Covalent Triazine Framework Nanoparticles via Size-Controllable Confinement Synthesis for Enhanced Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1002/anie.202007358

For metal-free, organic conjugated polymer-based photocatalysts, synthesis of defined nanostructures is still highly challenging. Here, we report the formation of covalent triazine framework (CTF) nanoparticles via a size-controllable confined polymerization strategy. The uniform CTF nanoparticles exhibited significantly enhanced activity in the photocatalytic formation of dibenzofurans compared to the irregular bulk material. The optoelectronic properties of the nanometer-sized CTFs could be easily tuned by copolymerizing small amounts of benzothiadiazole into the conjugated molecular network. This optimization of electronic properties led to a further increase in observed photocatalytic efficiency, resulting in total an 18-fold enhancement compared to the bulk material. Full recyclability of the heterogeneous photocatalysts as well as catalytic activity in dehalogenation, hydroxylation and benzoimidazole formation reactions demonstrated the utility of the designed materials.

Full text of DOI:10.1002/anie.202007358

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Accepted Article
Title: Covalent triazine framework nanoparticles via size-controllable
confinement synthesis for enhanced visible light photoredox
catalysis
Authors: Wei Huang, Niklas Huber, Shuai Jiang, Katharina Landfester,
and Kai A. I. Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202007358
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Covalent triazine framework nanoparticles via size-controllable
confinement synthesis for enhanced visible light photoredox
catalysis
Wei Huang,ǂ Niklas Huber,ǂ Shuai Jiang,ǂ Katharina Landfester, Kai A. I. Zhang*
Abstract: Construction of defined nanostructures is a key for
enhancing the efficiency of heterogeneous catalytic materials.
However, for metal-free, organic conjugated polymer-based
photocatalysts, due to their mostly amorphous nature or specific
synthetic routes, synthesis of defined nanostructures is still highly
challenging. Here, we report the formation of covalent triazine
framework (CTF) nanoparticles via a size-controllable confined
polymerization strategy. The uniform CTF nanoparticles exhibited
significantly enhanced activity in the photocatalytic formation of
dibenzofurans compared to the irregular bulk material. Moreover, the
optoelectronic properties of the nanometer-sized CTFs could be
easily tuned by copolymerizing small amounts of benzothiadiazole
into the conjugated molecular network. This optimization of electronic
properties led to a further increase in observed photocatalytic
efficiency, resulting in total an 18-fold enhancement compared to the
bulk material. Full recyclability of the heterogeneous photocatalysts
as well as catalytic activity in dehalogenation, hydroxylation and
benzoimidazole formation reactions demonstrated the utility of the
designed materials.
Among the organic photocatalytic systems being used,
covalent triazine frameworks (CTFs) have emerged as promising
heterogeneous polymer photocatalysts for visible light-promoted
chemical transformations.[9] They have been employed as
efficient platform for visible-light photoredox reactions such as
water splitting[9b, 10], carbon dioxide reduction[11] or organic
reactions.[12] The CTF systems comprise various advantageous
properties, such as high activity in photoredox reactions, low cost
and non-toxicity, excellent recoverability as well as stability in
repeated applications. Moreover, CTFs have proven to be easily
tuneable in their electronic, optical and chemical properties.[13]
However, a main challenge within photocatalytic CTF systems
remains in establishing mild synthetic procedures yielding
morphologically defined structures. Conventional liquid-phase
approaches in molten ZnCl2 or trifluoromethanesulfonic acid
(TfOH) solution are not suitable to give CTFs with regular
morphologies and defined optical properties. Recently, as one of
the few examples, our group reported on the use of silica
templates to form hollow or mesoporous CTFs.[8b, 14] The group of
Tan and Jin was able to synthesize hollow CTF nanospheres for
photocatalytic hydrogen evolution.[6b] The study emphasizes the
benefits of nanoscale features for catalytic applications as well as
the crucial role of morphology control in designing efficient
photoactive CTF materials. Even though initial progress could be
made, synthetic methods for self-standing covalent triazine
framework nanoparticles with defined size for photocatalytic
applications have not yet been documented.
