63
Synthesis and Photoinduced Electron Transfer of Donor-Sensitizer-Acceptor Systems Yunhua Xu Stockholm University 2005
1
Synthesis and Photoinduced
Electron Transfer of
Donor-Sensitizer-Acceptor Systems
Yunhua Xu
Stockholm University
2005
2
Synthesis and Photoinduced Electron Transfer of Donor-Sensitizer-Acceptor Systems
Akademisk avhandling
som för avläggande av filosofie doktorsexamen vid Stockholms Universitet, tillsammans med arbetena I-VII, offentligen kommer att försvaras i Magnélisalen, Kemiska övnings-laboratoriet, Svante Arrhenius väg 16, onsdagen den 20 april 2005, klockan 10.00.
Av
Yunhua Xu
Avhandlingen försvaras på engelska Institutionen för organisk kemi ISBN 91-7155-034-8 pp 1-53 Arrheniuslaboratoriet Stockholm 2005 Stockholms Universitet 106 91 Stockholm
Abstract
Artificial systems involving water oxidation and solar cells are promising ways for the conversion of solar energy into fuels and electricity. These systems usually consist of a photosensitizer, an electron donor and / or an electron acceptor. This thesis deals with the synthesis and photoinduced electron transfer of several donor-sensitizer-acceptor supramolecular systems. The first part of this thesis describes the synthesis and properties of two novel dinuclear ruthenium complexes as electron donors to mimic the donor side reaction of Photosystem II. These two Ru2 complexes were then covalently linked to ruthenium trisbipyridine and the properties of the resulting trinuclear complexes were studied by cyclic voltammetry and transient absorption spectroscopy. The second part presents the synthesis and photoinduced electron transfer of covalently linked donor-sensitizer supramolecular systems in the presence of TiO2 as electron acceptors. Electron donors are tyrosine, phenol and their derivatives, and dinuclear ruthenium complexes. Intramolecular electron transfer from the donor to the oxidized sensitizer was observed by transient absorption spectroscopy after light excitation of the Ru(bpy)3
2+ moiety. The potential applications of Ru2-based electron donors in artificial systems for water oxidation and solar cells are discussed. In the final part, the photoinduced interfacial electron transfer in the systems based on carotenoids and TiO2 is studied. Carotenoids are shown to act as both sensitizers and electron donors, which could be used in artificial systems to mimic the electron transfer chain in natural photosynthesis.
3
Synthesis and Photoinduced
Electron Transfer of
Donor-Sensitizer-Acceptor Systems
Yunhua Xu
Department of Organic Chemistry
Stockholm University
2005
4
Doctoral Dissertation 2005 Department of Organic Chemistry Arrhenius Laboratory Stockholm University Sweden
Abstract Artificial systems involving water oxidation and solar cells are promising ways for
the conversion of solar energy into fuels and electricity. These systems usually consist
of a photosensitizer, an electron donor and / or an electron acceptor. This thesis deals
with the synthesis and photoinduced electron transfer of several donor-sensitizer-
acceptor supramolecular systems.
The first part of this thesis describes the synthesis and properties of two novel
dinuclear ruthenium complexes as electron donors to mimic the donor side reaction of
Photosystem II. These two Ru2 complexes were then covalently linked to ruthenium
trisbipyridine and the properties of the resulting trinuclear complexes were studied by
cyclic voltammetry and transient absorption spectroscopy.
The second part presents the synthesis and photoinduced electron transfer of
covalently linked donor-sensitizer supramolecular systems in the presence of TiO2 as
electron acceptors. Electron donors are tyrosine, phenol and their derivatives, and
dinuclear ruthenium complexes. Intramolecular electron transfer from the donor to the
oxidized sensitizer was observed by transient absorption spectroscopy after light
excitation of the Ru(bpy)32+ moiety. The potential applications of Ru2-based electron
donors in artificial systems for water oxidation and solar cells are discussed.
In the final part, the photoinduced interfacial electron transfer in the systems based
on carotenoids and TiO2 is studied. Carotenoids are shown to act as both sensitizers
and electron donors, which could be used in artificial systems to mimic the electron
transfer chain in natural photosynthesis.
© Yunhua Xu ISBN 91-7155-034-8 pp 1-53
Intellecta Docusys AB, Sollentuna
5
Table of Contents
List of Publications .............................................................................................................. i
List of Abbreviations .......................................................................................................... ii Preface................................................................................................................................ iii
1 Artificial Photosynthesis and Dye-sensitized Solar Cells............................................. 1
1.1 Introduction ........................................................................................................... 1 1.2 Natural and Artificial Photosynthesis.................................................................... 2 1.3 Dye-sensitized Solar Cells..................................................................................... 3 1.4 Donor-Sensitizer-Acceptor Systems...................................................................... 5
1.4.1 Photosensitizers............................................................................................ 6 1.4.2 Electron Donors............................................................................................ 7 1.4.3 Electron Acceptors ....................................................................................... 7
2 Synthesis and Properties of Dinuclear Ruthenium Complexes as Electron Donors..... 9
2.1 Dinuclear Ruthenium Complexes........................................................................ 10 2.1.1 Synthesis and Characterization ................................................................... 11 2.1.2 Photophysical and Electrochemical Properties ........................................... 12 2.1.3 Conclusions ................................................................................................. 14
2.2 Dinuclear Ruthenium Complexes Covalently Linked to Ru(bpy)32+ .................. 15
2.2.1 Synthesis and Characterization ................................................................... 16 2.2.2 Properties of the Complexes ....................................................................... 18 2.2.3 Conclusions ................................................................................................. 22
3 Photoinduced Electron Transfers in Donor-Sensitizer-Acceptor Systems ................. 23
3.1 Tyrosine-Ru(bpy)32+ Anchored to TiO2 in Colloid Solution............................... 23
3.1.1 Synthesis and Sample Preparation .............................................................. 24 3.1.2 Photophysical Properties and Photoinduced Electron Transfer .................. 26 3.1.3 Conclusions ................................................................................................. 28
3.2 Substituted Tyrosine-Ru(bpy)32+ Anchored to TiO2 Films ................................. 28
3.2.1 Sample Preparation ..................................................................................... 29 3.2.2 Photoinduced Electron Transfer.................................................................. 29 3.2.3 Conclusions ................................................................................................. 31
3.3 Polyphenolate-Ru(bpy)32+ in the Presence of External Acceptors ...................... 31
3.3.1 Synthesis and Properties.............................................................................. 32 3.3.2 Photoinduced Electron Transfer.................................................................. 32 3.3.3 Conclusions ................................................................................................. 33
3.4 Ru2-Ru(bpy)3 Anchored to TiO2 Film................................................................. 34 3.4.1 Photoinduced Electron Transfer.................................................................. 34 3.4.2 Conclusions ................................................................................................. 35
4 Photoinduced Electron Transfer in Supermolecules Based on Carotenoid –TiO2 ..... 37
4.1 Carotenoid Anchored to TiO2 Nanoparticles....................................................... 37 4.1.1 Synthesis...................................................................................................... 38 4.1.2 Properties and Photoinduced Electron Transfer.......................................... 38 4.1.3 Conclusions ................................................................................................. 39
4.2 Carotenoid and Pheophytin Assembled on TiO2 Surface.................................... 40
6
4.2.1 Sample Preparation ..................................................................................... 40 4.2.2 Photoinduced Electron Transfer.................................................................. 41 4.2.3 Conclusions ................................................................................................. 42
5 Concluding Remarks................................................................................................... 43
6 Supplementary Information ........................................................................................ 45 Acknowledgements........................................................................................................... 47 References......................................................................................................................... 49
i
List of Publications This thesis is based on papers I-VII as follows: I. Mixed-valence Properties of an Acetate-Bridged Dinuclear Ruthenium(II,III)
Complex Reiner Lomoth, Ann Magnuson, Yunhua Xu and Licheng Sun. J. Phys. Chem. A, 2003, 107, 4373-4380.
II. Synthesis and Characterization of Novel Dinuclear Ruthenium Complexes Covalently Linked to Ru(II) Trisbipyridine: an Approach to Mimics of the Donor Side of PS II Yunhua Xu, Gerriet Eilers, Magnus Borgström, Jingxi Pan, Maria Abrahamsson, Ann Magnuson, Reiner Lomoth, Jonas Bergquist, Tomas Polivka, Licheng Sun, Villy Sundström, Stenbjörn Styring, Leif Hammarström and Björn Åkermark. Manuscript
III. Light-driven Tyrosine Radical Formation in a Ruthenium-Tyrosine Complex Attached to Nanoparticle TiO2 Raed Ghanem, Yunhua Xu, Jie Pan, Tobias Hoffmann, Johan Andersson, Tomas Polivka, Torbjörn Pascher, Stenbjörn Styring, Licheng Sun and Villy Sundström Inorg. Chem. 2002, 41, 6258-6266.
IV. Stepwise Charge Separation from a Ruthenium-Tyrosine Complex to a Nanocrystalline TiO2 Film Jingxi Pan, Yunhua Xu, Gabor Benkö, Yashar Feyziyev, Stenbjörn Styring, Licheng Sun, Björn Åkermark, Tomas Polivka and Villy Sundström J. Phys. Chem. B, 2004, 108, 12904-12910.
V. Synthesis and Photoinduced Electron Transfer Study of a Substituted Phenol Covalently Linked to Ruthenium Trisbipyridine with or without Four Ester Groups Yunhua Xu, Jie Pan, Ping Huang, Yashar Feyziyev, Reiner Lomoth, Leif Hammarström, Stenbjörn Styring, Tomas Polivka, Villy Sundström, Björn Åkermark and Licheng Sun. Manuscript
VI. Photoinduced Electron Transfer between a Carotenoid and TiO2 Nanoparticle Jie Pan, Gabor Benkö, Yunhua Xu, Torbjörn Pascher, Licheng Sun, Villy Sundström and Tomas Polivka J. Am. Chem. Soc. 2002, 124, 13949-13957.
VII. Carotenoid and Pheophytin on Semiconductor Surface: Self-Assembly and Photoinduced Electron Transfer Jingxi Pan, Yunhua Xu, Licheng Sun, Villy Sundström and Tomas Polivka J. Am. Chem. Soc. 2004, 126, 3066-3067.
