Ruthenium(II) Polypyridyl Complexes

Applications in Artificial Photosynthesis

Olof Johansson

Department of Organic Chemistry

Stockholm University 2004

Doctoral Dissertation 2004 Department of Organic Chemistry Arrhenius Laboratory Stockholm University Sweden

Abstract

Molecular mimics of PS II, which consist of a photosensitizer linked to electron acceptors/donors, are attractive candidates for the conversion of solar energy into chemical energy. Several different classes of sensitizers have been studied and among these, ruthenium(II) polypyridyl complexes continue to attract major attention.

The first part of this thesis presents the photophysical properties, stereochemistry, and general synthesis of ruthenium(II) complexes based on 2,2´-bipyridyl and 2,2´:6´,2´´-terpyridyl ligands. The second part deals with ruthenium(II) polypyridyl complexes linked to electron acceptors (benzoquinone, naphthalene diimide) and electron donors (phenothiazine, tyrosine, manganese complexes). Functionalized 2,2´-bipyridines and 2,2´:6´,2´´-terpyridines were synthesized and used in the stepwise assembly of the chromophore-quencher complexes. These were fully characterized and the redox properties were studied by cyclic and differential pulse voltammetry. The intramolecular charge-separated states formed after light excitation of the ruthenium(II) unit were observed by time-resolved absorption and EPR spectroscopy.

In the third part of this thesis, the synthesis and photophysical properties of a novel class of bistridentate ruthenium(II) polypyridyl complexes based on bipyridyl-pyridyl methane ligands are discussed. The solution structures of the homoleptic and heteroleptic ruthenium(II) complexes were studied by 2D NMR techniques. The X-ray structure of one of the homoleptic complexes has been solved. The effect on the excited state lifetime for these ruthenium(II) complexes compared to the parent [Ru(tpy)2]2+ is discussed. Finally, in one of the heteroleptic complexes an interesting reversible linkage isomerization was observed that could be induced either electrochemically or chemically.

© Olof Johansson ISBN 91-7265-852-5 pp 1-60

Intellecta Docusys AB, Sollentuna

Table of Contents

LIST OF PUBLICATIONS......................................................................................................... i

LIST OF ABBREVIATIONS ..................................................................................................... ii

PREFACE.............................................................................................................................. iv

1 GENERAL INTRODUCTION ............................................................................................ 1

2 RUTHENIUM(II) POLYPYRIDYL COMPLEXES .............................................................. 3

2.1 Photophysical and Redox Properties .................................................................... 3 2.2 Photo-induced Electron Transfer in Model Systems............................................ 6 2.3 Stereochemistry of Ruthenium(II) Polypyridyl Complexes................................. 8 2.4 Synthesis of Ruthenium(II) Polypyridyl Complexes ......................................... 12 2.5 Synthesis of Oligopyridines. General Strategies ................................................ 14 2.5.1 Synthesis of Substituted 2,2´-Bipyridines and 2,2´:6´,2´´-Terpyridines............. 16 2.5.2 Synthesis of [6-(2,2´-Bipyridyl)]-(2´-pyridyl)-methanes .................................... 17

3 DONOR/ACCEPTOR SYSTEMS BASED ON BENZOQUINONE ........................................ 19

3.1 Introduction ........................................................................................................ 19 3.1.1 Synthesis ............................................................................................................. 22 3.1.2 Electrochemical and Photochemical Properties ................................................ 25 3.1.3 Conclusions ........................................................................................................ 28

4 NAPHTHALENE DIIMIDE AS ACCEPTOR ..................................................................... 30

4.1 Introduction ........................................................................................................ 30 4.1.1 Preparation of Ru -NDI DyadsII .......................................................................... 31 4.1.2 Electrochemical and Photochemical Properties ................................................ 34 4.1.3 Conclusions ........................................................................................................ 36 4.2 A Hydrogen Bonded Tyrosine-Ruthenium(II)-Naphthalene Diimide Triad...... 37 4.2.1 Synthesis ............................................................................................................. 37 4.2.2 Results from the Electrochemical Study ............................................................. 39 4.2.3 Photochemical Properties .................................................................................. 40 4.2.4 Conclusions ........................................................................................................ 42 4.3 Properties of a Mn -Ru -Naphthalene Diimide Triad2

II/II II ................................... 43

5 BIPYRIDYL-PYRIDYL METHANES AS TRIDENTATE LIGANDS FOR RUTHENIUM(II).. 46

5.1 Enhancement of Luminescence Lifetimes in Bistridentate Ruthenium(II) Complexes.......................................................................................................... 46

5.1.1 Ligand Design and Synthesis of Ruthenium(II) Complexes ............................... 47

5.1.2 Electrochemical and Photophysical Properties ................................................. 50 5.1.3 Conclusions ........................................................................................................ 52 5.2 Linkage Isomerism in a Bistridentate Ruthenium(II) Complex ......................... 53 5.2.1 Electrochemical Behavior .................................................................................. 54 5.2.2 Influence of Base ................................................................................................ 55 5.2.3 Conclusions ........................................................................................................ 57

6 CONCLUDING REMARKS............................................................................................. 59

ACKNOWLEDGEMENTS ...................................................................................................... 60

List of Publications This thesis is based on the following papers, referred to in the text by their Roman numerials I-VI. I Electron Donor-Acceptor Dyads and Triads Based on

Tris(bipyridine)ruthenium(II) and Benzoquinone: Synthesis, Characterization, and Photoinduced Electron Transfer Reactions

Borgström, M.; Johansson, O.; Lomoth, R.; Berglund-Baudin, H.; Wallin, S.; Sun, L.; Åkermark, B.; Hammarström, L. Inorganic Chemistry, 2003, 42, 5173-5184.

II Electron Donor-Acceptor Dyads Based on Ruthenium(II) Bipyridine and

Terpyridine Complexes Bound to Naphthalenediimide Johansson, O.; Borgström, M.; Lomoth, R.; Palmblad, M.; Bergquist, J.; Hammarström, L.; Sun, L.; Åkermark, B. Inorganic Chemistry, 2003, 42, 2908-2918.

III Intramolecular charge separation in a hydrogen bonded tyrosine-

ruthenium(II)-naphthalene diimide triad Johansson, O.; Wolpher, H.;Borgström, M.; Hammarström, L.; Bergquist, J.; Sun,

L.; Åkermark, B. Chemical Communications, 2004, 194-195. IV A tridentate ligand for preparation of bisterpyridine-like ruthenium(II)

complexes with an increased excited state lifetime Wolpher, H.; Johansson, O.; Abrahamsson, M.; Kritikos, M.; Sun, L.; Åkermark,

B. Inorganic Chemistry Communications, 2004, 7, 337-340. V A New Strategy for Improvement of Photophysical Properties in

Ruthenium(II) Polypyridyl Complexes. Synthesis, Photophysical and Electrochemical Characterisation of Six New Mononuclear Ruthenium(II) Bisterpyridine Type Complexes

Abrahamsson, M.; Wolpher, H.; Johansson, O.; Larsson, J.; Kritikos, M.; Sun, L.; Åkermark, B.; Hammarström, L.

Manuscript VI Linkage Isomerism in a Bistridentate Ruthenium(II) Complex Johansson, O.; Lomoth, R.; Sun, L.; Åkermark, B. Manuscript Reprints were made with the permission of the publishers.

i

List of Abbreviations A electron acceptor AcOH acetic acid AIBN azobisisobutyronitrile AQ anthraquinone ATP adenosine tri-phosphate Boc t-butoxycarbonyl bpy 2,2´-bipyridine BQ benzoquinone Bu butyl cat catalyst COSY correlated spectroscopy CR charge recombination CS charge separation CV cyclic voltammetry D electron donor DCC N,N´-dicyclohexyl-carbodiimide DMA dimethylacetamide DMAP 4-dimethylamino-pyridine dmb 4,4´-dimethyl-2,2´-bipyridine DMF dimethylformamide DMSO dimethylsulfoxide dpa dipicolylamine DPV differential pulse voltammetry EPR electron paramagnetic resonance spectroscopy ESI-MS electrospray ionization masspectrometry Et ethyl eV electronvolt Fc ferrocene isc intersystem crossing L ligand LDA lithium diisopropylamide MC metal-centered Me methyl MLCT metal-to-ligand charge transfer MV2+ methyl viologen NBS N-bromosuccinimide NDI naphthalene diimide NOE nuclear Overhauser effect NPht phthalimide

ii

P photosensitizer pp any 2,2´-bipyridyl ligand ppp any 2,2´:6´,2´´-terpyridyl ligand PS II photosystem II PTZ phenothiazine py pyridine rt room temperature THF tetrahydrofuran tpy 2,2´:6´,2,´´-terpyridine TsOH p-toluene sulphonic acid ttpy 4´-p-tolyl-2,2´:6´,2´´-terpyridine Tyr tyrosine

iii

Preface

This thesis describes the work reported in the publications I-VI listed on the preceding page. My ambition has been to summarize all the results from the individual papers, involving both synthesis and photophysical or photochemical properties of various ruthenium(II) polypyridyl complexes.

As a PhD student, I have been involved in an artificial photosynthesis project that is a joint collaboration between five research groups in Sweden, the group of Professor Björn Åkermark and Associate Professor Licheng Sun at the Department of Organic Chemistry at Stockholm University, the groups of Professor Stenbjörn Styring (Biochemistry) and Assistant Professor Tomas Polivka (Chemical Physics) at Lund University, and the groups of Associate Professor Leif Hammarström (Physical Chemistry) and Professor Peter Lindblad (Physiological Botany) at Uppsala University.

I am responsible for the synthetic work in papers I, II, and VI, and parts of the synthesis in papers III, IV, and V. The electrochemical investigations of the complexes have been done by me or together with Reiner Lomoth at the Department of Physical Chemistry at Uppsala University. The major photophysical measurements involving time-resolved emission and absorption techniques in papers I-V have been done by Magnus Borgström, Maria Abrahamsson, and Helena Berglund-Baudin at Uppsala University. Nizamuddin Shaikh at the Department of Biochemistry at Lund University did the EPR studies described in Section 4.3. Jonas Bergquist at Analytical Chemistry at Uppsala University did the electrospray ionization masspectrometry investigations and Mikael Kritikos at Structural Chemistry at Stockholm University performed the X-ray spectroscopy in papers IV and V.

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General Introduction

1

Modern society has reached its complexity by consuming enormous amounts of energy each day. Fossil fuels have contributed as one of the major energy sources, but in recent years it has been realized that the supply is limited. Also, their use has led to air pollution and increased amounts of carbon dioxide in the atmosphere. Alternative energy sources have therefore been of major concern in the last decades, and solar energy has emerged as one of the most promising ways of producing a non-polluting fuel.

Photosynthesis is the most successful of all processes for the photochemical conversion and storage of solar energy (eq. 1). It is therefore natural that the concepts of photosynthesis are attractive to researchers in their efforts to create an artificial system that is capable of transforming solar energy into chemical energy.

H2O + CO2hν O2 + 1/6 (CH2O) (1)

The key process in natural photosynthesis is photo-induced charge separation.1 Upon absorption of a photon by photosystem II (PS II, a large membrane-bound enzyme in plants), an electron is transferred to a primary acceptor and then further to the final acceptor in the reaction center. The energy of this charge-separated state is subsequently used in the photosynthetic reactions. During the charge separation event, the reaction center in PS II is reduced by a nearby tyrosine (TyrZ) which in turn is reduced by electrons from a manganese cluster, electrons that are ultimately derived from water.

During the last 20 years, great efforts have been devoted to the study of artificial systems that mimic parts of the photosynthetic machinery. A long-term goal in this area is to construct a biomimetic supramolecular system that consists of various electron acceptors (A) and donors (D), which can carry out the function of natural photosynthesis (Figure 1). This would involve the formation of a long-lived charge-separated state (D+/A−), which subsequently can react with water to give oxygen and some high-energy substance (e.g. hydrogen).

1 (a) Barber, J.; Andersson, B. Nature 1994, 370, 31-34. (b) Recent crystal structure of PS II: Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata, S. Science 2004, 303, 1831-1838.

1

e− e−hν

PD A

H2O

O2

H+

H2

Figure 1. Electron transfer processes in an artificial system that consistsof a photosensitizer (P), electron acceptors (A), and donors (D).

In our work in the field of artificial photosynthesis, we have focused on synthetic

manganese complexes linked to ruthenium(II) polypyridyl photosensitizers (Figure 2). Recent studies have shown that synthetic manganese complexes can catalyze the conversion of water into oxygen,2 but these processes have so far not been coupled to photo-induced charge separation. We believe that light-driven electron transfer from a manganese moiety to a ruthenium sensitizer in “simple” supramolecular systems can be coupled to water oxidation. The knowledge we obtain from our artificial systems could hopefully lead to a better understanding of the natural photosynthesis, and also take us one step closer towards a less polluted society.

RuN

N

NN N

N

2+

Mn / TyrOH [Ru(bpy)3]2+ Acceptor

e− e−hν

1

Figure 2. Electron transfer in Mn-Ru dyads. 1 is [Ru(bpy)3]2+.

This thesis presents our latest efforts to extend our manganese-ruthenium

chemistry to include electron acceptors as additional components. In Chapter 2, ruthenium(II) polypyridyl complexes are discussed with the emphasis on photophysical and redox properties, stereochemistry, and synthesis. The following chapters describe the synthesis and properties of ruthenium(II) polypyridyl complexes linked to electron acceptors such as benzoquinone (Chapter 3) and naphthalene diimide (Chapter 4). The last part of this thesis deals with our strategy towards a new type of ruthenium(II) polypyridyl photosensitizers with improved photophysical properties.

2 (a) Limburg, J.; Vrettos, J. S.; Liable-Sands, L. M.; Rheingold, A. L.; Crabtree, R. H.; Brudwig, G. W. Science 1999, 283, 1524-1527. (b) Limburg, J.; Vrettos, J. S.; Chen, H.; de Paula, J. C.; Crabtree, R. H.; Brudwig, G. W. J. Am. Chem. Soc. 2001, 123, 423-430. (c) Shimazaki, Y.; Nagano, T.; Takesue, H.; Ye, B.-H.; Tani, F.; Naruta, Y. Angew. Chem. Int. Ed. 2004, 43, 98-100.

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Ruthenium(II) Polypyridyl Complexes

2 2.1 Photophysical and Redox Properties

Ruthenium(II) trisbipyridyl complexes have been thoroughly studied during the last 30 years as a result of their remarkable chemical stability and photophysical properties.3 The use of these chromophores has resulted in wide applications in different areas, from solar energy related research3,4 to molecular wires,5 sensors and switches,6 machines,7 and also as therapeutic agents.8

The [Ru(bpy)3]2+ (1) prototype has octahedral geometry of D3 symmetry. The absorption spectrum in the visible region is dominated by an intense metal-to-ligand charge transfer (1MLCT) band at 450 nm (ε ~ 104 M-1cm-1), caused by the transition from a dπ metal orbital to a ligand based orbital (πL*).9 The 1MLCT state rapidly decays, within 300 fs,10 via intersystem crossing (isc) to a 3MLCT excited state which has a lifetime (τ) of around 0.9 µs in acetonitrile at room temperature.3 This is long enough for the excited state of [Ru(bpy)3]2+ to efficiently transfer its energy to another molecule (a quencher), either by energy transfer or electron transfer. In the absence of a quencher, the excited state undergoes deactivation through both nonradiative and radiative decay pathways with an emission quantum yield (φ) of 0.06 (Figure 3).3

3 (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85-277. (b) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press Limited: London, 1992; pp 87-212. 4 (a) Campagna, S.; Di Pietro, C.; Loiseau, F.; Maubert, B.; McClenaghan, N.; Passalacqua, R.; Puntoriero, F.; Ricevuto, V.; Serroni, S. Coord. Chem. Rev. 2002, 229, 67-74. (b) Dürr, H.; Bossmann, S. Acc. Chem. Res. 2001, 34, 905-917. (c) Meijer, M. D.; van Klink, G. P. M.; van Koten, G. Coord. Chem. Rev. 2002, 230, 141-163. (d) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-170. (e) Sun, L.; Hammarström, L.; Åkermark, B.; Styring, S. Chem. Soc. Rev. 2001, 30, 36-49. 5 (a) Barigelletti, F.; Flamigni, L. Chem. Soc. Rev. 2000, 29, 1-12. (b) Robertson, N.; McGowan, C. A. Chem. Soc. Rev. 2003, 32, 96-103. (c) Ziessel, R.; Hissler, M.; El-ghayoury, A.; Harriman, A. Coord. Chem. Rev. 1998, 178-180, 1251-1298. 6 De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515-1566. 7 Ashton, P. R.; Ballardini, R.; Balzani, V.; Credi, A.; Ruprecht Dress, K.; Ishow, E.; Kleverlaan, C. J.; Kocian, O.; Preece, J. A.; Spencer, N.; Stoddart, J. F.; Venturi, M.; Wenger, S. Chem. Eur. J. 2000, 6, 3558-3574. 8 Metcalfe, C.; Thomas, J. A. Chem. Soc. Rev. 2003, 32, 215-224. 9 In the UV-region the spectrum is dominated by ligand-centered (LC) π → π* transitions at 285 nm (ε ~ 8×104 M-1cm-1). 10 McCusker, J. K. Acc. Chem. Res. 2003, 36, 876-887.

