An artificial model of photosynthetic photosystem II: Visible-light-derived O2 production from water by a di-μ-oxo-bridged manganese dimer as an oxygen evolving center

Article abstract of DOI:10.1039/c0cc03114c

Visible-light-derived O₂ production was yielded by conjugating water oxidation catalysis by [(OH₂)(terpy)Mn(μ-O) ₂Mn(terpy)(OH₂)]³⁺ as an oxygen evolving center model and photo-sensitization of [Ru(bpy)₃]²⁺ as a photoexcitation center model at an interlayer of mica.

Full text of DOI:10.1039/c0cc03114c

View Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
An artificial model of photosynthetic photosystem II: visible-light-derived
O2 production from water by a di-l-oxo-bridged manganese dimer as an
oxygen evolving centerw
Masayuki Yagi,*ab Mayuu Toda,a Satoshi Yamadaa and Hirosato Yamazakia
Received 8th August 2010, Accepted 3rd September 2010
DOI: 10.1039/c0cc03114c
Visible-light-derived O2 production was yielded by conjugating
water oxidation catalysis by [(OH2)(terpy)Mn(l-O)2Mn-
(terpy)(OH2)]3+ as an oxygen evolving center model and
photo-sensitization of [Ru(bpy)3]2+ as a photoexcitation center
model at an interlayer of mica.
Herein we report that visible-light-derived O2 production
is yielded by conjugating water oxidation catalysis by 1
and photo-sensitization of [Ru(bpy)3]2+ at an interlayer
of mica.
Mica is able to adsorb cationic 1 and [Ru(bpy)3]2+ by
cation exchange with Na+ (cation exchange capacity:
CEC = 1.2 meq gꢁ1). 1 was adsorbed onto mica from an
aqueous solution of 1, followed by similar adsorption of
[Ru(bpy)3]2+ to yield the mica adsorbate of 1 and [Ru(bpy)3]2+
(mica/1/[Ru(bpy)3]2+). The X-ray diffraction (XRD) data
indicate that either 1 or [Ru(bpy)3]2+ is intercalated into an
interspace between mica layers (Fig. S1, ESIw). For the diffuse
reflectance (DR) spectra of the mica/1/[Ru(bpy)3]2+ adsorbate,
the absorption at lmax = 475 nm assigned to MLCT transition
of [Ru(bpy)3]2+ was 5.2 times more intense than that for the
mica/[Ru(bpy)3]2+ adsorbate with the identical [Ru(bpy)3]2+
concentration (wRu = 20 mmol gꢁ1) (Fig. S2w). The specifically
intense absorption of the mica/1/[Ru(bpy)3]2+ adsorbate can
be explained by localization of [Ru(bpy)3]2+ close to the mica
surface. This means that [Ru(bpy)3]2+ is shallowly-intercalated
outside 1 in an interlayer of mica.
Light absorption of the chlorophyll photoexcitation center
so-called P680 induces electron transfer from P680 to pheophytin
as a primary electron acceptor and subsequently to two
quinones at phꢀotosystem II (PS II) in photosynthesis.1 To
the formed P680 + radical cation, an electron is donated from
an oxo-bridged tetra-manganese cluster so-called oxygen evolving
complex (OEC) through a TyrZ residue as an electron mediator.
Water oxidation to evolve O2 occurs upon accumulating
four oxidizing equivalents on OEC by the successive photo-
induced electron-transfer process. Development of a functional
PS II model is a challenging task in the related fields to provide
mechanistic insight into photosynthetic O2 production and
shed light on the promising way towards an artificial photo-
synthetic model.2–5 The most functional OEC models capable of
catalyzing water oxidation to O2 have been synthesized based on
ruthenium and iridium complexes.6–13 Synthetic manganese-oxo
complexes reported as a functional OEC model are few, though
OEC is composed of oxo-bridged manganese clusters.14–18
The extension of manganese-oxo complex-based OEC models to
a photochemical system is expected to yield a strikingly similar
PS II model.
The mica/1/[Ru(bpy)3]2+ adsorbate (3.3 mmol 1, 0.4 mmol
[Ru(bpy)3]2+, 20 mg mica) was suspended in an acetate buffer
solution (pH = 6.2, 2.0 ml) containing 15 mM S2O82ꢁ. When
visible light (l > 420 nm) irradiated the suspension with
stirring, O2 was significantly evolved, as shown in Fig. 1a. In
the absence of a component of 1, [Ru(bpy)3]2+ and S2O82ꢁ, O2
was not evolved (Fig. 1b–e), showing that these three
components are essential for the photoinduced O2 evolution.