Nanostructured materials have been key to catalytic research
in recent years.[1] By scaling a catalyst down to the nanoscale, the
materials surface-to-volume ratio increases and active interface
is exposed to the reaction media. In most cases, the downscaling
has beneficial effects on, for instance, activity, selectivity or
turnover of an applied catalyst.[2] For heterogeneous
photocatalytic systems using light as clean and highly sustainable
energy source, efforts to synthesize nanometer-sized
morphologies included nanoparticles[3], nanowires[4] as well as
hollow nanostructures.[5] Photocatalysts with improved
performance in catalysing hydrogen evolution from water[6],
pollutant degradation[7] or organic reactions[8] compared to bulk
materials have been reported. Clearly, precise morphological
control over the material synthesis constitutes a major interest in
the production of defined and highly efficient heterogeneous
photocatalysts. So far, a vast number of synthetic methods to form
nanostructured photocatalysts have been mainly applied to
inorganic materials. For pure organic heterogeneous
photocatalytic systems, due to their mostly amorphous nature or
specific synthetic routes, however, synthesis for defined
nanostructures is still highly challenging.
In this work, we present a confinement synthesis for size-
controllable covalent triazine framework nanoparticles, which is
inspired by the baking techniques for fine-shaped cookies. A
thiophene-derived dinitrile monomer can be encapsulated inside
a temporarily formed silica shell using an emulsion approach. Via
in-situ
trifluoromethanesulfonic
acid
(TfOH)-catalyzed
polymerisation followed by silica shell removal, uniform CTF
nanoparticles can be obtained. Variation of the emulsifying
parameters led to feature sizes between 80 and 550 nm. Apart
from controlling the morphology of, the electronic and optical
properties of the CTF photocatalysts can be further optimized
through copolymerizing electron-withdrawing benzothiadiazole
units into the CTF backbone and thereby boosting
photogenerated charge separation. Kinetic studies of the
photocatalytic dihydrobenzofuran synthesis proved that both
nanoscaling and copolymerization had a significant effect on
photocatalytic activity with an enhanced 18-fold efficiency
compared to bulk CTF. The photocatalysts’ robustness was
underlined by recycling experiments. The versatility of the CTF
nanoparticles were further investigated by conducting
dehalogenation of α-chloroacetophenone, hydroxylation of 4-
biphenylboronic acid as well as benzoimidazole formation from o-
phenylenediamine with excellent selectivity and efficiency.
[*]
Dr. W. Huang, N. Huber, Dr. S. Jiang, Prof. K. Landfester, Prof. K.
A. I. Zhang
Max Planck Institute for Polymer Research, Ackermannweg 10,
55128 Mainz (Germany)
E-mail: kai.zhang@mpip-mainz.mpg.de
Prof. K. A. I. Zhang
Department of Materials Science, Fudan University
200433 Shanghai (P. R. China)
E-mail: kai_zhang@fudan.edu.cn
Equal contributions.
ǂ
Supporting information for this article is given via a link at the end of
the document.
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In order to shade a light on the formation of the nanoparticles,
transmission electron microscopy (TEM) as well as scanning
electron microscopy (SEM) studies were conducted. As shown in
Figure 1a, TEM imaging indicated the successful confinement of
DCHT in the formed silica capsules. A control experiment, in
which the premier synthesis step was conducted with n-hexane
instead of DCHT yielded collapsed silica capsules after the
evaporation of hexane during the freeze-drying step (Figure S3).
The core-shell structure was retained after the TfOH-assisted
polymerization step as shown in Figure 1b. Energy-dispersive X-
ray spectroscopy (EDX) mapping on the encapsulated CTF-NPs
showed high prevalence of silicon and oxygen in the shell area,
whereas carbon and sulphur could be detected exclusively in the
core of the nanoparticles (Figure 1c). After the silica shell removal,
CTF NPs with different diameters ranging from circa 80 to 550 nm
(CTF80, CTF180, CTF550) on average were obtained (Figure 1d-f,
Figure S22-23), depending on the volume ratio between
dispersed phase and water phase in the miniemulsion synthesis.
Light scattering on the colloidal particles in a diluted THF
suspension (insert 1d), known as the Tyndall effect, further
underlined the successful synthesis of the uniform covalent
triazine framework nanoparticles.
Scheme 1. Synthetic route for covalent triazine framework nanoparticles in
confinement. In a combined sol-gel emulsion and TfOH vapour-assisted
polymerization approach CTF nanoparticles are formed. The resulting material
can be used for visible light-promoted organic redox photocatalysis.
The general synthetic route is outlined in Scheme 1 and
precisely described in the supplementary material. In detail, the
liquid monomer 2,5-dicyano-3-hexylthiophene (DCHT) was
specifically designed to ease the formation of a two-phase system
with tetraorthosilicate (TEOS) in water. In aqueous media, DCHT
and TEOS combine in a non-miscible oil phase and can, upon
ultrasonication, form a surfactant-stabilized miniemulsion. When
stirred at room temperature, TEOS in the droplets slowly
hydrolyses at the oil-water interphase and forms the silica shell as
confinement around the monomer DCHT. The silica
encapsulation of liquid monomers in a miniemulsion process was
reported in previous studies.[15] After freeze-drying, the resulting
particles are exposed to trifluoromethanesulfonic acid (TfOH)
vapour, leading to formation of the corresponding CTF inside the
silica capsule. To obtain pure CTF nanoparticles, the silica shell
is removed by etching with a NH4HF2 solution.
