Visible-light harvesting pyrene-based MOFs as efficient ROS generators

Article abstract of DOI:10.1039/c9sc03080h

The utilization of reactive oxygen species (ROS) in organic transformations is of great interest due to their superior oxidative abilities under mild conditions. Recently, metal-organic frameworks (MOFs) have been developed as photosensitizers to transfer molecular oxygen to ROS for photochemical synthesis. However, visible-light responsive MOFs for oxygen activation remains scarce. Now we design and synthesize two porous MOFs, namely, PCN-822(M) (M = Zr, Hf), which are constructed by a 4,5,9,10-(K-region) substituted pyrene-based ligand, 4,4′,4′′,4′′′-((2,7-di-tert-butylpyrene-4,5,9,10-tetrayl)tetrakis(ethyne-2,1-diyl))-tetrabenzoate (BPETB^4-). With the extended π-conjugated pyrene moieties isolated on the struts, the derived MOFs are highly responsive to visible light, possessing a broad-band adsorption from 225-650 nm. As a result, the MOFs can be applied as efficient ROS generators under visible-light irradiation, and the hafnium-based MOF, PCN-822(Hf), can promote the oxidation of amines to imines by activating molecular oxygen via synergistic photo-induced energy and charge transfer.

Full text of DOI:10.1039/c9sc03080h

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Visible-light Harvesting Pyrene-based MOFs as Efficient ROS
Generators
Yingmu Zhanga, Jiandong Panga, Jialuo Lia, Xinyu Yanga, Mingbao Fengc, Peiyu Caia, Hong-Cai
Received 00th January 20xx,
Accepted 00th January 20xx
Zhou*,a,b
DOI: 10.1039/x0xx00000x
The utilization of reactive oxygen species (ROS) in organic transformations is of great interest due to their superior oxidative
abilities under mild conditions. Recently, metal-organic frameworks (MOFs) have been developed as photosensitizers to
transfer molecular oxygen to ROS for photochemical synthesis. However, visible-light responsive MOFs for oxygen activation
remains scarce. Now we design and synthesize two porous MOFs, namely, PCN-822(M) (M = Zr, Hf), which are constructed
www.rsc.org/
by
a
4,5,9,10-(K-region)
substituted
pyrene-based
ligand,
4,4',4'',4'''-((2,7-di-tert-butylpyrene-4,5,9,10-
tetrayl)tetrakis(ethyne-2,1-diyl))-tetrabenzoate (BPETB4). With the extended π-conjugated pyrene moieties isolated on the
struts, the derived MOFs are highly responsive to visible light, possessing a broad-band adsorption from 225 – 650 nm. As a
result, the MOFs can be applied as efficient ROS generators under visible-light irradiation, and the hafnium-based MOF, PCN-
822(Hf), can promote the oxidation of amines to imines by activating molecular oxygen via synergistic photo-induced energy
and charge transfer.
coordination bonds.14, 15 Owing to their intrinsic porosity and
infinite tunability, MOFs have shown a vast and ever-expanding
array of applications, including gas storage, separation,
Introduction
Reactive oxygen species (ROS) are highly active molecules that
are commonly formed by normal metabolism of oxygen in
heterogeneous catalysis, chemical sensing, drug delivery, and
proton conduction.15-25 Recently, MOFs have been exploited as
a unique platform to integrate photosensitizers for light
harvesting and photocatalysts.26-31 For light-driven ROS
generation, many MOF-based systems employ predesigned
organic and metal-organic chromophores as linkers to impart
MOFs with photoactivities.32-34 The confinement and isolation
effect of the frameworks prevent the undesired self-
aggregation of the photosensitizers, reducing the non-
productive decay process of the excited states. Meanwhile, the
porous feature of the MOFs facilitates the diffusion of ROS,
2
aerobic life.1, With their superior oxidative abilities, ROS
present great potential in various applications, such as
industrial wastewater treatment, photochemical synthesis and
photodynamic therapy (PDT).3-6 So far, efforts have been
devoted to explore artificial chemical systems to produce ROS
in an environmentally benign condition. The biomimetic
process usually requires a photosensitizer (PS), which can
harvest solar energy and then generates ROS via energy transfer
and/or electron transfer.7 In particular, organic dyes and
transition metal complexes have been extensively studied as
photosensitizers to create ROS for organic transformations.8-12
However, most photosensitizers suffer from aggregation-cause
quenching (ACQ) at high concentrations, which undoubtedly
hampers their performance in solar-energy conversion.13
enhancing their further interaction with incoming reactants.35-
37
Metal-organic frameworks (MOFs), also known as porous
coordination polymers (PCPs), are an emerging class of
crystalline porous materials that are assembled with inorganic
secondary building units (SBUs) and organic linkers via
a. Department of Chemistry, Texas A&M University, College Station, Texas 77842-
3012, United States.
