Narrow-Bandgap Chalcogenoviologens for Electrochromism and Visible-Light-Driven Hydrogen Evolution

Article abstract of DOI:10.1002/anie.201711761

A series of electron-accepting chalcogen-bridged viologens with narrow HOMO–LUMO bandgaps and low LUMO levels is reported. The optoelectronic properties of chalcogenoviologens can be readily tuned through heavy atom substitution (S, Se and Te). Herein, in situ electrochemical spectroscopy was performed on the proof-of-concept electrochromic devices (ECD). E-BnV²⁺ (E=Se, Te; BnV²⁺=benzyl viologen) was used for the visible-light-driven hydrogen evolution due to the strong visible-light absorption. Remarkably, E-BnV²⁺ was not only used as a photosensitizer, but also as an electron mediator, providing a new strategy to explore photocatalysts. The higher apparent quantum yield of Se-BnV²⁺ could be interpreted in terms of different energy levels, faster electron-transfer rates and faster formation of radical species.

Full text of DOI:10.1002/anie.201711761

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Accepted Article
Title: Narrow Band Gap Chalcogenoviologens for Electrochromism and
Visible-Light-Driven Hydrogen Evolution
Authors: Gang He, Guoping Li, Letian Xu, Weidong Zhang, Kun Zhou,
Yousong Ding, Fenglin Liu, and Xiaoming He
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201711761
Angew. Chem. 10.1002/ange.201711761
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Angewandte Chemie International Edition
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COMMUNICATION
Narrow Band Gap Chalcogenoviologens for Electrochromism and
Visible-Light-Driven Hydrogen Evolution
[a]
[a]
[a]
[a]
[a]
[a]
[b]
Guoping Li, Letian Xu, Weidong Zhang, Kun Zhou, Yousong Ding, Fenglin Liu, Xiaoming He,
and Gang He*[a]
Abstract: A series of electron-accepting chalcogen-bridged viologen
with narrow HOMO–LUMO band gaps and low LUMO levels is
reported. The optoelectronic properties of chalcogenoviologens can
be readily tuned via heavy atoms substitution (S, Se and Te). Herein,
in-situ electrochemical spectroscopy was performed on the proof-of-
2+
concept electrochromic devices (ECD). E-BnV (E = Se, Te) was
used in visible-light-driven hydrogen evolution due to the strong visible
2+
light absorption. Remarkably, E-BnV was not only used as a
photosensitizer, but also as an electron mediator, providing a new
strategy to explore photocatalysts. The higher apparent quantum yield
2+
of Se-BnV could be interpreted in terms of different energy levels,
faster electron-transfer rates and faster formation of radical species.
2
+
The research and application of viologen analogs (RV ) have
developed rapidly,[1] particularly in fields such as electrochromic
display devices (ECD),[2] metallosupramolecular assemblies,[3]
host-guest complexes,[4] solar-to-fuel conversion,[5] energy
storage[6] and many other utilities.[7] The widespread use of
viologen analogs results from their unique properties, that is,
different redox states of these analogs can donate or accept
electrons, participate in the coordination from reversible redox
reaction and change color between purple and orange.[8]
Scheme 1. Synthesis of chalcogenoviologens.
Typically, polythiophene tends to generate p-doping charges
and capture photons which improves electrical conductivity,
electrochromism and drive photocatalysis.[ As members of the
chalcogen family, heavier Se and Te atoms have special
properties compared to S atom in terms of greater mass and
16]
The substitution of specific carbon atoms in the purely carbon-
based scaffolds by main-group heteroatoms has drawn significant
attention due to their unique synergistic electrical and optical
properties.[9] In recent years, several groups have made efforts to
combine heteroatoms and viologens. For example, rigid
phosphaviologens (P-RV)[10], doubly phosphorus-bridged
viologen (DPMV)[11] or disulfide viologen[12] were developed by
doping P or S into viologen scaffolds, which showed enhanced
polarizability, which are expected to improve the electron
mobilities.[
17]
It can be envisioned that introducing heavier
chalcogen atoms (Se and Te) into viologen skeleton should
significantly perturb the electronic structures of novel viologen
derivatives. Based on this consideration, we have now focused
on the synthesis of chalcogen-bridged viologens (chalcogeno-
2
+
viologens, E-RV ), which combine the nature of viologen and
chalcogenophene and extend their applications in electrochromic
devices and visible-light-driven hydrogen evolution.
electrochemical
and
electrochromic
properties.[13]
By
incorporation of silole/germole or thiazole with viologen, the solid-
state phosphorescent dipyridinometalloles[14] and fluorescent
bipyridinium thiazolothiazole derivatives[15] have been reported.
