Covalent organic frameworks as metal-free heterogeneous photocatalysts for organic transformations

Article abstract of DOI:10.1039/c7ta07691f

Two-dimensional covalent organic frameworks (2D-COFs) are excellent candidates for photocatalytic organic transformations due to their periodic columnar π-arrays and ordered nanochannels. Here, we present a novel 2D-COF that features permanent porosity wi

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ISSN 2050-7488
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Covalent organic frameworks as metal-free heterogeneous
photocatalysts for organic tranformations
Received 00th January 20xx,
Accepted 00th January 20xx
Yongfeng Zhia, Ziping Lia, Xiao Fengc, Hong Xiab, Yumin Zhanga, Zhan Shid, Ying Mua and Xiaoming
Liua*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Over the last decades, various kinds of dye molecule such as
transition metal complexes, organic dyes and inorganic
Two-dimensional covalent organic frameworks (2D-COFs) are
excellent candidates for photocatalytic organic transformations
arise from their periodic columnar π-arrays and ordered semiconductors have been proved to be highly efficient
nanochannels. Here, we present a novel 2D-COF that features a
permanent porosity with high surface area and large pore volume,
and exhibits a high crystllinity, an excellent stability. Indeed, the
new 2D-COF was found to be a highly efficient, metal-free,
recyclable heterogeneous photocatalyst for oxidative C-H
functionalizations under visible-light irradiation using O2 as a green
oxygen source.
photocatalysts in photoredox reactions via signal electron transfer
process.11 In order to overcome the important shortcomings of the
homogeneous photocatalysts including high price, tedious
separation and low recyclability, some photoactive solid catalysts
like graphitic carbon nitride, metal-organic frameworks and
mesoporous silica have also been explored for a serious of
photocatalytic organic reactions.12 From a standpoint of principle,
however, an ideal photocatalyst should have an excellent porosity,
columnar π-arrays, a strong crystallinity. On the other hand, the low
cost, metal-free, good recyclability and scale-up synthesis are still
important for industrial applications. In this context, predesignable
2D-COF can be an excellent scaffold for heterogeneous
photocatalysis of organic transformations. Herein, we report a
novel 2D-COF with high surface area, strong srystallinity and robust
stability (Scheme 1, COF-JLU5), and that exhibit excellent
heterogeneous photocatalytic performances for oxidative C-H
functionalizations under visible-light irradiation.
As a new generation of crystalline porous polymer materials,
covalent organic frameworks (COFs) are two-dimensional (2D) or
three-dimensional (3D) periodic architectures with permanent
porosity.1 Owing to their low density, large specific surface area and
high thermal stability, COFs have recently attracted huge attention
for promising applications in gas adsorption,2 catalysis,3 sensing,4
proton conduction,5 drug delivery,6 and energy storage.7 In
particular, the most remarkable feature for 2D-COFs is periodic
columnar π-arrays arise from layered stacking of 2D polymer sheets.
The ordered columns are ideal channels for exciton diffusion,
migration and separation,8 and 2D-COFs has proven as a powerful
platform for designing functional materials in optoelectronic
devices.9 Surprisingly, however, the field of 2D-COFs as
heterogeneous photocatalysts for organic transformations remains
largely untrapped up to now.10
In order to obtain an excellent COF-based heterogeneous
photocatalyst for organic transformations, a suitable structure was
rationally designed. First, select triazine based tri-amines as a knot,
because its plane structure can increase π-π interaction between
the layers to improve the crystallinity of the COF, but also it
possesses a good electron-deficient property. Second, introduction
an -OMe with electron-rich group adjacent to Schiff-base centers to
enhance the chemical stability of skeleton by integrating resonance
effect.3b Third, the construction of the electronic donor-acceptor
structure in the layer can adjust the photoelectric properties to
increase the photocatalytic efficiency of the COF. Therefore, a new
COF-JLU5 was synthesized under solvothermal conditions by
condensation of 1,3,5-tris-(4-aminophenyl)triazine (56.6 mg, 0.16
mmol) and 2,5-dimethoxyterephthaldehyde (46.6 mg, 0.24 mmol),
in the presence of 6 M aqueous acetic acid (0.2 mL) using
mesitylene/1,4-dioxane (2.0 mL, 4:1 by vol.) as the solvent at 120 oC
for 3 days, which afforded a yellow-green microcrystalline solid in
88% yield (Scheme 1). Fourier transform infrared (FT-IR) spectrum
The use of visible-light to drive organic transformations offers a
green and sustainable way for efficient synthesis of fine chemicals.