Nor was O2 evolved in the homogeneous solution containing
We previously reported that [(OH2)(terpy)Mn(m-O)2Mn-
(terpy)(OH2)]3+ (1) (terpy = 2,20:60,20 0-terpyridine) works as
a catalyst for water oxidation when it is adsorbed on layer
compounds of kaolin, montmorillonite and mica.15,19 This
encourages us to develop a functional PS II model using the
layer compound/1 adsorbate as an OEC model. It is well-
defined that with a metal-to-ligand charge transfer (MLCT)
photoexcited state of [Ru(bpy)3]2+ (bpy = 2,20-bipyridine) it
is thermodynamically possible to split water to O2 and H2.
2ꢁ
the same amounts of 1, [Ru(bpy)3]2+ and S2O8 (Fig. 1f).
This result suggests that 1 and [Ru(bpy)3]2+ adsorbed on mica
are effective for O2 evolution, which is consistent with the
conclusion for a chemical water oxidation system.19 5.5 mmol
of O2 was evolved by light irradiation of the suspension
(pH = 6.2, 2.0 ml) containing the mica/1/[Ru(bpy)3]2+
adsorbate for 17 h under the conditions (1.6 mmol (164 mmol gꢁ1
)
1, 0.25 mmol (25 mmol gꢁ1) [Ru(bpy)3]2+, 10 mg mica, 15 mM
S2O82ꢁ). The turnover numbers (TN) of 1 and [Ru(bpy)3]2+
were 3.4 and 88 during the 17 h catalysis, respectively.
This result corroborates that 1 and [Ru(bpy)3]2+ works as a
a Department of Materials Science and Technology, Faculty of
Engineering &Center for Transdisciplinary Research, Niigata
University, 8050 Ikarashi-2, Niigata, 950-2181, Japan.
E-mail: yagi@eng.niigata-u.ac.jp
b PRESTO (PRESTO: Precursory Research for Embryonic Science
and Technology), Japan Science and Technology Agency (JST),
4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
catalyst and
a photocatalyst in photochemical water
oxidation, respectively, because the TN numbers are more
than unity. Nevertheless, TN of 1 is considerably lower compared
with TN = 15–17 for a chemical water oxidation system.19
Instability of [Ru(bpy)3]2+ is supposed to be responsible for
the lower TN of 1. Most likely, [Ru(bpy)3]2+ might be
decomposed by oxidized 1 during the photochemical reaction.
w Electronic supplementary information (ESI) available: Experimental
details, X-ray diffraction (XRD) spectroscopic data of mica adsorbate,
UV-visible diffuse reflectance (DR) spectroscopic data of mica
adsorbates, electron-impact-ionization mass spectra (EI-MS) in
18O-labeling experiments, kinetic data of photochemical O2 evolution.
See DOI: 10.1039/c0cc03114c
c
8594 Chem. Commun., 2010, 46, 8594–8596
This journal is The Royal Society of Chemistry 2010

View Online
Fig. 2 Emission decays of photoexcited [Ru(bpy)3]2+ intercalated in
Fig. 1 Time courses of the amount (nO /mol) of O2 evolved in
2
mica in water using a single-photon counting technique. (a; black)
mica/[Ru(bpy)3]2+, (b; red) mica/[Ru(bpy)3]2+ in the presence of
photochemical water oxidation in an aqueous suspension (2.0 ml,
2ꢁ
pH = 6.2) of 0.2 M acetate buffer and 15 mM S2O8 (a) containing
S2O8
in a liquid phase, (c; blue) mica/1 (8 eq.)/[Ru(bpy)3]2+
.
2ꢁ
the mica/1/[Ru(bpy)3]2+ adsorbate, (b) the mica/1 adsorbate, (c) the
mica/[Ru(bpy)3]2+ adsorbate, (d) neat mica. (mica, 20 mg; 1, 3.3 mmol
(164 mmol gꢁ1); [Ru(bpy)3]2+, 0.4 mmol (20 mmol gꢁ1)). (e) In the
0.08 mmol (20 mmol gꢁ1) [Ru(bpy)3]2+, 4 mg mica, acetate buffer
(3.0 ml, pH = 6.2), 25 1C, Ar atmosphere.
2ꢁ
absence of S2O8 under the conditions of (a). (f) In a homogenous
solution (pH = 6.2, 2.0 ml) containing 3.3 mmol (1.6 mM) 1 and
solution. Nevertheless, the emission decay from the photo-
2ꢁ
0.4 mmol (0.2 mM) [Ru(bpy)3]2+ and 15 mM S2O8
.
