Electron-transfer processes photoreaction

Knowledge of photoiaduced electroa-transfer dyaamics is important to technological appUcations. The quantum efficiency, ( ), ie, the number of chemical events per number of photons absorbed of the desired electron-transfer photoreaction, reflects the competition between rate of the electron-transfer process, eg, from Z7, and the radiative and radiationless decay of the excited state, reflected ia the lifetime, T, of ZA ia abseace ofM. Thus,... 

The photochemistry of imides, especially of the N-substituted phthalimides, has been studied intensively by several research groups during the last two decades [233-235]. It has been shown that the determining step in inter- and intramolecular photoreactions of phthalimides with various electron donors is the electron transfer process. In terms of a rapid proton transfer from the intermediate radical cation to the phthalimide moieties the photocyclization can also be rationalized via a charge transfer complex in the excited state. 

There are only a few examples of singlet photoreaction between alkenes that are not tethered or constrained in close proximity. In these cases, calculations have suggested the presence of exciplex [6] and/or diradical intermediates [7]. The short lifetime of the typical alkene singlet excited state (on the order of 10-20 ns) [8] limits the chance for productive collisions. On the other hand, photochemistry between electron rich and electron poor alkenes such as tetracyanoethylene and methoxy substituted alkenes, provides evidence for an electron transfer process [9]. These matched pairs benefit from ground state attraction and a resulting preorientation that enhances the alkene orbital overlap. Alternatively, electron transfer pathways have been accessed by employing electron transfer sensitizers (see Sch. 2), DCA is dicyanoanthracene) [10]. 

Light absorption modifies the driving force for electron transfer processes in all kinds of materials. As photoactivated species are always better oxidants and reductants than their ground state equivalents, an enhanced redox reactivity is usually observed in the excited state. Photoreactions are therefore ideally suited to trigger, study, and mimic bioinorganic electron transfer. 

The photoreactions of some amines with Co(acac), have been described these lead to a novel type of coupling process between the amines and the P-diketone which occurs by an electron transfer process. Photoreduction of Co(acac)3 by 1-benzyl-1,4-dihydronicotinamide is promoted in SDS micelles. ... 

The ion-pair approach to the design of photosensitizers for electron transfer processes has been followed also in the case of [Co" (sep)] -oxalate system. In a deoxygenated solution, the excitation in the IPCT band of [Co" (sep)] +-HC204 causes the reduction of [Co" (sep)] + to [Co"(sep)] " and the oxidation of oxalate to carbon dioxide. The [Co"(sep)] + complex is a sufficiently strong reductant to reduce H+ to H2 at moderately acidic pH values. Thus, when the photoreaction is carried out in the presence of colloidal platinum catalyst, such a reaction indeed occurs, and H2 evolves from the solution in addition to carbon dioxide. Under such conditions, the overall reaction is the oxidation of oxalate, which plays the role of sacrificial agent, combined with the reduction of water to yield carbon dioxide and dihydrogen, according to Eq. 8. 

In an earlier report Mazzocchi and his coworkers reported that the photo-reaction of A) methylnaphthalimide (325) with phenyIcyclopropane involved the production of a radical cation/radical anion pair. The product from the reaction was the cyclic ether (326). - A study of the mechanism of this reaction using suitably deuteriated compounds has demonstrated that the reaction is not concerted and takes place via the biradical (327). - Other systems related to these have been studied. In the present paper the photoreactivity of the naphthalimide (328) with alkenes in methanol was examined. Thus, with 1-methylstyrene cycloaddition occurs to the naphthalene moiety to afford the adducts (329) and (330). The mechanism proposed for the addition involves an electron transfer process whereby the radical cation of the styrene is trapped by methanol as the radical (331). This adds to the radical anion (332) ultimately to afford the observed products. Several examples of the reaction were descr ibed. 

There are many photochemical redox reaction systems for which back electron transfer is so rapid that the equilibrium of the redox couple does not shift to the product side even under irradiation. In such systems, unlike the photoredox systems described in the last section, a photoresponse cannot be obtained at an electrode which is simply dipped into a mixed solution of the photoreaction couple. When the elefctrode is coated with one of the photoreaction couple, however, a photoresponse may be obtained at the coated electrode because the dynamics of the electron transfer process between the electrode and the photochemical reaction products may overcome the rapid back electron transfer reaction of the products. 

Since the fate of the redox pair must be back electron-transfer (in the absence of a diversionary reaction), interest centers on chemical reactions fast enough to obviate the back electron transfer process in Scheme V. Clearly, instability in the reduced acceptor or oxidized donor can promote efficient photoreaction. For example, oxidized alkylmetal donors are unstable, and the charge-transfer photolysis of R4M (M = Sn and Pb) in molecular complexes with TCNE is a convenient source of alkyl radicals [206] ... 

Such deviations can be due to a number of quantum mechanical contributions to the electron transfer process. The quantum yield for the photoreaction between Ru(bpj)3 and TI is 2. This value, which shows that the absorption of a single photon causes the formation of 2 moles of Ru(bp 3)3, results from the sequence of reactions shown in Eqs. (5.11) and (5.12), whereby the initially formed Tl ion oxidizes a ground state molecule of Ru(bp5)3 to Ru(bp3)3, thereby yielding the second molecule of product ... 

Photoreaction of 2-pyridone with aliphatic and aromatic amines leads to addition of the amines at C4 and C6 to give mixtures of dihydropyridone products (Scheme 15). All of these reactions appear to derive from a single electron transfer process. Tertiary aliphatic amines such as triethylamine 150 yield pyridones 151 and 152 (2 1) plus the reductively coupled pyridone dimer 153. Pyrrole 154 leads to a similar mixture of 4- and 6-substituted dihydropyridones 155 and 156 (1 1). Dimethylpyrrole 157 and indole 158 lead to analogous products of 4- and 6-substition of the pyridone (at C3 of the pyrrole 157). N-Methyl pyrrole 159, however, does not yield photoproducts. When the N-methyl pyrrole is alkyl substituted at C2 (160), addition to the pyridone yields 161 and 162 (1 1). ... 

---