Electron transfer reactions of fullerenes

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

Volume 2 is dedicated to a detailed description of the most important classes of electron transfer reactions involving organic molecules (Part 2.1) and organometallic and inorganic compounds (Part 2.2). In several cases the reactions described are important not only from the viewpoint of fundamental research on reaction mechanisms, but also for their catalytic and synthetic applications. The emerging fields of electron transfer reactions of fullerenes, electron-reservoir complexes, and biomi-metic electron transfer chemistry of porphyrins are discussed in depth. 

This review has described the fundamental electron-transfer properties of fullerenes together with a variety of electron-transfer reactions of fullerenes, which often lead to a new type of derivatization of fullerenes. The small reorganization energy of fullerenes, especially in electron-transfer reactions, and the tong triplet excited sate lifetimes has rendered fullerenes as useful components in the design of novel elec-... 

Table 7.5. Kinetic parameters of electron transfer reactions of fullerenes ...

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]. 

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. 

Sun et al. further investigated the photoinduced electron transfer reactions of C6o and triethylamine, both in deoxygenated solution and air saturated solution [79], Three types of cycloadducts of fullerenes 33 and 92a-b were obtained, whereas the formation of the monoalkylated l,2-dihydro[60]fullerene 29 as described by Liou et al. [230] in the reaction of trimethylamine and /V,/V-dimcthy-laniline with C6o, was not observed (see Fig. 32). 

In future work it would be advisable to study to a greater extent the kinetics of selected organic compounds as model reactants. Though one-electron transfer reactions of such compounds are frequently very fast, the application of submicroelectrodes in the measurement of the kinetic data of such processes should provide good results. Fullerenes are other good subjects for study (for a review see [304]). 

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. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Studies that are of a general chemical nature were also common in 1993. Electron transfer reactions of metal carbonyl anions are noted by Atwood and carbonyl anions are also covered in a study dealing with their reactions with alkyl halides. CgQ (fullerene) chemistry in conjunction with metal carbonyls receives another mention this year but this reporter s expectations of a flood of like-minded papers has not materialised. 

The first chemical transformations carried out with Cjq were reductions. After the pronounced electrophilicity of the fullerenes was recognized, electron transfer reactions with electropositive metals, organometallic compounds, strong organic donor molecules as well as electrochemical and photochemical reductions have been used to prepare fulleride salts respectively fulleride anions. Functionalized fulleride anions and salts have been mostly prepared by reactions with carbanions or by removing the proton from hydrofullerenes. Some of these systems, either functionalized or derived from pristine Cjq, exhibit extraordinary solid-state properties such as superconductivity and molecular ferromagnetism. Fullerides are promising candidates for nonlinear optical materials and may be used for enhanced photoluminescence material. 

However, the initial step of the electron transfer reaction strongly depends on the solvent polarity. By changing the solvent to less polar or nonpolar solvents like benzene or nonaromatic hydrocarbons the transient absorptions of 3C 0, G)0 and donor radical cation appear immediately after the laser pulse. The decay of all the absorptions is also completed at the same time. The fast appearance and the fast decay of the Go and donor radical cation absorption suggest that there is an interaction between fullerene and donor in less polar and nonpolar solvents before laser irradiation [120,125,133-139],... 

Amongst the radiolysis products of dichloromethane is the highly oxidizing radical cation CH2Cl2 [30]. Examples of its use in studies of electron transfer reactions are the oxidation of the fullerenes Ceo [18], 75, and C78 to the corresponding fullerene radical cations Cn , and also arenes to (arene) + [30]. By measurement of the rates of the reaction in Eq. 41 for several different (arene) +, clear evidence was obtained for the Marcus inverted region (see below) from a plot of log k vs... 

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]. 

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