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List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals:

I How Close Can You Get? Studies of Ultrafast Light-

Induced Processes in Ruthenium-[60]Fullerene Dyads with Short Pyrazolino and Pyrrolidino Links. Susanne Karlsson, Judit Modin, Hans-Christian Becker, Leif Hammarström and Helena Grennberg Inorganic Chemistry 2008, 47(16), 7286-7294

II Vectorial Electron Transfer in Donor-Photosensitizer-Acceptor Triads Based on Novel Bis-tridentate Ruthenium Polypyridyl Complexes Rohan J. Kumar, Susanne Karlsson, Daniel Streich, Alice Ro-landini Jensen, Michael Jäger, Hans-Chrsitian Becker, Jonas Bergquist, Olof Johansson and Leif Hammarström Chemistry- a European Journal 2010, 16, 2830-2842

III Towards [Ru(bpy)3]2+-Based Linear Donor(D)-Photo-sensitizer(P)-Acceptor(A) Arrays. Using the 5,5’-Positions in [Ru(bpy)3]2+-Benzoquinone Dyads. Michael Jäger, Susanne Karlsson, Alice Rolandini Jensen, Hans-Christian Becker, Leif Hammarström and Olof Johansson Manuscript

IV Double-Pulse Excitation of a Mn2-Ru(II)-di-Naphthalene Triad: Challenges for Accumulative Electron Transfer. Susanne Karlsson, Daniel Streich, Olof Johansson, Magnus Anderlund, Hans-Christian Becker and Leif Hammarström Manuscript in preparation

V Step-wise Accumulative Charge Separation Inspired by Photosynthesis Susanne Karlsson, Julien Boixel, Yann Pelegrin, Errol Blart, Hans-Christian Becker, Fabrice Odobel and Leif Hammarström Submitted manuscript

VI Multiple Excitation Studies of Ru(II)polypyridyl-

Oligotriarylamine Dye-Nanocrystalline TiO2 Systems for Photoinduced Charge Accumulation Susanne Karlsson, Julien Boixel, Yann Pelegrin, Hans-Christian Becker, Errol Blart, Fabrice Odobel and Leif Hammarström Manuscript in preparation

Related papers not included in this thesis:

VII Very Large Acceleration of the Photoinduced Electron Transfer in a Ru(bpy)3-Naphthalene Bisimide Dyad Bridged on the Naphthyl Core Frédérique Chaignon, Magnus Falkenström, Susanne Karlsson, Errol Blart, Fabrice Odobel and Leif Hammarström Chemical Communications 2007, 64-66

VIII Long-Range Electron Transfer in Zn(II)-phthalocyanine Based Donor-Acceptor Dyads Erik Göransson, Susanne Karlsson, Hans-Christian Becker, Fabrice Odobel and Leif Hammarström Manuscript in preparation

Reprints were made with permission from the respective publishers.

Comments on my participation

I was responsible for all ultrafast measurements and a major contributor to the nanosecond and steady state measurements in papers I–VI. In paper I, I also performed the electrochemical characterization and in paper II and V, I contributed to the spectroelectrochemistry. I was responsible for the double pulse excitation measurements in papers IV–VI. I was also main responsible for the writing of papers I, IV and VI and I contributed to the writing of papers II and III.

Contents

1. Introduction ............................................................................................... 11

2. Background ............................................................................................... 13 2.1 Water oxidation and fuel production .................................................. 13 2.2 Photoinduced electron transfer in natural photosynthesis .................. 14 2.3 Donor-photosensitizer-acceptor assemblies ....................................... 15 2.4 Ruthenium-polypyridine photosensitizers .......................................... 17 2.5 Beyond the first electron – Accumulative electron transfer ............... 18

3. On excited states, photophysics and photochemistry ................................ 19 3.1 Electronic absorption and excited states ............................................ 19

3.1.1 The fate of excited states ............................................................ 20 3.2 Photochemical quenching processes .................................................. 21

3.2.1 Electron transfer .......................................................................... 22 3.2.2 Energy transfer............................................................................ 26

4. Accumulative electron transfer ................................................................. 29 4.1 Accumulative vs. single electron transfer ........................................... 29

4.1.1 Examples of accumulative electron transfer ............................... 30 4.1.2 One-photon-two-electron processes ........................................... 31

4.2 Challenges in accumulative electron transfer ..................................... 32 4.2.1 Stategies for accumulative electron transfer ............................... 33

5. Results I. Single electron transfer in donor-photosensitizer-acceptor systems .......................................................................................................... 36

5.1 Energy and electron transfer pathways in Ru(II)polypyridine-C60 fullerene dyads with very short links ....................................................... 36 5.2 Electron transfer in linear donor-photosensitizer-acceptor arrays ...... 38

5.2.1 DPA assemblies based on bis-tridentate Ru(II)polypyridyl complexes ............................................................................................ 39 5.2.2 Ru(bpy)3 functinalized through the bipyridine 5,5'-positions ..... 41

5.3 Towards accumulative electron transfer: Multi-electron acceptors ... 43 5.3.1 C60 as a multi-electron acceptor .................................................. 43 5.3.2 Quinones: Two-electron two-proton acceptors ........................... 43

6. Results II. On the challenges of accumulative electron transfer: Double-pulse excitation studies of a Mn2-Ru(II)-di-Napthalenediimide triad ........... 46

6.1 Background and kinetics of single-electron transfer .......................... 46 6.2 Double-pulse excitation studies .......................................................... 48 6.3 Reflections on the challenges in accumulative electron transfer ........ 50

7. Results III. Accumulative electron transfer in semiconductor-Ru(II)-oligotriarylamine systems ............................................................................. 51

7.1 Photoinduced electron transfer in DPA systems with a nanocrystalline semiconductor acceptor ............................................................................ 51 7.2 Accumulation of holes upon multiple excitations .............................. 52 7.3 A successful approach to accumulative electron transfer ................... 55

8. Concluding remarks .................................................................................. 56

9. Methods .................................................................................................... 58 9.1 Ultrafast pump-probe spectroscopy .................................................... 59 9.2 Nanosecond transient absorption and emission spectroscopy ............ 60 9.3 Time-correlated single photon counting ............................................. 61

Acknowledgements ....................................................................................... 63

Summary in Swedish .................................................................................... 64 Enkel och ackumulativ elektronöverföring – en förutsättning för artificiell fotosyntes ................................................................................................. 64

References ..................................................................................................... 67

Appendix. Photoinduced electron transfer in Ru(II)polypyridyl-benzoquinone(BQ) dyads in aqueous and acetonitrile solutions. ................. 75

List of Abbreviations

A electron acceptor BET back electron transfer bpy 2,2'-bipyridine BQ benzoquinone D electron donor dqp 2,6-di(quinoline-8-yl)pyridine EnT energy transfer ET electron transfer IC internal conversion ISC intersystem crossing LC ligand-centered MC metal-centered MLCT metal-to-ligand charge transfer NDI naphthalenediimide OEC oxygen-evolving complex P photosensitizer Pheo pheophytin PQ plastoquinone PSI photosystem I PSII photosystem II PTZ phenothiazine Q quencher or quinone RET reverse electron transfer tpy 2,2',6',2''-terpyridine VR vibrational relaxation

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1. Introduction

By the beginning of the 20th century, it was already realized that light could be used in man-made chemistry.1 These findings founded the photochemistry of today, with important current applications in chemical synthesis, industry and medicine. For example, photochemistry is used in photodynamic therapy and UV-hardened coatings.2-3

The early visionaries also imagined that with the aid of photochemistry, a new industry powered directly from the sun would develop. In this new in-dustry coal could be replaced by sunlight. It was also known that if water was oxidized the gasses that would be produced, dioxygen and dihydrogen, had high energy content and could serve as energy carriers. In his late 19th century science fiction novel L’Île mystérieuse (The Mysterious Island), Jules Verne lets one of his characters express this intriguing idea:4

“(…) that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable. (...) Wa-ter will be the coal of the future.” (J. Verne, 1874)

Today, reports on climate change and the ever-increasing energy needs of the growing population of this planet have brought to the fore our need of energy carriers other than fossil fuels. In the words of the Intergovernmental Panel of Climate Change in the 4th Assessment Report:5

“Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations.” i (IPCC, 2007)

Experts are debating at what point the supplies of fossil fuels will be emptied or become too expensive to extract, but all agree that the supply is finite and alternative energy sources must be explored sooner or later. Solar energy is one of the more promising alternatives. In 2001, the yearly energy consump-tion was 4.1×1020 J, a number comparable to the energy delivered to earth by solar irradiation in one hour (4.3×1020 J).6 If we can capture, convert and store parts of this energy it can give a major carbon-neutral contribution to our future society. i Author’s remark: GHG is greenhouse gases.

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Unfortunately, efficient man-made production of fuels from sunlight and renewable substrates is still science fiction today. We split water to produce hydrogen by electrochemical means, much like Jules Verne suggested, but the energy penalty for this is high, although important progress towards re-ducing the penalty have been made in the development of water oxidation catalysts.7-13 The current needed for such electrocatalysis could in principle be obtained by photovoltaics, solar cells that produce electricity from sun light.14-18 However, this indirect route to solar fuels through current and elec-trolysis will involve inevitable losses. Direct production by photo-driven chemical catalysis could offer a more efficient solution to our problems.

Green plants, algae and cyanobacteria are way ahead of man in this mat-ter. The oxygenic photosynthesis of these organisms presents the very che-mistry we would like to perform; visible photons harvested by the antenna leads to charge separation in the Photosystems and to the four electron oxi-dation of water, while the accumulated redox agents are used in subsequent reactions to produce the fuel for the organism in the carbohydrate synthesis.19 One possible approach to solar fuel production would be to use photosynthetic organisms, e.g. cyanobacteria, to produce target fuels such as biohydrogen.20-22 Although promising in many aspects, this is another indi-rect approach and net energy losses (from the production of nutrients etc) are unavoidable. If we instead could mimic the key events of oxygenic photo-synthesis, direct production of solar fuels could become a reality. Over the last few decades there have been strong efforts to develop such chemistry in man-made systems.23-35 This molecular approach is referred to in the follow-ing as artificial photosynthesis.

The work presented in this thesis aims towards artificial photosynthesis in that the systems investigated and described herein all concern photoinduced electron transfer. In natural photosynthesis, photoinduced charge separationii is the means by which light is converted to electrochemical potential energy through the formation of reduced and oxidized species. Specifically, the work presented here aims towards accumulation of redox equivalents by successive photoinduced charge separation. This is a fundamental question to address for the direct coupling of photoinduced electron transfer with multi-electron catalytic processes, such as water oxidation, hydrogen pro-duction or carbon dioxide reduction.

ii The two terms photoinduced electron transfer and photoinduced charge separation are used synonymously throughout this thesis.

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2. Background

To understand the role of photoinduced electron transfer in artificial photo-synthesis, a basic understanding of the target chemistry and the natural sys-tem that we are attempting to mimic is necessary. In this chapter, the chemi-stry of water oxidation and fuel production is briefly reviewed, and some important aspects of natural photosynthesis are described. The concept of donor-photosensitizer-acceptor systems as a mimic of natural photosynthesis is introduced, as well as the Ruthenium photosensitizers used in the studies.

2.1 Water oxidation and fuel production The balanced reaction for electrochemical water oxidation (1) can be found in any textbook in basic chemistry. The tabulated standard potential (Eº) for this reaction is 1.23 V vs NHE, which means that the minimum energy re-quired to drive this reaction by electrolysis with an ideal normal hydrogen reference electrode as the counterpart is ca. 120 kJ/mol per mole electrons (�Gº = 1.23 eV or 28.4 kcal/mol, for reactants in their standard states). This is comparable to the energy carried by a visible photon (e.g. 680 nm light corresponds to 1.84 eV or 176 kJ/mol). The energy required for the reaction is higher since the making and breaking of bonds gives rise to an activation energy or overpotential. The role of catalysts, such as the oxygen-evolving center (OEC) in Photosystem II, is to minimize this overpotential.

2H2O � O2 + 4H+ + 4e- (1)

Photocatalytic water-splitting was demonstrated for the first time by Fuji-shima and Honda in 1972 by means of UV irradiation of titanium dioxide.36 The examples of catalysts for water oxidation driven by visible light in mo-lecular systems are however quite few.34,37-38 Much development is still needed in this field in terms of efficiencies and stability of the catalysts.

The electrons and protons produced in reaction (1) can be recombined to obtain molecular hydrogen by reaction (2). Hydrogen gas can serve as an energy carrier and a renewable fuel. The development of efficient molecular catalysts for photodriven hydrogen production is currently being pursued by several groups.39-44 Many of these molecular catalysts are inspired by hydro-

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genases, natural enzymes found for example in cyanobacteria, which are able to catalyze the production of hydrogen at low overpotential.45-46

2H+ + 2e- � H2 (2)

The electrons (and protons) released in water oxidation (1) can also be used to reduce the most oxidized form of carbon, carbon dioxide (CO2), in a series of reactions. Equations (3) and (4) describe the reduction of CO2 to formic acid and carbon monoxide, which could be further converted to fuels. Me-thanol or methane can be obtained in more complex reactions (6 or 8 elec-trons and protons).47-48 As for hydrogen production catalysts, molecular cata-lysts for photo-driven CO2 reduction is an active research field.48

CO2 + 2H+ + 2e- � HCOOH (3) CO2 + 2H+ + 2e- � CO + H2O (4)

Notably, all reactions described above (1-4) are multi-electron processes, i.e., two or more electrons are released or consumed in the reaction.

