jrli@bjut.edu.cn
them considerably attractive as advanced materials for
b.
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials
and Applications, School of Materials Science and Engineering, Zhejiang
luminescence sensing, in comparison with rigid porous
−
2
University, Hangzhou 310027, P. R. China
Electronic Supplementary Information (ESI) available: Detailed sample activation
sensors. One exciting example is [Zn (bdc) (dpNDI)] (bdc
2
=
2
†
and gas adsorption of BUT-62 and -63, stability test of BUT-62 and -63, N
1
,4-benzenedicarboxylate; dpNDI = N,N'-di(4-pyridyl)-1,4,5,8-
1
2
7
adsorption of acid or base treated BUT-62 and -63, temperature-dependent PXRD naphthalenediimide) reported by Kitagawa and co-workers.
of BUT-62 and -63, fluorescence measurements for pH sensing, fabrication of
This MOF has a twofold interpenetrated dynamic framework,
in which naphthalenediimide fluorophores are incorporated.
MOF/CMC plates can be found in ESI. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C8TC03036G
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Scheme 1 Schematic representations of intensity (a), lifetime or quantum yield (b), ratiometric intensity (c), and wavelength (d) based luminescence sensing.
6 4 8 2 4 2
Interestingly, it shows intense luminescent emission when Technology) and Zr O (OH) (H O) (CTNA) (BUT-63), which
adsorbing a class of aromatic compounds, while its guest-free were synthesized from intentionally chosen low symmetrical
framework is not luminescent. Particularly, its fluorescent ligands (trapezoid) with strong fluorophores, namely,
wavelength is dependent on the chemical substituents of the 4,4',4'',4'''-(9H-carbazole-1,3,6,8-tetrayl)-tetrabenzoic
aromatic guests. The observed emission turn-on and changes (H CTTB) and 6,6',6'',6'''-(9H-carbazole-1,3,6,8-
were proposed to be resulted from the dynamic movement of tetrayl)tetrakis(2-naphthoic acid) (H CTNA), as shown in Figure
two sets of interpenetrated networks by dislocation of their 1. The MOFs show high porosity, high chemical stability,
mutual positions, which leads to enhanced reversible guest-dependent structural flexibility, as well as
acid
4
4
naphthalenediimide-aromatic guest interactions and the stimuli responsive fluorescence upon interactions with guest
formation of complexes with charge transfer characteristics. molecules. Remarkably, evident fluorescent emission shifts
This MOF is thus potentially applicable in detecting series of were observed when the MOFs were treated with aqueous
aromatic molecules with small structural difference.
solutions with wide range of pH values. Especially, the
wavelengths of maximum emission (λmax) are well dependent
on the pH values of aqueous solutions ranging from 1 to 8 in a
linear relationship, rendering a new type of luminescence
sensing method by the determination of emission wavelength
instead of intensity, lifetime, or quantum yield (Scheme 1d). To
the best of our knowledge, the wavelength-based
luminescence sensing of pH realized by BUT-62 and -63 is
unprecedented in MOF chemistry or other luminescent
materials, which should be of high importance for
luminescence sensing in some circumstances as an alternative
option.
Nevertheless, most flexible MOFs are not stable in aqueous
media, restricting their applications in many circumstances
1
8-20
(e.g. physiological fluids).
The prerequisite of simultaneous
presence of dynamic long-range ordered framework, high
chemical resistance, as well as sensitive luminescence
response to external stimuli, thus challenges the construction
of porous flexible MOFs for applications in aqueous media as
advanced sensory materials. Chemically stable MOFs were
usually constructed by the combination of highly charged
2
1,22
metal ions with carboxylate-based ligands.
In the past few
years, zirconium-based MOFs (Zr-MOFs) have been
demonstrated to be a subclass of MOFs with both high
2
3-27
designability and exceptional stability.
Most of reported Zr-
+
2
. Results and discussion
4
MOFs were constructed by high-valent Zr species and anionic
O-containing organic ligands, including carboxylates,
phenolates, and phosphates. Due to the strong hard acid-hard
2
.1 Synthesis and Crystal Structures of BUT-62 and -63
4
+
Solvothermal reactions of ZrCl and the ligands (H CTTB or
4
4
base interaction between Zr and ligand O atoms, many highly
porous Zr-MOFs have been proven stable in organic solvents,
water, and even strong acidic and weak basic aqueous
H4CTNA) in N,N-dimethylformamide (DMF) in the presence of
an excess of modulator acid (acetic acid or formic acid) yielded
crystalline samples of as-synthesized BUT-62 and -63
28-31
solutions.
On the other hand, the chemical robustness of
(hereafter referred as BUT-62W and -63W, W stands for white
most Zr-MOFs is often accompanied with their framework
rigidness, limiting the occurrence of structural flexibility for
specific application.
color of the bulk sample), respectively. It should be pointed
out that in the synthesis of Zr-MOFs, additional acid modulator
usually plays an important role in controlling the formation of
products, as well as the nucleation and growth rate,
In this work, we present two flexible Zr-MOFs,
2
3,34
Zr O (OH) (H O) (CTTB) (BUT-62, BUT = Beijing University of
6
4
8
2
4
2
morphology, and crystallinity of the crystalline products.
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Without modulators, intergrown micro-sized aggregates of in a tetragonal antidipyramid coordination geometry. For the
crystals or amorphous phases were usually obtained, as shown two Zr atoms at the apical positions, the eight coordinated O
in SEM images (SI, Figure 11 and S12). Acetic acid or formic atoms are four μ
3
-OH/O group O atoms and four carboxylate O
4
–
acid was used as the modular reagent to obtain pure ones from four different but symmetrically equivalent CTTB
4
–
crystalline samples of BUT-62 and -63, which were confirmed or CTNA ligands. For the four Zr atoms at the equatorial
by powder X-ray diffraction (PXRD) (Figure 2b and c). Tuning positions, eight coordinated O atoms come from four μ -OH/O
the amounts of modulator led to single crystals big enough for groups, two terminal OH groups, and two carboxylates from
3
4
–
4–
the single-crystal X-ray diffraction in each case. In addition, different but symmetrically equivalent CTTB or CTNA
from the FT-IR spectra of BUT-62 and -63, slight blue shifts of ligands. Thus, a Zr (μ -O) (μ -OH) core is connected with eight
carbonyl group characteristic bands compared with CTTB or CTNA ligands through eight carboxylate groups.