b. Department of Materials Science and Engineering, Texas A&M University, College
Station, Texas 77842, United State.
c. Department of Environmental and Occupational Health, School of Public Health,
Texas A&M University, College Station, TX 77843, United State.
†Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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the four phenylacetylenic substituents are attached to the
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4,5,9,10-positions (K-region) of pyrene with the aim of
DOI: 10.1039/C9SC03080H
extending the π conjugation of the pyrene chromophore, more
importantly, red-shifting the wavelength of absorption. It has
been revealed that the electrophilic substitution of pyrene
prefers to occur at 1,3,6,8-positions.46 To tackle the synthetic
t
obstacle, two Bu groups were attached to the 2- and 7-
positions, leading to the K-region substitution (Scheme S1†).
Yellow single crystals of PCN-822(Zr) were obtained under
solvothermal reaction conditions between zirconium (IV)
chloride and H4BPETB with trifluoroacetic acid (TFA) as a
modulating reagent. Similar reaction with hafnium (IV) chloride
as a starting material afforded single crystals of PCN-822(Hf).
Powder X-ray diffraction (PXRD) studies demonstrated the
crystalline phase purity of both MOFs. As the MOFs share the
same framework structure confirmed by single-crystal
diffraction studies, the structure of PCN-822(Zr) is discussed as
a representative (Fig. 1 and Table S1†). PCN-822(Zr) crystalizes
푎푚푑
in the tetragonal I41/
space group. In the Zr6(μ3-O)4(μ3-OH)4
Fig. 1. Representative structure and topology of (a) the 8-connected Zr6/Hf6 clusters and
cluster core, six Zr atoms form an idealized octahedron where
eight edges are bridged by eight carboxylate groups from
BPETB4 ligands and the triangular faces are capped by eight μ3-
oxygen atoms. The symmetry of the cluster core is D2d, which is
distorted from the ideal cubic symmetry (Fig. 1a). In the BPETB
linker, each carboxylate connects with one Zr6 cluster and the
(b) the tetratopic BPETB4- that constitute (c) the (4,8)-connected PCN-822.
Among various photochromic compounds, pyrene and its
derivatives with the four-ring-fused planar electron-enriched
skeleton are well-known for their unique optical and
photoelectronic properties.38 They have been demonstrated to peripheral benzene rings have a dihedral angle of 10.99°
deviated from the pyrene plane (Fig. 1b). Consequently, PCN-
822(Zr) adopts a three-dimensional (3D) structure with a (4,8)-
connected sqc topology, which is isostructural to PCN-225.47
The framework contains two types of channels along the a and
b-axes, while no apparent pores are observed along c-axis. The
small channel with a size of 0.6 × 1.8 nm2 in quadrangle shape
is surrounded by two Zr6 clusters and two BPETB4 ligands,
whereas the larger one with a size of 0.8 × 3.2 nm2 in pear-like
shape is comprised of three Zr6 clusters and three BPETB4
ligands (Fig. 1c). The solvent accessible volume in fully
evacuated PCN-822(Zr) is 56.5 % calculated by Mercury with a
probe of 1.8 Å. Due to the relatively large 1D channels in two of
the three dimensions, activation attempts of the MOFs for N2
adsorption isotherm measurements were unsuccessful.