Further characterization was conducted using CTF80 (denoted
as CTF NPs) if not stated otherwise. The Fourier transformed
infrared (FT-IR) spectra of the obtained nanoparticles exhibited
three characteristic signals of the triazine group at 1482, 1385 and
824 cm-1. A comparably low-intensity signal at 2225 cm-1 hints at
the presence of terminal nitriles but also indicates the effective
formation of triazine networks with high degree of polymerization
(Figure S4). Similar conclusions could be drawn from the
corresponding solid state 13C cross-polarization magic-angle-
spinning (CP-MAS) NMR spectrum. A pronounced signal at ca.
168 ppm verified the presence of sp2 triazine carbons (-C=N),
whereas the terminal nitrile signal around 112 ppm vanishes into
the baseline signal. All signals could be matched with the
assumed structure, confirming the CTF synthesis from DCHT
(Figure S5). Powder X-ray diffraction (PXRD) patterns) showed a
broad peak at 13.0° for all materials, which becomes more
prominent with increasing BT content (Figure S6). The general
appearance of all CTFs was indeed rather amorphous.
Thermogravimetric analysis (TGA) showed that the CTF NPs stay
intact up to 400°C, which is comparable to CTFs from previous
studies and proves good thermal stability (Figure S7).[16]
To further optimize the optoelectronic properties of the as-
synthesized CTF NPs, copolymerizations of DCHT with 1, 2 and
4 mol% of dicyanobenzothiadiazole (DCBT) were carried out. The
aim was to incorporate the benzothiadiazole as additional
electron acceptor into the CTF backbone structure and further
improve the photogenerated charge separation. The resulting
CTF NPs were denoted as CTF-xBT NPs with x = 1, 2 or 4
depending on the benzothiadiazole (BT) unit content (molar
percentage) in the CTF networks. The experimental approach for
the nanoparticle synthesis remained unchanged; for experimental
details see the supporting information. The morphological
nanoparticle formation was confirmed by TEM imaging (Figure
S8). Higher amounts of BT seem to yield less uniform particles
after silica removal. FT-IR spectra of the CTF-xBT samples
Figure 1. TEM image of (a) monomer and (b) CTF-NPs confined in silica
capsules. (c) Elemental mapping shows enriched contents of silicon and oxygen
in the shell and carbon and sulphur in the core. (d-f) Different CTF-NP sizes of
80, 180 and 550 nm on average could be obtained. The inset in (d) shows the
Tyndall effect of CTF80 in THF (0.01 mg/mL).
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resemble the spectrum of the CTF nanoparticles without BT,
hinting at a retained chemical constitution and connectivity of the
materials despite the introduction of BT units (Figure S9). In the
13C CP MAS NMR spectra of the CTF-xBT samples, a minor peak
at ca. 155 ppm can be detected, which corresponds to the BT
imine carbon in accordance to previous reports (Figure S10).[8b]
These findings demonstrate the successful introduction of BT
units into the conjugated molecular networks.
photocurrent measurements, where CTF-2BT could offer more
effective photogenerated electrons during the catalytic process.
Impedance measurements showed the lowest charge resistance
for CTF-2BT, as displayed in Figure S15.
Time-resolved photoluminescence (TRPL) spectroscopy at
an excitation wavelength at 400 nm showed that by adding the
strong electron acceptor unit (BT), the fluorescence lifetime
decreased proportionally to the increasing BT content (Figure
S16). The lifetimes were 0.89 ns for CTF80, 0.57 ns for CTF-1BT,
0.62 ns for CTF-2BT and 0.29 ns for CTF-4BT, respectively. This
indicates that among the BT-containing CTFs, CTF-2BT might
possess the best balance of effective charge separation and
delocalization (non-radiative recombination of the excitons) within
the CTF-xBTs.
The optical properties of the CTF nanoparticles were then
characterized by UV/Vis diffuse reflection (DR) measurements.