Reprints were made with the permission of the publishers. Paper not included in this thesis:
Switching the Redox Mechanism: Models for Proton-Coupled Electron Transfer from Tyrosine and Tryptophan Martin Sjödin, Stenbjörn Styring, Henriette Wolpher, Yunhua Xu, Licheng Sun and Leif Hammarström J. Am. Chem. Soc. 2005, in press. Web release date: Feb. 25, 2005.
ii
List of Abbreviations
A electron acceptor ACN acetonitrile BPA N-(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2-pyridylmethyl)amine bpy 2,2′-bipyridine CV cyclic voltammetry D electron donor DMF dimethyl formamide DMSO dimethyl sulphoxide DPA N,N-bis(2-pyridylmethyl)amine DPV differential pulse voltammetry EnT energy transfer EPR electron paramagnetic resonance ESI-MS electrospray ionization mass spectrometry ET electron transfer EtOH ethanol Fc ferrocene LC ligand centered 1MLCT metal-to-ligand charge transfer (singlet) 3MLCT metal-to-ligand charge transfer (triplet) MeOH methanol MV2+ methyl viologen OEC oxygen-evolving center P photosensitizer Pht phthalimido PS I Photosystem I PS II Photosystem II Q quinone TyrZ tyrosineZ SCE saturated calomel electrode
iii
Preface
This thesis reports the work based on the papers I-VII in the List of Publications.
My project is a part of the collaboration of three universities in Sweden: The organic
chemistry department at Stockholm University, the physical chemistry department
and the biochemistry department at Uppsala University, and the chemical physics
department at Lund University. I am responsible for the synthesis in all papers and
some of the electrochemical and photophysical measurements in Paper II. Other
measurements were done at Uppsala University and / or Lund University. ESI-MS
was measured either by Jonas Bergquist at the analytical chemistry department at
Uppsala University, or by Jerker Mårtensson at Götborg University.
iv
1
1
Artificial Photosynthesis and Dye-sensitized Solar Cells 1.1 Introduction Our society is dependent on energy conversion and energy balance. In nature,
some organisms convert solar energy into chemical energy by reducing carbon
dioxide to organic compounds such as carbohydrates, fats, amino acid etc. by
photosynthesis.1 The chemical energy, which is stored in these compounds, can then
be used as renewable energy by all other organisms to develop and sustain life.
However, the energy demands of our society much exceed the present supply of
organic biomass. This leads mankind to use other energy sources, e.g. fossil fuels
(coal, oil, natural gas), nuclear power, wind power and hydroelectric power. The use
of certain energy sources, on the other hand, results in various problems. For example,
the supply of fossil fuels is limited and their combustion leads to very severe air
pollution; nuclear power has a different risk profile and seems to be unacceptable in
many countries. Thus, there is a challenge for scientists to find alternative sustainable
and environmentally friendly energy sources.
The production of renewable and non-polluting fuels and electricity via the direct
conversion of solar energy is a fascinating alternative. The splitting of water into
molecular oxygen and molecular hydrogen by visible light (Eq. 1.1) is one of the most
promising ways for this photochemical conversion and storage of solar energy,
because the raw material, water, is abundant and cheap.2
H2O ⎯⎯⎯ →⎯ light visible 21 O2 + H2 (1.1)
To develop efficient solar cells is another way to make use of solar energy. The
solar cell is a device made from semiconductor materials which directly converts
sunlight to electrical energy. It is based on the so-called photovoltaic effect which
describes how sunlight (photons) is absorbed to produce an electric potential.3,4
2
This thesis describes our attempts to develop supramolecular devices for
applications in the light-driven water-oxidation and dye-sensitized solar cells.
1.2 Natural and Artificial Photosynthesis The oxidation of water to molecular oxygen in green plants is one of the most
important and fundamental chemical processes in nature. It takes place in
Photosystem II (PS II),5-8 a large protein complex, located in the thylakoid membrane
of plant chloroplasts and in cyanobacteria.
4H+ + O2
Mn4/Ca2H2O
TyrZ P680
e- e-hν
Phe QA QB
e- e-e- e-
Figure 1.1. Schematic picture of PS II with involved redox components.
The main components involved in water oxidation are: a multimer of chlorophylls
(P680), a redox active amino acid tyrosineZ (TyrZ), and a manganese cluster (Fig.
1.1).5,8-10 After light absorption by P680 in PS II, electron transfer occurs from the
excited state (P680*) to the primary electron acceptor pheophytin (Phe) and
subsequently to two quinones, forming a P680+ radical cation. The unique oxo-bridged
Mn4 cluster, which is responsible for the catalytic water oxidation to generate oxygen,
serves as an electron donor to P680+, and this electron transfer is mediated by the TyrZ
residue.5,8,10-17
TyrZ plays an important role in PS II and is believed to be an electron transfer
intermediate between P680 and the Mn cluster.11,13,14,16,17 Babcock et al. even proposed
that TyrZ directly participates in the water oxidation chemistry. 11,14,18-20
Scientists have been devoting great efforts to mimic this natural photosynthesis
process by constructing artificial systems.2,13-15,17 A water-splitting mimic would
require transfer of four electrons to generate oxygen (Eq. 1.2) and two to generate
hydrogen (Eq. 1.3):
3
2H2O → O2 + 4H+ + 4e-, E0 (pH=7) = +0.82 V vs. NHE (1.2)
2H2O + 2e- → H2 + 2OH-, E0 (pH=7) = -0.41 V vs. NHE (1.3)
The reaction is thus a multielectron transfer process and the splitting of water
requires 1.23 eV per electron transferred. In principle, photons with λ< 1008 nm
corresponding to a minimum energy of 1.23 eV can induce the cleavage of water.
Because water does not absorb visible light, it can not be split directly by sunlight,
and catalysts are needed.2
What are the design requirements for an artificial reaction center for water
splitting? As its basic operation is photoinduced electron transfer, a model reaction
center normally consists of a photosensitizer (P) that absorbs visible light, an
additional electron donor (D) or/and an electron acceptor (A). These parts are
covalently linked to form a supermolecule (Fig. 1.2). Upon the absorption of light, the
excited state P* is formed, and then transfers an electron to the electron acceptor A to
store the excitation energy as redox energy in the P+-A- pair. The A- should then,
either directly or through a catalyst, reduce water to hydrogen. P+ is reduced by the
electron donor D and returns to the active state. The oxidized D+ will, either directly
or through another catalyst capable of storing electron holes, oxidize water to
oxygen.14,15
O2
D
2H2O
PH2
A
2H+
e- e-hν
Figure 1.2. Schematic presentation of an artificial photosynthetic device for water splitting.
1.3 Dye-sensitized Solar Cells There have been several approaches to light-to-electricity conversion during the
past half century: Silicon-based solar cells, thin film solar cells and dye-sensitized
4
solar cells.3,4,21,22 Silicon-based solar cells are based on a p-n junction, commonly
formed by diffusion of dopants into n-type and p-type silicon wafers.3,4 This cell
requires high purity and relative large amounts of materials, and therefore it is a costly
alternative for the large-scale energy production.3,4,22c Beside Si, other materials such
as GaAs, CdTe, Cu2S, Zn3P2, InP and CuInSe2 are suitable for solar cells.4,21
However, unlike Si, all these materials must be in thin film (only a few micrometers
film thickness) in order to effectively absorb the solar spectrum.4,21 These thin films
can be obtained by low-cost processes.
(P+/P)
(P+/P*)e-
hopping
e-
hν
×
CB
VB
semiconductor dye hole transmitting solid
counterelectrode
E
Figure 1.3 Schematic energy diagram of a dye-sensitized solar cell. CB: conduction band; VB:
valence band; P: dye.
Dye-sensitized solar cells consist of a wide-bandgap semiconductor in combination
with dye molecules (photosensitizers) and an electrolyte.22-24 This technology can
minimize manufacturing costs because wide-bandgap semiconductors are stable and
cheap. However, such a system can not form an efficient solar cell if only a
monolayer of dye on a flat semiconductor electrode is used since it does not absorb
more than a few percent of the incident light. Grätzel and co-workers made a
breakthrough in dye-sensitized solar cells by using porous nanocrystalline TiO2
electrodes which have a very high internal surface area so that a monolayer of dye
adsorbed on such an electrode is sufficient to absorb a major part of the solar
spectrum.22-24 In this new type of solar cells, a dye is anchored to the surface of
5
nanostructured semiconductors such as TiO2. Upon light irradiation, the dye
(photosensitizer, P) is photoexcited and an electron is injected from the excited state
of the dye into the conduction band of the semiconductor. The dye is then regenerated
by electron transfer from electrolyte or a hole-transmitting solid, e.g., an amorphous
organic arylamine. A schematic representation of the principle of dye-sensitized
heterojunction solar cell is shown in Figure 1.3.
Compared to the thin film cells and conventional silicon-based cells which have
efficiency around 20%,22c,25 the present dye-sensitized cells usually have lower
efficiencies (around 10%).22-25 However, the costs are relatively low and it seems
very probable that the properties of dye-sensitized cells can be improved
substantially. For example, when an internal electron donor in a dye is introduced
(Figure 1.4), the excited state of dye transfers an electron to the conduction band of
nano-TiO2, forming a charge separated state on the surface of TiO2. Instead of the
normal charge recombination, the photo-oxidized dye is reduced by intramolecular
electron transfer from the internal donor, moving the hole further away from the
surface of TiO2 and subsequently forming a longer-lived charge separated state. Such
a system could also be used in the artificial photosynthesis.
TiO2 Dye Donor
hve- e-
Figure 1.4. A schematic representation of the principle of two-step electron transfers to prolong the lifetime of charge separated state.
1.4 Donor-Sensitizer-Acceptor Systems
The purpose of this thesis is to develop a supramolecular system comprising donor-sensitizer-acceptor subunits separated by well-defined spacer groups. Such a system could not only be applied in solar cells, but also be used to catalyse photoelectrochemical oxidation reactions, for example, epoxidation, hydroxylation
6
and ultimately water oxidation.
