Synthesis and Photoinduced Electron Transfer of Donor-Sensitizer-Acceptor Systems

Synthesis and Photoinduced Electron Transfer of

Donor-Sensitizer-Acceptor Systems

Yunhua Xu

Stockholm University 2005

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.

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 2+ excitation of the Ru(bpy)3 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.

Synthesis and Photoinduced Electron Transfer of

Donor-Sensitizer-Acceptor Systems

Yunhua Xu

Department of Organic Chemistry Stockholm University 2005

Doctoral Dissertation 2005 Department of Organic Chemistry Arrhenius Laboratory Stockholm University Sweden

Abstract

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 2+ excitation of the Ru(bpy)3 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.

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+ 2.2 Dinuclear Ruthenium Complexes Covalently Linked to Ru(bpy)3 ...... 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 2+ 3.1 Tyrosine-Ru(bpy)3 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 2+ 3.2 Substituted Tyrosine-Ru(bpy)3 Anchored to TiO2 Films ...... 28 3.2.1 Sample Preparation ...... 29 3.2.2 Photoinduced Electron Transfer...... 29 3.2.3 Conclusions ...... 31 2+ 3.3 Polyphenolate-Ru(bpy)3 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

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

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.

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

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.

iii iv

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

visible light 1 H2O ⎯⎯→⎯⎯ 2 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

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.

- hν - - e e- e e e- e- + 4H + O2 P Q Q Mn4/Ca TyrZ 680 Phe A B

2H2O 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. 5,8-10 1.1). 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 11,13,14,16,17 intermediate between P680 and the Mn cluster. Babcock et al. even proposed 11,14,18-20 that TyrZ directly participates in the water oxidation chemistry. 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):

+ - 0 2H2O → O2 + 4H + 4e , E (pH=7) = +0.82 V vs. NHE (1.2) - - 0 2H2O + 2e → H2 + 2OH , E (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

e- hν e-

O2 H2 D P A + 2H2O 2H

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

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 4,21 as GaAs, CdTe, Cu2S, Zn3P2, InP and CuInSe2 are suitable for solar cells. 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.

E - e (P+/P*) CB

hν hopping e-

(P+/P) VB

semiconductor dye hole transmitting counterelectrode solid

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

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.

e- hv e-

TiO2 Dye Donor

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

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.

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 2+ acceptors. Acceptors such as [Co(NH3)5Cl] , 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 22,23,25,36,54 TiO2, SnO2 and ZnO are used as solid state acceptors. In this thesis, this kind of acceptors are used in most cases.

Energy

e- CB

hν

TiO2 Dye

Figure 1.5 Energy diagram for dye sensitization of TiO2.

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

Synthesis and Properties of Dinuclear Ruthenium Complexes as Electron Donors

+ 5,8- In PSII, the oxygen-evolving center (OEC) serves as an electron donor to P680 . 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 4+ [(bpy)2(H2O)RuORu(H2O)(bpy)2] 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 38-48,56-58 III IV catalysts. The trinuclear complex [(NH3)5Ru (µ-O)Ru (NH3)4(µ- III 6+ O)Ru (NH3)5] with a large excess of a Ce(IV) oxidant in an aqueous solution 56 III III 4+ induced O2 formation. Dinuclear complexes [(NH3)5Ru (µ-O)Ru (NH3)5] and III II 2+ 57 [(NH3)5Ru (µ-Cl)Ru (NH3)5] showed similar catalytic activities. Some III 3+ mononuclear ruthenium complexes, for examples [Ru (NH3)6] and III 2+ [Ru (NH3)5Cl] , 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

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).

Chart 2.1

N O N N ·2ClO ·ClO N Ru O 4 N Ru O 4 O O O O O O

N Ru O N Ru O N N N O

1 2

2.1.1 Synthesis and Characterization

The synthesis of dinuclear ruthenium complexes 1 and 2 is shown in Scheme 2.1.

Scheme 2.1

DMSO RuCl3·xH2O cis-Ru(DMSO)4Cl2 Reflux

O H N H NaBH N 4 NH2 N N 43% N DPA

H NaBH4 NHO OH 72% NH2 N HN

BPA

DPA N N OH N N Ru(DMSO)4Cl2 1 NaOAc / MeOH CH2O OH N N 55% 3 89%

OH 1) SOCl2 Ru(DMSO) Cl HO OH OH 4 2 N N 2 2) BPA NaOAc /MeOH OH OH N N 69% 81% 4

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

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- 63 Ru(DMSO)4Cl2 was prepared by refluxing RuCl3 in DMSO for 10 min. 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

3 -1 cm

-1 2 M 4 10 / ε 1

0 200 300 400 500 600 700 800 λ / nm Figure 2.1 Absorption spectra of 1 (---) and 2 (⎯) in acetonitrile.

