Structural Transformation in Metal–Organic Frameworks for Reversible Binding of Oxygen

Article abstract of DOI:10.1002/anie.201902810

Polycyclic aromatic derivatives can trap ¹O₂ to form endoperoxides (EPOs) for O₂ storage and as sources of reactive oxygen species. However, these materials suffer from structural amorphism, which limit both practical applications and fundamental studies on their structural optimization for O₂ capture and release. Metal–organic frameworks (MOFs) offer advantages in O₂ binding, such as clear structure–performance relationships and precise controllability. Herein, we report the reversible binding of O₂ is realized via the chemical transformation between anthracene-based and the corresponding EPO-based MOF. It is shown that anthracene-based MOF, the framework featuring linkers with polycyclic aromatic structure, can rapidly trap ¹O₂ to form EPOs and can be restored upon UV irradiation or heating to release O₂. Furthermore, we confirm that photosensitizer-incorporated anthracene-based MOF are promising candidates for reversible O₂ carriers controlled by switching Vis/UV irradiation.

Full text of DOI:10.1002/anie.201902810

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Accepted Article
Title: Structural Transformation in Metal-Organic Framework for
Reversible Binding of Oxygen
Authors: Jin-Yue Zeng, Xiao-Shuang Wang, Yong-Dan Qi, Yun Yu,
Xuan Zeng, and Xian-Zheng Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902810
Angew. Chem. 10.1002/ange.201902810
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Structural Transformation in Metal-Organic Framework for
Reversible Binding of Oxygen
Jin-Yue Zeng,[a]# Xiao-Shuang Wang,[a]# Yong-Dan Qi,[a]# Yun Yu,[a] Xuan Zeng,[a] and Xian-Zheng
Zhang[a,b]
*
Abstract: Polycyclic aromatic derivatives can trap 1O2 to form
endoperoxides (EPOs) for O2 storage and as sources of reactive
oxygen species. However, these materials suffer from structural
amorphism, which limit practical applications and fundamental studies
on the structural optimization for O2 capture and release. Metal-
organic framework (MOF) show advantages in the binding of O2, such
as clear structure-performance relationship and precise controllability.
Here, we report the reversible binding of O2 is controllably realized via
the chemical transformation between anthracene-based and the
corresponding EPO-based MOF. It is demonstrated that anthracene-
based MOF, the framework featuring linkers with polycyclic aromatic
structure, can rapidly trap 1O2 to form EPOs and can be restored upon
UV irradiation or heating for releasing O2. Furthermore, we confirm
that photosensitizer-incorporated anthracene-based MOF can be
promising candidates for reversible O2 carriers by switching Vis/UV
irradiation.
delivery systems.[7] The hybrid nature of MOF offers an almost
infinite suite of building blocks that can be manipulated to
construct functional MOF through introducing open metal
coordinate sites and functional groups into the framework.[8]
Generally, the chemical transformation of building blocks in MOF
involves irreversible or passive interaction, which may lead to the
damage of crystalline structure.[9] It is difficult to control the spatial
configuration transformation of linkers and the structure stability
of MOF, thus, the chemistry occurring at the interface between
the framework and an incoming substrate.[10] To our best
knowledge, the reversible binding of guest molecule triggered by
thermally and photochemically inducing chemical transformation
of ligands in MOF has rarely been reported.
Endoperoxides (EPOs) are capable of interesting photochemical
and thermodynamic properties.[1] Their potential applications for
reversible oxygen storage and as chemical sources of reactive
oxygen species, have made these compounds the focus of many
fundamental investigations.[2] Many polycyclic aromatic
derivatives could trap the singlet oxygen (1O2) to form EPOs.[3]
EPOs exhibit the exceptional performance of releasing oxygen,
generally in the excited singlet state, under heating or ultra violet
(UV) irradiation.[4] Functional materials constructed with polycyclic
aromatic compounds or EPOs can conduct the binding of oxygen
upon light irradiation or with thermal treatment.[5] Therefore, the
reversible chemical binding of oxygen has significant advantages,
such as better stability and more controllability, compared with
physical adsorption.[6] However, these materials suffer from
structural amorphism and poor solubility, which limit practical
applications and fundamental studies on the structural
optimization for the precise control of O2 capture and release.