1.4.1 Photosensitizers
The photosensitizer (dye) is an interface towards light. The excited state of such a
light-absorbing unit must be easily accessible and the photosensitizer must show
suitable redox behavior. Further requirements are: (a) high efficiency in light
absorption; (b) high quantum yield for the population of the reactive excited state; (c)
a long lifetime of the excited state; (d) stability towards thermal and photochemical
decomposition reactions. Some examples of useful photosensitizers are
porphyrins,22a,26-32 phthalocyanines,22a,29 and polypyridine complexes of d6 metal ions
such as Ru(II)15,22,25,27,31,32 and Os(II)31,32 which have intense metal-to-ligand charge
transfer (MLCT) transitions in the visible region.
Ruthenium(II) polypyridyl complexes are often used as photosensitizers in
artificial photosynthesis and dye-sensitized solar cells,15,22,25,27,31-33 since they are able
to absorb light in the near UV and visible region and have favorable properties such as
chemical stability and well-defined reversible redox behavior. Their excited states,
which can be formed rapidly (∼300 fs), are quite stable and sufficiently long-lived,
and they can undergo rapid electron-transfer reactions. The best photovoltaic
performance in terms of both conversion yield and long-term stability has so far been
achieved with ruthenium polypyridyl complexes cis-RuL2(NCS)2 known as the N3
dye.22b
Carotenoids are a class of natural pigments that have important functions in many
biological systems.34-36 They act as light-harvesting agents in almost all
photosynthetic organisms covering a region of the visible spectrum not accessible by
(bacterio)chlorophylls. Therefore they can be used as photosensitizers in artificial
systems.
This work will focus on the use of ruthenium trisbipyridine complexes and carotenoids as photosensitizers. In order to be anchored to nanocrystalline semiconductor, they have to be modified by introducing ester or carboxylic acid groups.
7
1.4.2 Electron Donors
Some metal complexes are good electron donors. In artificial systems, Mn
complexes are often used as final electron donors to mimic the structure and function
of the oxygen-evolving center (OEC).13-15,17 Ruthenium complexes are also attractive
as electron donors, since a number of ruthenium complexes are shown to be water
oxidation catalysts.13,14,17,37-48
A number of organic molecules can also serve as primary electron donors to the
oxidized sensitizers. Tyrosine and its derivatives can donate an electron to produce a
tyrosine radical and a proton.13-16,49 Carotenoids can play the role of electron donor in
the photosynthetic reaction center when a suitable electron acceptor is available.50,51
In the work described in this thesis, polyruthenium complexes, tyrosine and its
derivatives, and carotenoids are used as electron donors.
1.4.3 Electron Acceptors
Bipyridinium ions (viologens)52 and quinones53 are often used as exteneral
acceptors. Acceptors such as [Co(NH3)5Cl]2+, that can undergo irreversible
decomposition upon reduction, are also used to hinder undesired back reactions
between the oxidized forms of the photosensitizers and the reduced forms of the
acceptors.52f
Besides the acceptors mentioned above, wide bandgap semiconductors such as
TiO2, SnO2 and ZnO are used as solid state acceptors.22,23,25,36,54 In this thesis, this
kind of acceptors are used in most cases.
e-
Energy
Dye
hν
CB
VB
TiO2 Figure 1.5 Energy diagram for dye sensitization of TiO2.
8
Interfacial electron transfer between sensitizers and semiconductors has been
intensely studied recently.22,25,54 It involves the adsorption of a dye onto a
semiconductor surface and photoexcitation of the dye to induce interfacial electron
transfer (Fig. 1.5). When molecular components are anchored to semiconductors, the
interaction with the surface can greatly change the rate of the individual photophysical
processes. For example, when ruthenium polypyridyl complexes are bound to TiO2,
electron injection from the excited state into the conduction band of the
semiconductor is on the time scale of femtosecond to picosecond. On the other hand,
the back-electron-transfer process is several orders of magnitude slower than the
forward-electron-transfer reaction. As a result, an efficient and long-lived charge
separation is achieved. To generate such systems is one of the driving forces behind
the work carried out in this area.54
9
2
Synthesis and Properties of Dinuclear Ruthenium Complexes as Electron Donors
In PSII, the oxygen-evolving center (OEC) serves as an electron donor to P680+.5,8-
11 Also in artificial systems, it would be useful if the electron donor could be the
catalyst for water oxidation. Since a manganese cluster is the essential cofactor to
catalyze water-oxidation in PS II, a number of polynuclear manganese-oxo complexes
have been synthesized and studied in order to establish a structural analogue for the
active site of water oxidation,13-15,17 and some manganese complexes have been
covalently linked to photosensitizers as well.14,15,52,55 However, water oxidation with
such manganese complexes has not yet been achieved.14,15,17 It is interesting to note
that some dinuclear ruthenium complexes have been shown to perform water
oxidation to a reasonable extent via homogeneous catalysis.13,14,17 In 1982, Meyer and
his co-workers reported a dinuclear ruthenium complex
[(bpy)2(H2O)RuORu(H2O)(bpy)2]4+ that can catalyze water-oxidation although the
stability of the catalyst is limited to 10-25 turnovers.37 Since then, a variety of related
ruthenium complexes have been synthesized and shown to be water-oxidation
catalysts.38-48,56-58 The trinuclear complex [(NH3)5RuIII(µ-O)RuIV(NH3)4(µ-
O)RuIII(NH3)5]6+ with a large excess of a Ce(IV) oxidant in an aqueous solution
induced O2 formation.56 Dinuclear complexes [(NH3)5RuIII(µ-O)RuIII(NH3)5]4+ and
[(NH3)5RuIII(µ-Cl)RuII(NH3)5]2+ showed similar catalytic activities.57 Some
mononuclear ruthenium complexes, for examples [RuIII(NH3)6]3+ and
[RuIII(NH3)5Cl]2+, can also catalyze water-oxidation, although they are less efficient
than the multinuclear complexes.58 Recently Llobet and his co-workers presented a
new dinuclear ruthenium complex, that is capable of oxidizing water to O2 but does
not contain the Ru-O-Ru motif.47
With the aim of developing an alternative route towards artificial systems and
constructing an efficient internal electron donor for dye-sensitized solar cells, we have
prepared two dinuclear ruthenium complexes 1 and 2. These dinuclear complexes
have also been attached to a ruthenium trisbipyridine photosensitizer. In this section, I
10
will describe their photophysical and electrochemical properties and investigate the
possibility of their application in artificial systems and dye-sensitized solar cells.
2.1 Dinuclear Ruthenium Complexes (Paper I and Supplementary Information)
2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol (3, HBPMP) is a
binucleating ligand and has been widely used to synthesize dinuclear complexes with
a bridging phenoxo group. These complexes have a non-linear bridging structure with
a short metal-to-metal distance, and are different from those dimers with N-
heterocycle-bridges. The related 2,6-bis{[(2-hydroxy-3,5-di-tert-butylbenzyl)(2-
pyridylmethyl) amino]methyl}-4-methylphenol (4) is also a binucleating ligand but
has two additional phenolate groups, and has been used to prepare dinuclear Mn
complexes.61 The tert-butyl groups on the phenols should both increase the electron
donating effect and improve the solubility of the complexes formed with this ligand.
When coordinated to a metal ion, ligand 4 becomes a trianion and therefore can
stabilize higher oxidation states of the metal ion. This property of ligand 4 is of
interest since high-valent metal species are probably required for catalytic water
oxidation. Thus, we have used ligands 3 and 4 to make the dinuclear ruthenium
complexes, 1 and 2 (Chart 2.1).
1 2
N
N
O
Ru
Ru
O
O
N
N
N
N
O
O
·2ClO4 ·ClO4N
N
O
Ru
Ru
O
O
N
N
O
O
O
O
Chart 2.1
11
2.1.1 Synthesis and Characterization
The synthesis of dinuclear ruthenium complexes 1 and 2 is shown in Scheme 2.1.
1
2
NN OH
N N
N N
Scheme 2.1
OHOHN
NOH
N
N
NaOAc /MeOH
Ru(DMSO)4Cl2
NaOAc / MeOH
Ru(DMSO)4Cl2
3
4
NH2N
NH
O HN
NN
NaBH4
DPA
OH
DPA
CH2O
OH
OHHO
HN
N HONaBH4
BPA
HOH
1) SOCl2
2) BPA
RuCl3·xH2O cis-Ru(DMSO)4Cl2DMSO
NH2N
O
72%
89%
81%
55%
43%
69%
Reflux
Ligands 3 and 4 were prepared according to the published methods.60b,61 2-
Pyridylmethylamine was reacted with pyridine-2-carboxaldehyde and 3,5-di-tert-
butyl-2-hydroxybenzaldhyde respectively, followed by reduction by NaBH4, to afford
12
the secondary amines N,N-bis(2-pyridylmethyl)amine (DPA)62 and N-(2-hydroxy-
3,5-di-tert-butylbenzyl)-N-(2-pyridylmethyl)amine (BPA).61 Then the Mannich
reaction of p-cresol, DPA and paraformaldehyde gave the ligand 3.60b In principle,
ligand 4 can also be made in a similar way by Mannich reaction, however separation
of the product from the excess BPA would be difficult. Therefore an alternative way
was chosen to prepare 4 by reaction of BPA with 2,6-bis(chloromethyl)-4-
methylphenol.61
Complexes 1 and 2 were obtained by refluxing the mixture of cis-Ru(DMSO)4Cl2
and the free ligands 3 and 4, respectively, in MeOH in the presence of NaOAc,
followed by the addition of a saturated aqueous solution of NaClO4. Cis-
Ru(DMSO)4Cl2 was prepared by refluxing RuCl3 in DMSO for 10 min.63 Both
complexes 1 and 2 were well characterized by ESI-MS, elemental analysis,
electrochemistry and electron paramagnetic resonance (EPR) spectroscopy.
Interestingly one Ru(II) in 1 and two Ru(II) in 2 were air-oxidized to Ru(III) during
preparation of the complexes. This means that 1 and 2 are Ru2(II,III) and Ru2(III,III)
species, respectively.
