Photoinduced electron transfer intersystem crossing

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

The radical anions of tram- and c/s-stilbene can be distinguished by ESR [503]. A radical ion pair has been observed by this method using trial-kylamines in acetonitrile [497], Electron back transfer (i.e., a reaction of the radical anion of frans-stilbene with the radical cation of the donor) opens a new pathway for intersystem crossing to the tram triplet state. Time-resolved resonance Raman spectroscopy of photoinduced electron transfer from amines to frans-stilbene has been reported [497,504], In the photooxidation of frans-stilbene the radical cation has been observed by flash photolysis using cyanoanthracenes [505-507], The radical cations of cis-and frans-stilbene were also produced by electron transfer from biphenyl to excited 9,10-dicyanoanthracene and subsequent electron transfer from stilbene to the radical cation of biphenyl [508]. External magnetic field effects... 

In conclusion, photoinduced electron transfer fi om the sensitizer RuN3 to nanostructured Ti02 occurs along two pathways. The major part of the molecules ( 60 %) inject directly fiom the non-thermalized MLCT state prior to IVR and intersystem crossing, while the remaining part inject from the thermalized triplet state of the non-attached ligand, in a process controlled by interligand electron transfer. 

MEH-PPV. Upon adding Ceo, the triplet signal for the 1.35 eV PIA band is completely quenched. Instead a strong spin =1/2 signal dominates, indicated charged polarons as photoexcitation on the polymer donor (Figure 8.13 [59]). This confirms that the photoinduced electron transfer occurs on a time scale sufficiently fast to quench the intersystem crossing to the triplet state. 

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

Switching systems based on photochromic behavior,I29 43,45 77-100 optical control of chirality,175 76 1011 fluorescence,[102-108] intersystem crossing,[109-113] electro-chemically and photochemical induced changes in liquid crystals,l114-119 thin films,170,120-1291 and membranes,[130,131] and photoinduced electron and energy transfer1132-1501 have been synthesized and studied. The fastest of these processes are intramolecular and intermolecular electron and energy transfer. This chapter details research in the development and applications of molecular switches based on these processes. 

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