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).

ic (a)

(b)

10 µA

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 E / V vs 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 II,III III,III II,III II,II wave (Ru2 → Ru2 ) and one reversible reduction wave (Ru2 → Ru2 ). In comparison with 1, 2 displayed much richer redox properties. The CV of 2 III,III III,IV III,IV (Fig.2.2(b)) shows two reversible oxidation waves (Ru2 → Ru2 and Ru2 → IV,IV Ru2 ) 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

spectrum of the dinuclear ruthenium itself but generates an organic radical. The other III,III II,III two reduction waves are probably the expected redox processes Ru2 → Ru2 and II,III II,II Ru2 → Ru2 .

Table 2.1 Electrochemical data.

[b] [c] E1/2 [V] (∆Ep [mV])

II,III/II,II III,III/II,III -• III,IV/III,III IV,IV/III,IV Complexes Ru2 Ru2 X/X Ru2 Ru2

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 III,III II,III II,III II,II of the one-phenolate ligands. The potentials for the Ru2 /Ru2 and Ru2 /Ru2 couples of 2 are lower by 0.86 and 1.33 V than those of 1, respectively. In addition, III,IV III,III IV,IV III,IV two more oxidation processes, the Ru2 /Ru2 and Ru2 /Ru2 couples, are 2+ observed with 2 and it should be possible to drive them by photogenerated Ru(bpy)3

(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- IV,IV phenolate ligand, the high oxidation state Ru2 can easily be reached.

2.1.3 Conclusions II,III Complex 1 contains a mixed-valence Ru2 moiety, which can readily undergo reversible a one-electron reduction and a one-electron oxidation, resulting in the II,II III,III III,III Ru2 and Ru2 complexes, respectively. Complex 2 is a Ru2 complex and II,III II,II exhibits even richer electrochemistry. Reversible reduction to Ru2 and Ru2 and III,IV IV,IV oxidation to Ru2 and Ru2 could be observed with this complex.

2+ 2.2 Dinuclear Ruthenium Complexes Covalently Linked to Ru(bpy)3 (Paper II

and Supplementary Information)

Chart2.2

O O O O N N N N Ru Ru

N N OH N N N N O N N

COOEt COOEt

·2PF ONH ONH 6 ·4PF6

R R N N N N N N Ru Ru N N N N N N R R R R

R R

5: R=H 7: R=H 6: R=COOEt 8: R=COOMe 8a R=COOH

O O O O N N N N Ru Ru

HO O OH N OH N O N O N

·2PF 6 HN ·3PF6 HN

O N O N N N N N Ru Ru N N N N N N

910

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 15

2+ manganese complexes. The structures of the free ligands containing Ru(bpy)3 and the trinuclear ruthenium supramolecular complexes are shown in Chart 2.2.

Scheme 2.2

N N N N OH N N N N OH N N N N OH N N H DPA N CF3COOH COOEt 83% NH-Boc 55% COOEt COOEt 11 12 13 NH-Boc NH2

OH H OH H OH N Cl Cl OH O O 71% N N 1) NaBH CN / ZnCl CH N 3 2 2 CH2 CH2 N N 2) SOCl N OOCF3COOH OO 2 OO HO 86% 89% 14 15 16 17

OH NHO NN Cl Cl OH NN HN OH OHN N OH BPA N2H4/EtOH OHN N OH CH2 CH RT OON CH2Cl2, RT 2 OON NH2 82% 80% 17 18 19

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

52i chlorination with SOCl2 to give the dichloro compound 17. Alkylation of 17 with BPA, followed by cleavage of the resulting phthalimide 18 with hydrazine at room temperature, gave compound 19.

Scheme 2.3 COOEt

EtOOC N HOOC COOH EtOOC COOEt N Cl 1) SOCl2 RuCl3 Ru NN N Cl NN 2) EtOH 18% EtOOC N 20 94% 21 22 COOEt O EtOOC OH N N NN 23 O EtOOC N Ru N OH 55% N N 2PF6 EtOOC24 COOEt

N N

R N N OH N N

N N O COOEt R N Ru N OH SOCl NH2 13 N N 2 5: R=H (70%) 2PF6 6: R=COOEt (63%) RR 25: R=H 24: R=COOEt NN O OH OH OHN N OH

2PF6 N N NH2 N Ru N SOCl 19 2 9 N N 67%

5 Ru(DMSO)4Cl2 7 (66%) 4 eq. NaOH 6 8 (47%) 8 8a 9 NaOAc / MeOH 10 (63%) Acetone, reflux

Ru(4,4'-di-COOEt-bpy)2(4'-Me-4-COOH-bpy)(2PF6) (24) was prepared by the

64 reaction of 4'-Me-4-COOH-bpy (23) 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 65 prepared from 4',4-diCOOH-bpy (20) by chlorination with SOCl2 followed by 64 reaction with EtOH. Ru(bpy)2(4'-Me-4-COOH-bpy)(2PF6) (25) 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 2+ Ru2(II,III) moiety and a Ru(bpy)3 moiety while 10 has a Ru2(III,III) moiety and a 2+ Ru(bpy)3 moiety. Complex 8a was obtained without further purification by hydrolysis of 8 in acetone with NaOH.