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1MLCT [Ru(bpy)3]2+

3MLCT [Ru(bpy)3]2+

3MC-dd

[Ru(bpy)3]2+

knr´knr kr

kdd

isc

∆Eact

E

Photochemistry

Figure 3. Formation and deactivation pathways for the 3MLCT excited state of [Ru(bpy)3]2+.

The observed lifetime (τ) is given by eq. 2 where kr, knr, and kdd are the rate

constants for the radiative decay, non-radiative decay (governed by the energy gap law), and thermal population of the nearby metal-centered (3MC) state. The relationship between φ and kr is given by eq. 3. Thus, measuring the lifetime and emission quantum yield, kr can easily be obtained (ηisc, the efficiency of intersystem crossing, is normally considered unity).

φ = ηisc⋅kr⋅τ

1/τ = kr + knr + kdd

(3)

(2)

The thermal population of the nearby 3MC state has an important effect on the

observed lifetime. In the 3MC state, an electron occupies an anti-bonding metal-based orbital, which leads to large geometrical distortions and rapid non-radiative decay.11 In [Ru(bpy)3]2+, the 3MC state is located about 3600 cm-1 above the 3MLCT state,3 but in the related [Ru(tpy)2]2+ (tpy is 2,2´:6´,2´´-terpyridine) ∆Eact is only 1500 cm-1 which leads to rapid non-radiative decay and the excited state has a lifetime of only 250 ps.12 The weak ligand field is due to unfavorable bite angles of the terpyridyl ligands,12 and enhancing the photophysical properties of these complexes is currently being pursued by many research groups (Chapter 5).

11 Ligand dissociation in [Ru(bpy)3]2+ and related complexes may also occur. One early example: Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4803-4810. See also ref 3a. 12 Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. Rev. 1994, 94, 993-1019.

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In the cyclic voltammogram (CV) of [Ru(bpy)3]2+ (Figure 4), a reversible oxidation appears at 0.88 V (all redox potentials in this thesis are reported relative to the ferrocenium/ferrocene couple)13 which involves a metal-centered orbital (dπ), and three closely spaced ligand-localized reductions at –1.74, −1.93, and –2.17 V where each added electron is localized on a single ligand.3 In the 3MLCT excited state, formally [RuIII(bpy)2(bpy•−)]2+, the metal is oxidized and one ligand is reduced, and the complex is therefore both a strong reductant and oxidant. The excited state redox potentials can to a first approximation be determined from the excited state energy (E00) and the ground state potentials (eqs. 4 and 5).3 With an excited state energy of 2.12 eV,3 the redox potentials for oxidation and reduction of the 3MLCT state are calculated to –1.24 and 0.38 V respectively.

E°(Ru3+/2+*) = E°(Ru3+/2+) − E00

E°(Ru2+*/+) = E°(Ru2+/+) + E00

(4)

(5)

It has long been recognized that because of the redox properties of [Ru(bpy)3]2+,

this complex can function as a photocatalyst for the decomposition of water into hydrogen and oxygen. A unique combination of ground state and excited state stability, redox potentials, lifetime of the excited state, and also the synthetic chemistry available for ruthenium(II) polypyridyl complexes, render these molecules the most popular sensitizers in the field of artificial photosynthesis. In practice, however, [Ru(bpy)3]2+ in itself does not catalyze water cleavage and additional components are needed for the formation of high-energy substances.

E / V vs Fc+/0-2 -1 0 1

20 µA

Figure 4. CV of [Ru(bpy)3]2+ (1), 1.5 mM, CH3CN, N(n-C4H9)4PF6, 0.1 V/s.

13 The Fc+/0 redox couple is at 0.38 V vs SCE.

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2.2 Photo-induced Electron Transfer in Model Systems

In the early 70’s, Adamson and Gafney discovered that excited [Ru(bpy)3]2+ is oxidized in a bimolecular reaction with [Co(NH3)5Br]2+ to form [Ru(bpy)3]3+.14 Since then, several multimolecular systems capable of reducing or oxidizing water based on [Ru(bpy)3]2+ and various electron acceptors, donors, and catalysts, have appeared in the literature.15

One limitation in the multimolecular approach is that there is no spatial control of the photo-produced oxidant/reductant pair, and the charge recombination is often too rapid to allow secondary reactions. Therefore, another strategy has been explored which relies on covalently linked sensitizers and acceptors/donors, where the photo-produced oxidant/reductant pair is separated in space and available for further chemical transformations. A large number of these dyads, triads, and tetrads have been synthesized and investigated in detail, and the principles that govern long-range electron transfer are now well understood.16

The simplest covalently linked system for studying charge separation consists of a chromophore (P) (e.g. [Ru(bpy)3]2+) and an electron acceptor (A), Figure 5. Following light excitation, the P+-A− charge-separated state is formed (oxidative quenching) where the light energy is stored as redox energy. The energy of the charge-separated state can be estimated from the first redox potentials of the chromophore and the acceptor, obtained from electrochemical measurements.17 Using these data, the driving force for the charge separation can then be calculated (eq. 6).18 The forward electron transfer rate constant (kf) is given by eq. 7,19 where τobs is the measured excited state lifetime of the dyad and τ0 is the lifetime of a suitable model complex lacking the acceptor. The charge-separated state

P ALink hν P A P Akf* H2O

H2 + 1/2 O2

kb

cat

Figure 5. Electron transfer processes in a P-A dyad.

14 Gafney, H. D.; Adamson, A. W. J. Am. Chem. Soc. 1972, 94, 8238-8239. 15 See for example: (a) Amouyal, E. Sol. Energy Mater. Sol. Cells 1995, 38, 249-276. (b) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press Limited: London, 1992; pp 339-368, and references cited therein. 16 See for example: (a) Gust, D.; Moore, T. A.; Moore, A. L. Electron Transfer in Chemistry; ed. Balzani, V., WILEY-VCH: Weinheim, 2001; pp 272-336. (b) Scandola, F.; Chiorboli, C.; Indelli, M. T.; Rampi, M. A. Electron Transfer in Chemistry; ed. Balzani, V., WILEY-VCH: Weinheim, 2001; pp 337-408. (c) Wasielewski, M. R. Chem. Rev. 1992, 92, 435-461. (d) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood Limited: West Sussex, 1991. 17 Valid in polar solvents such as acetonitrile. 18 Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271. 19 Provided that electron transfer is the only quenching mechanism.

6

subsequently undergoes charge recombination (kb), unless other competing processes (such as water cleavage) occur.

−∆G° = E°(A0/−) − E°(P+/0) + E00 (6)

kf = (1/τobs) − (1/τ0) (7)

The link between the chromophore and the acceptor plays a very important role for

several reasons. Through the coordinating sites, it directly influences the photophysical and redox properties of the metal unit(s). The nature of the linker also influences the electronic communication between the chromophore and the acceptor, and it determines the distance and relative orientations between the two components. To mimic the electron transfer chain in natural photosynthesis, great efforts have been devoted to the synthesis of chromophore-quencher systems which are held together by noncovalent interactions such as hydrogen bonding,20 hydrophobic interactions,20 or even mechanical bonds.21,4b In our work, however, we have been primarily concerned with covalent linkages.

A significant advance in the field of artificial photosynthesis and multistep electron transfer was recently described by Moore, Gust, and Moore. Photo-induced electron transfer in one of their well characterized triads composed of a quinone acceptor, a porphyrin chromophore, and a carotene donor, resulted in a pH gradient across a lipid bilayer of a reconstituted liposome.22 The resulting pH gradient subsequently drives F0F1-ATP synthase to produce ATP.

20 (a) Piotrowiak, P. Chem. Soc. Rev. 1999, 28, 143-150. (b) Ward, M. D. Chem. Soc. Rev. 1997, 26, 365-375. 21 Chambron, J.-C.; Collin, J.-P.; Dalbavie, J.-O.; Dietrich-Buchecker, C. O.; Heitz, V.; Odobel, F.; Solladié, N.; Sauvage, J.-P. Coord. Chem. Rev. 1998, 178-180, 1299-1312. (b) Hu, Y.-Z.; Bossmann, S. H.; van Loyen, D.; Schwarz, O.; Dürr, H. Chem. Eur. J. 1999, 5, 1267-1276. (c) Hu, Y.-Z.; van Loyen, D.; Schwarz, O.; Bossmann, S.; Dürr, H.; Huch, V.; Veith, M. J. Am. Chem. Soc. 1998, 120, 5822-5823. 22 (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40-48. (b) Steinberg-Yfrach, G.; Rigaud, J.-L.; Durantini, E. N.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1998, 392, 479-482.

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2.3 Stereochemistry of Ruthenium(II) Polypyridyl Complexes

Trisbidentate metal complexes with octahedral geometry are chiral, and exist in two isomeric forms (∆ and Λ). This has implications when preparing multinuclear assemblies, where the isolated samples usually reflect statistical mixtures of the possible isomers. The preparation of complexes with predetermined chirality can be achieved using enantiomerically pure cis-[Ru(phen)2(py)2]2+,23 cis-[Ru(bpy)2(py)2]2+,24 or cis-[Ru(bpy)2(CO)2]2+,25 which can undergo substitution reactions with bidentate ligands with retention of configuration. Other approaches are aimed at introducing chirality in the 2,2´-bipyridyl ligands and thereby favoring the formation of one diasteromer,26 or resolving the racemic products by conventional techniques.27

Additional stereoisomerism has to be considered when two (or three) of the bipyridyl ligands are unsymmetrically substituted. For complexes of the type [Ru(bpyA)3]2+ (bpyA is an unsymmetrical ligand) fac and mer geometrical isomers are possible (Figure 6). If the substituents are different, the resulting [Ru(bpyA)(bpyB)(bpy)]2+ and [Ru(bpyA)(bpyB)(bpyC)]2+ complexes exist in four and eight different geometrical isomers respectively, all of which can have distinct photophysical properties.28

RuN

N

NN N

N

R

RR

RuN

N

NN N

N

R

R

R

fac mer

Figure 6. Geometrical isomers of [Ru(bpyA)3]2+ with unsymmetrical ligands.

Meyer and co-workers studied the donor-chromophore-acceptor triad

[Ru(dmb)(dmbPTZ)(dmbMV2+)]4+ (2),29 (dmb is 4,4´-dimethyl-2,2´-bipyridine, PTZ is

23 Bosnich, B.; Dwyer, F. P. Aust. J. Chem. 1966, 19, 2229-2233. 24 Hua, X.; von Zelewsky, A. Inorg. Chem. 1995, 34, 5791-5797. 25 Rutherford, T. J.; Quagliotto, M. G.; Keene, F. R. Inorg. Chem. 1995, 34, 3857-3858. 26 (a) Hayoz, P.; von Zelewsky, A.; Stoeckli-Evans, H. J. Am. Chem. Soc. 1993, 115, 5111-5114. (b) von Zelewsky, A.; Mamula, O. J. Chem. Soc. Dalton Trans. 2000, 219-231. 27 Two recent examples are: (a) Caspar, R.; Amouri, H.; Gruselle, M.; Cordier, C.; Malézieux, B.; Duval, R.; Leveque, H. Eur. J. Inorg. Chem. 2003, 499-505. (b) Patterson, B. T.; Keene, F. R. Inorg.Chem. 1998, 37, 645-650. 28 (a) Keene, F. R. Coord. Chem. Rev. 1997, 166, 121-159. (b) Rutherford, T. J.; Reitsma, D. A.; Keene, F. R. J. Chem. Soc. Dalton Trans. 1994, 3659-3666. 29 Treadway, J. A.; Chen, P.; Rutherford, T. J.; Keene, F. R.; Meyer, T. J. J. Phys. Chem. A 1997, 101, 6824-6826.

8

phenothiazine, and MV2+ is methyl viologen) where the trans isomer has the ideal separation between the donor and the acceptor in space (Figure 7). The isomers were separated and the rate of charge recombination (PTZ•+-RuII-MV•+ → PTZ-RuII-MV2+) was compared. The rate-constants were found to be of the same order of magnitude (kb = 4.5×106-8.7×106 s-1).

RuN

N

NN N

NRu

NN

NN N

NRu

NN

NN N

NRu

NN

NN N

N

N N

A A

A A

D

D D

D

N

SA D

Figure 7. Geometrical isomers of [Ru(dmb)(dmbPTZ)(dmbMV2+)]4+ (2).

To circumvent the problem with isomeric mixtures and non-optimized spatial

organization, Meyer and co-workers synthesized donor/acceptor complexes based on the trans-[Ru(bpy)2(L)2]2+ core (3).30 However, the room-temperature stability of these complexes is poor, and the charge-separated state is not accessible to a significant degree

Ru

N

NN

NRu

NN

NN N

NN

N

NS

HN

O

O

OOCH3

OCH3

3 4

2+ 2+

Figure 8. Approaches toward linear D-P-A assemblies.

30 Coe, B. J.; Friesen, D. A.; Thompson, D. W.; Meyer, T. J. Inorg. Chem. 1996, 35, 4575-4584.

9

at lower temperature. Sauvage and co-workers recently took another approach, using a bis-chelate based on 1,10-phenanthroline, designed to form octahedral complexes with a well-defined axis (4).31 This strategy has not been used for the incorporation of donor/acceptor units.

In contrast to ruthenium(II) trisbipyridine, the geometry of ruthenium(II) bisterpyridine offers the possibility to construct linear donor-chromophore-acceptor assemblies with trans configuration (Figure 9).12,32 However, due to the short inherent excited state lifetime of [Ru(tpy)2]2+ (250 ps), the use of this chromophore has been limited. Collin et al. studied the triad 5 and detected the charge-separated state, but only at 155 K where the deactivation of the excited state is suppressed.33 Another recent example of terpyridine based triads has been prepared by Amouyal and co-workers (6),34 who used the triaryl-pyridinium electron acceptor. The quantum yield of emission and excited state lifetime are improved but the charge-separated state is not formed.

N

N

N

Ru

N

N

N

N

H3CO

H3CO

CH2 N N

N

N

N

Ru

N

N

N

N N

5

6

4+

3+

Figure 9. D-P-A assemblies based on the [Ru(tpy)2]2+ core.

Even though great advances have been made in the design of linear donor-acceptor

assemblies, extensive research is still carried out in many laboratories, striving for the

31 (a) Pomeranc, D.; Heitz, V.; Chambron, J.-C.; Sauvage, J.-P. J. Am. Chem. Soc. 2001, 123, 12215-12221. (b) Goze, C.; Chambron, J.-C.; Heitz, V.; Pomeranc, D.; Salom-Roig, X. J.; Sauvage, J.-P.; Morales, A. F.; Barigelletti, F. Eur. J. Inorg. Chem. 2003, 3752-3758. 32 Collin, J.-P.; Gavina, P.; Heitz, V.; Sauvage, J.-P. Eur. J. Inorg. Chem. 1998, 1-14. 33 Collin, J.-P.; Guillerez, S.; Sauvage, J.-P.; Barigelletti, F.; De Cola, L.; Flamigni, L.; Balzani, V. Inorg. Chem. 1991, 30, 4230-4238. 34 (a) Lainé, P.; Bedioui, F.; Ochsenbein, P.; Marvaud, V.; Bonin, M.; Amouyal, E. J. Am. Chem. Soc. 2002, 124, 1364-1377. (b) Lainé, P.; Bedioui, F.; Amouyal, E.; Albin, V.; Berruyer-Penaud, F. Chem. Eur. J. 2002, 8, 3162-3176.

