Two-electron oxidation of water to form hydrogen peroxide sensitized by di(hydroxo)porphyrin GeIV complex under visible-light irradiation

Article abstract of DOI:10.1016/j.jphotochem.2015.06.011

Visible-light-induced two-electron oxidation of water to form hydrogen peroxide (H₂O₂) was investigated in a photovoltaic cell (PVC) composed of an ITO electrode coated by tetrakis-(4-carboxylphenyl) porphyrinatogermanium(IV) (1)-adsorbed titanium oxide (TiO₂) particles as an anode (1/TiO₂/ITO) and a Pt electrode as a cathode in an aqueous solution containing an electrolyte. The short-circuit photocurrent was observed under irradiation of the 1/TiO₂/ITO anode with the light of 550 nm under aerated conditions. Simultaneously, H₂O₂ was detected only in the anode chamber. After irradiation for 6 h, the turnover number (TON) and the faradaic efficiency for the formation of H₂O₂ were estimated to be 9.0 and 92%, respectively. The action spectrum for the formation of H₂O₂ was well consistent with the absorption spectrum of 1. These findings indicate that H₂O₂ is produced by the excitation of 1 on the anode through a quantitative process with the obtained faradaic current. It was concluded that water acted as an electron donor on the anode, while molecular oxygen acted as an electron acceptor under aerated conditions, and several different types of ions also acted as an electron acceptor under argon atmosphere on the cathode. It is suggested that the photooxidation of water to form H₂O₂ was initiated by a photoinduced electron transfer from the excited singlet state of 1 to TiO₂ to generate the porphyrin radical cation (1⁺), which underwent a proton dissociation from the hydroxo axial ligand to give a Ge-oxyl complex as a key intermediate for a two-electron activation of water to form hydrogen peroxide.

Full text of DOI:10.1016/j.jphotochem.2015.06.011

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JPC 9935 No. of Pages 6
Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Two-electron oxidation of water to form hydrogen peroxide sensitized
by di(hydroxo)porphyrin GeIV complex under visible-light irradiation
a
a
a
Tsutomu Shiragamia, , Haruki Nakamura , Jin Matsumoto , Masahide Yasuda ,
*
Yoshiyuki Suzurib, Hiroshi Tachibanab,c, Haruo Inoueb,c
Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki, Gakuen-Kibanadai, Miyazaki 889-2192, Japan
a
b
Department of Applied Chemistry, Graduate School of Urban Environmental Science, Tokyo Metropolitan University, Minami-Ohsawa, Hachioji, Tokyo 192-
0397, Japan
c
Center for Artificial Photosynthesis, Tokyo Metropolitan University, Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Visible-light-induced two-electron oxidation of water to form hydrogen peroxide (H2O2) was
investigated in a photovoltaic cell (PVC) composed of an ITO electrode coated by tetrakis-(4-
carboxylphenyl) porphyrinatogermanium(IV) (1)-adsorbed titanium oxide (TiO2) particles as an anode
(1/TiO2/ITO) and a Pt electrode as a cathode in an aqueous solution containing an electrolyte. The short-
circuit photocurrent was observed under irradiation of the 1/TiO2/ITO anode with the light of 550 nm
under aerated conditions. Simultaneously, H2O2 was detected only in the anode chamber. After
irradiation for 6 h, the turnover number (TON) and the faradaic efficiency for the formation of H2O2 were
estimated to be 9.0 and 92%, respectively. The action spectrum for the formation of H2O2 was well
consistent with the absorption spectrum of 1. These findings indicate that H2O2 is produced by the
excitation of 1 on the anode through a quantitative process with the obtained faradaic current. It was
concluded that water acted as an electron donor on the anode, while molecular oxygen acted as an
electron acceptor under aerated conditions, and several different types of ions also acted as an electron
acceptor under argon atmosphere on the cathode. It is suggested that the photooxidation of water to form
H2O2 was initiated by a photoinduced electron transfer from the excited singlet state of 1 to TiO2 to
generate the porphyrin radical cation (1+ꢀ), which underwent a proton dissociation from the hydroxo
axial ligand to give a Ge-oxyl complex as a key intermediate for a two-electron activation of water to form
hydrogen peroxide.