Instead, the porosity of the materials was confirmed by dye
adsorption in solution, which shows that the materials could
adsorb ~ two methyl orange (MO, ~ 1.42 nm  0.35 nm  0.19
nm) per formula unit (S6†).
possess a low energy triplet state as well as significant O2-
induced enhanced intersystem crossing (EISC), which are
favourable to generate ROS, especially singlet oxygen (1O2),
through energy transfer.39-41 Several pyrene-based MOFs have
1
been investigated for O2 generation.37, 42-44 However, as the
absorption bands of the reported materials mainly reside in the
UV region, they exhibit inferior performance upon visible-light
irradiation. Therefore, the development of a MOF-based system
that can take advantage of the merits of pyrene to produce 1O2,
while utilizing visible light, is desirable.
Bearing these considerations in mind, we report two visible-
light responsive MOFs, PCN-822(Zr/Hf), consist of a judiciously
designed pyrene-based ligand, 4,4',4'',4'''-((2,7-di-tert-
butylpyrene-4,5,9,10-tetrayl)tetrakis(ethyne-2,1-diyl))-
tetrabenzoate (BPETB4). Solvothermal reactions between
H4BPETB and Zr6/Hf6 cluster afforded PCN-822(Zr/Hf) with a
(4,8)-connected sqc topology. Due to the well-isolated pyrene
chromophores in the framework, the MOFs preserve the visible-
light absorption of H4BPETB and, simultaneously, show
improved efficiency in singlet oxygen generation. PCN-822(Hf)
can also be used as a photosensitizer for aerobic photo-
oxidation of amines to imines. The heavy Hf6 clusters enhance
the intersystem crossing (ISC) in the material, promoting the
generation of singlet oxygen. Furthermore, the photo-induced
charge separation was also observed in the system, which
Both MOFs possess excellent thermal and chemical stability.
Thermal stability of PCN-822(Zr/Hf) was evaluated by
thermogravimetric (TG) analyses under air stream. It shows that
the decomposition temperature of the two MOFs is around

triggers the O2 production. Combining the energy and charge
transfer, PCN-822(Hf) exhibits excellent photocatalytic activity
in transforming amines to imines.
Results and discussion
Fig. 2. PXRD patterns of (a) PCN-822(Zr) and (b) PCN-822(Hf) after water treatment for 24
hours. Both MOFs retain their crystallinity in aqueous solutions with pH values from 2-11.
The pyrene-based ligand was synthesized in a moderate
yield via modified Sonogashira coupling reaction.45 In H4BPETB,
2 | J. Name., 2012, 00, 1-3
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450°C and the weight loss before 320 °C corresponds to the loss
ARTICLE
The ability of PCN-822(Zr/Hf) in singlet oxygeVniewgeArntieclreaOtniloinne
DOI: 10.1039/C9SC03080H
of isolated N, N-dimethylformamide (DMF) molecules and was evaluated by photooxidation of 1,5-diphdroxynaphthalene
coordinated hydroxide groups/water molecules (Fig. S4†). (DHN), where DHN acts as an efficient 1O2 scavenger to produce
Water treatment tests were also performed to assess chemical 5-hydroxy-1,4-naphthalenedione (juglone) (Fig. 4a).49 The
stability of the MOFs. The as-synthesized samples were reactions were carried out with 0.036 mmol of DHN and 1 %
immersed into aqueous solutions with various pH values and PCN-822(Zr/Hf) in acetonitrile under the irradiation of visible
allowed to stay overnight. The PXRD patterns of the samples light and the UV-vis absorption of the mixture was recorded at
after water treatment remain unchanged, indicating the an interval of one hour. As the reaction occurred, the
crystallinity of the MOFs in the aqueous solutions with pH absorption peak of DHN at 301 nm decreased, whereas the
values ranging from 2 to 11 was preserved (Fig.2).
