Covalent Organic Frameworks Enabling Site Isolation of Viologen-Derived Electron-Transfer Mediators for Stable Photocatalytic Hydrogen Evolution

Article abstract of DOI:10.1002/anie.202016618

Electron transfer is the rate-limiting step in photocatalytic water splitting. Viologen and its derivatives are able to act as electron-transfer mediators (ETMs) to facilitate the rapid electron transfer from photosensitizers to active sites. Nevertheless, the electron-transfer ability often suffers from the formation of a stable dipole structure through the coupling between cationic-radical-containing viologen-derived ETMs, by which the electron-transfer process becomes restricted. Herein, cyclic diquats, a kind of viologen-derived ETM, are integrated into a 2,2′-bipyridine-based covalent organic framework (COF) through a post-quaternization reaction. The content and distribution of embedded diquat-ETMs are elaborately controlled, leading to the favorable site-isolated arrangement. The resulting materials integrate the photosensitizing units and ETMs into one system, exhibiting the enhanced hydrogen evolution rate (34600 μmol h^?1 g^?1) and sustained performances when compared to a single-module COF and a COF/ETM mixture. The integration strategy applied in a 2D COF platform promotes the consecutive electron transfer in photochemical processes through the multi-component cooperation.

Full text of DOI:10.1002/anie.202016618

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 9642–9649
International Edition:
German Edition:
doi.org/10.1002/anie.202016618
doi.org/10.1002/ange.202016618
Photocatalysis
Covalent Organic Frameworks Enabling Site Isolation of Viologen-
Derived Electron-Transfer Mediators for Stable Photocatalytic
Hydrogen Evolution
Zhen Mi, Ting Zhou, Weijun Weng, Junjuda Unruangsri, Ke Hu, Wuli Yang, Changchun Wang,
Kai A. I. Zhang, and Jia Guo*
+
Abstract: Electron transfer is the rate-limiting step in photo-
catalytic water splitting. Viologen and its derivatives are able to
act as electron-transfer mediators (ETMs) to facilitate the rapid
electron transfer from photosensitizers to active sites. Never-
theless, the electron-transfer ability often suffers from the
formation of a stable dipole structure through the coupling
between cationic-radical-containing viologen-derived ETMs,
by which the electron-transfer process becomes restricted.
Herein, cyclic diquats, a kind of viologen-derived ETM, are
integrated into a 2,2’-bipyridine-based covalent organic frame-
work (COF) through a post-quaternization reaction. The
content and distribution of embedded diquat-ETMs are
elaborately controlled, leading to the favorable site-isolated
arrangement. The resulting materials integrate the photosensi-
tizing units and ETMs into one system, exhibiting the enhanced
in the formation of a corresponding cationic radical, RV C.
The species preferentially assembles into a stacked dimer with
stable dipole structure via the intermolecular p-conjuga-
[13]
tion.
(SOMO)
Accordingly, the singly occupied molecular orbital
has emerged, as a consequence, with slightly
higher energy than that of the highest occupied molecular
orbital (HOMO). The electron transition in the RV C dimer
might be forbidden due to the low-lying SOMO level.
Therefore, a strategy to suppress the formation of p-stacked
cationic radicals, RV C dimers, is essential, in order to make
a full potential use of viologen-derivatives as stable ETMs.
Two-dimensional covalent organic frameworks (2D
COFs) have been highlighted as photocatalysts for water
[14]
+
+
[
15]
[
16]
splitting. They feature chemical tunability, uniform porosity
and high crystallinity for synergizing different functions and
À1 À1
[17–22]
hydrogen evolution rate (34600 mmolh g ) and sustained
facilitating charge transport under visible irradiation.
performances when compared to a single-module COF and
a COF/ETM mixture. The integration strategy applied in a 2D
COF platform promotes the consecutive electron transfer in
photochemical processes through the multi-component coop-
eration.