2.2 Photoinduced electron transfer in natural photosynthesis The photosynthetic process is generally divided into light-dependent reac-tions and light-independent (‘dark’) reactions. The photo-driven reactions take place in Photosystems I and II (PS I; PS II), two protein complexes found in the thylakoid membranes in the chloroplasts of green plants, algae and cyanobacteria.19,49 PS II contains the site for water oxidation and will therefore serve as our model for artificial photosynthesis.

The protein subunits and the cofactors involved in the electron transfer chain in PS II49-51 are depicted schematically in Figure 1. Light is collected by antenna complexes52 and the excitation is transferred to the chlorophyll unit P680, where photoinduced charge separation starts. The excited photo-sensitizer P680* is very quickly (1-30 ps) quenched by electron transfer to the primary acceptor pheophytin (Pheo).51 This results in the charge separated state P680

+Pheo-. The reduced Pheophytin has a lifetime of only ca. 300 ps since it is very quickly oxidized by the next acceptor in the electron transfer chain, a protein bound quinone named QA, resulting in the P680

+PheoQA-

charge separated state. The QA- sends the electron on to another quinone, QB,

on the opposite side of the protein. Upon successive excitation and charge separation, the doubly reduced QBH2 interchanges with the plastoquinone (PQ) pool and leaves PS II carrying two electrons and two protons. These

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redox equivalents are used in subsequent photosynthetic reactions, resulting in carbohydrate synthesis.

The oxidized photosensitizer P680+ is regenerated by hole transfer to a ty-

rosine (YZ) and further to the Calcium-Manganese cluster CaMn4 which is the site of water oxidation, also called the oxygen-evolving complex (OEC).50,53-54 The four redox equivalents required for reaction (1) are accu-mulated by oxidation of the CaMn4 cluster upon four successive excitations of the regenerated P680.55-56

Figure 1. Illustration of Photosystem II. The two main protein subunits hold the cofactors involved in charge separation, including the CaMn4 oxygen-evolving complex.

Each charge separation in PSII is reversible and the yield of charge separa-tion in each step is affected by the competition between forward electron transfer and recombination or other competitive processes (e.g. energy trans-fer to the CaMn4 site). Due to the kinetic fine-tuning of the system, the over-all yield of charge separation is very high.57 The intermediate acceptors and donors play an important role in this, since the electron and hole are very rapidly separated at long distances resulting in weak electronic coupling for the recombination electron transfer (this effect will be discussed in Chapter 3). Each forward electron transfer step is thermodynamically downhill and approximately 60% of the energy of one 680 nm photon is stored in the sin-gle charge separated state CaMn4

+QA-.57

2.3 Donor-photosensitizer-acceptor assemblies The different redox active cofactors in PS II can be divided into three cate-gories based on their functionalities: The photosensitizer P, acceptors A and

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donors D. The photocatalytic cycle can thus be described schematically as depicted in Figure 2. First, light is absorbed by the photosensitizer P (P680 in PS II). The excited photosensitizer P* is a more potent reductant (and oxi-dant) than in its ground state. P* is thus quenched by electron transfer to the acceptor A (pheophytin and quinones in PS II) by oxidative quenching.iii Thus the intermediate charge separated state P+A- is produced. The photo-sensitizer is then regenerated by hole transfer to the donor D. The resulting fully charge separated state is denoted D+PA-. This state may further reduce and oxidize catalysts to perform the target chemistry, e.g., water-splitting and fuel production, or the donor and acceptor unit may serve as catalysts themselves.

Figure 2. A general scheme for photocatalysis by oxidative electron transfer. The photosensitizer (P) absorbs a photon and is thus able to reduce an acceptor A. The photosensitizer is regenerated by electron transfer from a donor moiety D. If re-peated, this cycle can provide several redox equivalents for the catalytic oxidation and reduction of chemical substrates (Sred and S'ox, respectively).

To utilize this type of scheme for artificial photosynthesis it becomes neces-sary to develop systems for photoinduced electron transfer that hold these three functionalities. Such DPA assemblies are the object of study in this thesis. Sometimes one component may serve two functionalities in the as-sembly, e.g. the photosensitizer may also act as the donor in a molecular dyad.

All the systems discussed in this thesis are covalently linked, i.e. the D, P and A units are connected by bridging units so that they are all contained within the same molecule. It is also possible to use separate units in bimole-cular reactions to obtain the same overall reaction scheme. However, in bi-molecular reactions the overall yield of charge separation might suffer from kinetic limitations by diffusion controlled rates and it is difficult to control back reactions. Many of the topics discussed in this thesis are similar in bi-molecular and covalent systems. iii In principle, the photocatalytic cycle in Figure 2 could just as well be based on quenching by reductive electron transfer. Oxidative and reductive electron transfer is discusssed in sec-tion 3.2.

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2.4 Ruthenium-polypyridine photosensitizers Some important properties of the photosensitizer in a DPA assembly are the ground state absorption spectrum, redox properties of the ground state and the excited state, photostability, excited state lifetime and excited state ener-gy. Ruthenium-polypyridine compounds have been used extensively for these and related purposes.58-59 Ruthenium(II)-tris-bipyridine ([Ru(bpy)3]2+), here denoted Ru(bpy)3, is one of the most frequently used motifs for DPA arrays.59-60 In this thesis, the alternative bis-tridentate sensitizer Ruthe-nium(II) bis-diquinolinylpyridin61-62 ([Ru(dqp)2]2+) here denoted Ru(dqp)2, was also used (paper III).

The absorption spectrum of Ru(bpy)3 and Ru(dqp)2 are shown in Figure 3 together with the solar irradiance spectrum.63 The Ru(II)-polypyridine com-plexes absorb a significant fraction of the photons in the visible region. Both Ru(bpy)3 and Ru(dqp)2 are photostable and have high excited state energies.59,62 Unlike other bis-tridentate Ru(II) complexes, the Ru(dqp)2 has a remarkably long excited state lifetime, while the redox properties and ex-cited state energy are comparable to those of Ru(bpy)3.62

Figure 3. Structures and room temperature absorption spectra of Ru(bpy)3 (dashed line) and Ru(dqp)2 (solid line) in acetonitrile solution, with the solar irradiation spectrum.63

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2.5 Beyond the first electron – Accumulative electron transfer Since the first attempts to develop systems for artificial photosynthesis, many advanced supermolecular donor-acceptor assemblies which harvest energy by photoinduced charge separation have appeared in the literature.24,26,29,59-60,64-72 These dyads, triads and pentads successfully and beautifully mimic the primary events in photosynthesis. Studies of such sys-tems have been, and still are, indispensable in the development of our under-standing and control of photoinduced electron transfer. However, the vast majority of these studies concern charge separation of a single electron-hole pair only.

If the aim is set on multi-electron catalysis, e.g. water oxidation (1), hy-drogen production (2) or reduction of carbon dioxide to fuels (3) and (4), several charge separation events must occur to accumulate a sufficient num-ber of redox equivalents. As described above, this is what happens in PSII in natural photosynthesis. This accumulation of electrons and holes upon sev-eral successive photoinduced electron transfer events is essential in molecu-lar photocatalysis of multi-electron processes. This will be discussed further in chapter 4 of this thesis and is the process of interest in several of the stu-dies presented herein.

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3. On excited states, photophysics and photochemistry

The aim of this chapter is to acquaint the reader with some basic concepts of photophysics and photochemistry to the extent necessary for the reading of this thesis. The concepts of electronically excited states, radiative and non-radiative transitions, quenching, electron transfer and electron exchange energy transfer are introduced. More extensive information can be found in textbooks on photochemistry and photophysics73-74 and in original articles on electron and energy transfer.75-78 The factors that govern the rate of electron transfer and energy transfer will be described. This is the basis for the tuning of photoinduced processes in DPA assemblies as to obtain charge separation upon photoexcitation.

3.1 Electronic absorption and excited states To understand the absorption of light by matter on the atomic or molecular level, it is necessary to consider quantized states with defined energies. If light is described as a particle with quantized energy, a photon, it is possible to imagine that this photon may ‘hit’ an atom or molecule and provide addi-tional electronic, vibrational, rotational and translational energy, creating an excited state with higher energy than the ground state. If light is instead de-scribed as electromagnetic radiation, the transition from one quantized state to another can be described as the result of oscillating dipole interactions of the molecular or atomic states with the oscillating electric field component of the electromagnetic wave.

Electronically excited states are the result of a transition from a lower to a higher electronic quantum state upon the absorption of light. Visible light (400-700 nm) typically carries the amount of energy that is required to in-duce transitions between electronic states related to the valence electrons of an atom or a molecule. This is in contrast to X-rays, which typically interact with core electrons and infrared light, which often cannot induce electronic transitions but gives vibrational and rotational transitions.

Electronically excited states are often termed according to the nature of the transitions involved and the total spin of the excited state. For example, in the Ru(II)polypyridine metal-organic complexes used here the lowest

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excited state is typically a 3MLCT* state, which refers to an excited state(*) where the two unpaired electrons have parallel spins (3, triplet), formed by a Metal-to-Ligand Charge Transfer (MLCT) transition. The terms, e.g., MLCT transitions, Metal-centered (MC) transitions and Ligand-centered (LC) tran-sitions, depend on the nature of the molecular orbitals coupled to a specific excitation.

3.1.1 The fate of excited states It is said that what goes up must come down, and this is particularly true for photoexcited states. The electronically excited state produced upon absorp-tion will lose the excess energy provided by the photon and decay to the ground state by one of the general pathways described in Figure 4. The exci-tation typically produces a thermally excited state. The excited molecules will lose their excess energy by vibrational and rotational cooling and colli-sions with other molecules, including the solvent. This type of non-radiative decay, called vibrational relaxation (VR), occurs on the order of 1-10 ps in solution.

Figure 4. Jablonski diagram showing radiative and non-radiative decay pathways from a singlet excited state S1. After excitation (1) and vibrational relaxation (VR), the molecule may transfer to the vibrationally and rotationally excited ground state (S0) by internal conversion (IC) or to the lowest excited triplet state (T1) by intersys-tem crossing (ISC). The radiative transitions from the S1 and T1 state are called fluo-rescence (2) and phosphorescence (3), respectively.

In the ensemble of electronically excited molecules, some will make the non-radiative transition to the ground state (S0 in Figure 4). The term used for this process is intersystem crossing (IC) and it is typically followed by vibrational relaxation all the way back to the thermally relaxed ground state. There is also a non-negligible probability that the excited molecules undergo a spin transition, e.g. from a singlet to a triplet state. Changing the spin state

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is quantum mechanically forbidden but spin-spin interactions can make spin-flips allowed. In the presence of a heavy nucleus such as Ruthenium, the singlet-triplet transition occurs with high probability, for which the dominat-ing effect is spin-orbit coupling.79 Thus, in our ensemble of excited states a few molecules or many may change spin state by intersystem crossing (ISC). Intersystem crossing is typically followed by vibrational relaxation to the lowest excited state of the new spin configuration, as depicted in Figure 4.

The decay pathways described above are non-radiative, where energy is dissipated in a series of successive transitions and no radiation is emitted. However, the excited state can also return to the ground state spontaneously in one single step, by releasing the excess energy as an emitted photon. The emissive decay pathways are shown in Figure 4 by wiggly arrows. If the emissive transition does not involve a change of spins, it is called fluores-cence and if a spin-flip is involved, it is called phosphorescence.

The number of molecules that decay by non-radiative and radiative path-ways respectively is dependent on the probabilities for each process. By measuring the total emission of an ensemble of photoexcited molecules, the relative probabilities for radiative vs. non-radiative transitions (correspond-ing to rates kr and knr, respectively) can be obtained. This is often evaluated in terms of the emission quantum yield, �em, defined as the ratio of emitted to absorbed photons. Related to the probability of emissive decay is the life-time of the excited state, defined as the time at which the emission intensity of an ensemble of molecules has decreased to 1/e of its initial value, where e is the irrational number e � 2.718.80

For a complete description of the fate of excited states, stimulated emis-sion must also be considered. If an excited state interacts with a photon of the right wavelength this may trigger the emissive decay back to the ground state. This is a light-triggered process in contrast to the spontaneous emis-sion described above. This phenomenon is actively utilized in lasers; LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.

3.2 Photochemical quenching processes The excited state P* may also decay via pathways provided by other mole-cules, or adjacent moieties in the same molecule. The effect of such addi-tional decay pathways is to shorten the emissive lifetime resulting in de-creased (quenched) overall emission intensity. The quenching processes relevant for the systems described in this thesis are electron transfer (ET; equation (5) and (6)) and energy transfer (EnT; equation (7)). The overall yield of electron transfer (�ET) and energy transfer (�EnT) depends on the branching of the excited state population into the different decay pathways, which is determined by the rates of electron and energy transfer kET and kEnT, and the rates of radiative and non-radiative decay (kr and knr) discussed

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above. In the following, different factors that affect kET and kEnT will be dis-cussed.

P* + A � P+ + A- oxidative ET (5) P* + D � P- + D+ reductive ET (6) P* + Q � P + Q* EnT (7)

3.2.1 Electron transfer Equations (5) and (6) describe excited state quenching by electron transfer. Electron transfer can be discussed in general terms of a donor (D) that gives an electron to an acceptor (A). In the equation for oxidative ET given above (5), the excited photosensitizer P* acts as the donor, and in the case of reduc-tive ET (6), P* is the acceptor.