Metal-organic frameworks (MOFs) are powerful porous
crystalline materials and used extensively for a wide range of
applications from basic research to successful practical
utilizations, including adsorption and separation of guest
molecules, providing active catalytic sites and fabrication of drug
Scheme 1. Schematic diagram of the structural transformation in MOF for
reversible binding of oxygen. Photo-physical processes consist of energy
transfer (ET), intersystem crossing (ISC). S0, S1 and T1 corresponds to the
ground states, the lowest excited states and triplet states, respectively.
Our strategy to obtain the reversible binding is based on the
chemical transformation between anthracene-based MOF and
the corresponding endoperoxide-based MOF (Scheme 1).
Anthracene-based derivatives exhibit excellent luminescent
property and photoresponse, are widely used in supramolecular
systems and reversible conversion materials.[11] Here, we report
on the controlled synthesis, structural and spectroscopic
characterization, and reversible binding studies of oxygen on
anthracene-based MOF, which can convert into endoperoxide-
[a]
[b]
Dr. J.-Y. Zeng, X.-S. Wang, Y.-D. Qi, Y. Yu, Dr. X. Zeng, Prof. Dr.
X.-Z. Zhang
Key Laboratory of Biomedical Polymers of Ministry of Education &
Department of Chemistry, Wuhan University
Wuhan 430072, China
based MOF by the photo-oxygenation involving
a [4+2]-
cycloaddition of 1O2 on electron-rich carbons of anthracene-based
linker. Meanwhile, the resulting endoperoxide-based MOF can
release the binding oxygen and restore to parent anthracene-
based MOF through the cycloreversion of peroxidic bond upon
UV irradiation or heating. In this process, the crystalline structure
of MOFs well maintain due to the robust coordination interaction
between Zr6 cluster and the linker. Furthermore, we investigated
that the photosensitizer was incorporated into anthracene-based
E-mail: xz-zhang@whu.edu.cn
Prof. Dr. X.-Z. Zhang
The Institute for Advanced Studies, Wuhan University
Wuhan 430072, China
# These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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MOF to form core-shell nanoplatform for trapping and releasing
the oxygen by switching Vis/UV irradiation.
investigated (Figure 2a). The EPOs of DPA-MOF, EPO-MOF, can
be obtained by the [4+2]-cycloaddition with 1O2, which was
produced by 5,10,15,20-Tetraphenylporphyrin (TPP) under
irradiation of 660 nm light-emitting diode (LED). The cross-
polarization magic-angle-spinning 13C nuclear magnetic
resonance (CP/MAS 13C NMR) spectrums of DPA-MOF and
EPO-MOF showed separate peaks for C5 and C5’, C6 and C6’ as
well as for C7 and C7’ as a result of the formation of peroxidic
bond, characteristic of the unique carbon atoms of DPA and EPO,
respectively (Figure 2b). Upon exposure to 1O2, a new peak
centered at 84 ppm emerged, consistent with formation of
peroxidic bond in EPO-MOF. The electrospray ionization mass
spectrometry (ESI-MS) data of phosphate-digested EPO-MOF
showed the quasi-molecular ion peak at mass-to-charge ratio
(m/z) of 450.1, indicating that the ligands in DPA-MOF were
capable of trapping 1O2 to form endoperoxide-based linkers
(Figure S13).
Figure 1. (a) Schematic representation of DPA-MOF, in which 12-connected Zr6
cluster and DPA were simplified as blue nodes and black line, respectively. (b)
Schematic illustration of DPA-MOF on the basis of thermodynamic regulation,
yielding different sizes. (c) The cavities of DPA-MOF (c1: octahedral, c2:
tetrahedral).