Molecular engineering is the predominant strategy, often
applied on 2D COFs to give prominent photoactivity. The
introduction of heteroatom-containing functional groups into
[23]
[
24]
[25]
photosensitizing COFs such as sulfone,
triazine
and
[26]
benzothiadiazole benefits the modulation of energy band
structures, improving light harvesting and facilitating electron
transfer toward proton-reduction sites. To further strength-
en the electron push-to-pull effect, a built-in electric field
has been created by accommodation of metal nanoparticles to
[
27]
Introduction
[28]
Electron-transfer mediators (ETMs) are well known for
their dual role to accept and donate electrons in a dynamic
[29]
cooperate with 2D COFs.
The composites promote the
electron/hole dissociation and free charge transport.
[
1]
[2]
[30]
equilibrium and to sustain electron-transfer processes for
As
[3]
[4]
improving reaction efficiency. ETMs such as TEMPO,
compared with the hybrid strategy, organic ETM modules
offer possibility to constitute an all-in-one system that allows
the pre-designed integration of various modules into one
COF framework. Furthermore, the predictable location of
each unit in the periodic framework would realize the spatial
isolation of single ETM molecule with each other for the
desirable electron-transfer properties. So far, ETMs have
[
5]
[6]
phenylene and reduced graphene oxide have been em-
[7]
ployed in a plethora of advanced applications, including
single electron-transfer reaction, zymoid catalysis and
water splitting.
RV ) is one of distinctive ETMs,
[
8]
[9]
[
10]
Of known varieties, viologen derivative
2+
[11]
(
which can render
prominent electron-transfer capability to improve reaction
[
12]
2+
kinetics. Once a single RV accepts an electron, this results
[
*] Z. Mi, T. Zhou, W. Weng, Prof. Dr. W. Yang, Prof. Dr. C. Wang,
Prof. Dr. J. Guo
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science
Fudan University
Dr. J. Unruangsri
Department of Chemistry
Chulalongkorn University
Phayathai Road, Bangkok 10330 (Thailand)
Prof. Dr. K. Hu
Department of Chemistry
Fudan University
2
005 Songhu Road, Shanghai 200438 (China)
E-mail: guojia@fudan.edu.cn
Prof. Dr. K. A. I. Zhang
Department of Materials Science
Fudan University
2005 Songhu Road, Shanghai 200438 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016618.
2
005 Songhu Road, Shanghai 200438 (China)
9
642
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ring closure with tunable chain lengths (from -(CH ) - to
2
2
-
(CH ) -). The optimized conditions were then applied with
2 4
the Tp-BPy-COF to afford the quaternized COFs, namely
2+
Tp-nC/BPy -COF (n = 2, 3, 4), in ꢀ 90% yield.
The quaternization conversions were assessed by the
potential titration analysis to quantify the amount of counter
À
anions, Br , in the diquats-containing COFs. It was found that
the relative molar ratios of cyclic diquats to 2,2’-bipyridine
2
+
moieties in Tp-nC/BPy -COF largely depended on the
reaction time (Table S1 and Figure S1 in the Supporting
Information). The higher quaternization conversion was
achieved with prolonged reaction time under identical con-
ditions. X-ray photoelectron spectroscopy (XPS) was per-
formed to analyze the signals of N 1s and Br 3d, which could
2
+
Scheme 1. Schematic diagram for BPy -accelerated photocatalytic
hydrogen evolution from water splitting by virtue of electron-transfer
effect.
2
+
evidence that the BPy moieties were formed with pendent
alkyl groups either in linear or cyclic forms. Compared with
the parent Tp-BPy-COFs, the broad peaks of N 1s signals for
often been used as additives in photocatalysis but have rarely
been embedded in 2D COFs for a synergistic strategy.