The theory of electron transfer in weakly coupled systems described here originates from the electron transfer theory developed in the 1950’s by R. A. Marcus.76,81-82 The semi-classical Marcus equation (8) describes the rate of electron transfer, kET, as a function of three parameters; 1) the free energy change, or driving force, of electron transfer �Gº, 2) the reorganization energy � and 3) the electronic coupling between the donor and acceptor, VDA.76,83-85 The meaning of ‘weakly coupled systems’, also referred to as non-adiabatic systems, for which this equation is valid, is discussed below.

��

��

�� (8)

The transition from the reactant state (DA) to the product state (D+A-), where the electron has transferred from the donor to the acceptor, is typically asso-ciated with various changes in bond lengths, atomic positions, etc of the donor, acceptor and the surroundings (solvent). In Marcus theory, all rele-vant changes the system has to go through to move from the DA state to the D+A- state are put into one single variable called the reaction coordinate. The free energy associated with the system moving along this coordinate can be described by two harmonic parabolas representing the DA and D+A- states, respectively. This is schematically depicted in Figure 5. The graphical repre-sentation of the driving force, reorganization energy and electronic coupling is also given in this figure.

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Figure 5. Illustration of the reactant (DA) and product (D+A-) parabolas described in the text. The reorganization energy �, driving force �Gº, crossing-point energy �G‡ and electronic coupling VDA are illustrated. In the weakly coupled systems described herein, VDA is small and the DA and D+A- can be seen as separate surfaces (non-adiabatic ET).

Reorganization energy The reorganization energy � can be understood as the energy required to move all nuclei of the reactants and solvent into the positions of the product state (the relaxed charge separated state), without actually transferring the electron. The total reorganization energy can be described as the sum of two contributions; the inner (�in) and outer (�out) reorganization energy, which are usually evaluated from equations (9) and (10), respectively.76,84 The �in stems from the internal changes in bond lengths and bond angles within and be-tween D and A upon giving away or accepting an electron, and can thus be evaluated from the normal mode vibrational force constants ki and the nuc-lear displacements of the normal vibrations �qi.

The outer reorganization energy �out arises from the solvent molecules ad-justing to local changes in the electric field upon electron transfer. Accord-

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ing to (10), the magnitude of �out is affected by the donor-acceptor distance rDA and the solvent polarity, represented by the optical (�op) and static (�s) dielectric constants. This dielectric continuum model of the surroundings does not take into account the molecular nature of the solvent.83 Other fac-tors that go into Eq. (10) are the charge transferred by one electron, �e,iv and the radii of the donor and acceptor �D and �A, modeled as hard spheres. In non-polar solvents the �out contribution is generally very small, while polar solvents typically give an outer reorganization energy of 0.5-1.0 eV.84

Experimentally, the reorganization energy can be obtained from the tem-perature dependence of the electron transfer rate by equation (8).

Driving force of electron transfer The total free energy change associated with moving from the relaxed DA state to the relaxed D+A- state is here termed �Gº. In Figure 5, the system has to pass through a state of higher energy in order to cross between the two surfaces (if the probability for nuclear tunneling is low, see below). In the free energy parabola approximation the crossing point energy �G‡ can be calculated from the driving force �Gº and reorganization energy � as shown in equation (11).

� ! � �� (11)

An interesting consequence of the semi-classical Marcus equation (8) is that if the driving force for electron transfer increases while � and VDA are held constant, kET will reach a maximum at –�Gº = �, where the transition from the DA state to the D+A- state is barrierless (�G‡ = 0). At more exergonic reaction conditions (–�Gº > �) the system enters the so-called inverted re-gion, where the barrier starts to increase again and thus the electron transfer rate decreases with increasing driving force. This principle is illustrated in Figure 6.

The inverted region was not demonstrated experimentally until 1984, when G. Closs and J. Miller published studies on a series of donor-acceptor dyads with similar coupling but gradually increasing driving force.86-87 The effect of slower rates of electron transfer in the inverted region can be active-ly used in the design of molecular DPA systems to slow down the recombi-nation of the electron and hole in a charge separated state (back electron transfer) and thus prolong the charge separated state lifetime. This can be obtained either by a large driving force �Gº or small � for the unwanted back reaction.

iv In equation (10) it is assumed that the D and A have no net charge in the reactant state.

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Figure 6. Illustration of the normal and inverted region of electron transfer predicted by R A Marcus. In the inverted region the rate of electron transfer decreases as the magnitude of the driving force –�Gº increases. The maximum electron transfer rate is obtained at –�Gº � �. The dotted line schematically illustrates the effect of nuclear tunneling in the inverted region.

One reason for the late experimental observations of the inverted region is nuclear tunneling effects. Nuclear tunneling is the process where the whole system moves from the DA state directly to the D+A- state by tunneling, without going through the crossing point. Thus, the rate of electron transfer is no longer directly dependent on the transition state energy �G‡,84,88 which is the basis for the electron transfer rate predicted by equation (8). Instead, it can be described by means of the vibrational overlap between donor and acceptor and the Huang-Rhys factor that relates to the nuclear displacement between the DA and D+A- state, described in detail elsewhere.85 If the inter-section point can be reached thermally and the vibrational spacings are small (kBT >> h�), it can generally be assumed that nuclear tunneling is negligi-ble.83,85,89

The driving force for photoinduced electron transfer is typically evaluated from the reduction potentials of the donor and acceptor, Eº(D+/0) and Eº(A0/-) respectively, by equation (12).90 In the case of photoinduced electron trans-fer, the excited state energy E00 of the photosensitizer contributes to its re-ducing and oxidizing potential. In addition, the electrostatic interaction be-tween the electron and hole given by the Coulombic work term �w stabilizes the charge separated state, a contribution that is particularly important at short electron-hole distances.

�� " � � #$"�%"&'� � $"�(�&"�) � �* (12)

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Electronic coupling The rate of electron transfer also depends on the probability that the electron will transfer when the system is at the crossing point. This is contained in the pre-exponential factor in equation (8), introducing the third parameter that is important for the rate of electron transfer: The electronic coupling, VDA.

The electronic coupling originates from the orbital overlap between donor and acceptor and thus it decreases exponentially as the DA distance rDA in-creases (13). The pre-exponential factor V0 is the coupling at a centre to cen-tre distance limited by the radii of the donor and acceptor. The attenuation factor � is a system-specific parameter, strongly dependent on the medium linking the donor and acceptor, and of the electron tunneling barrier.72

�� "�� #�+

� ,��) (13)

If the donor and acceptor are contained within the saturated environment of a protein or covalently linked by a bridge, the medium between D and A may facilitate electron transfer.66,72,87,91-94 Such bridge-mediated electron transfer can significantly enhance electron transfer rates at long distances. The bridge either acts as an intermediate acceptor and donor by a step-wise hopping mechanism, or it mediates the electron transfer by enhancing the electronic coupling between A and D through its valence orbitals in the so-called su-per-exchange mechanism.66,72,95-96

It was mentioned above that the equations and theory of electron transfer discussed here are valid only for weakly coupled systems. The meaning of this is that the probability of crossing from the DA state to the D+A- state is low (<< 1) when the system is at the crossing point of the potential energy surfaces. Non-adiabatic conditions are obtained when the electronic coupling is small compared to the thermal energy, VDA < kBT.83-84

Out of the three factors that influence the rate of electron transfer, the electronic coupling is the most difficult to assess experimentally, especially in weakly coupled systems. It can in principle be evaluated from its tempera-ture-dependence or driving force-dependence 87,94 by equation (8), or as the solvent-independent factor in electron transfer rates.97

3.2.2 Energy transfer Very often electron transfer competes with radiationless excitation energy transfer, where the excited state energy of the photosensitizer is transferred to a quencher as described by equation (7). The general requirement for these types of quenching processes is that the donor and acceptor have isoe-nergetic states. This is evaluated by the spectral overlap of the P* emission and the Q absorption.

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There are two different mechanisms for energy transfer, differentiated by the nature of the interaction between P* and Q. The first is called electron-exchange energy transfer, originally described by D. L. Dexter.77 The elec-tron exchange mechanism requires orbital overlap and therefore it is typical-ly relevant at short distances, up to ca. 10 Å. At longer distances, up to ca. 100 Å, the Coulombic interaction mechanism, first described by T. Förster,78 is usually dominant. Förster energy transfer occurs by means of dipole-dipole interactions of the P* � P and Q � Q* transitions. The following discussion will focus on electron exchange energy transfer as this is of most relevance for the systems described in this thesis.

Figure 7. Illustration of the frontier orbitals of the photosensitizer P and quencher Q and the apparent exchange of electrons in the Dexter energy transfer mechanism. The excited state energy is transferred from the excited photosensitizer P* to the quencher Q, as described by equation (7).

The mechanism of Dexter energy transfer can be pictorially described as shown in Figure 7. The excited state energy of the photosensitizer P* is transferred to the ground state acceptor Q by exchange of the unpaired elec-trons. This results in the ground state P and the excited state Q*. The energy transfer rate kEnT varies exponentially with the PQ distance rPQ (14).

��-� � .�/01/2%�� #� �

3 ,45) (14)

A and L are system-specific constants. The spectral overlap integral JDexter is dependent on the fluorescence spectrum of the excited photosensitizer P* and the normalized molar absortivity of the energy acceptor Q, as mentioned above. However, JDexter is not dependent on the oscillator strength, as the probability of the transitions is unimportant for this mechamism.

There are clear parallels between electron-exchange energy transfer and electron transfer.98-100 The overall distance dependence at long distances are similar, as seen in the comparison of equations (13) and (14). However, elec-tron-exchange energy transfer is less affected by the reorganization energy, since there is no change of formal charge.

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Energy transfer is a very important photochemical process. For example, in the light harvesting antenna complexes in plants and cyanobacteria the collected photons are directed onto the primary chromophores in Photosys-tems I and II by energy transfer.52 Energy transfer can also be used in sensi-tization of a system to light of particular wavelengths. If the quencher Q has very low molar absorptivity in the visible region, a photosensitizer absorbing visible light might be able to sensitize Q by energy transfer, producing Q* that can act as the photosensitizer in subsequent processes.

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4. Accumulative electron transfer

In chapter 2, the multi-electron chemistry of water oxidation, hydrogen pro-duction and carbon dioxide reduction was described. To drive such catalytic processes by photoinduced electron transfer in molecular systems, the effec-tive accumulation of electrons and holes is required. In this chapter, accumu-lative electron transfer and related concepts are introduced. The specific challenges in the accumulation of redox equivalents by step-wise photoin-duced charge separation and different strategies that could be used to over-come these challenges are discussed.

Figure 8. Schematic diagram of the charge separated state energies and electron transfer steps involved in single electron transfer (left) and accumulative electron transfer (right). Solid arrows represent the preferred electron transfer processes and dashed arrows indicate unproductive decay pathways. Energy transfer pathways are not included in this diagram.

4.1 Accumulative vs. single electron transfer As mentioned in chapter 2, the vast majority of studies of photoinduced elec-tron transfer in DPA assemblies concern separation of a single electron-hole pair, often in multiple steps.24,26,29,59-60,64-72 A general energy scheme for sin-gle-electron photoinduced electron transfer in DPA arrays is given to the left in Figure 8. As discussed in the previous chapter, the yield of charge separa-tion stems from the branching ratio of electron transfer vs. the competing

30

processes, e.g. radiative and non-radiative decay of P* and energy transfer. The yield and lifetime of the final charge separated state depends on the rate of back electron transfer of the final and intermediate steps, depicted by dashed arrows.

If the lifetime of the first charge separated state D+PA- is sufficiently long for another excitation to occur before it has recombined to the ground state, a second charge separation can take place. This is schematically depicted to the right in Figure 8. If the unproductive pathways (again presented by dashed arrows) are avoided, the system arrives at the second charge sepa-rated state D2+PA2-, where two holes are found at the donor site and two electrons at the doubly reduced acceptor. This process is referred to in this thesis as accumulative electron transfer.

4.1.1 Examples of accumulative electron transfer There are but a few examples of accumulative electron transfer such as gen-erally described above in the current literature. The first relevant example was presented by M. Wasielewski and co-workers.101 This system consisted of two porphyrin (H2P) photosensitizers covalently linked to the same pery-lene diimide (PDI) acceptor. It was shown that at high pulse intensities, exci-tation of both sensitizers lead to the formation of the doubly reduced accep-tor (PDI2-) and the two singly oxidized photosensitizers (H2P+), a state that was reported to recombine in ca. 5 ns. This system was presented as a mole-cular switch.

K. Brewer and co-workers have reported two steps of charge separation in Ru-Ir-Ru and Ru-Rh-Ru systems under irradiation in the presence of a sa-crificial electron donor.102-103 In these systems, the Ru moieties act as photo-sensitizers and the Ir or Rh core is doubly reduced. These systems were re-cently used in photocatalytic hydrogen production.104

From F. MacDonnell and co-workers comes dinuclear Ru-complexes where the ligand is reduced in four separate steps under irradiation in the presence of a triethylamine sacrificial donor.105-107 The electrons are accumu-lated at the bridging ligand. Charge accumulation was also studied in aqueous media, and proton-coupled processes were observed.