2.1.2 Photophysical and Electrochemical Properties
λ / nm
200 300 400 500 600 700 8000
1
2
3
4
ε / 1
04 M-1
cm
-1
Figure 2.1 Absorption spectra of 1 (---) and 2 (⎯) in acetonitrile.
13
UV-Vis. For complex 1 (Fig. 2.1), there is a fairly weak absorption at ca 400 nm and a
stronger absorption in the UV region with λmax = 246 nm. For 2 (see also Fig. 2.1),
there are weak broad absorptions between 200 nm and 450 nm with two peaks λmax =
291 nm and 333 nm. In addition, there is a fairly weak broad absorption in the region
500 – 800 nm.
Electrochemistry. Redox properties of the complexes 1 and 2 in dry acetonitrile were
studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). All
potentials are referenced vs saturated calomel electrode (SCE).
Figure 2.2. Cyclic voltammograms (ν= 100 mV s-1) of (a) 1 (1 mM) and (b) 2 (1 mM)
in acetoniltrile with 0.1 M [N(nC4H9)4]PF6 as supporting electrolyte.
CVs of 1 and 2 are shown in Figure 2.2 and the assignments of the redox waves are
summarized in Table 2.1. The CV of 1 (Fig. 2.2(a)) shows one reversible oxidation
wave (Ru2II,III→ Ru2
III,III) and one reversible reduction wave (Ru2II,III→ Ru2
II,II). In
comparison with 1, 2 displayed much richer redox properties. The CV of 2
(Fig.2.2(b)) shows two reversible oxidation waves (Ru2III,III→ Ru2
III,IV and Ru2III,IV→
Ru2IV,IV) and three reversible reduction waves. The nature of the first reduction wave
(E1/2 = -0.623 V) is not clear since reduction of 2 at –0.70 V does not change the EPR
E / V vs SCE
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
ia
ic
10 µA
(a)
(b)
14
spectrum of the dinuclear ruthenium itself but generates an organic radical. The other
two reduction waves are probably the expected redox processes Ru2III,III→ Ru2
II,III and
Ru2II,III→ Ru2
II,II.
Table 2.1 Electrochemical data.
E1/2 [V][b] (∆Ep [mV])[c]
Complexes Ru2II,III/II,II Ru2
III,III/II,III X/X-• Ru2III,IV/III,III Ru2
IV,IV/III,IV
1[a] -0.230 (70) 0.470 (70) - - -
2[a] -1.095 (67) -0.867 (81) -0.623 (67) 0.756 (125) 1.016 (150)
[a] As ClO4–salt. [b] Versus SCE in CH3CN solution with 0.1 M [N(nC4H9)4]PF6 as supporting
electrolyte, ±0.02V. [c] ν= 100 mVs-1.
Interestingly the phenolate ligands strongly influence the redox behaviour and
considerably stabilize the higher oxidation states of the Ru ions compared to the case
of the one-phenolate ligands. The potentials for the Ru2III,III/Ru2
II,III and Ru2II,III/Ru2
II,II
couples of 2 are lower by 0.86 and 1.33 V than those of 1, respectively. In addition,
two more oxidation processes, the Ru2III,IV/Ru2
III,III and Ru2IV,IV/Ru2
III,IV couples, are
observed with 2 and it should be possible to drive them by photogenerated Ru(bpy)32+
(E1/2 = 1.32 V). Since the Ru-Ru interaction seems weak according to the EPR data,
the major reason for the observed differences in redox potentials is probably the
introduction of negatively charged ligands. The result also shows that with the tri-
phenolate ligand, the high oxidation state Ru2IV,IV can easily be reached.
2.1.3 Conclusions
Complex 1 contains a mixed-valence Ru2II,III moiety, which can readily undergo
reversible a one-electron reduction and a one-electron oxidation, resulting in the
Ru2II,II and Ru2
III,III complexes, respectively. Complex 2 is a Ru2III,III complex and
exhibits even richer electrochemistry. Reversible reduction to Ru2II,III and Ru2
II,II and
oxidation to Ru2III,IV and Ru2
IV,IV could be observed with this complex.
15
2.2 Dinuclear Ruthenium Complexes Covalently Linked to Ru(bpy)32+ (Paper II
and Supplementary Information)
NN
N NO
O NH
COOEt
RuRu
N
NN
NRu
OONN
NN
OO
·4PF6
R
R
R
R
NN
N NOH
O NH
COOEt
N
NN
NRu
NN
NN
·2PF6
R
R
R
R
Chart 2.2
5: R=H6: R=COOEt
7: R=H8: R=COOMe8a R=COOH
N
N
N NOH
O
HN
NN
NN
Ru
NN
OH HO
·2PF6
N
N
N NO
O
HN
RuRu
NN
NN
Ru
OONN
OO
O O
·3PF6
9 10
This part describes the complexes where the dinuclear ruthenium complexes have
been covalently linked to ruthenium trisbipydine photosensitizers. The properties of
the trinuclear complexes were studied and compared with those of the corresponding
16
manganese complexes. The structures of the free ligands containing Ru(bpy)32+ and
the trinuclear ruthenium supramolecular complexes are shown in Chart 2.2.
OH
NH-BocCOOEt
DPA N NOH
NN
NN
NH-BocCOOEt
CF3COOH N NOH
NN
NN
NH2
COOEt
OH
CH2
ClCl
NO O RTCH2Cl2, RT
OH
NH2
NN
N N
OH OH
OH
CH2
NN
N N
OH OH
NO O
HN
N HO
NH
NN
Scheme 2.2
11 12 13
17 19
N2H4/EtOH
18
OH
CH2
ClCl
NO O
17
OH
CH2NO O
16
OH
CH2
OO
NO O
HH
BPA
OH
HO
1) NaBH3CN / ZnCl22) SOCl2
NN
NN
CF3COOH
1514
71%
86% 89%
80% 82%
55% 83%
2.2.1 Synthesis and Characterization
The preparations of the trinuclear ruthenium complexes are shown in Schemes 2.2
and 2.3. Compound 13 was prepared by the Mannich reaction between
di(pyridylmethyl)amine (DPA) and the tert-butoxycarbonyl(Boc)-protected L-tyrosine
ester (11), followed by deprotection of the amino group.15,52f 19 was prepared starting
from commercially available 4-hydroxybenzyl alcohol (14). 2-(4-Hydroxy-benzyl)-
isoindole-1,3-dione (15), obtained by several steps from 14, was formylated via Duff
reaction to afford the diformylated phenol 16.52i 16 was then reduced to the
corresponding alcohol with NaBH3CN in the presence of ZnCl2 followed by
17
chlorination with SOCl2 to give the dichloro compound 17.52i Alkylation of 17 with
BPA, followed by cleavage of the resulting phthalimide 18 with hydrazine at room
temperature, gave compound 19.
HOOC COOH
N N
EtOOC COOEt
N N
Cl
COOEt
COOEt
EtOOC
EtOOCCl
NN
NN
RuRuCl3
21202) EtOH
1) SOCl2
2294%
18%
N N
OOH
23 N
NO
OHNN
N
N
Ru
EtOOC
EtOOC
EtOOC COOEt
2PF6
24
55%
SOCl2
N NOH
NH2
COOEt
NN
NN
13
N
NO
OHNN
N
N
Ru
R
R
R R
2PF6
25: R=H24: R=COOEt
5: R=H (70%)6: R=COOEt (63%)
N N
OOH
N
NN
N Ru
2PF6
SOCl2 19 9
OH
NH2
NN
N N
OH OH
25
67%
Ru(DMSO)4Cl2
NaOAc / MeOH
7 (66%)8 (47%)10 (63%)
569
4 eq. NaOH
Acetone, reflux8 8a
Scheme 2.3
Ru(4,4'-di-COOEt-bpy)2(4'-Me-4-COOH-bpy)(2PF6) (24) was prepared by the
18
reaction of 4'-Me-4-COOH-bpy (23)64 with Ru(4,4'-di-COOEt-2,2'-bpy)2Cl2 (22) that
was obtained by refluxing 4,4'-di-COOEt-2,2'-bpy (21) and RuCl3 in DMF. 21 was
prepared from 4',4-diCOOH-bpy (20)65 by chlorination with SOCl2 followed by
reaction with EtOH. Ru(bpy)2(4'-Me-4-COOH-bpy)(2PF6) (25)64 and Ru(4,4'-di-
COOEt-bpy)2(4'-Me-4-COOH-bpy)(2PF6) (24) were first chlorinated with thionyl
chloride and then reacted with 13 to afford 5 and 6, respectively. Ligand complexes 5
and 6 were refluxed with Ru(DMSO)4Cl2 in methanol in the presence of NaOAc,
followed by addition of NH4PF6, to afford the trinuclear ruthenium complexes 7 and
8, respectively. Complex 10 was made in a similar way starting from 25 and 19. EPR,
ESI-MS and elemental analysis show that both complexes 7 and 8 contain a
Ru2(II,III) moiety and a Ru(bpy)32+ moiety while 10 has a Ru2(III,III) moiety and a
Ru(bpy)32+ moiety. Complex 8a was obtained without further purification by
hydrolysis of 8 in acetone with NaOH.
2.2.2 Properties of the Complexes
A
λ / nm
200 300 400 500 600 700 800
Abs
(a.u
.)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
λ / nm
200 300 400 500 600 700 800
ε / 1
04 M-1
cm
-1
0
2
4
6
8
10
12
14
16
B
Figure 2.3 Absorption spectra of (A) 7 and (B) 10 in acetonitrile.
UV-Vis. The spectra of 7 and 10 are basically superpositions of those from the
dinuclear ruthenium moiety and the Ru(bpy)32+ unit66. There is a broad low-energy
MLCT band with a maximum for 7 (Fig. 2.3) at 453 nm in the visible region and two
π→π* transition absorption maxima at 289 nm and 247 nm in the UV region. For 10
the corresponding maxima (Fig. 2.3) are found at 457 nm in the visible region, and
19
289 nm and 245 nm in the UV region.
Electrochemistry. Cyclic voltammograms of 7 and 10 are presented in Figure 2.4.