2.2.2 Properties of the Complexes

16 1.4 14 1.2 12 1.0 -1 10 cm

0.8 -1 8 M 4

Abs (a.u.) 0.6 6 / 10 ε

0.4 4

0.2 2 A B 0.0 0 200 300 400 500 600 700 800 200 300 400 500 600 700 800 λ / nm λ / nm

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 2+ 66 dinuclear ruthenium moiety and the Ru(bpy)3 unit . 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

289 nm and 245 nm in the UV region.

Electrochemistry. Cyclic voltammograms of 7 and 10 are presented in Figure 2.4. 2+ 66 The CVs of 7 and 10 consist of the waves due to the [Ru(bpy)3] moiety 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 II,III to less cathodic potentials. For instance, the Ru2 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 2+ reason for this difference could be the positive charge on the [Ru(bpy)3] moiety.

i (a) c

(b)

20 µA

-2 -1 0 1

E / V vs SCE

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.

Table 2.2 Electrochemical data of complexes 7 and 10.

[b] [c] E1/2 [V] (∆Ep [mV])

0/- +/0 2+/+ II,III/II,II III,III/II,III -• III,IV/III,III IV,IV/III,IV 3+/2+ Comp. Ru(bpy)3 Ru(bpy)3 Ru(bpy)3 Ru2 Ru2 X/X Ru2 Ru2 Ru(bpy)3

7[a] -1.723 -1.589 -1.196 -0.191 0.495 1.275 - - - (74) (72) (62) (62) (69) (72)

10[a] -1.721 -1.474 -1.269 -1.037 -0.764 -0.593 0.774 1.019 1.274

(67) (101) (60) (58) (65) (64) (139) (129) (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.

2+ Lifetimes and quenching of the excited state of Ru(bpy)3 moieties. The excited state lifetime of the complexes is a crucial property related to the possibility of 2+ oxidative quenching of the Ru(bpy)3 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 0.15 (74%) 0.4 (75%) <0.05 (43%) 0.4 (52%)

(rel. amplitudes) 1.2 (15%) 3.0 (27%) 0.5 (22%) 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

quenching mechanisms have to be considered: electron transfer (ET) and energy transfer (EnT) quenching. Studies by means of transient absorption spectroscopy 2+ show that the quenching of the Ru(bpy)3 excited state in the investigated complexes occur by different mechanisms, depending on the oxidation state of the Ru2 moiety. II,III For the Ru2 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 2+ Ru(bpy)3 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 III,III Ru2 . 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 2+ electron transfer product was found and the Ru(bpy)3 excited state decays with time constants of 290 and 55 ps (Fig. 2.5A).

0.1 0 3b 3a 0.0 5 -1 -0.1 τ = 250 ps τ = 55 ps rise 1 A 4 B C τ = 350 ps rise τ = 1490 ps -2 τ = 290 ps A (mOD) -0.2 decay 2 3 τ =1580 ps ∆ decay A (mOD)

A (mOD) -0.3 ∆ ∆ -3 2

-0.4 -4 1 3 -0.5 0 0 2000 4000 6000 8000 0 500 1000 1500 2000 0 2000 4000 6000 8000 Time (ps)

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

2+ of the Ru(bpy)3 by an external acceptor, as we have observed in our previous studies.

2.2.3 Conclusions

2+ The Ru(bpy)3 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.

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 2+ 2+ internal or external. Methylviologen (MV ) and [Co(NH3)5Cl] 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 2+ dinuclear ruthenium complexes were used as electron donors; modified Ru(bpy)3 complexes were used as photosensitizers. Depending on the properties of the 2+ 2+ supramolecules, either external electron acceptors such as MV and [Co(NH3)5Cl]

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.

2+ 3.1 Tyrosine-Ru(bpy)3 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

studies showed that, in the presence of an external electron acceptor (methylviologen, 2+ MV ), a model complex Ru(II)(bpy)2(4-Me-4'-CONH-L-tyrosine ethyl ester-2,2'- 15,52c,52e,49,68 bpy)•2PF6 can mimic the TyrZ-P680 functional units in PSII. 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.

Chart 3.1

COOEt COOEt O O HN HO HN

NN NN R N Ru N R HOOC N Ru N COOH N N N N

R R HOOC COOH 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

complexes 28 and 28a were characterized by 1H NMR and electrospray ionization mass spectrometry (ESI-MS).