Herein, we propose a conveniently accessible method to
engineer the site-isolated ETM modules on a 2,2’-bipyridine-
based COF by a post-synthetic route. As shown in Scheme 1,
a typical b-ketoenamine linked COF is constructed using 2,2’-
bipyridine as the strut, which is then quaternized with
dibromo alkanes into cyclic diquats, that is, BPy . The
resulting COF constitutes an integrative platform containing
,2’-bipyridine as a photosensitizer and cyclic diquat as an
ETM for photocatalytic water splitting. Despite the coaxial
stacking of COF layers, the cationic diquats are likely to be
positioned at the different individual layers due to strong
electrostatic repulsion. Meanwhile, the purposeful incorpo-
ration of different alkyl chains from -(CH ) - to -(CH ) - on
2
+
the Tp-nC/BPy -COF could be deconvoluted into one
additional nitrogen form corresponding to the quats at
402.0 eV (Figure S2), which proved the formation of quater-
2
+
nized BPy moieties in the COFs. For the signal of Br 3d,
there was one symmetric single peak found at 67.6 eV for all
2
+
three Tp-nC/BPy -COFs with ꢀ 20 mol.% quaternization
(Figure 1a). This was assigned to the Br anion, indicating that
the cyclic quaternization was dominated. When the quater-
2+
2
+
2
nization was over 50 mol.%, Tp-3C/BPy -COF (53.91%)
[
31]
2+
and Tp-4C/BPy -COF (58.49%) gave the two individual
deconvoluted peaks of Br 3d, which could be ascribed to Br
2
2
2 4
the cyclic diquats further generates the steric hindrance to
distance the species apart. Especially, with the controlled
quaternization (conversion < 50%), the p-stacked coupling
of cyclic diquats moieties could rarely occur enabling the
electron transfer of single cyclic diquat. It is thus desirable
that the dual-module COF could not only generate excitons
but also mediate efficient electron/hole dissociation and rapid
charge transfer from the embedded ETM to the co-catalysts
for enhancing the whole reaction kinetics.
Results and Discussion
Synthesis and Characterizations of Dual-Functional COFs
A typical b-ketoenamine linked 2D COF was synthesized
by an aldimine reaction in a mixed solvent of mesitylene and
dioxane (v/v of 1:1) at 1208C for 5 days, using 6 M HOAc
[32]
solution as a catalyst.
The resulting Tp-BPy-COF was
obtained in a 72.4% isolated yield. As found in our earlier
[
33]
report, a Brønsted acid catalyst facilitates the interconver-
sion of trans-BPy to the more stable mono-cationic cis-BPy.
This, in turn, promotes the effective cyclization and quater-
nization of Tp-BPy-COF with various dibromo alkanes in
a controlled manner. In an independent study, the model
reaction testified that 2,2’-bipyridine could be successfully
quaternized under reflux in acetonitrile for 4 h. The adjacent
two pyridine nitrogen are bound with alkyl chains forming
Figure 1. a) XPS spectra, b) FT-IR spectra, c) PXRD patterns, and d) N2
2+
sorption isotherms of Tp-BPy-COF and three quaternized Tp-nC/BPy
-
COFs (n=2, 3, 4) with approximately 20 mol.% of cyclic diquats.
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2
À1
anion (67.6 eV) and bromine from the C-Br bond (70.5 eV).
The latter signifies the formation of linear alkylated quats, but
surface areas from 1359 to 1226 and to 1028 m g was
+
2
2+
observed for Tp-2C/BPy -COF (19.10%), Tp-3C/BPy -
COF (19.36%) and Tp-4C/BPy -COF (21.98%). Also,
2+
2+
it was negligible in the XPS spectrum of Tp-2C/BPy -COF
57.89%) (Figure S3). The result points toward the prefer-
(
BET surface areas were correspondingly reduced with an
increase in quaternization conversion (Figure S9). The pore-
size distributions of Tp-nC/BPy -COF ( ꢀ 20%) were much
broader than that of the parent Tp-BPy-COF and shifted to
the micropore range of 1–2 nm (Figure S10), which suggests
the presence of alkyl chains in the pore channels.
ential formation of the six-membered cyclic diquats over the
seven-, eight-membered cyclic diquats as well as linear
alkylated quats.
