Photochemistry and Photophysics of Coordination Compounds

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162 S. Campagna et al. (for similar systems, see Sect. 4.4) [194]. A recently investigated, closely related tetranuclear dendritic-shaped compound, where the bridging ligand is the asymmetric PHEHAT ligand (PHEHAT = 1,10-phenanthrolino[5,6b]-1,4,5,8,9,12-hexaazatriphenylene), displays similar photophysical properties [274]. Mixed-metal Os(II)–Ru(II) dendrimers whose bridging ligands contain ether linkages have also been reported: compounds 38 and 39 are two examples [275, 276]. The tetranuclear species in 38 exhibits quantitative energy transfer from the Ru(II) chromophores to the Os(II) core [275], whereas for 39 the efficiency of the energy transfer process is highly temperature dependent; in fact the Ru-to-Os energy transfer is highly efficient at low temperature, whereas at room temperature it does not compete with the intrinsic decay of the Ru(II)-based subunits [276]. In most of the mixed-metal dendrimers featuring energy transfer and containing Ru(II) subunits, the Ru(II) centers play the role of the energy donor components (and usually Os(II) centers are the acceptors). Examples of photoactive dendrimers in which Ru(II) subunits behave as acceptor components are the tetranuclear compound 40 [277] and its higher-generation, Y-shaped octanuclear species, in which four additional Ir(III) chromophores have been connected at the two peripheral Ir(III) subunits of the compound 40 (JAG Williams, personal communication). Here, efficient energy transfer takes place for the Ir(III) cyclometallated subunits toward the single Ru(II) polypyridine chromophore, which acts as the energy trap of the assemblies. The Ru(II) subunit then emits with relatively high quantum yield (0.12 in degassed acetonitrile at room temperature) and long lifetime (1.6 µs) [277].
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 163 The fluoro substituents which are present on the peripheral Ir chromophores have the role of increasing the excited-state energy levels of the outer Ir chromophores with regard to the inner ones, so generating the correct energy gradient. Multi-ruthenium dendrimers of the 2,3-dpp family discussed above have also been functionalized with organic electron donor subunits, to yield integrated light-harvesting antennae/electron donor systems for charge separation. In particular, a triruthenium dendron has been coupled with a tetrathiafulvalene (TTF) derivative [278] and the tetranuclear OsRu3 has been functionalized at the peripheral bpy ligands with up to six phenothiazine subunits [279]. In both cases, the light-harvesting antenna emission was totally quenched by reductive electron transfer from the electron donor subunit. Interestingly, the electron donor quenchers were not directly linked to the energy trap of the antenna; however, the electron transfer process was fast and
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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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