The CVs of 7 and 10 consist of the waves due to the [Ru(bpy)3]2+ moiety66 and the
waves related to the Ru2(II,III) moiety. Data for the redox processes in 7 and 10 are
compiled in Table 2.2. In comparison with the data of 1 and 2, the redox potentials for
oxidations and reductions of the dinuclear ruthenium moieties in 7 and 10 are shifted
to less cathodic potentials. For instance, the Ru2II,III redox potential for the dinuclear
cluster in 7 is found at higher potential than in 1 ( +0.495 and +0.470, respectively).
The difference for the same redox process in 10 and 2 is even higher (0.103 V). The
reason for this difference could be the positive charge on the [Ru(bpy)3]2+ moiety.
E / V vs SCE-2 -1 0 1
ia
ic
20 µA
(a)
(b)
Figure 2.4. Cyclic voltammograms (ν= 100 mV s-1) of (a) 7 (1 mM) and (b) 10 (1 mM) in acetoniltrile with 0.1 M [N(nC4H9)4]PF6 as supporting electrolyte.
20
Table 2.2 Electrochemical data of complexes 7 and 10.
E1/2 [V][b] (∆Ep [mV])[c]
Comp. Ru(bpy)30/- Ru(bpy)3
+/0 Ru(bpy)32+/+ Ru2
II,III/II,II Ru2III,III/II,III X/X-• Ru2
III,IV/III,III Ru2IV,IV/III,IV Ru(bpy)3
3+/2+
7[a] -1.723
(74)
-1.589
(72)
-1.196
(62)
-0.191
(62)
0.495
(69) -
- - 1.275
(72)
10[a] -1.721
(67)
-1.474
(101)
-1.269
(60)
-1.037
(58)
-0.764
(65)
-0.593
(64)
0.774
(139)
1.019
(129)
1.274
(78)
[a] As PF6– salt. [b] Versus SCE in CH3CN solution with 0.1 M [N(nC4H9)4]PF6 as supporting
electrolyte, ±0.02V. [c] ν= 100 mVs-1.
Lifetimes and quenching of the excited state of Ru(bpy)32+ moieties. The excited
state lifetime of the complexes is a crucial property related to the possibility of
oxidative quenching of the Ru(bpy)32+ excited state by an external electron acceptor.
Unfortunately all the trinuclear ruthenium complexes 7, 8 and 10 have very short
lifetimes (Table 2.3).
Table 2.3 Emission Lifetimes of complexes 7, 8 and 10.
Complexes 7 8 8a 10
τ / ns
(rel. amplitudes)
0.15 (74%)
1.2 (15%)
0.4 (75%)
3.0 (27%)
<0.05 (43%)
0.5 (22%)
0.4 (52%)
1.5 (41%)
The lifetimes of these Ru-Ru2 complexes are substantially shorter than those of the
corresponding Ru-Mn2 complexes. For example, the corresponding dinuclear
manganese complexes of ligand 5 and a ligand similar to 9 but without the tert-butyl
groups, have lifetimes of 110 ns and 2 ns, respectively,52b,52i while the lifetimes of the
corresponding ruthenium complexes 7 and 10 are much shorter (Table 2.3). The
reason for the substantial difference in lifetimes of the excited states of trinuclear
ruthenium complexes and the corresponding manganese complexes is not clear.
Perhaps different quenching mechanisms are involved.
To elucidate the mechanisms responsible for this difference, two types of
21
quenching mechanisms have to be considered: electron transfer (ET) and energy
transfer (EnT) quenching. Studies by means of transient absorption spectroscopy
show that the quenching of the Ru(bpy)32+ excited state in the investigated complexes
occur by different mechanisms, depending on the oxidation state of the Ru2 moiety.
For the Ru2II,III state in complexes 7, 8 and 8a, the dominating quenching mechanism
is either exchange (Dexter type)67 EnT or oxidative ET from the excited state of the
Ru(bpy)32+ to the Ru2 unit, but we could not discriminate between them; the minor
quenching mechanism, which accounts only for ca. 15-25% of the total quenching
reaction in 8 and 2-3% in 8a, is a reductive quenching.
Only the minor, reductive quenching generated detectable products, Ru(bpy)3+ and
Ru2III,III. In the case of 8, reductive quenching is strongest, and electron transfer occurs
with a time constant of ~350 ps and the lifetime of the charge-separated state is
~1.6 ns (Fig 2.5B). Similar behaviour was observed for the case of 8a, although the
efficiency of electron transfer was less than for 8 (Fig. 2.5C). For 7, however, no
electron transfer product was found and the Ru(bpy)32+ excited state decays with time
constants of 290 and 55 ps (Fig. 2.5A).
0 500 1000 1500 2000
-4
-3
-2
-1
0
∆A (m
OD
)
3
τ1 = 55 ps
τ2 = 290 ps
A
0 500 1000 1500 2000
-4
-3
-2
-1
0
∆A (m
OD
)
3
τ1 = 55 ps
τ2 = 290 ps
A
0 2000 4000 6000 8000
0
1
2
3
4
5
τrise= 350 ps
τdecay=1580 ps
∆A (m
OD
)
3a
B
0 2000 4000 6000 8000
0
1
2
3
4
5
τrise= 350 ps
τdecay=1580 ps
∆A (m
OD
)
3a
B
0 2000 4000 6000 8000-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
∆A (m
OD
)
3b
τrise= 250 ps
τdecay= 1490 ps
Time (ps)
C
0 2000 4000 6000 8000-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
∆A (m
OD
)
3b
τrise= 250 ps
τdecay= 1490 ps
Time (ps)
C
Figure 2.5. Kinetics (A) indicating no electron transfer product was formed for 7. Kinetics
(B) and (c) recorded at 530 nm showing formation and decay of the electron transfer products
for 8 and 8a, respectively.
The quenching mechanism for complex 10 is probably similar to that for 7.
One possible way to reduce quenching by both exchange EnT and oxidative ET is
that the bridging bipyridine should be designed without electron-withdrawing groups,
so that the MLCT state is more strongly localized on the non-bridging bipyridines. In
this case, the excited state lifetime may be long enough for the desired photooxidation
22
of the Ru(bpy)32+ by an external acceptor, as we have observed in our previous
studies.
2.2.3 Conclusions
The Ru(bpy)32+ excited state in all trinuclear complexes has very short lifetime due
to strong quenching by the dinuclear Ru2 moiety. This makes it difficult to observe the
desired electron transfer in solution to an external acceptor such as methyl viologen.
However, by attachment of the complexes to the semiconductor TiO2 as electron
acceptors, this problem could be overcome (see next chapter). Since the dinuclear
ruthenium complexes are more stable and easier to handle than the corresponding
manganese complexes, they may offer an alternative as electron donors, at least in
dye-sensitized solar cells.
23
3
Photoinduced Electron Transfer in Donor-Sensitizer-Acceptor Systems
In order to build a supermolecule aiming for artificial PSII or dye-sensitized solar
cells, the system should consist of an additional electron acceptor that could be either
internal or external. Methylviologen (MV2+) and [Co(NH3)5Cl]2+ can be used as
external electron acceptors in some cases.52 However, the use of an external
(sacrificial) electron acceptor has the disadvantage of diffusion controlled electron
transfer rate from the excited state of the sensitizer to the external electron acceptor.68
Under certain conditions, for example, if there are fast competing processes such as
energy transfer or inverse electron transfer to the electron donor, electron transfer
from the excited state of the sensitizer to the electron acceptor could be less efficient
or even non-existent (see previous section about quenching). In such a case, the use of
a nanocrystalline TiO2 semiconductor as an electron acceptor can favour the desired
electron transfer from the internal donor to the oxidised sensitizer to generate efficient
and long-lived charge separation.
In this section, photoinduced electron transfer in various donor-sensitizer systems
was studied in the presence of electron acceptors. Tyrosine and its derivatives or
dinuclear ruthenium complexes were used as electron donors; modified Ru(bpy)32+
complexes were used as photosensitizers. Depending on the properties of the
supramolecules, either external electron acceptors such as MV2+ and [Co(NH3)5Cl]2+
or internal acceptors, such as nano-crystalline TiO2, were used. The use of
semiconductors makes it possible to assemble supermolecules for further application
in devices such as dye-sensitized solar cells.
3.1 Tyrosine-Ru(bpy)32+ Anchored to TiO2 in Colloid Solution (Paper III)
TyrZ is believed to play a crucial role in the process of photosynthetic water
oxidation, and has been extensively studied in artificial PSII models.49,68 Our previous
24
studies showed that, in the presence of an external electron acceptor (methylviologen,
MV2+), a model complex Ru(II)(bpy)2(4-Me-4'-CONH-L-tyrosine ethyl ester-2,2'-
bpy)•2PF6 can mimic the TyrZ-P680 functional units in PSII.15,52c,52e,49,68 A tyrosyl
radical is formed after intramolecular electron transfer from the tyrosine moiety to the
photogenerated Ru(III), and it can oxidize a dinuclear manganese cluster.52e Here we
will use TiO2 as acceptors and measure the true rate of electron transfer.
A new complex 26 was synthesized together with the reference complex 26a
(Chart 3.1), which can be attached to nanocrystalline TiO2 via four carboxylic acid
groups. Multistep electron transfer rates in these systems have been determined with
time-resolved transient absorption spectroscopy.
N N
N
N
N
N
Ru
OCOOEt
HNHO
COOH
HOOC COOH
HOOC
N N
N
N
N
N
Ru
OCOOEt
HN
R
R R
R
Chart 3.1
26 26a R=COOH
3.1.1 Synthesis and Sample Preparation
The synthesis of 26 (Scheme 3.1) started with the ligand 4-methyl-4'-carboxy-2,2'-
bipyridine (23)64. Conversion of the carboxylic acid 23 into the acid chloride,
followed by the reaction with L-tyrosine ethyl ester hydrochloride in acetonitrile
solution in the presence of triethyl amine as base, led to the formation of 27. Ligand
27 was then subjected to the coordination reaction with ruthenium (II) (4,4'-di-
COOEt-2,2'-bpy)2Cl2 (22) to afford 28. The complex with the carboxylic acid groups
is usually difficult to purify. Therefore, purification was performed in its ester form 28
by normal column chromatography. No attempts of further purification were made
after the hydrolysis of 28 to give the final product 26. The reference complex 26a was
synthesized in a route similar to that for 26, using L-alanine ethyl ester hydrochloride
as the starting material instead of L-tyrosine ethyl ester hydrochloride. Both
25
complexes 28 and 28a were characterized by 1H NMR and electrospray ionization
mass spectrometry (ESI-MS).