In this work, an anthracene-based MOF (DPA-MOF) was
prepared by 9,10-anthracenyl bis(benzoic acid) (DPA) for
reversible binding of O2. The crystal structure of DPA-MOF
showed the cubic crystal system with Fm-3m space group and
lattice parameter of a = 32.9382 Å (Figure S1, S2 and Table S1).
The framework was constructed from the secondary building units
(SBUs) of Zr6(μ3-O)4(μ3-OH)4(carboxylate)12 connected by the
linear anthracene-based linkers, which composed by one
octahedral cavity and two tetrahedral cavities (Figure 1a, 1c). The
high connectivity on Zr6 cluster could increase the linker loading
efficiency for trapping oxygen. In MOF, the ligand spatially
isolated by the framework, thus greatly maintaining its molecular
property. This feature would allow for maximization of its potential
application by optimizing the dimension of MOF.[12]
To control the size of DPA-MOF, thermodynamic analysis was
performed along with the manipulations of reaction conditions,
such as temperature, time and substrate concentration (Figure 1b,
Table S2-S4). The size and morphology of DPA-MOF were
measured by Scanning Electron Microscope (SEM). In principle,
the formation of MOF undergone nucleation and crystal growth,
which could be simplified as the process of coordination chemical
equilibrium.[13] Increasing the synthetic temperature T was an
approach to raise nucleation rate and facilitate the crystal growth
process.[14] When the temperature was varied with a ∼10 K
interval from 363 to 403 K, the sizes of DPA-MOF increased from
220 to 650 nm (Figure S3, S4). The growth of DPA-MOF was
further tuned by diluting the system, while aiming to produce more
nucleation, which would generate smaller nanoparticles. When
the system was diluted from 1 to 30 times, based on its primary
synthetic condition, the sizes of DPA-MOF reduced from 190 to
45 nm (Figure S5, S6). As confirmed by Powder X-ray diffraction
(PXRD) patterns, DPA-MOF with different sizes retain their crystal
structure (Figure S9-S11). The reaction kinetic for DPA in the
growth of DPA-MOF indicated that DPA-MOF nucleated and
growed with reaction time and the crystallinity became better
(Figure S7, S8 and S12).
Figure 2. (a) Schematic illustrating the chemical transformation between DPA-
MOF and EPO-MOF for reversible binding of O2 (Atom color, C, black, O, red,
Zr, blue). (b) CP/MAS 13C NMR spectra of activated DPA-MOF before (bottom)
and after (top) photo-oxygenation.
To investigate the structure of EPO-MOF, the structural model
was built from DPA-MOF, which adopts the cubic crystal system
with Fm-3m space group and lattice parameter of a = 32.7767 Å.
Such a structural model is supported by the similarity of the
experimental PXRD pattern as compared to that obtained from
parent DPA-MOF (Figure S14). The long-range order structure of
EPO-MOF was confirmed by small angle XRD, which showed the
similarity compared to DPA-MOF (Figure S15). Furthermore, we
found that EPO-MOF exhibited similar O2 and N2 adsorption
isotherms compared to that of DPA-MOF, suggesting that EPO-
MOF well maintained porosity (Figure S16, S17). Therefore,
Having confirmed the unaltered crystal property of different
sizes, the chemical bonding of oxygen in the framework was then
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these results demonstrated that DPA-MOF transformed into the
corresponding EPO-MOF through photo-oxygenation for trapping
oxygen and the crystal structure of MOF well maintained.
fluorescence analysis and observed by the fluorescence
resonance energy transfer from the ligands in MOF to the guest
fluorescent molecules (Figure S19-S22). The emission of DPA-
MOF gradually vanished with the increase of irradiation time,
suggesting that anthracene-based linkers have quantitatively
reacted with 1O2 to form EPOs (Figure 3c). Meanwhile, the
temperature of thermolysis on EPO-MOF was more than 333 K,
the fluorescence gradually recovered and the cycloreversion rate
of EPO-MOF increased with reaction temperature (Figure 3d, 3e).