Evidently, combination of main group elements and viologen is an
efficient strategy to develop viologen derivatives with broad
promising applications.
The synthesis of chalcogenoviologens were obtained by
treatment of 1[
10a]
with nBuLi, followed by addition of S
Cl
2
,[18]
2
SeCl [
19]
or TeCl
.
[20]
Thieno[2,3-c:5,4-c']bipyridine (2a) was
21]
2
4
[
obtained in 36% yield; selenopheno[2,3-c:5,4-c']bipyridine (2b)
and telluropheno[2,3-c:5,4-c']bipyridine (2c) were synthesized for
the first time with the moderate yield of 44% and 57%,
1
respectively. Low-field shift of H resonances was observed from
[
a]
G. Li, L. Xu, W. Zhang, K. Zhou, Y. Ding, Dr. F. Liu, Prof. Dr. G. He
Frontier Institute of Science and Technology, Xi’an Jiaotong
University, Xi'an, Shaanxi Province, 710054 (China)
E-mail: ganghe@mail.xjtu.edu.cn.
9.28 to 9.18 on the α-pyridine protons with the introduction of
heavier atoms. Chalcogen-bridged bipyridine 2 was converted
into the chalcogen-MV2+ 3 via reaction with methyl triflate (MeOTf),
which were obtained as a solid (3a: off-white, which was reported
previously,[22] 3b: dark green, 3c: dark red) in a high yield of ca.
80%. We hypothesized that the basicity of the nitrogen centers of
[
b]
Prof. Dr. X. He
School of Chemical Science and Engineering, Tongji University,
Shanghai, 200092 (China)
This work was supported by the Natural Science Foundation of China
(21704081), National 1000-Plan program, the Cyrus Chung Ying Tang
2
would increase along with the introduction of electron-donating
Foundation and start funding from Xi’an Jiaotong University. We also
thank Prof. Yu Fang, Prof. Eric Rivard and Prof. Fengting Lv for help
discussion. We are grateful to Prof. Yanzhen Zheng for his help with
X-ray crystallographic analysis. We thank Dr. Gang Chang and Yu
Wang at Instrument Analysis Center of Xi'an Jiaotong University for
their assistance with HRMS and Elemental analysis and
photoluminescence (PL) spectra.
[10b]
chalcogen contrary to 3,7-diazadibenzophosphole oxide,
so
that the N-benzylation reaction could carry out at room
temperature instead of microwave conditions. The strong
association of 2 with trifluoroacetic acid (TFAH) verified our
prediction (Figure S1−S3). Based on this, we quaternized the
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nitrogen atoms via benzylation in mild conditions, resulting in
dicationic salts 4a/4b/4c with moderate yields (70−81%). More
soluble 5a/5b/5c were synthesized by anion exchanges between
−1.10 V). The reduction potentials of 5a/5b/5c (Figure 1c and S11)
were significantly lower than those of both the starting materials
2a/2b/2c and chalcogen-MV2 (3a/3b/3c). Although 5 had 70-150
mV higher reduction potential than P-BnV, the first reduction
events were still 60−140 mV lower than the corresponding parent
analog, 1,1’-bis(benzyl)-4,4’-bipyridinium salt (BnV), and showed
desirable electron-accepting characters (Figure S13a). This was
attributed to a more rigid and conjugated structure due to the
introduction of heteroatoms. Clearly, both the ionization and
incorporation of the chalcogen decrease the energy of the LUMO-
levels, facilitating the reduction reaction. From the electron-
transfer constants of 5a (kET1 = 1.17  10-2 cm·s-1 and kET2 = 2.13
+
4a/4b/4c and excess MeOTf (Scheme 1).