Received 14 April 2015
Received in revised form 8 June 2015
Accepted 11 June 2015
Available online xxx
Keywords:
Artificial photosynthesis
Water oxidation
Germanium porphyrin
Visible-light
ã 2015 Elsevier B.V. All rights reserved.
1. Introduction
transfer with a photochemical process. It is, thus, difficult to
achieve these processes under such conditions due to the short
It is most desirable to develop water oxidation systems for the
construction of artificial photosynthesis [1–3]. There are many
reports on the electrochemical oxidation of water to induce oxygen
evolution through four-electron transfer processes using metal
complexes as a catalyst [4–10]. However, there are few studies on
the photochemical oxidation of water through the four-electron
transfer [11]. In the photochemical process, for example, a period
of time of the order of seconds is required to achieve the stepwise
four-electron oxidation of a metal complex, because the photon-
flux is very rarefied in natural sunlight [12]. Therefore, the unstable
highly oxidized state of the metal complex must survive during the
second-order timescale in order to induce the four-electron
lifetime of the oxidized species, even in the one-electron-oxidized
case. This problem is referred to as the “photon-flux-density
problem” [12].
On the other hand, we have focused on the two-electron
oxidation of water using metal complexes as an alternative
pathway for water activation and found that the oxygenation of
alkenes to form epoxides can be sensitized by metalloporphyrins
such as di(hydroxo) porphyrininato SbV [13], SnIV complexs
[14–16], or carbonyl-coordinated ruthenium porphyrin complex
[17–19] in the presence of water as both an electron and oxygen
atom donor. In this system, a key intermediate capable of
activating the water molecule is thought to be a metal-oxo
complex formed by photoinduced electron transfer (PET) to an
electron acceptor and the subsequent proton dissociation of the
axial hydroxo ligand. We elucidated that these reactions proceed
with a two-electron oxidation process using the energy of one
* Corresponding author. Fax: +81 985 58 7315.
E-mail address: t0g109u@cc.miyazaki-u.ac.jp (T. Shiragami).
http://dx.doi.org/10.1016/j.jphotochem.2015.06.011
1010-6030/ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Shiragami, et al., Two-electron oxidation of water to form hydrogen peroxide sensitized by di(hydroxo)
porphyrin GeIV complex under visible-light irradiation, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphoto-
chem.2015.06.011

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photon, so that they are free from the “photon-flux-density
problem” [20]. Accordingly, it can be expected that it would be
possible to achieve the two-electron oxidation of water to form
hydrogen peroxide (H2O2), if a photochemical water oxidation
system were properly fabricated using a metal complex.
experiment was measured by a microammeter (8340A, ADC
Corporation, Japan). Light of 550 nm (4.4 mW cmÀ2) was isolated
from the output of a 500-W Xe lamp using a monochromator
(L12194, Hamamatsu Photonics K. K, Japan).
Recently, we have found that the two-electron oxidation of
methanol to formaldehyde can be sensitized by a di(hydroxo)
porphyrin GeIV complex, suggesting that the oxidation proceeds
2.2. Photovoltaic cell
Transparent ITO glass electrodes (20 mm  16 mm  3.2 mm)
coated with TiO2 particles (0.8 cm2 of the coating area) were
purchased from Dyesol Industries Pty. Ltd. (Australia). The
preparation of 1/TiO2/ITO was performed by a dipping TiO2/ITO
for 2 h in a methanol solution of 1. The amount of adsorbed 1 was
estimated to be 7.0 Â10À8 mol cmÀ2. The area of 1-modified TiO2
electrode used for the photoreaction was 0.8 cm2. The PVC was
composed of 1/TiO2/ITO as an anode, ÀPt wire as a cathode, and
aqueous solution (12 cm3) of Et4N+BF4 (0.1 M) as an electrolyte.
