Oxidative and reductive quenching

By using pulsed laser sources and fast measuring devices, direct observation of redox products in flash photolysis experiments provides evidence regarding oxidative and reductive quenching mechanisms in Ru(bpy)3+. 

Oxidative and reducing quenching of excited polypyridine complexes produce strong ground state oxidants and reductants, respectively (Figure 2). For example, [Ru(bpy)3]3+ ( ,/2(3+/2+) = +0.85 V) and [Ru(bpy)3]+ (. 1/2(2+/+) = -1.76 V) are formed by oxidative and reductive quenching, respectively, of [Ru(bpy)3] . Very often, these primary products of excited state electron transfer are the proper oxidants and reductants for the substrate, that is B or Y in Figure 3. 

The MLCT excited state is both a strong oxidant and a strong reductant and it can be quenched by either electron acceptors (oxidative) or donors (reductive quenching) [65, 71]. Table 9 gives rate constants for oxidative and reductive quenching of the MLCT excited state of a number of /ac-Re(a-diimine)(CO)3X complexes. 

Table 9. Oxidative and reductive quenching of MLCT excited state of Re(a-diimine)(C0)3X by Q in MeCN at room temperature.

A Study of the photophysical and photochemical properties of alkylidyne complexes such as W(CPh)Cl(CO)2(tmeda) was reported by Bocarsly et al. (see also Section II,B) 24). Quenching experiments with aromatic hydrocarbons indicate that a significant amount of triplet character is associated with the emissive excited state. Bimolecular oxidative and reductive quenching is observed as well. Methylviologen dichloride is reduced and iV.A, A, A, -tetramethyl-p-phenylenediamine is oxidized by W(CPh)Q-(CO)2(tmeda) on irradiation with 488 nm light. Photosubstitution of two CO ligands in W(CPh)Cl(CO)(dppe) by dppe to give W(CPh)Cl(dppe)2 was observed as well. 

Flash photolysis of [Ru(OEP)CO] has been investigated. The lifetime of the excited state species [Ru(porph)CO] (porph = TPP derivatives) has been measured as 30 /is with emission occurring near 730 nm. Oxidative and reductive quenching of [Ru(TPP)CO] and related species has been reported, for the [Ru(TPP)CO] redox couple being assessed as — 0.57 V vs. SSCE. ... 

Wallentin C-J, Nguyen JD, EinkbeinCT P, Stephenson CRJ (2012) Visible light-mediated atom transfer radical additimr via oxidative and reductive quenching of photocatalysts. J Am Chem Soc 134 8875-8884... 

Figure 10.1 Oxidative and reductive quenching cycles for photocatalysts.

The redox potential diagram in eq. 1 illustrates that the effect of optical excitation is to create an excited state which has enhanced properties both as an oxidant and reductant, compared to the ground state. The results of a number of experiments have illustrated that it is possible for the excited state to undergo either oxidative or reductive electron transfer quenching (2). An example of oxidative electron transfer quenching is shown in eq. 2 where the oxidant is the alkyl pyridinium ion, paraquat (3). 

The TTF-porphyrin dyad 3 was described by the group of Odense.11 The fluorescence of 3 is significantly quenched by the photoinduced electron transfer process. Notably, the fluorescence intensity of dyad 3 increases largely after addition of Fe3 + that oxidizes TTF into TTF" +. Successive reduction of TTF" + is not reported. Nevertheless, it is anticipated that the fluorescence of dyad 3 can be reversibly modulated by redox reactions. In fact, the fluorescence of the supramolecule 4, formed between Zn-tetraphenylporphyrin and a pyridine-substituted TTF (TTF- ), can be reversibly tuned by sequential oxidation and reduction of the TTF moiety in 4.12 It should be noted in this context that the synthetically challenging system associating a porphyrin ring fused to four TTFs (5) was also reported.13... 

Ferrocene has been widely investigated as an electron donor and its electron donating ability can be tuned by redox reactions. As anticipated, when a ferrocene unit is covalently connected to an electron acceptor moiety that shows intrinsic fluorescence, the fluorescence of the acceptor moiety would be largely quenched because of the photoinduced electron transfer between ferrocene and the fluorescent acceptor. For instance, triad 15 that contains perylene diimide flanked by two ferrocene moieties, shows rather weak fluorescence due to the photoinduced electron transfer between perylene diimide and ferrocene units. Either chemical or electrochemical oxidation of ferrocene unit lead to fluorescence enhancement. This is simply because the electron donating ability of ferrocene is reduced after oxidation and accordingly the photoinduced electron transfer is prohibited. In this way, the fluorescence intensity of 15 can be reversibly modulated by sequential electrochemical oxidation and reduction. Therefore, a new redox fluorescence switch can be established with triad 15.25... 

Recently we observed eel of the binuclear platinum complex tetra-kis(diphosphonato)diplatinate(II) (Pt (pop) ) (37). This anion has attracted much attention due to its intense green luminescence in room temperature solution (38-40) (excited state of this complex undergoes oxidative (42) and reductive quenching (41). From the quenching experiments the redox potentials were estimated to be E° = -1.4 V vs. SCE for the reduction and E° 1 V for the oxidation of Pt2(pop) - (41). The potential difference of 2.4 V almost matches the energy of the phosphorescing triplet ( 2.5 eV) of Pt -(pop) . Consequently, it should be possible to observe eel of this... 

Quenching of an excited state by electron transfer needs electronic interaction between the two partners and obeys the same rules as electron transfer between groimd state molecules (Marcus equation and related quantum mechanical elaborations (7)), taking into account that the excited-state energy can be used, to a first approximation, as an extra free energy contribution for the occurrence of both oxidation and reduction processes. 

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