6
4–
3
4
3
4
4
–
4
–
4–
corresponding ligands were observed, illustrating the metal Each CTTB or CTNA ligand links four Zr
coordination of carboxylate groups of ligands (SI, Figure S2 and carboxylate group coordinating with two neighboring Zr atoms
S3). of one Zr6 cluster in a bimonodentate fashion. Alternate
6
clusters with each
4
–
4–
connection of the CTTB or CTNA ligands and the Zr clusters
6
Single-crystal structure analysis revealed both BUT-62W
results in the 3D framework of BUT-62 and -63 with
intersecting channels (Figure 1c and i). The largest diamondoid
channels are along the crystallographic b direction and
relatively small ones in the a and c directions. Two diagonals
for cross-section of the large diamondoid channels are related
to the unit cell a and c axes, respectively, and the lengths are
estimated to be ca. 10 and 23 Å for BUT-62W, and 13 and 29 Å
for BUT-63W (taking into account the van der Waals radii of
the atoms). Diameters of the small channels along the a and c
and -63W crystallize in the space group Immm and have
isoreticular three-dimensional (3D) frameworks consisted of 8-
8
connected [Zr O (OH) (H O) ] clusters (hereafter referred as
+
6
4
8
2
4
4
Zr ) which are interconnected by 4-connected CTTB and
–
6
4
–
CTNA ligands, respectively (Figure 1a, b, g, and h). In the Zr6
cluster, six Zr atoms arrange in a distorted octahedral
geometry, and each facet of the Zr
6
octahedron is capped by a
-OH group, forming a Zr (μ -O) (μ -OH) core. The
six Zr atoms are all eight-coordinated, each with eight O atoms
2
−
−
μ
3
-O or μ
3
6
3
4
3
4
Fig. 1 Construction, crystal structures, and underlying net topology of BUT-62 and -63. Ligands H
clusters (b and h) as 8-connected nodes (e) to generate 3D networks of BUT-62 and -63 (c and i) with the scu-a topology (f), respectively (in crystal structures, H atoms are omitted
for clarity and color codes are set as Zr, purple; C, O, and N, green; those in Zr cluster are Zr, powderblue; C, grey; and O, red).
4 4 6
CTTB (a) and H CTNA (g) as 4-connected linkers (d) are used to assemble with Zr
6
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Recently, two examples of rigid Zr-MOFs (NU-902 and NUPF-
directions are 4~14 and 3~14 Å in BUT-62W and 7~17 and
4
3
1
) with the scu-a topology were reported, which are
constructed by D4h symmetric porphyrin ligands and distorted
Zr clusters with reduced symmetry.
6
~17 Å in BUT-63W, respectively. The total solvent accessible
volumes of BUT-62W and -63W are calculated to be 74.6 and
0.0% of their crystal volumes by PLATON, respectively.
6
8
Clearly, a lot of –OH/O groups on the Zr-based clusters
decorate the pore surfaces of the frameworks, which can
2
.2 Porosity and Stability
provide the opportunity of forming host-guest interactions Permanent porosity of the BUT-62 and -63 was confirmed by
with guest molecules. Topologically, when considering each Zr
the N sorption measurements at 77 K. After the guest solvent
6
2
4
–
4–
cluster and each CTTB or CTNA ligand as a 8- and 4- molecules (DMF and water) being removed through the guest
connected nodes, respectively, the frameworks of BUT-62 and exchange with acetone and the subsequent evacuation under
-
2
63 can be simplified into an augmented 4,8-connected scu vacuum at 100 °C, N adsorption isotherms were recorded at
4
scu-a) network with the point symbol of (4 .6 )
2
16 12
(4 .6 ) 77 K for the guest-free phases of BUT-62 and -63 (hereafter
referred as BUT-62A and -63A, A stands for activated sample).
(
2
(Figure 1d-f).
3
–1
4
It was noted that the CTTB and CTNA ligands in BUT-62
–
4–
Saturated N2 uptakes of 652 and 910 cm
g
(STP) are
achieved, and the BET surface areas of BUT-62A and -63A are
–1
estimated to be 2401 and 3060 m g , respectively (Figure 2a).
and -63 are in a planar trapezoidal coordination configuration
with the C2v symmetry, while all previously reported planar
tetratopic carboxylate ligands in Zr-MOFs are rectangular or
square usually with the D2h or D4h symmetry, respectively, such
2
3
–1
The pore volumes are calculated to be 1.03 and 1.49 cm g ,
respectively, which are lower than the values calculated (1.22
3
–1
4
–
28,32,35-37
38
39 and 1.75 cm g ) based on single-crystal structure data of
BUT-62W and -63W. The lowered pore volumes suggest
possible structural alteration after removing guest molecules
as TCPP in PCN-22X (X = 1~5),
40
MOF-525, and NU-902, Py-XP in NU-1101, PTBA in NU-
MMPF-6, MOF-545,
4–
3
9
4–
27
3
3
4–
8
41
1
100, and ETTC in PCN-94 and PCN-128, etc. (SI, Figure
from the as-synthesized samples, which will be discussed
S1 and Table S7). For the D2h and D4h symmetric ligands, Zr
clusters with higher connectivities seem to be more
thermodynamically stable, which often give rise to rigid
6
below.