Photophysical Properties
characteristic peak of juglone at 410 nm increased gradually.
With PCN-822(Hf) as a photosensitizer, the absorption peak of
DHN disappeared completely in 6 hours, indicating the
consumption of the starting material (Fig. 4b). However, when
PCN-822(Zr) was used as a photosensitizer, it took longer time
for the DHN absorption band to decay, meanwhile, the new
absorption band of juglone developed more slowly than that
sensitized by PCN-822(Hf) (Fig. 4c and Fig. S16†). Clearly, PCN-
822(Hf) exhibits higher capability in 1O2 generation. The better
performance might be ascribed to the heavy-metal effect of
hafnium, which enhances the intersystem crossing of the
materials. The reliability of the measurements was further
Given a good photosensitizer is supposed to have efficient
absorption of light, especially in the visible range, we initially
collected the solid-state absorption of PCN-822(Zr), PCN-
822(Hf) and H4BPETB.48 According to the UV-vis spectra,
H4BPETB exhibits a broad absorption band ranging from 225 to
700 nm, while PCN-822 (Zr/Hf) have slightly narrowed
absorption bands from 225 to 650 nm (Fig. 3a, dashed lines).
Obviously, the electrons of the MOFs could be sufficiently
promoted to an excited state upon blue-green light irradiation.
Density functional theory (DFT) calculations were conducted to
study the optical band gap of the materials (Fig. S15†). It has
been shown that the highest occupied molecular orbital
(HOMO) of H4BPETB in free state is mostly delocalized over the
central π-conjugated pyrene core, and the lowest unoccupied
molecular orbital (LUMO) is formally delocalized over both the
central π-conjugated pyrene core and the exterior
phenylacetylenic carboxylate units. In comparison with free-
state H4BPETB, BPETB4 shows similar frontier molecular orbital
distributions in the MOFs, which explains the almost unchanged
absorption of the MOF fragments.
As pyrene is a conventional fluorophore, the photo-
luminescence (PL) properties of H4BPETB and the MOFs were
also studied. H4BPETB emits green light centred at 554 nm,
while the MOFs show emission maxima at 547 nm and 538 nm
for PCN-822(Zr) and PCN-822(Hf), respectively (Fig. 3a, solid
lines). The small blue-shifts as well as the similar emission
profiles between the ligand and the MOFs indicate that the
emission is ligand-based. The green fluorescence of MOFs was
further confirmed by the confocal scanning microscopic images
shown in Fig. 3b.
tested by
a control experiment with no addition of
photosensitizer. The absorption spectra show slightly increase
at 410 nm and negligible changes at 301 nm, revealing the
indispensable role of photosensitisers (Fig. S17†).
Oxidation of Various Amines
With the excellent performance in 1O2 generation, PCN-
822(Hf) was chosen as photosensitizer for aerobic oxidation of
amines to imines. Imines and derivatives are versatile
intermediates in organic transformations and they are widely
Photooxidation of 1,5-Dihydroxynaphthalene (DHN)
Fig. 4. (a) Photooxidation of DHN sensitized by PCN-822 and H4BPETB in the present of
oxygen and light irradiation. (b) Changes of UV-vis spectra for oxidation of DHN
sensitized by PCN-822(Hf). Insert: changes of juglone absorbance (at 419 nm) with
reaction time. (c) Comparison of PCN-822(Hf) and PCN-822(Zr) in juglone development.
used in industry for the synthesis of fine chemicals and
pharmaceuticals. Traditional synthetic protocols of imines
generally involve unstable aldehydes, dehydrating agents as
well as Lewis acid catalysts. Hence, significant efforts have been
dedicated to develop mild and practical methodologies for
51
imine formation.50,
Among all the methodologies,
photoinduced aerobic oxidation of amines turns out as an
effective and sustainable approach from the view of green
Fig. 3. (a) UV-vis spectra of PCN-822(Zr/Hf), H4BPETB (dashed line) and solid-state
photoluminescent spectra of PCN-822(Zr/Hf) and H4BPETB (solid line). (b) Fluorescent
microscopic images of PCN-822(Zr/Hf) upon excitation at 488 nm.