N N
OCl
COOEt
HNHO
N
COOEtEtOOC
EtOOC COOEt
N
N
N
N
N
Ru
O
N N
OHO
COOEt
HNHO
N N
OCOOEt
NH2HO
COOEt
HN
N
COOEtEtOOC
EtOOC COOEt
N
N
N
N
N
Ru
O
COOEt
HN
N N
OCOOEt
NH2
Cl
COOEtEtOOC
EtOOC COOEt
Cl
N
N
N
N
Ru
SOCl2
HCl
HCl
2PF6 2PF6
26
27
23
28a
27a
28
26a
22
4 eq. NaOH 4 eq. NaOH
27 27a
Scheme 3.1
86% 60%
MeCN, NEt3
MeCN, NEt3
34%
69%
Nanocrystalline colloidal TiO2 particles were prepared by a controlled hydrolysis
of TiCl4.69 The adsorption of the dye molecules to the TiO2 surface is a result of
strong electrostatic interaction between the dye and TiO2. The sample solution was
prepared by adding TiO2 colloidal solution to freshly prepared solutions of 26 or 26a.
26
3.1.2 Photophysical Properties and Photoinduced Electron Transfer
Photophysical properties. The absorption spectra of 26 and 26a exhibit the
characteristic band due to Ru(bpy)32+.32,66 The intense ligand-centered band (LC) (π to
π* transition) and the metal-to-ligand charge transfer (MLCT) (d to π* transition)
appear at 301 and 475 nm, respectively (Figure 3.1). The lowest MLCT excited state
displays an intense emission band at 650 nm at pH 1 and 625 nm at pH 7 (inset in
Figure 3.1).
300 400 500 600 700 800
300 450 600 7500
1
Wavelength [nm]
Inte
nsity
[a.u
]
Figure 3.1 Normalized absorption and emission spectra of complexes 26 and 26a (inset) recorded at pH
7 (-) and pH 1 (···). The emission spectra were excited at 450 nm. The luminescence intensities of 26 and 26a are both decreased with the increase of
the TiO2 concentration (data not shown), meaning that the MLCT excited states are
quenched due to the electron transfer to the semiconductor.
Photoinduced electron transfer. The photoinduced electron transfer processes in 26-
TiO2 are shown in Figure 3.2. Excitation of the MLCT band of Ru(II) promotes an
electron from a Ru d orbital to a π* orbital of the ligand, from which an electron can
be injected into the conduction band of TiO2 and the dye cation Ru(III) is formed.
This dye cation Ru(III) will return to the Ru(II) ground state either by back electron
transfer from TiO2 (charge recombination) or by intramolecular electron transfer from
27
the linked tyrosine moiety, which forms a tyrosyl radical. These two ways for the
recovery of Ru(II) take place on a similar time scale with an average rate of 4.4×105
s-1 and hence it is difficult to clearly separate the two processes.
N N
N
N
N
N
Ru
OCOOEt
HNO
COOH
-OOC COO-
HOOC
N N
N
N
N
N
Ru
OCOOEt
HNHO
COOH
-OOC COO-
HOOC
TiO2 TiO2
hv
e-
e-
e-
- H+
(i)
(ii)
Figure 3.2 Reaction scheme proposed for the photo-induced electron transfer in the 26-TiO2 system.
The formation of tyrosyl radical was confirmed by the appearance of a new
positive band at 410 nm in the transient absorption spectra of adsorbed complex 26 on
TiO2 (Fig. 3.3).70
3 5 0 4 0 0 4 5 0 5 0 0 5 5 0
-8
-6
-4
-2
0
B
∆ A
2 0 µs 4 0 µs 8 0 µs 1 2 0 µs 1 6 0 µs
3 5 0 4 0 0 4 5 0 5 0 0 5 5 0-3
-2
-1
0
1
A
∆A [m
OD
]
2 0 µs 4 0 µs 8 0 µs 1 2 0 µs 1 6 0 µs 3 0 0 p s
Wavelength (nm) Wavelength (nm)
3 5 0 4 0 0 4 5 0 5 0 0 5 5 0
-8
-6
-4
-2
0
B
∆ A
2 0 µs 4 0 µs 8 0 µs 1 2 0 µs 1 6 0 µs
3 5 0 4 0 0 4 5 0 5 0 0 5 5 0-3
-2
-1
0
1
A
∆A [m
OD
]
2 0 µs 4 0 µs 8 0 µs 1 2 0 µs 1 6 0 µs 3 0 0 p s
Wavelength (nm) Wavelength (nm)
Figure 3.3 Transient absorption spectra of adsorbed complexes 26 (A) and 26a (B) at pH 2.4 and 15 µM with 120 µM of TiO2 at different delay times after excitation at 450 nm. In panel (A), the transient absorption spectrum recorded at 300 ps was normalized to allow a direct comparison with the spectra recorded at microsecond delays.
28
The yield of Ru(II)–tyrosyl radical conversion, however, was limited to ca. 15%
due to the fast competing charge recombination between Ru(III) and photo-injected
electrons in the TiO2.
3.1.3 Conclusions
Attachment of complex 26 to nanocrystalline TiO2 results in ultrafast electron
injection from the excited MLCT state into the conduction band of TiO2. This
simplifies the study of the second intramolecular electron transfer because this step is
now rate limiting. The intramolecular electron transfer from the tyrosine moiety to the
Ru(III) occurs on a similar time scale as the charge recombination, and the average
rate constant for these two processes is 4.4×105 s-1 which is greater than that (5×104 s-
1) observed earlier for the Tyr-Ru(bpy)3 system in the presence of MV2+ in
solution.15,49
3.2 Substituted Tyrosine-Ru(bpy)32+ Anchored to TiO2 Films (Paper IV)
In PSII, TyrZ most probably is hydrogen-bonded to a histidine residue, His190 in
the D1 polypeptide.71 The strong hydrogen bonding is believed to aid the electron
transfer.8,9 To mimic this natural process, we prepared complex 5 (Chart 2.3)
containing two N,N-di(2-pyridylmethyl)amine (DPA) arms which can form hydrogen-
bonding with the proton of the phenol group.15,52b,f This modification makes tyrosine
an efficient electron donor and results in fast electron transfer from the tyrosine
moiety to the photo-generated Ru(III) with a rate of at least 100 times greater than that
of the complex without the two DPA arms. Actually this intramolecular electron
transfer is so rapid that the overall rate is limited by the initial quenching of the
excited state of Ru(bpy)32+ by the external electron acceptor.15,52f
In this part, we will employ complex 6 (Chart 2.3) in which four additional ester
groups in two bipyridines make it possible to attach this complex onto the TiO2
surface, to study the photoinduced electron transfer by means of time-resolved
absorption spectroscopy together with EPR spectroscopy. Complex 28a (Scheme 3.1)
29
containing alanine instead of tyrosine is used as a reference.
3.2.1 Sample Preparation
Dye sensitization of the TiO2 film was carried out by soaking the prepared film in
an acetonitrile solution of the dye and incubating at room temperature for about 24 h.
The excess dye was washed off with acetonitrile. The resulting dye-sensitized TiO2
films were studied by time-resolved spectroscopy and EPR.
3.2.2 Photoinduced Electron Transfer
The photoinduced electron transfer process in 6-TiO2 is illustrated in Figure 3.4.
After light excitation, the Ru(II) ground state is converted to its 3MLCT excited state.
An electron is injected into the conduction band of TiO2, generating Ru(III) that is
then reduced to the Ru(II) ground state by intramolecular electron transfer from the
tyrosine moiety and / or by back electron transfer from TiO2.
N N
N
NO
ONH
COOEt
N
NN
N Ru
N
N
N
N
EtOOC COOEt
H
TiO2
O
OEt
O
EtO
I
II
III
IV
e-
hν
KET ~2×106 s-1 e-
e-
N N
N
NO·
ONH
COOEt
N
NN
N Ru
N
N
N
N
EtOOC COOEt
TiO2
O
OEt
O
EtO
e-
H+
Figure 3.4: Proposed photoinduced electron transfer in the 6-TiO2 System. (I) Light irradiation; (II) MLCT; (III) electron injection from the MLCT excited state to the conduction band of TiO2; (IV)
intramolecular ET from the hydrogen-bonded tyrosine moiety to Ru(III).
The transient absorption spectra (Figure 3.5) show that the recovery of the Ru(II)
ground state is much faster in 6-TiO2 than in 28a-TiO2, indicating that the Ru(III) is
30
quickly reduced by an electron from the attached tyrosine-dpa moiety in 6-TiO2.
0 3 6 9
-0.05
0.00
0 10 20 30
-0.02
0.00
400 450 500 550 600
-0.04
-0.02
0.00
-0.04
-0.02
0.00
∆A
Time (µs)
∆A
Time (µs)
B
Wavelength (nm)
A
∆A
Figure 3.5 The time-resolved absorption difference spectra recorded after pulsed light excitation at 450 nm of the dye-sensitized films in 0.1 M LiClO4 acetonitrile. (A), the data for 28a-TiO2 were recorded at 50 ns (□), 500 ns (О), 2 µs (∆), and 20 µs (∇). (B), the data for 6-TiO2 were recorded at 50 ns (□),
200 ns (О), and 2 µs (∆). The insets display the recovery kinetics at 470 nm.
Magnetic Field (G)3450 3460 3470 3480 3490 3500
Am
plitu
de (a
.u.)
Figure 3.6 EPR spectrum recorded during illumination of 6-TiO2 at room temperature. Microwave power: 50 mW; field modulation amplitude: 3 G; time constant: 20 ms.