Scheme 3.1 O HO COOEt COOEt 23 O NN HO NH2 HCl HO HN

SOCl2 O MeCN, NEt3 34% 27 NN Cl

COOEt COOEt NN O HN NH2 HCl

MeCN, NEt3 69% 27a NN

Cl Cl

EtOOC N Ru N COOEt

N N

EtOOC COOEt 22 27 27a

86% 60% COOEt COOEt O O HO HN HN

2PF6 2PF6 N N N N

EtOOC N Ru N COOEt EtOOC N Ru N COOEt

N N N N

EtOOC 28 COOEt EtOOC 28a COOEt

4 eq. NaOH 4 eq. NaOH

26 26a

Nanocrystalline colloidal TiO2 particles were prepared by a controlled hydrolysis 69 of TiCl4. 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.

3.1.2 Photophysical Properties and Photoinduced Electron Transfer

Photophysical properties. The absorption spectra of 26 and 26a exhibit the 2+ 32,66 characteristic band due to Ru(bpy)3 . 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).

0 300 450 600 750

Intensity [a.u]

300 400 500 600 700 800 Wavelength [nm] 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

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.

COOEt COOEt O hv O HO HN O HN e- (ii) NN NN

HOOC N Ru N COOH HOOC N Ru N COOH - H+ N N N N (i) e- -OOC COO- -OOC COO- e- TiO2 TiO2

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 70 TiO2 (Fig. 3.3).

1 0

0 -2

∆ A

-1 -4

A [mOD] ∆

20 µs -6 -2 40 µs 20 µs 80 µs 40 µs 120 µs -8 80 µs 160 µs A 120 µs B -3 300 ps 160 µs 350 400 450 500 550 350 400 450 500 550 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.

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 2+ ) observed earlier for the Tyr-Ru(bpy)3 system in the presence of MV in solution.15,49

2+ 3.2 Substituted Tyrosine-Ru(bpy)3 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 2+ 15,52f excited state of Ru(bpy)3 by the external electron acceptor.

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)

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 N H+ H N N N N O O· N N IV N N 6 -1 - KET ~2×10 s e O COOEt O COOEt hν NH NH

I N N N N

EtOOC N Ru N COOEt EtOOC N Ru N COOEt

N II N N N e- O III O O O OEt EtO OEt EtO - - TiO2 e TiO2 e

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

quickly reduced by an electron from the attached tyrosine-dpa moiety in 6-TiO2.

0.00

A -0.02 ∆ A ∆

-0.02

0 102030 -0.04 A Time (µs)

0.00

-0.02 0.00 A ∆

-0.05 -0.04 B 0369 Time (µs) 400 450 500 550 600 Wavelength (nm)

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.

Amplitude (a.u.)

3450 3460 3470 3480 3490 3500 Magnetic Field (G)

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.

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 6 -1 tyrosine with KET ~2×10 s , 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- 2+ Ru(bpy)3 supermolecules 9 (Chart 2.3) and 29 (Chart 3.2) is studied. Since these 2+ phenolates do not quench the Ru(bpy)3 excited state (see below), the long-lived 2+ 3 Ru(bpy)3 MLCT state could facilitate the desired oxidative quenching reaction by external acceptors.

Chart 3.2 OH

N O OH NH N

N ·2PF6 NN

EtOOC N Ru N COOEt OH

N N

EtOOC COOEt 29

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.

Scheme 3.3

NN OH EtOOC OHN N OH

N N O EtOOC N Ru N NH2 OH SOCl2 19 N N 29 2PF6 53% EtOOC COOEt 24

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

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:

2+ 2+ 3+ +• *Ru(bpy)3 - Ph-OH + MV → Ru(bpy)3 - Ph-OH + MV (1)

3+ 2+ + Ru(bpy)3 - Ph-OH → Ru(bpy)3 - Ph-O• + H (2)

2+ + +• 2+ 2+ Ru(bpy)3 - Ph-O• + H + MV → Ru(bpy)3 - Ph-OH + MV (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. 33

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 2+ moiety, the lifetimes of the Ru(bpy)3 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 II,III II shows the transient absorption spectra of 8-TiO2 film (Ru2 -Ru -TiO2) after excitation. Although no spectral features resembling absorption spectra of the III,III expected product (Ru2 ) 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 III,III known absorption of the Ru2 moiety (see Paper I) which has a long lifetime of ~1 II,III ms. This is the obvious evidence that the Ru2 moiety is oxidized by the 3+ photogenerated Ru(bpy)3 .

200 ps 300 ns

0 A (a.u.) A ∆

0 200 400 600 800 Time (µs)

500 550 600 650 700 Wavelength (nm)

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.

