CO2-Triggered Switchable Hydrophilicity of a Heterogeneous Conjugated Polymer Photocatalyst for Enhanced Catalytic Activity in Water

Article abstract of DOI:10.1002/anie.201711773

Water compatibility for heterogeneous photocatalysts has been pursued for energy and environmental applications. However, there exists a trade-off between hydrophilicity and recyclability of the photocatalyst. Herein, we report a conjugated polymer photocatalyst with tertiary amine terminals that reversibly binds CO₂ in water, thereby generating switchable hydrophilicity. The CO₂-assisted hydrophilicity boosted the photocatalytic efficiency in aqueous medium with minimum dosage. When CO₂ was desorbed, the photocatalyst could be simply regenerated from reaction media, facilitating the repeated use of photocatalyst. Hydrophilicity/hydrophobicity control of the polymer photocatalyst was successfully showcased through a variety of organic photoredox reactions under visible-light irradiation in water.

Full text of DOI:10.1002/anie.201711773

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Accepted Article
Title: CO2-Triggered Switchable Hydrophilicity of Heterogeneous
Conjugated Polymer Photocatalyst for Enhanced Catalytic
Activity in Water
Authors: Jeehye Byun, Wei Huang, Di Wang, Run Li, and Kai A. I.
Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201711773
Angew. Chem. 10.1002/ange.201711773
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10.1002/anie.201711773
Angewandte Chemie International Edition
COMMUNICATION
CO2-Triggered Switchable Hydrophilicity of Heterogeneous
Conjugated Polymer Photocatalyst for Enhanced Catalytic
Activity in Water
Jeehye Byun, Wei Huang, Di Wang, Run Li, Kai A. I. Zhang*[a]
Abstract: Water-compatibility of heterogeneous photocatalysts has
been pursued for energy and environmental applications. However,
there exists a trade-off between hydrophilicity and recyclability of the
photocatalyst. Herein, we report a conjugated polymer photocatalyst
with tertiary amine terminals that reversibly binds CO2 in water,
photocatalysts, which may cause difficulty in recovering and
regenerating the photocatalyst after reaction. Indeed, there has
been
a trade-off between water-compatibility and efficient
recovery of photocatalyst.
One strategy for making water-compatible photocatalyst
without compromising its recyclability is to apply ‘switchable’
hydrophilicity to polymer photocatalyst. In other words, the
wettability of polymer photocatalyst is reversibly switched from
hydrophilic to hydrophobic depending on reaction sequence.
Lately, CO2 gas, as an abundant and cheap reagent, has been
realized as an external stimulus for controlling wettability of
generating
a
switchable hydrophilicity. The CO2-assisted
hydrophilicity boosted up the photocatalytic efficiency in aqueous
medium with minimum dosage. When CO2 was desorbed, the
photocatalyst could be simply regenerated from reaction media,
facilitating
the
repeated
use
of
photocatalyst.
The
hydrophilicity/hydrophobicity control of the polymer photocatalyst has
successfully showcased through a variety of organic photo-redox
reactions under visible light irradiation in water.
materials such as membrane[17], colloid dispersion[18], solvent[19]
,
surfactant[20], polymer nanoparticle[21], block copolymer[22], and
small molecular homogenous catalyst[23]. The CO2-responsive
functional groups, i.e. amidine, guanidine and amine, can
reversibly form a charged cation by binding CO2 in a presence of
water, resulting in better wettability of the materials.
Notwithstanding the effectiveness, CO2-assisted hydrophilicity
control of heterogeneous photocatalytic system has yet been
unveiled.