The formations of these compounds were also confirmed by X-
ray crystallography (Figures 1, S6−S10). Single crystal structure
of 2c exhibits a planar backbone with two similar Te−C bond
lengths (i.e., 2.10 Å), the packing in 2c shows a strong Te−Te
interaction (i.e., 3.99 Å) as well as a layered motif with two
different face-to-face interactions (Figure 1b). Single crystals of
3b, 3c, 4c and 5c were obtained from their concentrated
acetonitrile solutions. The packing in 3b shows Se−O interactions,
however no Se-Se interactions were found. Strong interactions
from both Te−Te and Te−O bonds can be observed from the
packing in 3c, which showed a similar packing model to 2c. In the
structure of 5c, it is worth noting that the triflate anions are
disordered over two positions, in which the one is interacting
closely with the  system of bipyridine skeleton at a distance of
 10-2 cm·s ), 5b (kET1 = 1.55  10 cm·s and kET2 = 2.16  10
-1
-2
-1
-2
-
1
-2
-1
-2
cm·s ), and 5c (kET1 = 0.93  10 cm·s and kET2 = 1.67  10
-1
cm·s ), it is clear that the electron-transfer rates of 5b are faster
than 5a and 5c (Nicholson method, Table S7). Notably, in 5c,
there is another semi-reversible reductive wave at −2.66 V due to
the introduction of tellurophene rings; this is in a good agreement
with the reported dibenzo[b,d]tellurophene (DPhTe) and the
precursor 2c. This demonstrates the existence of extra redox
centers of Te-BnV2+ (Scheme S1).
3.09 Å, while the other triflate anion displays Te−O interaction at
a distance of 3.06 Å (Figure 1a). Compared to 3c, there are no
face-to-face interactions or close contacts between the skeletons
in 5c due to the presence of bulky benzyl groups. TGA analysis
indicated that chalcogen-bridged viologens are all stable under
The UV–vis spectra of chalcogenoviologens were studied to
evaluate their light absorption properties (Figures S12 and 1d),
and their photophysical data were summarized in Table S6. The
maximum absorptions of 2a/2b/2c in DMF were measured to be
200 °C (Figure S4 and S5).
345, 355 and 385 nm, respectively. The HOMO-LUMO energy
gaps were calculated to be 3.50 (2a) to 3.38 (2b) to 3.08 eV (2c).
The maximum absorptions for chalcogen-BnV2+ 5a/5b/5c in DMF
solution were observed at 395, 422 and 491 nm. There is a
pronounced redshift of optical absorption from 5a to 5b to 5c,
compared to precursors 2 and 3 (Figure S13b). This change in
absorption is accompanied by a reduced energy gap from 3.00
(
5a) to 2.78 (5b) to 2.33 eV (5c). These results can be explained
by the stronger electron-donating character of the chalcogen
atoms and decrease in aromaticity from S-viologen to Se-viologen
to Te-viologen.[16b] Additionally, this transformation is in good
agreement with the first singlet excitation energy obtained by
time-dependent DFT (TD-DFT) calculations (Figure S28−S42).
Raised HOMO levels are also observed from 5a to 5b to 5c
because the electronegativity of the heteroatom is inversely
proportional to the HOMO of the heterocycles. Compared to the
known P-BnV, chalcogenoviologens showed raised HOMO levels
and narrowed band gaps, while keeping electron-accepting
characters, which is destined to be extraordinary for applications.
We used the proof-of-concept ECD aiming to verify
electrochromic and photophysical behaviors of 5a/5b/5c.
Fluorine-doped tin oxide (FTO)-coated glasses were utilized as
the electrodes and 5a/5b/5c were used as active components
Figure 1. Crystal structure of (a) 5c and (b) 2c (thermal ellipsoids set at 50%
probability, H atoms are omitted for clarity). (c) Cyclic voltammograms of 5c (c
-
3
=
10 M) in DMF solution, [n-Bu
4
N][PF
6
] (0.1 M) as supporting electrolyte, vs.
+
2+
-4
Fc/Fc . (d) UV−vis spectra of chalcogen-BnV in DMF, c~10 M; extrapolated
g
optical band gaps (E ) are shown as insets.