III,III Therefore a reaction scheme is proposed for the formation of Ru2 :

h ν - e- e

II,III II II,III III - III,III II - Ru2 -Ru -TiO2 Ru2 -Ru -TiO2(e ) Ru2 -Ru -TiO2(e )

II,III 3+ The rate constant for the second electron transfer from Ru2 to Ru(bpy)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 2+ electron injection from the Ru(bpy)3 excited state to semiconductor TiO2 can occur. 3+ The photogenerated Ru(bpy)3 can oxidize the dinuclear ruthenium complex from the II,III III,III Ru2 state to the Ru2 state by intramolecular electron transfer. These properties II,III II make the Ru2 -Ru -TiO2 system a promising sensitizer for the Grätzel type solar 22-24 cells, 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.

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.

Chart 4.1

COOH 30

4.1.1 Synthesis

30 was prepared by oxidation of trans-8'-apo-β-caroten-8'-al with silver oxide as 73 shown in Scheme 4.1. 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.

Scheme 4.1

CHO Ag2O 30 78% trans-8'-apo-β-caroten-8'-al

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.

E (V vs. SCE)

TiO2 -1 Kinj = 1/360 fs e- 2 S2 CB 3 6 S1 τ = 18 ps

7 1 -0.5 4 }} 8 T1 0 τ = 7.3 µs 5 9 S 30 0

+2.7 VB

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 -1 S1 state, and the rate constant of kinj is 1/360 fs .

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

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.

Chart 4.2 O O

OCH3 N O H

N O N

H N 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 36 Carotenoid 30 is used to achieve efficient attachment to the TiO2 surface. 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

O O O O

- - - - - COO- COO COO- COO COO COO- COO COO TiO 2

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.

0.01

0.00 1 850 nm

0 A [OD]

∆ -0.01 520 nm -1 A (Normalized)

∆ 012345 -0.02 Time [µs] 500 600 700 800 900 1000 Wavelength [nm]

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.

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 2+ and covalently linked them to the photosensitizer Ru(bpy)3 . 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 3+ photogenerated Ru(bpy)3 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.

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.

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.

References

1. Lehninger, A.L.; Nelson, D.L.; Cox, M.M. Principles of Biochemistry, Worth Publishers, Inc., 2nd Ed., 1993, p.571. 2. Amouyal, E. Solar Energy Materials and Solar Cells, 1995, 38, 249-276. 3. Zweibel, K. Basic Photovoltaic Principles and Methods, Van Nostrand Reinhold Company Inc., New York, 1984. 4. Neville, R. C. Solar Energy Conversion: The Solar Cell. Elsevier North-Holland Inc., New York, 1978. 5. Renger, G. Biochim. Biophys. Acta, 2001, 1503, 210-228. 6. Britt, R.D. Oxygenentic Photosynthesis: The light reactions, Eds. Ort, R.D. and Yocum, C.F., Kluwer Academic Publishers, Dordrecht, The Netherlands, 1996, pp. 137-159. 7. Andersson, B. and Styring, S. in Current Topics in Bioenergeetics, Ed. Lee, C.P., Academic Press, San Diego, 1991, 16, p.1. 8. McEvoy, J. P.; Brudvig, G. W. Phys. Chem. Chem. Phys. 2004, 6, 4754-4763. 9. Grotjohann, I.; Jolley, G.; Fromme, P. Phys. Chem. Chem. Phys. 2004, 6, 4743-4753. 10. Barber, J.; Ferreira, K.; Maghlaoui, K.; Iwata, S. Phys. Chem. Chem. Phys. 2004, 6, 4737- 4742. 11. Messinger, J.; Lubitz, W. Phys. Chem. Chem. Phys. 2004, 6, E11-E12. 12. Yachandra, V.K.; Sauer, K.; Klein, M.P. Chem. Rev. 1996, 96, 2927-2950. 13. Ruttinger, W.; Dismukes, G. C. Chem. Rev. 1997, 97, 1-24. 14. Yagi, M.; Kaneko, M. Chem. Rev. 2001, 101, 21-35. 15. Sun, L.; Hammarström, L.; Åkermark, B.; Styring, S. Chem. Soc. Rev. 2001, 30, 36-49. 16. Ferreira, K.N.; Iverson, T.M.; Maghlaoui, K.; Barber, J.; Iwata, S. Science, 2004, 303, 1831- 1838. 17. Mukhopadhyay, S.; Mandal, S.K.; Bhaduri, S.; Armstrong, W. Chem. Rev. 2004, 104, 3981- 4026. 18. Hoganson, C.W.; Babcock, G.T. Science, 1997, 277, 1953-1956. 19. Tommos, C.; Hoganson, C.W.; Di Valentin, M.; Lydakis-Simantiris, N.; Dorlet, P.; Westphal, K.; Chu, H.A.; McCracken, J.; Babcock, G.T. Curr. Opin. Chem. Biol. 1998, 2, 244-252. 20. Tommos, C.; Babcock, G.T. Acc. Chem. Res. 1998, 31, 18-25. 21. Chopra, K.L.; Das, S.R. Thin Film Solar Cells, Plenum Press, New York, 1983. 22. (a) Kalyanasundaram, K.; Grätzel, M. Coord. Chem. Rev. 1998, 177, 347-414. (b) Hagfeldt, A.; Grätzel, M. Acc. Chem. Res. 2000, 33, 269-277. (c) Grätzel, M. Nature, 2001, 414, 338- 344. 23. Hagfeldt, A.; Grätzel, M. Chem. Rev. 1995, 95, 49-68. 24. Hagfeldt, A.; Boschloo, G.; Lindström, H.; Figgemeier, E.; Holmberg, A.; Aranyos, V.; Magnusson, E.; Malmqvist, L. Coord. Chem. Rev. 2004, 248, 1501-1509.