L. Hammarström, S. Styring and co-workers have demonstrated the ac-cumulation of holes on the Mn-site in a Ru-Mn2 dyad upon excitation of the Ru moiety in the presence of a sacrificial acceptor.108-109 These particular studies are part of the motivation for the studies described in paper IV and will be discussed later in this thesis.

More recent examples are polyoxymetallate systems presented by A. Har-riman, F. Odobel and co-workers110 and the proposed accumulation of elec-trons in phthalocyanine sensitized carbon nanotubes presented by F. Zhang and co-workers.111-112

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A related system has been presented by D. Gust and co-workers, who demonstrated the buildup of charge over a synthetic membrane driven by the combined (but not coupled) photoinduced charge separation in several do-nor-acceptor pentads.113 This system is different from the others presented here in that it is based on the buildup of a proton gradient rather than the accumulation of electrons or holes at specific sites.

It is worth noting that sacrificial agents were used in the majority of the above mentioned studies. In the context of molecular photoinduced catalysis, it is necessary that the energy of the charge separated state is conserved to ultimately provide redox equivalents to the target substrates (water, protons or carbon dioxide). This means that the charge separated state must be main-tained without the aid of sacrificial donors or acceptors. Also, the accumula-tion of either holes108-109 or electrons101-103,105-107,110 was demonstrated, but the localized accumulation of both oxidizing (D2+) and reducing (A2-) equiva-lents in the same molecular assembly, as indicated in Figure 8, has not been shown. Ideally, the sites of electron and hole accumulation should be access-ible for further chemical processes.

There are also examples on photocatalytic fuel production or water oxida-tion in bimolecular systems7,9-10,37-42,44,48 which involve the accumulation of redox equivalents, again by using sacrificial agents. These studies are typi-cally performed under steady-state illumination conditions by photoproduc-tion of a redox agent, and in general very little mechanistic detail is pro-vided. The quantum yield for fuel production is typically very low. Most likely, some of these unidentified losses are connected with electron and hole accumulation. To better understand the mechanisms behind successful accumulation of charge and ultimately integrate this understanding in the design of new systems, fundamental studies on accumulative electron trans-fer are needed.

The perhaps most important example of accumulative electron transfer is given in natural photosynthesis. In Photosystem II the photocycle is repeated several times with minimum yield losses.51,57 Perhaps there are a few proper-ties of the natural system that can provide inspiration when approaching accumulative electron transfer in man-made systems.

4.1.2 One-photon-two-electron processes The above discussion concerns accumulation of charge by one-photon-one-electron processes, as presented in Figure 8. Direct two-electron transfer induced by a single photon, which has been demonstrated in molecular sys-tems,39,114-115 is a mechanistically interesting alternative. However, these processes are either dissociative (non-reversible) or require high-energy (UV) photons. Such systems are less suitable for direct solar fuels applica-tions. The focus of the work presented in this thesis is on the fundamental

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steps of accumulation of redox equivalents by successive one-photon-one-electron processes.

4.2 Challenges in accumulative electron transfer The literature also provides interesting examples of systems with specific decay pathways that could prevent accumulative electron transfer. For ex-ample, M. Borgström et al. reported zero charge separation yield of the po-tential second electron transfer step in a RuIIRuIINDI triad upon double-pulse excitation.116 L. Flamigni et al. have shown how multiple excitations lead to a decrease of the overall charge separation yield in a IrII-based DPA triad.117 It can also be noted that some molecular switches utilizes the tendency for reversed direction of electron transfer compared to the ground state in excita-tion of the first charge separated state, as discussed below.118-119

Looking in detail on the scheme for accumulative electron transfer given in Figure 8 (right), we find that compared to single-electron transfer (left in Figure 8) there are additional decay pathways from the second excited state D+P*A- and from the following intermediate states. The specific study of these decay pathways may provide important insights on the limitations and challenges encountered in accumulative electron transfer. An example of such a fundamental study is presented in paper IV.

One of the potential complications in this accumulative electron transfer scheme (Figure 8) stems from the high reducing and oxidizing ability of the excited photosensitizer P*. In the D+P*A- state, P* is surrounded by the oxi-dized donor D+ and reduced acceptor A-, that can act as oxidant and reduc-tant, respectively. Thus, in the D+P*A- state P* is likely to be quenched by either D+ or A- as indicated by the dashed arrows in Figure 9. This process is referred to in this thesis as reverse electron transfer (RET) to distinguish it from back electron transfer (BET) to the ground state.

Additional decay pathways related to the recombination of electrons and holes are available in the intermediate states D+P+A2- and D2+P-A-. The doub-ly oxidized donor D2+ may recombine directly with the singly reduced accep-tor A- or vice versa (A2-–D+ recombination). Thus, the electronic coupling between the different units may become even more important for the overall yield of the doubly charge separated state D2+PA2- than in the formation of the first charge separated state.

Also, the accumulation of electrons and holes at specific sites present an electrostatic problem: the negative charge at the singly reduced acceptor A- will repel the arriving electron by Coulombic repulsion. The corresponding situation holds for the holes at the donor site. This electron-repulsion energy is why the second reduction (or oxidation) of a redox species is normally more energy demanding than the first.120 At the same time, the available redox potential to drive the charge separation is that of the excited photosen-

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sitizer P*. This gives certain system-specific restrictions for the redox poten-tials of the donor and acceptor units.

In addition to these electron transfer decay pathways, energy transfer may occur, depending on the specific properties of D, P, and A. Also, paramag-netic quenching may become important, since the presence of unpaired spins in D+ and A- can influence the rate of intersystem crossing,79 competing with or promoting the formation of D+P+A2-, D2+P+A- and D2+PA2-. This latter effect is not considered further here, as all the investigated systems described in this thesis are based on Ru(II)polypyridine complexes, where ISC already occurs in almost unity yield.

Figure 9. Left: Frontier orbitals of the donor, photosensitizer and acceptor in the first excited state DP*A. The arrows indicate the forward electron transfer process, re-sulting in the first charge separated state D+PA-. Right: The same frontier orbitals in the second excited state D+P*A-. Reverse electron transfer, where the acceptor is reoxidized or the oxidized donor reduced (dashed arrows) is an additional decay pathway from this state.

4.2.1 Stategies for accumulative electron transfer The challenges discussed above arise upon the second, or higher order, pho-toexcitation in the accumulative electron transfer scheme (see Figure 8). These challenges most likely have to be met in the design of molecular sys-tems for accumulative electron transfer. In this section, a set of strategies to promote the accumulation of redox equivalents in high yields, and prevent unproductive pathways, are proposed.

Multi-electron donors and acceptors The first question in designing molecular systems for accumulative electron transfer is the choice of donors and acceptor units. The oxidizing or reducing potential of the excited photosensitizer P* is the limit within which the donor and acceptor must be able to donate or accept two or more electrons. For accumulative electron transfer, the donor and acceptor moieties should ideal-ly be oxidized and reduced in several steps within a narrow potential range.

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This is reflected in the choice of donors and acceptors throughout this thesis (paper I–VI).

Coupled electron transfers If electron transfer is coupled to structural rearrangements or charge com-pensation, such as proton transfer, this can compensate for the electron-repulsion energy and facilitate the accumulation of electrons and holes. It can also help prevent back reaction from the charge separated state.

In Photosystem II, structural changes in the CaMn4 oxygen-evolving complex upon oxidation can be correlated with the accumulation of four holes on this site within a narrow potential range.56 Similar structural rear-rangements have been observed to facilitate the oxidation of the Mn2 dimer donor used in the studies presented in paper IV.35,109

On the acceptor side of PSII, the final acceptor in the electron transport chain, QB, acts as a two-electron two-proton acceptor, interchanging with the plastoquinone pool.51,121 The proton-coupled reduction of QB is facilitated by charge-compensation from the protons. The benzoquinone acceptors utilized in paper II-III could serve as two electron-two-proton acceptors in a similar way. The kinetics of electron transfer however showed to be less than optim-al for such studies, as will be discussed in chapter 6.

Rapid initial electron transfer Reverse electron transfer (see Figure 9) stems from the interaction of the excited photosensitizer P* with the reduced acceptor A- and oxidized donor D+. In papers V-VI, a strategy that involves rapid initial electron transfer from the D+P*A- state is employed to overcome this problem. Since P* is rapidly quenched by productive electron transfer, reverse electron transfer cannot compete with the primary productive charge separation. A parallel to this in PSII is the initial charge separation between P680 and the primary ac-ceptor pheophytin (Pheo), which is very fast.51

Electronic donor-acceptor coupling It was discussed above that additional recombination pathways are available from the intermediate charge separated states in the accumulative electron transfer scheme (Figure 8). Decoupling of the electron and hole is necessary to avoid these unproductive pathways. This can be obtained by increased donor-acceptor distance or, in general, by choosing DPA systems where the electronic coupling has previously shown to benefit the forward electron transfer direction in single electron transfer. This is also one of the strategies behind the design of the systems described in papers V–VI.

Intermediate donors and acceptors The intermediate redox active cofactors in PSII combine the ideas presented in the previous two paragraphs. Intermediate acceptors ensure fast forward

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electron transfer, as to prevent quenching by reverse electron transfer. At the same time, each step in the donor-acceptor cascade increases the electron-hole distance, which leads to decreased electronic coupling and helps pre-vent charge recombination of the electron-hole pair. Such multi-step electron transfer has been studied on the single electron-hole pair level. It may be that the use of intermediate donors and acceptors in DPA1A2 or D1D2PA systems is a good strategy to achieve accumulative electron transfer.

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5. Results I. Single electron transfer in donor-photosensitizer-acceptor systems

The studies described in this chapter concern the fundamental step of pho-toinduced electron transfer on the single electron-hole pair level. If this step fails, there can be no accumulation of redox equivalents and no subsequent photocatalysis. Despite our understanding of the different factors that govern the rates and yields of photoinduced charge separation (see chapter 3), the a priori evaluation of the influence of these factors on a specific system is not a simple task. Thus, fundamental experimental studies of photoinduced processes in DPA assemblies are needed.

The the ideal DPA system for the purpose of photocatalysis should have the following properties: 1) fast formation of the charge separated state; 2) high yield of charge separation, i.e. minimum losses by competing pathways such as energy transfer; 3) long-lived charge separated states, so that the target chemistry has time to occur before the charge separated state recom-bines to the ground state and 4) multi-electron donor or acceptor units, for future use in accumulative electron transfer and multi-electron catalysis. These different aspects will be exemplified below in the results presented in papers I-III.

5.1 Energy and electron transfer pathways in Ru(II)polypyridine-C60 fullerene dyads with very short links In paper I, competing energy and electron transfer pathways in a set of Ru(II)polypyridyl-C60 dyads were investigated. The C60 fullerene has served as the acceptor in many donor-acceptor assemblies for photoinduced single electron transfer.69,122-125 The first reduction of C60 is readily accessible from most Ru(II)polypryridyl excited states (E1/2(C60/C60

-) = -0.6 V vs SCE ac-cording to Echegoyen et al).126-127 However, surprisingly few examples of photoinduced electron transfer in Ru(II)polypyridyl-C60 dyads have been reported.128-131 The main reason for this is the low-lying fullerene triplet ex-cited state 3C60*, with a reported state energy of 1.5 eV.126 In many

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Ru(bpy)3-C60 assemblies, this results in energy transfer from the Ru 3MLCT excited state to the 3C60* state.132-137

The purpose of this study was to investigate the branching of electron and energy transfer pathways upon photoexcitation in the Ru(II)polypyridine-C60 dyads presented in Figure 10. These dyads have unusually short donor-acceptor distances. While exchange energy transfer and electron transfer display similar overall distance dependence,v the rate of electron transfer is more influenced by the reorganization energy, which is also distance depen-dent (see equation (10)).vi In addition, the driving force for electron transfer is distance dependent, which is reflected in the Coulombic work term in equation (12). Thus, the overall effect of the short donor-acceptor distances in the dyads shown in Figure 10 might well be to give very fast electron transfer, favoring this reaction pathway over energy transfer by kinetic com-petition.

Figure 10.Structures and schematic energy level diagram for the Ru(II)polypyridine-C60 dyads studied in paper I.

Ultrafast transient absorption spectroscopy of the complexes presented in Figure 10 revealed very fast formation of the 3C60* state (kEnT = 1×1011-2×1012 s-1) from the Ru 3MLCT state. These energy transfer rates are signifi-cantly higher than reported previously for other Ru(II)polypyridine-C60 fulle-rene dyads, and suggests an energy transfer yield close to unity. The trend within the series of compounds was that the shorter the donor-acceptor dis-tance, the faster the energy transfer rate. The possibility of a transient popu-

v See equation (8), (13) and (14) in chapter 3. vi Equation (10) describes the distance dependence of the outer reorganization energy under the approximation that the donor and acceptor are separate spheres at a certain distance in a dielectric continuum. This is most probably not a satisfactory model to describe the donor-acceptor systems in Figure 10.

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lation of the RuIIIC60- charge separated state was investigated by probing in

the 1000 nm region, but we did not observe the C60- optical signature126 and

thus concluded that energy transfer is the dominating decay mechanism. This study shows that short donor-acceptor linkers is not a viable way to

favor electron transfer over electron-exchange energy transfer in Ru(II)polypyridine-C60 assemblies. Instead, the fullerene was efficiently sensitized to 300-500 nm light.

