Fullerene ground state complex

Kim et al. observed a very fast ion pair formation (below their detection limit of about 1 ps) from transient absorption spectra of fullerenes in the presence of aromatic amines such as /V,/V-dimcthyl- or /V,/V-dicthyl-anilinc, corresponding to a rate > 1 X 1012 M-1 s-1. An explanation for such extremly fast electron transfer is most likely a ground-state complex of fullerene and amine. Excitation leads to the neutral aminc/ C 0 contact pair followed by electron transfer. The decay of the both transient absorption from Cfo and Qo/amine occurs with the same rate suggesting that charge recombination is the major nonradiative relaxation channel [138],... 

The formation of fullerene-amine 1 1 ground state complexes was examined by use of a chemometric method principal component analysis (PCA) [89]. With the digitization of n observed absorption spectra to form a data matrix Y... 

Apart from zinc(II) (na)phthalocyanines, the ruthenium(II) analogs have also been used to complex with pyridyl fullerenes. Arrays 33-35 show electronic coupling between the two electroactive components in the ground state as shown by UV-Vis spectroscopy and electrochemical measurements [45], The use of ruthe-nium(II) instead of zinc(II) phthalocyanines reduces the undesired charge recombination, increasing the lifetime of the radical ion pair state (on the order of hundreds of nanoseconds). 

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. 

A similar treatment in terms of a Perrin-type formulation for static quenching was applied to the fullerene-donor systems without invoking contributions due to the excitation of ground state fullerene-donor complexes [88], In room-temperature toluene, upward curvatures were observed in quenchings of Cgo and C70 fluorescence intensities by DMA and DEA (Fig. 30). The upward curvatures were also treated by including static fluorescence quenching in terms of the equation as follows [88,94]. 

The extreme solvent sensitivity of the exciplex fluorescence is very interesting. Fullerene-amine exciplex emissions observed in saturated hydrocarbon solvents are absent in solvents such as benzene and toluene (27,84,88,101), which has been explained in terms of solvent polarizability effects [101]. However, there has also been an explanation [84] that the formation of exciplexes in a solvent such as benzene is hindered by specific solute-solvent interactions that result in complexation between the fullerene and solvent molecules. The two explanations are fundamentally different. In the former, the exciplex state is effectively quenched through a radiationless decay pathway facilitated by a stronger dielectric field of the solvent. However, the latter assumes that the ground state fiillerene-solvent complexation prevents the formation of fullerene-donor exciplexes. In order to understand whether the extreme solvent sensitivity is solvent specific (limited to benzene, toluene, and other aromatic solvents) or solvent property specific (solvent polarity and polarizability), fluorescence spectra of C70-DEA were measured systematically in mixtures of hexane and a polar solvent (acetone, THF, or ethanol) with volume fraction up to 10% [101]. The results are consistent with the explanation of solvent polarity and polarizability effects. 

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. 

The incorporation of the functionalized fullerene into a host molecule, such as a y-cyclodextrin or surfactants is an elegant way to bypass the aggregation of C6oC(COO )2-As demonstrated in studies with [60]fullerene this host can accommodate only a single fullerene molecule, which still has access to the solvent phase. The ground state spectrum of this guest-host complex shows the same narrow bands as, for example, monomeric C6oC(COOEt)2 or Cgo/Y-CD and clearly differs from the presumed CeoCfCOO )2 n cluster. 

Yoshida et al. reported that Qo forms the l 2-complex with y-CD (Fig. 16), which shows good solubility in water [74]. Andersson et al. reported that C70 also forms the complex with y-CD [75]. Since the spectral shapes of the ground state and triplet excited state are not changed by the inclusion in y-CD, the interaction between fullerenes and y-CD is quite weak. It was revealed that the bimolec-ular excitation-relaxation and electron-transfer processes of the inclusion complex of fullerene in y-CD are changed in comparison to the pristine fullerenes [29]. For example, the rate constants for the triplet-triplet annihilation processes of C6q and C70 in y-CD are much smaller than... 

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. 

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. 

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