25. Bignozzi, C.A.; Argazzi, R.; Kleverlaam, C.J. Chem. Soc. Rev. 2000, 29, 87-96. 26. Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res., 2001, 34, 40-48. 27. Collin, J.-P.; Harriman, A.; Heitz, V.; Odobel, F.; Sauvage, J.-P. J. Am. Chem. Soc., 1994, 116, 5679. 28. Osuka, A.; Marumo, S.; Mataga, N.; Taniguchi, S.; Okada, T.; Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki, K. J. Am. Chem. Soc., 1996, 118, 155. 29. Wasielewski, M. R. Chem. Rev., 1992, 92, 435. 30. Kuroda, Y.; Sugou, K.; Sasaki, K. J. Am. Chem. Soc., 2000, 122, 7833. 31. Ziessel, R.; Hissler, M.; El-ghayoury, A.; Harriman, A. Coord. Chem. Rev. 1998, 178-180, 1251-1298. 32. Kalyanasundaram, K. Photochemistry of Polypyridine and porphyrin complexes, Academic Press Limited, London, 1992. 33. (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev., 1996, 96, 759. (b) Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Plenum: New York, 1994; pp 165-215. (c) Balzani V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: Chichester, 1991. (d) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic: New York, N. Y., 1970. (e) Meyer, T. J. Pure Appl. Chem. 1990, 62, 1003. (f) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (g) Kalyanasundaram K. Coord. Chem. Rev. 1982, 46, 159. 34. Ritz, T.; Damjanovic, A.; Schulten, K.; Zhang, J.-P.; Koyama, Y. Photosynth. Res. 2000, 66, 125-144. 35. van Amerongem, H.; van Grondelle, R. J. Phys. Chem. B, 2001, 105, 604-617. 36. Polivka, T.; Sundström, V. Chem. Rev., 2004, 104, 2021-2071. 37. Gersten, S. W.; Samuels, G. J.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4029-4030. 38. Gilbert, J.A.; Eggleston, D.S.; Murphy, W.R., Jr.; Geselowitz, D.A.; Gersten, S.W.; Hodgson, D.J.; Meyer, T.J. J. Am. Chem. Soc. 1985, 107, 3855-3864. 39. Raven, S.J.; Meyer, T.J. Inorg. Chem. 1988, 27, 4478-4483. 40. Geselowitz, D.; Meyer, T. J. Inorg. Chem. 1990, 29, 3894-3896. 41. Schoonover, J.R.; Ni, J.; Roecker, L.; White, P.S.; Meyer, T.J. Inorg. Chem. 1996, 35, 5885- 5892. 42. Chronister, C.W.; Binstead, R.A.; Ni, J.; Meyer, T.J. Inorg. Chem. 1997, 36, 3814-3815. 43. Lebeau, E.L.; Adeyemi, S.A.; Meyer, T.J. Inorg.Chem. 1998, 37, 6476-6484. 44. Binstead, R. A.; Chronister, C. W.; Ni, J.; Hartshorn, C. M.; Meyer, T. J. J. Am. Chem. Soc. 2000, 122, 8464-8473. 45. Yamada, H.; Siems, W.F.; Koike, T.; Hurst, J.K. J. Am. Chem. Soc., 2004, 126, 9786-9795. 46. Yang, X.; Baik, M.-H. J. Am. Chem. Soc., 2004, 126, 13222-13223. 47. Sens, C.; Romero, I.; Rodriguez, M.; Llobet, A.; Parella, T.; Benet-Buchhola, J. J. Am. Chem. Soc., 2004, 126, 7798-7799. 50

