Photochemistry and Photophysics of Coordination Compounds

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170 S. Campagna et al. parent first-order rate constant of 6.6 × 10 3 s –1 . This species is produced with alowquantumyield(0.5%), stores about 1.3 eV, and recombines to the ground state with a lifetime exceeding 2 ms. In spite of the low efficiency of the charge-separation process, there are several interesting points: (1) in the absence of myoglobin, charge separation is not obtained at all; (2) the complete process is pH dependent; (3) the final charge-separated state lifetime is comparable with that of natural photosynthetic reaction centers; and (4) back electron transfer is regulated by protonation/deprotonation of distal histidine moieties, which appears to be needed to reduce the Mb(Fe IV =O) subunit. The low efficiency of the overall process is mainly attributed to charge recombination within the Mb(Fe III OH2) + -Ru 2+ -BV 3+ state, which efficiently competes with the deprotonation processes. This study highlights the potential of mixed synthetic–natural systems for obtaining long-lived charge separation. 5.4 Polyads Based on Oligoproline Assemblies The interesting results obtained by organizing D–P–A triads on the structure of the amino acid lysine (Sect. 5.3) prompted the preparation of D–P–A systems assembled on oligoproline scaffolds, by means of solid-state peptide synthesis [294, 297–299]. One such system is 46. In this species, a phenothiazine (PTZ) group acts as the electron donor and an anthraquinone (Anq) subunit plays the role of the electron acceptor. Oligoprolines were selected, since it is known that oligoproline chains of nine or more proline units fold into stable helices even with large functional groups on the proline units. The terminal segments allow the helix to begin and end with capped Pro3 turns which prevent unwinding of the helix. For 46, a fully developed charge-separated state is gained in acetonitrile solution with good efficiency (53%). The charge-separated state stores 1.65 eV relative to the ground state and returns to the ground state with a rate constant of 5.7 × 10 6 s –1 (τ = 175 ns) [299, 300]. Quenching of the Ru-based excited state is domi-
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 171 nated by reductive electron transfer involving the PTZ electron donor, in a process which is largely solvent dependent, as is the driving force of the process. Then, there is a fast electron transfer from the reduced metal chromophore to the Anq subunit, which yields the charge-separated state. There is a strong solvent-dependent competition between such a second electron transfer, which allows for fully developed charge separation and back electron transfer in the D + –P – –A intermediate. The consequence of this solvent dependence is that going from 1,2-dichloroethane to dimethylacetamide, the efficiency of charge separation changes from 33 to 96%. Also, the charge recombination is solvent dependent, and the electronic coupling between PTZ + and Anq – was calulated to be about 0.13 cm –1 [300]. A more elaborated polyad based on the formerly described systems is the D–P–P–A tetrad 47 [301]. This is the evolution of a system quite related to 46, where substituents on the terminal bpy ligands of the metal chromophore are used to favor the thermodynamics of the (reductive) first electron transfer step. This modification led to an efficiency of charge separation of 90% in acetonitrile for the corresponding triad. In the tetrad, 13 proline spacers are present between PTZ and Anq. The efficiency of formation of the charge-separated state in the tetrad is 60% and its lifetime is 2 µs (kCR = 5.0 × 10 5 s –1 ). Excitation can occur in both the Ru chromophores, but apparently the result is not identical. Excitation of the Ru(II) complex adjacent to the PTZ electron donor gives the D + –P – –P–A system. To produce the fully developed D + –P–P–A + species, it is proposed that a stepwise mechanism occurs, with the species D + –P–P – –A as an intermediate. Efficient, isoergonic electron transfer between the two chromophores is therefore foreseen. Excitation of the Ru(II) complex adjacent to the electron acceptor Anq subunit would be unproductive in a direct sense, since oxidative electron transfer by Anq is unfavorable thermodynamically and direct quenching from the PTZ unit is unlikely because of the large distance. However, even excitation of this Ru(II) chromophore can become productive, provided that isoergonic energy transfer to the Ru(II) chromophore adjacent to the PTZ unit takes place. Since the quantum efficiency of formation of the charge-separated state is 60%, and considering that excitation of the two identical Ru(II) chromophores
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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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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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