In spite of their high porosity, both BUT-62 and -63 were
frameworks, such as 4,12-connected ftw-a network of PCN-94, found to be tolerant in neutral water, weak base, and strong
2
7,33
PCN-228, PCN-229, PCN-230, NU-110X (X = 0~5),
MOF-525, acid aqueous solutions. As shown in Figure 2b and c, PXRD
535, CPM-99, Zr-BTBA, and Zr-PTBA, and shp-a network patterns suggest that the two MOFs remain highly crystalline
3
9
42
24
24
-
3
2
of PCN-223. The 4,6-connected she-a network of PCN-224, after being immersed in the neutral water, pH = 9 or 2.0 M HCl
which can be regarded as a partial ligand missing ftw-a type aqueous solution for 24 h at room temperature, respectively.
network, is rigid as well. Furthermore, the assembly of D2h and Note that after BUT-62A and -63A being treated with water or
D
4h symmetric ligands with 8-connected Zr
6
clusters also tends weak base, the PXRD patterns obviously changed, particularly
4
3
to form csq-a type rigid framework (e.g. NU-1000, MOF-545, for the latter, in comparison with those of BUT-62W and -63W,
MMPF-6, PCN-222, and PCN-128). The rigidity of these MOF respectively, implying structural transformation. This will be
frameworks should be related with their intrinsic topologies. discussed in the following sections in detail. Furthermore, after
As pointed out by Feréy, a topology cannot be flexible when these treated samples being reactivated by following the same
4
4
2
odd cycles of inorganic bricks exist in the network. In all these procedure as that for as-synthesized samples, N sorption
ftw-a, shp-a, she-a, and csq-a type Zr-MOFs, triangular isotherms were found to nearly overlap with those of the
channels (or ligand-linked circle subunits) can be observed. It pristine samples (Figure 2a), further confirming the high
should be point out that PCN-128 with a csq-a network and stability of BUT-62 and -63. However, it was found that the
NU-1104 and -1105 with a ftw-a network are slightly flexible, two MOFs gradually degraded in the aqueous solutions with
which could be attributed to the rotation freedom of their pH > 9 (SI, Figure S4 and S5). The chemical stability of BUT-62
ligands but not the breathing of whole frameworks.
and -63 is thus similar to many reported Zr-MOFs, which are
stable in acidic condition but unstable in high alkaline
When the symmetry of the tetratopic ligand is reduced to
2
6,28,35
solution.
Zr-MOFs in strong base is the result of the substitution of
It is commonly believed that the degradation of
C , rigid frameworks with higher connectivities are excluded,
2v
and a scu-a network becomes more thermodynamically
favored. The scu-a type network of BUT-62 and -63 shows only
quadrangular channels. Additionally, the 8-connected Zr6
clusters in BUT-62 and -63 are distorted with a lower
2
−
carboxylate ligands and O entities by hydroxide groups,
which results in more stable Zr-hydroxide with relatively
37,38
stronger Zr–Ohydroxide bonds.
symmetry of C2v compared with the 12-connected Zr clusters
6
In addition, thermal gravimetric analysis (TGA) curves
with O symmetry in UiO-66 or 8-connected Zr clusters with indicate that as-synthesized BUT-62W and -63W are thermally
h
6
D4h symmetry in PCN-222. Each Zr cluster contains 1, 2, and 3 stable up to about 400 and 450 °C, respectively (SI, Figure S6),
6
symmetrically independent Zr atoms in UiO-66, PCN-222, and and temperature-dependent PXRD measurements reveal that
BUT-62 (or BUT-63), respectively. These structural they remain highly crystalline and their frameworks are
characteristics (low symmetries of both ligands and metal basically persisted up to 270 °C, suggesting good thermal
clusters) might be responsible to the considerable structural stability of the two MOFs (SI, Figure S7 and S8). Since the two
flexibility of BUT-62 and -63, which will be discussed below. MOFs have isoreticular structures with close Zr–O bond
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strength, the observed different thermal stabilities of them 62A sample further became deeper after they were soaked in
can mainly attribute to the different decomposition neutral water, to form the water-included phase BUT-62Y (Y
temperatures of the corresponding ligands, which are related stands for yellow color); BUT-63 changes color from white to
with their compositions and molecular weights. Similar green yellow and finally to luminous yellow. Furthermore,
2
6
situation has been observed in the reported literature.
BUT-62Y and -63Y samples underwent negligible color change
when immersed in DMF or acetone. However, when they were
immersed into strong acid (2 M HCl aqueous solution), the
color changed to white immediately. The color change of the
BUT-62 and -63 samples suggests that the electronic band
structures of the corresponding frameworks are changed.
Zhou and co-workers observed the color change of UiO-66
sample from white to yellow, and then to brown with
4
4
increasing amount of missing-linker defects. The color
change mechanism should be different for the two cases,
because in BUT-62 and -63 the color change is reversible, while
defective UiO-66 cannot change to defect-free UiO-66. The
color change for the UiO-66 samples might mainly relate with
the coordination change of Zr clusters, while that in BUT-62
6
and -63 might be resulted from the configurational change of
the organic ligands.
Besides directly observed color changes, different phases
of BUT-62 and -63 also represent distinct fluorescence
emissions, respectively. Carbazole is an attractive fluorophore
4
7
due to its rigid structure and large conjugated plane, and the
−
MOFs constructed by carbazole-based ligands, such as CTTB
4
4
–
in BUT-62 and CTNA in BUT-63, are expected to show intense
fluorescence. The solid-state fluorescent properties of H CTTB,
CTNA, BUT-62W, -62Y, -63W, and -63Y were studied. As
shown in Figure 3a, H CTTB gives a bright-blue emission with
4
H
4
4
the wavelength of emission maximum (λmax) of 465 nm when
excited at 370 nm at room temperature. Under the same
excitation, BUT-62W emits pale-blue light with λmax = 425 nm,
while BUT-62Y gives a green emission with λmax = 522 nm.
