Charge transfer complexes with fullerenes

Owing to their electron-seeking properties, fullerenes form charge-transfer complexes with the substances capable for donating their electrons [16-19]. 

Electrochemical studies have shown that Ceo is easily reduced (E1/2 = —0.21 and 0.33 V vs Ag/AgCl in tetrahydrofuran and benzonitrile, respectively [52] and —0.42 V vs SCE in benzonitrile [53, 54]). Up to six electrons can be added reversibly [55]. Several authors have shown that the fullerenes form charge-transfer complexes with amines [33, 56-59]. Wudl et al. have shown that Cgo reacts chemically with amines, giving various substitution products [60, 61]. Since the reduction potential of Ceo should be higher than that of the ground state by the amount of the triplet energy [62,63], its first reduction potential should be near 1.14 V vs SCE in benzonitrile [64]. The triplet should therefore be easily reduced by electron transfer from electron donors of lower oxidation potential. 

Fullerenes are excellent electron acceptors. The early examples for the high electron affinity of fullerenes include efficient nucleophilic addition reactions of fullerenes with electron donors such as primary and secondary amines. Since then, there have been many studies of electron transfer interactions and reactions involving fullerene molecules. It is now well established that both ground and excited state fullerene molecules can form charge transfer complexes with electron donors. The photochemically generated fullerene radical anions as a result of excited state electron transfers serve as precursors for a wide range of functionalizations and other reactions. 

Tetrakis(dimethylamino)ethylene (TDAE) has a strong donor ability to form strong charge-transfer complexes with electron acceptors. It has been reported that charge-transfer complex salts of fullerenes with TDAE show ferromagnetism at low temperature [5]. It was revealed that in polar solvents can be reduced in the presence of TDAE without photoirradiation (Fig. 12) [70]. From the relation between the... 

Compared with the variety of existing carbon or nitrogen nucleophiles that were subjected to nucleophilic addition to there are few examples for phosphorus nucleophiles. Neutral trialkylphosphines turn out to be to less reactive for an effective addihon to Cjq even at elevated temperatures [114], Trialkylphosphine oxides show an increased reactivity. They form stable fullerene-substituted phosphine oxides [115] it is not yet clear if the reaction proceeds via a nucleophilic mechanism or a cycloaddition mechanism. Phosphine oxide addition takes place in refluxing toluene [115], At room temperature the charge-transfer complexes of with phosphine oxides such as tri-n-octylphosphine oxide or tri-n-butylphosphine oxide are verifiable and stable in soluhon [116],... 

Hence Q)0 fullerene molecules in aromatic solvents can a priori enter into intermolecular charge-transfer interactions with aromatic hydrocarbon molecules to form complexes of the donor-acceptor type. 

Tens of conductive LB films have been developed so far, including metallic and superconductive LB films. These LB films are classified into the categories anion radical salt, charge-transfer complex, cation radical salt, conducting polymer, and transition metal complex in this section. The LB films, with metallic temperature dependences of conductivity, and the fullerene LB films, which exhibit a superconducting transition, are discussed separately. 

Fullerene-Doped Polyvinylcarbazole. Fullerenes are known to be good electron acceptors. In the presence of electron donors such as aromatic amines, weakly bonded charge-transfer complexes can be formed [115]. Through virtual excitation, the existence of charge-transfer interaction can enhance the second-order optical nonlinearity of fullerenes [116], With direct excitation, excited state electron transfer between fullerenes and various electron donors such as aromatic amines [115,117], semiconductor colloids [118], porphyrin [119], and polymers [101, 103, 120] can occur. This electron-accepting property led to the development of fullerene-doped polymeric photoconductors [101,103]. 

Fullerene-amine charge transfer complex formation has also been studied at different temperamres to determine thermodynamic parameters of the equilibria. For Cg4-DEA as an example, observed absorbances decrease systematically with increasing temperature, which is due to a shift in the complex formation equilibrium with temperature, because molar absorptivities of in toluene are essentially temperature independent [92]. 

Ground state and C70 also react readily with a tertiary aliphatic amine triethylamine (TEA) at high TEA concentrations [87]. The reaction of Cgg and TEA results in the formation of a new absorption band in the blue region, which was initially mistaken as the absorption of a Cgg-TEA charge transfer complex [85,87]. The reaction products appear to be complicated as well, whose separations and identifications remain to be completed. In the photoexcited states of fullerenes, however, reactions with TEA are more efficient even at low TEA concentrations [66,71,118]. The reaction mixture can be divided into two fractions in terms of the solubility in toluene. The relative quantities of the two fractions are somewhat dependent on irradiation time. 

Tsai, H., Xu, Z., Pai, R.K., Wang, L., Dattelbaum, A.M., Shreve, A.P., et al. Structural dynamics and charge transfer via complexation with fullerene in large area conjugated polymer honeycomb thin filmsf. Chem. Mater. 23, 759-761 (2010)... 

Morana et al. investigated the effect of ODT on the formation of the charge transfer complex (CTC) for C-PCPDTBT and Si-PCPDTBT [91]. Despite the pristine C-PCPDTBT, no changes were observed in the absorption spectrum of the Si-PCPDTBT films prepared with ODT. Enhanced phase segregation in the C-PCPDTBT films upon addition of ODT caused increase in the molecular luminescence to CT luminescence ratio. This is due to the reduced concentration of CT complexes by a decrease in the contact area between the polymer and the fullerene because of phase separation. 

In this review article, conductivity and superconductivity in doped fullerenes are described with reference to important literature. The review covers the geometry of the fullerenes (Cgo and tubules), and the structure of their solids before and after doping, being followed by the electronic properties found in fullerenes and doped fullerenes. Conductivity and superconductivity of alkali-metal C o (AxC o, here A is the alkali-metal and X can generally vary from 1 to 6) fullerides are discussed and compared with those encountered in conventional conductors and superconductors. The formation of charge-transfer complexes of C o with other organic systems and structural/physical properties reported for them are also discussed. Lowdimensional polymer phases made in the stoichiometry... 

Fullerenes are also known to form the inclusion complexes with calixarenes. In the case of the water-soluble inclusion complex of Qo and calixarene (cationic homoox-acalix[3]arene), substantial interaction between fullerenes and the calixarene was observed in the ground state absorption spectrum [76]. Increase in the absorption intensity around 400-500 nm of C q in calixarene can be attributed to the charge-transfer complex formation due to the n -electron system of calixarene. Strong interaction between the calixarene and fullerenes was also observed in the excited states. The triplet absorption peak of 60 in the calixarene appeared at 545 nm, which is largely blueshifted compared to that of pristine Cgg- The triplet lifetime is as short as 50 ns. The substantial interaction between calixarene and included Qo was also observed in the singlet excited state as a large blueshift of the fluorescence peak. 

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