5.2 Electron transfer in linear donor-photosensitizer-acceptor arrays As mentioned in chapter 2, the Ruthenium-tris-bipyridine (Ru(bpy)3) motif is one of the most popular phototosensitizers in molecular DPA assemblies. Its photostability, high redox potential, high excited state energy and long ex-cited state lifetime make it nearly ideal for the purpose of photoinduced elec-tron transfer. However, it has been argued that the stereoisomeric diversity of DPA arrays based on Ru(bpy)3 and similar tris-bidentate complexes present complications in such applications. Firstly, the average charge sepa-rated state lifetime may be shortened by contributions from isomers with short donor-acceptor distances. Secondly, in molecular arrays the overall directionality of electron transfer in space becomes important.61,138

Three different isomers typically obtained in the synthesis of a Ru(bpy)3-based DPA triad functionalized at the conventional 4,4’-position of the bipy-ridine are shown in Figure 11 (upper row). The electron transfer distance for stepwise forward electron transfer is obviously similar in all these isomers. However, the through-space donor-acceptor distance for the overall recom-bination of the charge separated state is different. The average lifetime of an ensemble of molecules may thus be shortened by the contribution from some isomers, typically those where the effective donor-acceptor distance is short. It has been demonstrated that different isomers can be associated with differ-ent charge separated state lifetimes in DPA triads.139

One approach to overcome this isomeric problem is through stereo-selective synthesis and isolation of the different isomers, which has been demonstrated.140 A less demanding approach is to redesign the photosensi-tizer to obtain the desired geometry.61,141-144 In the following, a DPA triad and a dyad based on the alternative photosensitizer motifs shown in the bot-tom row of Figure 11 are presented. These photosensitizer motifs enable the construction of linear DPA assemblies with defined donor-acceptor geome-try. The studies are found in paper II and III, respectively.

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Figure 11. Top: Three isomers of DPA assemblies based on Ru(bpy)3 by functiona-lization at the 4,4’-positions of the bipyridines, with different donor-acceptor dis-tances. Bottom: The bis-tridentate photosensitizer Ru(dqp)2 (left) or the functionali-zation of Ru(bpy)3 at the 5,5’-posititions of the same bipyridine ligand (right) enables linear DPA assemblies with well-defined geometry and donor-acceptor distances.

5.2.1 DPA assemblies based on bis-tridentate Ru(II)polypyridyl complexes Paper II describes the photophysical investigations of DPA triads based on the novel Ruthenium(II)bis-diquinolinyl-pyridine photosensitizer Ru(dqp)2 developed by Abrahamsson et al.61-62 Triads based on bis-tridentate com-plexes have a defined donor-acceptor arrangement, but have been less suc-cessful as photosensitizers due to the short excited state lifetime, exemplified by Ru(II)bis-terpyridine (Ru(tpy)2) that has a room temperature lifetime of only 250 ps.138 Charge separation yields in DPA assemblies based on bis-tidentate complexes are typically low, since electron transfer has to compete with fast nonradiative transitions from the excited state.138,145-147

The reason for the short excited state lifetime of conventional bis-tridentate complexes has been identified as rapid deactivation of the 3MLCT excited state though metal-centered (MC) states, followed by fast non-radiative decay to the ground state. The low-lying MC states can be ex-plained by a weak ligand field due to the poor bite angle of the tridentate ligands (158� for terpy).148 Since the ligand bite angle of dqp is close to 180� (178� as determined from X-ray crystallography), the ligand field splitting is strong and thus the MC states are pushed towards higher energies. This re-sults in a room-temperature excited state lifetime of 3 �s for Ru(dqp)2.62 At

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the same time, the 3MLCT state energy and redox potentials are comparable to those of Ru(bpy)3.

The structures of the two DPA triads that were investigated in paper II are shown in Figure 12 below. These triads were the first DPA assemblies to be constructed based on the Ru(dqp)2 photosensitizer motif and serve as a proof of principle that the Ru(dqp)2 photosensitizer can be functionalized and that photoinduced charge separation can be obtained in these systems. The donor used in this study was a phenothiazine (PTZ) while benzoquinone (BQ) was chosen as the acceptor. Two different photosensitizer-acceptor linker motifs, an amide bridge and a phenyl linker, where used. The compounds with a phenyl linker were generally more stable during preparation and purification than the amide-linked compounds.

Figure 12. Left: Structures of the PTZRu(dqp)2BQ triads studied in paper III. Note the different linker motifs; di-methyl phenyl (top) and amide (bottom). Right: Tran-sient absorption spectra upon excitation by 460 nm pulses of ~120 fs pulse duration of the phenyl linked triad in acetonitrile. The inset shows the double-difference spectrum for the fully developed signals of the charge separation products BQ- (at 440 nm) and PTZ+ (at 510 nm).

Upon MLCT excitation of the Ru moiety of the RuII(dqp)2BQ reference dyads, the excited state was found to be quenched in both time-resolved emission and transient absorption measurements. However, the expected spectroscopic signature of the reduced electron acceptor (BQ-) was not ob-served in transient absorption. This was interpreted as inverted kinetics, where the back electron transfer from the RuIII(dqp)2BQ- state was much faster than the forward reaction, so that no detectable concentration of the charge separated state could be observed in the transient absorption mea-surements. The rate of electron transfer was determined to kET = 1×109 s-1 for the phenyl linked dyad and kET = 5×108 s-1 in the amide-linked compounds. Since the benzoquinone excited states are thermodynamically uphill from the Ru(dqp)2* excited state,149-150 quenching by energy transfer was excluded.

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In the case of the DPA triads shown in Figure 11, electron transfer from the PTZ donor unit was fast enough to compete with the RuIII(dqp)2BQ- back electron transfer. The concomitant growth of oxidized phenothiazine (PTZ+) and reduced benzoquinone (BQ-) spectral features could be observed in the ultrafast transient absorption spectra, as shown for the phenyl-linked triad in Figure 13. The decay of the fully charge separated state PTZ+RuII(dqp)2BQ- was followed on the nanosecond timescale (see Figure 13). Charge separated state lifetimes of 140 ns and 200 ns for the phenyl-linked and amide-linked triads, respectively, were found. Monoexponential charge recombination kinetics were observed, in line with the single donor-acceptor distance.

Figure 13. Transient absorption at selected wavelengths upon RuII MLCT excitation on different timescales to follow the kinetics of forward electron transfer (left) and charge separated state recombination (right) in the di-methyl-phenyl-linked triad (top structure in Figure 12) in acetonitrile. The growth and decay of the reduced benzoquinone (BQ-) and oxidized phenothiazine (PTZ+) give rates of forward elec-tron transfer kET = 1×109 s-1 and recombination kBET = 5×106 s-1 by global fits (solid lines).

The yield of charge separation was determined to � 95% in the amide-linked triad and � 50% in the (di-methyl-)phenyl-linked triad. This is the first ex-ample known to us of assemblies based on a bis-tridentate photosensitizer that gives charge separation yields comparable to those of Ru(bpy)3-based triads.151-154 This was made possible by the intrinsically long excited state lifetime of the Ru(dqp)2 photosensitizer.

5.2.2 Ru(bpy)3 functinalized through the bipyridine 5,5'-positions The most common functionalization of Ru(bpy)3 is through the synthetically versatile 4,4'-positions of the 2,2'-bipyridine, as shown in the upper row in Figure 11. A 5,5'-functionalization was explored in paper III in the study of the Ru(bpy)3-benzoquinone(BQ) dyads shown in Figure 14.

When both donor and acceptor are attached to the same ligand it may be-come necessary to lower the electronic coupling through the ligand to avoid

42

very fast recombination of the fully charge separated state. A tool for such decoupling may be asymmetric ligands, that push and pull the electrons of the excited and charge separated states toward the acceptor. This is the pur-pose of the amide linker used in this study, which has an electron withdraw-ing side (carbonyl) and an electron donating side (amine). Such effects have been explored previously by Kincaid and co-workers and by Meyer and co-workers.155-156 If there is significant push-and-pull effects, this should be reflected in differences in the rate of electron transfer to the benzoquinone acceptor for the different directions of the amide linker.

Figure 14. Structures and transient absorption spectra upon 480 nm excitation in acetonitrile of the Ru(bpy)3BQ dyads studied in paper III. The obtained rates of electron transfer was kET = 0.90×109 s-1 (left) and kET = 2.4×109 s-1 (right).

The emission of the RuII excited state in the Ru(bpy)3BQ dyads was found to be quenched by electron transfer. As in the study described above (paper II), the spectral features of the reduced benzoquinone BQ- were not observed due to inverted kinetics (kBET >> kET).The rates of forward electron transfer was kET = 0.90×109 s-1 and kET = 2.4×109 s-1, respectively (see Figure 14). The small differences in electron transfer rates indicate that there is no sig-nificant push-pull effect of the amide link in the different directions, or that other factors are more important for the effective rate of electron transfer in this case, such as small differences in driving force. One possible interpreta-tion is that the push-pull effects in the 5,5'-functionalization is less pro-nounced than that demonstrated for the 4,4'-functionalization. The role of the amide linker remains unclear, but the possibility to utilize the 5,5'-functionalization for linear donor-acceptor assemblies based on Ru(bpy)3 was demonstrated.

It has been observed previously that the isolation of quinone-containing Ru(II)-polypyridyl compounds free from hydroquinone impurities can be challenging.157-158 In the two studies described here, stability problems were encountered in particular with the amide bridged benzoquinone compounds.

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5.3 Towards accumulative electron transfer: Multi-electron acceptors Some acceptors and donors that have been previously used in DPA assem-blies for single-electron transfer studies have promising properties for accu-mulative electron transfer. In the systems described above, the acceptor units in the Ru(bpy)3-C60 fullerene dyads (Figure 11) and Ru(II)polypyridine-benzoquinones (Figure 12, Figure 14) were chosen in part due to their po-tential function as multi-electron acceptors.

Figure 15. Lef: Ladder scheme for the stepwise reduction and protonation of 1,4-benzoquinone. Right: Schematic Pourbaix diagram for the proton-coupled reduction of 1,4-benzoquinone in aqueous solution.

5.3.1 C60 as a multi-electron acceptor Pristine C60 fullerene has been reported to accumulate up to six electrons within a 2 V potential range upon successive reductions127,159 which makes it potentially interesting for accumulative electron transfer studies. To put a second electron onto the fullerene a more reducing potential is needed (E1/2(C60

-/C602-) = -0.99 V vs SCE).126-127 This is verging on what the excited

state of conventional and modified Ru(II)polypyridine complexes can pro-vide.59 Other sensitizer motifs might be more suitable for such studies, e.g. porphyrins and phthalocyanines.

5.3.2 Quinones: Two-electron two-proton acceptors Quinones have been used quite extensively in donor-acceptor assemblies for photoinduced electron transfer.24,26,64,152,157,160-163 However, these studies all concern single electron transfer and do not explore the possibility of using quinones as a two-electron acceptor. This could be particularly interesting since the quinones can also accept two protons, as shown in Figure 15.164-165

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In a protic solvent, electron transfer to quinones is coupled to proton transfer in a quite wide pH range, which compensates for the electronic repulsion energy in the second reduction step and actually inverts the order of the elec-trode potentials.120,166 This is shown in the schematic Pourbaix diagram for 1,4-benzoquinone in aqueous solution in Figure 15.

The Ru(dqp)2-benzoquinone dyads presented in paper II were also inves-tigated in aqueous solvents. The results of these studies are summarized in the Appendix (page 75–77). For these dyads, inverted kinetics was observed in acetonitrile and the rate of charge recombination could not be resolved. Compared to in acetonitrile (�Gº = -0.3 eV), the redox potentials in aqueous solution are shifted so that the driving force for photoinduced electron trans-fer is higher, ca. �Gº(pH 7) = -0.7 eV and �Gº(pH 2) = -0.8 eV. This should lead to increased rates of electron transfer, something which was also ob-served; the rate of forward electron transfer in aqueous solution of the two dyads was kET = 8.0×109 s-1 and kET = 7.4×109 s-1 which is ca. 10 times faster than the corresponding rates measured in acetonitrile. Inverted kinetics were observed, but for one of the dyads the rate of back electron transfer could be determined kBET = 8.0×109 s-1. This corresponds to a maximum population yield of the RuIIIBQ- charge separated state of ca. 20 % at 40 ps. The overall same behavior was observed at pH 2, where no significant increase of the charge separated state lifetime due to protonation could be observed. Most likely, the protonation of the semiquinone is disfavored by the fast recombi-nation of RuIIIBQ-. The inverted kinetics behavior makes these dyads less than optimal for charge accumulation studies.

Investigations of a Ru(bpy)3-benzoquinone dyad previously studied in acetonitrile by M. Borgström et al152 was also performed. As above, the rates of forward and back electron transfer increased in aqueous solvent compared to acetonitrile. This was ascribed to the increased driving force for forward electron transfer and decreased recombination driving force, assuming that the back reaction occurs in the Marcus inverted region. The same overall behavior was observed at pH 2. The maximum population of the RuIIIBQ- charge separated state was obtained at 20 ps as determined by kinetic simu-lations. Due to the fast recombination a covalently linked donor would be required to achieve multiple excitations and accumulative charge separation in the system. Moreover, the semiquinone/hydroquinone signature overlaps with the Ru(bpy)3 ground state bleach and RuIII signature, which means that it may be difficult to follow the redox and protonation state of the benzoqui-none directly in optical spectroscopy. In all, this Ru(bpy)3BQ dyad was also found to be non-optimal for charge accumulation studies.

