Selective visible-light-driven oxygen reduction to hydrogen peroxide using BODIPY photosensitizers

Article abstract of DOI:10.1039/c7cc09383g

Selective visible-light-driven O₂ reduction to H₂O₂ was realized using BODIPY photosensitizers (PS) in the presence of ferrocene (Fc) as the reductant and acetic acid as the proton source. Mechanistic studies suggested that O₂ could be activated by ³PS? through an energy transfer pathway to give singlet oxygen (¹O₂) in the absence of Fc. However, with Fc, ³PS? was first reductively quenched to PS^-, which was able to reduce O₂ to the superoxide radical form in a subsequent electron transfer step.

Full text of DOI:10.1039/c7cc09383g

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Received 00th January 20xx,
Selective visible-light-driven oxygen reduction to hydrogen
peroxide using BODIPY photosensitizers
Xiaojun Guo,a Xialiang Li,a Xiao-Chen Liu,c Ping Li,a Zhen Yao,b Jianfeng Li,b Wei Zhang,a Jian-Ping
Zhang,c Dong Xue*a and Rui Cao*ac
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: Selective visible-light-driven O2 reduction to H2O2 was (3) the high yield of long-lived triplet excited state; and (4) the
realized by using BODIPY photosensitizers (PS) in the presence of easily tuned photophysical feature by structural modification.
ferrocene (Fc) as the reductant and acetic acid as the proton Importantly, in the reactions, organic substrates were oxidized
source. Mechanistic studies suggested that O2 could be activated by O2, and H2O2 was identified as the reduction product. On
by 3PS* through an energy transfer pathway to give singlet oxygen the basis of these results, we are interested in examining the
(1O2) in the absence of Fc. However, with Fc, 3PS* was first possibility of light-driven ORR using BODIPY. Herein we report
−
reductively quenched to PSꢀ , which was able to reduce O2 to the selective visible-light-driven O2 reduction to H2O2 using various
superoxide radical form in a subsequent electron transfer step.
BODIPY photosensitizers in the presence of Fc as the reductant
and acetic acid as the proton source. Mechanistic studies were
performed to better understand the reaction pathways leading
to O2 activation. This work is important to show an example of
selective light-driven O2 reduction to H2O2 and also to shed
light on photocatalytic oxidation of organic substrates with O2
using BODIPY photosensitizers.
The oxygen reduction reaction (ORR) is significant in both
biology and energy conversion systems.1-7 O2 can be reduced
either by two electrons to H2O2 or by four electrons to H2O.8
Although the 4e− pathway is more energetically favored,1 the
2e− reduction of O2 to H2O2 has been attracting increasing
attention.9-11 First, H2O2 is an alternative fuel.12 H2O2 fuel cell
has a 1.09 V output potential, a value that is comparable to the
1.23 V output potential of a H2 fuel cell. Compared with H2,
H2O2 is much easier for storage and transportation. Second,
H2O2 has been extensively used as an oxidant in industry.13 The
standard redox potential of H2O2 to H2O is 1.78 V, a value even
larger than that of O2 to H2O at 1.23 V. Moreover, the product
from the oxidation with H2O2 is water.
A variety of BODIPY derivatives were examined (Scheme
1). Typically, O2-saturated acetonitrile containing BODIPY (0.5
mM), Fc (50 mM), and acetic acid (500 mM) was irradiated
with a xenon lamp (280 W, λ = 400-780 nm) in a water bath at
25 °C. The formed ferrocenium cation (Fc+) was identified by
its characteristic UV-vis absorption band at 620 nm (Fig. S1).
The amount of produced H2O2 was obtained by the iodometric
titration method (Fig. S2). Control experiments showed that
BODIPY, light, Fc, and acetic acid were all essential for catalytic
O2 reduction (Table S1). The plots of formed Fc+ and H2O2
versus time are depicted in Fig. 1a. From the slopes of these
lines, the formation rates of Fc+ and H2O2 was determined to
be 0.35 and 0.17 μmol min−1, respectively. The quantity of
formed Fc+ was nearly two times the quantity of formed H2O2,
a ratio that is consistent with the reaction stoichiometry
shown in Eqn (1). This result indicates the selective 2e−
reduction of O2 in this light-driven system with BODIPY. This
selectivity was further confirmed by calculating the amount of
O2 consumed and the amount of H2O2 generated (see
Electronic Supplementary Information ESI for details).