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of COF-JLU5 exhibited a strong vibration band at 1584 cm-1, TEM) image showed the direct visualization of the layered
indicating the formation of C=N linkages (Fig. S1). The precise nanomorphology and hexagonal pore texture (Fig. S3d and e).
connections were assessed by solid state 13C CP/MAS NMR analysis.
The permanent porosity of COF-JLU5 was evaluated by
The peak at about 151 ppm is assigned to the carbon atoms of measuring nitrogen gas adsorption isotherm of the activated
imine groups, and the high-field peak at about 53 ppm can be samples at 77 K. As shown in Fig. 2A, the sorption profile was
attributed to the methoxy groups (Fig. S2). Elemental analysis of the classified as typical type IV isotherm with a rapid uptake under at a
COF-JLU5 confirmed that the C, H, N contents were 71.53, 4.94, and low relative pressure of p/p0 < 0.1 and a step in rang of p/p0 = 0.05-
14.15, which were in good agreement with the calculated values of 0.20, indicating that COF-JLU5 is a mesoporous material. The
73.08, 4.60, and 14.20 expected for an infinite 2D sheet.
Brunauer-Emmett-Teller (BET) surface area of COF-JLU5 was
evaluated to be 1632 m2 g-1. Based on the non-local density
functional theory method, the average pore size was calculated to
be 2.7 nm (Fig. 2B), which is reasonably close to the theoretical
value form the crystal simulation. The total pore volume of COF-
JLU5 was still estimated to be 1.59 cm3 g-1 at p/p0 = 0.99.
Fig. 1 (A) Observed PXRD pattern (black line), the Pawley refined pattern
(red line), the difference plot (green line) and the simulated pattern (blue)
for AA-stacking model of COF-JLU5. Space filling models showing AA stacking:
top view (B) and side view (C); Carbon (grey), Nitrogen (blue), Oxygen (red),
hydrogen is omitted for clarity.
Scheme 1. Synthesis of COF-JLU5 by imine condensation reaction.
The crystalline structure of COF-JLU5 was determined by powder
X-ray diffraction (PXRD) analysis in connection with structural
simulations (Fig. 1). The experimental PXRD pattern exhibits a
strong diffraction peak at 2.76o, and relatively weak signals at 5.84o,
7.61o, 9.71o, and 24.9o, which can be assigned to the (100), (200),
(210), (220) and (001) reflections, respectively (black curve). The d
spacing was found to be 3.57 Å, which implied the exitence of π-π
Fig. 2 (A) Nitrogen adsorption (point) and desorption (circle) isotherm of
COF-JLU5 at 77 K, (B) Pore size distribution of COF-JLU5 on the N2 adsorption
interaction of the adjacent layers. The structural simulation of PXRD isotherm.
The stability of material is an important parameter for practical
pattern suggests that COF-JLU5 possesses preferably the AA
stacking mode (blue curve). In this model, the Pawley refined applications. Such, we tested the stability of COF-JLU5 under
profile (red curve) is in good agreement with observed pattern, as various conditions. Thermogravimetric analysis (TGA) displayed that
the COF has an excellent thermal stability up to 450 oC under
evidenced by their negligible difference (green curve, Rwp = 5.40%
nitrogen atmosphere (Fig. S4). To research the chemical stability of
and Rp = 3.92%, respectively). The expected crystalline morphology
of the COF was evidenced by field-emission scanning electron COF-JLU5, we dispersed the pristine samples in different organic
solvents, aqueous HCl (12 M) and NaOH (14 M) solutions at 25 oC
microscopy (FE-SEM). The SEM image revealed that COF-JLU5 with
for three days. Then the samples were collected by filtration,
a flowerlike morphology (Fig. S3a and b), and each independent
flower can be deemed as the result of aggregation of abundant washed with methanol, and dried under vacuum. To our delight, all
petals with lengths in the nanometer size (200-300 nm) (Fig. S3c). In the samples also show strong peaks in XRD patterns, and the similar
skeleton connection in FT-IR profiles and almost no wight loss (Fig.
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addition, high-resolution transmission electron microscopy (HR-
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S5). Even under the additional visible-light irradiation (blue LED: 460 COF-JLU5 was easily isolated from the reaction mixtures by
nm, 30 W) for methanol suspension of COF-JLU5, the strong centrifugation, washed, dried and reused at least ten cycles without
crystallinity is still retained in the COF sample after treatment (Fig. any obvious loss of catalytic activity and the original structure (Fig.
S6). Therefore, the experimental results indicated unambiguously S9). It is mainly attributed to the outstanding stability of the COF.
that the new COF possesses an excellent thermal, chemical and Furthermore, we separated COF-JLU5 form the reaction solution at
photochemical stability.
