Oxidative electron transfer quenching

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). 

Since the excited state may act as an electron donor or as an electron acceptor, two electron-transfer quenching pathways, reductive and oxidative, are possible. 

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

The fluorescence of 3-t (113-117) and 3-7 (118) is quenched by secondary and tertiary amines. Rate constants for quenching of It by tertiary amines increase with decreasing amine ionization or oxidation potential (Table 11), as expected for the formation of a charge-transfer stabilized exciplex in which the amine serves as the electron donor. Electron transfer quenching in nonpolar solvent is calculated to be exothermic for amines with E 2 < 1 34 V. Thus, it is not surprising that secondary and tertiary amines quench 3-t with rate constants which approach or even exceed the rate of diffusion. The inefficient quenching of It and 3-7 by primary amines is consistent with their higher oxidation potentials. 

Photo-electroswitching takes place in substances where an electrochemical change induces more or less pronounced variations in light absorption or emission properties. This occurs namely for metal complexes subjected to redox interconversion between their various oxidation states. A two-component device combining a luminescent centre and an electroactive unit may function as a photo-electroswitch in which the emission properties are modulated by redox interconversion via energy or electron transfer quenching (Figure 30). 

The sensitization of semiconductors is a special example of electron transfer quenching and may prove to be very important. A photoexcited electron may, for example, be injected with high quantum yield into the semiconductor conduction band, to produce a photovoltaic device. The hole that is left behind may then perform some useful oxidation process. 

When dealing with organic anions, a problem often encountered is to find correct values for the oxidation potential. Electrochemical methods frequently lead to irreversible values generally related to the high reactivity of the radical species formed after oxidation. One way to circumvent this problem is to use a series of electron transfer quenching measurements involving the anion and to adjust the oxidation potential in order to fit the results to a Marcus correlation. This has been done using the Weller equation to calculate the oxidation potentials of a series of borate anions [93] and the Marcus model in the case of naphtholate anions [96], A reasonable estimation of E0I, with discrepancies of 0.2 V as a maximum can be obtained by this method. 

Ru(II)tris(bipyridine) [Ru(bpy)3 +] as a photosensitizer, triammonium ethylene-diaminetetraacetic acid [(NH4)3EDTA] as a sacrificial electron donor and the enzyme ferredoxin NADP+ reductase (FDR) [215, 216]. Oxidative electron-transfer quenching of the excited Ru(bpy)3 + yields the A,A -dimethyl-4,4 -bipyridinium radical cation (reduced methylviologen, MV+), which mediates the reduction of NADP+ in the presence of FDR as a biocatalyst (Figure 32A). The quantum efficiency for NADH production corresponds to = 1.9 x 10 . A related system that includes Zn(II)wc50-(A-tetramethylpyridinium)porphyrin (Zn-TMPyP +) as a photosensitizer, mercaptoethanol as a sacrificial donor and lipoamide dehydrogenase (LipDH) as a biocatalyst has been applied for the photochemical reduction of NAD+ to NADH (Figure 32B). 

By the second approach, the enzyme is immobilized in a redox polymer assembly (Figure 39B). Electron-transfer quenching of the photosensitizer by the polymer matrix generates an electron pool for the activation of the enzyme. Photoreduction of nitrate to nitrite was accomplished by the physical encapsulation of NitraR in a redox-functionalized 4,4 -bipyridinium acrylamide copolymer [234]. In this photosystem, Ru(bpy)3 + was used as a photosensitizer and EDTA as a sacrificial electron donor. Oxidation of the excited photosensitizer results in electron transfer to the redox polymer, and the redox sites on the polymer mediate further electron transfer to the enzyme redox center, where the biocatalyzed transformation occurs. The rate constant for the MET from the redox polymer functionalities to the enzyme active site is — (9 + 3) x 10 s. Similarly, the enzyme glutathione reductase was electrically wired by interacting the enzyme with a redox polymer composed of polylysine modified with A-methyl-A -carboxyalkyl-4,4 -bipyridinium. The photosensitized reduction of oxidized glutathione (GSSG) (Eq. 21) ... 

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