Photoelectron transfer reactions

The interface between two immiscible electrolyte solutions is well suited to studying artilicial photosynthesis. Indeed, the products of a photoinitiated electron-transfer reaction can be separated on either side of the interface, thereby breaking the cage effect that is dominant in bulk solutions. 

Back in 1988, Girault s group started to investigate photocurrent measurements at polarized I l lES using Ru(bpy)3 as a sensitizer [218,219]. This early work was followed by a series of papers of photoinduced electron reactions using porphyrin sensitizers [220-226]. 

In part I of the series, it was demonstrated that photocurrent responses associated with the heterogeneous quenching of water-soluble ZnTPPC by ferrocene and DFCET are potential dependent, proportional to light intensity, and that the action spectrum followed the absorption spectrum of the porphyrin [220]. 

Zinc meso-tetra-sulphonatophenyl porphyrin ZnTPPS 

Zinc meso-tetrakis-carboxyphenyl porphyrin ZnTPPC 

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Closely related reactions have been accomplished by photoelectron-transfer reactions of allylsilanes and benzylsilanes, and a similar mechanism involving the cation radical intermediate is suggested [29]. Chemical oxidation of allylsilanes [27] and ferrocenylsilanes [30] also cleaves the C-Si bond and mechanism of these reactions seem to closely relate to that of the electrochemical process. 

DNA mediated photoelectron transfer reactions have been demonstrated60 . Binding to DNA assists the electron transfer between the metal-centered donor-acceptor pairs. The increase in rate in the presence of DNA illustrates that reactions at a macromolecular surface may be faster than those in bulk homogeneous phase. These systems can provide models for the diffusion of molecules bound on biological macromolecular surfaces, for protein diffusion along DNA helices, and in considering the effect of medium, orientation and diffusion on electron transfer on macromolecular surfaces. 

The classic precursors used for generating diradicals are cyclic, bicychc and polycyclic diazenes. However, diradicals have also been made by Norrish type I photochemical extrusion of CO from cychc ketones, by thermal cleavage of vinyl and divinylcyclopropanes, by pinacol reactions of diketones, by Bergman-type cyclisations of endiynes, by several types of photoelectron transfer reactions and in other ways. Most synthetic applications have started with a derivative of 2,3-diazabicyclo[2.2.1]hept-2-ene which on heat-... 

Both direct and sensitized photoelectron-transfer reactions of amines have been utilized for bringing about synthetically useful transformations of amines. 

Topics that have formed the subjects of reviews this year include contemporary issues in electron transport research, dynamics of bimolecular photoelectron transfer reactions, photophysical properties of functionalised fullerene derivatives, carbon-carbon bond formation via radical ions, photoinduced electron transfer processes in ketone, aldehyde, and ester synthesis, photochemical reactions between arenenitriles and benzylic donors, photo-oxidation of conjugated dienes, photoredox reactions of aromatic nitro compounds, electron transfer-mediated photochemistry of some unsaturated nitrogen-containing compounds, reactions of 02( Ag), carbon dioxide activation by aza-macrocyclic complexes, and photochromism of chalcone derivatives. ... 

The distance dependencies of photoinduced electron transfer rates have been examined in anthracene-spacered porphyrin-quinone cyclophanes, and the same authors have also discussed the distance dependencies of photo-induced electron-transfer rates in benzene-, naphthalene-, and anthracene-spacered porphyrin-quinone cyclophanes and biphenylene-spacered porphyrin-quinone cyclophanes. Photoelectron transfer reactions of the porphyrin-quinone cyclophanes (3) and their zinc complexes have been examined, and in some cases at least interaction of the quinone carbonyl group with the zinc atom may be an alternative to through-space electron transfer. A study of intramolecular photoinduced electron transfer for the quinone-porphyrin cyclophane type (4) containing the especially strong acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) has appeared." The distance dependence of the TCNQ and porphyrin is of particular interest, and to this end the corresponding 2,8-naphthalenediyl-TCNQ-porphyrin has been synthesised. 

Figure 16.3 shows a schematic representation of a photoelectron transfer reaction where a sensitizer (S ) in one phase is quenched by an electron donor (Q) in the adjacent phase. A charge-transfer complex [S - Q+] is formed at the inta face. In a bulk solution, recombination often occurs due to the cage effect formed by the solvent molecules. At soft interfaces, the dissociation of the charge transfer complex into photoproducts can be favored by the presence of the static electric field, and this is still a very important point to quantify in the coming years. 

Photoelectron transfer reactions can be considered to occur by a mechanism that involves a pair of encounter complexes. Thus, the reductive quenching reaction shown in Eq. (1.37) can be expanded to include both a first- and a second-encounter complex. This first-encounter complex is formed between the excited state M and the quencher Q. Electron transfer then occurs within this complex to give the ionic second-encounter complex, which then finally dissociates to give the separated product ions... 

Ruthenium hexacyanides have been used in these photoelectron transfer reactions because of the availability of the (Ru(II)/Ru(III)) redox system, which differs by one electron. For the cyanide complex, (RuCCNji ) " =0.86 V. An example... 

The dimeric carbonyl complexes M2(CO)io (M = Mn, Re) also undergo photoelectron transfer reactions. Such reactions occur with quinones where both ortho and para quinones react with the two primary photoproducts M(CO)5 and M2(C0)9. Toward M2(CO)9 these quinones (Q) coordinate as a donor ligand, but toward the 17-electron M(CO)5 they act as a one-electron oxidant to give a complexed semiquinone (SQ) (Scheme 6.2). ... 

The quantum yields for this photoelectron transfer reaction decrease regularly as the value of n increases, with electron transfer being facile for intermetallic separations of up to 7-8... 

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