The anode and cathode chambers were separated by a glass filter
(Fig. 1).
through
a mechanism involving hydrogen abstraction from
methanol with a Ge-oxyl complex formed by PET to an electron
acceptor, with the subsequent proton dissociation of the axial
hydroxo ligand [21]. We have also succeeded in the generation of
the Ge-oxyl complex in a photovaltaic cell (PVC) composed of an
ITO electrode coated by tetrakis-(4-carboxylphenyl) porphyrina-
togermanium(IV) (1)-adsorbed titanium oxide (TiO2) particles
(1/TiO2/ITO) as an anode and a Pt electrode as a cathode. In this PVC
system, it was elucidated that the short-circuit photocurrent was
obtained by PET to TiO2 from the excited singlet state of 1 to form a
Ge-oxyl complex, with subsequent hydrogen abstraction from
aliphatic alcohols by the Ge-oxyl complex [22]. These results
indicate that the photooxidation of water with a Ge-oxyl complex
can be expected to be induced by this PVC system. Here, we report
the formation of hydrogen peroxide (H2O2) as a two-electron
oxidation product from water using the 1/TiO2/ITO electrode in the
presence of various electron acceptors (Scheme 1).
2.3. Photoreaction
The short-circuit photocurrent was normally measured under
irradiation of the anode with monochromic light (550 nm) under
air or argon atmospheres in the presence of electron acceptors in
the cathode chamber at 20 ꢁC. The identification and quantitative
analysis of H2O2 was carried out for the irradiated solution in the
anode chamber by absorption spectrophotometry using a Ti-oxo
porphyrin complexing reagent [23].
2. Experimental
2.1. Materials and instruments
3. Results and discussion
H2O2 aqueous solution,
(III) nitrate hydrate (Fe(NO3)3
(Ce(NH4)2(NO3)6) tetraethylammonium
L
-ascorbic acid, potassium iodate, iron
9H2O), ceric ammonium nitrate
tetrafluoroborate
Á
3.1. Formation of H2O2 under aerated conditions
(Et4N+BF4À), germanium tetrachloride (GeCl4) and organic solvents
were purchased from Wako Pure Chemical Industries as guaran-
teed reagent grade (GR) and used without further purification. Oxo
[5,10,15,20-tetra(4-pyridyl) porphyrinato]titanium(IV) (Ti-oxo
porphyrin) was purchased from Tokyo Kasei as GR reagent for
the quantitative analysis of the produced hydrogen peroxide.
Potassium hexachloroplatinate (IV) (K2[PtCl6]) and tetraphenyl-
porphyrin (H2tpp) were also purchased from Aldrich as GR reagent.
When the 1/TiO2/ITO anode was irradiated with 550-nm light in
aqueous electrolyte solution under aerated conditions in the PVC
system, the short-circuit current was observed to respond to on-off
irradiation switching (Fig. 2). The observed photocurrent was
nearly constant, even under continuous irradiation for 1 h (Fig. 3).
The short-circuit current density (Jsc) and the incident photon-to-
current conversion efficiency (IPCE) were estimated to be 5.5 mA
cmÀ2 and 0.28%, respectively. After irradiation for 1 h, H2O2
(75 nmol) was detected only in the anode chamber, not in the
cathode chamber, as shown in run 1 of Table 1. On the other hand,
when the aqueous electrolyte solution was bubbled with argon gas,
the Jsc was much smaller (IPCE = 0.05%) than that under aerated
conditions. Moreover, only trace amounts of H2O2 (5 nmol) were
detected in the anode chamber, whereas no H2O2 was produced in
the cathode chamber (run 2). Although a larger photocurrent
1
was synthesized according to the literature [22]. UV–vis
absorption of the solutions was obtained on a JASCO V-500
spectrophotometer. The short-circuit current of the PVC
Scheme 1. 1-sensitized oxidation of water to form H2O2 with various electron
Fig. 1. Photovoltaic cell (PVC) with 1/TiO2/ITO and Pt wire as an anode and cathode,
acceptors (A).
respectively.
Please cite this article in press as: T. Shiragami, et al., Two-electron oxidation of water to form hydrogen peroxide sensitized by di(hydroxo)
porphyrin GeIV complex under visible-light irradiation, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphoto-
chem.2015.06.011

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3
Off
10
8
4
0
0
Fig. 4. Action spectrum for the formation of H2O2 (^) along with absorption
spectrum of 1 (——).
100
200
300
400
On
Time/sec
Fig. 2. Short-circuit current observed under visible light irradiation (550 nm) and in
the dark in aqueous electrolyte solution. Irradiation was started at the mark “ON”
and stopped at the mark “OFF”.
