Anions photoinduced electron transfer

It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25, 26]. Photoinduced electron-transfer and photoelectrochemical reactions using the C o clusters have been extensively reported because of the interesting properties of C o clusters [25,26]. The M F Es on the decay of the radical pair between a Cgo cluster anion and a pyrene cation have been observed in a micellar system [63]. However, the MFEs on the photoinduced electron-transfer reactions using the Cgo cluster in mixed solvents have not yet been studied. 

The cationie dye was assoeiated with the anion (7,8,9,10,11,12 Brg-l-CBnHg) in order to dissolve it in the organie phase. The polarizable window available for photoinduced electron transfer in this system extended over 100 mV for the eonditions specified in Fig. 13. The photoeurrent responses were measured under ehopped light and lock-in detection at 8.4 Hz. Figure 13(b) shows that some photoresponses oeeur only in the presence of the dye speeies, whieh the authors attributed to the transfer of Ru(bpy)3" to the aqueous phase as a result of interfaeial polarization induced by the ehopped light [130]. Upon addition of in the aqueous phase, an inerease in the amplitude of the photo-... 

Soumillion, J.-P. Photoinduced Electron Transfer Employing Organic Anions. 168, 93-141 (1993). 

Photoinduced electron transfer processes between Ru(L)32+ and several quenchers (Scheme 6) have been investigated in the presence of anionic PAMAM dendrimers [16,18, 19, 23]. 

J. L. Habib Jiwan and J. P. Soumillion, Electron transfer photochemistry initiated from a twisted intramolecular charge transfer state used as an electron donor and as an acceptor, J. Photochem. Photobiol. A Chem. 64, 145-158 (1992) J. P. Soumillion, Photoinduced electron transfer implying organic anions, in Topics in Current Chemistry, Vol., Photoinduced Electron Transfer (J. Mattay, ed.), Springer Verlag, Berlin, pp. 93-141 (1993). 

Some cation-radicals can appear as hydrogen acceptors. Thus, fullerene Cgg is oxidized to the cation-radical at a preparative scale by means of photoinduced electron transfer. As in the case of anion-radical, the fullerene Cgo cation-radical bears the highly delocalized positive charge and shows low electrophilicity. This cation-radical reacts with various donors of atomic hydrogen (alcohols, aldehydes, and ethers) yielding the fullerene 1,2-dihydroderivatives (Siedschlag et al. 2000). 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

VII. METAL-ION-PROMOTED PHOTOINDUCED ELECTRON TRANSFER VIA BINDING OF RADICAL ANIONS WITH METAL IONS... 

As described earlier, the reactivity of photoinduced electron transfer is remarkably enhanced by the complexation of excited states with metal ions. Even if there is no direct interaction between excited states and metal ions, however, metal ions can enhance the reactivity of photoinduced electron transfer when the radical anion produced in photoinduced electron transfer binds with metal ions [11,12,25]. For example, although there is no direct interaction between the triplet excited state of Ceo ( Ceo ) and Sc(OTf)3, an efficient electron transfer occurs from Ceo to p-chloranil (CI4Q) to produce Ceo" and the p-chloranil radical anion CUQ -Sc complex [135]. In contrast to the facile reduction of Ceo,... 

A similar reaction has been observed experimentally (a) M. Chakraborty, C. A. Tessier, and W. J. Youngs, J. Org. Chem. 1999, 64, 2947. A photochemical C1C5 endiyne cyclization has recently been observed, but follows probably not a biradical mechanism via a fulvenediyl, but involves photoinduced electron transfer to the endiyne with subsequent cyclization of the radical anion (b) I. V. Alabugin and S. V. Kovalenko, J. Am. Chem. Soc. 2002, 124, 9052. 

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. 

These are photoinduced electron transfer reactions between two ions. The closed-shell ions then form free radicals which can be charged or neutral, these primary photochemical products being very reactive. One example of this process is the electron transfer between a uranyl cation and a nitrate anion... 

Since most organic compounds, in particular n acceptors, have small reorganization energies, the change of redox potentials by the interaction of the corresponding radical anions with M may be the main factor to accelerate the rates of photoinduced electron transfer. Thus, any material M that can stabilize the radical anions by complexation may act as an efficient... 

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