10

optimal photophysical and geometrical properties of the ruthenium(II) polypyridyl complexes. In many cases, one has succeeded to address a certain limitation such as the short excited state lifetime in bistridentate ruthenium(II) complexes. However, this achievement has frequently resulted in losses in some other important properties of the sensitizer such as redox potentials or excited state energy. The greater opportunity to vary the coordinating ligands in trisbidentate complexes renders these types of sensitizers the most frequently used today.

11

2.4 Synthesis of Ruthenium(II) Polypyridyl Complexes

The most common synthetic approach to ruthenium(II) polypyridyl complexes starts from the commercially available RuCl3⋅xH2O. Refluxing RuCl3⋅xH2O in DMF with a thermally insensitive bipyridyl (pp) ligand, transforms it into cis-Ru(pp)2Cl2.35 A third chelate is then easily introduced to give the bisheteroleptic [Ru(pp)2(pp´)]2+ complex. With heat-sensitive ligands, Ru(DMSO)4Cl2

36 or Ru(CH3 4 2CN) Cl 26a are convenient intermediates, since the synthesis is performed under less vigorous conditions.

In order to tune the spectroscopic and redox properties of the ground state and excited state of the ruthenium(II) polypyridyl complexes, the incorporation of three different pp ligands into the coordination sphere may be necessary. Several synthetic routes for the preparation of trisheteroleptic [Ru(pp)(pp´)(pp´´)]2+ complexes are now available.37−42 The method developed by Meyer and co-workers starting from the oligomer [Ru(CO)2Cl2]n, has been extensively investigated.39 Another interesting approach was recently presented by Mann and co-workers,42 who used the facile photochemical displacement of benzene in [(benzene)Ru(pp)Cl]Cl to give [Ru(pp)(CH3CN)3(Cl)]Cl and Ru(pp)(CH3CN)2Cl2. Stepwise introduction of two additional bidentate ligands is then easily achieved.

In our synthesis of trisheteroleptic complexes, we used the protocol developed by Grätzel and co-workers (Scheme 1).40 Starting from Ru(DMSO)4Cl2, the first bipyridyl ligand is introduced in refluxing chloroform. The choice of solvent is crucial since protic solvents (methanol or ethanol) or high boiling solvents (DMF or DMSO) were reported to give mixtures of mono- and bis-substituted products. Subsequent reactions are performed in refluxing DMF and ethanol respectively.

35 Lay, P. A.; Sargeson, A. M.; Taube, H. Inorg. Synth. 1986, 24, 291-299. 36 Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc. Dalton Trans. 1973, 204-209. 37 Thummel, R. P.; Lefoulon, F.; Chirayil, S. Inorg. Chem. 1987, 26, 3072-3074. 38 Juris, A.; Campagna, S.; Balzani, V.; Gremaud, G.; von Zelewsky, A. Inorg. Chem. 1988, 27, 3652-3655. 39 (a) Anderson, P. A.; Deacon, G. B.; Haarmaan, K. H.; Keene, F. R.; Meyer, T. J.; Reitsma, D. A.; Skelton, B. W.; Strouse, G. F.; Thomas, N. C.; Treadway, J. A.; White, A. H. Inorg. Chem. 1995, 34, 6145-6157. (b) Strouse, G. F.; Anderson, P. A.; Schoonover, J. R.; Meyer, T. J.; Keene, F. R. Inorg. Chem. 1992, 31, 3004-3006. (c) Treadway, J. A.; Meyer, T. J. Inorg. Chem. 1999, 38, 2267-2278. 40 Zakeeruddin, S. M.; Nazeeruddin, Md. K.; Humphry-Baker, R.; Grätzel, M. Inorg. Chem. 1998, 37, 5251-5259. 41 Hesek, D.; Inoue, Y.; Everitt, S. R. L.; Ishida, H.; Kunieda, M.; Drew, M. G. B. Inorg. Chem. 2000, 39, 308-316. 42 Freedman, D. A.; Evju, J. K.; Pomije, M. K.; Mann, K. R. Inorg. Chem. 2001, 40, 5711-5715.

12

RuIIICl3 xH2O RuII(DMSO)4Cl2

RuII(pp)2Cl2

RuII(pp)2(pp´)

RuII(pp)(DMSO)2Cl2

RuII(pp)(pp´)Cl2

RuII(pp)(pp´)(pp´´)

DMSO

ppDMFLiCl

EtOH pp´

CHCl3 pp

DMF pp´

EtOH pp´´

RuII(pp)2Cl2

EtOH pp´

RuII(pp)2(pp´)∆

∆

∆

∆

∆

∆

∆

DMF∆

pp

Scheme 1. Synthesis of bis- and tris-heteroleptic ruthenium(II) trisbipyridyl complexes. pp is a 2,2´-bipyridyl ligand.

The bistridentate ruthenium(II) complexes are prepared in procedures similar to

those developed for the synthesis of trisbidentate complexes (Scheme 2). Ru(tpy)Cl3 is a frequently used precursor (left route),43 but a reductant (NR3) is needed in the second step. Hence, the other route is preferred for oxidation sensitive ligands. In our work, we prepared the Ru(ttpy)(DMSO)Cl2 precursor (7) for the “RuII” route (ttpy is 4´-(p-tolyl)-2,2´:6´,2´´-terpyridine). This compound was isolated in a 1.5:1 mixture of two products that may reflect the spatial orientation of the DMSO ligand.44

N

N

N

Ru

Cl

Cl

N

N

N

Ru

Cl

Cl

7

DMSO

DMSO

RuIIICl3 xH2O RuII(DMSO)4Cl2

RuIII(ppp)Cl3

DMSO

ppp

EtOH

∆

∆

∆

RuII(ppp)(DMSO)Cl2

ppp∆

RuII(ppp)(ppp´)

EtOH∆

EtOH EtOH

NR3

ppp´ ppp´

Scheme 2. Left: Synthesis of ruthenium(II) bisterpyridyl complexes. ppp is a 2,2´:6´,2´´-terpyridyl ligand. Right: Two possible isomers of 7.

43 Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19, 1404-1407. 44 Linkage isomers cannot be excluded.

13

2.5 Synthesis of Oligopyridines. General Strategies

Since the discovery of 2,2´-bipyridine in the 19th century and the first synthesis of 2,2´:6´,2´´-terpyridine in the 1930’s,45 the coordination chemistry of these and other oligopyridines has been extensively studied. Their redox stability, ease of functionalization and capability to complex metal ions, have resulted in wide applications in chemistry.

There is a great variety of methods for the synthesis of 2,2´-bipyridines and 2,2´:6´,2´´-terpyridines.46 Various coupling methodologies have been applied in the preparation of 2,2´-bipyridines, such as oxidative coupling of pyridines using raney nickel or palladium on charcoal.47,48 The yield of 2,2´-bipyridine, however, is small and only minor amounts of 2,2´:6´,2´´-terpyridine are formed. The copper mediated Ullmann coupling of halopyridines has also been used, but generally gives low yields.49 Higher yields have been obtained using nickel catalyzed couplings of halopyridines,50 or the ipso-substitution on sulfoxide-pyridyl compounds by 2-pyridyllithium.51

More recent coupling strategies for the preparation of oligopyridines have relied on the palladium catalyzed cross-coupling reactions of pyridyl-boron (Suzuki),52 pyridyl-tin (Stille),52,53 and pyridyl-zinc (Negishi) reagents.54 Appropriately functionalized ruthenium(II) polypyridyl complexes have recently been shown to function as organic synthons in nickel catalyzed homo-couplings and palladium catalyzed hetero-couplings of various heterocycles.55,56 This building-block approach is likely to expand in the near future for the synthesis of highly complex structures containing various oligopyridyl ligands.

2,2´-Bipyridine and its derivatives are readily available, either commercially or via one of the coupling strategies mentioned above. Terpyridines, however, are expensive and

45 Morgan, G. T.; Burstall, F. H. J. Chem. Soc. 1932, 20-30. 46 Two excellent recent reviews for the preparation of 2,2´:6´,2´´-terpyridines are: (a) Cargill Thompson, A. M. W. Coord. Chem. Rev. 1997, 160, 1-52. (b) Fallahpour, R.-A. Synthesis 2003, 155-184. 47 Hagelin, H.; Hedman, B.; Orabona, I.; Åkermark, T.; Åkermark, B.; Klug, C. A. J. Mol. Catal. A: Chem. 2000, 164, 137-146. 48 Badger, G. M.; Sasse, W. H. F. J. Chem. Soc. 1956, 616-620. 49 Fanta, P. E. Synthesis 1974, 9-22. 50 Tiecco, M.; Testaferri, L.; Tingoli, M.; Chianelli, D.; Montanucci, M. Synthesis 1984, 736-738. 51 Uenishi, J.; Tanaka, T.; Wakabayashi, S.; Oae, S. Tetrahedron Lett. 1990, 31, 4625-4628. 52 Lehmann, U.; Henze, O.; Schlüter.A.D. Chem. Eur. J. 1999, 5, 854-859. 53 (a) Schubert, U. S.; Eschbaumer, C.; Heller, M. Org. Lett. 2000, 2, 3373-3376. (b) Hanan, G. S.; Lehn, J.-M.; Kyritsakas, N.; Fischer, J. Chem. Commun. 1995, 765-766. 54 Savage, S. A.; Smith, A. P.; Fraser, C. L. J. Org. Chem. 1998, 63, 10048-10051. 55 (a) Fanni, S.; Di Pietro, C.; Serroni, S.; Campagna, S.; Vos, J. G. Inorg. Chem. Commun. 2000, 3, 42-44. (b) Johansson, K. O.; Lotoski, J. A.; Tong, C. C.; Hanan, G. S. Chem. Commun. 2000, 819-820. 56 (a) Dunne, S. J.; Constable, E. C. Inorg. Chem. Commun. 2001, 1, 167-169. (b) Fraysse, S.; Coudret, C.; Launay, J.-P. J. Am. Chem. Soc. 2003, 125, 5880-5888. (c) Goze, C.; Kozlov, D. V.; Castellano, F. N.; Suffert, J.; Ziessel, R. Tetrahedron Lett. 2003, 44, 8713-8716.

14

only formed in small quantities in coupling reactions. Therefore, there has been a need for developing other options for the preparation of 2,2´:6´,2´´-terpyridine and its derivatives that rely on ring synthesis. The most widely used strategy is based on the assembly of the central ring in the ligand.46 Pioneering work by Hantsch utilized the one-pot condensation of two equivalents of 3-oxocarboxylate esters with one equivalent aldehyde to give a dihydropyridine ester (Scheme 3). Oxidation followed by removal of the carboxylic esters gives the 2,6-substituted pyridine.

N

H H

H

OEt

O

EtO

O

R R

ox

N

OEt

O

EtO

O

R R NR R

EtO

OR

O

OEt

O R

O

NH3

H H

O

Scheme 3. Pyridine synthesis according to Hantsch.

More recently, multistep syntheses have been developed by the groups of

Kröhnke,57 Potts,58 and Jameson (Scheme 4).59 They all depend on the isolation of a suitably substituted enone. Subsequent reaction with the enolate of acetylpyridine in the presence of NH4OAc gives the corresponding 2,2´:6´,2´´-terpyridine. The advantage of these methods lies in the opportunity to introduce asymmetry in the terpyridyl ligand.

NO

NO

SMe+

NH4OAcN

NN

SMe

SMe

NO

NO

NMe2+

NN

NNH4OAc

NO

N

I

NO

R+

NN

N

R

NH4OAc

Potts

Jameson

Kröhnke

Scheme 4. Ring synthesis of 2,2´:6´,2´´-terpyridines.

57 Kröhnke, F. Synthesis 1976, 1-24. 58 (a) Potts, K. T.; Cipullo, M. J.; Ralli, P.; Theodoridis, G. J. Org. Chem. 1982, 47, 3027-3038. (b) Potts, K. T.; Usifer, D. A.; Guadalupe, A.; Abruna, H. D. J. Am. Chem. Soc. 1987, 109, 3961-3967. 59 Jameson, D. L.; Guise, L. E. Tetrahedron Lett. 1991, 32, 1999-2002.

15

From the discussion above, the opportunities in the design of a specific oligopyridine are obvious. A handful of synthetic approaches has led to the preparation of a multitude of mono-nucleating bipyridyl or terpyridyl ligands and to the synthesis of multi-nucleating ligands which contain several oligopyridine units.60

2.5.1 Synthesis of Substituted 2,2´-Bipyridines and 2,2´:6´,2´´-Terpyridines

In the Kröhnke protocol, the nucleophilic components are acylpyridinium salts that act as synthetic equivalents to enolates. The pyridinium group facilitates deprotonation of the α-protons, and additionally, functions as an internal oxidant of the dihydropyridine intermediate. The enones 11 and 12 were synthesized from 2-acetylpyridine (10), and p-methylbenzaldehyde (8) and p-nitrobenzaldehyde (9), respectively (Scheme 5).61 A subsequent reaction with N-(1-(2´-pyridyl)-1-oxo-2-ethyl)pyridinium iodide (13)62 in the presence of NH4OAc gave the 2,2´:6´,2´´-terpyridines 14 and 15.63

+N

O

R

ON

N

N

N

R

R

O

8, R=CH39, R=NO2

11, R=CH3 (72 %)12, R=NO2 (80 %)

14, R=CH3 (65 %)15, R=NO2 (48 %)

10

OH-, EtOH/H2O, rt

NO

N

IR

ON

11, R=CH312, R=NO2

13

+NH4OAc, AcOH, reflux

Scheme 5. Synthesis of 4´-arylsubstituted 2,2´:6´,2´´-terpyridines.

60 Selected references for the synthesis of multi-nucleating ligands: (a) Baba, A. I.; Wang, W.; Kim, W. Y.; Strong, L.; Schmehl, R. H. Synth. Commun. 1994, 24, 1029-1036. (b) Grosshenny, V.; Romero, F. M.; Ziessel, R. J. Org. Chem. 1997, 62, 1491-1500. (c) Phillips, I. G.; Steel, P. J. Inorg. Chim. Acta 1996, 244, 3-5. (d) Kaes, C.; Katz, A.; Hosseini, M. W. Chem. Rev. 2000, 100, 3553-3590. (e) Cargill Thompson, ref 46a. 61 The enones were prepared as described by Baba et al. for similar compounds, ref 60a. 62 Priimov, G. U.; Moore, P.; Maritim, P. K.; Butalanyi, P. K.; Alcock, N. W. J. Chem. Soc. Dalton Trans. 2000, 445-449. 63 Terpyridines 14 and 15 were prepared as described by Kröhnke for similar compounds, ref 57.

16

The corresponding 4-substituted 2,2´-bipyridines 21 and 22 were synthesized from sodium pyruvate (16), and p-methylbenzaldehyde and p-nitrobenzaldehyde, respectively (Scheme 6).64 The α,β-unsaturated ketoacids 17 and 18 were reacted with 13 in the presence of NH4OAc, to give 19 and 20. A subsequent decarboxylation gave 21 and 22. The 2,2´:6´,2´´-terpyridines and 2,2´-bipyridines described in this section were used for the preparation of naphthalene diimide substituted ruthenium(II) polypyridyl complexes (Chapter 4).

R

O

+ OO

O

NaR

OOH

O

N

N

R

COO NH4

N

N

R

8, R=CH39, R=NO2

17, R=CH3 (48 %)18, R=NO2 (60 %)

19, R=CH3 (65 %)20, R=NO2 (79 %)

21, R=CH3 (45 %)22, R=NO2 (74 %)

16

OH-, EtOH/H2O, 0°C

13, NH4OAc, H2O, reflux ∆

Scheme 6. Synthesis of 4-arylsubstituted 2,2´-bipyridines.