As displayed in Figure 2b, the pristine CTF NPs exhibited visible
light absorption which is considered characteristic of
semiconductors. Interestingly, the absorption onset at around 550
nm shifts towards longer wavelengths, with increasing the
amounts of DCBT. The color of the materials changes with
increasing BT content from yellow to brownish orange. This
observation could be confirmed by the Kubelka-Munk-
transformed reflectance spectra, which gave band gaps of 2.82
eV for the pristine CTF NPs, 2.79 eV for CTF-1BT, 2.76 eV for
CTF-2BT and 2.66 eV for CTF-4BT, respectively (Figure S11). In
order to assess the location of the lowest unoccupied molecular
orbitals (LUMO) of the materials in this study, cyclic voltammetry
(CV) measurements were conducted. As depicted in Figure S12,
a higher BT content shifts the LUMO energy to more positive
potentials versus the saturated calomel electrode standard. This
trend is consistent with B3LYP/6-31G(d) density functional theory
(DFT) calculations for triazine structures analogous to the CTFs
in this study (Figure S13). By adding the strong electron acceptor
2,1,3-benzothiadiazole, D-A charge transfer and therefore
electron localization can be further promoted.[17] The consequent
LUMO energy decrease and the reduction of the HOMO/LUMO
gap account for the observations made in the optoelectronic
characterizations above.
Moreover, photoluminescence (PL) spectra with an excitation
wavelength of 380 nm were recorded. The PL intensity of the
CTF-xBT NPs significantly decreased with the emission
maximum shifting to longer wavelengths compared to the pristine
CTF NPs (Figure 2c). This could be due to a promoted charge
carrier separation in CTF-xBTs leading to other non-radiative
charge recombination pathways being favored. The observed
redshift in the catalysts emission characteristics could be derived
from the changed UV/Vis absorption behavior. The lowering
LUMO levels and bandgaps led to the CTF-xBT NPs emitting from
an excited state with lower energy. To further investigate the
photogenerated electron separation in CTF-xBT NPs,
photocurrent measurements were conducted (Figure 2d). A
significant increase of photocurrent for CTF-xBT NPs compared
to pristine CTF NPs was observed, with CTF-2BT showing the
highest photocurrent intensity. This indicates an improved light-
induced charge mobility within the copolymerized CTF. In addition,
electron paramagnetic resonance (EPR) measurements were
carried out. As shown in Figure S14, a single Lorentzian line
centered at a g-value of 2.0031 was observed for all CTF NPs.
This signal originates from unpaired electrons in the π-conjugated
system. Particularly, the EPR intensity of CTF-2BT showed
considerable enhancement compared to the other CTF NPs
under visible light irradiation. This might confirm the finding of the
Figure 2. (a) Molecular structure, (b) diffuse reflectance UV/Vis spectra, (c)
steady state photoluminescence spectra with λexc=380 nm as well as (d)
photocurrent measurement under visible light irradiation for CTF-xBT NPs.
We then examined the photocatalytic activity of the CTF NPs
in the photocatalytic synthesis of dibenzofuran derivatives. The
photocatalytic oxidative [3+2] cycloaddition was first published by
Yoon and coworkers, using ruthenium-based homogeneous
photocatalysts.[18] The 2,3-dihydrobenzofuran core is a common
motif in bioactive natural products and poses a complex
photocatalytic synthesis target.[19] In a typical experiment, trans-
anethol was reacted with mequinol in nitromethane using CTF
NPs as photocatalyst under blue light irradiation (Figure 3a).
Ammonium peroxodisulfate was added to act as terminal oxidant.
Pristine CTF80 NPs exhibited a highly enhanced photocatalytic
activity compared to the bulk material (Figure 3b-c). After 10h of
reaction time, for example, the reaction yield is increased 8-fold.
This effect could be attributed to the decreased particle size and
increased number of reactive sites having a positive impact on
photocatalytic activity. CTF180 and CTF550 NPs showed slightly
lower activity than CTF80. Looking at the copolymerized
nanoparticles, CTF-2BT showed the highest photocatalytic
efficiency with a conversion of about 90% after 10h, which equals
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an 18- and 2.3-fold increase compared to the bulk and pristine
CTF80 NPs, respectively. Considering the similar BET surface
area (Figure S17) and particle size of the CTF-xBT samples, the
superior catalytic efficiency of CTF-2BT could be attributed to
improved photoelectronic properties as indicated previously. The
amount of copolymerized electron accepting unit appears to have
optimal beneficiary effects on photoelectronic properties at 2
mol% BT, since CTF-1BT and CTF-4BT exhibit inferior
photocatalytic efficiency in the benchmark reaction. Based on
these observations, the nanoscaling (size effect) as well as the
introduction of benzothiadiazole into the CTF backbone
(electronic effect) synergistically contribute to a substantial
improvement in photocatalytic performance.
