Sacrificial donors

Figure 12.12 Building blocks of an artificial photosynthesis system for hydrogen production using a chromophore (C), an electron acceptor (A) and a sacrificial donor(D)...

Photoelectrochemical hydrogenation of double and triple bonds has been reported with sulfide anion acting as a sacrificial donor, Eq. (35)With CdS metallized with platinum or rhodium as the photocatalyst, hydrogenation was found to be about... 

An organic molecule can be used as the sacrificial donor in a reduction half reaction. Generally there is no net energy storage, but (depending on the reaction) hydrogen may be evolved at the same time as a surplus reaction product. For example, the reaction... 

One such study, undertaken by Morita et al. [79], investigated photocurrent generation in the helical peptide SAMs illustrated in Figure 5.47. The photocurrent could be switched from anodic to cathodic by changing the sacrificial donor-acceptor species. 

Figure 6.9 Photoinduced electron transfers via (a) direct or (b) photosensitized processes. R, reactant Sens, sensitizer D and sD, donor and sacrificial donor, respectively A and sA, acceptor and sacrificial acceptor, respectively. Reductive PET (photoinduced electron transfer) schemes are drawn in black, whereas oxidative schemes are in red...

A sacrificial donor (acceptor) is a molecular entity that acts as the electron donor (acceptor) in a photoinduced electron transfer process and is not restored in a subsequent redox process, but is destroyed by irreversible chemical conversion [29]. 

Moreover, in the membrane environments the tocopherols are present, which remove the toxic oxygen species (02,102, HOO and OH ). The tocopherols act here as sacrificial donors, ie scavengers, which are not restored in a subsequent reduction process but are destroyed by irreversible chemical conversion [125],... 

When part of a delocalized aromatic molecule, the anion, after excited state electron transfer, gives rise to a not too reactive radical so that if one plays with the sequence of hydrogen abstraction from solvent followed by proton abstraction from the base, the initial anion may be restored and the solvent usefully replaces the sacrificial donor. This is the case with naphtholate as anion, certainly an interesting anionic sensitizer to test in other systems. Since the solvent is by definition at high concentration, its recovering role is played with a maximum efficiency reaction between the solvent and one of the radicals arising from the electron transfer is a good way to circumvent the back electron transfer. 

It is worth noting that similar processes could be in occurring with compounds 11, 19, 20 and 23 in which the oxidized sacrificial donor (e.g., TEA, DMA) deproto-nate to form a neutral radical species with good reducing power. While it is difficult to rule such a possibility out, MacDonnell and co workers have shown that the sing ly reduced version of compound 11, [(phen)2Run(tatpp )Run(phen)2]3+ can be iso lated and, when subject to photochemical reduction, cleanly undergoes the second reduction. Thus, while the overall reduction of 11 to 22 could include a thermal redox reaction, it does not require one. 

Another dye, hydroxoaluminiumtricarboxymonoamide phthalocyanine (AlTCPc), adsorbed on Ti02 particles at different loadings was tested for Cr(VI) photocatalytic reduction under visible irradiation in the presence of 4-CP as sacrificial donor. Direct evidence of the one-electron reduction of Cr(VI) to Cr(V) was also obtained by EPR experiments. The inhibition... 

Fig. 19. Possible scheme for use of TICT compounds as catalysts in photoreduction (e.g. photodechlorination of organic compounds ArCl). >, is a sacrificial donor to reduce the TICT cation radical back to the starting material in the ground state, TICT compounds can also be used as photooxydation catalysts as shown for the example of norbornadiene(Dq)-isomerization [13]. In this case, direct back electron transfer in the ground state takes place as shown in the figure...

The most essential pathways of pollutant photodegradation start from photocatalyst absorption followed by photosensitization or photoassisted reactions. The former consists in the energy or charge transfer from an excited photosensitizer to the substrate (quencher) molecule, whereas the latter results in generation of a catalyst for one cycle, called photoinitiator. To guarantee the continuity of the charge transfer reactions both photosensitizers and photoinitiators should be recycled the common practice is their regeneration by means of adequate electron donors and/or acceptors. These are not restored in a subsequent redox process but destroyed by irreversible chemical conversion thus they are called sacrificial donors and acceptors, respectively. 

The most common sacrificial electron acceptor in the environment is molecular oxygen, whereas the main sacrificial donors are organic compounds. In consequence, the self-cleaning processes consist in oxidation of organic pollutants by molecular oxygen in its triplet ground state the reactions are driven by energy from solar radiation. In nature, many different photoinitiators or photosensitizers are reactive, but the most common environmental photosensitizers include hiunic substances (HS), whereas the best photoinitiators are transition metal complexes. 

Analogous photoelectrochemical hydrogenations of double and triple bonds have also been reported with other oxidizable sacrificial donors (Eq. 27) [166]. 

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). 

The Ru generated in reaction (4) is rapidly reduced by a sacrificial donor such as EDTA to prevent back electron transfer and allow Ru cyt c to transfer an electron to compound I [Eq. (5)]. At low ionic strength (2 mM) and a 1 1 ratio of compound I to horse cyt c, reduction of the... 

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