Similarly, H
nm when excited at 375 nm. Under the same excitation, BUT-
3W emits pale-blue light with λmax = 433 nm, while BUT-63Y
4
CTNA shows a bright-blue emission with λmax = 465
6
gives an orange emission with λmax = 623 nm. For better
understanding the fluorescence, we also recorded the
4 4
fluorescence spectra of the ligands H CTTB and H CTNA after
they are dissolved in different solvents (DMF, water, methanol,
and toluene). The results are shown in the Figure S35. It can be
observed that the emission shifting is dependent on the nature
of the solvents, and the energy shifts are evident, revealing a
solvatochromic effect of the ligands. Particularly, when the
ligands are dissolved in DMF, their emission peaks are at
4 4
positions (432 nm for H CTTB, and 470 nm for H CTNA) quite
Fig. 2 Porosity and stability of BUT-62 and -63. (a) N
2
adsorption isotherms recorded at
close to those for the as-synthesized BUT-62W (432 nm) and
BUT-63W (466 nm). From the fluorescence spectra, the ligands
and the MOFs share very similar profile of emission.
Furthermore, we also carried out DFT calculations for the
energies of HOMO and LUMO orbitals of the free ligands, as
well as the ligands in the MOFs. The results are given in Figure
S29 and Table S9. It was found that the energy gaps are
different for the ligands in different statuses, which is
consistent with the observations in fluorescence spectrum
7
7 K and (b, c) PXRD patterns of BUT-62 and -63 samples treated under different
conditions, to check their chemical stability. Note that BUT-62A and -63A could adsorb
some water from air during recording their PXRD patterns.
2
.3 Optical Signal and Fluorescence
Interestingly, obvious color changes were observed when we
were preparing the BUT-62A and -63A samples for gas sorption
experiments and stability tests (Figure 3a). White crystalline
powders of the BUT-62W sample turned to light-yellow after
they being activated into BUT-62A. The yellow color of BUT-
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studies. Since Zr(IV) atoms are not fluorescent metal ions, we speculate that the fluorescence of the MOFs should be based
Fig. 3 Varied color and fluorescence of BUT-62 and -63 samples. Schematic illustration for transitions (a) among H
4
CTTB, BUT-62W, -62A, and -62Y and (b) among H
3W, -63A, and -63Y, and photographs for colors/fluorescences of the MOF samples; (c) intensity normalized emissions of BUT-62W, -62A, -62Y, and H CTTB upon the excitation at
70 nm, and (d) those of BUT-63W, -63A, -63Y, and H CTNA upon the excitation at 375 nm.
4
CTNA, BUT-
6
3
4
4
on the ligands. Notably, the emissions of BUT-62Y and -63Y are luminescence of the breathing Zr-MOFs can be fine-tuned by
significantly red-shifted with respect to those of BUT-62W and the modification of the ligands.
-
63W by almost 100 and 190 nm wavelengths, respectively.
2
.4 Structural Transformation and Flexibility
The obvious shift of fluorescent emissions indicates that
energy gaps between ground state and excited state of the
Above color and fluorescence changes stimulate us to study
the structural transformations between different phases of
BUT-62 and -63 in detail. As the two MOFs have similar
structures and optical signals, we are mainly focusing on the
former here. As mentioned above, PXRD patterns clearly show
the structural change of BUT-62 at different states, while
1
1,48
associated systems are changed.
Such a large difference in
fluorescence emission wavelength in each case should be
related with the change of ligand configuration of the MOFs
during the structural transformation from BUT-62W to BUT-
6
2Y, or BUT-63W to BUT-63Y.
It should also be pointed out that the guest/structure- remaining good crystallinity in each case (Figure 2b). BUT-62W
dependent fluorescence change of BUT-63 is more sensitive transformed into BUT-62A after all guest molecules were
than that of BUT-62. As shown in Figure 3c and d, the shift of removed, and then into BUT-62Y when water molecules were
fluorescence emissions induced by the structural re-adsorbed. After treated with strong acid, BUT-62Y then
transformation after water exchange is more pronounced for transformed to a phase with PXRD pattern similar to that of
BUT-63 than that for BUT-62 (190 vs. 97 nm). The different BUT-62W. Further studies also confirm the structural
sensitivity of the fluorescence change between the two MOFs transformations are reversible to some extent. These results
4
−
should be related with their different ligands. Both CTTB and thus suggest that the framework of BUT-62 is flexible with
4
−
CTNA contain a carbazole core, but four peripheral arms are reversibility.
different, phenyl groups for the former while larger naphthyl
The structural flexibility of BUT-62 is also approved by the
groups for the latter. Compared to phenyl rings, the
conjugated naphthyl rings are larger in size, and commonly
recognized as better fluorescent groups. Here, the
replacement of phenyl groups by naphthyl groups not only
enlarges the pore size, but also enhances the sensitivity of
guest-dependent fluorescent property of the resulting MOF.
This result thus suggests that the guest-dependent
temperature-dependent PXRD measurements starting from
BUT-62W sample. From the PXRD patterns, it is obvious that
the framework gradually transformed when the sample was
heated. It was also found that all PXRD peaks shifted to higher
angles after heating, suggesting the shrinkage of the
framework upon the guest molecules removal (SI, Figure S7
6
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and S8). The structural shrinkage thus explains the and -63 remain almost the same porosity, chemical stability,
experimentally observed lower pore volume of BUT-62A (or - and stimuli responsive fluorescence after multiple stretching.
