Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
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170 S. Campagna et al.<br />
parent first-order rate constant <strong>of</strong> 6.6 × 10 3 s –1 . This species is produced with<br />
alowquantumyield(0.5%), stores about 1.3 eV, <strong>and</strong> recombines to the<br />
ground state with a lifetime exceeding 2 ms. In spite <strong>of</strong> the low efficiency<br />
<strong>of</strong> the charge-separation process, there are several interesting points: (1) in<br />
the absence <strong>of</strong> myoglobin, charge separation is not obtained at all; (2) the<br />
complete process is pH dependent; (3) the final charge-separated state lifetime<br />
is comparable with that <strong>of</strong> natural photosynthetic reaction centers; <strong>and</strong><br />
(4) back electron transfer is regulated by protonation/deprotonation <strong>of</strong> distal<br />
histidine moieties, which appears to be needed to reduce the Mb(Fe IV =O)<br />
subunit. The low efficiency <strong>of</strong> the overall process is mainly attributed to<br />
charge recombination within the Mb(Fe III OH2) + -Ru 2+ -BV 3+ state, which efficiently<br />
competes with the deprotonation processes. This study highlights the<br />
potential <strong>of</strong> mixed synthetic–natural systems for obtaining long-lived charge<br />
separation.<br />
5.4<br />
Polyads Based on Oligoproline Assemblies<br />
The interesting results obtained by organizing D–P–A triads on the structure<br />
<strong>of</strong> the amino acid lysine (Sect. 5.3) prompted the preparation <strong>of</strong> D–P–A systems<br />
assembled on oligoproline scaffolds, by means <strong>of</strong> solid-state peptide<br />
synthesis [294, 297–299]. One such system is 46. In this species, a phenothiazine<br />
(PTZ) group acts as the electron donor <strong>and</strong> an anthraquinone (Anq)<br />
subunit plays the role <strong>of</strong> the electron acceptor. Oligoprolines were selected,<br />
since it is known that oligoproline chains <strong>of</strong> nine or more proline units<br />
fold into stable helices even with large functional groups on the proline<br />
units. The terminal segments allow the helix to begin <strong>and</strong> end with capped<br />
Pro3 turns which prevent unwinding <strong>of</strong> the helix. For 46, a fully developed<br />
charge-separated state is gained in acetonitrile solution with good efficiency<br />
(53%). The charge-separated state stores 1.65 eV relative to the ground<br />
state <strong>and</strong> returns to the ground state with a rate constant <strong>of</strong> 5.7 × 10 6 s –1<br />
(τ = 175 ns) [299, 300]. Quenching <strong>of</strong> the Ru-based excited state is domi-