Conjugated polymers have emerged as a new generation of
heterogeneous photocatalysts for utilizing visible light as a cheap
and environmentally-friendly energy source[1]. By virtue of
structural robustness, non-toxicity, and high photocatalytic
efficiency, the conjugated polymer photocatalysts have been
suggested as a sustainable alternative to resolve a chronic
drawback of small molecular photocatalysts, i.e., transition-metal
complexes[2] and organic dyes[3]. Due to the fact of being a
polymer, the conjugated polymer photocatalysts can be designed
for target application, in which their electronic, optical, and
chemical properties can be tuned by doping method[4], molecular
engineering[5], surface functionalization[6] etc. In this regard, there
has been an enormous effort to construct efficient polymer
photocatalysts, to name a few, carbon nitride[7], conjugated
microporous polymer[8], covalent triazine framework[9], and
Herein, we aim to control the hydrophilicity of heterogeneous
conjugated polymer photocatalysts in water by a simple CO2/N2
switch.
A tertiary amine-tethered conjugated polymer was
designed to exhibit a reversible hydrophilicity change triggered by
CO2, which enhances its photocatalytic efficiency and recyclability
in water. The polymer photocatalysts having switchable
hydrophilic nature were demonstrated for photocatalytic reactions
in aqueous medium under visible light illumination, i.e. photo-
degradation of organic dyes and organic transformations of water-
soluble molecules.
planarized conjugated polymer[10]
nanostructure[11]
,
conducting polymer
.
Besides the material design viewpoint, a more facile approach
to environmentally-friendly reaction would be a utilization of green
solvents. Water, in particular, is considered as a cheap, safe, and
eco-friendly green solvent for chemical reactions[12]. Furthermore,
the employment of water opens up a potential for energy,
environmental, and biological applications such as water
splitting[7b, 13], anti-bacterial treatment[4b], photodynamic therapy[14]
,
11a]
and water treatment[6,
.
Accordingly, water-compatible
heterogeneous polymer photocatalysts have been recently
achieved through transformation[4b, 13b], surface functionalization[6,
15], and protonation via acid treatment[16]. However, such synthetic
protocols accompanied
a permanent wettability change of
[a]
Dr. J. Byun, W. Huang, D. Wang, R. Li, Dr. K. A. I. Zhang
Max Planck institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany
E-mail: kai.zhang@mpip-mainz.mpg.de
Scheme 1. Schematic illustration of the switchable hydrophilicity and the
photocatalytic applications of the conjugated polymer photocatalyst in water by
CO2/N2 swing.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201711773
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COMMUNICATION
The reversible hydrophilicity/hydrophobicity of the conjugated
polymer photocatalyst is illustrated in Scheme 1. The conjugated
polymer P-BT-DEA bearing tertiary amine was simply generated
through post-modification of hydrophobic P-BT-Br with
diethylamine (DEA) (Figure S1). Elemental analysis indicated a
91.3 % conversion of bromine on P-BT-Br to amine on P-BT-DEA
(Table S1). As revealed in Figure 1a, the P-BT-DEA stays
hydrophobic in water, however once being treated with CO2 gas
for 1 h, DEA at the terminal of the polymer bound CO2, producing
bicarbonate salt in a presence of water. The formation of
bicarbonate salt boosts up water compatibility of the polymeric
chain, leading to good polymer dispersion in water (the solution
was named as P-BT-DEA-CO2 hereafter). The absorbed CO2 can
be removed by N2 treatment under mild condition, and the
elimination of bicarbonate salt causes precipitation of
hydrophobic P-BT-DEA which can be filtered through normal
paper filter (Figure S2). Such a switchable hydrophilicity could be
also observed in two-dimensional system, in which P-BT-DEA film
on glass substrate exhibited smaller contact angle after the
treatment with CO2 in water (Figure 1b).
5.5 during the CO2 treatment owing to the formation of carbonic
acid (H2CO3). The pH of the aqueous polymer dispersion was
recovered to 8.25 under N2 condition, which indicates all the CO2
was desorbed from the solution. The pH change was also
reversible under the different gas atmosphere. The dynamic light
scattering (DLS) of aqueous P-BT-DEA-CO2 gave an average
diameter of 339 ± 96 nm, corresponding to the microscopic
observation (Figure S3).
The reversible formation of bicarbonate salt on P-BT-DEA
was able to be monitored with FT-IR spectroscopy (Figure 1d-g).