(
Figure S15 and 2a). For 5c, with the applied potential of -1 V, the
absorption in the visible region increased and the color gradually
changed to dark blue (Figure 2c) due to the formation of radical
species (5c’). This process resembles chemical reduction of Zn.
Moreover, the decreased absorption in the visible region was
observed due to the accumulation of neutral species 5c” by
applying higher voltages of -2 V (Figure 2d); this process
corresponds to the chemical reduction of Na. Similarly,
electrochromism and identical absorption profiles of 5a and 5b
were observed (Figure S16−S20). Reversibility of color change
was displayed using etched FTO glass or diffusion of air into the
The electrochemical characteristics of chalcogenoviologens
were probed via cyclic voltammetry (CV, Figure S11), and the
results were summarized in Table S6. For example, 2c presents
redox potential Ered1,1/2= −2.11 V (ELUMO = −2.69 eV) and Ered2,1/2
=
−2.56 V. All dicationic species show two reversible one-electron
reductions due to the presence of bipyridine skeleton. The
reduction potentials of new chalcogen-BnV2+ were measured for
5a (Ered1,1/2= −0.60 V and Ered2,1/2= −1.05 V), 5b (Ered1,1/2= −0.63 V
and Ered2,1/2= −1.05 V), and 5c (Ered1,1/2= −0.68 V and Ered2,1/2
=
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COMMUNICATION
Figure 2. (a) Solution-based ECD with 5c. (b) Gel-based ECD with 5c. (c) Spectroelectrochemistry of 5c for first reduction and chemical reduction with Zn is shown
as the inset. (d) Spectroelectrochemistry of 5c for second reduction and chemical reduction with Na is shown as the inset. (e) EPR spectrum and spin densities of
radical species of chalcogen-BnV2+ (5a’, 5b’ and 5c’).
cell. The CV stability test of 5c indicated good electronic stability
even after 400-scan cycles, and the slight transmittance changes
for ECD of 5c at 661 and 730 nm showed good electrochromic
switching (Figure S17). In gel-based ECD, lower voltages were
needed due to the presence of a thinner material layer with higher
electrical conductivities (Figure 2b).
LUMO levels (Figure S25). Therefore, the water-soluble E-BnV2+
was used both as a photosensitizer and as an electron mediator
(Figure 3a). In contrast to reported systems based on MV or P-
RV, this system does not require an additional photosensitizer.
EDTA was used as a sacrificial electron donor and colloidal
platinum particles stabilized by polyvinylpyrrolidone (colloids of
PVP-Pt) as a catalyst. The proposed mechanism was shown in
Interestingly, chalcogen-BnV2+ showed weak fluorescence in
2
+
DMF solutions (Figure S21, lifetime () for 5a: 8.93 ns; 5b: 5.39
Figure S26. Firstly, the E-BnV was excited to excited states (E-
2
+
2+
ns and 5c: 5.18 ns). The quantum yields (
1
5
F
) were all less than
%, due to the strong heavy atom effect. When radical species
c’ were exposed to air, emission intensity gradually increased
BnV *) under visible light, where E-BnV * accepted one electron
+
•
+
from EDTA and changed to radical cation E-BnV . Then H was
reduced to hydrogen on the surface of PVP-Pt and E-BnV+• was
oxidized to E-BnV2+ to finish the catalytic cycle.[24]
and the maximum emission wavelength was redshifted until
reaching the wavelength of the original dication 5c. This process
was also monitored by UV-vis spectroscopy and H NMR (Figure
Taking 5b as an example, a mixture of 5b (5 mmol), EDTA (50
mmol), colloidal PVP-Pt (4 mg) and 1 mL distilled water were
sealed in a Pyrex bottle. After bubbling with argon for 30 minutes,
the bottle was exposed to the xenon lamp with a filter ( > 400 nm)
at 100 mW. The hydrogen quantity was calculated according to
1
S23). The same trends were also observed in 5a and 5b,
suggesting most of the radical species converted to original
dication over time in air (Figure S22). Simultaneously, radical
characters of radical species were evaluated via electron
paramagnetic resonance (EPR) spectroscopy. The resulting
signal displayed a considerable amount of hyperfine coupling
the H
generation are shown in Figure 3b. Hydrogen can continuously
generate for more than ten hours (total about 13.03 μmol of H
2