10
8
4
Fig. 5. Time-course plot for the formation of H2O2 in 1-sensitized PVC system.
increased nearly linearly vs. irradiation time, without an induction
period. The turnover number (TON) for the formation of H2O2 was
estimated to be 9.0 after 6 h of visible light irradiation. Further-
more, the faradaic efficiency (FE) for the production of H2O2 was
estimated to be 92 % (run 1 of Table 1). These results indicate that
the production of H2O2 should proceed with both a catalytic cycle
of 1 and a nearly quantitative process for the obtained faradaic
currents. After irradiation for 6 h, however, 1 was decomposed and
desorbed from TiO2. The decomposition of 1 was not occurred with
backward reactions of formed H2O2, because the formation of H2O2
was observed even in the presence of small amount of H2O2
0
12
24
36
48
60
0
Time/min
Fig. 3. Time-course plots of the short-circuit currents generated by continuous
visible light irradiation (550 nm) under air (—) and argon (——) atmospheres.
Table 1
Production of H2O2 with various electron acceptors in 1-sensitized PVC system.a
Run
Acceptor
H2O2 (nmol)
FE (%)b
IPCE (%)
(0.5 mmol) using new 1/TiO2/ITO anode under the irradiation.
c
1
2
3
4
5
6
O2
75
5
51
55
48
54
92
30
88
92
91
99
0.28
0.05
0.20
0.21
0.15
0.19
d
It is expected that the production of H2O2 strongly depends on
the change of pH in the aqueous electrolyte solution, because two
protons are released in the production of one H2O2 from two water
molecules in an overall oxidation (Scheme 1). In fact, it was
observed that the production of H2O2 in the PVC system tended to
decrease with the increase of proton concentration (Fig. 6). This
result indicates that the production of H2O2 can be induced by the
O2
Ce4+
Fe3+
À
IO3
[PtCl6]À
a
Under argon atmosphere.
Faradaic efficiency.
Under air atmosphere.
b
c
d
Low concentration of O2 in aqueous electrolyte solution under argon
atmosphere.
(Jsc = 130 m L-ascorbic acid as an
A cmÀ2) was obtained using
electron donor in the anode chamber, H2O2 was not detected at
all in the cathode chamber under argon atmosphere. These results
strongly suggest that water acted as an electron donor on the
anode and molecular oxygen acted as an electron acceptor as a
consequence of the four-electron reduction to water on the Pt
cathode. Fig. 4 shows the action spectrum for the production of
H2O2 under aerated conditions. The action spectrum is well
consistent with the absorption spectrum of 1; this consistency
strongly indicates that the observed production of H2O2 was
induced by the photochemical sensitization of 1.
Fig. 5 shows the time-course plot for the production of H2O2
from water in the PVC system. H2O2 was produced in amounts that
Fig. 6. pH effect upon the formation of H2O2 in 1-sensitized PVC system.
Please cite this article in press as: T. Shiragami, et al., Two-electron oxidation of water to form hydrogen peroxide sensitized by di(hydroxo)
porphyrin GeIV complex under visible-light irradiation, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphoto-
chem.2015.06.011

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oxidation of water. Although an enhancement of H2O2 production
was expected for irradiation under basic conditions, the desorption
of 1 from TiO2 was accelerated under at pH > 8, resulting in the
complete absence of formation of H2O2 under basic conditions. The
observed desorption may be rationalized by the repulsive force
between the carboxylate anion group of 1 and the anionic TiO2
surface under basic conditions.
3.2. Formation of H2O2 in the presence of various electron acceptors
under argon atmosphere
The production measurements of H2O2 from water were
performed in the presence of various electron acceptors except
molecular oxygen under argon atmosphere in the PVC system (runs
3–6 of Table 1). When Ce4+, Fe3+, IO3À and [PtCl6]À, which are well
known as typical effective electron acceptors, were used in the
cathode chamber, H2O2 was produced in the anode chamber for all
cases. Their IPCE and FE values were nearly the same as those for the
case of molecular oxygen as an electron acceptor. In particular, FE
reached99% in the caseof [PtCl6]À. However, the catalytic reactionof
1 at the anode can be rate-determining in the PVC system, since the
production of H2O2 did not depend on the type of electron acceptor
in general. These results again strongly suggest that the oxygen
source of H2O2 is water molecules themselves, because H2O2 was
obtained even in the absence of molecular oxygen.