2.5.2 Synthesis of [6-(2,2´-Bipyridyl)]-(2´-pyridyl)-methanes

The short excited state lifetime of the [Ru(tpy)2]2+ chromophore due to the weak ligand field has motivated research groups to search for alternative ruthenium(II) polypyridyl complexes as sensitizers. One approach involves the destabilization of the 3MC states by introducing ligands that can coordinate in a more octahedral fashion than ordinary 2,2´:6´,2´´-terpyridines. In our approach, we designed tridentate ligands where an extra carbon is inserted between two pyridines to give mixed bipyridyl-pyridyl ligands. These ligands were subsequently used in the preparation of bistridentate ruthenium(II) complexes and are discussed in Chapter 5.

The [6-(2,2´-bipyridyl)]-(2´-pyridyl)-methanes (Scheme 8) were synthesized via addition of 6-lithio-2,2´-bipyridine to 2-acetylpyridine (10) and 2-pyridinecarboxaldehyde (27), respectively. The 6-lithio-2,2´-bipyridine was prepared via metal-halogen exchange 64 Berg, K.; Tran, A.; Raymond, M. K.; Abrahamsson, M.; Wolny, J.; Andersson, M.; Sun, L.; Styring, S.; Hammarström, L.; Toftlund, H.; Åkermark, B. Eur. J. Inorg. Chem. 2001, 1019-1029.

17

of 6-bromo-2,2´-bipyridine (26),65 obtained in three steps from 2,2´-bipyridine (23) (Scheme 7).

N

N

N

N

I

N

NO

N

NBr

25

26

2423

MeI, CH3CN, 45°C63 %

K3Fe(CN)6, OH-, 5°C33 %

Br2, PPh3, CH3CN-5°C then reflux

51 %

Scheme 7. Preparation of 6-bromo-2,2´-bipyridine.

First attempts to perform the metal-halogen exchange reaction at –78 °C in

tetrahydrofuran, resulted in several products among which an addition product dominated. In a diethylether/tetrahydrofuran (4:1) mixture, however, only minor addition occurred and using 2-pyridinecarboxaldehyde as electrophile 28 was isolated in 64 % yield. Employing 2-acetylpyridine as the electrophile, 29 was isolated in lower yield reflecting the competition between addition and α-proton abstraction, which resulted in a ~2:1 mixture of 29 and 2,2´-bipyridine. Both compounds were subsequently methylated to give 30 and 31.

N

N

N

NBr

NR

O N

ROH

N

N

N

RO

N

N

N

ROH

27, R=H 10, R=CH3

26

28, R=H (64 %) 29, R=CH3 (52 %)

30, R=H (73 %)31, R=CH3 (64 %)

28, R=H29, R=CH3

1) n-BuLi, Et2O/THF, -78°C

2)

1) tBuOK, THF, rt2) MeI

Scheme 8. Preparation of bipyridyl-pyridyl ligands.

65 Norrby, T.; Börje, A.; Zhang, L.; Åkermark, B. Acta Chem. Scand. 1998, 52, 77-85.

18

Donor/Acceptor Systems Based on Benzoquinone

(Paper I)

3 3.1 Introduction

In the Swedish Consortium for Artificial Photosynthesis, we have previously

demonstrated intramolecular electron transfer from covalently linked tyrosine and manganese complexes to photogenerated RuIII (e.g. 32 and 33).66,67 The external acceptor, either a cobalt(III) complex or methyl viologen (MV2+), has been involved in ligand

RuN

N

NN N

Nhν

HN O

O NN

N N

NNMn Mn

O OO O

1e-

2

e-

MV2+

RuN

N

NN N

N

hν

1e-

2

e-

MV2+

NN Mn(H2O)2Cl2

32

33

N N

EtO2C

2+ 3+

66 (a) Sjödi3936. (b) SY.; MagnuChem. Soc.67 (a) Sun, P.; PhilouzMagnuson,Hammarstrour work, s

Figure 10. Intramolecular electron transfer in Mn-RuII dyads.

n, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarström, L. J. Am. Chem. Soc. 2000, 122, 3932-un, L.; Burkitt, M.; Tamm, M.; Raymond, M. K.; Abrahamsson, M.; LeGourriérec, D.; Frapart, son, A.; Kenéz, P. H.; Brandt, P.; Tran, A.; Hammarström, L.; Styring, S.; Åkermark, B. J. Am. 1999, 121, 6834-6842. L.; Hammarström, L.; Norrby, T.; Berglund, H.; Davydov, R.; Andersson, M.; Börje, A.; Korall, e, C.; Almgren, M.; Styring, S.; Åkermark, B. Chem. Commun. 1997, 607-608. (b) Huang, P.; A.; Lomoth, R.; Abrahamsson, M.; Tamm, M.; Sun, L.; van Rotterdam, B.; Park, J.; öm, L.; Åkermark, B.; Styring, S. J. Inorg. Biochem. 2002, 91, 159-172. For a recent update of ee: Hammarström, L. Curr. Opin. Chem. Biol. 2003, 7, 666-673.

19

exchange reactions and intermolecular charge recombination reactions respectively. No effort had previously been made to utilize the reducing equivalents for the production of high-energy substances. An extension to our earlier work would be the attachment of an electron acceptor to the supramolecules, and ultimately, a metal complex capable of proton reduction.

The most common electron acceptor in ruthenium(II) polypyridyl based dyads and triads is the bipyridinium or viologen type acceptor.68 The charge-separated state lifetimes are usually rather short (time scale of ps), even though recent work on noncovalent or mechanically linked ruthenium(II) polypyridyl-viologen dyads were shown to effectively stabilize the charge-separated state.4b,21b,21c In order to mimic natural photosynthesis, but also to minimize energy losses in the primary charge separation, we choose the quinone as acceptor instead.

Quinones have been extensively used as electron acceptors in studies involving porphyrins,16a,16c and in some cases even to ruthenium(II) polypyridyl complexes.69−71,30 Meyer and co-workers studied ruthenium(II) trisbipyridine based dyads and triads using linked anthraquinone (AQ) as electron acceptor.70,30 In all dyads,72 the excited state is quenched by the quinone but the charge-separated state was never detected due to rapid charge recombination. With phenothiazine as donor, however, the PTZ•+-RuII-AQ•− state is formed in decent yields in the corresponding triads (26-40 %) and could be observed by transient absorption or time-resolved Raman spectroscopy.73 In contrast to the anthraquinone based dyads, Lehn and co-workers studied a [Ru(bpy)3]2+-benzoquinone (BQ) dyad where the charge recombination is slower than that of charge separation.69d

68 Selected references dealing with RuII-viologen type assemblies: (a) Danielson, E.; Michael Elliott, C.; Merkert, J. W.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 2519-2520. (b) Kelly, L. A.; Rodgers, M. A. J. J. Phys. Chem. 1995, 99, 13132-13140. (c) Klumpp, T.; Linsenmann, M.; Larson, S. L.; Limoges, B. R.; Bürssner, D.; Krissinel, E. B.; Elliott, C. M.; Steiner, U. E. J. Am. Chem. Soc. 1999, 121, 1076-1087. (d) Larson, S. L.; Elliott, C. M.; Kelley, D. F. J. Phys. Chem. 1995, 99, 6530-6539. (e) Lomoth, R.; Häupl, T.; Johansson, O.; Hammarström, L. Chem. Eur. J. 2002, 8, 102-110. (f) Yonemoto, E. H.; Saupe, G. B.; Schmehl, R. H.; Hubig, S. M.; Riley, R. L.; Iverson, B. L.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 4786-4795. 69 (a) Beer, P. D.; Timoshenko, V.; Maestri, M.; Passaniti, P.; Balzani, V. Chem. Commun. 1999, 1755-1756. (b) Berthon, R. A.; Colbran, S. B.; Moran, G. B. Inorg. Chim. Acta 1993, 204, 3-7. (c) Beyeler, A.; Belser, P. Coord. Chem. Rev. 2002, 230, 29-39. (d) Goulle, V.; Harriman, A.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1993, 1034-1036. (e) Storrier, G. D.; Colbran, S. B.; Craig, D. C. J. Chem. Soc. Dalton Trans. 1998, 1351-1363. (f) Schanze, K. S.; Sauer, K. J. Am. Chem. Soc. 1988, 110, 1180-1186. 70 (a) Mecklenburg, S. L.; McCafferty, D. G.; Schoonover, J. R.; Peek, B. M.; Erickson, B. W.; Meyer, T. J. Inorg. Chem. 1994, 33, 2974-2983. (b) Opperman, K. A.; Mecklenburg, S. L.; Meyer, T. J. Inorg. Chem. 1994, 33, 5295-5301. 71 (a) Arounaguiri, S.; Maiya, B. G. Inorg. Chem. 1999, 38, 842-843. (b) Kim, M.-J.; Konduri, R.; Ye, H.; MacDonnell, F. M.; Puntoriero, F.; Serroni, S.; Campagna, S.; Holder, T.; Kinsel, G.; Rajeshwar, K. Inorg. Chem. 2002, 41, 2471-2476. (c) Konduri, R.; Ye, H.; MacDonnell, F. M.; Serroni, S.; Campagna, S.; Rajeshwar, K. Angew. Chem. Int. Ed. 2002, 41, 3185-3187. 72 Except for the dyad based on the [Ru(bpy)2(L)2]2+ core (Figure 8). 73 Schoonover, J. R.; Strouse, G. F.; Chen, P.; Bates, W. D.; Meyer, T. J. Inorg. Chem. 1993, 32, 2618-2619.

20

Several studies on multi-component systems involving [Co(bpy)3]3+ as catalyst for CO2 reduction have been reported.74 RuII-CoIII dinuclear complexes have recently been shown to undergo light-induced electron transfer from excited RuII to the CoIII acceptor.75 In this context, we designed a novel RuII-CoIII dinuclear complex linked by a redox-active benzoquinone to further investigate such systems. The rather slow recombination in the similar [Ru(bpy)3]2+-benzoquinone dyad reported by Lehn and co-workers,69d would ideally lead to a further charge shift to generate the RuIII-BQ-CoII state.

We envisaged that a donor linked to the RuII unit would further stabilize the charge-separated state where the reducing equivalent is localized on the benzoquinone. Instead of using our tyrosine derivatives for manganese complexation, we decided to use the phenothiazine donor. In order to have a suitable model complex for the triad, the trisheteroleptic complex [Ru(bpy)(dmb)(dmbPTZ)]2+ was also synthesized. This PTZ-RuII complex differs from similar dyads reported by Meyer (Figure 11),29,70b in that all three bipyridyl ligands are different and, therefore, would be expected to have unique properties different from the previously addressed dyads. In 34 and 35, the forward electron transfer rates were calculated from the measured emission decay of the chromophore, giving kf ≥ 2.8×108 and 4×105 s-1 respectively.76

RuN

N

NN N

N

N

S

2+

RuN

N

NN N

N

N

S

2+

34 35

Figure 11. PTZ-RuII dyads studied by Meyer and co-workers.

74 See for example: (a) Creutz, C.; Sutin, N. Coord. Chem. Rev. 1985, 64, 321-341. (b) Keene, F. R.; Creutz, C.; Sutin, N. Coord. Chem. Rev. 1985, 64, 247-260. 75 (a) Komatsuzaki, N.; Himeda, Y.; Hirose, T.; Sugihara, H.; Kasuga, K. Bull. Chem. Soc. Jpn. 1999, 72, 725-731. (b) Song, X.; Lei, Y.; Van Wallendal, S.; Perkovic, M. W.; Jackman, D. C.; Endicott, J. F.; Rillema, D. P. J. Phys. Chem. 1993, 97, 3225-3236. (c) Yoshimura, A.; Nozaki, K.; Ikeda, N.; Ohno, T. J. Am. Chem. Soc. 1993, 115, 7521-7522. (d) Yoshimura, A.; Nozaki, K.; Ikeda, N.; Ohno, T. J. Phys. Chem. 1996, 100, 1630-1637. 76 Only the excited state lifetime for 35 was given in ref 29. kf was estimated using eq. 7.

21

3.1.1 Synthesis

The synthetic strategy was based on the preparation of a symmetric bis-2,2´-bipyridyl ligand, linked by a suitable quinone precursor (Scheme 9).

N

N N

N N

N

O

O

OH

O

OH OH O

O

OH OH O

O

Br Br

N

N N

N

O

O

36 37 38

39 40

41

MeI, K2CO3, acetone, reflux

52 %HBr/AcOH, 10°C

75 %

1) LDA, THF, -78°C2) 0.5 equiv. 38

78 %

TsOH, CeIV, CH3CN, rt68 %

Scheme 9. Synthesis of a bis-2,2´-bipyridyl substituted quinone.

The 2,6-dihydroxymethyl p-methoxyphenol 3677 was selectively alkylated at the

phenolic oxygen giving 37,78 which was subsequently brominated in HBr/AcOH to furnish 38. Two equivalents of deprotonated 4,4´-dimethyl-2,2´-bipyridine (39) were reacted with 38 to give the symmetrical bis-2,2´-bipyridine 40. The oxidation of 40 with cerium(IV) in an acetonitrile/water mixture failed, due to the formation of an uncharacterized Ce-complex, but in the presence of acid this was prevented and 41 was isolated in 68 % yield.

The reaction of an excess of 40 and Ru(bpy)2Cl2·2H2O (42) at reflux in ethanol gave 43, which was oxidized with cerium(IV) to give the RuII-BQ dyad 44. Alternatively, 44 was prepared from Ru(bpy)2Cl2·2H2O and 41 in a similar yield. The RuII-BQ-CoIII triad 45 was prepared from 44 and [Co(bpy)2Cl2]Cl in refluxing methanol (Scheme 10).

77 Sun, L.; von Gersdorff, J.; Sobek, J.; Kurreck, H. Tetrahedron 1995, 51, 3535-3548. 78 Moran, W. J.; Schreiber, E. C.; Engel, E.; Behn, D. C.; Yamins, J. L. J. Am. Chem. Soc. 1952, 127-129.

22

NN

N

N

NN

RuN

N

O

OCl

N

N

NN

Ru

Cl

+

2+

NN

N

N

NN

RuN

N

O

O

2+

67 %40

42

72 %

43

44

72 %

NN

N

N

NN

RuN

N

O

O

NN

N

NCo

5+

45

EtOH, reflux

CeIV, H2O/CH3CN, rt [Co(bpy)2Cl2]Cl, MeOH, reflux

Scheme 10. Synthesis of the RuII-BQ dyad and the RuII-BQ-CoIII triad.

Compound 48 (dmbPTZ) was prepared as described by Meyer.79 Bromination of

4,4´-dimethyl-2,2´-bipyridine to give 47 can be achieved either via benzylic oxidation with SeO2 followed by reduction and bromination of the intermediate alcohol 46,80 or by direct radical bromination with NBS.81 Although studies from the literature reported much higher yields for the oxidation/reduction sequence, the yields obtained were comparable for the two pathways. Subsequent substitution with deprotonated phenothiazine gave 48.

Following the protocol developed by Grätzel and co-workers, the stepwise introduction of three different 2,2´-bipyridyl ligands starting from Ru(DMSO)4Cl2 via Ru(bpy)(DMSO)2Cl2 (49) gave the trisheteroleptic PTZ-RuII-BQ triad 50 and PTZ-RuII dyad 51 in 10 % and 21 % yields respectively. The low yields were mainly attributed to purification difficulties, and in the synthesis of triad 50 the intermediate bis-bipyridine compound Ru(bpy)(dmbPTZ)Cl2 also contained some Ru(dmbPTZ)2Cl2 from ligand scrambling.

79 Maxwell, K. A.; Sykora, M.; DeSimone, J. M.; Meyer, T. J. Inorg. Chem. 2000, 39, 71-75. 80 (a) Geren, L.; Hahm, S.; Durham, B.; Millett, F. Biochemistry 1991, 30, 9450-9457. (b) Börje, A. PhD Thesis, KTH, Stockholm, 1997. 81 Gould, S.; Strouse, G. F.; Meyer, T. J.; Sullivan, B. P. Inorg. Chem. 1991, 30, 2942-2949.