6
3A) than that calculated value based on the single-crystal
In order to further investigate the structural flexibility and
associated optical properties, we have attempted to
determine the crystal structures of BUT-62W transformed
phases. Probably due to large void and/or high flexibility of the
BUT-62 framework, after BUT-62W was activated into BUT-
structure of BUT-62W (or -63W) as stated above. Interestingly,
the PXRD pattern recovered after the heated sample being
immersed in a mixture of DMF and acidic water. These results
indicate that the framework of BUT-62 can display a reversible
breathing behavior. In addition, water vapor adsorption of
BUT-62A was carried out at room temperature. The isotherm
6
2A by the guest removal or after BUT-62A was transformed
into BUT-62Y by immersing BUT-62A in water, the resulting
crystals all gave weak/bad single-crystal X-ray diffraction
patterns, although they look like transparent and well-shaped,
and their PXRD peaks are strong. Nevertheless, we found that
the BUT-62W could also transform into BUT-62Y through
direct guest exchange with pure water as confirmed by PXRD
patterns, and obtained BUT-62Y crystals could gave much
better single-crystal X-ray diffraction data (SI, Table S2 and S3).
shows a steep water uptake at low pressure range (P/P
.05) followed by a gradual increase until 0.25 P/P , and then
another steep uptake between 0.25 and 0.35 P/P , followed by
0
<
0
0
0
again gradual increase at higher pressure range (SI, Figure
S17). This stepwise adsorption isotherm also implies the
structural change of BUT-62 induced by the water
4
7
adsorption. Similar vapor adsorption isotherms have been
4
observed in several reported flexible MOFs.
9,50
2
Ar, CO , and
Single-crystal structure determination and analysis shows
that the obtained BUT-62Y remains in the same space group of
BUT-62W with both the c and b axes slightly lengthened by
methanol vapor adsorption isotherms were also recorded for
BUT-62A (SI, Figure S14-16). It was found that almost no
hysteresis between adsorption and desorption branches was
3
.58 and 3.80%, respectively, but the a axis obviously
shortened by 17.90%, thus the unit cell volume decreased by
1.80%. Size changes of the MOF crystals during the phase
2 2
observed for Ar and CO . The interaction between Ar or CO
molecules with the framework is thus not so strong to trigger
the flexibility of the MOF. But for methanol, an obvious
hysteresis was observed, which should be related with the
flexibility of the MOF, as well as the high polarity and
hydrogen-bonding potential of the methanol molecules.
1
transition could also observed from their SEM images and
optical photographs, although it is quite challenging to
precisely record (SI, Figure S13). As a result, the large
diamondoid channels along the b direction are shrunken,
which is reflected by the change of the dihedral angles
between the carbazole rings on adjacent channel walls from
Additionally, the flexible property of these two MOFs is
also studied by mechanical extrusion. When compression was
applied, BUT-62W changed color immediately from white to
yellow, indicating the occurrence of a phase transformation.
Moreover, after treated with acidic DMF, the yellow sample
transformed back to a phase with white color similar to that of
as-synthesized BUT-62W, indicating a fully reversible flexibility
of the MOF. As shown in Figure S9a and b, the structural
flexibility of BUT-62 was also verified by the pressure-
dependent PXRD measurements. From the PXRD patterns, it is
obvious that the framework gradually changed when the
sample was compressed. It was found that all PXRD peaks
shifted to higher angles after compressing, suggesting the
shrinkage of the framework. Furthermore, the PXRD pattern
recovered after the compressed sample being immersed in a
mixture of DMF and acidic water. These results further
indicate that the framework of BUT-62 can display a reversible
breathing behavior. We also carried out repeat tests for the
structure transformation of the BUT-62 at varied pressures
1
14.2(4)° in BUT-62W to 139.0(3)° in BUT-62Y (SI, Figure S27a
and b), as well as two diagonals of the channels from 10 and
2
3 Å to 13 and 29 Å.
To better understand the structural transformation and the
structure-dependent optical property of BUT-62, structures of
BUT-62W and BUT-62Y are carefully compared. As shown in
Figure S27c and d, the Zr6 octahedron in BUT-62W is nearly
ideal. The distances of two diagonal Zr atoms at equatorial
positions and of two at apical positions are 4.99(1) and 4.90(3)
Å, respectively. However, in BUT-62Y, the Zr6 octahedron is
obviously distorted. The related distances of diagonal Zr atoms
are 4.85(2) and 5.03(3) Å, respectively. Accordingly, the cuboid
shaped [Zr (μ -O) (μ -OH) (OH) (−COO) ] SBU also changes in
6
3
4
3
4
4
8
shape. The geometrical dimension of the cuboid defined by
eight carboxylate C atoms changes from 6.43(5) × 4.24(2) ×
4
.41(3) Å to 6.49(3) × 4.40(2) × 4.24(1) Å (SI, Figure S27e and f).
In addition, the Zr–Ocarboxylate bond lengths in BUT-62W and -
2Y are in the range of 2.17(3)~2.28(5) and 2.15(9)~2.29(8) Å,
(from 0 to 1 GPa for BUT-62) for 3 cycles (where the samples
6
were recycled by treatment with acidic DMF and dried under
vacumm). As shown in Figure 9a, the PXRD patterns of BUT-62
remain nearly unchanged after 3cycles. These results indicated
that the structural flexibility of BUT-62 is well reversible and
reproducible. Similar phenomena was also observed in BUT-63,
according to the results from PXRD measurement (Figure S9b),
as discussed above for BUT-62, BUT-63 has similar structural
flexibility and reversibility to that of BUT-62 under mechanical
respectively, being almost not changed during the structural
transformation. Clearly, the change of the related bond angles
contributes more to the distortion of the Zr SBU. It was also
6
found that the framework transformation is resulted from not
only the change of linkages between the metal atoms and the
ligands but also the configurational change of the ligands
themselves. In BUT-62W, the dihedral angles between two
types of peripheral phenyl rings and the central carbazole are
extrusion. As shown in Figure S10, the PXRD patterns, N
2
4
–
4
5.2 and 39.1°, respectively. The geometry of the CTTB
adsorption, and fluorescence studies demonstrate that BUT-62
defined by four carboxylate C atoms is a trapezoid with three
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distinct edge lengths of 9.38(5), 12.34(3), and 13.31(3) Å, treated phase, leading to opened diamondoid channels close
respectively. In BUT-62Y, however the corresponding dihedral to those of BUT-62W. For comparison, distances between H-
angles and edge lengths change, being 47.4 and 37.6°, and bonding donor-acceptor atoms in BUT-62W are also evaluated
10.06(3), 11.92(4), and 13.71(4) Å, respectively. These changes and compared with those in BUT-62Y (SI, Figure S26 and Table
can be regarded as that the “arms and legs” of the ligand open S8). It was found that the H-bonding donor-acceptor distances
slightly from BUT-62W to -62Y, which can be well illustrated by in BUT-62Y are shorter than those in BUT-62W, implying
the superposed representation of the ligands as shown in different strengths of associated H-bonding interactions of
Figure S27. We also measured the single crystal diffraction water and DMF molecules.
