Photochemistry and Photophysics of Coordination Compounds

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150 S. Campagna et al. Another example showing an “active” role of the bridge in mediating intercomponent transfer processes involving Ru(II) species is evidenced by 23 [233]. In this species, there are two close-lying MLCT states per metal center involving the bridging ligand (leaving aside the MLCT state involving the peripheral ligands), because of the particular nature of the bridge (see Sect. 5.9). The higher energy of such MLCT states (MLCT1) involves a bridging ligand orbital mainly centered in the bpy-like coordinating site (LUMO+1), and the lower energy one (MLCT0) islocalizedonthecentral phenazine-like site (LUMO). Light excitation of the Ru-based chromophore populates the singlet MLCT1 state, which rapidly decays to its triplet counterpart. Direct light excitation into the singlet MLCT0 level (and successive population of its triplet) is inefficient because of the negligible oscillator strength of the transition. For Ru-to-Os energy transfer, two possible pathways are possible: (1) Ru-to-Os energy transfer at the 3 MLCT1 level (EnT), followed by 3 MLCT1-to- 3 MLCT0 relaxation within the Os(II) chromophore (a sort of intraligand electron transfer, ILET, within the Os(II) subunit); and (2) 3 MLCT1-to- 3 MLCT0 relaxation within the Ru(II) chromophore (ILET in Ru(II) subunit), followed by Ru-to-Os energy transfer at the 3 MLCT0 level. The situation is schematized in Fig. 13 [210, 233]. Interestingly, ultrafast spectroscopy shows that pathway 1 is followed in dichloromethane and pathway 2 prevails in the more polar acetonitrile solvent. Oligophenyl bridges are reported to play “active” roles in the dinuclear Ir(III)–Ru(II) species 24–27 [234]. In this series of complexes, the Ru-based component is the energy transfer acceptor subunit. Indeed, Ru-based emission takes place in all the species at about 625 nm (lifetime about 200 ns) in aerated acetonitrile at room temperature and at about 590 nm (lifetime about 6 µs) in butyronitrile at 77 K, whereas the high-energy Ir-based chromophore has a very short excited-state lifetime, determined by time-resolved emission and subpicosecond transient absorption spectroscopy, slightly dependent on the bridge. The energy transfer rate constant is very weakly slowed down by increasing the bridge length, passing from 8.3 × 10 11 s –1 for the species with two phenyls as spacer to 3.3 × 10 11 s –1 for the species with five interposed phenyls. The apparent attenuation parameter β for energy transfer rate con-
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 151 Fig. 13 Energy transfer pathways in a dinuclear Ru(II)–Os(II) species containing a πextended bridge stant would be 0.07 ˚A –1 . However, increasing the bridge length changes the excited-state energy level of the bridge itself, and it is proposed that the MLCT excited state of the Ir center assumes an increasing LC character involving the bridge orbitals as the number of phenyls increases. As a consequence, metal–metal separation does not reflect the effective donor–acceptor separation for the energy transfer process; with the donor excited state largely involving the oligophenyl spacer, the energy transfer takes place by an in-
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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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