The possibility of proton-coupled electron transfer as a means to compen-sate for the electron repulsion energy in accumulative electron transfer was discussed in chapter 4. It is the author’s opinion that the proton-coupled chemistry of quinones holds a significant promise for charge accumulation studies. It is however important to first identify systems that have more ad-

45

vantageous behavior in the first charge separated state, e.g. long charge sepa-rated state lifetime in aqueous solution, distinguishable spectroscopic signa-tures and suitable redox properties.

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6. Results II. On the challenges of accumulative electron transfer: Double-pulse excitation studies of a Mn2-Ru(II)-di-Napthalenediimide triad

As was discussed in chapter 4, there are documented examples of processes that can prevent accumulative electron transfer, or lower the yield of the doubly charge separated state (D2+PA2- in Figure 8).116-118,167 These processes are related to the potential unproductive decay pathways in accumulative electron transfer. It is of outmost importance to better understand these un-productive pathways in order to construct systems where these losses are avoided. In paper IV, a triad previously investigated in single electron trans-fer studies was exposed to multiple excitations, to investigate the productive and unproductive decay mechanisms.

6.1 Background and kinetics of single-electron transfer This study builds on previous results from M. Borgström et al, who reported photoinduced electron transfer in the Mn2-Ru(II)-di-naphthalene-diimide(NDI2) triad shown to the left in Figure 16.168 The Mn2

II,IIIRuIINDI--NDI charge separated state, as followed by optical spectroscopy and EPR, was found to be very long-lived with a multiexponential decay on the �sms time scale. This was attributed to an unusually large activiation energy, that stems from the large inner reorganization energy of the Manganese dimer in the Mn2

II,IIINDI- recombination (for which � ~ 2.0 eV). The Manganese dimer’s initial oxidation state is Mn2

II,II. Studies of a re-lated Ru(bpy)3Mn2

II,II dyad,108-109 identical to the triad in Figure 16 apart from the acceptors, showed that in the presence of a sacrificial electron ac-ceptor ([Co(NH3)5Cl]2+) the formation of Mn2

III,IV could be observed by EPR (at 77 K). It was also shown that photo-oxidized Ru(bpy)3 could retrieve electrons from the Mn2

II,II and chemically prepared Mn2II,III in bimolecular

reactions at room temperature.109 Thus, in the Mn2Ru(bpy)3NDI2 triad the multi-electron donor Mn2 and the two single-electron acceptors NDI could in principle give the doubly charge separated state Mn2

III,IIIRuIINDI-NDI- upon two successive excitations, as shown in Figure 17. This motivated the

47

double-pulse excitation studies of the Mn2Ru(bpy)3NDI2 triad presented in paper IV.

Figure 16. Left: Structure of the Mn2Ru(bpy)3NDI2 triad. Right: Ground state elec-tronic absorption spectrum in butyronitrile (solid line, left scale) and the transient absorption spectrum recorded at 1 �s after single pulse excitation at 460 nm (dotted line, right scale) assigned to Mn2

II,IIIRuIINDI-NDI.

Figure 17. Schematic energy diagram over relevant states in the Mn2Ru(bpy)3NDI2 triad upon double pulse excitation (Mn2* states omitted). The kinetics of single charge separation (1), (2) and recombination (3) agrees well with previously pub-lished results on the same system.168 Kinetics for process (4), (5) and (6) are taken from published studies on relevant model compounds.109,167 Processes (7) and (8) are proposed in paper IV.

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6.2 Double-pulse excitation studies In Figure 17 some of the relevant states for single- and double-pulse excita-tion of the Mn2Ru(bpy)3NDI2 triad are given (states related to electron trans-fer and energy transfer involving NDI are shown, states related to energy transfer to the Mn2 unit are omitted for clarity). The kinetics given in the figure are from the single-pulse and double-pulse measurements in this study (paper IV), together with data from previous studies of relevant model com-plexes.109,167 The visible absorption of the Manganese dimer is weak, but the NDI- radical and Ru oxidation state could be followed by optical spectrosco-py. The evaluation was therefore primarily based on the yield of NDI- and RuII* excited state decay.

The transient absorption spectra collected upon 460 nm double-pulse ex-citation were identical to those obtained in single-pulse measurements within the timescale of the experiments, apart from the spectral amplitudes. The quenching of RuII* and growth of the NDI- radical feature was followed by the RuII* emission at 610 nm and transient absorption. Figure 18 presents the transient absorption at 475 nm, the NDI- radical absorption maximum, upon 460 nm single-pulse (thin line) and double-pulse (bold line) excitation. The NDI- growth and concomitant RuII ground state recovery kinetics were found to be identical to the single-pulse measurements. However, the yield of NDI- upon the second excitation appeared to be significantly depleted. This can be understood mainly as an effect of direct excitation of the NDI- by the second excitation pulse, as shown by changing the second pulse exci-tation wavelength to 605 nm, where only NDI- absorbs.

Figure 18. Transient absorption decay at 475 nm upon 460 nm single-pulse excita-tion (thin line) and double-pulse excitation (bold line).

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The RuII* decay as followed by the emission at 610 nm after the first and second excitation pulse (both at 460 nm) is shown in Figure 19. The decay kinetics after the first and second pulse were identical. However, the emis-sion amplitude after the second excitation pulse was ca. 10% lower than after the first pulse. A similar difference was observed upon close inspection of the RuII* ground state bleach at early times. This difference cannot be as-cribed only to variations of the pump intensities (± 2% in an average of 10 flashes), nor the direct excitation of the NDI- and its screening of RuII in the second excitation pulse as described above (corresponding to only ca. 2-3% of the total number of RuII in the probe volume). In detailed quantitative evaluation, it was found that the small but not insignificant fraction (ca. 1.5% of molecules in the probe volume) that form the Mn2

II,III(RuII)*NDI--NDI excited state must be included to account for the observed losses of RuII*. This points to a fast quenching process from the Mn2

II,III(RuII)*NDI--NDI state that cannot be resolved on the timescale of these experiments (� 10 ns).

Figure 19. Ru(II) excited state decay probed by the emission at 610 nm upon 460 nm single-pulse excitation (dotted line) and double-pulse excitation (solid line).

The possible quenching mechanisms presented in paper IV are energy trans-fer from Ru(II)* to NDI- or Mn2

II,III, followed by fast decay, or reverse elec-tron transfer to the Mn2

II,IIRuIIINDI-NDI or Mn2II,IIIRuI(NDI)2 states. To re-

solve which of these processes that dominates the quenching, studies of pre-oxidized and pre-reduced model compounds are suggested.

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6.3 Reflections on the challenges in accumulative electron transfer The Mn2Ru(bpy)3NDI2 triad studied in paper IV seemed to have several of the required properties to achieve accumulative electron transfer upon mul-tiple excitations. The Mn2 dimer had shown several oxidations within a nar-row potential range, coupled to ligand reorganization.109 Also, the long-lived charge separated state gives plenty of time for a second excitation event to take place. However, this study showed that the Ru(II)* excited state was rapidly quenched upon the second excitation, instead of leading to formation of the doubly charge separated state Mn2

III,IIIRuIINDI-NDI-. These results illustrate how the manifold of decay pathways that are

available from the second excited state (D+P*A- in Figure 8) compete with the productive charge separation in the direction of the doubly charge sepa-rated state (D2+PA2- or, in this case, Mn2

III,IIIRuIINDI-NDI-). A further com-plication in this case, compared to the scheme in Figure 8, is that energy transfer provides additional unproductive decay pathways.

To prevent the unproductive pathways from the Mn2II,III(RuII)*NDI-NDI

excited state it seems that it is necessary to decouple the excited photosensi-tizer from the oxidized donor and reduced acceptor. Whether the competing reactions are reverse electron transfer or energy transfer, this could be achieved by increasing the donor-photosensitizer and acceptor-photosensitizer distance.

This study also serves as an example of the importance of optimization of the first, single-electron, charge separation process. The lifetime of the Mn2

II,IIIRuIINDI-NDI charge separated state is sufficiently long for multiple excitations to occur, but the low yield is a complication as the total yield of the doubly charge separated state stems from the yield of the first charge separation.

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7. Results III. Accumulative electron transfer in semiconductor-Ru(II)-oligotriarylamine systems

This chapter describes the successful accumulation of holes and electrons upon step-wise excitation in the semiconductor-Ru(II)-oligotriarylamine systems investigated in papers V and VI. A doubly charge separated state was obtained in high overall yield, and no sacrificial agents were required to obtain electron and hole accumulation. Also, the photosensitizer was used in a regenerative fashion, in mimic of P680 in PSII. The key strategies for accu-mulative electron transfer employed in these systems are 1) fast initial elec-tron transfer, that promotes the productive electron transfer direction over reverse electron transfer, and 2) decoupling of the electrons and holes, which prevents recombination from intermediate charge separated states.

7.1 Photoinduced electron transfer in DPA systems with a nanocrystalline semiconductor acceptor The studies in papers V and VI concern photoinduced electron transfer in Ru(II)-oligotriarylamine dyes (RuOTA) covalently attached to nanocrystal-line titanium dioxide (TiO2) by carboxylic anchoring groups, as illustrated in Figure 20. The choice of TiO2 as the acceptor was based on the documented fast electron injection from photoexcited Ru(II)-polypyridyl complexes into the conduction band of TiO2, especially in the presence of small cations such as Li+.169-176 The resulting TiO2

(-)RuIII charge separated state in such systems is known to be very long-lived.171,175,177

The OTA donor unit is suitable for multiple hole transfers, since it dis-plays several oxidation peaks within a quite narrow potential range, of which at least two accessible from the oxidized photosensitizer. Moreover, the oxi-dation states can be differentiated by their optical absorption spectra, as shown by spectroelectrochemical studies in paper V and VI.

The dyes under investigation here are shown in Figure 20. The Ruthe-nium dye RuImid and ‘first generation’ imidazole-linked dye with a single triarylamine acceptor, RuImidTA, were used as references. The two oligo-amine complexes RuOxaOTA and RuImidOTA differ only in the link be-

52

tween the Ru moiety and the OTA, which is either an oxazol or an imida-zole. When adsorbed on TiO2, photoexcitation of the Ru(II) moiety in a transparent electrolyte (propylene carbonate, 1 M LiClO4) gave rapid injec-tion of electrons into TiO2 (� 10 ps) corresponding to formation of the charge separated state TiO2

-RuIII. In RuImidTA, RuImidOTA and RuOxaO-TA, this initial electron transfer was followed by hole transfer from RuIII to the triarylamine donor unit on the nanosecond timescale.

Figure 20. Illustration of the oxazole-linked dye RuImidOTA anchored on a TiO2 nanocrystal (not to scale). To the right, the structures of the other RuII dyes used in this study are shown.

Other DPA systems with nanocrystalline TiO2 as the acceptor have been reported, as dyes for Grätzel-type solar cells.178-181 In those studies, the main purpose of the donor D was to slow down charge recombination by the in-creased electron-hole distance, resulting in a lowered coupling for the back reaction. This was observed also in our studies, in particular for the RuImid-TA and RuImidOTA dyes. The overall decay of the singly charge separated states TiO2

-RuIIOTA+ and TiO2-RuIITA+ were found to be multi-exponential,

with complete recombination occurring on the millisecond timescale.

7.2 Accumulation of holes upon multiple excitations In the case of the RuOTA dyes, there is a possibility to re-excite the photo-sensitizer in the first charge separated state and produce the double charge separated state TiO2

2-RuIIOTA2+, as depicted in Figure 21. However, for this to work, it is necessary that the productive pathways leading to the second charge separated state are kinetically favored over unproductive pathways, as discussed in chapter 4.

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Figure 21. Schematic energy level diagram for the TiO2RuOxaOTA system.

The RuOxaOTA and RuImidOTA dyes were exposed to double excitations by two 10 ns pulses delayed by 1 �s. The obtained transient absorption spec-tra for the TiO2RuOxaOTA system collected directly upon the first and second excitation pulse are shown in Figure 22. The second excitation pulse acted on both the TiO2

-RuIIOTA+ charge separated state produced by the first excitation pulse and on the dyes that did not interact with the first pulse. When this latter fraction, which forms the TiO2

-RuIIOTA+ charge separated state from the ground state in the second pulse, was subtracted, the differ-ence spectrum shown by open circles in Figure 22 was obtained. This differ-ence spectrum fits well with the difference absorption spectrum for the doub-ly oxidized OTA2+ obtained from spectroelectrochemistry. It was thus con-cluded that the effect of the excitation of the TiO2

-RuIIOTA+ charge sepa-rated state was that of further charge separation, to form the doubly charge separated state TiO2

2-RuIIOTA2+ where electrons have accumulated in the TiO2 and two holes are accumulated at the OTA donor. The same effect could be observed in single-pulse measurements at high excitation intensi-ties. Quantitative evaluation of the degree of excitation and the concentration of the TiO2

2-RuIIOTA2+ state gave unity yield of formation from the TiO2--

(RuII)*OTA+ state.

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Figure 22. Transient absorption spectra obtained upon single-pulse (bold dots) and double-pulse (thin dots) excitation by 480 nm 10 ns pulses of RuOxaOTA on TiO2 film in a propylene carbonate electrolyte (1 M LiClO4). The double difference spec-trum (open circles) matches well the difference absorption spectrum obtained for the doubly oxidized OTA2+(solid line), as obtained by electrochemical oxidation of RuOxaOTA on TiO2 film.