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Table 1. Oxidation of various amines. [a]
Entry
1
Substrate
Product
Yield (%)
93
Sel. (%)
98
90
91
98
98
2
3
81
90
4
5
83
96
____
____
____
6
7
8
79
76
84
83
82
79
92
86
85
95
9
10
11
[a] Reaction conditions: amine (0.1 mmol), PCN-822(Hf) (1.0 mol%), acetonitrile (2.0 mL), 1 atm O2, rt., LED light ( = 450ꢀnm), 100 mW/cm2. Yields were determined by
1H NMR with mesitylene as internal standard.
chemistry, as it can efficiently utilize sunlight, requiring relative material could be efficiently converted to the imine-based
mild and neat conditions.52-55 At the outset, dibenzylamine was product with the highest yield in 6 h (entry 6). Furthermore,
chosen as a model substrate to optimize the reaction conditions control experiments were conducted without light irradiation or
(Table S2†). The reaction was generally performed under O2 O2 atmosphere (entry 7,8). As expected, no product was
atmosphere upon excitation of visible light at room observed under these reaction conditions. The necessity of
temperature. It turned out that solvents exerted remarkable photosensitizer was also confirmed by entry 9 with no addition
influence on oxidation of dibenzylamine, where acetonitrile was of PCN-822(Hf) and it gave no yield of imine. Interestingly, when
proved to be the most suitable medium compared with other the esterified ligand, Me4BPETB, was used as photosensitizer,
types of solvents (entry 1-5). It was also noted that prolonged only 22.2% yield of product was achieved. The dramatic
reaction deteriorated the formation of target product, leading decrease in the formation of imine might be partially ascribed
to the dominating aldehyde byproducts (entry 4). Finally, with to the aggregation-caused quenching (ACQ) effect of the
1.0 mole % of PCN-822(Hf) photosensitizer, the starting pyrene-based ligand in the solution (entry 10).
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PCN-822(Hf) also shows excellent chemical stability and
recyclability. The MOFs separated from the reaction mixture
could be reused two more times without loss of catalytic
activity. The PXRD patterns of the samples recovered from each
recycle showed no essential change in the profiles from the
pristine ones, suggesting the intact crystallinity of the MOFs
under the catalytic conditions (Fig. S18 and S19†).
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DOI: 10.1039/C9SC03080H
Under the optimized conditions, the oxidation of various
amines was further performed to evaluate the general
capability of PCN-822(Hf). As shown in table 1, both primary and
secondary amines could be oxidized to the corresponding
imines with satisfactory yields. Primary amines underwent
oxidative coupling reaction photocatalyzed by PCN-822(Hf). It
was noted that para-substituted benzylamines with electron-
donating groups (CH3 and OCH3) afforded higher yields of imines
than those with electron-withdrawing groups (Cl and CF3),
which indicates the remarkable electronic effect on the
oxidation reactions (entries 1-5). In addition, no product was
observed when catalysing aniline, suggesting that the coupling
reaction experiences an abstraction of -hydrogen of the
substrates (entry 6). Compare to primary amines, secondary
amines gave imines by oxidative dehydrogenation. Due to the
possible oxidative cleavage of the C=N bond, they presented
relatively lower yields and selectivity in imine formation (entries
7-10). Substrates with bulky substitutes also reacted smoothly,
which might be facilitated by the large channels of the MOF
(entry 8-10). Besides aromatic amines, heterocyclic amines such
as 1,2,3,4-tetrahydroisoquinoline could also be oxidized to
dehydrogenated imine with good yield and high selectivity
(entry 11).