31
Although no spectral features around 410 nm due to the tyrosyl radical70 were
detected here, the formation of tyrosyl radical is confirmed by EPR study.
Illumination of the powder sample of 6-TiO2 in acetonitrile generated a weak EPR
signal (Figure 3.6) that originates from a deprotonated phenoxy radical produced by
intramolecular ET reaction in 6-TiO2.
3.2.3 Conclusions
Attachment of 6 onto the nanocrystalline TiO2 film leads to ultrafast light-induced
electron injection from the MLCT state of 6 to the conduction band of TiO2. The
photogenerated Ru(III) is then reduced by intramolecular electron transfer from
tyrosine with KET ~2×106 s-1, moving the positive holes further away from the surface
of TiO2. The electron transfer efficiency is as high as 90%. The intramolecular
hydrogen bonding between the phenolic hydroxyl group and the dpa arms in 6 is
believed to be the reason for this efficient and fast electron transfer. This
supramolecular system can be used not only in artificial PSII to mimic the donor side,
but also in dye-sensitized solar cells to prohibit charge recombination and transfer the
hole to the redox mediator.
3.3 Polyphenolate-Ru(bpy)3 in the Presence of External Acceptor (Paper V)
As seen in previous section, the polyphenolate ligands can stabilize higher
oxidation states of multinuclear manganese or ruthenium complexes compared with
the BPMP ligand. This is of interest for water oxidation. In this part, electron transfer
from the phenolate ligands to the photogenerated Ru(III) in the ployphenolate-
Ru(bpy)32+ supermolecules 9 (Chart 2.3) and 29 (Chart 3.2) is studied. Since these
phenolates do not quench the Ru(bpy)32+ excited state (see below), the long-lived
Ru(bpy)32+ 3MLCT state could facilitate the desired oxidative quenching reaction by
external acceptors.
32
N N
ONH
N
N
N
NOH
N
NN
N Ru
OH
OH
·2PF6
29
COOEt
COOEtEtOOC
EtOOC
Chart 3.2
3.3.1 Synthesis and Properties
Synthesis. The preparation of complex 9 was described in section 2.3. The synthesis
of 29 is similar to that for 9, as shown in Scheme 3.3. Chlorination of the carboxylic
acid in 24 with SOCl2 gave the acid chloride which further reacted with the amino
compound 19 to afford 29.
N
NO
OHNN
N
N
Ru
EtOOC
EtOOC
EtOOC COOEt
2PF6
Scheme 3.3
24
SOCl2 19 29
OH
NH2
NN
N N
OH OH
53%
Photophysical properties. The lifetimes of the 3MLCT state emissions of 9 and 29 in
acetonitrile solution are 1.42 µs and 1.23 µs, respectively, which are long enough to
facilitate the desired oxidative quenching reaction by an external acceptor.
3.3.2 Photoinduced Electron Transfer
33
Photoinduced electron transfer was studied by time-resolved absorption
spectroscopy. After excitation of 9 or 29 in acetonitrile in the presence of an external
electron acceptor MV2+, the electron transfer processes could be described as follows:
*Ru(bpy)32+ - Ph-OH + MV2+ → Ru(bpy)3
3+ - Ph-OH + MV+• (1)
Ru(bpy)33+ - Ph-OH → Ru(bpy)3
2+ - Ph-O• + H+ (2)
Ru(bpy)32+ - Ph-O• + H+ + MV+• → Ru(bpy)3
2+ - Ph-OH + MV2+ (3)
The phenol radical signal could not be clearly seen in the transient absorption at
410 nm since it overlaps with the strong absorption of the long-lived MV+• radical at
390 nm. However the formation of the phenol radical Ph-O• was confirmed by a
separate experiment using SnO2 as electron acceptor (data not shown).
An important message obtained from the transient absorption spectra of 9-MV2+ and
29-MV2+ (data not shown) is that in the case of 9, electron transfer from the phenols
to the photogenerated Ru(III) is slower than the quenching of the Ru(II) excited state
by MV2+ while in the case of 29, electron transfer from the phenols to the
photogenerated Ru(III) is fasterer than the quenching of the Ru(II) excited state by
MV2+. Thus intramolecular electron transfer from the phenols to the photogenerated
Ru(III) (reaction 2) is the rate-limited step for the 9-MV2+ system whereas the
diffusion controlled bimolecular reaction (reaction 1) is rate-limiting for the 29-MV2+
system and all the kinetics are driven by this step.
3.3.3 Conclusions
We have demonstrated that in the presence of MV2+ as external electron acceptor,
intramolecular electron transfer from the phenol moiety to the photogenerated Ru(III)
in the two complexes 9 and 29 occurs at rates of 3.8 × 106 s-1 and >1.7 × 107 s-1,
respectively, which is two orders of magnitude faster than the rate observed for the
Ru(bpy)3-Tyr complex. The driving force for this dramatic increase in electron
transfer rates is probably the introduction of BPA arms which can form hydrogen
bonding with the phenols.
34
3.4 Ru2-Ru(bpy)3 Anchored to TiO2 (Paper II)
As discussed in section 2.3, due to strong quenching by the dinuclear ruthenium
moiety, the lifetimes of the Ru(bpy)32+ excited states in the trinuclear ruthenium
complexes are too short to initiate the desired photoinduced electron transfer in the
presence of external electron acceptors. Here crystalline TiO2 is used as an electron
acceptor to efficiently compete with the quenching by the diruthenium moiety and
establish the desired photoinduced electron transfer in complex 8 (see Chart 2.3).
3.4.1 Photoinduced Electron Transfer
The electron transfer was studied by transient absorption spectroscopy. Figure 3.7
shows the transient absorption spectra of 8-TiO2 film (Ru2II,III-RuII-TiO2) after
excitation. Although no spectral features resembling absorption spectra of the
expected product (Ru2III,III) are observed at 200 ps, the absorption at 300 ns is
completely different. A new transient absorption band around 600 nm fits well to the
known absorption of the Ru2III,III moiety (see Paper I) which has a long lifetime of ~1
ms. This is the obvious evidence that the Ru2II,III moiety is oxidized by the
photogenerated Ru(bpy)33+.
500 550 600 650 700
0
0 200 400 600 800
0
200 ps 300 ns
∆A (a
.u.)
Wavelength (nm)
Time (µs)
Figure 3.7 Transient absorption spectra of 8-TiO2. Femtosecond excitation at 490 nm was used to obtain the transient spectrum at 200 ps, while 7 ns excitation pulses centered at 480 nm were used for measurements at nanosecond time scale. The inset shows decay of the product monitored at 600 nm.
35
Therefore a reaction scheme is proposed for the formation of Ru2III,III:
Ru2II,III-RuIII-TiO2(e-) Ru2
III,III-RuII-TiO2(e-)Ru2II,III-RuII-TiO2
h νe- e-
The rate constant for the second electron transfer from Ru2II,III to Ru(bpy)3
3+ lies in
the interval 109 s-1 > k > 107 s-1. However, the yield of the fully charge separated state
is less than 10% due to the poor efficiency of the initial electron injection.
3.4.2 Conclusions
The trinuclear ruthenium complex with ester groups can be anchored to TiO2, and
electron injection from the Ru(bpy)32+ excited state to semiconductor TiO2 can occur.
The photogenerated Ru(bpy)33+ can oxidize the dinuclear ruthenium complex from the
Ru2II,III state to the Ru2
III,III state by intramolecular electron transfer. These properties
make the Ru2II,III-RuII-TiO2 system a promising sensitizer for the Grätzel type solar
cells,22-24 as the fast secondary electron transfer removes the hole far from the TiO2
surface, thereby preventing charge recombination, leading to the millisecond lifetime
of the charge-separated state. Of course, if a proper dinuclear ruthenium complex
could be found, it is also possible to construct an efficient artificial system for water
oxidation with this kind of Ru2-Ru(bpy)3-TiO2 systems.
36
37
4
Photoinduced Electron Transfer in Supermolecules Based
on Carotenoid-TiO2 Carotenoids have been studied in artificial systems to mimic either antenna
complexes or reaction centers, due to their ability to act as light-harvesting agents in
the photosynthetic antenna pigment-protein complexes, and their potential to serve as
electron donors in some photosynthetic reaction centers.36 In the early 1980s, a
carotenoid covalently linked to a porphyrin molecule was shown to have both antenna
(singlet-singlet energy transfer from carotenoid to porphyrin) and photoprotective
(quenching of the porphyrin triplet state) functions.72 Since then, carotenoid-based
triads or even pentads have been studied and proven to be excellent models for
artificial reaction centers.36 While extensive studies of energy transfer processes
involving carotenoids have been carried out,34,35 the photoinduced electron transfer
from an excited carotenoid molecule is much less investigated. In this section, this
process will be studied in several systems. Because of the short lifetime of the
carotenoid excited states, we used a semiconductor TiO2 as electron acceptor to
successfully compete with intramolecular energy relaxation processes.
4.1 Carotenoid Anchored to TiO2 Nanoparticle (Paper VI)
The terminal carboxylate group of 8'-apo-β-caroten-8'-oic-acid (30, see Chart 4.1)
can be anchored to the TiO2 surface. The resulting system makes it possible to study
the interfacial electron transfer between carotenoid and the TiO2 colloidal
nanoparticles by means of transient absorption spectroscopy.
COOH30
Chart 4.1
38
4.1.1 Synthesis
30 was prepared by oxidation of trans-8'-apo-β-caroten-8'-al with silver oxide as
shown in Scheme 4.1.73 To the solution of carotenal in toluene was added Ag2O
suspended in EtOH containing NaOH. The mixture was stirred at room temperature
overnight, neutralized with 4N HCl, and then extracted with diethyl ether. After ether
was removed, the crude product was run column on Al2O3 using diethyl ether as
eluent to remove unreacted starting material first, and then 10% acetic acid in diethyl
ether to elute the product. Recrystallization from a mixture of pentane and diethyl
ether gave the desired pure product 30.