2+
The quaternization of Tp-BPy-COF was further validated
by different spectroscopy techniques. The FT IR spectra
À1
showed the CÀH stretching bands around 3000 cm , which
newly emerged compared to that of the parent Tp-BPy-COF
(
Figure 1b), indicative of the alkyl groups as part of the COF
Electron-Transfer Capability
13
skeleton. Meanwhile, the solid-state C CP/MAS NMR
spectra manifested the presence of alkyl carbon signals
around 30–50 ppm as well as the other resonances at 185,
The space charge limited current (SCLC) method was
utilized to assess the charge carrier mobility of the COF
materials. The current density-voltage curves were examined
in two different regions, which were differentiated into an
ohmic region (I / V) at low voltages and an SCLC region (I /
1
42 and 107 ppm, assigned to the carbon atoms in the
b-ketoenamine linkages (Figure S4a). Also, they could be
2+
assigned by the spectrum of Tp-2C/BPy model compound
Figure S4b), confirming the targeted structure again. Be-
2
[36]
(
V ) at high voltages. The carrier mobilities (m) of Tp-2C/
+
2
2+
sides, the positively charged skeletons of these COFs were
responsible for the better water affinity, offering much lower
contact angles at 43.88 for Tp-2C/BPy -COF (19.10%), 44.38
for Tp-3C/BPy -COF (19.36%) and 40.58 for Tp-4C/BPy -
COF (21.98%), all of which are in sharp contrast to that of
BPy -COF (19.10%), Tp-3C/BPy -COF (19.36%) and
+
Tp-4C/BPy -COF (21.98%) were estimated to be 0.41, 0.16
and 0.027 cm V s , respectively, in the SCLC region
governed by the Mott-Gurney law (Figure 2a). All the values
of m obtained by the quaternized COFs are larger than that of
2
2+
2
À1 À1
2
+
2+
[
34]
2
À1 À1
Tp-BPy-COF (62.18) (Figure S5). As reported,
the en-
Tp-BPy-COF (0.0086 cm V s ), demonstrating that the
2
+
hanced hydrophilicity of COFs is beneficial to elevate the
photocatalytic hydrogen production in aqueous media.
Powder X-ray diffraction (PXRD) gave the typical
crystalline patterns for all of the quaternized COFs (Fig-
ure 1c), which were similar to that of the parent Tp-BPy-
COF. However, the dominated X-ray peak corresponding to
the (100) facet was broadened and the relative intensity
declined with an increase in quaternization conversion,
inherent BPy moiety indeed enhances the electron-transfer
capability in the resulting COFs. Meanwhile, the electron-
transfer rate can be tuned by the length of alkyl chains in
2
+
cyclic diquats. Among these, 2C/BPy has the fastest charge
carrier mobility; hence it is the best candidate as ETM.
2+
especially for Tp-4C/BPy -COF (Figure S6). This can be
attributed to a number of alkyl chains linked onto the BPy
2+
moieties of the COF skeletons, leading to the partial
disordering in structure and weakening the diffraction
[
35]
2+
intensities.
When the Tp-2C/BPy -COF was directly
2+
synthesized via a multicomponent reaction of 2C/BPy -,
BPy- and Tp-based building blocks, the (100) peak intensity
significantly decreased (Figure S7) and the product yield was
as low as 34.2%. The relative molar ratio of 2C/BPy to BPy
moieties in the resulting COF was merely 6.2 mol.% (Fig-
ure S8), far lower than the feeding content of 2C/BPy -
containing monomer (20 mol.%). When the added 2C/BPy
increased up to 50 mol.%, the product became completely
amorphous, giving 47.4% yield and 29.1 mol.% 2C/BPy .
The control experiments reveal that the bottom-up synthesis
of the quaternized COFs is unable to efficiently regulate the
quantity of BPy components. Meanwhile, the electrically
neutral building blocks are more appropriate for the crystal-
lization of COF frameworks. When they are copolymerized
with the quaternized BPy components, the evolution of
layer-by-layer ordering arrangement is greatly impaired due
to the strong electrostatic repulsion.
2+
2+
2+
2+
2+
2+
Figure 2. a) Current density-voltage curves, b) UV-vis reflectance spec-
2+
tra and c) 8-h HERs of Tp-BPy-COF and three quaternized Tp-nC/BPy
-
COFs (n=2, 3, 4) with roughly 20 mol.% of cyclic diquats. d) Photo-
catalytic hydrogen generation with Tp-2C/BPy -COF (19.10%) upon
a 48-h visible irradiation (l>420 nm). All the photocatalytic experi-
ments used 10 mg photocatalyst, 3 wt.% Pt as a co-catalyst and 0.1 M
ascorbic acid as a sacrificial reagent.