Next, we investigated the cycloreversion reactions of photolysis
on EPO-MOF. Previous studies revealed that the photo-
cycloreversion of EPOs originated from upper excited singlet
state ππ* and the decomposition occurs from the lowest excited
singlet state S1 (π*σ*), corresponding to the locally excited
peroxide chromophore (Figure S23).[15] We found that EPO-MOF
exhibited two UV-Vis absorption bands, including a high-energy
absorption centered at 264 nm, and less intense low-energy
absorption at wavelength above 350 nm (Figure S24). Based on
this, it was demonstrated that EPO-MOF could restore to parent
DPA-MOF under 254 nm UV irradiation through photo-
cycloreversion (Figure 3f). As confirmed by solid-state 13C NMR,
ESI-MS of phosphate-digested MOF and fluorescence analysis,
EPO-MOF can efficiently be restored to parent DPA-MOF and
release O2 through the cycloreversion of peroxidic bond upon
thermolysis and photolysis (Figure S25-S28). Recurring multiple
cycles of photo-oxygenation, heating and UV irradiation revealed
that the emission and the porosity as well as crystal structure of
MOF have no significant changes in reversible chemical
transformation (Figure S29-S32).
Figure 3. (a) Schematic diagram of structural transformation between DPA-
MOF and EPO-MOF for reversible binding of oxygen. (b) Schematic illustrating
chemical transformation of upon thermolysis or photolysis. (c) Changes in
fluorescence spectrum over time for photo-oxygenation on DPA-MOF. (d)
Fluorescence intensity at 450 nm upon thermolysis on EPO-MOF at different
temperatures. Changes in fluorescence spectrum over time for (e) thermolysis
at 373 K and (f) photolysis upon 254 nm UV irradiation on EPO-MOF.
Fluorescence spectrums were measured with 50 μg/mL of MOFs in DMF
(excitation, 365 nm).
As DPA-MOF could chemically convert into the corresponding
EPO-MOF by trapping 1O2, we investigated EPO-MOF for O2
release under heating and UV irradiation (Figure 3a). Considering
the rearrangement pathway of hemolytic cleavage, the transition
state of linker involves a distance change between Zr6 clusters
from 14.3 Å of EPO-MOF to 9.0 Å of transition state (Figure S18).
The hemolytic cleavage of peroxidic bond induces the severe
twist of ligands in the transition state of MOF, and results in the
collapse of crystal structure. Therefore, it can be inferred that
cycloreversion is greatly favored over hemolytic cleavage by the
robust coordination interaction between Zr6 cluster and the linker
(Figure 3b). Furthermore, we found that the reversible chemical
Figure 4. (a) Schematic illustrating TCPP@DPA-MOF for reversible binding of
oxygen. (b) SEM image and TEM image of DPA-MOF for the construction of
TCPP@DPA-MOF. (c) SEM image and (d) TEM image of core-shell
TCPP@DPA-MOF, scale bar = 400 nm. (e) HAADF-STEM image and the
corresponding STEM-EDS elemental mapping images, scale bar = 200 nm (e1,
C; e2, N; e3, Zr, e4, Pd-TCPP).