23

N

N

N

N

OH Br

N

N

N S

75 %

N

N

46 47

26 %

85 %

48

1) SeO2, dioxane, reflux2) NaBH4, MeOH, 0°C

30 %

NBS, AIBN, CCl4, reflux

HBr/H2O, H2SO4, reflux

1) n-BuLi, THF,-78°C2) 47

N

S

H

39

Cl

N

N

DMSORu

Cl

DMSO

NN

N

N

NN

RuN

N

O

O

2+

NS

NN

N

N

NN

Ru

NS

2+

4950

Cl

N

N

DMSORu

Cl

DMSO

49

51

1) 48, DMF, reflux2) 6 equiv. 41, EtOH, reflux

10 %

1) 39, DMF, 120°C2) 48, EtOH, reflux

21 %

Scheme 11. Synthesis of the PTZ-RuII-BQ triad and the PTZ-RuII dyad.

The signals in the 1H NMR spectrum of 45 are slightly broadened, reflecting the mixture of isomers in the sample (∆Λ,ΛΛ + enantiomers). Otherwise no difference in the photophysical properties is expected. The presence of geometrical isomers of 50 and 51 was indicated by their 1H NMR spectra, where the –CH2− resonance in the dmbPTZ ligand gave rise to four and two singlets respectively, of approximately equal intensity (Figure 12).

mixture of isomers in the sample (∆Λ,ΛΛ + enantiomers). Otherwise no difference in the photophysical properties is expected. The presence of geometrical isomers of 50 and 51 was indicated by their 1H NMR spectra, where the –CH2− resonance in the dmbPTZ ligand gave rise to four and two singlets respectively, of approximately equal intensity (Figure 12).

24

RuN

N

NN N

NRu

NN

NN N

N

RuN

N

NN N

NRu

NN

NN N

N

A A

A A

D

D

D

D

N

S

A

D

RuN

N

NN N

NRu

NN

NN N

N

D

D

N

N

O

O

Figure 12. Top: Geometric isomers in complexes 50 (left) and 51 (right). Bottom: The corresponding NMR spectra of the complexes highlighted inthe 5.0-6.7 region (CD3CN, 298 K, 400 MHz). Singlet at 5.45 ppm isresidual CH2Cl2 in the samples.

3.1.2 Electrochemical and Photochemical Properties

The redox potentials for the various components of the dyads and triads in this study are summarized in Table 1. Also included are the absorption data of the 1MLCT bands of the ruthenium chromophore as well as the emission quantum yield relative to [Ru(bpy)3]2+. All absorption spectra can be described as the sum of the individual units, indicating that the interactions between the components in the ground state are weak. The absence of charge-transfer bands in 50 indicates that direct electron transfer from PTZ to BQ does not play a role in the formation of the charge-separated state.

25

Table 1. Electrochemical and absorption/luminescence data. E½ / V[b] Absorption[c] Emission

Complex[a] BQ0/− Co3+/2+ PTZ+/0 Ru3+/2+ λmax (ε⋅10-4) λmax[d] (φrel)[e]

RuII-BQ (44) –0.93 — — 0.84 454 (1.2) 584 (<0.01) RuII-BQ-CoIII (45) –0.87 –0.10 — 0.84 454 (1.2) 584 (<0.01) PTZ-RuII-BQ (50) –0.93 — 0.39 0.81 459 (1.5) 589 (<0.01) PTZ-RuII (51) — — 0.40 0.81 458 (1.5) 589 (0.10) [Ru(bpy)3]2+ (1) — — — 0.88 451 (1.4)39a 582 (1.0) [Co(bpy)3]3+ — −0.08 — — 318 (sh, 2.6) —

a) As PF6− salt. b) CH3CN, N(n-C4H9)4PF6 (0.1 M), 0.1 V/s, vs Fc. c) CH3CN. d) Recorded at 77 K.

e) Relative to [Ru(bpy)3]2+, 298 K, CH3CN.

In the cyclic voltammogram of 51, two reversible anodic waves were observed at

0.40 and 0.81 V, assigned to the PTZ+/0 and RuIII/II redox couples respectively (Figure 13). For 50, however, the phenothiazine oxidation was irreversible. In addition, a reversible cathodic wave was observed at –0.93 V associated with the reduction of the quinone unit. The quinone reduction was only reversible in the first scan. Subsequent scans over the potential range where both PTZ and BQ are affected led to additional peaks that increased at the expense of the quinone and the phenothiazine peaks (not shown). It seems therefore, that chemical reactions occur between the phenothiazine radical cation and the neutral benzoquinone on the timescale of the electrochemical experiment.

b

E / V vs Fc+/0-1,0 -0,5 0,0 0,5 1,0

a

20 µA

Figure 13. CV of 51 (a) and 50 (b), 1 mM, CH3CN, N(n-C4H9)4PF6, 0.1 V/s.

The emission from the ruthenium chromophore in both 44 and 45 is efficiently

quenched by the appended quinone (Scheme 12). In 44, the forward (kf) and back (kb) electron transfer rate-constants are 5×109 s-1 and 4.5×108 s-1 respectively. In 45, the transient signals are identical to those from 44 and no charge shift to the CoIII unit was

26

observed. Apparently, the charge shift reaction (ksh) is unable to compete with the charge recombination (kb).82

Scheme 12.

*RuII-BQ RuIII-BQkf

*RuII-BQ-CoIII RuIII-BQ -CoIIIkf

kb RuII-BQ

RuIII-BQ-CoII

RuII-BQ-CoIII

ksh

kb

(44)

(45)

The emission in 51 is 10 % of that of [Ru(bpy)3]2+ with a lifetime of 90 ns

(Scheme 13). Thus, one would think that this time constant reflects the primary charge separation (PTZ-RuII → PTZ•+-RuII(bpy•−)). However, the transient absorption data indicate a mixture of the charge transfer state (CS), PTZ•+-RuII(bpy•−), and the excited state, PTZ-*Ru, in a ~1:4 mixture which decays with a single time constant (90 ns). This suggests that an equilibrium between the excited state and the charge transfer state is formed, and the charge separation rate is higher than that obtained from the emission decay (eq. 7). All rate-constants determined for 51 are included in Figure 14.

Scheme 13.

*RuII kf

kbCS

kRu* kCR

The rate-constants for the electron transfer steps as determined for 50 and/or the

different model complexes are shown in Figure 14. For 50, there are two possible pathways for the charge separation reaction (Scheme 14): Either via initial oxidative quenching by the BQ unit (1), or via initial reductive quenching of the PTZ unit (2).

P

1.

2.

82 ReductioRuII-CoIII d

Scheme 14.

PTZ-*RuII-BQ PTZ-RuIII-BQ

TZ-*RuII-BQ PTZ -RuII(bpy )-BQ

PTZ -RuII-BQ

PTZ -RuII-BQ

n of t2g

6-CoIII to give t2g5eg

2-CoII is known to be slow due to large structural changes. Studies of inuclear complexes indicated that electron transfer might be fast to form the excited doublet CoII.

27

PTZ-*RuII-BQPTZ -RuII(bpy )-BQ

PTZ-RuIII-BQ

PTZ -RuII-BQ

PTZ-RuII-BQ

2.17 eV2.10 eV

1.74 eV

1.32 eV

0.00 eV

1)

2)

3)

4)

5)

6)

7) 8)

1) k = 2.5×107 s-1 2) k = 6.8×107 s-1 3) k = 5×109 s-1 4) k > 5×109 s-1 5) k = 4.5×108 s-1 6) k = 1.25×107 s-1

7) k = 5.7×107 s-1 8) k = 1.1×106 s-1

Figure 14. Kinetic scheme that summarizes the electron transfer rates andenergetics in triad 50.

The charge-separated state PTZ•+-RuII-BQ•− is formed with a rate constant of kf =

5×109 s-1 and no intermediate state can be detected. Since the reductive quenching rate constant in 51 is 2.5×107 s-1, it can be concluded that the charge separation occurs via oxidative quenching and that the following charge shift occurs with a rate constant >5×109

s-1. The PTZ•+-RuII-BQ•− state subsequently decays to the ground state with kb = 1.25×107

s-1 (80 ns). In 50, 1.32 eV (63 %) of the excitation energy is stored as redox energy. The charge separation yield is close to 100 %, far better than most previously reported triads based on ruthenium(II) polypyridyl complexes.

A new species with properties very similar to 51 was also detected. The concentration of this “PTZ-RuII” like complex increased upon light exposure, suggesting that the diradical in the charge-separated state can undergo coupling with the formation of a PTZ-hydroquinone species. This unexpected reaction discouraged us from developing the system further. 3.1.3 Conclusions

The dyad 44 shows efficient quenching of the emission in analogy with a related ruthenium(II) trisbipyridine-benzoquinone dyad previously reported.69d In an attempt to further develop this system, a cobalt(III) trisbipyridine complex was covalently linked to give triad 45. However, the desired secondary charge shift to the CoIII acceptor is unable to compete with charge recombination. To stabilize the charge-separated state, the

28

frequently used electron donor phenothiazine was linked to the ruthenium unit giving triad 50. Indeed, the lifetime increases 40-fold compared to the corresponding dyad (2 ns vs. 80 ns) with a charge separation yield of nearly 100 %. The oxidative quenching mechanism in 50 was unambiguously determined by a comparison with the PTZ-RuII dyad 51. Interestingly, an equilibrium between the excited state and the charge transfer state is established after excitation of 51, a possibility that may have been overlooked in other similar systems.

29

Naphthalene Diimide as Acceptor (Papers II and III) 4

This chapter describes our efforts to utilize the naphthalene diimide (NDI) as primary acceptor. The first section describes the synthesis and properties of some ruthenium(II) polypyridyl-naphthalene diimide dyads with the aim of optimizing the primary charge separation event. In the second part, the incorporation of one of these RuII-NDI units in a dpaTyr-RuII-NDI triad is described (dpaTyr is 2,6-bis(N,N-dipicolylamino) tyrosine ethyl ester). This precursor was further used for complexation of MnII to give the first trinuclear Mn2

II/II-RuII complex containing intramolecular acceptors.

4.1 Introduction

The choice of the naphthalene diimide acceptor was motivated from the extensive use of imide acceptors in porphyrin-based models for the photosynthetic reaction center. Even though our quinone-based systems were proven to efficiently generate long-lived charge-separated states, the lack of a characteristic absorption from the reduced quinone made it difficult to identify the PTZ•+-RuII-BQ•− state. In contrast, the reduced diimide acceptors often have intense absorption spectra that allow easy detection. In addition, the imide acceptors are chemically more robust than the quinones, and can be linked to other components in ways that restrict conformational mobility.

A number of aromatic imide acceptors have been used previously in porphyrin-based and other organic chromophore systems,83,84 The choice of the naphthalene diimide in our systems was made from the expected redox potential of the acceptor, which should ensure sufficient driving force for efficient oxidative quenching, but at the same time is

83 Selected references dealing with porphyrin-imide assemblies: (a) Debreczeny, M. P.; Svec, W. A.; Wasielewski, M. R. Science 1996, 274, 584-586. (b) Hayes, R. T.; Wasielewski, M. R.; Gosztola, D. J. Am. Chem. Soc. 2000, 122, 5563-5567. (c) Ohkohchi, M.; Takahashi, A.; Mataga, N.; Okada, T.; Osuka, A.; Yamada, H.; Maruyama, K. J. Am. Chem. Soc. 1993, 115, 12137-12143. (d) O´Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines III, G. L.; Wasielewski, M. R. Science 1992, 257, 63-65. (e) Tan, Q.; Kuciauskas, D.; Lin, S.; Stone, S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. B 1997, 101, 5214-5223. 84 Selected references dealing with other organic chromophore-imide assemblies: (a) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R. J. Am. Chem. Soc. 1996, 118, 6767-6777. (b) Lukas, A. S.; Miller, S. E.; Wasielewski, M. R. J. Phys. Chem. B 2000, 104, 931-940.

30

low enough to allow only small losses of the incident photon energy. This acceptor has earlier been utilized in ruthenium(II) polypyridyl assemblies. Hossain et al. reported a charge separation yield of 75 % in a mixed RuII-NDI-OsIII complex with flexible alkyl linkers, giving the redox isomer RuIII-NDI-OsII state.85 Dixon et al. studied a flexible RuII-NDI dyad and its intercalation into DNA.86 This complex showed multiexponential emission decay both in solution and in the presence of DNA, which probably originates from different conformers of the dyad. To circumvent such complications, we designed rigid systems where the distances and orientations between the donor and the acceptor are restricted. Since earlier work reported oxidative quenching in “RuII-bpy” type dyads on the nanosecond timescale, it seemed possible to accomplish electron transfer also in [Ru(tpy)2]2+-based systems. Therefore, two sets of RuII-NDI dyads were prepared, one based on ruthenium(II) trisbipyridine and the other on ruthenium(II) bisterpyridine.

4.1.1 Preparation of RuII-NDI Dyads

The synthetic strategy relied on the condensation reaction between amine-functionalized oligopyridines and the naphthalene monoanhydride precursor 63 (Scheme 16). A subsequent reaction with an appropriate ruthenium precursor would give the RuII-NDI dyads.

The primary amines were prepared by the Gabriel procedure from 4´-(p-tolyl)-2,2´:6´,2´´-terpyridine (14), 4-(p-tolyl)-2,2´-bipyridine (21), and 4,4´-dimethyl-2,2´-bipyridine (39) (Scheme 15). These were converted to the corresponding bromides 52, 53, and 60, and then reacted with potassium phthalimide to give 54, 55, and 61. Treatment with H2NNH2 gave 56, 57, and 62.87 The nitrophenyl-functionalized oligopyridines 15 and 22 were reduced with SnCl2 or H2NNH2 over Pd/C to furnish 58 and 59.34a,88

85 Hossain, D.; Haga, M.; Monjushiro, H.; Gholamkhass, B.; Nozaki, K.; Ohno, T. Chem. Lett. 1997, 573-574. 86 Dixon, D. W.; Thornton, N. B.; Steullet, V.; Netzel, T. Inorg. Chem. 1999, 38, 5526-5534. 87 Hamachi, I.; Tanaka, S.; Tsukiji, S.; Shinkai, S.; Oishi, S. Inorg. Chem. 1998, 37, 4380-4388. 88 Ng, W. Y.; Gong, X.; Chan, W. K. Chem. Mater. 1999, 11, 1165-1170.

31

N

NNPht

N

NBr

N

N NH2

R R

N

N

14, R=py 21, R=H

R

N

N NO2

R R

N

N

NPhtN

N

Br

60 61

N

NNH2

R

N

N

NH2

62

52, R=py (70 %) 53, R=H (56 %)

54, R=py (76 %)55, R=H (92 %)

15, R=py 22, R=H

58, R=py (56 %) 59, R=H (88 %)

56, R=py (84 %) 57, R=H (92 %)

NBS, AIBN, CCl4, refluxor alternatively,NBS, (PhCOO)2, CCl4, reflux

N

O

O

K

DMF, 80°C

SnCl2, HCl, 60°C (58)Pd/C, H2NNH2, EtOH, reflux (59)

H2NNH2, EtOH, reflux

N

O

O

K

DMF, 80°C85 %

H2NNH2, EtOH, reflux

92 %

Scheme 15. Synthesis of amine-functionalized oligopyridines.

Due to the electron deficient pyridines, fairly forcing conditions had to be

employed in the condensation of the aniline derivatized oligopyridines 58 and 59 with the anhydride precursor 63. Attempts to perform this reaction in refluxing toluene in the presence of DCC/DMAP resulted in no conversion. Instead, refluxing in dimethylacetamide followed by addition of pyridine and acetic anhydride to affect the cyclodehydration resulted in moderate yields of 66 and 67.89 In contrast, the aminomethyl aryl derivatives were much more reactive and the reactions could be performed in toluene at reflux to give compounds 64, 65,and 68.

89 Storrier, G. D.; Colbran, S. B. J. Chem. Soc. Dalton Trans. 1996, 2185-2186.

32

N O

O

O

O

O

NO

O

NO

O C8H17

N

N

N

N N

O

O

O

N

O

C8H17

N

N

N

O

O

O

N

O

C8H17

R

R

64, R=py (65 %) 65, R=H (63 %)

68 (68 %)

O O

O

O

O

O

63 66, R=py (45 %) 67, R=H (55 %)

DMF, 140°C38 %

1. 58 or 59, DMA reflux2. Pyridine, (AcO)2O, 80°C

NH2

+

56 or 57, toluene,reflux

62, toluene, reflux

Scheme 16. Synthesis of naphthalene diimide substituted oligopyridines.