As shown in Figure 1d, Tp-BPy-COF features a type IV
sorption isotherm, indicative of a mesopore character.
Brunauer-Emmett-Teller (BET) surface areas were calculat-
2+
2
À1
ed to be as high as 1673 m g . A decreasing trend of BET
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Visible Photocatalytic H Evolution in Water splitting
Photophysical Characterizations
2
UV-vis reflectance spectra of the COFs were measured in
the solid state. The absorption onset of the quaternized COF
was slightly red-shifted by about 30 nm compared to that of
the parent Tp-BPy-COF (Figure 2b). With an increase in the
amount of cyclic diquats, the absorption onset was further
red-shifted (Figure S11) and in turn, the band gaps became
narrower as revealed by the Tauc plots (Figure S12). Accord-
ing to Mott-Schottky plots (Figure S13–S16), the positive
slopes of the plots imply that all the COFs are n-type
semiconductors. Thus, the bottom of conduction band (CB)
To quantify the spectral distribution of photocatalytic
activity, the apparent quantum efficiency (AQE) was mea-
sured as a function of the wavelength of the incident light
using band-pass filters with central wavelengths at 420, 450,
2
+
500, 550 and 650 nm, respectively. The AQEs of Tp-2C/BPy -
COF (19.10%) was wavelength-dependent (Figure 3a) and
the maximum of 6.93% appeared at 420 nm. Compared with
the unmodified Tp-BPy-COF, the 420-nm AQEs of Tp-2C/
2
+
2+
BPy -COF(19.10%),
Tp-3C/BPy -COF(19.36%)
and
2+
Tp-4C/BPy -COF(21.98%) were 2-, 1.4- and 1.1-times high-
er than that of Tp-BPy-COF, respectively (Figure S24).
[
37]
could be roughly analogous to the flat band position. When
the diquat contents increased, the CB position was going
2+
deeper from À0.78 to À0.54 V (vs. RHE) for Tp-2C/BPy -
2+
COFs, À0.74 to À0.53 V for Tp-3C/BPy -COFs, and À0.79 to
2+
À0.57 V for Tp-4C/BPy -COFs (Figure S17). The findings
corroborate that the potentials of their CB minima are more
+
negative than the potential of H /H , thereby thermodynami-
2
cally allowing the hydrogen evolution in photocatalytic
reaction. In addition, the use of ascorbic acid (H A) as an
2
appropriate sacrificial electron donor for the hole filing is
suggested through the band-structure investigation.
The photocatalytic tests were performed under visible
irradiation (l > 420 nm) in the presence of H A as a sacrificial
2
donor and Pt as a co-catalyst. As shown in Figure 2c, the
hydrogen evolution rate (HER) of Tp-BPy-COF reaches
À1
1
83 mmolh (per 10 mg) under the 8-h illumination. With the
2+
identical conditions, the HERs of Tp-2C/BPy -COF
19.10%), Tp-3C/BPy -COF (19.36%) and Tp-4C/BPy -
COF (21.98%) are increased to 346, 274 and 223 mmolh (per
2+
2+
(
À1
1
0 mg), respectively. All the values are higher than that of the
pristine Tp-BPy-COF, indicating that the electron-transfer
efficiency plays the pivotal role in promoting the photo-
2+
catalytic performance. Furthermore, Tp-2C/BPy -COF
19.10%) displays the best photocatalytic activity, reflecting
(
2+
again that 2C/BPy moiety is the most efficient ETM among
the derivatives. Amid the representative COF-based photo-
catalysts, the Tp-2C/BPy -COFs presented here are ranked
among the top materials generating hydrogen when irradiated
with visible light (Table S2).
Figure 3. a) Wavelength-dependent AQEs for photocatalytic hydrogen
evolution with Tp-2C/BPy -COF (19.10%). b) Transient photocurrent
2+
2+
responses, c) Nyquist plots from electrochemical impedance spectro-
scopy, and d) time-correlated single-photon counting measurement of
2
+
Tp-BPy-COF and three quaternized Tp-nC/BPy -COFs (n=2,3,4) with
roughly 20 mol.% of cyclic diquats.
