Electron transfer cosensitization

With respect to photoinitiation, generally, it is important to be very careful in one s choice of sensitizers. For example, attempts to initiate the cyclization of homobenzylic ethers failed if 1,4-dicyanobenzene was used as a sensitizer. Rapid regeneration of the starting material by back-electron transfer from the dicyanobenzene anion-radical to the substrate cation-radical was the cause of cyclization inefficiency. To slow this unproductive process, a mixture of A-methylquinolinium hexafluorophosphate (sensitizer), solid sodium acetate (buffer), and tert-butylbenzene (cosensitizer) in 1,2-dichloroethane was employed. This dramatically increased the efficiency of the reaction, providing cyclic product yields of more than 90% in only 20 min (Kumar and Floreancig 2001, Floreancig 2007). 

Sensitized PET reactions are often very slow and have low quantum yields due to dominating back-electron transfer. In these cases, the addition of cosubtrates (e.g., biphenyl or phenanthrene to DCA- or DCN-sensitized reactions) is useful. The use of such an additive is called cosensitization. In these reactions, the substrate is not oxidized (or reduced) by the excited sensitizer but by the radical ion of the cosensitizer (ET, ). This is a thermal electron-transfer step without the problems of back-electron transfer. The key step is the primary PET process (ETJ in which the cosensitizer radical ion is formed. The main characteristic of cosensitization systems is the high quantum yield of the free-radical ion (e.g., overall quantum yield is high and the reaction is fast (Scheme 7). 

An additional effect of the cosensitization may lead to different products or product ratios, which is caused by the efficient sensitizer radical ion—substrate ion separation. This separation inhibits the early back-electron transfer to the substrate radical ion or early intermediates and favors products of complex reaction pathways (late ETc)... 

Mattay and coworkers extended the investigations of photoinduced electron tansfer from C6o to excited sensitizers and cosensitizers (Scheme 4) [173-175], They used dicyanoanthracene (DCA), dicyanonaphthaline (DCN), A-methylacri-dinium hexafluorophosphate (MA+), and triphenylpyrylium tetrafluoroborate (Tpp+) as sensitizers. In the case of DCA, DCN, and MA+, the addition of a cosensitizer (biphenyl) was necessary to produce the fullerene radical cation in sufficiently high yields [175], Otherwise, fast back-electron transfer seems to be predominant. However, by using TPP+ the formation of Q,o could be detected by EPR measurements even in the absence of a cosensitizer. This can be explained by (1) the high reduction potential (Ejed = 2.53 V vs SCE) and (2) the neutral form of the reduced sensitizer (electron shift) [174-176], Nevertheless, no influence of the cosensitizer on the EPR signal was observed under irradiation [175],... 

As a first example, the photochemical synthesis of substituted 1,2-dihydro-[60]fullerenes will be discussed. These compounds can be synthesized by various photochemical reaction pathways. In the first one the radical cation Qo is involved in the reaction. In 1995, Schuster et al. reported the formation of C6o radical cations by photosensitized electron transfer that were trapped by alcohols and hydrocarbons to yield alkoxy or alkyl substituted fullerene monoadducts as major products [211], Whereas Foote et al. used N-methylacridinium hexafluorophos-phate NMA+ as a sensitizer and biphenyl as a cosensitizer [167], Schuster et al. used 1,4-dicyanoanthracene (DCA) as a sensitizer for the generation of C 6o- The... 

The above considerations suggest that the most effective cosensitizers for DCA-sensitized reactions will be those with oxidation potentials in the range 1.8-2.0 V vs SCE. In fact, in this range the cosensitizers will poorly quench the DCA fluorescence, but will have high enough oxidation potentials to maximize the cage escape efficiency of the radical cation from the pair in order to facilitate secondary electron-transfer processes with the substrates. 

Photoinduced electron transfer (PET) is of current interest and numerous electron transfer sensitizers have been employed not only for mechanistic but also for synthetic purposes [135], effectiveness of PET being further improved by cosensitization technique [136]. PET and Chemical Electron Transfer (CET) have been conducted in solution and within zeolite cavities for the bicyclo [2.1.0] pentanes [137]. The results indicated the CET method to be comparatively better and providing mechanistic insights on the regioselectivity (effective charge localization) and... 

Schaap, A.P., Siddiqui, S., Pradad, G., Palomino, E., and Lopez, L., Cosensitized electron transfer photo-oxygenations. The photochemical preparation of 1,2,4-trioxolanes, 1,2-dioxolanes and l,2,4-dioxazohdines,/. Photochem., 25,167,1984. 

Liu, Z., Zhang, M., Yang, L., Liu, Y, Chow, Y.L., and Johansson C.I., Electron transfer photoisomerization of norbomadiene to quadricyclane cosensitized by dibenzoyhnethanatoboron difluoride and aromatic hydrocarbons, /. Chem. Soc., Perkin Trans. 2, 585-590,1994. 

Schaap, A. R, Siddiqui, S., Gagnon, S. D., and Lopez, L., Stereoselective formation of cis-stilbene ozonide from the cosensitized electron-transfer photooxygenation of cis- and trans-2,3-dipheny-loxiranes, /. Am. Chem. Soc., 105, 5149,1983. 

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