4
–
data for BUT-62 loaded with toluene molecules In order to verify whether carbazole NH groups in the CTTB
Toluene@BUT-62). Structural analysis and comparison reveal ligands have an important effect on the flexible behavior of
that the conformation of ligand also changed when the guests BUT-62, the protons of the NH groups in BUT-62W were
in BUT-62W was exchanged with toluene molecules (SI, Figure substituted with methyl groups by post-synthetic
(
a
S28). As a result, the changes of these structural parameters modification. After substitution, the N atoms are expected less
should be responsible to the observed color and fluorescence hydrophilic. Experimentally, an as-made sample of BUT-62W
changes in BUT-62W and -62Y samples.
was treated with iodomethane in DMF in the presence of
K
2
CO
3
at 40 °C for 3 days, producing a phase referred as BUT-
In addition, water molecules strongly interacted with the
framework in BUT-62Y can be modeled by using the single-
4
. It was found that the CTTB ligands were
–
6
2W-CH
3
completely converted into methyl-substituted derivatives as
1
confirmed by H NMR spectrum of the NaOH aqueous solution
2
crystal diffraction data (referred as BUT-62Y-H O in SI). It was
found that the adsorbed water molecules in the voids of the
framework show relatively high electron densities (c.a. 1.3~2.8
3
digested sample of BUT-62W-CH (SI, Figure S30). BUT-62W-
–
3
CH3 retains the same structure as that of BUT-62W, as
confirmed by single-crystal diffraction and PXRD patterns (SI,
e
Å ) and reasonable atomic displacement parameters,
suggesting that they could strongly interact with the
framework surface. The primary water adsorption sites were
3
Table S5 and Figure S33). After the BUT-62W-CH sample being
2
–
activated by removing guest molecules in its pores and
subsequently immersed in water for 10 min, it was found that
it also changed color from white to yellow, forming the phase
3 6
found to be near the μ -O atoms of the Zr clusters in a
distance of ca. 2.27~2.42 Å, and the carbazole NH groups with
the N···O distance of c.a. 2.75 Å (SI, Figure S26). It should be
pointed out that the distances between the adsorbed water O
3
referred as BUT-62Y-CH . PXRD measurements reveal that a
2
–
structural transformation occurred from BUT-62W-CH to BUT-
3
atoms and μ
3
-O atoms should be slightly underestimated
2
–
–
62Y-CH3 as observed in the phase change from BUT-62W to
because of the symmetrical disorder of μ
3
-O and μ
-O atom is closer to the center of Zr cluster
-OH atom, but the two types of O atoms
3
-OH
2
–
BUT-62Y (SI, Figure S33). Fluorescence emission of BUT-62Y-
CH3 was also significantly red-shifted compared with that of
entities. The μ
compared to the μ
3
6
–
3
BUT-62W-CH
3
(SI, Figure S31). These results suggest that the
are not distinguished in the structure. Obviously, strong H-
bonding interactions are involved between these adsorbed
host-guest interactions only between adsorbed water
2
molecules and hydrophilic sites on the Zr clusters (μ -O , μ -
–
2
–
–
6
3
3
water molecules and the hydrophilic O or OH groups on the
framework. Additional adsorbed water molecules might also
–
OH , carboxylate O atoms, and coordinated OH and H
–
2
O) can
–
–
3
also induce the structural breathing behavior of BUT-62-CH ,
3 2
locate near the μ -OH , carboxylate O, terminal OH /H O, and
4
–
and the hydrophilic NH groups in CTTB ligand is not
determinative for the flexibility of BUT-62. According to the
results from PXRD measurement, water vapor adsorption, and
solid-state fluorescence study (Figure 2c, 3b, 3d and SI, Figure
S8, and S20), as discussed above for BUT-62, BUT-63 has
similar structural flexibility to that of BUT-62.
primarily adsorbed water. It is proposed that the H-bonding
interactions between the adsorbed water molecules in the
channels and the hydrophilic sites on the channel walls might
lead to the shrinkage of the large diamondoid channels in BUT-
62Y compared with those in BUT-62W. Based on this
observation, we speculate that the activated phase BUT-62A
might have the most expanded channels, because no solvent
molecules present in their pores to cause structure shrinkage
2
.5 pH Sensing of Aqueous Solution
due to H-bonding or other interactions. However, good single Inspired by above confirmed high chemical stability and
crystallinity could not retain after guest molecules were sensitive guest-dependent fluorescence response with
removed from BUT-62W and BUT-62A showed a PXRD pattern pronounced shift of emissions, the fluorescent properties of
with weak and broad peaks, which limits its structural BUT-62 and -63 responding to the pH value of aqueous
refinement. Similar structural transformation induced by the solution were investigated, to explore their potential
3
+
water inclusion has also been observed in MIL-53(Al or application in sensing the pH value of water system. As shown
The structural transformation of BUT-62Y to a phase in Figure 4a, the fluorescence spectra were recorded after the
3+ 51,52
Cr ).