3.3. DFT calculation of the one-electron oxidized species of 1
The generation of the photocurrent induced by the excitation of
1 adsorbed on the TiO2 nanoparticles clearly indicates that there is
an electron injection from the excited state of 1 into the conduction
band of TiO2 to form the one-electron oxidized form of 1. As we had
already reported,
a Ge-oxyl complex (one-electron-oxidized
radical form) was presumed to be the key intermediate in the
photochemical C-H activation sensitized by 1 [21] and the
photochemical fuel cell device by 1 [22]. In the current PVC
system of forming H2O2, the one-electron-oxidized radical form,
Ge-oxyl complex, is also supposed to be the key intermediate. To
rationalize this hypothesis, the electronic structure of the one-
electron oxidized form of 1 was estimated by theoretical analysis
with DFT calculations (Gaussian09 UB3LYP/6-31G* SCRF (PCM,
solvent = water). Taking the axial ligands of 1 to be dihydroxo
groups, the DFT calculation of the one-electron-oxidized Ge
complex (cation radical: I) in water indicates that the electron
spin density is mostly delocalized on the porphyrin ring, while its
deprotonated species (II) has a dominant spin population on the
axial ligand oxygen, as shown in Figs. 7a and b, where the spin
population was expressed by the size of the blue lobe on each atom
by use of the Jmol software package [24]. Species II thus has mostly
oxygen radical character in its axial ligand (Fig. 7b), and the
hydrogen abstraction from a proper hydrogen atom donor would
be rationalized by the electronic structure, as presumed [21]. Here,
it should be the pointed out that there is a question as to whether
or not the formation of H2O2 is feasible in the expected process
between the Ge-oxyl radical and the hydroxide ion. To obtain
deeper insight into this point, each heat of formation before and
after the reaction of the Ge-oxyl complex (II) with the hydroxide
ion was calculated. Very interestingly, the reaction of Ge-oxyl
radical (II) with hydroxide ion to form Ge-hydroperoxy complex
Fig. 7. Electronic structures and spin densities of (a) the one-electron oxidized form
of 1 (Intermediate I), (b) the deprotonated form of I (Intermediate II), and (c) the
reaction product between II and hydroxide ion (Intermediate III).
spin in complex III is indicated to be delocalized on the porphyrin
ring, but mostly on the meso-position of the ring (ꢂ20%
population); specifically, complex III has an electronic structure
as the radical anion of the porphyrin. This strongly suggests that
the Ge-hydroperoxy complex (III) as the radical anion of the
porphyrin can readily inject the second electron to the conduction
band of TiO2. As a conclusion, the sensitized one-electron injection
from the excited state of 1 to the conduction band of TiO2 can
initiate the reaction with the hydroxide ion to form Ge-hydro-
peroxy complex (III) in the form of the radical anion on the
porphyrin ring. The complex (III) would subsequently inject the
second electron to TiO2, leading to a two-electron activation of
water to form H2O2, as discussed below.
(III) is fairly exothermic, with
DH = À31.41 kcal/mol. The formation
of hydroperoxy complex (III) is thus well rationalized. Another
curious point would be how the formation of H2O2 actually
proceeds on the anode. The electronic structure analysis should
provide crucial information about this. As shown in Fig. 7c, the
Ge-hydroperoxy complex (III) generated through the reaction of
Ge-oxyl complex (II) has still be in a doublet state with a spin. The
Please cite this article in press as: T. Shiragami, et al., Two-electron oxidation of water to form hydrogen peroxide sensitized by di(hydroxo)
porphyrin GeIV complex under visible-light irradiation, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphoto-
chem.2015.06.011

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5
3.4. Plausible reaction mechanism of water oxidation to H2O2
4. Conclusion
On the basis of the experimental results and theoretical
analysis, a plausible reaction mechanism is shown in Scheme 2.
Upon visible light irradiation, an electron transfer process from the
excited singlet state of 1 to TiO2 takes place to form the radical
cation of 1 (intermediate I), which is followed by deprotonation to
give a Ge-oxyl complex (intermediate II). A nucleophilic attack of
water to complex II proceeds to form the intermediate III, which
has the character of a porphyrin radical anion, as analyzed above.