Fig. 5. Proposed mechanisms of oxidation of amines.
radicals, like electron-hole pairs (Fig. S21†). To further ascertain
the photoinduced electron-transfer mechanism, the energy
diagram of PCN-822(Hf) was estimated by UV-vis spectra
combined with cyclic voltammetry (S10†). According to cyclic
voltammogram, the reduction potential of PCN-822(Hf) was
0.55V vs. standard hydrogen electrode (SHE), corresponding
to LUMO level (Fig. S22†). The relatively more negative
potential theoretically enables the material to reduce molecular
oxygen to O2 (E (O2/ O2) = 0.33 V vs. SHE). The optical band
gap of PCN-822(Hf) was calculated to be 2.17 eV on the basis of
Tauc plot, from which HOMO level was determined to be 1.62
V vs. SHE (Fig. S12†). As the E1/2 (M/M) of amines is around
1.47 V vs. SHE, the more positive HOMO level of PCN-822(Hf)
should be sufficient for the amine oxidation.
Mechanism Analysis
Conclusions
Although the mechanism underlying the oxidation reaction
remains disputable, two possible pathways are proposed to
explain the excellent photocatalytic performance of PCN-
822(Hf) (Fig. 5). Firstly, as PCN-822(Hf) has been proved to be
an excellent singlet oxygen generator, a mechanism is proposed
In summary, a tetratopic pyrene-based ligand (H4BPETB)
was rationally designed to construct two novel porous MOFs,
PCN-822(Zr/Hf). The MOFs have a broad adsorption of visible
light up to 650 nm. The desirable optical adsorption of the
materials is mainly ascribed to the 4,5,9,10-positions (K-region)
substitution of the pyrene-derived ligand, which leads to an
extended π-conjugated moiety. Merging the merits of pyrene in
singlet oxygen generation with visible-light harvesting, PCN-
1
where the material produces O2 species to oxidize amine via
energy transfer upon the light irradiation. The generation of the
singlet oxygen was further confirmed by electron paramagnetic
resonance (EPR) spectra, which displayed featured signals in the
(Fig. S20†). Regarding to the semiconductor nature of MOFs, an
alternative mechanism involving photoinduced electron-
transfer process is also operable. It is assumed that the reaction
is initiated by the excitation of PCN-822(Hf) upon visible-light
irradiation, which gives rise to electron (e) and hole (h) pairs
separated in the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) of the
material. The photogenerated electron reduces molecular
oxygen to its activated superoxide radical species, O2, while
the hole oxidizes amine to a cationic amine radical. The amine
radical and O2 then react with each other to produce imine via
proton and hydrogen transfer. For primary amine, the formed
imine serves as an intermediate, which is further attacked by
another amine to give the final coupled product. EPR spectra
showed an enhanced signal upon light irradiation relative to
that taken under darkness, indicating the photogenerated
1
822(Zr/Hf) exhibit an efficient capability in activating O2 to O2
under the excitation of visible light. Furthermore, PCN-822(Hf)
is demonstrated to be highly active in oxidation of various
amines to imines. Possible dual pathways are proposed for the
light-driven transformation, involving the energy transfer-
based 1O2 generation and photo-induced charge separation.
The present work not only exemplifies novel pyrene-based
MOFs as inherent visible-light harvesting photosensitizers
without the aid of other chromophores, but also illustrates the
necessity of the structural design at the molecular level in the
MOF-based photosynthesizing system.
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Conflicts of interest
There are no conflicts to declare.
DOI: 10.1039/C9SC03080H
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Acknowledgements
This work was supported by the Centre for Gas Separations, an
Energy Frontier Research Centre 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
H.-C. Z. (A-0030). The authors also acknowledge the financial
support of NPRP award NPRP9-377-1-080 from the Qatar
National Research Fund. Use of the TAMU Materials
Characterization Facility is also acknowledged.
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