68% conversion, the filtrate did not display further reactivity, which
Electron absorption spectrum of the COF-JLU5 in the solid state indicate unambiguously the heterogeneous photocatalytic feature
exhibited an absorption band at 470 nm (Fig. 3A, blue line), which is of the new COF photocatalyst. We also examined the possibility of a
red-shifted from that its monomers (Fig. S7). Upon excitation at 470 larger scale synthetic using 1a (0.5 g, 2.39 mmol) as a substrate in
o
nm, COF-JLU5 displayed an intense orange-red luminescence band 30 mL of methanol at 25 C for 10 h. An excellent yield (95 %) was
at about 580 nm (Fig. 3A, red line). Cyclic voltammetry afforded in the scaled-up reaction (Scheme S1).
measurements revealed an onset potential at 0.90 V (vs SCE),
Scheme 1. Photocatalytic C-C/C-P coupling reaction of N-
attributable to the redox potential of the COF-JLU5+/COF-JLU5
phenyltetrahydroisoquinonlines with various nucleophiles using
COF-JLU5 as photocatalysta
couple (Fig. 3B). On the basis of a free energy change (E0-0) between
the ground state and the vibrationally related excited state of 2.40
eV (516 nm, Fig. 3A), the redox potential of the excited state COF-
JLU5+/COF-JLU5* couple was calculated as -1.50 V. The potentials
are comparable to that of the excited state of classical transition
metal complexes,11 as a result COF-JLU5 was expected to be a
candidate in photoredox catalytic organic transformations.
Fig. 3 (A) Normalized absorption (blue line) and emission spectra (red line)
of COF-JLU5 excited at 470 nm. (B) Solid-state cyclic voltammetry of COF-
JLU5 with a scan rate of 100 mV s-1.
The COF-JLU5 offers an excellent porosity, a high crystallinity, an
exceptional stability, and the columnar π-arrays, as well as the
promising photoredox properties, and serves as
a unique
heterogeneous photocatalyst for organic transformations. In order
to validate our hypothesis, we conducted the aerobic cross-
dehydrogenative
coupling
(CDC)
reaction
of
N-
aryltetrahydroisoquinolines, which is one of the most classical
photocatalytic transformation.13 We first examined the reaction of
N-phenyl-1,2,3,4-tetrahydroisoquinoline
(THIQ,
1a)
with
nitromethane (2a) containing a catalytic amount of COF-JLU5 under
irradiation of blue LEDs at 25 oC in the presence of O2 as green
oxygen source. Compared with toluene and cyclohexane, the rapid
conversion rate and high yield (99%) were obtained in methanol
solvent (Fig. S8). The results of control experiments revealed that
the light, oxygen gas and photocatalyst COF-JLU5 are all essential
for this reaction (Table S1). However, the TPB-DMTP COF without
the triazine core only achieved low yield (42 %, Table S1, entry 8)
under the optimized conditions, which implying the rationality of
the structural design for COF-JLU5. Besides catalytic activity, the
recyclibity of catalyst is the most crucial advantage in the
heterogeneous reaction for commercial use. The photocatalyst
[a] Reaction conditions: COF-JLU5 (4.0 mg), 1 (0.2 mmol), 2 (0.6 mmol),
methanol (2.0 mL), O2 (1 bar), 30 W blue LEDs with 460 nm, 25 oC, the yields
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observed upon irradiation of the air-saturated CH3CN mixture of
COF-JLU5 and TEMP. However, the 1O2 signal disappeared upon the
addition of the substrate 1a to the mixture of TEMP and COF-JLU5
detected by NMR spectroscopy and the isolated yields are given are in
parentheses.
Further, we explored the substrate scope of the visible-light
•—
mediated COF catalyzed aerobic CDC reaction. As shown in Scheme
1, in general, different THIQ derivatives 1a-c with nitroalkanes were
subjected to the aerobic aza-Henry reaction, giving the desired
reaction products 3a-h in excellent yields. The presence of
electronic properties for the substituent group in the phenyl ring
and alkyl attached on nitroalkanes had little influence on the yields.
Then we researched the applicability of other nucleophiles. To our
delight, higher substituted dialkyl malonates were still able to
perform this coupling reaction showing slightly reduced activities.
Subsequently, utilizing acetone as nucleophile, we investigated the
(Fig. 4A, blue line). By contrast, when DMPO, an O2 scavenger,
was used instead of TEMP in the same air-saturated solution, a
•—
clear characteristic signal of O2 was detected (Fig. 4B, blue line).
These results suggest that the effective electron transfer between
excited COF-JLU5 and 1a can be effectively suppressed the
production of 1O2.15
Accordingly, a plausible reaction mechanism for the aerobic
photocatalytic C-H functionalizations by COF-JLU5 is proposed (Fig.
