Photochemistry and Photophysics of Coordination Compounds

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118 S. Campagna et al. 5.8.1 PhotogenerationofHydrogen......................... 180 5.8.2 OtherPhotocatalyticSystems ......................... 182 5.9 Photoactive Molecular Machines Able to Perform Nuclear Motions . . . . 183 6 Ruthenium Complexes and Biological Systems ............... 185 7 Dye-Sensitized Photoelectrochemical Solar Cells .............. 188 7.1 GeneralConcepts................................ 188 7.2 Ruthenium-SensitizedPhotoelectrochemicalSolarCells .......... 191 7.3 SupramolecularSensitizers .......................... 193 8 Miscellanea ................................... 196 References ....................................... 200 Abstract Ruthenium compounds, particularly Ru(II) polypyridine complexes, are the class of transition metal complexes which has been most deeply investigated from a photochemical viewpoint. The reason for such great interest stems from a unique combination of chemical stability, redox properties, excited-state reactivity, luminescence emission, and excited-state lifetime. Ruthenium polypyridine complexes are indeed good visible light absorbers, feature relatively intense and long-lived luminescence, and can undergo reversible redox processes in both the ground and excited states. This chapter presents some general concepts on the photochemical properties of Ru(II) polypyridine complexes and gives an overview of various research topics involving ruthenium photochemistry which have emerged in the last 15 years. In particular, aspects connected to supramolecular photochemistry and photophysics are discussed, such as multicomponent systems for light harvesting and photoinduced charge separation, systems for photoinduced multielectron/hole storage, and photocatalytic processes based on supramolecular Ru(II) polypyridine species. Interaction with biological systems and dye-sensitized photoelectrochemical cells are also briefly discussed. Keywords Ruthenium · Luminescence · Electron transfer · Energy transfer · Solar energy conversion · Light-powered molecular machines · Dye-sensitized solar cells 1 Introduction The photochemistry of ruthenium complexes has undergone an impressive growth in the last few decades. The prototype compound [Ru(bpy)3] 2+ (bpy =2,2 ′ -bipyridine) has certainly been one of the molecules most extensively studied and widely used in research laboratories during the last 30 years. A unique combination of chemical stability, redox properties, excited-state reactivity, luminescence emission, and excited-state lifetime has attracted the attention of many researchers, first on this molecule and then on some hundreds of its derivatives. The study of this class of complexes has stimulated the growth of several branches of chemistry. In particular, Ru(II) polypyridine complexes have played and are still playing a key role in the development of
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 119 photochemistry, photophysics, photocatalysis, electrochemistry, photoelectrochemistry, chemi- and electrochemiluminescence, and electron and energy transfer. Mostly in the last 15 years, Ru(II) polypyridine complexes have also contributed highly to the development of supramolecular photochemistry, and in particular to its aspects related to photoinduced electron and energy transfer processes within multicomponent (supramolecular) assemblies, including luminescent polynuclear metal complexes, light-active dendrimers, artificial light-harvesting antennae, photoinduced charge-separation devices, luminescent sensors, and light-powered molecular machines. Because of the enormous number of Ru(II) complexes investigated from a photochemical viewpoint and the variety of multicomponent structures prepared and light-based functions explored, it is impossible to make an exhaustive review. In this chapter, we recall some basic concepts on ruthenium photochemistry and discuss in some detail a few selected topics, particularly those that have developed or emerged during the last 15 years. In this way we also hope to give an overview of some research directions which ruthenium photochemistry allows to be explored. An exhaustive review [1] published about 20 years ago collects photochemical, photophysical, and redox data of several hundreds of Ru(II) polypyridine complexes. Another extensive review was published about 10 years ago [2], dealing with the luminescence properties of polynuclear transition metal complexes, most of them containing Ru(II) polypyridine subunits (interestingly, in the former review [1] less than ten polynuclear Ru complexes were reported). A review focused on the photophysical properties of Ru(II) complexes with tridentate polypyridine ligands [3] has also been published. All these review articles contain more or less comprehensive tables of data. Enlightening articles on some basic properties of Ru(II) polypyridine complexes are also available [4–8]. The very large majority of photochemical investigations on ruthenium complexes deal with Ru(II) polypyridine species. For such a reason, as also implicitly suggested above, we will limit our discussion to these species. Other photoactive compounds containing ruthenium metals, including ruthenium porphyrins, are not included in this article. 2 Structure, Bonding, and Excited States of Ru(II) Polypyridine Complexes Ru2+ is a d6 system and the polypyridine ligands are usually colorless molecules possessing σ donor orbitals localized on the nitrogen atoms and π donor and π∗ acceptor orbitals more or less delocalized on aromatic rings. Following a single-configuration one-electron description of the excited state in octahedral symmetry (Fig. 1a), promotion of an electron from a πM metal ligand orbitals gives rise to metal-to-ligand charge transfer orbital to the π∗ L (MLCT) excited states, whereas promotion of an electron from πM to σ∗ M or-
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X Preface nation Compounds. The ven
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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