Additionally, recycling experiments demonstrated that CTF-
2BT could be used for five reaction cycles without apparent loss
of its catalytic efficiency. No obvious change in FT-IR, UV/Vis DR
spectra and morphology could be observed (Figure S20). The
heterogeneous photocatalyst therefore reveals excellent stability
and recyclability in its photocatalytic application. The versatility of
the photocatalytic material was investigated by employing
different alkenes and phenols in the cycloaddition reaction (Figure
S21). Apart from depicted synthesis, further organic photoredox
reactions could be catalyzed using the nanoparticle catalyst.
Furthermore, as shown in the Figure 3d, dehalogenation of α-
chloroacetophenone, hydroxylation of 4-biphenylboronic acid as
well as benzoimidazole formation from o-phenylenediamine as
example of photooxidation, reduction and redox reactions could
be catalyzed with excellent yields of above 95%. This
demonstrates general applicability of the CTF nanoparticles in the
application as photocatalysts for organic photoredox reactions.
Control experiments conducted in the absence of light,
photocatalyst and (NH4)2S2O8 only gave trace amounts of product,
indicating their indispensable roles in current catalytic system
(Table S1). The catalytic mechanism is proposed in accordance
to the literature (Figure S18) and was supported by scavenger
experiments employing CuCl2 as electron and KI as hole
scavengers.[18] Upon visible light irradiation, the excited
photocatalyst oxidizes the phenol and generates a corresponding
radical cation. In the same time, S2O82- quenches the generated
excited electrons from the LUMO level of the photocatalyst.
Through hydrogen abstract transfer (HAT), the radical cation is
converted to a resonance-stabilized phenoxonium cation, which
reacts with anethole to give the desired dibenzofuran, after losing
one proton. The apparent quantum yield (AQY) in the reaction is
wavelength dependent and showed a maximum of 1.89% (Figure
S19) at 385 nm for CTF-2BT.
In conclusion, we report the size-controlled confinement
synthesis of covalent triazine framework nanoparticles and their
application in organic photoredox catalysis. The CTF
nanoparticles were obtained with a uniform size distribution and
could be varied in diameter between 80 and 550 nm. Additionally,
the optoelectronic properties of the CTF NPs could easily be
tuned by copolymerizing electron-withdrawing benzothiadiazole
units. The photocatalytic activity of the CTF NPs was tested in the
oxidative [3+2] cycloaddition of trans-anethol and mequinol as
benchmark reaction. Both of the decreased particle size and the
modified photoelectronic properties through BT-introduction
synergistically contributed to its superior photocatalytic efficiency
compared to the bulk CTF material. Moreover, the CTF NPs could
be employed as efficient photocatalyst in further dibenzofuran
syntheses as well as dehalogenation, hydroxylation and
benzoimidazole formation reactions. In summary, the materials
depict a compelling example for the morphological, compositional
and electronic engineering of advanced polymer-based
photocatalysts. We believe that this study could pave a way for
defined nanometer-sized CTF materials in photocatalytic
applications. The innovative and mild assembly route to obtain
CTF nanoparticles could be adapted using various monomers
and therefore be relevant for a broader application range.
Acknowledgements
The authors thank the Max Planck Society for financial support.
N.H. acknowledges the Kekulé fellowship of Fonds der
chemischen Industrie (FCI) and the fellowship of the Gutenberg-
Akademie of the Johannes Gutenberg University of Mainz. This
work is part of the research conducted by the MaxSynBio
consortium that is jointly funded by the Federal Ministry of
Education and Research of Germany (BMBF) and the Max Planck
Society (MPG). The synthetic contribution of Sabrina Brand is
greatly acknowledged.
Figure 3. (a) Benchmark [3+2] cycloaddition reaction of trans-anethol and
mequinol. (b) Kinetic study using different CTF nanoparticles as photocatalysts
over the course of 24h and (c) at the specific time point of 10h reaction time. (d)
Three photoredox reactions catalyzed by CTF-2BT.
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10.1002/anie.202007358
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Making polymer photocatalysts in
a nano baking dish: Synthesis of
size-controllable covalent triazine
framework nanoparticles through a
confined polymerisation strategy
within silica capsules that show
manifold increase in photocatalytic
activity compared to the bulk system
W. Huang, N. Huber, S. Jiang,
K. Landfester, K. A. I. Zhang*
Page No. – Page No.
Covalent triazine framework
nanoparticles via size-
controllable confinement
synthesis for enhanced visible
light photoredox catalysis
This article is protected by copyright. All rights reserved.