Excited-state reactions fullerenes

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

Mittal JP (1995) Excited-States and Electron-Transfer Reactions of Fullerenes. Pure and Applied Chemistry 67 103-110. 

In principal, electron transfer reactions with fullerenes could occur via both the singlet- and triplet-excited state. However, due to the short singlet lifetime and the efficient intersystem crossing, intermolecular electron transfer reactions usually occur with the much longer lived triplet-excited state. The result of the electron transfer is a radical ion pair of fullerene and electron donor or acceptor. 

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. 

As a matter of fact, we may assume that the singlet excited state energies of all oFLs (2.70 eV) are quantitatively transduced to C o (1.76 eV). This is followed by an efficient intersystem crossing to yield the fullerene triplet excited state. The energy transfer reaction was quantified by comparing the C60 fluorescence of 5 and 6 in, for example, toluene with that of Ceo-reference 1 (6.0 x 10-4). Herby, the latter served as an internal reference when exactly the same experimental conditions are applied. Quantum yields close to 6.0 x 10 1 speak for a quantitative energy transfer in all the tested systems. 

Use of photoexcited fullerenes (i.e., the singlet or triplet excited state) widens the scope of electron-transfer reactions. This assumption is because excitation of fullerenes enhances both the electron-acceptor and -donor behavior of the photoexcited fullerenes. For example, the triplet excited state of C o, which is formed by efficient intersystem crossing (i.e. with a quantum yield close to unity) [18, 19] has a reduction potential of E°red = 1.14 V relative to the SCE [18, 19]. This potential is clearly more positive than the reduction potential of the ground state (—0.43 V) [20]. Thus, the triplet excited state of Ceo can be reduced with a variety of organic compounds yielding the Cgo radical anion and the oxidized donor [18]. 

Photoinduced electron transfer from dilferent electron donors to the triplet excited states of Ceo and C70 occurs efficiently and is typically associated with a small reorganization energy [18, 19, 21-27]. Consequently, the occurrence of fast electron-transfer events involving the fullerene excited states has been well established as giving rise to small intrinsic barriers. In contrast with the fast electron-transfer reactions of the triplet excited state of Ceo, an extremely slow electron-transfer reaction has been reported for the reaction of Ceo in its ground state with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to produce Ceo in benzonitrile. The latter can be followed even by conventional Vis-NIR spectroscopy [28]. In this instance, however, it is not clear whether the generation of Ceo is directly related to electron transfer from DBU to Ceo, or if Ceo evolves from the produet of a secondary reaction. 

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