Photoinduced electron transfer sensitization

Arylcyclopropanes and their heterocyclic analogues are liable to electron transfer induced fragmentation of a carbon-carbon bond that in some cases leads to synthetically useful products. Thus, 1,2-diarylcyclopropanes [240-243] as well as 2,3-diaryloxirans [244-246] and -aziridines (in the last case, also 2-monophenyl derivatives) [247,248] are cleaved upon photoinduced electron transfer sensitization. The final result, after back electron transfer, is trans-cis isomerization of the ring. In the presence of a suitable trap, however, a cycloaddition reaction takes place, involving either the radical cation or the ylide. Thus, dioxoles, ozonides or azodioxoles, respectively, are formed in the presence of oxygen and oxazolidines have been obtained from cyclopropanes in the presence of nitrogen oxide (Sch. 23). 

A Ci-symmetric 1,2,3,4-adduct with a Cs-symmetric bridge102 between C(2) and C(3) (( )-167, Figure 1.39) has been reported as minor product resulting from the reaction between 2-methylpropan-2-ol and C6o +, generated by irradiation in the presence of the PET-(photoinduced electron transfer) sensitizer 2,4,6-triphenylpyrylium tetrafluoroborate.323 Its formation can be understood in terms of a further transformation of the initially formed monoadduct l-([60]fullerenc-l (2//)-yl)-2-mcthylpropan-2-ol. 

The described problem was encountered in investigations of cis-trans isomeriza-tions and cycloadditions of donor olefins D and acceptor olefins A in acetonitrile. All polarizations, both of the cycloadducts and of the starting and isomerized olefins, could be traced to radical ion pairs D" + A" formed by photoinduced electron transfer. As, however, exciplexes are frequently discussed as percursors to the products in such systems,and CIDNP does not respond to exciplexes because no diffusive separation is possible, the question as to the relative contributions of the radical ion and exciplex pathways arose. To answer it, we employed photoinduced electron transfer sensitization (PET-sensitization). 

Direct photolysis of 50-53 in 02-saturated acetonitrile solution also leads to the corresponding carbazoles with a quantum yield of ca 0.64 in all cases (equation 13)161. Apparently, substituents have only a little effect on the chemical yield of carbazole produced by steady-state irradiation in aerated acetonitrile. However, an attempt to carry out such a photocyclization reaction by using photoinduced electron-transfer sensitization has failed, presumably due to fast back electron transfer that quenches the net reaction. It is also interesting to note that chemical oxidation and electrochemical oxidation of 50-53 does not result in carbazoles. Instead, benzidine products are formed. These results are consistent with the AMI calculations, which suggest that the cyclization reaction is both kinetically and thermodynamically more favorable from the triplet state than from the cation radical or dication. 

Ultrafast photoinduced electron transfer in semiconducting polymers mixed with controlled amounts of acceptors this phenomenon has opened the way to a variety of applications including high-sensitivity plastic photodiodes, and efficient plastic solar cells ... 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

The advantage of employing periodic perturbation of light intensity, e.g., using a chopper, and phase-sensitive detection are beyond a simple enhancement of the signal-to-noise ratio. For photoinduced electron-transfer mechanisms, as schematized in Fig. 11, the... 

As described above, the arrangement of the various functional moieties was controlled spatially across the films at molecular dimensions in the form of LB films. In a series of folded type of sensitizer (S) and electron-donor (D) dyads in a previous work, however, the dyad molecules in the LB films can take many conformations due to flexibility of the longer alkyl chain of the dyads so that clear dependence of the photoinduced electron transfer rate on the alkyl chain length, i.e. S-D distance, was not observed [2], By this reason, we are studying the chain length dependence by using a series of linear type S-D dyads, in which the S and D moieties were linked by a single alkyl chain. In the closely packed LB films, the alkyl chain was considered to be extended and the distance between S and D to be... 

Designing a conjugated polymer sensor based on FQ, however, is not only a matter of making a fluorescent polymer for which the photoinduced electron transfer reaction is energetically favorable. There are other important factors that must be considered and requirements that must be met to rehably detect any analyte of interest, including TNT, from the vapor phase. In the broadest sense, these considerations distill to the two primary considerations for any sensing system, sensitivity and selectivity. 

The photochemical addition of some cyclic oligosilanes to Ceo has also been found interesting. Scheme 8.8 shows some examples of such a transformation. Irradiation (X > 300 nm) of a toluene solution of disilirane 36 with Ceo afforded the fullerene derivative 37 in a 82% yield [37]. The reaction mechanism is still unknown. When toluene is replaced by benzonitrile the bis-silylated product of the solvent was obtained in good yields. In these experiments a photoinduced electron transfer between 36 and Ceo is demonstrated, indicating the role of Ceo as sensitizer [38]. The photoinduced reactions of disilirane 36 with higher fullerenes such as C70, Cv8(C2v)and CuiDi) have also been reported... 

Highly efficient and stereoselective addition of tertiary amines to electron-deficient alkenes is used by Pete et al. for the synthesis of necine bases [26,27], The photoinduced electron transfer of tertiary amines like Af-methylpyrrolidine to aromatic ketone sensitizers yield regiospecifically only one of the possible radical species which then adds diastereospecifically to (5I )-5-menthyloxy-2-(5//)-furanone as an electron-poor alkene. For the synthesis of pyrrazolidine alkaloids in approximately 30% overall yield, the group uses a second PET step for the oxidative demethylation of the pyrrolidine. The resulting secondary amine react spontaneously to the lactam by intramolecular aminolysis of the lactone (Scheme 20) [26,27]. 

Mattay et al. examined the regioselective and stereoselective cyclization of unsaturated silyl enol ethers by photoinduced electron transfer using DCA and DCN as sensitizers. Thereby the regiochemistry (6-endo versus 5-exo) of the cyclization could be controlled because in the absence of a nucleophile, like an alcohol, the cyclization of the siloxy radical cation is dominant, whereas the presence of a nucleophile favors the reaction pathway via the corresponding a-keto radical. The resulting stereoselective cis ring juncture is due to a favored reactive chair like conformer with the substituents pseudoaxial arranged (Scheme 27) [36,37]. 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

C-C Cleavage in Epoxides. Radical cations generated by photoinduced electron transfer from epoxides (130) or aziridlnes (131) also ring open, giving oxidative products in the presence of oxygen. For example, dicyanoanthracene sensitizes the conversion of aryl epoxides to ozonides, eq. 48,... 

Photoinduced Electron Transfer. Monolayer organizates are particularly suited for the investigation of photoinduced electron transfer, since the molecules are fixed and the distance between the planes at which the donor and the acceptor molecules, respectively, are located can be well defined. Therefore, complex monolayers have been arranged in order to study the distance dependence of electron transfer in these systems (2, 20). This strategy has also been used to elucidate the relative contributions of electron injection and energy transfer mechanisms in the spectral sensitization of silver bromide (21). 

Photoinduced electron transfer in the presence of a sensitizer (9,10-diphenylan-thracene) also generates the same anion radical. However, its disintegration proceeds within the solvent (acetonitrile) cage. Inside the cage, 4-nitrobenzyl radical and thiocyanate ion unite anew, but in this case by their soft-to-soft ends. This nucleophilic reaction takes place faster than the electron back-transfer occurs. The final, stable product of the whole process is 4- n i tro benzyl- iso- th iocy an ate (Wakamatsu et al. 2000) ... 

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