CHO Ag2O
trans-8'-apo-β-caroten-8'-al
30
Scheme 4.1
78%
The TiO2 powder was prepared and tested as described earlier. To form a colloidal
TiO2 solution, a suspension of 0.8 g/L TiO2 was prepared by dissolving the desired
amount of TiO2 powder into a mixture of ethanol and water (97% EtOH). Before
experiments, the ethanol solution of 30 was added to the TiO2 colloidal solution, and
the mixture was degassed by nitrogen prior to measurements.
4.1.2 Properties and Photoinduced Electron Transfer The photophysical properties of 30 bound to TiO2 and photoinduced electron
transfer between 30 and TiO2 are shown in Figure 4.1, by using a simplified energy
level diagram.
39
}}
E (V vs. SCE)
TiO2
e-
S2
S1
T1
S0
1
23
4
5
6
7
8
9
30
VB
CB
+2.7
0
-0.5
Kinj = 1/360 fs-1
τ = 7.3 µs
τ = 18 ps
Figure 4.1. Schematic energy level diagram showing electron-transfer processes between 30 and the TiO2 particle. The different processes are indicated as follows: (1) photoexcitation; (2) electron injection; (3) electron relaxation and trapping within the CB (<100 fs); (4) trapping/detrapping of the electron in states below the CB (>1 ns); (5) electron recombination to S0; (6 and 7) internal conversion from S2 to S1 and S1 to S0, respectively; (8 and 9) electron recombination to S0 via T1.
After excitation (pathway 1), electron injection from the carotenoid excited state
into the conduction band of TiO2 (pathway 2) occurs, forming the long-lived
carotenoid radical (30+•) which has a strong absorption band with a maximum at ~854
nm in the transient absorption spectra.
Interestingly electron injection is from the initially excited S2 state other than the
S1 state, and the rate constant of kinj is 1/360 fs-1.
4.1.3 Conclusions
When 30 is bound to the surface of TiO2, 40% of the excited S2 state injects
electrons into the conduction band of the semiconductor on a time scale of a few
hundreds of femtoseconds while the rest undergoes competitive internal conversion to
the S1 state which does not inject electrons but relaxes to the ground state. The cation
radical 30+• recombines with conduction band electrons to regenerate the ground state
40
of 30.
4.2 Carotenoid and Pheophytin Assembled on TiO2 Surface (Paper VII)
Under certain condition, carotenoid 30 (Chart 4.1) and pheophytin a (31, Chart
4.2) can self-assemble into a supramolecular system on the surface of nanocrystalline
TiO2. Such a system makes it possible to study the energy / electron transfer between
carotenoid and pheophytin.
NN
HN
NH
OO
OCH3O
O
Chart 4.2
31
4.2.1 Sample Preparation.
The synthesis of the carotenoic acid 30 was described previously. Pheophytin a was
obtained by treating chlorophyll a with dilute HCl to remove the central magnesium.74
Carotenoid 30 is used to achieve efficient attachment to the TiO2 surface.36 The
proposed self-assembled system by 30 and 31 on the surface of nanocrystalline TiO2
is shown in Scheme 4.2. The molar ratio of 30 and 31 self-assembled on TiO2 film is
estimated to be approximately 8.6:1. Interestingly 31 can not be attached to TiO2
without 30, probably due to its weak interaction with the hydrophilic oxide surface.74
41
COO-
OO
COO- COO-
OO
COO- COO- COO- COO- COO-
TiO2 Scheme 4.2. Proposed self-assembled system of 30 and 31 on the surface of nanocrystalline TiO2.
4.2.2. Photoinduced Electron Transfer
Excitation of the carotenoid moiety generates the long-lived carotenoid radical
cation (30•+) that has absorption with a maximum at 860 nm (data not shown). This
radical is formed by electron injection from the carotenoid S2 state into the conduction
band of TiO2, as discussed in section 4.1.
Excitation of the pheophytin moiety also produces the radical cation 30+•
immediately, giving rise to a strong absorption at 850 nm (Figure 4.2). However this
30+• is generated as a result of reductive quenching of 131 by 30, forming a charge-
separated state (31•--30+•-TiO2). The interesting thing is that no bleaching of
pheophytin is observed on a longer time scale, probably because 31-• injects an
electron into the TiO2 conduction band through the carotenoid layer.
42
500 600 700 800 900 1000
-0.02
-0.01
0.00
0.01
0 1 2 3 4 5
-1
0
1
∆A [O
D]
Wavelength [nm]
520 nm
850 nm
∆A (N
orm
aliz
ed)
Time [µs]
Figure 4.2 Time-resolved absorption difference spectra recorded after pulsed laser excitation of the
deoxygenated 30-31 film at 670 nm: 0.1 µs (■), 0.5 µs (●), 5 µs (▲). Inset: Kinetic traces at selected wavelengths after 670-nm laser light excitation.
4.2.3 Conclusions
In a self-assembled carotenoid-pheophytin system, a carotenoid can reductively
quench the pheophytin moiety efficiently, to form a long-lived charge-separated state.
Such a "self-assembling" strategy may be also applied in dye-sensitized solar cells
and other artificial systems related to electron transfer.
43
5
Concluding Remarks
Water oxidation to oxygen by light and direct conversion of sunlight to electricity
by solar cells are two of the most promising ways for scientists to find alternative
energy sources. This thesis describes several donor-sensitizer-acceptor
supramolecular systems that could be applied in these fields.
Electron donors based on tyrosine or phenol derivatives and their metal complexes
are used to mimic the donor side of PS II. Since nature employs the manganese cluster
to catalyze water oxidation, we have no doubt that water oxidation could be achieved
by an artificial system based on highly active manganese complexes. However, as an
alternative way, we could also search for other metal complexes as electron donors
and water oxidants. With this aim, we prepared two dinuclear ruthenium complexes
and covalently linked them to the photosensitizer Ru(bpy)32+. In spite of the short
lifetime of the photosensitier due to quenching by Ru2 moieties, we managed to
achieve the desired intramolecular electron transfer from the Ru2 to the
photogenerated Ru(bpy)33+ by using nanocrystalline TiO2 as electron acceptors.
Although it is far away, this approach is a promising starting point for the
development of an entirely ruthenium-based system for mimicking the donor side
reaction of PS II. Such a system could be also used in the dye-sensitized solar cells to
prohibit the charge recombination and to achieve a long-lived charge separated state.
However we should consider how to minimize the quenching between the Ru2 and the
sensitizer, for example, by designing the bridging bipyridine ligand without electron-
withdrawing groups.
Studies on the systems based on carotenoids provided a possibility to employ
carotenoids as efficient electron donors for artificial systems.
To design artificial systems which are capable of converting sunlight into fuel or
electricity is an exciting but difficult task that probably requires massive research
efforts. Our results bring the attempts some way, and hopefully other groups will be
inspired to join in.
44
45
6
Supplementary Information Synthesis of dinuclear ruthenium(III,III) complex 2. The appropriate ligand 461
(157 mg, 0.2 mmol) was dissolved in 8 mL of methanol, the solution was degassed
and NaOAc (150 mg, 1.8 mmol) and Ru(DMSO)4Cl2 (196 mg, 0.4 mmol) were
added. The yellow solution was refluxed over night under nitrogen in the dark. The
resulting red-brown solution was cooled to room temperature and a saturated aqueous
solution of NaClO4 (1 mL) was added to precipitate the complex as ClO4- salt. A
dark-green preciptate was formed, which was collected by filtration, washed with
water and ether and dried in vacuum to give 195 mg (81 %) of pure complex 2. ESI-
MS (m/z): 1103.4 (calcd. for [M-ClO4-], 1103.4). Anal. Calcd for
C55H71Cl1N4O11Ru2•6H2O (%): C, 50.43; H, 6.39; N, 4.28; Ru, 15.43. Found: C,
50.48; H, 6.44; N, 4.09; Ru, 15.61.
Synthesis of trinuclear ruthenium(II,III,III) complex 10. To a solution of 9 (157
mg, 0.092 mmol) in MeOH (15 mL) were added cis-Ru(DMSO)4Cl2 (90 mg, 0.186
mmol) and NaOAc (102 mg, 1.24 mmol). The mixture was refluxed for 20 h under N2
in the dark. A saturated solution of NH4PF6 (1 mL) was added to the resulting red-
brown solution to precipitate the complex as the PF6- salt. The dark green crystal was
filtered, washed with water and dried. Yield: 125 mg (63%). ESI-MS (m/z): 2018.4
(calcd. for [M-PF6-], 2018.4) and 936.7 (calcd. for [M-2PF6
-], 936.7). Anal. Calcd for
C87H96N11O8P3Ru3•NH4PF6 (%): C, 44.95; H, 4.34; N, 7.23. Found: C, 45.07; H,
4.60; N, 7.16.
46
47
Acknowledgements
First of all, I would like to thank my supervisors Licheng Sun and Björn Åkermark
for accepting me as a graduate student and for all the support and encouragement you
have given me.
All members in our group (former and present): Anh Johansson, Olof Johansson,
Jacob Fryxelius, Henritte Wolpher, Magnus Anderlund, Josefin Utas, Hoa Tran,
Jesper Ekström, Hanna Jonsson, Lennart Schwartz, and Sasha Ott, Xiaojun Peng,
Xichuan Yang, Xiaobing Zhang, Shiguo Sun, Susan Schofer, Ferenc Korodi and
Sabolcsz Salyi.
All people at the Department of Organic Chemistry.
All members (former and present) of the Consortium for Artificial Photosynthesis,
especially Stenbjörn Styring, Villy Sundström, Leif Hammarström, Tomas Polivka,
Reiner Lomoth, Ann Magnuson. Raed Ghanem, Jie Pan, Jingxi Pan, Gerriet Eilers and
other co-authers for insightful help.
Licheng Sun, Björn Åkermark, Jan-Erling Bäckvall and Jacob Fryxelius for
comments on this thesis.
Henritte Wolpher for helps on this thesis.
Leif Hammarström, Tomas Polivka, Reiner Lomoth, Stenbjörn Styring and Villy
Sundström for helps on the preparation of manuscripts.
Jonas Bergquist, Jerker Mårtensson and Mikael Kritikos for measurements.
The Swedish Energy Agency and the Swedish Research Council (VR) for financial
support.
My family for love and support.
48
49
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