48. Howells, A.R.; Sankarrja, A.; Shannon, C. J. Am. Chem. Soc., 2004, 126, 12258-12259. 49. Magnusson, A.; Berglund, H.; Korall, P.; Hammarström, L.; Åkermark, B.; Styring, S.; Sun, L. J. Am. Chem. Soc., 1997, 119, 10720-10725. 50. Tracwell, C.A.; Cua, A.; Stewarr, D.H.; Bocian, D.F.; Brudvig, G.W. Biochemistry, 2001, 40, 193-203. 51. Faller, P.; Pascal, A.; Rutherfold, A.W. Biochemistry, 2001, 40, 6431-6440. 52. (a) Sun, L.; Hammarström, L.; Norrby, T.; Berglund, H.; Davydov, R.; Andersson, M.; Börje, A.; Korall, P.; Philouze, C.; Almgren, M.; Styring, S.; Åkermark, B. Chem. Commun. 1997, 6, 607-608. (b) Sun, L.; Raymond, M.K.; Magnuson, A.; LeGourrierec, D.; Tamm, M.; Abrahamsson, M.; Kenez, P.H.; Mårtensson, J.; Stenhagen, G.; hammarström, L.; Styring, S.; Åkermark, B. J. Inorg. Biochem. 2000, 78, 15-22. (c) Hammarström, L.; Sun, L.; Åkermark, B.; Styring, S. Biochim. Biophys. Acta, 1998, 1365, 193-199. (d) Sun, L.; Berglund, H.; Davydov, R.; Norrby, T.; Hammarström, L.; Korall, P.; Börje, A.; Philouze, C.; Berg, K.; Tran, A.; Andersson, M.; Stenhagen, G.; Mårtensson, J.; Almgren, M.; Styring, S.; Åkermark, B. J. Am. Chem. Soc. 1997, 119, 6996-7004. (e) Magnuson, A.; Frapart, Y.; Abrahamsson, M.; Horner, O.; Åkernark, B.; Sun, L.; Girerd, J. J.; Hammarström, L.; Styring, S. J. Am. Chem. Soc. 1999, 121, 89-96. (f) Sun, L.; Burkitt, M.; Tamm, M.; Raymond, M. K.; Abrahamsson, M.; LeGourrierec, B.; Frapart, Y.; Magnuson, A.; Kenez, P. H.; Brandt, P.; Tran, A.; Hammarström, L.; Styring, S.; Åkermark, B. J. Am. Chem. Soc. 1999, 121, 6834-6842. (g)Abrahamsson, M. L. A.; Baudin, H. B.; Tran, A.; Philouze, C.; Berg, K. E.; Raymond- Johansson, M. K.; Sun, L.; Åkermark, B.; Styring, S.; Hammarstöm, L. Inorg. Chem. 2002, 41(6), 1534-1544. (h) Berg, K. E.; Tran, A.; Raymond, M. K.; Abrahamsson, M.; Wolny, J.; Redon, S.; Andersson, M.; Sun, L.; Styring, S.; Hammarström, L.; Toftlund, H.; Åkermark, B. Eur. J. Inorg. Chem. 2001, 4, 1019-1029. (i) Johansson, A.; Abrahamsson, M.; Magnuson, A.; Huang, P.; Mårtensson, J.; Styring, S.; Hammarström, L.; Sun, L.; Åkermark, B. Inorg. Chem. 2003, 42, 7502-7511. 53. (a) Ward, M. D. Chem. Soc. Rev. 1997, 26, 365-375. (b) Wasielewski, M. R.; Gaines, G. L., III; Wiederrecht, G. P.; Svec, W. A.; Niemczyk, M. P. J. Am. Chem. Soc. 1993, 115, 10442. 54. (a)Asbury, J. B.; Hao, E.; Wang, Y.; Ghosh, H. H.; Lian, T. J. Phys. Chem. B 2001, 105, 4545-4557. (b)Hirata, N.; Lagref, J.-J.; Palomares, E.J.; Durrant, J.R.; Nazeeruddin, M.K.; Gratzel, M.; Censo, D.D. Chem. Eur. J. 2004, 10, 595-602. (c) Rego, L.G.C.; Batista, V.S. J. Am. Chem. Soc. 2003, 125, 7989-7997. (d) Bignozzi, C.A.; Biancardo, M.; Schwab, P.F.H. Heterosupramolecular Devices Based on Nanocrystalline Semiconductors, in Semiconductor Photochemistry and Photophysics; Ramamurthy, V.; Schanze, K.S. eds., Marcel Dekker, Inc., New York, 2003; p2. (e) Grätzel, M.; Moser, J.-E. Solar Energy Conversion, in Electron Transfer in Chemistry; Balzani V.; Gould, I., eds.; vol. V, Wiley-VCH, Weinheim, 2001; p589. 55. (a) Burdinski, D.; Wieghardt, K.; Steenken, S. J. Am. Chem. Soc. 1999, 121, 10781-19787. (b) 51