2ꢁ
excited state did not change if 15 mM S2O8 is contained in
an aqueous suspension of the mica/[Ru(bpy)3]2+ adsorbate,20
In order to identify an oxygen atom source for O2 evolution,
18O-isotope labeling experiments for photochemical water
oxidation were carried out in a H218O medium, and the
evolved gas was analyzed using an electron-impact-ionization
mass spectroscopic (EI-MS) technique. In a 99% H218O
medium, EI-MS spectra gave a main peak at m/z = 36
assigned to 18O2 and a minor peak (4.3%) at m/z = 34
assigned to 18O16O in a range of m/z = 25–40, in contrast to
a single peak at m/z = 32 for 16O2 observed for the experiment
in an abundant water medium (Fig. S3w). This result corroborates
that the oxygen atoms in O2 evolved are originated exclusively
from water.
showing that the photoexcited state in mica is not quenched by
2ꢁ
2ꢁ
S2O8 in the liquid phase (Fig. 2b). S2O8 anions in the
liquid phase can not be allowed to come close to [Ru(bpy)3]2+
in an anionic interspace between mica layers for quenching
due to electrostatic repulsion. In contrast, the emission decay
is significantly fast for the mica/1/[Ru(bpy)3]2+ adsorbate, as
shown in Fig. 2c. This indicates that the photoexcited state is
quenched by 1 in mica. The electron transport for photo-
chemical O2 evolution in the mica/1/[Ru(bpy)3]2+ adsorbate is
initiated from the photoexcitation of [Ru(bpy)3]2+, followed
most probably by electron transfer from 1 to the excited
[Ru(bpy)3]2+ to form oxidized 1 and [Ru(bpy)3]+. The
reductive quenching of the excited [Ru(bpy)3]2+ is well-known
from previous reports,21,22 though we do not have any spectro-
scopic evidence for [Ru(bpy)3]+ formation yet. The successive
photoinduced electron-transfer processes could generate oxidizing
equivalents that are used for water oxidation catalysis. On the
The initial O2 evolution rate (vO /mol sꢁ1) was given from the
2
initial slope of the time course of the amount (nO /mol) of O2
2
evolved. vO increased linearly with the light intensity and deviated
2
downward above 95 mW cmꢁ2. This result indicates that the O2
evolution is rate-determined by the subsequent reactions rather
than the photochemical process above 95 mW cmꢁ2 under the
conditions employed (Fig. S4w). The following experiments were
conducted under the light intensity of 127 mW cmꢁ2. The second
other hand, [Ru(bpy)3]+ could be re-oxidized to [Ru(bpy)3]2+
2ꢁ
by S2O8
in a liquid phase to suppress a back electron
transfer from [Ru(bpy)3]+ to the oxidized 1, as reported in
the literature.23 However, since [Ru(bpy)3]+ formed close to 1
in mica can not react directly with S2O82ꢁ, an electron on
[Ru(bpy)3]+ must be relayed to [Ru(bpy)3]2+ close to the mica
surface for the re-oxidation by S2O82ꢁ possibly by a successive
self-exchange electron-transfer reaction between [Ru(bpy)3]+
and [Ru(bpy)3]2+ (electron hopping) in mica (Fig. 3). The
electron relay by electron hopping could occur efficiently at the
higher wRu conditions to achieve photochemical O2 evolution.
order dependency of vO with respect to the concentration of 1
2
(wMn/mol gꢁ1) was given (Fig. S5w), suggesting that two
equivalents of 1 are required for photochemical O2 production.
This is consistent with the cooperative catalysis reported in the
chemical water oxidation system.19 Although O2 was hardly
evolved below wRu = 5 mmol gꢁ1 under the constant wMn
(164 mmol gꢁ1) conditions, vO critically increased at more than
2
w
Ru = 7.5 mmol gꢁ1 (Fig. S6w). Light harvesting by [Ru(bpy)3]2+
should be an important factor to yield photochemical O2
This could be a possible explanation for the critical vO
2
evolution. However, it can not explain the critical vO increase
increase with wRu.
2
with wRu reasonably, because the light absorption by [Ru(bpy)3]2+
increases linearly with wRu for the mica/1/[Ru(bpy)3]2+ adsorbate.
The photoexcited state of [Ru(bpy)3]2+ is well-known to be
For preliminary experiments, photochemical O2 production
was not yielded under the same conditions (164 mmol gꢁ1 1,
20 mmol gꢁ1 [Ru(bpy)3]2+, 20 mg mica, 15 mM S2O82ꢁ) using
two control adsorbates: (i) the mica/[Ru(bpy)3]2+/1 adsorbate
2ꢁ
sufficiently quenched by S2O8 in a homogeneous aqueous
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8594–8596 8595

View Online
Notes and references
1 J. P. McEvoy and G. W. Brudvig, Chem. Rev., 2006, 106, 4455–4483.
2 G. C. Dismukes, R. Brimblecombe, G. A. N. Felton,
R. S. Pryadun, J. E. Sheats, L. Spiccia and G. F. Swiegers,
Acc. Chem. Res., 2009, 42, 1935–1943.
