Photochemical electron transfer,

M. GiAtzel, Heterogenous Photochemical Electron Transfer, CRC Press, Boca Raton, Ela., 1988. 

Gratzel, M., Heterogeneous Photochemical Electron Transfer Reactions, CRDC Press, Baton Rouge, Florida, USA (1987). 

Photochemical electron transfer occurs from the excited singlet state of the primary donor. 

J.R. Bolton In solution most photochemical electron transfer reactions occur from the triplet state because in the collision complex there is a spin inhibition for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs in such systems but the back electron transfer is too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as in a collision complex, yet it is still close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer is inhibited by the distance and molecular structure between the two ends. Our future work will focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode . 

It is important to note that the reactions are fundamentally different from similar radical cation Diels-Alder reactions initiated with the use of a photochemical electron-transfer reaction [35, 36]. In photochemical reactions, a one-electron oxidation of the substrate leads to a cycloaddition that is then terminated by a back electron transfer . No net change is made in the oxidation state of the substrate. However, the reaction outlined in Scheme 13 involves a net two-electron oxidation of the substrate. Hence, the two pathways are complementary. 

Weller A (1982) Exciplex and radical pairs in photochemical electron-transfer. Pure Appl Chem 54 1885... 

Unlike thermal [2 + 2] cycloadditions which normally do not proceed readily unless certain structural features are present (see Section 1.3.1.1.), metal-catalyzed [2 + 2] cycloadditions should be allowed according to orbital symmetry conservation rules. There is now evidence that most metal-catalyzed [2 + 2] cycloadditions proceed stepwise via metallacycloalkanes as intermediates and both their formation and transformation are believed to occur by concerted processes. In many instances such reactions occur with high regioselectivity. Another mode for [2 + 2] cyclodimerization and cycloadditions involves radical cation intermediates (hole-catalyzed) obtained from oxidation of alkcnes by strong electron acceptors such as triarylammini-um radical cation salts.1 These reactions are similar to photochemical electron transfer (PET) initiated [2 + 2] cyclodimerization and cycloadditions in which an electron acceptor is used in the irradiation process.2 Because of the reversibility of these processes there is very little stereoselectivity observed in the cyclobutanes formed. 

Although hole-catalyzed (cycloaddilions involving radical cation intermediates) and PET (photochemical electron transfer) mixed [2 -I- 2] cycloadditions have been reported from electron-rich alkenes, the only report of a cyclodimerization is that of (E)-4-(prop-l-enyl)anisole which gives stereoisomeric mixtures of the head-to-head dimers 1 and 2.12... 

Gratzel, M. Heterogeneous Photochemical Electron Transfer, CRC Press Boca Raton, FL, 1989. 

Because of this difference in electron-donorand electron-acceptor properties, excited states have very different redox properties from those of related ground states. The effect is so marked that many photochemical processes begin with a complete transfer of an electron from (or to) an excited state (1.2), and the subsequent chemistry is that of radical cations and radical anions, species that are regarded as unusual in ground-state organic reactions. The importance of photochemical electron transfer is underlined by its extensive involvement in photobiological processes such as photosynthesis. 

L. Mattes and S. Farid, in A. Padwa (ed.). Organic Photochemistry, vol. 6, Dekker (1983). The photochemical electron transfer reactions of olefins are described in this account, which provides an interesting perspective that cuts across classifications based on an overall reaction type. 

Radical reactions are facilitated by the fact that pyrroles and indoles can form reasonably stable radical cations in some cases. For instance, photoarylation of indole by 2-iodopyridine is controlled by a photochemical electron-transfer reaction leading to the combination of the indole radical cation and the 2-pyridyl radical. The direction of attack is controlled by the relative spin densities of the possible radical cations. In polar solvents, substitution is favored at positions 3,6, and 4, whilst in nonpolar solvents, there is a preference for substitution at positions 2 and 7 (Scheme 35) (88CPB940). 

Mechanistic studies are consistent with photochemical electron transfer from the carbyne complex to chloroform followed by H atom abstraction. Ring expansion then occurs to give a metallacyclopentene, which undergoes carbonyl insertion. Finally, reductive elimination yields the cyclopentenone complex that slowly releases the free enone (equation 119)158. 

Woodbury, N. W., M. Becker, D. Middendorf, and W. W. Parson, Picosecond kinetics of the initial photochemical electron transfer reaction in bacterial photosynthetic reaction centers. Biochem. 24 7516, 1985. Fast spectrophotometric techniques are used to follow the initial steps in reaction centers purified from photosynthetic bacteria. 

Photoinduced Sj l aromatic substitutions apparently involve primary photochemical electron transfer. Hoz and Bunnett, for example, have characterized the mechanistic details of photoinitiation in the reaction of iodobenzene with diethyl phosphite as involving either electron exchange, eq. 61 (188),... 

Crucial to the success of reactions designed to produce hydrogen via intermolecular electron transfer reactions is the addition of an efficient redox catalyst which allows reduction of protons by the reduced form of the relay (e.g. MV+) formed by the initial photochemical electron transfer. 