Burdinski, D.; Bothe, E.; Wieghardt, K. Inorg. Chem. 2000, 39, 105-116. 56. Yagi, M.; Tokita, S.; Nagoshi, K.; Ogino, I.; Kaneko, M. J. Chem. Soc., Faraday Trans. 1996, 92, 2457-2461. 57. (a) Nagoshi, K.; Yagi, M.; Kaneko, M. Bull. Chem. Soc. Jpn. 2000, 73, 2193-2197. (b) Yagi, M.; Osawa, Y.; Sukegawa, N.; Kaneko, M. Langmuir 1999, 15, 7406-7408. 58. (a) Yagi, M.; Sukegawa, N.; Kasamastu, M.; Kaneko, M. J. Phys. Chem. B 1999, 103, 2151- 2154. (b) Yagi, M.; Nagoshi, K.; Kaneko, M. J. Phys. Chem. B 1997, 101, 5143-5146. 59. Gavrilova, A.L.; Bosnich, B. Chem. Rev. 2004, 104, 349-383. 60. (a) Torelli, S.; Belle, C.; Gautier-Luneau, I.; Pierre, J.L. Inorg. Chem. 2000, 39, 3526-3536. (b) Lubben, M.; Feringa, B. J. Org. Chem. 1994, 59, 2227-2233. (c) Diril, H.; Chang, H.-R.; Nilges, M.J.; Zhang, X.; Potenza, J.A.; Schugar, H.J.; Isied, S.S.; Hendrickson, D.N. J. Am. Chem. Soc. 1989, 111, 5102-5114. (d) Holman, T.R.; Wang, Z.; Hendrich, M.P.; Que, L. Jr. Inorg. Chem. 1995, 34, 134-139. (e) Diril, H.; Chang, H.-R.; Zhang, X.; Larsen, S.K.; Potenza, J.A.; Pierpont, C.G.; Schugar, H.J.; Isied, S.S.; Hendrickson, D.N. J. Am. Chem. Soc. 1987, 109, 6207-6208. (f) Seo, J. S.; Sung, N.-D.; Hynes, R. C.; Chin, J. Inorg. Chem. 1996, 35, 7472-7473. (g) Borovik, A.S.; Papaefthymiou, V.; Tayor, L.F.; Anderson, O.P.; Que, L.Jr. J. Am. Chem. Soc. 1989, 111, 6183-6195. 61. Lomoth, R.; Huang, P.; Zheng, J.; Sun, L.; Hammarström, L.; Åkermark, B.; Styring, S. Eur. J. Inorg. Chem. 2002, 2965-2974. 62. Larsen, S.; Michelsen, K.; Pedersen, E. Acta Chem. Scand. 1986, A 40, 63-76. 63. Evans, I.P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973, 204-209. 64. Peek, B.M.; Ross, G.T.; Edwards, S.W.; Meyer, G.J.; Meyer, T.J. Int. J. Peptide Protein Res. 1991, 38, 114-123. 65. McCafferty, D.G.; Bishop, B.M.; Wall, C.G.; Hughes, S.G.; Mecklenberg, S.L.; Meyer, T.J.; Erickson, B.W. Tetrahedron, 1995, 51, 1093-1106. 66. Johansson, O.; Borgström, M.; Lomoth, R.; Palmblad, M.; Bergquist, J.; Hammarström, L.; Sun, L.; Åkermark, B. Inorg. Chem. 2003, 42, 2908-2918. 67. ref.32, p57. 68. Sjödin, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarström, L. J. Am. Chem. Soc. 2000, 122, 3932-3936. 69. Kormann, C.; Bahnemann, W.; Hoffman, R. J. Phys. Chem. 1988, 92, 5196-5201. 70. Land, E.J.; Prutz, W.A. Int. J. Radiat. Bio. 1979, 36, 75-83. 71. (a) Svensson, B.; Etchebest, C.; Tuffery, P.; van Kan, P.; Smith, J.; Styring, S. Biochemistry 1996, 35, 14486-14502. (b) Hays, A.-M. A.; Vassiliev, I. R.; Golbeck, J. H.; Debus, R. J. Biochemistry 1998, 37, 11352-11365. (c) Mamedov, F.; Sayre, R. T.; Styring, S. Biochemistry 1998, 37, 14245-14256. 72. Bensasson, R.V.; Land, E.J.; Moore, A.L.; Crouch, R.L.; Dirks, G.; Moore, T.A.; Gust, D. Nature, 1981, 290, 329. 52

73. Singh, H.; John, J.; Cama, H.R. Int. J. Vitam. Nutr. Res. 1973, 43, 147-149. 74. Kay, A.; Grätzel, M. J. Phys. Chem. 1993, 97, 6272-6277.