with PXRD pattern similar to that of BUT-62W in strong acid BUT-62Y samples being suspended in aqueous solutions with
might thus also be related with the involved H-bonding pH values from 0 to 9 for 1 min, respectively. It was found that
interactions. Due to the protonation of adsorbed water the emission spectra are similar for the samples treated with
molecules and probably O/N atoms on the channel walls, the the pH = 9, 8, 7, and 6 aqueous solutions. As the acidity of the
H-bonding interactions in the channels are weaken in the acid aqueous solution increased from pH = 6 to 0, the emission
8
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peaks of the fluorescence spectra gradually blue-shifted from pH sensing which is commonly accomplished by the response
5
,37,53
λ
max = 522 to 426 nm. Considering the complicated conditions of emission intensity.
in possible real applications, interference experiments fluorescence intensity of the sensing material can be
including the temperature and ionic strength) on pH sensing influenced by not only pH value but also some other factors
were performed. As shown in Figure S42, the wavelength of (cations, anions, impurities, optical path length, drift of
max only slightly shifts with different temperatures (25, 35, optoelectronic system, etc.) in aqueous systems. A fluorescent
and 45 °C) at pH = 7 for the two MOFs, which suggests that the sensing material with pH dependent λmax would thus be quite
max shifts of the two MOFs are not profound at varied advantageous because the profile of emission peak normally
Especially, in some cases,
(
λ
λ
temperatures. The fluorescence emissions of the two MOFs at maintains no matter whether its emission intensity is
different pH (2.0, 5.0 and 7.2) in phosphate buffered saline quenched or enhanced. It should be noted that the
(PBS; ionic strength = 0.05 M) solution are also very similar to fluorescence intensity of BUT-62 also changed as the pH value
those of their suspensions in aqueous solution, indicating of checked aqueous solution varied (SI, Figure S36), however, a
strong anti-interference ability of pH sensing functionality with clear relation between its λmax and the acidity of the aqueous
different ionic strength (Figure S43). Moreover, when different solution can still be withdrawn.
acids (HCl, HNO , and H SO ) were used in experiments, the
3
2
4
For BUT-63, the fluorescence spectra are almost
overlapped for the BUT-63Y derived samples after being
suspended in the aqueous solutions of pH values at 8 and 9,
respectively. However, the λmax gradually blue-shifted from
fluorescence changes were substantially the same (Figure S23
and S24), indicating that the acidity of the aqueous solution is
the key factor for changing fluorescence emission, but not
associated anions or other chemicals. These results suggest
that the pH sensing of the two MOFs was not obviously
influenced by the temperature, ionic strength or other
chemicals. More interestingly, the λmax and the pH values of
the aqueous solutions are basically linear-dependent in the pH
range of 1~6 for BUT-62 (Figure 4c).
6
03 to 422 nm as the pH values decreased from 8 to 0. And,
the λmax and pH values of the aqueous solutions are well
linearly dependent in the almost full pH range measured (pH =
8
to 1) (Figure 4c). Owing to the wider emission shift in BUT-63
181 nm) than that of BUT-62 (96 nm), the sensing sensitivity
of the former is thus enhanced by over 1.3 times relative to
(
A linear relationship between the λmax and the pH value that of the latter, which can be indicated by the slope values
suggests the feasibility of quantitatively determining pH value (24.63 vs. 18.4) for the linear fittings of the data. In addition,
of an aqueous media by measuring the λmax of a fluorescent how fast the λmax shifts as the pH varies is also an important
material. It would be an alternative approach for fluorescence factor for such pH sensors. Additional experiments were
Fig. 4 Fluorescent pH sensing of aqueous solution with BUT-62 and -63. Intensity normalized luminescent spectra of BUT-62 (a) and -63 (b) treated in aqueous solutions with
different pH values at room temperature; c) the pH dependence of the wavelength of emission maximum (λmax) and the linear fits for BUT-62 (black) and -63 (red); and
photographs of BUT-62Y/CMC plate (d) and BUT-63Y/CMC plate (e) under 365 nm UV irradiation after being dipped in aqueous solutions with different pH values.
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configuration changes of ligands. We further measured the
PXRD patterns of the two MOFs after they are treated in
aqueous solutions with different pH values. The results are
shown in the Figure S38 and S39. The PXRD patterns are
different, showing that the structures of the MOFs
transformed with respect to different pH values. This is similar
to the observations from PXRD patterns of the MOFs loaded
carried out to check this. It was found that the shifting of λmax
could completed within 10s after changing the pH values of
the suspensions, suggesting a fast kinetics for the pH sensing
of the MOFs. To the best of our knowledge, BUT-62 and -63
represent the first MOF example capable of quantitatively
sensing/detecting the pH value of aqueous solution by
measuring the λmax of the fluorescent emission, but not only
the intensity.
with different guest molecules. Large
λmax shifts in
fluorescence for the MOFs are thus probably due to the
sensitive response of the ground (LUMO) and excited states
Reusability is an important feature for an effective sensing
material. To check if the samples of BUT-62 and -63 could be
reused to test multiple pH values, we carried out repeated
tests for the fluorescence of the MOFs in varied pH aqueous
solutions (pH from 6 to 1 for BUT-62, and pH from 8 to 1 for
BUT-63) for 3 cycles, where the samples were recycled by
centrifugation and washing with deionized water after each
measurement. As shown in Figure S40, the fluorescence
spectra of the two MOFs are reproducible after regeneration
with a simple treatment.
(HOMO) of the ligands to stimuli, such as their conformation
changes as well as different host-guest interactions. Under the
stimuli, the energy gaps between HOMO and LUMO of the
systems changed, resulting in the fluorescence wavelength
shifts.