normalized curves (Figure S24). The plots of the hydrogen
2
,
-
1
-1
(
Figure 2e), suggesting delocalization of the radical over a large
hydrogen generation rate: 0.31 mmol•h •g ), and apparent
−
3
portion of the molecular scaffold. 5a’, 5b’ and 5c’ had expected
g-factor values of 2.0031, 2.0042 and 2.0049, respectively,
supporting the organic nature of the radicals. Typically, the spin
density on the chalcogen element is gradually increased from 5a’
to 5b’ to 5c’, which was confirmed through DFT calculations
quantum yield (AQY) was 1.71 × 10 , which is higher than the
previously reported results (Table S8). For Te-BnV2+ (5c),
hydrogen was generated for more than twenty-two hours with the
total amount of ca. 4.73 μmol (the first twelve hours was listed in
Figure 3b, hydrogen generation rate: 0.056 mmol•h •g ), and
lower AQY of 3.39 × 10−4 compared to 5b. For S-BnV2+ (5a), there
was only 0.12 μmol hydrogen generated after thirty hours, while
-1
-1
(
insets of Figure 2e). The SOMO of the radical cations are similar
to the LUMO of the dicationic species, and the radicals are
delocalized and stabilized over the whole -system of the scaffold.
Considering of the strong visible light absorption of Se/Te-BnV
2
+
there was no hydrogen evolution in the case of BnV .
To understand this phenomenon, the formation speeds of
radical species were investigated in the absence of PVP-Pt. As
shown in Figure 3c, the radical species, 5b’ were generated much
faster than 5c’ (1 min vs. 2 h), while 5a’ and BnV’ did not form
under this condition. This is attributed to the more negative HOMO
level of 5b compared to 5c. In HOMO level, in case of 5b the
electrons are distributed over the entire skeleton in contrast to the
solely bypiridine part (in particular, Te atom) in case of 5c.
Furthermore, 5b showed faster electron-transfer rates than 5c.
This may lead to capture of more photon flux from the solar
(
  400 nm) in combination with their facile radical formation
properties, visible-light-driven hydrogen evolution system was
designed to uncover the new application of viologens. Previous
2
+
[5b]
investigations show that Ru(bpy)
3
or polythiophene
2
+
derivatives (PT) can be used as photosensitizers and MV
served as an electron relay for photoreduction of water.[23] In our
tests, Se-BnV2+ (5b) and Te-BnV2+ (5c) possessed similar
properties to the photosensitizer. Meanwhile, 5a’/5b’/5c’ have
+
sufficient driving forces for H reduction based on their negative
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COMMUNICATION
spectrum to further improve hydrogen evolution efficiency in 5b.
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In conclusion, a series of novel chalcogenoviologens were
successfully synthesized by combining the viologen with
chalcogenophenes. By introducing different chalcogen atoms (S,
Se and Te), the HOMO−LUMO band gaps narrowed gradually,
along with increased HOMO levels, which caused the red-shift of
absorptions to the visible range. The multiple redox peaks were
found (especially for 5c), which were promising in many electron-
related fields. The proof-of-concept solution-based/gel-based
ECD were fabricated successfully and their electrochemical
performance was studied in situ. These results lay a solid
foundation for flexible low-voltage electrochromic devices and
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Keywords: viologen • chalcogenoviologens • electrochromism •
visible region • hydrogen evolution
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Angewandte Chemie International Edition
10.1002/anie.201711761
COMMUNICATION
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COMMUNICATION
Narrow band gap
Guoping Li, Letian Xu,
chalcogenoviologens were
developed by a combination
of the viologen with
chalcogenophenes, which
showed the fascinating
application in
electrochromism and
visible-light-driven hydrogen
evolution.
Weidong Zhang, Kun Zhou,
Yousong Ding, Fenglin Liu,
Xiaoming He, and Gang He*
Page No. – Page No.
Narrow Band Gap
Chalcogenoviologens for
Electrochromism and
Visible-Light-Driven
Hydrogen Evolution
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