Although transition-metal-based complexes have been
examined as ORR catalysts,14-28 few were shown to be selective
for the 2e− reduction of O2 to H2O2. In addition, transition
metal complexes can typically catalyze the disproportionation
of H2O2 to O2 and H2O. As a result, the stability of produced
H2O2 in the presence of metal catalysts is an issue. Recently,
Fukuzumi and co-workers reported light-driven 2e− ORR using
various organic electron donor-acceptor molecules.29,30 These
works are important to show the generation of H2O2 from a
metal-free and light-driven ORR system. Photocatalytic H2O2
production from H2O and O2 by metal-based31-34 and metal-
free carbon nitride catalysts35-37 have also received attention.
Boron-dipyrromethene (BODIPY) is a class of metal-free
dyes, which have been used for labelling and sensing.38,39
Recently, BODIPY has been used as photosensitizers for light-
driven oxidation of organic substrates with O2.40-43 BODIPY has
the following properties: (1) the strong absorption in the
visible light range; (2) the high stability under photoirradiation;
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3b). This result suggests that O2 − was produced in the solution
of PS- and Fc upon irradiation. We proposed that excited PS-
was first reductively quenched by Fc and the resulted PS-1ꢀ
ꢀ
Scheme 1 Molecular structures of used BODIPY derivatives.
1
1
−
−
ꢀ
Eqn (1) transferred one electron to O2 to give PS-1 and O2 .
The ORR turnover numbers (TONs) with different BODIPY
derivatives were determined (Table S2). All BODIPY derivatives
examined are active by functioning as photosensitizers for the
visible-light-driven ORR. It is worth noting that the maximum
TON expected under experimental conditions is 50.0 due to
the solubility of Fc in acetonitrile. Several conclusions can be
made. (1) PS-
derivatives examined. (2) The increase of irradiation time, acid Fig. 2 (a) The emission spectra of PS-
concentration or light intensity, leads to the increase of TON. at 25 °C with increasing amounts of Fc (0-10 mM) under a 446
(3) BODIPY is less stable in entries using strong acids, such as nm excitation. (b) The Stern-Volmer plot of PS- vs [Fc].
hydrochloric and perchloric acids. In addition, for PS- , the
1 is the most efficient one among BODIPY
1
(0.5 mM) in acetonitrile
1
1
apparent quantum yield for H2O2 production was 0.86% under
the irradiation of λ = 520 nm (see ESI for details).
The stability of BODIPY photosensitizer was evaluated. As
shown in Fig. 1b, the UV-vis spectra of the reaction mixture
before and after ORR under 4-h illumination display negligible
difference. In addition, BODIPY can be recovered and reused
for many cycles, further showing its stability by functioning as
photosensitizer in the visible-light-driven ORR.
Fig. 1 (a) Plots of formed H2O2 and Fc+ versus time in an O2- Fig. 3 EPR studies. (a) PS-
1
(0.1 mM) and DMPO (20 mM); (b)
(0.1
(0.1 mM), Fc (1.5 mM) and
saturated acetonitrile with PS-
1 (0.5 mM), Fc (50 mM), and PS-1 (0.1 mM), Fc (1.5 mM) and DMPO (20 mM); (c) PS-1
acetic acid (500 mM) under irradiation (λ = 400-780 nm). (b) mM) and TEMP (0.12 M); (d) PS-
1
UV-vis spectra of PS-
1
in acetonitrile before and after ORR.
TEMP (0.12 M). All measurements were done in O2-saturated
acetonitrile at 90 K under irradiation by a xenon lamp (280 W,
In order to better understand the mechanism, we studied 400-780 nm) for 600 s.