Compounds 64, 65, and 68 are poorly soluble in common organic solvents. The

methylene group between the oligopyridine part and the naphthalene diimide allows coplanarity of the units and efficient π-stacking. In contrast, compounds 66 and 67 are much more soluble as the N-phenyl group is expected to be twisted out of the plane of the naphthalene unit. Reacting 64 and 66 with the 2,2´:6´,2´´-terpyridine based ruthenium

N

N

N

Ru

N

N

N

N

O

O

N

O

C8H17

O

2+

CH2 n

69, n=0 70, n=1

N

N

N

Ru

N

N

N

N

O

O

N

O

C8H17

O

2+

CH2 n

71, n=0 72, n=1

Figure 15. Terpyridine based RuII-NDI dyads.

33

precursors Ru(ttpy)(DMSO)Cl2, or Ru(tpy)Cl3 in the presence of NEt3 as reductant, gave the RuII-NDI dyads 69-72 (Figure 15) in 31-46 % yield. The reaction of 65, 67, and 68 with Ru(bpy)2Cl2·2H2O gave the corresponding 2,2´-bipyridine based RuII-NDI dyads 73-75 (Figure 16) in 75-87 % yield.

N N

N

N N

N

Ru N

O

O

N

O

C8H17

O

2+

CH2 n

73, n=0 74, n=1

N N

N

N N

N

Ru CH2

75

N

O

O

N

O

C8H17

O

2+

Figure 16. Bipyridine based RuII-NDI dyads.

4.1.2 Electrochemical and Photochemical Properties

The cyclic voltammograms of the RuII-NDI dyads all displayed reversible RuIII/II

redox couples at 0.85-0.90 V and the first NDI reduction at –0.97 V. In 75, four additional reductions were observed while in 69-74 the complexes were adsorbed at the electrode after reduction to overall neutral charge (Figure 17). As for the RuII-BQ complexes discussed above, the observed absorption spectra of all RuII-NDI dyads can be constructed from the sum of the individual units which indicates that there is no significant interaction between the chromophore and the acceptor in the ground state.90 This is true also for 69, 71, and 73, and further supports the idea of substantial dihedral angles between the π-systems.

90 The lowest energy transition for the 1,4,5,8-naphthalene diimide in CH3CN is at 377 nm (ε = 2.8⋅104 M-

1cm-1). See for example: Barros, T. C.; Brochsztain, S.; Toscano, V. G.; Filho, P. B.; Politi, M. J. J. Photochem. Photobiol. A: Chem. 1997, 111, 97-104.

34

E / V vs Fc+/0-2 -1 0 1

20 µA

a

b

Figure 17. CV of 73 (a) and 75 (b), 1 mM, CH3CN, N(n-C4H9)4PF6, 0.1 V/s.

The appended naphthalene diimide unit only marginally affects the intrinsic

excited state decay in the terpyridine based dyads 69-72 and no significant electron transfer quenching occurs. For the 2,2´-bipyridine based dyads the situation is drastically different (Table 2). In complex 75, the forward electron transfer rate constant (kf) is 5×109

s-1 and the NDI•− was readily detected by transient absorption. The quantum yield is close to 100 % and the charge-separated state decays with a time constant of 7×109 s-1.

Table 2. Electrochemical and absorption/luminescence data.

E½ / V[b] Absorption[c] Emission

Complex[a] NDI0/− Ru3+/2+ λmax (ε⋅10-4) E00[d] (τem, ns)[e]

[Ru(bpy)3]2+ — 0.88 451 (1.4) 2.12 (890) 73 −0.97 0.87 456 (1.7) 2.08 (6.4) 74 −0.97 0.87 456 (1.7) 2.09 (7.9) 75 −0.96 0.85 455 (1.4) 2.12 (0.20)

a) As PF6− salt. b) CH3CN, N(n-C4H9)4PF6 (0.1 M), 0.1 V/s, vs Fc. c) CH3CN.

d) Calculated from the emission at 77 K. e) At 298 K, CH3CN.

For 73 and 74, the forward reactions are slower as a result of the greater distance between the chromophore and the acceptor. In the transient absorption spectra though, only features of the 3NDI were visible,91 formed by energy transfer from the ruthenium 3MLCT state in 20 % yield. This state subsequently evolved to give the final RuIII- 91 The absorption features of 3NDI and NDI•− are very similar at ~480 nm, but the NDI•− has a pronounced absorption also at 605 nm (ε ~ 6.4⋅103 M-1cm-1). Rogers, J. E.; Kelly, L. A. J. Am. Chem. Soc. 1999, 121, 3854-3861.

35

RuII-NDI

*RuII-NDIRuII-3NDI

RuIII-NDI

2.08 eV2.03 eV

1.84 eV

0.00 eV

1) k = 3.1×107 s-1

2) k = 1.2×108 s-1

3) k = 2.6×107 s-1

4) k > 1.2×108 s-1

1)

2)3)

4)

Figure 18. Kinetic scheme that summarizes the electron transfer rates and energetics in 73.

NDI•− state. The major fraction of the *Ru-NDI state (80 %) however, is quenched by direct electron transfer to the NDI unit, but this state can not be observed due to rapid charge recombination.92 A kinetic scheme summarizing the events following excitation of the chromophore for dyad 73 is shown in Figure 18.

4.1.3 Conclusions

Two sets of ruthenium(II) polypyridyl complexes linked to the electron acceptor naphthalene diimide have been synthesized. In the 2,2´:6´,2´´-terpyridine based dyads 69-72, no significant electron-transfer to the naphthalene diimide acceptor occurs. This is in contrast to the 2,2´-bipyridine based dyads 73-75, where the emission is almost entirely quenched by the appended diimide. In 73 and 74, 20 % of the charge-separated state is formed via population of the naphthalene diimide triplet, while electron transfer totally out competes energy transfer in 75. Due to the rigidity of the naphthalene diimide-oligopyridyl ligand 67 and the rather slow charge recombination in 73, this unit was to be incorporated in more complex structures involving ligands capable of coordinating manganese ions.

92 It is probably slower than in 75 though, due to the greater distance between the components.

36

4.2 A Hydrogen Bonded Tyrosine-Ruthenium(II)-Naphthalene Diimide Triad

In our previous work, the importance of efficient hydrogen bonding for rapid

tyrosine oxidation has been addressed.66,93 It was shown that electron transfer from bis-substituted dipicolylamine (dpa) tyrosine ethyl ester to RuIII is faster than in a related ligand where two hydrogen bonded pyridines are replaced by phenol moieties. Inspired by these results, we designed the dpaTyr-RuII-NDI triad containing two naphthalene diimide acceptors and one dpaTyr unit linked to RuII. The expected rapid intramolecular electron transfer process from dpaTyr to RuIII would ideally compete with the charge recombination in the dpaTyr-RuIII-NDI•− state. 4.2.1 Synthesis

The synthesis of the dpaTyr-RuII-NDI triad 87 together with its model complexes is outlined in Scheme 17. The precursor bpyCOOH (76) was obtained by SeO2/Ag2O oxidation of 4,4´-dimethyl-2,2´-bipyridine as described by Meyer and co-workers.94 This ligand was subsequently coupled with L-alanine ethyl ester and L-tyrosine ethyl ester, respectively, to give 77 and 78.95 The bis-dipicolylamine substituted tyrosine ethyl ester (79) was prepared by a Mannich reaction between Boc-protected tyrosine ethyl ester, p-formaldehyde, and dipicolylamine.66b

Complex 82 was prepared by the reaction of two equivalents of 67 with Ru(DMSO)4Cl2 in refluxing DMF in the presence of excess LiCl to suppress the formation of tris-substituted products. Using standard procedures, 82 was then reacted with compounds 39 and 76-78 to give 83-86 in moderate yields.96 Complex 84 was further treated with SOCl2 and subsequently coupled with 79 in the presence of NEt3 to give 87.

93 Johansson, A.; Abrahamsson, M.; Magnuson, A.; Huang, P.; Mårtensson, J.; Styring, S.; Hammarström, L.; Sun, L.; Åkermark, B. Inorg. Chem. 2003, 42, 7502-7511. 94 McCafferty, D. G.; Bishop, B. M.; Wall, C. G.; Hughes, S. G.; Mecklenburg, S. L.; Meyer, T. J.; Erickson, B. W. Tetrahedron 1995, 51, 1093-1106. 95 Ghanem, R.; Xu, Y.; Pan, J.; Andersson, J.; Polivka, T.; Pascher, T.; Styring, S.; Sun, L.; Sundström, V. Inorg. Chem. 2002, 41, 6258-6266. 96 Complex 82 was not purified which can explain the poor yields in these reactions.

37

OH NN

N N

NN

N

N

HNO

CO2Et

N

N

HNO

CO2Et

CO2Et

NH2

77 78 79

OH NN

N N

NN

CO2Et

NH-Boc

80

O NN

N N

NN

CO2Et

NH2

81

N

N

HOO

76

56 %

1) tBuOK, THF, rt2) MeI

CF3COOH, CH2Cl2, rt93 %

OH

NN

N

N

NN

Ru

N N

N NC8H17

O

O

O

O

C8H17

O

O

O

O

R

HN O

CO2Et

HN OHN OR = -CH3 -COOH

83 (31 %) 84 (30 %) 87, x=H (51 %) 88, x=CH3 (27 %)

86 (35 %)85 (29 %)

CO2Et CO2Et

OH O NN

N N

NN

Ru(DMSO)4Cl2 RuL2Cl2, L=67

X

82

2+

EtOH, reflux,39, 76, 77, 78respectively

67, LiCl, DMF, reflux

Scheme 17. Synthesis of the dpaTyr-RuII-NDI triad 87 and model complexes.

38

To interpret the results of dpaTyr-RuII-NDI, an analogous triad was synthesized in which the phenol is alkylated. This modification blocks possible phenol oxidation by RuIII, which leaves the tertiary amines as the only potential donors. Selective alkylation of 80 was achieved in tetrahydrofuran using potassium tert-butoxide followed by addition of CH3I. After deprotection, 81 was isolated and subsequently used in the preparation of 88.

The stereochemical outcome for the bisheteroleptic complexes 84-88 containing three unsymmetrical 2,2´-bipyridine ligands is depicted in Figure 19. Complex 83, containing one symmetrical 2,2´-bipyridyl ligand, exists in only three different geometrical isomers, while four isomers are possible for 84-88. It is assumed that statistical mixtures of the isomers are present in all isolated samples.

RuN

N

NN N

N

A

RuN

N

NN N

N

A

RuN

N

NN N

N

A

A

RuN

N

NN N

N

A

A

A

AD D D

D

RuN

N

NN N

N

A

RuN

N

NN N

N

A

RuN

N

NN N

N

A

A

A

A

Figure 19. Top: The four geometric isomers of complexes 84-88. Bottom: The three geometric isomers of 83.

4.2.2 Results from the Electrochemical Study

We had previously assigned the lowest oxidation in the related dpaTyr-RuII dyad to phenol oxidation by comparison with model compounds.66b The results from the present study support this assignment. The differential pulse voltammogram (DPV) of 85, 87, and 88 at positive potentials are shown in Figure 20. For 88 (long dash), the two oxidation peaks at 0.62 and 0.70 V are related to the formation of the amine cations, split by weak coulombic interactions. In complex 87, in contrast (solid line), an oxidation was observed

39

already at 0.43 V, assigned to the hydrogen bonded phenol.97 In addition, an amine oxidation at 0.58 V and a peak at ca. 0.85 V could be detected below the RuIII/II redox couple. From these data, it is clear that both the hydrogen bonded phenol and the tertiary amines are potential donors to photogenerated RuIII. The importance of hydrogen bonding is evident by a comparison with the electrochemical properties of 86. In this complex, the phenol oxidation was observed at a higher potential than the RuIII/II redox couple.98

E / V vs Fc+/00,0 0,2 0,4 0,6 0,8 1,0

E / V vs Fc+/00,0 0,2 0,4 0,6 0,8 1,0

10 µΑ

4.2.3 P

Thsubstitutemaximumsection.90

which caligands.33

a conseq(−∆G0 = 0

97 In the 1H98 The phen

Figure 20. DPV of 87 (solid), 88 (long dash), and 85 (short dash). Inset: CV of 87. 1 mM, CH3CN, N(n-C4H9)4PF6, 0.1 V/s.

hotochemical Properties

e absorption spectra of 83-88 are those expected for bis-naphthalene diimide d complexes (Figure 21), with an extinction coefficient of the absorption of the acceptor twice as high as for the dyads discussed in the previous

A small red-shift and an increase in intensity of the 1MLCT band were observed, n be explained by the additional phenyl substitution at the 2,2´-bipyridyl This decrease in energy is mirrored in the emission maximum (Table 3), and as uence the driving force for oxidative quenching is slightly lowered in 85-88 .20 eV) compared to 73 (−∆G0 = 0.24 eV).

NMR spectrum of 87, the hydrogen bonded phenolic proton appeared at 11.0 ppm (CD3CN). ol oxidation in 86 was observed as a shoulder slightly above the RuIII/II redox couple.

40

Wavelength (nm)300 400 500 600 700

ε (M

-1cm

-1)

0

20x103

40x103

60x103

80x103

100x103

120x103

MLCT

NDILC

NDI

Figure 21. Absorption spectra of 83 (solid) and 73 (dashed) in CH3CN.

The emission is efficiently quenched in 83 and 85-88, probably by oxidative

quenching in analogy to the RuII-NDI dyads. In contrast to the dyads and 83, the emission is biexponential for 85-88 with τ1 = 27 ns (75 %) and τ2 = 7 ns (25 %). This is an interesting observation, since the electron in the excited state will mainly be localized towards the acceptor in 83 while the opposite holds for 85-88.

Table 3. Electrochemical and absorption/luminescence data. E½ / V[b] Absorption[c] Emission

Complex[a] NDI0/− Ru3+/2+ λmax (ε⋅10-4) E00[d] (τem, ns)[e]

[Ru(bpy)3]2+ — 0.88 451 (1.4) 2.12 (890) 83 −0.97[f] 0.83 464 (2.2) 2.06 (4)

85-88 −0.97[f] 0.88 465 (2.4) 2.05 (27, 75 %) (7, 25 %)

a) As PF6− salt. b) CH3CN, N(n-C4H9)4PF6 (0.1 M), 0.1 V/s, vs Fc. c) CH3CN.

d) Calculated from the emission at 77 K. e) At 298 K, CH3CN. f) Irreversible.

In contrast to 83, 85, and 86, the naphthalene diimide radical was observed in ~10

% yield for 87 and ~5 % yield for the phenol ether 88.99 The decay is biexponential in 87 with kb = 3.3×106 s-1 and 1.0×105 s-1, both with equal amplitudes. A third fraction, ~30 % of the signal, is irreversible. In 88, only the irreversible process is observed. These results suggest that the irreversibility originates from amine oxidation and that in complex 87 both the tertiary amines and the hydrogen bonded phenol can act as parallel donors with comparable rates. The biexponential decay in 87 could possibly reflect different geometrical isomers. The processes that occur after excitation of 87 are shown in Figure 22.

99 An absorption at 605 nm confirmed this assignment, see note 91.

41

dpaTyr-RuII-NDI

dpaTyr-*RuII-NDI

dpaTyr-RuIII-NDI

(dpaTyr) -RuII-NDI

2.05 eV

1.85 eV

0.00 eV

1) k = 3.7×107 s-1

2) k > 3.7×107 s-1

3) k = 3.3×106 s-1 and k = 1.0×105 s-1

1)

2)90 %

3)

10 %

Figure 22. Kinetic scheme that summarizes the electron transfer rates and energetics in 87.

4.2.4 Conclusions

In a continuation of our previous work with hydrogen bonded phenols linked to RuII, we synthesized two bis-dipicolylamine substituted tyrosine-RuII-NDI triads, 87 and 88, differing only by the state of the phenol (methylated in 88). The analogous alanine (85) and tyrosine (86) substituted complexes were also synthesized. After excitation of the sensitizer in 87, the charge-separated state dpaTyr•+-RuII-NDI•− is formed in 10 % yield, showing that the hydrogen bonded phenol is a sufficiently fast donor to RuIII to compete with the charge recombination in the Ru-NDI unit.