After the formation of complex III, there may be two probable
routes leading to the formation of H2O2. One of the processes
would be that the axial hydroperoxo ligand of III is exchanged with
a hydroxo ligand to form intermediate IV, resulting in the release of
H2O2. The intermediate IV returns to the original state of 1 by
electron transfer from IV to TiO2 (route of A). The other pathway
would be an electron transfer from III to TiO2 at first to form
Ge-hydroperoxy complex (V) followed by substitution of the ligand
with water to recover the starting complex I (route of B). As
discussed above, the electron injection into TiO2 proceeded via the
route of B preferently. The electrons injected into TiO2 can move to
the Pt cathode through the external circuit to reduce the electron
acceptor ions or molecular oxygen, resulting in the short-circuit
photocurrent.
It was elucidated that the photochemical two-electron oxida-
tion of water to H2O2 could be selectively induced by using a
Ge-oxyl complex generated by PET from 1 to TiO2 under visible
light irradiation in a PVC system. The strong electrophilicity of the
Ge-oxyl complex can induce the activation of water, leading to the
production of H2O2 through a stepwise two-electron transfer to
TiO2. This is the first example of a water oxidation by a
photochemical process using a GeIV-porphyrin complex. Among
the studies on water oxidation, either transition metal-complexes
[8,10] or noble metal complexes [7,9] have been nearly always been
employed as the catalyst. Therefore, this study, in which water
oxidation was achieved by using a non-transition metal (main
group element) complex, may be significant from the viewpoint of
an “elemental strategy “ in photocatalysis.
Acknowledgements
This work was supported by both a Grant-in Aid for Scientific
Research on Innovative Area (Artificial photosynthesis (No. 2460)
(No. 25107520) and a Grant-in Aid for Challenging Exploratory
Research (No. 26620158) from the Japan Society for the Promotion
of Science (JSPS).
References
[1] A.J. Bard, M.A. Fox, Artificial photosynthesis: solar splitting of water to
hydrogen and oxygen, Acc. Chem. Res. 28 (1995) 141–145.
[2] A. Harriman, Artificial photosynthesis, J. Photochem. Photobiol. A 51 (1990)
41–43.
[3] T.J. Meyer, Chemical approach to artificial photosynthesis, Acc. Chem. Res. 22
(1989) 163–170.
[4] S.W. Gersten, G.J. Samuels, T.J. Meyer, Catalytic oxidation of water by an oxo-
bridge ruthenium dimer, J. Am. Chem. Soc. 104 (1982) 4029–4030.
[5] T. Wada, K. Tsuge, K. Tanaka, Electrochemical oxidation of water to dioxygen
catalyzed by the oxidized form of the bis(ruthenium-hydroxo) complex in
H2O, Angew. Chem. Int. Ed. 29 (2000) 1479–1482.
[6] J.J. Concepcion, J.W. Jurss, J.L. Templeton, T.J. Meyer, One site is enough.
Catalytic water oxidation by [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)
(OH2)]2+, J. Am. Chem. Soc. 130 (2008) 16462–16463.
[7] M. Yagi, A. Syouji, S. Yamada, M. Komi, H. Yamazaki, S. Tajima, Molecular
catalyst for water oxidation toward artificial photosynthesis, Photochem.
Photobiol. Sci. 8 (2008) 139-137.
[8] D.K. Dogutan, R. McGuire, D.G. Nocera, Electrocatalytic water oxidation by
cobalt (III) hangman
9180.
b-octafluoro corroles, J. Am. Chem. Soc. 133 (2011) 9178–
[9] R. Cao, W. Lai, P. Du, Catalytic water oxidation of single metal site, Energy
Environ. Sci. 5 (2012) 8134–8157.
[10] S. Kai, L.L. Ayensu-Mensah, P.H. Dinolfo, Evidence for catalytic water oxidation
by a dimanganese tetrakis-Schiff base macrocycle, Inorg. Chim. Acta 423
(2014) 201–206.
[11] T. Nakazono, A.R. Parent, K. Sakai, Cobalt porphyrin as homogeneous catalysts
for water oxidation, Chem. Commun. 49 (2013) 6325–6327.
[12] H. Inoue, T. Shimada, Y. Kou, Y. Nabetani, D. Masui, S. Takagi, H. Tachibana, The
water oxidation bottleneck in artificial photosynthesis: how can we get
through it. An alternative route involving
a two-electron process,
ChemSusChem 4 (2011) 173–179.