The formation of TiO22-RuIIOTA2+ was found to occur within the response of

the 10 ns excitation experiment. The doubly charge separated state was as-sumed to form through injection followed by hole transfer, as mapped out in Figure 21, in parallel with the formation of the first charge separated state. The TiO2

2-RuIIOTA2+ signature was found to contribute to the spectrum up to at least 100 �s delay after the second excitation pulse. The recombination rates of the doubly oxidized state could however not be isolated from the multi-exponential decay of the TiO2

-RuIIOTA+ charge separated state de-scribed above.

For the TiO2RuImidOTA system, excitation at sufficiently high intensities gave unique spectral features upon the second excitation. However, for this dye the obtained difference spectrum did not satisfactorily match the differ-ence absorption signatures expected for the doubly oxidized donor OTA2+. This spectrum was instead tentatively assigned to the oxidized imidazole (Imid+). This indicates that the TiO2

2-RuIIImid+OTA+ state may actually lie below the TiO2

2-RuIIIImidOTA+ state and hole transfer to the imidazole may occur upon the injection of a second electron. It is proposed in paper VI that the oxidation of the imidazole is facilitated by the proximity of the OTA unit, and possibly also by deprotonation of the imidazole which have been shown to shift the oxidation potential of the imidazole in similar

55

complexes,182 promoted by the OTA+. The evolution of the proposed TiO22-

RuIIImid+OTA+ state will be followed on longer timescales to investigate whether further hole transfer to form the TiO2

2-RuIIImidOTA2+ can occur.

7.3 A successful approach to accumulative electron transfer As mentioned above, the design strategies employed to promote accumula-tion of electrons and holes in these systems are that of rapid primary electron transfer in the forward direction, and weak electronic coupling between the electrons and holes to avoid recombination in intermediate states, as dis-cussed on a general level in chapter 4.

The different decay pathways that are relevant in the RuOxaOTA system are indicated in the energy level diagram in Figure 21. From the second ex-cited state TiO2

-(RuII)*OTA+ the injection of a second electron into the TiO2 conduction band competes with the reverse electron transfer pathways. By these competing reactions (approximate driving force 1-1.5 eV), formation of the TiO2(RuI)*OTA+ or TiO2

-(RuIII)OTA states may occur. However, the injection is faster, and this gives the intermediate charge separated state TiO2

2-RuIIIOTA+ in high yield. In paper VI, it is proposed that the electronic coupling favors injection over the reverse electron transfer pathways due to the excited state localization on the bipyridine ligand bound to the TiO2 na-noparticle.

From the TiO22-RuIIIOTA+ state, recombination to the TiO2

-RuIIOTA+ state or the TiO2

-RuIIIOTA state, which subsequently gives TiO2-RuIIOTA+,

can occur. In analogy with the first level of charge separation, the electrons in TiO2 and holes on the OTA moiety are sufficiently decoupled for these back reactions to be slow. Instead, the fully charge separated state TiO2

2-

RuIIOTA2+ was formed in high yield. This state was shown to have a long lifetime, ascribed to the long donor-acceptor distance.

In the systems described here, photoaccumulation of electrons and holes in two discrete steps was shown, using the photosensitizer regeneratively. In contrast to the systems described in the previous chapter, there were no losses from unproductive decay pathways and the doubly charge separated state was obtained in high yield. This system is one of the few to show that accumulation of holes and electrons can be achieved without the aid of sa-crificial donors or acceptors. The key step, and probably most important for the overall yield, is the fast initial electron transfer. In the studies presented in paper V and VI, we have shown that this is a propitious design strategy for accumulative electron transfer.

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8. Concluding remarks

The studies described in this thesis aims to elucidate the possibilities and challenges of photoinduced electron transfer in molecular systems. The sys-tems presented in paper I-III illustrate the challenges of obtaining charge separation of a single electron-hole pair in high yields, minimizing side reac-tions such as energy transfer, and to obtain long-lived charge separated states. In paper I, the possibility to obtain electron transfer in a series of Ru(bpy)3-C60 fullerene dyads was investigated. It was found that it is very difficult to avoid energy transfer in such systems, due to the low-lying triplet excited state C60*. From the linear Ru(dqp)2-based triads presented in paper II it was shown to be possible to obtain long-lived charge separated states in high yields in systems based on bis-tridentate Ru(II)polypyridine complexes. This was ascribed to the unique photophysical properties of the Ru(dqp)2 photosensitizer, that was here used for the first time in donor-acceptor as-semblies. In paper III, we explored the possibilities to use a 5,5'-bipyridine functionalization to construct linear DPA assemblies based on Ru(bpy)3. The studies of photoinduced electron transfer on the single electron-hole pair level have implications in the context of artificial photosynthesis, as dis-cussed herein, but may also contribute in the development of molecular elec-tronic devices, photovoltaics and similar applications.

The multi-excitation studies presented in paper IV-VI aims towards the accumulation of redox equivalents upon successive photoinduced electron transfer events. The relevant context of these investigations is photodriven catalysis of multi-electron processes, e.g., solar fuel production. The detailed kinetic investigation of such systems provides important information on the intramolecular photochemistry leading to accumulation of charge, or the dissipation of the energy to other molecular states within a donor-acceptor assembly. The latter is exemplified in the Mn2Ru(bpy)3NDI2 triad (paper IV), where accumulation of charge was prevented by intramolecular recom-bination pathways in the triad, despite previous results on several successive oxidations in bimolecular reactions with the corresponding Mn2Ru(bpy)3 dyad and a sacrificial acceptor.109 The study described in paper IV also illu-strates the importance of optimization of the single-electron charge separa-tion as to obtain the first charge separated state in high yield.

Finally, accumulation of two holes and two electrons without sacrificial agents was demonstrated in the TiO2RuOxaOTA system described in paper V–VI. The key component for the successful charge accumulation in this

57

system was identified as fast primary electron transfer, which could compete with the rate of reverse electron transfer. This is one of the design strategies for the promotion of accumulative electron transfer suggested in chapter 4, which proved to be successful here. A future challenge is to couple the cur-rent system to catalytic units. It would also be of fundamental interest to try to repeat this in a system that is not based on a nanocrystalline TiO2 semi-conductor, perhaps by means of intermediate acceptors as to rapidly remove the electrons from the proximity of the sensitizer and the oxidized donor.

The work presented herein will hopefully contribute to the identification and understanding of a scientific challenge that has so far not gained much attention in the field of photoinduced electron transfer research, namely the accumulation of redox equivalents in molecular systems in successive pho-toinduced electron transfer steps, here called accumulative electron transfer. This may be a very important question to address to arrive at the target mul-ti-electron chemistry of artificial photosynthesis; solar fuels production and water oxidation. Future work on this could concern the continued study of the ‘unproductive’ decay pathways that are specific for accumulative elec-tron transfer processes, to better understand the challenges and find new solutions. In chapter 4, a number of strategies to overcome the identified challenges were suggested, among which there are many possibilities to try out. In particular the possibility of coupled electron transfers in accumulative electron transfer has not yet been extensively explored.

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9. Methods

The most important techniques used in this thesis are transient absorption spectroscopy and time-resolved emission. In these techniques, the sample is excited by a laser pump pulse and the resulting excited state dynamics and photochemistry are followed by either monitoring the absorption of the sam-ple or the emission of the excited state as a function of time. The instrumen-tal response and the laser pulse duration determine the timescale of the expe-riment.

The transient absorption signal is the difference between the ground state absorption (before excitation) and the absorption at a certain delay time after the pump. Thus, only those molecules that are affected by the pump contri-bute to the transient absorption signal. This technique was used to follow excited state dynamics and intermediates in the photochemical reactions, such as electron transfer products. The transient absorption signal was ob-tained as

�%67 � �89 #:;<=:>= ) � �89 �:;<=:?@A� B 89 � :>=:?@A� (15)

where IGS, IPIS and Iref are the intensity of the probe light after passing through the non-excited sample (IGS), after passing through the photo-induced state (IPIS) and after passing through a reference medium not con-taining the sample (Iref).

Emissive excited states can also be probed by their fluorescence or phos-phorescence. Time-resolved emission spectroscopy on different time scales provided complementary information on the kinetics of electron and energy transfer. The emission quantum yield, �em, was obtained from steady-state emission measurements. In steady-state techniques, the absorption or emis-sion of the sample under continuous illumination is monitored.

Redox potentials were determined by cyclic voltammetry and differential pulse voltammetry. In these techniques, the current in an electrochemical cell is monitored as a function of the applied voltage. In spectroelectroche-mistry, the steady-state absorption is monitored simultaneously with the electrochemical oxidation or reduction by chronoampereometry. In this way, intermediates and products of an electrochemical reaction can be studied and the optical signature of stable redox species can be obtained.

59

Figure 23. Schematic setup for ultrafast transient absorption spectroscopy (pump-probe).

9.1 Ultrafast pump-probe spectroscopy The schematic setup for the ultrafast transient absorption measurements is shown in Figure 23. A Ti:sapphire laser system (Coherent) produced laser pulses at 800 nm with ~120 fs pulse duration at a repetition rate of 1 kHz. One fraction of the laser output was taken into an optical parametric amplifi-er (TOPAS), where non-linear mixing and doubling gave tunable output in the 480-790 nm range. Pump light in the 420-480 nm range was typically obtained by sum-frequency mixing of the TOPAS output (signal and 2 output) in an external crystal. The pump was taken though a chopper and a half-wave plate to adjust the polarization before passing through the vertical-ly moving sample.

Broadband probe light (350-700 nm) was produced in a CaF2 crystal from a small fraction of the Ti:sapphire 800 nm output, after passing through a movable delay line (delay 0-10 ns, step accuracy 10 �m/30 fs) and a polariz-er. For probe light in the near-IR region (> 800 nm) a stationary sapphire plate was used instead or, in paper I, the near-IR signal output of a TOPAS White (not shown), that was directed through a movable delay line (delay 0-2 ns) before arriving at the sample and detector.

60

A 500 Hz chopper blocked every second laser pulse to make possible the measurement of IGS in every second measurement (15). The reference me-dium for Iref was air. The signal was detected on a diode array (DA) after passing through a spectrograph. Typically, averages of 5000-10000 shots were used for each time point.

9.2 Nanosecond transient absorption and emission spectroscopy Time-resolved spectroscopy on the tens of nanoseconds and longer time-scales was performed with the experimental setup schematically described in Figure 24. In all experiments, the signal was taken though a monochromator and detected at right angle to the incoming laser light. Probe light was pro-vided by a pulsed 150 W Xe arc lamp for measurements up to ca. 1000 �s. A Tungsten/Iodine lamp was used to provide continuous probe light for longer timescale measurements. Transient traces were collected by a photomultip-lier tube (PMT) connected to a wide bandwidth oscilloscope. Alternatively, a spectrograph and CCD camera was used to collect transient spectra.

Figure 24. Schematic experimental setup for nanosecond transient absorption spec-troscopy and time-resolved emission.

In single pulse measurements, a frequency-tripled Nd:YAG laser with an optical parametric oscillator (OPO) was used to produce excitation light of ~10 ns pulse duration, tunable in the range 410 nm to 680 nm. The laser flashlamp and Q-switch triggering was controlled from the detection soft-ware. For the nanosecond double-pulse measurements used in paper IV–VI, two equivalent pulsed Nd:YAG lasers with separate OPOs were used, as

61

shown in Figure 24. The effective delay between laser pulses was adjusted by an external electronic delay, that triggered the laser flashlamps and Q-switches for both lasers individually. Direct comparison of single pulse and double pulse measurements was readily accomplished by blocking one of the pump lasers.

For nanosecond time-resolved emission measurements, the same pulsed Nd:YAG laser(s) with OPO described above was used to obtain excitation pulses of ~10 ns pulse duration in the 410–680 nm range. The probe light was blocked and the signal detected as described above.

9.3 Time-correlated single photon counting Time-resolved emission on the ultrafast time scale (10 ps100 ns) was meas-ured by time-correlated single-photon counting (TCSPC). Excitation pulses of ~150 fs pulse duration was obtained from a 200 kHz Ti:sapphire pulsed laser system (Coherent) depicted in Figure 25. The 800 nm laser output was doubled in a BBO crystal to provide 400 nm pump pulses. The pump light was attenuated before the sample chamber to assure a low photon count on the detector.

Figure 25. Schematic setup for the time-correlated single photon counting (TCSPC) measurements.

A cooled micro-channel plate photomultiplier (Hamamatsu) was used to detect emitted photons perpendicular to the incoming pump. Combinations of band-pass and long-pass filters were used to eliminate excitation contami-

62

nation of the decays. The emission detector was set to magic angle (54.7°) vs. the vertically polarized pump light.

A small fraction of the pump was reflected onto a photodiode to provide the start signal. Start and stop pulses were collected by a time-to-amplitude converter (TAC) and multichannel analyzer (MCA). The time-to-amplitude converter registers the time separation of the excitation and emission events. After collection of a large number of datapoints, a histogram showing the number of photons at different times was obtained, which is a statistical measure of the emission lifetime.183

63

Acknowledgements

First and foremost, I would like to thank my advisors Leif Hammarström and Hans-Christian Becker for all the things I have learned from you during the production of this thesis, for your readiness to answer and discuss my ques-tions on science and science-related things and for your general (and some-times particular) support.