A long-term photocatalytic activity for H production was
2
studied using the best performed catalytic system. As
displayed in Figure 2d, no significant attenuation is observed
in photocatalytic H production under the 48-h irradiation for
Tp-2C/BPy -COF (19.10%). Afterwards, the recycled COF
was subjected to the systematic characterizations including
The photocurrent response and electrochemical impe-
dance spectroscopy (EIS) were used to examine the insightful
photophysical mechanism. Upon switching the photoirradia-
tion on/off, all of the COFs responded to the incident light and
2
2
+
PXRD, N sorption, TEM, FT IR, XPS and UV-vis reflec-
2
2
+
tance spectra (Figure S18–S23). All the data confirmed that
there were no remarkable changes in compositions, structures
and photophysical properties before and after the long
catalytic cycles. Note that the weakened crystallinity and
porosity can be recovered to some extent after the solvo-
thermal treatment, as similar as the other reported COF-
Tp-2C/BPy -COF (19.10%) generated a largest photocur-
+
2
rent (Figure 3b). Concurrently, the Tp-2C/BPy -COF
(19.10%) also demonstrated the lowest resistance reflected
in the smallest diameter of semicircle in Nyquist plot (Fig-
ure 3c). Time-correlated single-photon counting (TCSPC)
was applied to estimate the excited-state lifetimes for these
materials in aqueous dispersions (Figure 3d). The results
[38]
based photocatalysts.
2
+
showed that Tp-2C/BPy -COF (19.10%) had the longer
excited-state lifetimes (t = 7.71 ns) than the others (Fig-
3
ure S25). Furthermore, the full transient absorption spectrum
2
+
of Tp-2C/BPy -COF (19.10%) showed a significant positive
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absorption band peaked at around 670 nm after nanosecond
pulsed laser excitation (Figure S26). The transient absorption
feature is indicative of the photoinduced electron localized on
2
+
the 2C/BPy moiety that lasted over microseconds, demon-
strating a clear charge separated state. All the findings
2
+
underline that the 2C/BPy moieties in the quaternized COF
is the most appropriate ETM module conferring the most
efficient electron transfer and intimately cooperating with
other functional modules to enhance the photocatalytic
activity.
Superiority of ETM-Embedded Dual-Module COFs on
Photocatalysis
To investigate the significance of the built-in ETMs, the
two independent hydrogen evolution tests of the ETM-
embedded dual-module COF (system I) and the single-
module COF mixed with external ETM (system II) were
designed (Figure 4a). Specifically, the comparison has been
2
+
made for Tp-2C/BPy -COF (19.10%) and a mixture of Tp-
BPy-COF and ETM, that is, [2,2’-BPy(CH ) ]Br , to show
2
2
2
their relative H evolution in 12 h. As shown in Figure 4b, the
2
2
+
H evolution of Tp-2C/BPy -COF (19.10%) is maintained at
an average HER of 340 mmolh (per 10 mg) over the course
2
À1
of 12 h, while the mixed species give a constant fall of HER
with the irradiation time. This is presumably due to the
+
coupling between the cationic radicals (BPy C) through the
aromatic p-interaction in the bulk ETMs, which lessens the
electron-transfer activity in the system. After 8 h, the HERs
of the system II became constant with the values similar to
that of the Tp-BPy-COF.