A stretching vibration of C-Br at 560 cm-1 was disappeared on P-
BT-DEA, indicating a complete amination (Figure 1d). The P-BT-
DEA showed three peaks at 1200, 1365, and 2790 cm-1, which
are attributed to tertiary aliphatic amine C-N stretching, N-CH-
deformation, and N-CH3 stretching vibration, respectively (Figure
1e). As Figure 1f shows, when treated with CO2 in water, the P-
BT-DEA-CO2 displayed intense peaks at 1635 and 3350 cm-1
-
which correspond to HCO3 (aq.)[25] and OH stretching vibration,
respectively. As the bicarbonate peak was evolved, the peaks
from DEA were weakened, which may be originated from a
shielding effect of bicarbonate ion at the position of DEA[26]. Once
the P-BT-DEA-CO2 was regenerated with N2, the FT-IR spectrum
became identical to that of pristine P-BT-DEA (Figure 1g). The
formation of bicarbonate on P-BT-DEA was also observed by 13
NMR spectrum (Figure S4).
C
The UV-vis diffuse reflectance (DR) spectrum of P-BT-DEA
showed a broad absorption in the visible region up to 560 nm
(Figure S5a), which is a typical behavior of benzothiadiazole (BT)-
anchored porous conjugated polymer photocatalysts[27]. The
optical band gap (Eg) of 2.19 eV was derived from the Kubelka-
Munk-transformed reflectance spectrum of P-BT-DEA (Figure
S5b). The electronic property of P-BT-DEA was further
characterized by cyclic voltammetry (CV) measurement (Figure
S5c). The LUMO of P-BT-DEA was located at -0.96 V vs. SCE,
thus the HOMO was calculated to be 1.23 V vs. SCE according
to its optical band gap.
The switchable hydrophilicity and semi-conductive nature of
P-BT-DEA allow us to explore water-based photocatalytic
reactions. Firstly, the feasibility of P-BT-DEA was demonstrated
by photocatalytic degradation of water-soluble organic dyes. In a
typical experimental set-up, P-BT-Br, P-BT-DEA, and P-BT-DEA-
CO2 were dispersed in aqueous solution of rhodamine B (RDB)
and irradiated under white LED light. As shown in Figure 2a, the
hydrophobic P-BT-Br and P-BT-DEA exhibited 17 % and 47.6 %
of RDB degradation after 1 h of light exposure, respectively (see
Figure S6 for the detailed degradation kinetics). There was a
slight enhancement in degradation efficiency with P-BT-DEA than
with the original P-BT-Br, which may be attributed to relatively
hydrophilic amine group in P-BT-DEA than bromine in P-BT-Br
counterpart. Meanwhile, P-BT-DEA-CO2 exhibited a complete
photo-degradation of RDB after 30 min of light irradiation, which
is well comparable to a state-of-the-art conducting polymer
photocatalyst[11a]. It is worth to note that thanks to the high affinity
toward water, the P-BT-DEA-CO2 dosage was 10 times lower
than the condition of previous studies[11a, 28]. Control experiments
with P-BT-DEA and P-BT-Br under chemically acidic condition
and CO2-blown condition, respectively, proved the primary effect
Figure 1. (a) Photographs of P-BT-DEA in water showing switchable hydro-
philicity/hydrophobicity. (b) Contact angle of P-BT-DEA film before and after
CO2 treatment for 1 h in H2O. (c) Reversible zeta potential of P-BT-DEA
dispersion in water by CO2/N2 treatment and concurrent pH change. FT-IR
spectra of (d) P-BT-Br, (e) P-BT-DEA, (f) P-BT-DEA-CO2, and (g) regenerated
P-BT-DEA after N2 treatment.