To further verify the sensitive pH dependent fluorescence
variation of BUT-62 and -63, and make the pH detection easy
and feasible, fluorescent test plates with a thin layer of
MOF/CMC (CMC = carboxymethyl cellulose, commonly used in
making thin layer chromatography plates) mixture on glass
slide substrate were fabricated. The prepared plates were
dipped in the aqueous solutions with the pH value ranging
from 9 to 0 for a few seconds, and then dried in air for few
minutes. Under the irradiation of UV light, the fluorescent
images of the plates were recorded by digital photograph. As
shown in Figure 4d, the fluorescence of the plates for BUT-62
changed from yellow green to green, and finally blue as the
acidity of the aqueous solutions increased from pH = 9 to 0.
The fluorescence change for the BUT-63 plates is more
marked, from yellow at pH = 9 and 8, green at pH = 4, to blue
at pH = 0 (Figure 4e). These results are in accordance with
those observed in fluorescence spectra for the BUT-62 and -63
suspensions in acidic aqueous solutions, and also demonstrate
a rapid and sensitive fluorescent response of them to the pH
values of aqueous solutions.
For better understanding the sensing mechanism of the
MOFs for pH values, we performed the control experiment on
BUT-62-CH3. The fluorescence spectra of BUT-62-CH3
suspended in aqueous solutions with different pH values were
recorded. As shown in Figure S32, the λmax of BUT-62-CH3
shifted on varying the pH values, similar to that observed in
BUT-62, but the emission shifting for BUT-62-CH (515 nm at
3
pH = 7 to 438 nm at pH =1) is slightly weakened in comparison
with that for BUT-62 (522 nm at pH = 7 to 426 nm at pH =1).
This result thus confirms that the presence N-H group in the
MOFs is not a determinative factor for their fluorescence
response, but has slight influence on the florescence of MOFs.
We have also checked the fluorescence spectra of pure organic
ligands H CTTB and H CTNA in water with different pH values.
4
4
As shown in Figure S41, for both of the two ligands, λmax
slightly shifts with varying pH values. For H CTTB, the λ
4
max
changes from 459 to 432 nm when the pH values change from
to 9. For H CTNA, it changes from 495 to 466 nm when the
0
4
pH values change from 0 to 9. Clearly, the λmax shifts of the two Conclusions
ligands are not profound in comparison with those of their
In summary, we have synthesized two highly porous and
corresponding MOFs, BUT-62 (from 426 to 522 nm) and -63
from 432 to 603 nm) in the pH ranges from 0 to 9.
chemically stable Zr-MOFs by using newly-designed trapezoid
organic linkers with a carbazole core as main fluorophore. The
two MOFs exhibit structural breathing behavior responsive to
adsorbed guest molecules in their pores, due to the changed
interactions between the frameworks and the guest
molecules. Accompanied with slight configurational changes of
the ligands during the framework breathing as demonstrated
by single-crystal structure analysis, these MOFs represent
sensitive fluorescence changes in the wavelengths of emission
peaks. Based on the guest-dependent structural flexibility and
associated fluorescence change, as well as the chemically
robust nature of the frameworks, the two MOFs have been
tested in aqueous media for pH sensing in the pH values
ranging from 0 to 9. The observed linear relationship between
the fluorescence λmax and the pH value of aqueous solution
suggests the capability of these MOFs in quantitatively sensing
the pH value in a unique manner, which is only based on the
(
Particularly, it was noted that for the ligands, λmax are red-
shifted as decreasing of pH values, in contrast, those for BUT-
62
and -63 are blue-shifted as decreasing pH values.
Furthermore, considering that the ligands H CTTB and H CTNA
4
4
are deprotonated and coordinated with metal atoms in the
MOFs, checking the fluorescence of their esterified partners
should be more informative. Fluorescence spectra of the
methyl-esterified H CTTB (referred as M CTTB) and H CTNA
4
4
4
(referred as M CTNA) dissolved in water with different pH
4
values were then recorded. As shown in Figure S41, the
fluorescence emissions of M CTTB and M CTNA are nearly
4
4
independent on the pH values. Above results suggest that the
pH sensing mechanism of the MOFs should not be simply
resulted from the pH responding behavior of ligands
themselves. The pH sensing effect of the two MOFs might be
attributed to their structural flexibility, which leads to the
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emission wavelengths other than intensities. They are thus 22 A. J. Howarth, Y. Liu, P. Li, Z. Li, T. C. Wang, J. T. Hupp, O. K.
Fara, Nat. Rev. Mater. 2016,
3 J. Liang, R. -P. Chen, X. -Y. Wang, T. -T. Liu, X. -S. Wang, Y. -B.
Huang, R. Cao, Chem. Sci., 2017, , 1570-1575.
4 S. B. Kalidindi, S. Nayak, M. E. Briggs, S. Jansat, A. P.
Katsoulidis, G. J. Miller, J. E. Warren, D. Antypov, F. Cora, B.
Slater, M. R. Prestly, C. Marti-Gastaldo, M. J. Rosseinsky,
Angew. Chem. Int. Ed., 2015, 54, 221-228.
1, 15018.
expected to be potentially useful in the wavelength-based
fluorescence pH sensing for environmental and biological
systems.
2
2
8
Conflicts of interest
2
5 T. -F. Liu, D. Feng, Y. -P. Chen, L. Zou, M. Bosch, S. Yuan, Z.
Wei, S. Fordham, K. Wang, H. -C. Zhou, J. Am. Chem. Soc.,
There are no conflicts to declare.
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TOC
Flexible Metal-Organic Frameworks for Wavelength-Based
Luminescence Sensing of Aqueous pH
Xiu-Liang Lv, Lin-Hua Xie, Bin Wang, Minjian Zhao, Yuanjing Cui and Jian-Rong Li*
Two new metal-organic frameworks (MOFs) constructed from trapezoidal carbazole-core based
ligands show high porosity, good chemical stability, reversible guest-dependent structural
flexibility, as well as sensitive stimuli responsive fluorescence. It is demonstrated that these
MOFs are highly potential in the wide-range quantitative pH sensing by facilely determining the
wavelengths of maximum emission.