Fullerene derivatives electron transfer

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. 

The structure-property relations of fullerenes, fullerene-derived solids, and carbon nanotubes are reviewed in the context of advanced technologies for carbon-hased materials. The synthesis, structure and electronic properties of fullerene solids are then considered, and modifications to their structure and properties through doping with various charge transfer agents are reviewed. Brief comments are included on potential applications of this unique family of new materials. 

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-... 

Guldi DM, Hungerbuhler H, Asmus KD (1999) Inhibition of cluster phenomena in truly water soluble fullerene derivatives bimolecular electron and energy transfer processes. Journal of Physical Chemistry B 103 1444-1453. 

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... 

These examples demonstrate the well-known process of polymerization initiated by anion-radicals. Our next consideration is devoted to an unusual case of initiation. Intercalation of fullerenes by metals results in the formation of fullerene-metal derivatives. Paramagnetic metallofullerenes (anion-radicals) are the fullerenes doped with endohedral metal. According to calculations and structural studies, LaCs2, for example, contains La in the center of one hexagonal ring of the fuller-ene cage (Akasaka et al. 2000, Nishibori et al. 2000, Nomura et al. 1995). Intrafullerene electron transfer in metallofullerenes is possible (Okazaki et al. 2001). 

The Sc -promoted photoinduced electron transfer can be generally applied for formation of the radical cations of a variety of fullerene derivatives, which would otherwise be difficult to oxidize [135]. It has been shown that the electron-transfer oxidation reactivities of the triplet excited states of fullerenes are largely determined by the HOMO (highest occupied molecular orbital) energies of the fullerenes, whereas the triplet energies remain virtually the same among the fullerenes [135]. 

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. 

A similar supramolecular approach, in which both n-n stacking stacking of pyrene on the SWNT surface and alkyl ammonium-crown ether interactions were used in the self-assembly process of a fullerene derivative with SWNTs, was recently reported (Scheme 9.22).72 The nanohybrid integrity was probed with various spectroscopic techniques, , and electrochemical measurements. Nanosecond transient absorption studies confirmed electron transfer as the quenching mechanism of the singlet excited state of C60 in the nanohybrid resulting in the formation of SWNT"1"/ Pyr-NH3 + /crown- charge-separated state. 

Considering the large variety of fullerene adducts, only a few investigations on photochemical processes of fullerene adducts have been reported. These studies deal mainly with reductive processes due to the easy reduction of fullerenes and its adducts. Studies on electron transfer processes of several pyrrolidino fullerenes (Fig. 20) with donors such as dimethylaniline (DMA) or tetrakis(dimethy-lamino)ethylene (TDAE) show that the electron transfer rate constants decrease compared to that of C6o (3.5 X 109 M 1 s 1 for C6o, 0.5 X 109 M 1 s 1 for the derivatives (Table 6, Fig. 21) [179], This can be interpreted in terms of decreasing u-conjugation of the C60 moiety, which causes an increase in LUMO energy level and a decrease in the lowest triplet energy. Therefore, the substituents influ-... 

Changing the solvent from polar to less polar solvents effects not only the electron transfer but also the back-electron transfer. Back-electron transfer rate constants are in less polar solvents larger than those in polar solvents, which can reasonably be interpreted in terms of desolvation process and loose in ion pair formation. The transient absorptions of the pyrrolidino fullerene radical anions are slightly blue-shifted compared to that of Qo (Qo 1076 nm, derivatives radical anions 991-1002 nm) [179],... 

Mittal et al. investigated the electron transfer from diphenylamine (DPA) or triphenylamine (TPA) to two pentaphenylated fullerene derivatives (PPF). The... 

Therefore, inhibiting cluster formation is a possible way to avoid the problem of suppressed electron transfer. There are several methods to reduce cluster formation, e.g., by capping the surface with surfactants like lauryl-sulfate or cetyltrimethylammonium chloride, or by incorporating the fullerene derivatives into the cavity of -y-cyclodextrines [182-185,187], Transient absorption spectroscopy show that excitation to the singlet-excited state and intersystem crossing to the triplet are not effected by surfacting or incorporating fullerene derivatives... 

It was clearly shown by EPR measurements that irradiation of aqueous suspensions of Ti02 and surfacted or encapsulated fullerene derivatives leads to the formation of fullerene radical anions by electron transfer from excited Ti02. The reduction of the embedded derivative compared to the surfacted one is more efficient [183], However, in comparison to organic solvents, the reduction yield of the fullerenes radical anions in aqeous media is lower. This may be explained by the influence of the aquatic environment and the stability of the radical anions formed [183,184], Nevertheless, electron transfer still occurs from the donor to the fullerenes triplet-excited state [182,185,186],... 

Bunker et al. also reported approximately three aminofullerene derivatives (3, 32, 33, Fig. 25) that show intramolecular electron transfer from the amino group to the singlet excited state of the fullerene core [189], Although the intramolecular electron transfer shares some characteristics with the classical twisted intramolecular charge transfer in molecules represented by p-N,N-dimethylaminobenzonitrile the amino-C6o derivatives are in fact better classified as redoxdyads [189],... 

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