42

4.3 Properties of a Mn2II/II-RuII-Naphthalene Diimide Triad

The Mn2

II/II-RuII trinuclear complex 89 containing two linked naphthalene diimide electron acceptors was synthesized from Mn(OAc)2 and 87 in ethanol (Figure 23). The formation of 89 was confirmed by ESI-MS which showed the double-charged ion (89-2PF6

−)2+ at 1258.9 and the triple-charged ion (89-3PF6−)3+ at 790.9.

The electron transfer events that occur after excitation of the ruthenium chromophore in 89 at 140 K in butyronitrile were followed by time resolved absorption techniques. The formation of the naphthalene diimide radical at 140 K was observed at 480 nm where the radical absorbs (Figure 24, solid circles).99 After the initial bleaching (resulting from a loss of RuII), a positive absorption from the NDI radical appeared. In contrast, no long-lived radical was formed for the alanine-substituted complex 85 (Figure 24, open circles). Hence, electron transfer from the Mn2

II/II unit in 89 can compete with the charge recombination of the Mn2

II/II-RuIII-NDI•− state to form the Mn2II/III-RuII-NDI•− state

(~10 % yield).100 The charge-separated state is very long-lived as evident from the decay of the NDI radical on the millisecond time-scale (Figure 24).101

RuN

N

NN N

N

hν

N

NC8H17

O

O

O

O

N

NC8H17

O

OO

O

HN O

O NN

N N

NNMn Mn

O OO O

1e−

2

e−

3

e−

89

EtO2C

3+

Figure 23. Structure of one of the four possible geometrical isomers of the Mn2

II/II-RuII-NDI triad.

100 Note the different donor in 89 compared to 87 and 88. 101 No degradation of the complex was observed.

43

0 5-0.100

-0.075

-0.050

-0.025

0.000

0.025

time/ms

∆abs

time/µs0 20 40 60 80

0.00

0.01

0.02

0.03

0.04

Figure 24. Transient absorption kinetics probed at 480 nm (140 K,butyronitrile) for 89 (solid circles) and 85 (open circles). The two time-scales show the formation (left) and decay (right) of the NDI radical in 89.

The EPR spectrum of 89 (at 11 K) before illumination (Figure 25a) was dominated

by a Mn2II/II species at approximately 2600 G. Also observed was a 16 line Mn2

III/IV signal that was present in approximately 20 %.102 After 50 flashes at 200 K (532 nm), the Mn2

II/II

signal decreased and a new signal assigned to the naphthalene diimide radical appeared (Figure 25b).103 Increasing the number of flashes resulted in a further decrease of the Mn2

II/II signal amplitude, as well as a build up of the NDI radical and of a signal attributed

a

b

c

Mn2II/II

Mn2II/III

NDI•−

a

b

c

Mn2II/II

Mn2II/III

NDI•−

Figure 25. Spectra recorded in dry argon-purged CH3CN at 11.2 K, power 10.03 mW, receiver gain 65, modulation amplitude 10 G. (a) Darkspectrum. (b) After 50 flashes at 200 K. (c) After 250 flashes at 200 K.

102 Such a species could not be observed in the ESI-MS of 89. 103 Brochsztain, S.; Rodrigues, M. A.; Demets, G. J. F.; Politi, M. J. J. Mater. Chem. 2002, 12, 1250-1255.

44

to Mn2II/III (Figure 25c). When the flashed sample was incubated at 200 K in the dark for

one minute, the spectral features from the NDI radical and MnII/III disappeared completely. This was correlated to the reappearance of an EPR spectrum of MnII/II. However, the newly formed spectrum was altered, probably due to structural changes, which complicates quantitative analysis.

These results conclude the discussion about naphthalene diimide as electron

acceptor. We have demonstrated intramolecular charge separation in a novel Mn2II/II-RuII-

NDI triad with a lifetime exceeding 100 ms at 140 K. Further investigations of this fascinating molecule are currently in progress.

45

Bipyridyl-pyridyl Methanes as Tridentate Ligands for Ruthenium(II) (Papers IV,V, and VI) 5 5.1 Enhancement of Luminescence Lifetimes in Bistridentate

Ruthenium(II) Complexes

The excited state lifetimes of ruthenium(II) polypyridyl complexes are critical for applications in various practical devices. Much effort has therefore been devoted to the synthesis of new chromophores with extended lifetimes, especially for the structurally more appealing bisterpyridine type complexes. Most of these approaches are aimed at increasing the energy gap between the 3MLCT state and the upper-lying 3MC state, thereby minimizing their interaction. This has been achieved by modification of the 3MLCT state, either by substitution104,105 or with the use of cyclometalating ligands,106 or by delocalization of the π-acceptor system.107 An alternative approach, based on covalently linked organic chromophores with triplet energy levels close to the 3MLCT state of the metal chromophore, has recently been very successful in achieving a long-lived ruthenium-based luminescent lifetime.108−110 A system of this type was described by

104 (a) Constable, E. C.; Cargill Thompson, A. M. W.; Armaroli, N.; Balzani, V.; Maestri, M. Polyhedron 1992, 11, 2707-2709. (b) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill Thompson, A. M. W. Inorg. Chem. 1995, 34, 2759-2767. 105 (a) Duati, M.; Fanni, S.; Vos, J. G. Inorg. Chem. Commun. 2000, 3, 68-70. (b) Duati, M.; Tasca, S.; Lynch, F. C.; Bohlen, H.; Vos, J. G.; Stagni, S.; Ward, M. D. Inorg. Chem. 2003, 42, 8377-8384. (c) Indelli, M. T.; Bignozzi, C. A.; Scandola, F.; Collin, J.-P. Inorg. Chem. 1998, 37, 6084-6089. 106 (a) Beley, M.; Collin, J.-P.; Louis, R.; Metz, B.; Sauvage, J.-P. J. Am. Chem. Soc. 1991, 113, 8521-8522. (b) Beley, M.; Collin, J.-P.; Sauvage, J.-P. Inorg. Chem. 1993, 32, 4539-4543. (b) Collin, J.-P.; Beley, M.; Sauvage, J.-P.; Barigelletti, F. Inorg. Chim. Acta 1991, 186, 91-93. 107 (a) Fang, Y.-Q.; Taylor, N. J.; Hanan, G. S.; Loiseau, F.; Passalacqua, R.; Campagna, S.; Nierengarten, H.; Van Dorsselaer, A. J. Am. Chem. Soc. 2002, 124, 7912-7913. (b) Hammarström, L.; Barigelletti, F.; Flamigni, L.; Indelli, M. T.; Armaroli, N.; Calogero, G.; Guardigli, M.; Sour, A.; Collin, J.-P.; Sauvage, J.-P. J. Phys. Chem. A 1997, 101, 9061-9069. (c) Hissler, M.; El-ghayoury, A.; Harriman, A.; Ziessel, R. Angew. Chem. Int. Ed. 1998, 37, 1717-1720. 108 Although very interesting, this approach is not relevant for our purpose since the excitation energy is to a large extent removed from the ruthenium chromophore. 109 (a) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2917-2920. (b) Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun. 1999, 735-736. (c) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak, P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11022. (d) Tyson, D. S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J. Phys. Chem. A 2001, 105, 8154-8161. 110 (a) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Chem. Eur. J. 1999, 5, 3366-3381. (b) Passalacqua, R.; Loiseau, F.; Campagna, S.; Fang, Y.-Q.; Hanan, G. S. Angew. Chem. Int. Ed. 2003, 42, 1608-1611.

46

Hanan and co-workers, in which a bisterpyridine type complex with appended anthracenes displayed a lifetime of 1.8 µs.110b

In our approach to enhance the lifetime of the emissive 3MLCT state, we aimed at modifying the coordination geometry around the metal center. A more octahedral geometry than in normal ruthenium(II) bisterpyridyl complexes would ideally lead to an increase in the ligand field strength. The complex would not suffer from energy losses of the 3MLCT state and/or decreased ground state redox potentials, common problems associated with the strategies mentioned above. More than 20 years ago, Meyer and co-workers reported an excited state lifetime of 500 ns in cis-[Ru(bpy)2(py)2]2+, but the monodentate ligands were labile and the complex readily underwent ligand displacement.111 Inspired by these results, we designed tridentate ligands where the pyridyl moieties are linked to the 2,2´-bipyridyl ligands. A potential extension to this methodology would lead to linear D-P-A assemblies with the steric advantage of normal ruthenium(II) bisterpyridyl complexes. 5.1.1 Ligand Design and Synthesis of Ruthenium(II) Complexes

To favor meridional coordination to the ruthenium(II) in the homoleptic complexes, the link between two pyridines was restricted to one carbon atom. A longer link could possibly lead to facial coordination and thereby a mixture of isomers (Figure 26).

NN

N

NN

N

NN

N

NN

N*

*

*

*

NN

N

NN

N

*

*

Figure 26. Meridional (left) and facial (middle and right) coordination.N* is the isolated pyridine ring.

The bipyridyl-pyridyl ligands used in this study are shown in Figure 27.

Compound 90 was synthesized via a Wolff Kischner reduction of ketone 91.112 The corresponding [Ru(ttpy)(L)]2+ and [Ru(L)2]2+ complexes were subsequently prepared from

111 (a) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620-6627. (b) Durham, B.; Walsh, J. L.; Carter, C. L.; Meyer, T. J. Inorg. Chem. 1980, 19, 860-865. 112 Parks, J. E.; Wagner, B. E.; Holm, R. H. J. Organomet. Chem. 1973, 56, 53-66.

47

N

N

N

CH3OH

N

N

N

HOCH3

N

N

N

CH3OCH3

N

N

N

HH

29 30 9031

N

N

N

91

O

N

N

N

Ru

HaHb

N

N

N

HH

N

N

N

Ru

N

N

NHH

N

N

N

Ru

H3CH3CO

N

N

N

OCH3CH3

N

N

N

Ru

N

N

N N

N

N

Ru

N

N

NH3CHO

N

N

N

Ru

N

N

N

9392

95 96 97

94

H3CH3CO O

2+2+

2+

2+

2+ 2+

Figure 27. Structures of ligands and complexes.

Ru(DMSO)4Cl2 or Ru(ttpy)(DMSO)Cl2 (Figure 27).113 The reaction between 30 and Ru(ttpy)(DMSO)Cl2 resulted in a mixture of [Ru(ttpy)(Lx)]2+ (Lx = 30) and [Ru(ttpy) (Ly)]2+ (Ly = 91) that was difficult to separate, and no further effort was made to isolate the former complex.

Due to the C1 symmetry of complexes 95 and 96, the 1H NMR spectra of these complexes consist of 23 coupled spins in an interval of only 2 ppm (All spectra recorded in CD3CN). The complete assignment is possible and was achieved by a combination of COSY and NOESY techniques. This study led to some insight into the general appearance of the spectra of all complexes. In 96, the groups of four protons on the terminal pyridine rings could be determined from a COSY experiment. A subsequent NOESY experiment was performed to determine the connectivity between the different pyridine rings. The NOE between H3 protons on different pyridines (Figure 28), but also the lack of interactions between H3a and H3b established the groups of four protons belonging to pyridines (a) and (c). (An interaction between the –OH proton and H3a/H3b also confirmed this assignment). Furthermore, an NOE was observed between H6d and the −CH3 group on

113 Two donor sets are conceivable in 96, either N6 or N5O, but only the former was observed.

48

N

N

N

RuN

NN

OHH3a

H4aH5a

H6a

H4b

H3b

H3b´

H3cH6c

H3dH3e

H3e´H3f

H6d

NN N

N

N

NOH

CH3

H6d

96

Figure 28. Proton numbering and NOE correlations in 96. The p-tolyl group in the right figure is omitted for clarity.

the orthogonal ligand which enabled us to differentiate between pyridines (d) and (f) (Figure 28).

The four coupled spins from pyridines (c) and (f) in 96 resemble those usually observed for bistridentate ruthenium(II) complexes with the lowest field resonance for H3 followed by H4, H6, and H5. The most dramatic change is for the H6 protons which experience a large upfield shift (~1 ppm) upon coordination due to shielding from the nearby pyridine ring on the other ligand.114 Interestingly, due to the above mentioned interaction between H6d and the –CH3 group, this proton does not experience the large upfield shift observed for the analogous complex 93 (δ = 8.10 for H6d in 96 and δ = 7.63 for H6d in 93), indicating substantial differences in the coordination geometry between the two complexes.

A general trend of the proton resonances of the isolated pyridine (a) was observed in 92, 93, 95, and 96. The H5a proton is always at highest field resonance of all pyridine protons in the complexes, positioned at δ = 6.89-6.96. Proton H3a, usually positioned furthest downfield (of H3-H6), is shifted upfield compared to normal 2,2´:6´,2´´-terpyridines due to the presence of the extra carbon atom between pyridines (a) and (b). In complexes 92 and 93 therefore, H4a is most downfield followed by H3a, H6a, and H5a. In 95 and 96, this effect is counterbalanced by the spatial proximity of the oxygen atom.

In contrast to 95 and 96, the heteroleptic 93 and 97 have CS symmetry and only show eighteen proton resonances. The terminal pyridine rings of the 2,2´:6´,2´´-terpyridine ligand are equivalent by symmetry leading to a more simplified spectrum. 92 has even higher symmetry (C2). Upon coordination, the Ha and Hb protons (Figure 27) that are enantiotopic in the free ligand, become diasterotopic and appear as a pair of doublets. In accordance with the high symmetry of 92, the 1H NMR reveals only 11 resonances. For

114 Constable, E. C.; Cargill Thompson, A. M. W.; Tocher, D. A.; Daniels, M. A. M. New J. Chem. 1992, 16, 855-867.

49

94, the spectrum is complex due to the existence of several isomers as a result of the additional stereogenic centers in the ligands.

The anticipated meridional coordination of two tridentate ligands was confirmed by X-ray analysis of complex 92 (Figure 29). A coordination geometry closer to octahedral than that in [Ru(tpy)2]2+ is apparent, with N-Ru-N angles of 168.3(2)° (N1-N3), 168.4(2)° (N4-N6), and 173.06° (N2-N5) as compared to 158°, 158°, and 178° for [Ru(tpy)2]2+.115 The bond-lengths are as expected for a mixed bipyridyl-pyridyl complex, with the former distances slightly shorter (2.034-2.059 Å) than the latter (2.114-2.109 Å).

C19

C20

C18 C21

C17

C22

N4C24

C23

C16

C15

C25

N3

C14

N5Ru1

C6

C12

C13

N2

C7

C11

C5

C26

N1

C27

C10

C8

C4

C9

C1

C3

C2

N6

C28

C32C29

C31C30

Figure 29. ORTEP view (30 % probability ellipsoids) of 92.

5.1.2 Electrochemical and Photophysical Properties

In Table 4, the results from the photophysical and electrochemical investigation of 92-97 are presented. The RuIII/II redox couple is generally shifted to lower potential than that of the corresponding [Ru(bpy)3]2+ and [Ru(tpy)2]2+, except for complex 97 which contains an electron-withdrawing carbonyl group. The effect on the excited state energy is small, however, as evident from the 77 K emission.

115 Lashgari, K.; Kritikos, M.; Norrestam, R.; Norrby, T. Acta Cryst. 1999, C 55, 64-67.

50

Table 4. Electrochemical and absorption/luminescence data. E½ / V[b] Absorption[c] Emission[d] (298 K) (77 K) ∆Eact

Complex[a] Ru2+/1+ Ru3+/2+ λmax (ε⋅10-4) λmax τ (ns) φ λmax (cm-1) [Ru(bpy)3]2+ −1.74 0.88 451 (1.4) 615 890 0.06[c] 582 3600 [Ru(tpy)2]2+ 12 −1.65[e] 0.89[e] 476 (1.8) — 0.25 — 598 1500

92 −1.67 0.78 477 (0.82) 655 15 6×10-4 609 3480 93 −1.61 0.82 486 (1.5) 655 1.4 2×10-4 637 2560 94 −1.64 0.81 475 (0.82) 627 [f] 2×10-4 627 [f]

95 −1.60 0.81 482 (1.7) 650 0.47 1×10-4 639 1300 96 −1.59 0.79[g] 482 (1.5) 655 0.14 2×10-5 650 660 97 −1.12 0.95 474 (0.87) [f] 50 [f] 696 [f]

a) As PF6− salt. b) CH3CN, N(n-C4H9)4PF6 (0.1 M), 0.1 V/s, vs Fc. c) CH3CN. d) EtOH/MeOH.

e) Calculated from reported values. f) Not determined. g) Anodic peak potential.