[13] S. Takagi, M. Suzuki, T. Shiragami, H. Inoue, Photochemical P-450 oxygenation
of cyclohexene with water sensitized by dihydroxy-coordinated
(tetraphenylporphyrinato) antimony (V) hexafluorophosphate, J. Am. Chem.
Soc. 119 (1997) 8712–8713.
[14] H. Inoue, T. Okamoto, M. Komiyama, M. Hida, Photochemical epoxidation of
alkenes by visible light in a redox system involving metalloporphyrins and
water, J. Photochem. Photobiol. A 65 (1992) 221–227.
[15] S. Takagi, H. Morimoto, T. Shiragami, H. Inoue, Visible light induced
oxygenation of cyclohexene with activation of water sensitized by
dihydroxy coordinated
tetraphenylporphyrinatotin(IV), Res.
Chem.
Intermed. 26 (2000) 171–183.
[16] K. Kurimoto, T. Yamazaki, Y. Suzuri, Y. Nabetani, S. Onuki, S. Takagi, T. Shimada,
H. Tachibana, H. Inoue, Hydrogen evolution coupled with photochemical
oxygenation of cyclohexene with water sensitized by tin(IV) porphyrins by
visible light, Photochem. Photobiol. Sci. 13 (2014) 154–156.
[17] S. Funyu, T. Isobe, S. Takagi, D.A. Tryk, H. Inoue, Highly efficient and selective
epoxidation of alkenes by photochemical oxygenation sensitized by
ruthenium(II) porphyrin with water as both electron and oxygen donor, J.
Am. Chem. Soc. 125 (2003) 5734–5740.
a
Scheme 2. Two plausible mechanisms (routes of A and B) of water oxidation to
H2O2 in 1-senstized PVC system.
Please cite this article in press as: T. Shiragami, et al., Two-electron oxidation of water to form hydrogen peroxide sensitized by di(hydroxo)
porphyrin GeIV complex under visible-light irradiation, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphoto-
chem.2015.06.011

G Model
JPC 9935 No. of Pages 6
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[18] H. Inoue, S. Funyu, Y. Shimada, S. Takagi, Artificial photosysnthesis via two-
electron conversion: photochemical oxygenation sensitized by ruthenium
porphyrins with water as both electron and oxygen atom donor, Pure. Appl.
Chem. 77 (2005) 1019–1033.
[21] T. Shiragami, A. Kuroki, J. Matsumoto, M. Yasuda, CH bond activation with Ge-
oxyl complex generated by photoinduced-electron-transfer of di(hydroxo)
porphyrin GeIV complex, J. Porphyrins Phthalocyanines 18 (2014)
529–534.
[19] S. Funyu, M. Kinai, D. Masui, S. Takagi, T. Shimada, H. Tachibana, H. Inoue, Key
reaction intermediates of the photochemical oxygenation of alkenes
sensitized by RuII-porphyrin with water by visible light, Photochem.
Photobiol. Sci. 9 (2010) 931–936.
[20] T. Shimada, A. Kumagai, S. Funyu, S. Takagi, D. Masui, Y. Nabetani, H. Tachibana,
D.A. Tryk, H. Inoue, How is the water molecule activated on
metalloporphyrins? Oxygenation of substrates induced through one-
photon/two-electron conversion in artificial photosynthesis by visible light,
Faraday Discuss. 155 (2012) 145–163.
[22] T. Shiragami, H. Ozawa, K. Kajihara, J. Matsumoto, M. Yasuda, Visible-light
driven fuel cell constructed by geranium(IV) porphyrin/TiO2 electrode using
alcohols as an electron donor, Chem. Lett. 44 (2015) 265–267.
[23] C. Matsubara, N. Kawamoto, K. Takamura, Oxo[5,10,15,20-tetra(4-pyridyl)
porphyrinato]titanium(IV): An ultra-high sensitivity spectrophotometric
reagent for hydrogen peroxide, The Analyst 117 (1992) 1781–1784.
[24] An open-source Java viewer for chemical structures in 3D. http://www.jmol.
org/.
Please cite this article in press as: T. Shiragami, et al., Two-electron oxidation of water to form hydrogen peroxide sensitized by di(hydroxo)
porphyrin GeIV complex under visible-light irradiation, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphoto-
chem.2015.06.011