Attachment of the ruthenium complex to the polysoap leads to even greater rate enhancements and it is believed that sequential passage of the electron from the ruthenium attached viologen after the photochemical electron transfer to neighbouring viologen units and then on down the chain of local viologens leads to efficient charge separation.149... 

For added viologens, photochemical electron transfer from inner pools containing EDTA or K2C204 and [Ru(bipy)3]2+ or [ZnTMPyP]4+ to methylene blue, [Fe(CN)fi]3, [ Ru(bipy)2(H20) 20]4+ or [SiMo,2042]8 was observed and in the absence of an electron acceptor in the bulk, viologen cation radical was formed. Again, tunnelling of an electron from a reduced viologen on the inner surface to one on the other surface is believed to occur. Quantum yields up to 0.1 have been observed.340... 

The geometric isomerization of olefins via photochemical electron transfer is well known28,29 and can be divided into two categories (a) isomerization via the radical cation, in which case the olefin is the donor in the presence of an excited electron acceptor (b) isomerization via the radical ion pair, which leads to the triplet-excited olefin, and in this mechanism the olefin is the acceptor. This subject is not discussed in this chapter because of space limitations. However, several reviews30 can be consulted in this regard. 

M. Gratzel, Heterogeneous photochemical electron transfer, CRC Press, Inc. (1989)... 

Pogozelski WK, Tullius TD (1998) Oxidative strand scission of nucleic acids Routes initiated by hydrogen abstraction from the sugar moiety. Chem Rev 98 1089-1107 Poole JS, Eladad CM, Platz MJ, Fredin ZP, Pickard L, Guerrero EL, Kesser M, Chowdhury G, Kotande-niya D, Gates KS (2002) Photochemical electron transfer reactions of tirapazamine. Photochem Photobiol 75 339-345... 

Photochemical electron-transfer can be effected by irradiation of the charge-transfer absorption band of the electron donor-acceptor complex.15 Alternatively, photochemical electron-transfer may proceed by actinic activation of RH followed by quenching with A, or by the reverse sequence involving activation of A and quenching with RH. 

Figure 11.15 Photochemical electron transfer mediated by n stacking interactions with an aromatic guest.

Banerjee, A. Falvey, D. E. Protecting groups that can be removed through photochemical electron transfer mechanistic and product studies on photosensitized release of carboxyl-ates from phenacyl esters. J. Org. Chem. 1997, 62, 6245-6251. 

Protein matrices are believed to play an important role in the accomplishment of photochemical electron transfer reactions in biological systems. First, the fragments... 

In order to utilize photochemical electron transfer, investigators have replaced the Fe-containing porphyrin active sites in several redox proteins with equivalent photoactive porphyrins. 

In several photochemical electron transfer reactions, addition products are observed between the donor and acceptor molecules. However, the formation of these products does not necessarily involve direct coupling of the radical ion pair. Instead, many of these reactions proceed via proton transfer from the radical cation to the radical anion, followed by coupling of the donor derived radical with an acceptor derived intermediate. For example, 1,4-dicyanobenzene and various other cyanoaromatic acceptors react with 2,3-dimethylbutene to give aromatic substitution products, most likely formed via an addition-elimination sequence [140]. 

Several interesting rearrangements (stereomutations) were observed for a series of l-anisyl-2-vinylcyclopropanes (32, Ar = CgH -OCty. These reorganizations have been elaborated in greater detail than most other radical cation reactions. For this reason, we will consider them in somewhat greater detail, even though they were initiated primarily by chemical, rather than photochemical electron transfer. Treatment of the m-isomer (cis-32) with aminium or dioxygenyl salts at... 

In the following years, several groups postulated the involvement of triplexes in various photochemical electron transfer reactions [104]. 

AEth is the activation energy for the thermal electron transfer. AEop is the Franck-Condon energy for the photochemical electron transfer... 

In the last few years there has been an increasing interest in solar energy utilization. Photochemical processes (and particularly photochemical electron transfer processes) are very suitable to convert light energy into more useful energy forms103-106). 

The generation of cr-radical cations from saturated hydrocarbons requires very strong SET oxidizers. The oxidation reactions can be accomplished by chemical electron transfer (CET), photochemical electron transfer (PET), and anodic oxidation. The oxidation potentials of stable, organic CET oxidants, e.g., commercially available tris(4-bromophenyl)aminium hexachloroantimonate (TBA +SbCI<,) or tris(2,4-dibromo-phenyl)aminium hexachloroantimonate (TDA +SbCl6 ), are too low (1.06 and 1.50 V... 

Hoffmann, N., Bertrand, S., Marinkovic F S. and Pesch, J. (2006) Efficient radical addition of tertiary amines to alkenes using photochemical electron transfer. Pure and Applied Chemistry, 78, 2227-2246. 

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

Scheme 5.2 [2 + 2] Photocycloaddition induced by photochemical electron transfer.

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