the reductive quenching of PS- with Fc in acetonitrile under
N2. We found that PS- showed strong luminescence with an
emission peak at 569 nm in the absence of Fc at room an O2-saturated acetonitrile displayed a strong signal assigned
1
1
In a separate experiment, the mixture of PS-1 and TEMP in
temperature (Fig. 2a). However, the emission of PS-1
gradually to the TEMP-1O2 adduct (Fig. 3c). However, in the presence of
decreases with the addition of Fc. Importantly, the quenching Fc, the TEMP-1O2 signal significantly decreased (Fig. 3d). This
constant of K = 168 M−1 could be determined (Fig. 2b), result suggests that in the absence of Fc, energy transfer from
indicating the presence of interaction between the excited excited PS-
state of PS- and Fc. The reductive quenching of excited PS-
formation of 1O2. However, in the presence of Fc, excited PS-
by Fc is therefore possible in the light-driven ORR. could be reductively quenched. Subsequent electron transfer
1 to O2 is the dominant process, leading to the
1
1
1
Electron paramagnetic resonance (EPR) spectroscopy was to O2 could happen to give O2 −. The latter process has been
used to probe intermediates formed during ORR. Complexes identified in the above DMPO-based experiments. Importantly,
5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2,2,6,6- Fukuzumi and co-workers reported that electron transfer from
ꢀ
−
1
Fc to O2 is endergonic (∆G0 = 0.26 eV).44 Therefore, it is not
ꢀ
tetramethylpiperidine (TEMP) were used as probes for O2
and 1O2, respectively. The mixture of PS- and DMPO in an O2- likely that 1O2 is first formed by the energy transfer from
saturated acetonitrile displayed no signal (Fig. 3a), indicating excited PS- to O2 and is then consumed by Fc. It is worth
that the oxidative quenching of excited PS- by O2 to produce noting that even in the presence of Fc, the energy transfer
PS-1ꢀ and O2 was not likely under experimental conditions. from excited PS-
to O2 still exists because of the detection of
However, in the presence of Fc, the above solution displayed a the TEMP-1O2 EPR signal. This competition between electron
et
1
1
1
+
−
ꢀ
1
signal assigned to the DMPO-O2 adduct upon irradiation (Fig. and energy transfer has also been reported previously.45-47
−
ꢀ
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Nanosecond
time-resolved
transient
difference and triplet excited states of PS-
1
is thermodynamically possible
absorption spectra are used to detect 3PS-
1* (Fig. 4a). With (see ESI for details). Considering that the lifetime of a triplet
pulsed excitation at 532 nm, bleaching at 520 nm is observed. excited state is much longer than that of a singlet excited
Transient absorption at 420 nm and 550-700 nm are state, we believe that the electron transfer pathway is from Fc
3
−
characteristics for BODIPY.48 The lifetime of PS-
1
* is 13.1 μs to 3PS-
1
*. Subsequent electron transfer from PS-1ꢀ to O2
3
(Fig. 4b). Electron transfer from Fc to PS-
and O2 −. The resulted O2 − can be further
ꢀ
ꢀ
1
* was monitored by happens to give PS-
1
the decay at 520 nm at various [Fc] (Fig. 5a). The rate obeyed reduced by one electron to give H2O2 or can disproportionate
pseudo-first-order kinetics, and the rate constant increased in the presence of acids to give H2O2 and O2. It is extremely
linearly with [Fc]. The second-order rate constant of electron difficult, if it is possible, to study the reaction details of the
3
−
ꢀ
transfer from Fc to PS-
1
* is 1.05 × 1010 M−1 s−1 (Fig. 5b). The conversion from O2 to H2O2 because of the difficulty to keep
3
energy transfer from PS-
* to O2 was also monitored by the O2 − in a stable state.50,51
ꢀ
1
decay at 520 nm at various [O2] (Fig. 5c). The rate obeyed
pseudo-first-order kinetics, and the rate constant increased in
proportional to [O2]. The second-order rate constant of energy
1
transfer from 3PS- * to O2 is 9.1 × 108 M−1 s−1 (Fig. 5d).
Fig. 4 (a) Nanosecond time-resolved transient difference
absorption spectra of PS- (15 μM) after pulsed laser
excitation (λex = 532 nm). (b) Decay of PS-
1
1
at 520 nm at 23 °C.
Scheme 2 Proposed mechanism for the selective O2 reduction
to H2O2 with PS-
1.
In conclusion, we report the selective visible-light-driven
O2 reduction to H2O2 in a metal-free system. A variety of
BODIPY derivatives were used as the photosensitizer. EPR
measurements manifested that O2 could be activated to 1O2 by
excited PS-
1
through an energy transfer pathway. However, in
was first reductively quenched.
the presence of Fc, exited PS-
1
The resulted PSꢀ− was able to reduce O2 to give O2 −, which was
able to convert to H2O2 through further one-electron reduction
or disproportionation. This work is an example showing the
selective generation of H2O2 from ORR in a metal-free and
light-driven system. Our results are also valuable to shed light
on photocatalytic oxidation of organic substrates with O2 using
BODIPY photosensitizers.
ꢀ
3
Fig. 5 (a, c) Normalized decay time profile of PS-
1* (produced
We are grateful for the support from the “Thousand
by laser excitation of 15 μM PS-1 at λex = 532 nm) at 520 nm at Talents Program” of China, the National Natural Science
various concentrations of Fc and O2, respectively. (b, d) Plot of Foundation of China under Grants 21101170, 21573139 and
pseudo-first-order rate constant (kobs) for electron transfer 21372148, the Fundamental Research Funds for the Central
from Fc to 3PS-
1
* against [Fc] and for energy transfer from 3PS- Universities (GK201704006), and the Research Funds of
1* to O2 against [O2].
Shaanxi Normal University.
On the basis of these results, we proposed a mechanism
Notes and references
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