The CO2-assisted wettability change could be monitored by
zeta potential change of polymer dispersion in water. As shown in
Figure 1c, the original P-BT-DEA exhibited ca. 3 mV of zeta
potential in water, indicating unstable colloid with a rapid
flocculation behavior[24]. On the other hand, the P-BT-DEA-CO2
showed c.a. 20 mV of zeta potential, demonstrating the increase
of dispersion stability due to the CO2-triggered hydrophilicity. The
zeta potential displayed a reversible pattern with respect to CO2
and N2 swing, implying that the P-BT-DEA-CO2 can be recycled
for repeated use. There existed obvious pH change of P-BT-DEA
aqueous solution under the CO2/N2 shift (Figure 1c). The initial pH
of P-BT-DEA in water was about 8.4, which further went down to
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10.1002/anie.201711773
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COMMUNICATION
of hydrophilicity in RDB degradation (Figure S7). The general
applicability of P-BT-DEA-CO2 for dye degradation was then
demonstrated via using methylene blue and crystal violet.
Similarly, the water-compatible polymer P-BT-DEA-CO2 showed
higher photo-degradation efficiencies than the hydrophobic
polymers of P-BT-Br and P-BT-DEA (Figure S8).
2-furoic acid, well-known as an 1O2 trap[30], was successfully
oxidized into 5-hydroxy-2(5H)-furanone with a conversion of >
99 % for 24 h (see NMR spectra of product in Figure S11). Oxygen
gas was applied during the reaction to facilitate O2 generation,
and a control experiment in the closed air condition exhibited 35 %
1
of conversion. The P-BT-DEA-CO2 showed
a quantitative
conversion of 2-furoic acid up to five repeated cycles, despite of
the possible loss of the catalyst by cycles (Figure S12).
P-BT-DEA-CO2 was subjected to repeated cycles of RDB
degradation through CO2/N2 switching. Up to 7th cycles, there was
no perceptible loss in RDB degradation efficiency, demonstrating
a high stability of P-BT-DEA photocatalyst (Figure 2b). The
switchable hydrophilicity of the heterogeneous photocatalyst has
promoted recovery efficiency as well as water compatibility.
Control experiments in the presence of a series of scavengers
Table 1. Photocatalytic conversion of organic molecules using P-BT-DEA-CO2
in aqueous media. All the reactions were conducted under white LED
irradiation.
Entry
(1)a
Reaction
Conv.
>99 %
-
revealed that reactive species of e-, •OH, h+, and •O2 played a
major role for RDB photo-degradation, well corresponding to
Oxidation
.
previous reports[29] The e- scavenger affected the most,
(2)b
Reduction
decreasing the photo-degradation efficiency down to 51.9 %,
96 %
85 %
-
since the excited e- involves in generating both •O2 and •OH
species[11a]. When a general radical scavenger was utilized, i.e.
vitamin C, the overall degradation efficiency was far deteriorated
(Figure S9).
(3)c Redox
Reaction conditions: [a] 2-Furoic acid (1 mmol), P-BT-DEA-CO2 (0.1 mg mL-1),
H2O (10 mL), r.t., 24 h. Conversion determined by NMR spectroscopy. [b] 4-
Nitrophenol (2 mM, 1 mL), P-BT-DEA-CO2 (0.2 mg mL-1), H2O (9 mL), NaBH4
(10 mg), 1 M NaOH (10 μL), r.t., 1 h. Conversion calculated by UV-vis
spectroscopy. [c] Caffeine (0.3 mmol), 4-methoxybenzenediazonium
tetrafluoroborate (0.2 mmol), P-BT-DEA-CO2 (0.5 mg mL-1), H2O (10 mL), r.t.,
42 h. Conversion observed by GC-MS.