I would also like to express my gratitude towards my collaborators and co-authors, without whom this work would not have been possible: Prof Fabrice Odobel, Dr Julien Boixel, Dr Yann Pellegrin and Dr Errol Blart at the University of Nantes, Prof Helena Grennberg and Dr Judit Modin at Uppsala University and, also in Uppsala, Dr Olof Johansson, Dr Rohan J Kumar and Dr Michael Jäger. I would like to thank Alice Rolandi Jensen, Daniel Streich, Dr Magnus Falkenström (former Borgström) and Erik Göransson, with whom I collaborated closely. It has been a pleasure working with you all.

I have been fortunate to work within the Swedish Consortium for Artifi-cial Photosynthesis and this has been very stimulating. Thank you, past and present CAP-members and in particular Prof Stenbjörn Styring for all input, feedback and discussions. I would also like to thank Erik Göransson and Dr Todd Markle for proof-reading and commenting on earlier versions of this thesis, and Dr James Gardner for the back cover photograph.

Working at the Department of Photochemistry and Molecular Science has been very enjoyable and for this I wish to thank all colleagues. In particular I thank past and present members of the Chemical Physics group for all scien-tific discussions and all the fun in and outside the lab. Thank you, Prof Jan Davidsson for generously sharing advice on teaching and science. Thank you Gunilla Hjort, Sven Johansson, Åsa Furberg and Susanne Söderberg for all the things you do to keep this place running smoothly.

I started my Ph D studies at the former Department of Physical Chemistry and I would like to acknowledge my colleges there who gave me a good start to my studies. Thank you, Anna Lundquist and Malin Morin for the alumni society events that we worked on together, and our nice lunches.

Liljewachls stipendiefond are acknowledged for funding contributions for conferences and travels.

Last, but definitely not least, I would like to thank my friends (including some former and present colleagues, you know who you are!) and family for you invaluable support. No-one mentioned, no-one forgotten.

64

Summary in Swedish

Enkel och ackumulativ elektronöverföring – en förutsättning för artificiell fotosyntes

Det går nästan inte att öppna en tidning eller titta på nyheterna idag utan att se något inlägg som handlar om klimat och annalkande klimatförändringar. Samtidigt baserar vi en stor del av vår ekonomi och vår energiutvinning på fossila bränslen. Jordens befolkning växer och en förutsättning för att alla dessa människor ska få en rimlig levnadsstandard är tillgång till billig och ren energi, helst från förnyelsebara resurser. En av de ännu underutnyttjade energikällorna är solen, som varje timme förser jorden med ungefär lika mycket energi som förbrukas under ett år, baserat på 2001 års globala ener-gikonsumtion.6 En ökad användning av solenergi förutsätter dock att vi kan lagra energi på ett effektivt och praktiskt sätt, för att ha tillgång till den även nattetid och vid dåliga väderförhållanden. Att lagra energi i kemisk form, d.v.s. i form av förnyelsebara bränslen, är ett av de mer attraktiva alternati-ven, inte minst för transporter och uppvärmning.

Sedan länge har forskare funderat på om man skulle kunna använda den energi som finns i solljus för att producera förnyelsebara bränslen.1 Om man spjälkar vatten genom att plocka ut elektroner får man syrgas och protoner. De elektroner och protoner som frigörs i vattenoxidationen skulle kunna användas för att framställa till exempel vätgas (H2) eller myrsyra (HCOOH) och kolmonoxid (CO), som man kan erhålla från koldioxid (CO2), enligt reaktionsformlerna på nästa sida. Vätgas kan sedan användas som bränsle och både CO och HCOOH kan omvandlas till användbara bänslen.

De här reaktionsformlerna hittar man i nästan vilken kemibok som helst, men trots det är kemin ganska svår att genomföra. Det som inte syns i form-lerna är den extra energi, den så kallade överpotentialen, som måste tillföras eftersom bindningar måste sträckas, brytas och bildas under de kemiska re-aktionernas gång. Denna överpotential gör att det inte är tillräckligt effektivt att spjälka vatten och framställa vätgas med hjälp av enkel elektrolys. Även om man lyckas utveckla bra katalysatorer8,11,14 medför de olika stegen att först omvandla solljus till ström och sedan använda strömmen för elektrolys alltid vissa förluster. Det vore bättre att direkt skapa kemiska redox-ekvivalenter, så att vattenoxidation och bränsleproduktion kan drivas i ett molekylärt system utan mellansteg.

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2H2O � O2 + 4H+ + 4e- vattenoxidation 2H+ + 2e- � H2 vätgasproduktion CO2 + 2H+ + 2e- � HCOOH reduktion av koldioxid CO2 + 2H+ + 2e- � CO + H2O Ett sådant system finns faktiskt redan i gröna växter, cyanobakterier och

alger. De här organismerna utför fotosyntes, där vatten spjälkas med hjälp av solljus samtidigt som cellens bränsle, NADPH och sockerarter, bildas.19 Genom att titta på de olika funktioner som finns i den naturliga fotosyntesen kan vi lära oss vilka olika enheter som behövs för att göra samma sak på konstgjord väg, genom så kallad artificiell fotosyntes.35,184

Arbetet som beskrivs i den här avhandlingen handlar om att rekonstruera det steg i fotosyntesen där ljusenergi från solen tas upp av ett färgämne och sedan omvandlas till kemiska redoxekvivalenter genom en process som kal-las fotoinducerad elektronöverföring. I fotosystem II, det proteinkomplex där vatten oxideras i den naturliga fotosyntesen, är det en klorofyllmolekyl som tar upp ljuset. Genom den extra energi molekylen får när den absorberar ljus kan den reducera och oxidera andra molekyler i sin närhet, varpå energin omvandlas till kemiska redoxekvivalenter. I våra artificiella system är kloro-fyll ersatt av andra färgämnen, Rutenium(II)polypyridin-komplex, som i sin omedelbara närhet har en acceptor, som kan reduceras av det fotoexciterade färgämnet, och en donor som i sin tur reducerar färgämnet igen. Undersök-ningarna som beskrivs i avhandlingen handlar om hur man kan optimera laddningsseparationen i den här typen av system.

När ljuset har absorberats av färgämnet gäller det att det laddningssepare-rade tillståndet bildas med så högt utbyte som möjligt. Om det finns konkur-rerande reaktioner, till exempel energiöverföring, blir utbytet mindre än 100% vilket minskar effektiviteten för hela systemet. Ett av projekten (arti-kel I) som beskrivs här handlar om energiöverföring kontra elektronöverfö-ring i ett molekylärt system.

Det är också viktigt att det laddningsseparerade tillståndet är tillräckligt långlivat, det vill säga att elektronen och hålet inte hittar tillbaka till varandra innan man kan använda det laddningsseparerade tillståndet till katalys. En del av avhandlingen (artikel II och III) handlar om undersökningar av linjära system, där acceptorer och donatorer ligger så långt ifrån varandra som möj-ligt, för att just åstadkomma långlivade laddningsseparerade tillstånd, och för att det för vissa användningar är önskvärt att ha samma riktning för elek-tronöverföring i en ensemble av molekyler.

I fotoinducerad elektronöverföring är det typiskt så att när en foton absor-beras av färgämnet leder det till ett laddningsseparerat tillstånd på en-elektronnivå. Men både vattenspjälkning och molekylär bränsleproduktion, som beskrivs ovan, är flerelektronprocesser. Därför måste ytterligare en eller flera fotoner absorberas en efter en, med efterföljande elektronöverföring,

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för att driva den katalytiska processen. Elektronerna och hålen måste byggas upp i systemet, som följd av flera successiva cykler av ljusabsorption och laddningsseparation, vilket också sker i fotosystem II. Detta kallas här för ackumulativ elektronöverföring.

De flesta studier som gjorts på system för fotoinducerad elektronöverfö-ring har gjorts på en-elektronnivå, och handlar bara om enkel elektronöver-föring. Flera av de system som presenteras i den här avhandlingen har enhe-ter som lämpar sig för ackumulativ elektronöverföring. Det finns dock flera saker som gör ackumulativ elektronöverföring mer komplicerad än enkel elektronöverföring, vilket diskuteras i avhandlingen. Därför är det en utma-ning att designa system där ackumulativ elektronöverföring faktiskt kan ske. Arbetena som presenteras ger exempel på när ackumulation inte sker (t.ex. artikel IV) trots att förutsättningarna finns. Men det ges också exempel på lyckad ackumulering av två elektroner och två hål (artikel V och VI) vid stegvis absorption av två fotoner. Det system som uppvisade ackumulativ elektronöverföring är unikt i sitt slag, eftersom vi kan visa att ett dubbelt laddningsseparerat tillstånd med lång livstid har bildats med mycket högt utbyte, utan att behöva använda externa acceptorer eller donatorer som för-brukas i processen. Förhoppningsvis kan dessa studier bidra till utvecklingen av fotokatalys av flerelektronprocesser, vilket är vad vi vill uppnå i artificiell fotosyntes.

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75

Appendix. Photoinduced electron transfer in Ru(II)polypyridyl-benzoquinone(BQ) dyads in aqueous and acetonitrile solutions.

This appendix presents data from the studies on Ru(II)-benzoquinone dyads (structures as in Scheme 1) in aqueous solution discussed in chapter 5 (Section 5.3.2).

Scheme 1. Structures of the Ru(dqp)2-BQ dyads 1-2 and the Ru(bpy)3-BQ dyad 3 investigated here. Synthesis performed by Rohan J Kumar (dyads 1 and 2)1,2 and Olof Johansson (dyad 3) 3,4. Dyad 3 was previously studied in acetonitrile by M. Borgström et al.4

Figure 1. Transient absorption of 1 in unbuffered aqueous solution (left) and aceto-nitrile (right) upon 490 nm ~120 fs excitation. Solid lines represent global fits of the data, corresponding to 1 = 39 ps (growth in the 450-540 nm region), 2 = 125 ps (decay), 3 = inf in aqueous solution and 1 = 1.9 ns (decay), 2 = inf in acetonitrile.

.

76

Table 1. Rate constants for forward (kET) and back (kBET) electron transfer in dyads 1-3 upon Ru(II) MLCT excitation, as determined from ultrafast transient absorption measurements. Acetonitrile samples were prepared from PF6

- salts of 1-3, and aqueous solution samples were prepared from Cl- salts.

Solvent

(�Gº)[a]

CH3CN

(-0.3 eV)

H2O[b]

(-0.7 eV)

Aq. pH 2[c]

(-0.8 eV)

1 [d] kET = 5.1×108 s-1

kBET >> kET

kET = 8.0×109 s-1

kBET = 2.6×1010 s-1

kET = 7.4×109 s-1

kBET = 2.6×1010 s-1

2 [d] kET = 1.1×109 s-1

kBET >> kET

kET = 7.4×109 s-1

kBET >> kET

n.d.

3[e] kET = 5.0×109 s-1

kBET = 4.5×108 s-1

kET = 1.3×1011 s-1

kBET = 1.1×1010 s-1

kET = 3.1×1011 s-1

kBET = 1.4×1010 s-1 [a]The driving force for electron transfer in acetonitrile was calculated from the pre-viously reported electrochemical data2,4 and excited state energies E00(Ru(bpy)3) = 2.1 eV; E00(Ru(dqp)2) = 1.84 eV. The driving force in aqueous solution was esti-mated from E(BQ-/BQ, pH 7) = 0.08 V vs. NHE5 and E(BQ-/BQ, pH 2) = 0.2 V vs. NHE.6 An assumingly pH-independent value of E(RuIII/II) = 1.5 V vs. NHE for [Ru(bpy)3

2+](aq) was used.7 The redox potentials of the [Ru(dqp)22+] complexes

were assumed to have a corresponding shift in aqueous solvent. The Coulombic work term was assumed to be negligible. [b]Unbuffered solutions. [c]Compound 1 was measured in 10 mM HCl (aq), unbuffered solution; compound 3 in a pH 2 phosphate buffer (2 mM NaH2PO4, aq). [d]The determination of the rate constants in acetonitrile were supported by time-correlated single photon counting measurements. [e]Alice Rolandini Jensen8 contributed to the results on compound 3 in unbuffered aqueous solution. References and notes. (1) Rohan J Kumar, [email protected], when at the Department of Photoche-

mistry and Molecular Science, Uppsala University, Uppsala, Sweden (2) Kumar, R. J.; Karlsson, S.; Streich, D.; Rolandini Jensen, A.; Jäger, M.; Becker,

H.-C.; Bergquist, J.; Johansson, O.; Hammarström, L. Chem. Eur. J. 2010, 16, 2830-2842.

(3) Olof Johansson, [email protected], Department of Photochemistry and Molecular Science, Uppsala University, Sweden

(4) Borgström, M.; Johansson, O.; Lomoth, R.; Berglund Baudin, H.; Wallin, S.; Sun, L.; Åkermark, B.; Hammarström, L. Inorg. Chem. 2003, 42, 5173-5184.

(5) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637-1657. (6) The benzoquinone reduction potential at pH 2 is calculated from a pKa(BQ) = 4

and a 60mV/pH-unit increase in reduction potential, according to ref 5.

77

(7) Juris, A.; Balzani, V.; Barigeletti, F.; Campagna, S.; Belser, P.; von Zelewski, A. Coord. Chem. Rev. 1988, 84, 85-277.

(8) Alice Rolandini Jensen, [email protected], when at the Department of Photo-chemistry and Molecular Science, Uppsala University, Uppsala, Sweden

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