Then we investigated the effect of site-isolated ETMs in
COFs on the photocatalytic performance. As displayed in
2
+
Figure 4c, the HERs increase with the content of BPy rising
from 9.59% to 19.10%; nevertheless, the higher contents of
diquats result in a dramatic reduce of HERs. Particularly,
Figure 4. a) Schematic diagram for the dual-module COF (System I)
and the mixture of single-module COF with unembedded ETM (System
II). b) Contrastive hydrogen evolution test of the integrated Tp-2C/
2
+
Tp-2C/BPy -COF (57.89%) nearly gave no H production
2
2
+
under visible irradiation. It is worth noting that the post-
quaternization is achieved at the expense of BPy moiety,
which is responsible for light harvesting; this thus lessens the
light absorbability in the COFs. Furthermore, the highly
BPy -COF (19.10%) and the bare Tp-BPy-COF mixed with additional
2+
2C/BPy used as ETM under visible-light (l>420 nm) irradiation. The
orange integral area roughly reflected the hydrogen evolution differ-
ence between the two photocatalytic systems. c) HERs of Tp-2CPBy
2+
-
2
+
COF (10 mg) with different quaternization conversions. d) Computa-
quaternized COFs possessing vertically adjacent BPy units
can trap the photogenerated electrons forming cationic
radicals. These species can then be stabilized through the
p-p interactions yielding the stable dipole structure, accom-
panying the generation of the low-lying SOMO band. As
a consequence, there is no sufficient free photogenerated
electrons participating in the proton reduction process. In-
deed, the photocurrent measurement manifested that few of
tional studies of molecular orbital adiabatic potentials for BPy (I),
2+
BPy (II), and the stable dipole structure (III–V) using B3LYP density
functional.
a study using the highly quaternized COFs with numerous
embedded ETMs. After the 12-h irradiation under the
2
+
identical conditions, the recycled Tp-2C/BPy -COF
(57.89%) gave the characteristic broad peak in a near-
infrared absorption region (Figure S29). This remarks the
2
+
photogenerated electrons in Tp-2C/BPy -COF (57.89%)
were transferred (Figure S27). The findings highlight the
significant role of the site-isolated ETMs in the efficient
2
+
transformation of 2C/BPy moieties into stable cationic
radicals. Therefore, the COFs containing a specific range of
embedded-ETM contents express the superior electron-trans-
fer property enabling the excellent HERs and sustained
2
+
electron-transfer process. For the other Tp-nC/BPy -COFs
n = 3, 4), the hydrogen evolution activity is correlated with
the degree of quaternization in the similar manner as those
(
2
+
found in Tp-2C/BPy -COF (Figure S28).
photocatalytic H production over a long-term irradiation.
2
As it was very challenging to characterize the presence of
cationic radical dimers in the reaction mixture, we conducted
The insightful electron-transfer process was evaluated by
considering the energy levels of BPy (I), BPy (II) and the
2
+
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2
+
+
2+
p-stacked dimers (III–IV) of BPy and BPyC , respectively,
using B3LYP density functional (Figure 4d). For the acces-
sible photocatalytic H2 evolution, it is essential that the
LUMO of the ETM should be lower than that of the
photosensitizer, that is, BPy (I), but should concurrently be
higher than the potential of proton reduction (À4.3 eV vs.
vacuum). The computed results revealed that the cyclic
model hexagons derived from Tp-2C/BPy -COF were con-
[40]
structed by Multiwfn
and Visual Molecular Dynamics
(VMD). Based on the hole-electron theory, the S/D value is
[41]
[42]
applied to evaluate the charge-transfer-like mode, where S
stands for the overlap integral of hole-electron distribution
and D means the distance between centroid of hole and
electron. When the smaller S is paired with the larger D, it can
indicate the more evident charge-transfer character. As
displayed in Figure 5a, when the quaternization conversions
increase from 0 to 1/6, 1/3, 2/3 and 1, the minimum S/D value
is given by the [2C/BPy ] -containing hexagon, indicating
the possibility of most efficient electron-hole separation.
This is well in line with the result that the optimal photo-
2
+
diquats (2C/BPy , II) has the LUMO energy of À4.1 eV
allowing electrons transferred from BPy (II) to reduce
2
+
2
+
proton. However, as the two 2C/BPy moieties are stacked
by p-p interaction, the electron-accepted dimers (IV) and (V)
give the lower-lying SOMO levels, which forbid the electron
transfer to the higher energy level for proton reduction. This
emphasizes that the stable p-stacked dipole structure loses
the function as ETMs after accepting electrons. Therefore, the
site-isolating concept is essential, for BPy moieties as ETMs
integrated in COFs, to ensure the fast and efficient electron
transfer in the photocatalytic hydrogen production.
