Photochemistry and Photophysics of Coordination Compounds

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246 M.T. Indelli et al. spectively. A clear advantage of the tethered Ru/DNA/Rh system is that both the donor and acceptor are covalently held in a well-defined fixed distance range. On the other hand, the report of Murphy et al. was limited by the exclusive use of steady-state emission spectroscopy. A lower limit for the photoinduced electron transfer rate (> 3 × 10 9 s –1 ) has been obtained measuring the quenching of the Ru(II) metal-to-ligand charge transfer (MLCT) emission by the tethered Rh(III) acceptor. In a subsequent investigation, untethered Ru/DNA/Rh and related systems were studied by Barton et al. [160] using ultrafast laser spectroscopy. The study was focused mainly on the system shown in Fig. 14 constituted by ∆- Ru(phen)2dppz 2+ as excited donor, ∆-Rh(phi)2bpy 3+ as acceptor intercalated in the calf thymus DNA with the aim to determine the rate of excited-state electron transfer (ket) that occurs from the lowest-lying MLCT state of the Ru donor, and the recombination electron transfer reaction (krec). Fig. 14 Photoinduced electron transfer processes taking place between Ru(phen)2dppz 2+ and Rh(phi)2bpy 3+ DNA intercalators Efficient and rapid quenching of luminescence of the Ru complex in the presence of Rh complex, even at surprisingly low acceptor loading on the DNA duplex was observed. All the experimental observations were consistent with complete intercalation of the donor and acceptor in DNA. A comparative experiment employing Ru(NH3) 3+ 6 complex as electron acceptor, clearly indicates that much less efficient quenching is observed when the quencher is groove bound rather intercalated. To deepen the understanding of the mechanism of the electron transfer processes, the authors examined the photoinduced charge separation (ket) and recombination electron transfer (krec) reactions on the picosecond time scale by monitoring both the kinetics of the emission decay and the kinetics of the recovery of ground state absorption of Ru(II) donor (Fig. 14). Time-correlated single photon counting failed to detect the lifetime of the excited state, clearly indicating that luminescent quenching by electron transfer proceeds faster (ket > 3 × 10 10 s –1 ) than the time resolution of the instrument (ca. 50 ps). Ultrafast transient absorption measurements, on the other hand, revealed bleaching of the MLCT band of the Ru(II) complex in a picosecond time scale, assigned by the authors to the
Photochemistry and Photophysics of Coordination Compounds: Rhodium 247 Table 1 Rate constants for charge recombination following electron transfer from DNAbound, photoexcited donors to ∆-Rh(phi)2(bpy) 3+ a Donor DNA krec [10 9 s –1 ] –∆G ◦ [V] ∆-Ru(phen)2(dppz) 2+ Calf thymus 9.2 1.66 rac-Ru(bpy)2(dppz) 2+ Calf thymus 7.1 1.69 ∆-Ru(dmp)2(dppz) 2+ Calf thymus 11 1.59 ∆-Ru(phen)2(F2dppz) 2+ Calf thymus 7.7 1.68 rac-Ru(phen)2(Me2dppz) 2+ Calf thymus 9.2 1.67 ∆-Os(phen)2(dppz) 2+ Calf thymus 11 1.21 Λ-Ru(phen)2(dppz) 2+ Calf thymus 4.5 ∆-Ru(phen)2(dppz) 2+ Poly-d(AT) 7.4 ∆-Ru(phen)2(dppz) 2+ Poly-d(GC) 0.21 a Based on [144] presence of Ru(III) oxidized donor. The rate constants for charge recombination process (krec) were obtained from the decay of this signal. The data for seven donor–acceptor pairs are given in Table 1. Within this series, the driving force (∆G ◦ ) is comparable but the donors vary with respect to intercalating ligand, ancillary ligands, chirality, and metal center. Despite such a range of chemical properties, the rate observed is centered around 10 10 s –1 . A significant difference in rate is observed, however, when the absolute configuration of the donor is varied. For the right-handed ∆-Ru(phen)2dppz 2+ the value is 2.5 times higher with respect the left-handed enantiomer indicating a deeper stacking of this complex into the double helix. This result, according to the authors, clearly suggests that the electron transfer process required the intervening aromatic base pairs. The notion of highly efficient ET through the stack of DNA base is also strongly supported by the finding that the largest change in electron transfer rate is observed when the sequence of the DNA bridge is changed: for the same donor and acceptor reactants the rate with poly d(AT) is 30 times higher than with poly d(GC). This is an importantresultthatindicatesthattheπ-stacked bases of the DNA provide an effective pathway for electron transfer reactions. However, the crucial point of this study that involves an untethered Ru/DNA/Rh system concerns the distances over which fast ET occurs. The question is: do the donor and acceptor complexes contact each other, or does electron transfer occur at long range? Two models were considered by the authors to interpret the experimental results: i) a cooperative binding model with ET over short D–A distance and ii) a random binding model with ET over long distances. On the basis of DNA photocleavage experiments, the first hypothesis was reject in favor of a rapid long range ET with a shallow distance dependence [160]. On the other hand, soon thereafter, Barbara [161] reinterpreted the experimental results on a quantitative basis using computational simulation procedures and demon-
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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X Preface nation Compounds. The ven
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