3 W. Ruettinger and G. C. Dismukes, Chem. Rev., 1997, 97, 1–24.
4 M. Yagi and M. Kaneko, Chem. Rev., 2001, 101, 21–35.
5 C. W. Cady, R. H. Crabtree and G. W. Brudvig, Coord. Chem.
Rev., 2008, 252, 444–455.
6 M. Yagi, A. Syouji, S. Yamada, M. Komi, H. Yamazaki and
S. Tajima, Photochem. Photobiol. Sci., 2009, 8, 139–147.
7 S. W. Gersten, G. J. Samuels and T. J. Meyer, J. Am. Chem. Soc.,
1982, 104, 4029–4030.
Fig.
3 Illustration of possible vectorial electron transport for
8 C. Sens, I. Romero, M. Rodriguez, A. Llobet, T. Parella and
J. Benet-Buchholz, J. Am. Chem. Soc., 2004, 126, 7798–7799.
9 T. Wada, K. Tsuge and K. Tanaka, Angew. Chem., Int. Ed., 2000,
39, 1479–1482.
10 R. Zong and R. P. Thummel, J. Am. Chem. Soc., 2005, 127,
12802–12803.
photochemical O2 production. Electrons are relayed from 1 to
[Ru(bpy)3]2+ close to the mica surface by a successive self-exchange
electron-transfer reaction between [Ru(bpy)3]+ and [Ru(bpy)3]2+
2ꢁ
(electron hopping) in mica, further transferring to S2O8
liquid phase.
in a
11 J. J. Concepcion, J. W. Jurss, J. L. Templeton and T. J. Meyer,
J. Am. Chem. Soc., 2008, 130, 16462–16463.
12 N. D. McDaniel, F. J. Coughlin, L. L. Tinker and S. Bernhard,
J. Am. Chem. Soc., 2008, 130, 210–217.
13 L. Duan, A. Fischer, Y. Xu and L. Sun, J. Am. Chem. Soc., 2009,
131, 10397–10399.
14 J. Limburg, J. S. Vrettos, L. M. Liable-Sands, A. L. Rheingold,
R. H. Crabtree and G. W. Brudvig, Science, 1999, 283, 1524–1527.
15 M. Yagi and K. Narita, J. Am. Chem. Soc., 2004, 126, 8084–8085.
16 R. Brimblecombe, G. F. Swiegers, G. C. Dismukes and L. Spiccia,
Angew. Chem., Int. Ed., 2008, 47, 7335–7338.
17 A. K. Poulsen, A. Rompel and C. J. McKenzie, Angew. Chem., Int.
Ed., 2005, 44, 6916–6920.
18 R. Brimblecombe, A. Koo, G. C. Dismukes, G. F. Swiegers and
L. Spiccia, J. Am. Chem. Soc., 2010, 132, 2892–2894.
19 K. Narita, T. Kuwabara, K. Sone, K. Shimizu and M. Yagi,
J. Phys. Chem. B, 2006, 110, 23107–23114.
prepared by an opposite adsorption order: [Ru(bpy)3]2+
followed by 1, and (ii) the mica/(1+[Ru(bpy)3]2+
)
adsorbate prepared by co-adsorption from the mixed solution
of 1 and [Ru(bpy)3]2+. The DR spectra for these control
adsorbates did not exhibit the specifically intense absorption
at 475 nm due to [Ru(bpy)3]2+ in contrast to the case for the
mica/1/[Ru(bpy)3]2+ adsorbate, implying that [Ru(bpy)3]2+ is
not shallowly-intercalated outside 1 in an interlayer for
these control adsorbates (Fig. S2w). These results suggest that
the arrangement of 1 and [Ru(bpy)3]2+ in an interlayer is
important for photochemical O2 evolution. The arrangement
of inside
1
and outside [Ru(bpy)3]2+ could allow for
efficient vectorial electron transport from 1 to S2O82ꢁ. The
present work demonstrates the illustration of artificial PS II
model for 1 and [Ru(bpy)3]2+ to act as OEC and P680 models,
respectively. We are undertaking an advanced project to build
up the artificial photosynthetic model by assembling the PS II
model and a H2 production catalyst system.
20 The multi-exponential emission decay could be ascribed to self-
quenching by neighboring [Ru(bpy)3]2+ concentrated in mica.
21 C. R. Bock, J. A. Connor, A. R. Gutierrez, T. J. Meyer,
D. G. Whitten, B. P. Sullivan and J. K. Nagle, J. Am.
Chem. Soc., 1979, 101, 4815–4824.
22 K. Kalyanasundaram, Coord. Chem. Rev., 1982, 46, 159–244.
23 F. Gaillard, Y.-E. Sung and A. J. Bard, J. Phys. Chem. B, 1999,
103, 667–674.
c
8596 Chem. Commun., 2010, 46, 8594–8596
This journal is The Royal Society of Chemistry 2010