Photo-reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-
AP) has been conducted by using P-BT-DEA-CO2. As described
in Table 1 (Entry 2), the reduction of 4-NP was completed in 1 h
of light illumination even with small amount of the catalyst, leading
to an average turnover frequency (TOF) of 22.95 h-1 (see Figure
S13a for conversion kinetics, Figure S14 for NMR and GC spectra
of product). The P-BT-DEA-CO2 exhibited
a
successful
conversion of 4-NP over the repeated cycles, resulting in 93 % of
conversion efficiency after 3rd cycle (Figure S15). Control
experiment showed that no 4-AP was detected in the absence of
P-BT-DEA-CO2, implying a critical role of photocatalyst for the
reduction of 4-NP (Figure S13b). When sodium borohydride
(NaBH4) was absent, the reduction of 4-NP rarely occurred, rather
the degradation of 4-NP took place over time (Figure S13c). This
indicates the role of NaBH4 as essential hydrogen and electron
donor[31]. The plausible mechanism for the reduction of 4-NP
involves electron and proton transfer processes[32]. Upon light
irradiation, the borohydride was oxidized from photo-induced hole,
and at the same time, 4-NP was activated to anionic radical
intermediate through the excited electron. The activated substrate
extracted the proton from the oxidized borohydride, finally giving
4-aminophenol (4-AP) as a product.
Figure 2. (a) Photocatalytic degradation of RDB in a presence of P-BT-Br, P-
BT-DEA, and P-BT-DEA-CO2. C0 is initial concentration of RDB after reaching
adsorption/desorption equilibrium under dark condition. C is the concentration
of RDB after light irradiation for a certain time period. (b) Recyclability of P-BT-
DEA-CO2 for repeated degradation of RDB.
The water compatibility of P-BT-DEA-CO2 was further
examined for a variety of organic transformation reactions under
visible light; i.e. photo-oxidation, photo-reduction, and
photocatalytic heteroaryl-aryl coupling reaction. P-BT-DEA-CO2
was able to efficiently generate singlet oxygen (1O2) (see EPR
spectra in Figure S10), thus oxidation reaction through the photo-
generated 1O2 was carried out. As displayed in Table 1 (Entry 1),
The photo-oxidation and photo-reduction performance of P-
BT-DEA-CO2 prompted us to conduct a reaction involving photo-
redox catalysis. We then investigated the radical-mediated
arylation of heteroarene under visible light to further verify
photocatalytic activity of P-BT-DEA-CO2 in water. As a model
reaction, the coupling of caffeine and aryldiazonium
tetrafluoroborate was chosen as an important example for the
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10.1002/anie.201711773
Angewandte Chemie International Edition
COMMUNICATION
direct production of biologically active compounds[33] (Table 1
Entry 3). Under the given condition, the cross-coupling product
was obtained after 42 h of reaction, giving 85.2 % of conversion
efficiency (see NMR spectra of product in Figure S16). Repeating
experiments of the heteroaryl-aryl coupling revealed that the P-
BT-DEA-CO2 could be used up to extra five 24 h-cycles without
losing the conversion efficiency (Figure S17). When the amount
of caffeine was predominant up to 10 equivalents to that of the
aryldiazonium counterpart, the conversion efficiency stayed to be
73.4 %. It may be attributed to the lower solubility of excess
caffeine in water, where previous studies had to be carried out in
harsh acidic condition to dissolve the extra heteroarenes[33]. This
demonstrates that the utilization of CO2 gas in this work was
advantageous in terms of not injecting additional acid, while
making the overall condition slightly acidic via formation of
carbonic acid in water.
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Keywords: Hydrophilicity switch• Conjugated polymer •
Heterogeneous photocatalysis • Carbon dioxide • Photo-redox
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COMMUNICATION
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10.1002/anie.201711773
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
On/Off hydrophilicity of
J. Byun, W. Huang, D. Wang, R. Li, K.
A. I. Zhang*
photocatalyst: A tertiary amine-
tethered conjugated polymer
photocatalyst was designed to exhibit
a switchable hydrophilicity via a
simple CO2/N2 trigger. The reversible
hydrophilicity of the heterogeneous
polymer photocatalyst enhances
catalytic efficiency and recyclability for
organic photo-redox reactions in
water.
Page No. – Page No.
CO2-Triggered Switchable
Hydrophilicity of Heterogeneous
Conjugated Polymer Photocatalyst
for Enhanced Catalytic Activity in
Water
This article is protected by copyright. All rights reserved.