After confirming the reversible chemical transformation
between DPA-MOF and EPO-MOF, we envisioned that
anthracene-based MOF could be switched through Vis/UV
irradiation by adopting photosensitive systems into the
frameworks (Figure 4a). Based on this, Tetrakis(4-
carboxyphenyl)-porphyrin (TCPP), which was used as
transformation
could
be
quantitatively
detected
by
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photosensitive system and could convert 3O2 into 1O2, was
incorporated into DPA-MOF due to their potential coordination to
Zr6 cluster. The optimized synthetic condition could produce the
core-shell TCPP@DPA-MOF rather than the mixture of different
MOFs. Transmission electron microscopy (TEM) and SEM
images of TCPP@DPA-MOF show approximately 510 nm of
core-shell nanoparticles with high uniformity and narrow size
distribution (Figure 4b-4d). It is reasoned that the thermodynamic
stability of DPA-MOF inhibited possible formation of TCPP-MOF,
which was also controlled by the low dose of TCPP. The
photosensitizer of TCPP, containing multiple carboxylates, could
readily participate in the coordination of Zr6 clusters by partially
substituting DPA linkers during the post-growth of DPA-MOF.
Once TCPP was installed inside the framework, the “ship-in-a-
bottle” effect resulting from the pore windows of DPA-MOF and
the robustness Zr-COO bonds could prevent the leaching of
TCPP (Figure S33).[16] The elemental mapping of C, N, Zr and
Pd(TCPP) in a single TCPP@DPA-MOF showed the core-shell
structure in terms of both geometrical and compositional
distributions (Figure 4e). Meanwhile, TCPP@DPA-MOF exhibited
additional peaks from TCPP absorbance in UV/Vis spectrum,
demonstrating that TCPP was incorporated into DPA-MOF
(Figure S34).
upon photochemical induction and heating (Figure 5a). It was
found that the fluorescence emission of TCPP@DPA-MOF at 450
nm gradually quenched upon 660 nm LED irradiation, suggesting
1
that TCPP@DPA-MOF not only converted O2 into O2 upon 660
nm LED irradiation, but also trapped 1O2 to form the
corresponding TCPP@EPO-MOF (Figure 5b, 5c). Meanwhile,
TCPP@DPA-MOF was capable of trapping O2 in water by
monitoring O2 concentration upon visible light irradiation (Figure
S35). Furthermore, we confirmed that TCPP@EPO-MOF could
release O2 under 254 nm UV irradiation, and restore to parent
TCPP@DPA-MOF (Figure 5d). The reversibility of TCPP@DPA-
MOF was performed by alternating cycles of switching between
different light exposures (Figure 5e). The structure and
morphology of TCPP@DPA-MOF showed no significant changes
in reversible chemical transformation (Figure S36, S39 and S40).
Additionally, TCPP@EPO-MOF could also converted into parent
TCPP@DPA-MOF upon heating for the release of oxygen (Figure
S37, S38).
In summary, we demonstrated that DPA-MOF could trap 1O2 to
form the corresponding EPO-MOF, which could be restored to
parent DPA-MOF upon UV irradiation or heating. Benefitting from
the robust coordination interaction between Zr6 clusters and the
linkers, the cycloreversion of EPO-MOF is favored over hemolytic
cleavage. Therefore, recurring multiple cycles of photo-
oxygenation and subsequent UV irradiation or heating, the
crystalline structure of MOF well retained and the performance for
reversible binding of O2 exhibited no significant changes.
Furthermore, we confirmed that TCPP could be incorporated into
DPA-MOF for the fabrication of core-shell TCPP@DPA-MOF,
which showed reversible binding oxygen through switching
Vis/UV irradiation.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (51833007, 51873162 and
51690152).
Keywords: metal-organic framework • structural transformation
• chemical binding • endoperoxide • oxygen
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Table of Contents
Reversible binding of oxygen can
Jin-Yue Zeng, Xiao-Shuang Wang,
Yong-Dan Qi, Yun Yu, Xuan Zeng, and
Xian-Zheng Zhang*
be controllably obtained through the
chemical transformation between
anthracene-based MOF and the
corresponding endoperoxide-based
MOF, which is able to trap and
release oxygen by switching Vis/UV
irradiation.
Page No. – Page No.
Structural Transformation in Metal-
Organic Framework for Reversible
Binding of Oxygen
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