Photosensitization by electron

A priori, photosensitization by electron transfer may occur in either of two modes, as shown in Schemes 8 and 9. 

Schemes 8 and 9 differ by whether the photosensitizer functions as the electron donor (Scheme 8) or acceptor (Scheme 9). In both cases, amplification of photons by a chain reaction is possible. The energetics of photosensitization by electron transfer have been discussed in Volume 4, Chapter 20, including a consideration of chain reactions. A comprehensive report on light-induced electron transfer reactions of unsaturated compounds, including dimerization, is available." ...

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

Kavamos GJ, Turro NJ (1986) Photosensitization by reversible electron transfer theories, experimental evidence, and examples. Chem Rev 86 401 -49... 

Corma, A., Fornes, V., Garcia, H., Miranda, M.A., Primo, J. and Sabater, M.-J. (1994). Photoinduced electron transfer within zeolite cavities eiv-Stilbcnc isomerization photosensitized by 2,4,6-triphenylpyrylium cation imprisoned inside zeolite Y. J. Am. Chem. Soc. 116, 2276-2280... 

Menadione (79) has been shown by Krishna et al. to act as a photosensitizer for thymine degradation. Spin-trapping experiments with several nucleosides showed that the photosensitizations occurred by electron transfer from the substrate to the excited triplet state of menadione to form the cation radical of the substrate and the anion radical of menadione, both of which were detected... 

Buettner, G. R., KeUey, E. E., and Bums, C. P., 1993, Membrane hpid free radicals produced from LI 210 murine leukemia ceUs by photofrin photosensitization an electron paramagnetic resonance spin trapping study. Cancer Res. S3 3670-3673. 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

The electron transfer sensitized photooxygenation reactions have been the subjects of numerous investigations and of several reviews [181-183]. In the presence of oxygen, CP is oxygenated during the DCA photosensitized reaction via the reaction of CP"+ and 0 2", which is formed by electron transfer from DCA to 02, to give the corresponding dioxolanes (Scheme 20) [173] ... 

Organic radical cations generated by electron transfer photosensitization are species with greatly enhanced pK a s compared with their closed shell precursors (142). If a C-H bond is located adjacent to the site of predominant charge density, rapid deprotonation will ordinarily ensue, producing a radical whose fate determines the identity of the isolated products. ... 

Douki T, Cadet J (1999) Modification of DNA bases by photosensitized one-electron oxidation. Int J RadiatBiol 75 571-581... 

Oxidation of guanine and 8-oxo-7,8-dihydroguanine with a Mn(IV)=0 species, a two-electron oxidant and riboflavin, a known photosensitizer and a one-electron oxidant, was studied. A quantification of the ratio between one- and two-electron oxidation mechanisms of guanine oxidation by electron transfer led to the conclusion that one-electron oxidation predominates and the two-electron oxidation process is a minor pathway.288... 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

Dannenberg, J. J., and J. H. Richards Photosensitization by ferrocene. Photochemistry of higher electronic excited states. J. Amer. chem. Soc. 87, 1626 (1965). 

As follows from the data from Sect. 2, the primary photochemical stage in the majority of the membrane systems studied is the redox quenching of the excited photosensitizer by an electron acceptor or donor leading to electron transfer across the membrane // water interface. For electron transfer to occur from the membrane-embedded photosensitizer to the water soluble acceptor, it is necessary for the former to be located sufficiently close to the membrane surface, though the direct contact of the photosensitizer with the aqueous phase is not obligatory. For example, Tsuchida et al. [147] have shown that electron transfer to MV2 + from photoexcited Zn-porphyrin inserted into the lecithin membrane, is observed only until the distance from the porphyrin ring to the membrane surface does not exceed about 12 A. 

Figure 2.21 Schematics illustrating photosensitized heterogeneous electron transfer mechanisms (a) a directly bound photosensitizer, which photoinjects directly to the electrode (b) a remotely bound bridge sensitizer, which undergoes a prior photoinduced electron transfer, followed by ground-state heterogeneous electron transfer to the electrode...

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