2
+
1/6
[43]
2
+
catalytic performance is obtained on Tp-2C/BPy -COF
(19.10%), which in structure resembles the [2C/BPy ] -
2
+
1
/6
2
+
containing hexagon.
Figure 5b exhibits the photoelectrochemically derived
energy levels of VB and CBex measured by the chopped
current-potential curves (Figure S34), both which coincide
with the theoretical energy levels derived by quantum-
chemical calculations (Table S3–S12 and Figure S35–S48 in
[44]
Electron-Transfer Properties of Model Compounds
the Supporting Information). Excited electrons in the CB of
ex
2
+
To reveal the electron-transfer origin in the quaternized
COFs, the model compound I was synthesized with Tp and
monoamine-functionalized bipyridine. Then a complete cyclic
quaternization was conducted to convert the compound I into
compound II. Both of them could represent the two kinds of
Tp-2C/BPy -COF (19.10%) have the largest thermodynamic
2
+
ideal repeating units in the Tp-BPy-COF and Tp-2C/BPy -
COF, respectively. By the analysis with cyclic voltammetry
(
Figure S30), the compound II showed the two reversible
single-electron reductions due to the redox character of cyclic
diquats. Meanwhile, the reduction potential of compound II
À1
(
Ered1, 1/2 = À0.29 V and Ered2, 1/2 = À0.7 V at 0.5 Vs ) was
much lower than that of the compound I (E
0
= À0.71 V at
red, 1/2
À1
.5 Vs ), indicating the stronger electron-accepting ability.
The electron-transfer constants kET were determined by the
peak current linear fitting using the Nicholson method
[
39]
(
Figure S31). It is evident that the kET1 of compound II is
almost 8 times higher than that obtained by compound I (k
ET
À1
À1
=
0.23 cms , k
= 1.78 cms and kET2 com-
compound I
ET1 compound II
À1
=
1.30 cms ). Besides, the photocurrent enhancement
pound II
illustrated that the mobility of the photoexcited charge
carriers is promoted; this is in line with the trend of
electron-transfer constants (Figure S32). Accordingly,
a marked decrease of Nyquist plot diameter for compound
II is observed in Figure S33, demonstrating that the electronic
resistance of compound II was smaller than that of compound
I due to its abundant charge carriers. The electronic proper-
ties reflected by the representative model compounds are in
good agreement with those of the corresponding COFs,
validating that the enhanced electron-transfer ability is
Figure 5. a) Real-space representation of hole and electron distribu-
tions (isovalue=0.001) of Tp-2C/BPy hexagons, calculated on TD-
PBE0-D3/def2-SVP//PBE0-D3/def2-SVP level of theory. The quaterniza-
tion conversions are varied from 0, 1/6, 1/3, 2/3 to 1. Blue and orange
regions denote the hole and electron distributions, respectively. b) En-
2+
2
+
indeed caused by the cyclic diquats (BPy ), instead of the
defects or doping effect in COFs.
ergy levels of VB and CB of Tp-BPy-COF and three quaternized Tp-nC/
ex
2
+
BPy -COFs (n=2, 3, 4) with roughly 20 mol.% of cyclic diquats. Green
lines indicate the measured data obtained by photo-electrochemistry,
and orange lines indicate the calculated data of the corresponding
models using TD-PBE0-D3/def2-SVP//PBE0-D3/def2-SVP method.
Electron-hole Distribution and Energy-Band Structures
For the insightful understanding of electron transfer in the
ETM-COF system, the electron and hole distributions of the
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[
[
[
[
[
[
Conclusion
In summary, we explore an ETM-integrated 2D COF by
a post-synthetic route for boosting electron transfer in
photocatalytic hydrogen production. Cyclic diquats are in
situ generated on the 2,2’-bipyridine moieties at the strut of
the COF framework. Given the combined effect of electro-
static repulsion and steric hinderance, they are individually
immobilized on the different sites and isolated with each
other, thereby suppressing the formation of stable p-stacked
cationic radicals during the electron-transfer process. Com-
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Manuscript received: December 14, 2020
Accepted manuscript online: January 22, 2021
Version of record online: March 11, 2021
[
6
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