5). Under visible-light irradiation, the COF-JLU5 generates the
excited species COF-JLU5*, which is reductively quenched by 1a via
a single electron transfer (SET) process to form an radical cation A
and radical anion COF-JLU5•—. Then generated radical anion COF-
JLU5•— delivered an electron to molecular O2 to form the
oxidative Mannich reaction, which offers
a powerful way to
valuable β-amino-ketones. In the presence of proline as Lewis base
to activate the ketone, the reaction also proceeded smoothly by
COF-JLU5 to prove the desired products 3m-o in high yield. In
addition to the C-C bond formation, notably, the COF
heterogeneous photocatalytic system could also directly accelerate
the C-P bond crossing reaction. And good yields of α-amino dialkyl
phosphonates 3p-v were obtained. Indeed, COF-JLU5 is an
outstanding photocatalyst for the aerobic CDC reaction, and its
photocatalytic activity is better than that of the reported solid
photocatalyst, comparing the best results of the homogeneous
photocatalysts (Table S2 and S3).
•—
superoxide anion O2 and simultaneously to regenerate COF-JLU5
•—
to its ground state. The active oxygen species O2 subsequently
abstracts a proton from A to produce the the hydroperoxide radical
and the intermediate B. Further, the obtained hydroperoxide
radical can abstract
a hydrogen atom from 1a to afford
intermediate B alone with the formation of H2O2. The B lose one
electron to afford the iminium ion C. Finally, the C undergoes
nucliophilic addition to give the desired product D.15
Fig. 4 (A) ESR spectra of the mixture COF-JLU5 (1.0 mg mL-1) and TEMP (0.1
M) in air-saturated CH3CN in dark (black line) and upon light irradiation (red
line); ESR spectra of the mixture COF-JLU5 (1.0 mg mL-1), 1a (1.0 × 10-2 M)
and TEMP (0.1 M) in air-saturated CH3CN in dark (green line) and upon light
irradiation (blue line); (B) ESR spectra of the mixture COF-JLU5 (1.0 mg mL-1)
and DMPO (0.1 M) in air-saturated CH3CN in dark (black line) and upon light
irradiation (red line); ESR spectra of the mixture COF-JLU5 (1.0 mg mL-1), 1a
(1.0 × 10-2 M) and DMPO (0.1 M) in air-saturated CH3CN in dark (green line)
and upon light irradiation (blue line).
Fig. 5 Proposed reaction mechanism for the aerobic photocatalytic C-H
functionalizations by COF-JLU5.
Conclusions
In summary, we have developed
a photoactive 2D-COF for
photocatalytic organic transformations. The COF-JLU5 can catalyze
oxidative C-H functionalizations under visible-light, and exhibit high
catalytic activity, broad substrate adaptability and outstanding
recyclability, owing to combining some key features such as
permanent porosity, high crystallinity and columnar π-arrays as well
as an outstanding stability. The new COF as a heterogeneous
photocatalyst possesses some distinct advantages over other
known catalytic materials that its inherent pores provide space for
the transport and transformation of matter, columnar π-arrays
offer ideal channels for charge conduction and separation, the
strong crystallinity presents a possibility for structural tailoring and
performance adjustments. Considering the inherent advantages of
According to previous results in the literature,14 molecular O2 is
converted into active oxygen species (single oxygen 1O2 and
superoxide radical anion O2•—) through energy or electron transfer
from the excited state of macromolecular photosensitizers. To
study the active oxygen species generated by COF-JLU5, electron
spin resonance (ESR) measurements were performed in which
2,2,6,6-tetramethylpiperiding (TEMP) and 5,5-dimethyl-1-pyrroline-
N-oxide (DMPO) were employed as singlet oxygen 1O2 and
superoxide radical anion O2•— scavengers, respectively. As shown in
Fig. 4 , the typical characteristic of 1O2 (Fig. 4A, red line) was
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(a) C. R. DeBlase, K. E. Siberstein, T. T. Truong, H. D. Abruña, W.
R. Dichtel, J. Am. Chem. Soc., 2013, 135, 16821. (b) C. R.
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reactions, the present work not only demonstrates the possibility of
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organic transformations, but also expands the application scope of
COFs and will promote greatly the development of the
photocatalytic field.
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Journal of Materials Chemistry A
Page 6 of 6
View Article Online
DOI: 10.1039/C7TA07691F
A robust two-dimensional covalent organic framewor (COF-JLU5), which exhibits
excellent heterogeneous photocatalytic performances for oxidative C-H
functionalizations under visible-light irradiation using O2 as a green oxygen source, is
reported.