Photochemistry and Photophysics of Coordination Compounds

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128 S. Campagna et al. of stabilization of the cationic [Ru(bpy)3] 2+ species, whereas formation of neutral [Ru(bpy)2X2] complexes is favored in low-polarity solvents. Photoracemization of [Ru(bpy)3] 2+ [86] also occurs with low quantum yield (2.9 × 10 –4 in water at 25 ◦ C). This process can be accounted for by a rearrangement of the square pyramidal primary photoproduct into a trigonal bipyramidal intermediate which can lead back to either the ∆ or the Λ isomer [64]. Ligand photodissociation is, of course, a drawback for the use of [Ru (bpy)3] 2+ in practical applications. To avoid ligand photodissociation one should prevent population of 3 MC and/or ligand dissociation from 3 MC. Population of 3 MC can be prevented or at least reduced by: (a) addition of sufficient quencher to capture 3 MLCT before surface crossing to 3 MC can occur; (b) working at low temperature; (c) increasing the energy gap between 3 MLCT and 3 MC; and (d) increasing pressure [87, 88]. Ligand dissociation from 3 MC can also be reduced by (e) avoiding coordinating anions in solvent of low dielectric constant and (f) linking together the three bpy ligands so as to form a single caging ligand which encapsulates the metal ion. Point (a) is experimentally difficult, since thermal equilibration is quite a fast process. Points (c) and (f) are particularly interesting and much effort has been made along such directions [1, 89, 90]. It should be considered that in most of the [Ru(bpy)3] 2+ derivatives, the 3 MLCT state is shifted to lower energies [1], whereas the energy of the 3 MC state usually does not change. This leads to an increased energy gap between MLCT and MC states and decreased photolability. As a consequence, photosubstitution is a minor problem in most ruthenium polypyridine complexes. It should be considered, however, that decreasing the energy of the 3 MLCT level increases the Franck–Condon factors for radiationless decay to the ground state, leading to decreased luminescence lifetimes and quantum yields. The rate of radiationless decay can be decreased by extending the delocalization of the promoted electron on suitable aromatic ligands [78, 91, 92]. Finally, it should also be noted that the photolabilization of ligands can be a profitable photochemical process: for example, a synthetic route to trisheteroleptic Ru complexes involves photosubstitution of ligands [93] and photochemical, reversible ligand exchange has been proposed to be used to photoswitch the complexation activity in a ruthenium complex containing a scorpionate terpyridine ligand [94]. 3.5 Quenching of the 3 MLCT Excited State: Energy and Electron Transfer Processes The lowest 3 MLCT excited state of [Ru(bpy)3] 2+ lives long enough to encounter other solute molecules (even when these are present at relatively low concentration) and possesses suitable properties to play the role of energy
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 129 Fig. 10 Molecular quantities of [Ru(bpy)3] 2+ relevant for energy and electron transfer processes. ∗∗ [Ru(bpy)3] 2+ indicates higher-energy spin-allowed excited states and ∗ [Ru(bpy)3] 2+ indicates the lowest spin-forbidden excited state ( 3 MLCT). Reported potentials are in aqueous solution vs SCE donor, electron donor, or electron acceptor. As is also shown in Fig. 10, the energy available to ∗ [Ru(bpy)3] 2+ for energy transfer processes is 2.12 eV and its reduction and oxidation potentials are + 0.84 and – 0.86 V(aqueoussolution, vs SCE). It follows that ∗ [Ru(bpy)3] 2+ is at the same time a good energy donor (Eq. 2), a good electron donor (Eq. 3), and a good electron acceptor (Eq. 4): ∗ [Ru(bpy)3] 2+ +Q→ [Ru(bpy)3] 2+ + ∗ Q energy transfer (2) ∗ [Ru(bpy)3] 2+ +Q→ [Ru(bpy)3] 3+ +Q – ∗ [Ru(bpy)3] 2+ +Q→ [Ru(bpy)3] + +Q + oxidative quenching (3) reductive quenching . (4) The direct observation of redox products represents the strongest evidence to support the occurrence of oxidative and reductive quenching mechanisms. These observations can be performed in a few cases with continuous irradiation [95, 96] and more often in flash photolysis experiments, because usually the redox products rapidly decay either by back electron transfer reactions to re-form the starting materials or by secondary reactions to form other products. In practice, the possibility to observe transient absorptions is related to the changes in the optical density of the solution caused by the photoreaction. Bleaching and recovering of the [Ru(bpy)3] 2+ spectrum can be used for kinetic measurements. The absorption band at 680 nm typical of [Ru(bpy)3] 3+ is too weak to detect small [Ru(bpy)3] 3+ concentrations, so that in oxidative quenching processes one is forced to use the absorption spec-
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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