The absorption spectra of 94 and 95 are shown in Figure 30.116 The spectra of 93,

95, and 96 in the visible region are dominated by the 1MLCT bands at approximately 480 nm originating from dπ → π* (ttpy) transitions. In 92 and 94, additional features were observed at higher energy, one of which originates from dπ → π* (py) transitions (350 nm).117,118 In 97, the 1MLCT band is much broader with a shoulder at 530 nm reflecting the lower energy of the π* orbital in the carbonyl-substituted bipyridyl-pyridyl ligand (not shown).

Wavelength (nm)300 400 500 600 700

ε (M

-1cm

-1)

0

20x103

40x103

60x103

80x103

Figure 30. Absorption spectra of 94 (dashed) and 95 (solid) in CH3CN.

116 The spectra of the analogous 92 and 93 are similar. 117 The dπ → π* (py) transitions were observed also in 93, 95, and 96, Figure 30. 118 (a) Krause, R. A. Inorg. Chim. Acta 1977, 22, 209-213. (b) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J.; Peedin, J. Inorg. Chem. 1979, 18, 3369-3374.

51

In comparison with the parent [Ru(tpy)2]2+, the luminescence lifetime is improved in complexes 92 and 93. The luminescence lifetime is 15 ns in 92, which is close to two orders of magnitude greater than for [Ru(tpy)2]2+. However, the lifetime is shorter for the methyl-substituted complexes 95 and 96. As shown by a temperature dependence study, the difference in lifetime in 92 and 93 from [Ru(tpy)2]2+ originates from an increased energy separation between the 3MLCT and 3MC states (∆Eact). The increase in ∆Eact is not related to a stabilization of the 3MLCT state, but rather to an increase in the ligand field strength induced by the more octahedral geometry. In contrast to 92 and 93, ∆Eact in 95 and 96 is smaller than that of [Ru(tpy)2]2+. An explanation for this behavior could be a distorted structure due to the interaction between the methyl group and the orthogonal ligand (Figure 28). Geometric distortions, induced by substituents in the 6-position of 2,2´-bipyridyl ligands in ruthenium(II) trisbipyridyl complexes, have earlier been reported to cause a lowering of the 3MC state (small ∆Eact) and a reduction of the emission quantum yield.119 For complexes 94 and 97 ∆Eact was not determined. 5.1.3 Conclusions

We have prepared a series of bistridentate ruthenium(II) complexes in an attempt to improve the photophysical properties compared to [Ru(tpy)2]2+. The crystal structure of the homoleptic complex 92 shows a more octahedral geometry than normal ruthenium(II) bisterpyridine complexes, and the excited state lifetime is close to two orders of magnitude greater than for the parent [Ru(tpy)2]2+.

119 (a) Barigelletti, F.; Juris, A.; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1983, 22, 3335-3339. (b) Fabian, R. H.; Klassen, D. M.; Sonntag, R. W. Inorg. Chem. 1980, 19, 1977-1982. (c) Kelly, J. M.; Long, C.; O´Connell, C. M.; Vos, J. G.; Tinnemans, A. H. A. Inorg. Chem. 1983, 22, 2818-2824.

52

5.2 Linkage Isomerism in a Bistridentate Ruthenium(II) Complex

During our work with the bipyridyl-pyridyl ligands, we became interested in the

coordination mode of ligand 29. The presence of four possible coordinating atoms, the three pyridine nitrogens and the tertiary alcohol, leaves an opportunity for linkage isomerism. We reasoned that an appropriate stimulus on the RuN6 complex 96, either chemical, electrochemical, or photochemical, could potentially induce a bond rotation and subsequent oxygen coordination to form the RuN5O complex (Figure 31). Such systems, which can undergo reversible structural changes upon an external stimulus, have been of considerable interest because of their implications in switching and memory devices.120

N

N

Ru

N

N

NN

N

N

Ru

N

N

NH3CHO

96

O

2+ 2+

H3CN

H

Figure 31. Possible linkage isomerism in 96.

Acid induced linkage isomerization in an alizarin-bisbipyridyl ruthenium(II) complex (99) was observed by Lever and co-workers (Figure 32).121 The process is reversible upon addition of base, but only after extensive heating. Recently, ruthenium(II) polypyridyl dimethylsulfoxide complexes have been of particular interest.122 Keene and co-workers reported photo-triggered Ru(S)2 → Ru(O)2 linkage isomerization in [Ru(bpy)2(DMSO)2]2+ which reverts slowly back to the starting state in the dark.122e An electrochemically triggered linkage isomerization was reported by Llobet and co-workers, who studied ruthenium(II) complexes containing the dinucleating ligand bis(2-pyridyl)pyrazole (100) (Figure 32).122d These processes can both be rationalized by a

120 See for example: (a) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P. Acc. Chem. Res. 1999, 32, 846-853, and references cited therein. (b) Sano, M.; Taube, H. Inorg. Chem. 1994, 33, 705-709. 121 (a) DelMedico, A.; Auburn, P. R.; Dodsworth, E. S.; Lever, A. B. P.; Pietro, W. J. Inorg. Chem. 1994, 33, 1583-1584. (b) DelMedico, A.; Fielder, S. S.; Lever, A. B. P.; Pietro, W. J. Inorg. Chem. 1995, 34, 1507-1513. (c) DelMedico, A.; Pietro, W. J.; Lever, A. B. P. Inorg. Chim. Acta 1998, 281, 126-133. 122 (a) Rack, J. J.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2001, 123, 2432-2433. (b) Rack, J. J.; Mockus, N. V. Inorg. Chem. 2003, 42, 5792-5794. (c) Rack, J. J.; Rachford, A. A.; Shelker, A. M. Inorg. Chem. 2003, 42, 7357-7359. (d) Sens, C.; Rodríguez, M.; Romero, I.; Llobet, A.; Parella, T.; Sullivan, B. P.; Benet-Buchholz, J. Inorg. Chem. 2003, 42, 2040-2048. (e) Smith, M. K.; Gibson, J. A.; Young, C. G.; Broomhead, J. A.; Junk, P. C.; Keene, F. R. Eur. J. Inorg. Chem. 2000, 1365-1370.

53

NNCl

ClRu

N

N

OS

OS

H

RuII-S

RuIII-S RuIII-O

RuII-O

-e− +e−

(unstable)

(unstable)

O

OO

O Ru(bpy)2

O

OOH

O

Ru(bpy)2

H+OH−, ∆

99

100

Figure 32. Acid induced (left) and electrochemically induced (right) linkage isomerization in ruthenium(II) polypyridyl complexes.

change in the metal oxidation state. During irradiation, the excited ruthenium, formally RuIII(bpy•−), will preferentially bind to hard donor atoms (e.g. oxygen in the dimethyl sulfoxide ligand). Therefore, the ambidentate ligand switches its mode of coordination during irradiation, but reverts back in the dark since the RuII ion prefers the softer sulfur atom. 5.2.1 Electrochemical Behavior

The redox properties of 96 were studied by cyclic voltammetry (Figure 33). Following oxidation of the all-pyridine coordinated RuII (Ep,a = 0.79 V), the corresponding cathodic wave was not observed. Instead, a cathodic peak appeared at Ep,c = 0.32 V (∆Ep = 470 mV). This is indicative of a linkage isomerization where a change in coordination

E / V vs Fc+/0-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0

10 µΑ

Figure 33. CV of 96, 1 mM, CH3CN, N(n-C4H9)4PF6, 0.1 V/s.

54

occurs in the RuIII state to give an oxygen coordinated species, 98-H+, stabilized by the hard oxygen donor. Subsequent scans over the same potential range were identical, showing that the complex changes back to all-pyridine coordination after reduction to RuII (Scheme 18).

N

N

RuIII

N

N

N

N

N

N

RuII

N

N

NH3CHO

OH3C

2+

3+

-e-

N

N

N

RuIII

N

N

NH3CHO

3+

NH

+e-N

N

RuII

N

N

NOH3C

2+

NH

(unstable)

(unstable) 98-H+

Scheme 18. Electrochemically induced linkage isomerization in 96.

At high scan rates (ν ≥ 50 V/s), the cathodic wave of 96 started to appear

suggesting that the re-reduction can compete with the isomerization (not shown). This peak became more pronounced as the scan rate increased, and at ν > 1500 V/s the cathodic wave of 98-H+ had almost completely disappeared, indicating that the isomerization process occurs on the millisecond timescale. 5.2.2 Influence of Base

Following the standard procedure in preparing ruthenium(II) bisterpyridyl complexes, the ambidentate ligand 29 preferentially binds via the pyridine nitrogen to form a six-membered chelate. Treating 96 with tert-butoxide in acetonitrile resulted in a drastic change in the color of the solution, suggesting a change in coordination from the N6 to the N5O donor set. The spectral changes were monitored by UV-Vis absorption, and showed isosbestic points at 350, 425, and 500 nm (Figure 34). The red-shift of the 1MLCT

55

band, from 492 nm to 533 nm (ε = 12300 M-1cm-1),123 is expected due to a stabilization of the RuIII(ttpy−) and RuIII(bpy−) excited states by the oxygen donor in 98.124 Even though ruthenium(II) usually has a preference for π-acidic ligands, isomerization occurs to form the five-membered chelate. Most importantly, the process is fully reversible and the starting spectrum was recovered instantaneously upon addition of acid (TsOH).

Wavelength (nm)300 400 500 600 700

Abs

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Figure 34. Absorption changes of an acetonitrile solution containing 96(2.5×10-5 M) and tBuOK (2 equivalents) measured after 0, 10, 20, 35, 55, 80 min, and complex 98.

Complex 98 was readily isolated after treatment of a concentrated solution of 96 in

DMF with tert-butoxide and subsequent precipitation by the addition of diethylether. As discussed above, the protons of the free ligand undergo substantial changes in the 1H NMR spectrum upon formation of a ruthenium(II) polypyridyl complex. The H6a proton in the non-coordinating pyridine appears at 8.40 ppm (∆δ = 1.0). The effect on H3a is even larger (∆δ = −1.3), presumably due to a shielding effect from the other 2,2´:6´,2´´-terpyridyl ligand (Figure 35).125

123 The extinction coefficient for 98 was estimated from the value determined for 96. 124 (a) Dovletoglou, A.; Adeyemi, S. A.; Meyer, T. J. Inorg. Chem. 1996, 35, 4120-4127. (b) Norrby, T.; Börje, A.; Åkermark, B.; Hammarström, L.; Alsins, J.; Lashgari, K.; Norrestam, R.; Mårtensson, J.; Stenhagen, G. Inorg. Chem. 1997, 36, 5850-5858. (c) Holligan, B. M.; Jeffery, J. C.; Norgett, M. K.; Schatz, E.; Ward, M. D. J. Chem. Soc. Dalton Trans. 1992, 3345-3351. 125 The conformation of 98 in Figure 35 was supported by MMFFs calculations performed by Norrby, P.-O. using MacroModel (www.schroedinger.com).

56

N

NRuN

NN

N

H6a

H5aH4a

O

98

1+

H3a

E / V vs Fc+/0-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

10 µΑ

a

b

98 96 H3a 6.95 8.27 H4a 7.10 7.68 H5a 6.95 6.86 H6a 8.40 7.40

Figure 35. Proton numbering in 98 and 1H NMR shifts for the isolated pyridine in 98 and 96 (CD3CN).

In the cyclic voltammogram of 98, the RuIII/II redox couple was observed at –0.10

V (Figure 36). In contrast to 96, the process is reversible with ∆Ep = 73 mV. In comparison to 96 that has the N6 donor set, the RuIII/II redox couple in 98 has decreased by 0.89 V due to the σ-donating alkoxide ligand of the N5O donor set. The lower potential for the base-generated N5O species 98 as compared to the electrogenerated N5O isomer 98-H+ is expected due to pyridine protonation.

Figure 36. CV of 98 (a), ~0.8 mM, and 96 (b), 1 mM, CH3CN, N(n-C4H9)4PF6, 0.1 V/s.

5.2.3 Conclusions

Linkage isomerism in a bistridentate ruthenium(II) complex containing a bipyridyl-pyridyl type ligand has been described. Upon oxidation, the N6 donor set in 96 rearranges to a N5O donor set that better stabilizes the RuIII state. After reduction, the complex

57

changes back to all-pyridine coordination. A similar rearrangement occurs after treatment with base to give 98. The process is completely reversible and 96 is fully recovered after addition of acid. Since the change in donor set is accompanied by a color change from red to purple, the system acts as a chromophoric molecular switch.

58

Concluding Remarks

6

Solar energy conversion continues to attract considerable attention in the search for alternative fuels. In this context, we prepared several donor-acceptor assemblies based on ruthenium(II) polypyridyl complexes as photosensitizers linked to various acceptors and/or donors. In many of the complexes studied, efficient intramolecular electron transfer occurred after light excitation of the ruthenium(II) unit. In particular, a long-lived charge-separated state was obtained in a trinuclear manganese(II,II)-ruthenium(II)-naphthalene diimide triad, which constitutes an important step toward a functional photosynthetic model.

A new approach towards bistridentate ruthenium(II) polypyridyl complexes was taken in attempt to improve the photophysical properties of the parent [Ru(tpy)2]2+. The effect of inserting a carbon atom between two pyridine rings of the 2,2´:6´,2´´-terpyridine ligand was investigated. Two complexes showed prolonged excited state lifetimes which were attributed to an increase in the ligand field strength due to more octahedral geometry than in [Ru(tpy)2]2+. In one of these complexes, an interesting reversible linkage isomerization occurred that was induced by either an electrochemical or chemical stimulus. Single molecules that can interconvert between two stable states upon some external stimulus are of increasing interest due to their potential use as switching devices.

The quest for photo-induced charge separation in supramolecular systems

continues to occupy chemists in the field of artificial photosynthesis. In the near future, more attention will probably be focused on various catalysts for the water-splitting reactions. The recent crystal structure of PS II will reveal valuable insights into the mechanism of water oxidation, and it will give inspiration in the design of synthetic analogous. For the reduction of protons into hydrogen, sulfur-containing dinuclear iron complexes are particularly interesting, as they closely resemble the hydrogenase enzymes that regulate the production and consumption of hydrogen in microorganisms.

The energy needed for both oxidizing and reducing water might be too high for a single chromophore. Therefore, it may well be necessary to divide the process into two half-reactions. The challenge for an organic chemist of such an approach is of course great, but all our efforts toward a less polluted society are worthwhile doing.

59

Acknowledgements First of all I would like to thank my supervisors Licheng Sun and Björn Åkermark

for accepting me as a graduate student and for always having time. I would also like to thank past and present members of the Consortium for

Artificial Photosynthesis, especially Leif Hammarström and Stenbjörn Styring for your enthusiasm and knowledge in the fields of artificial and natural photosynthesis. Magnus Borgström, Reiner Lomoth, Malin Abrahamsson, Nizamuddin Shaikh, and Helena Berglund-Baudin for excellent cooperations.

All other people that have been involved in the projects described in this thesis, in

particular Jonas Bergquist and Mikael Kritikos.

All members of the BÅ group. Sascha Ott, Magnus Anderlund, Henriette Wolpher, and Yunhua Xu for fruitful cooperations.

Björn Åkermark, Sascha Ott, Hans Adolfsson, Licheng Sun, Leif Hammarström,

and Stenbjörn Styring for valuable comments regarding this thesis. All people at the Department of Organic Chemistry. Hilda Rietz´, L Namowitzkys, Knut and Alice Wallenberg, and C.F. Liljevalch

foundations are greatly acknowledged for financial support. Friends outside the university. My family for love and support.

60