Photochemistry and Photophysics of Coordination Compounds

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158 S. Campagna et al. and MV 2+ indicated that the first-generation dendritic structure of this class of dendrimers is unable to shield the Ru-based core from bimolecular energy and electron transfer reactions. On the contrary, quenching by dioxygen was attenuated in going from [Ru(dmb)3] 2+ to larger dendrimers, suggesting an effect of the dendritic structure. As this effect is not observed with MA, PTZ, and MV 2+ , which are certainly much bulkier species, the shielding effect observedinthecaseofdioxygenwasattributedtolowerO2 solubility within the dendritic structure [250]. 5.2.2.2 Multimetallic Dendrimers For the class of dendrimers where metal complexes are the branching centers, a key role is played by polytopic chelating ligands (bridging ligands), which can control the shape of the polynuclear array and the electronic interaction between metal chromophores. The largest family of these dendrimers is based on the 2,3-bis(2 ′ - pyridyl)pyrazine (dpp) bridging ligand. Within such a series, the largest species contain 22 metal centers [251–253]. The decanuclear compound 35 is
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 159 a second-generation dendrimer of this family (in the sketch of the compound, the peripheral ligands, schematized as NˆN, stand for 2,2 ′ -bipyridine) [254]. For the dendrimers of this series, the modular synthetic strategy [252] allows a high degree of synthetic control in terms of the nature and position of metal centers, bridging ligands, and terminal ligands. Since the excited-state level of each metal center in the dendrimer depends on the nature of the metal, of its coordination sphere (which in its turn depends on the metal position, inner or outer, within the dendritic array) and on the ligands, each metal-based subunit is characterized by specific excited-state properties, which because of the symmetry of the dendritic structure are usually identical for each metalbased subunit belonging to the same dendritic layer. Therefore, the synthetic control translates into control of specific properties, such as the direction of electronic energy flow within the dendritic array (antenna effect) [245]. For example, in 35 the lowest excited-state level involves the peripheral subunit(s), and the emission of the species (acetonitrile, room temperature: λmax = 780 nm; τ = 60 ns; Φ = 3 × 10 –3 ) is assigned to a (bpy)2Ru→ µ-dpp MLCT triplet state [254]. Excitation spectroscopy indicates that quantitative energy transfer takes place from inner subunits to the peripheral ones [254]. Because of the energy gradient between the dendritic layers, the energy transfer is ultrafast (see later), occurring in the femtosecond timescale. On the basis of the above discussion, it is not surprising that all the homometallic dendrimers of the same family, independent of the number of Ru subunits (i.e, tetranuclear [255, 256], decanuclear [253, 254], and docosanuclear [251– 253], as well as hexanuclear [257, 258], heptanuclear [259], and tridecanuclear species [260], which have particular connections/geometries), exhibit practically identical photophysical properties, since the lowest-energy subunit(s) is in all cases the identical peripheral (bpy)2Ru(µ-dpp) MLCT state(s). A nonanuclear species has also been prepared [261], but its photophysical properties have not yet been reported. On increasing nuclearity, a unidirectional gradient (center-to-periphery or vice versa) for energy transfer is hardly obtained with only two types of metals (commonly, Ru(II) and Os(II)) and ligands (bpy and 2,3-dpp). In fact, by using a divergent synthetic approach starting from a metal-based core it becomes unavoidable that metal-based building blocks with high-energy excited states (high-energy subunits) are interposed between donor and acceptor subunits of the energy transfer processes [245]. For example, while in the tetranuclear [Os{(µ-dpp)Ru(bpy)2}3] 8+ (OsRu3) species, in which a central {Os(µ-dpp)3} 2+ subunit is surrounded by three {Ru(bpy)2} 2+ subunits, only the osmium-based core emission is obtained (acetonitrile, room temperature: λmax = 860 nm; τ = 18 ns; Φ = 1 × 10 –3 ) [262], indicating quantitative energy transfer from the peripheral Ru-based chromophore to the central Os-based site; for the larger systems the peripheral Ru-based emission is not quenched [245, 253]. This result highlights that although downhill or even isoergonic energy transfer between nearby building blocks in the dendrimers
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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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