Fullerene radical-anion

The mechanism of polyamine hydrogenation (Fig. 6.8) is believed to involve successive electron transfer (from polyamine to fullerene) - proton transfer (from polyamine radical cation to fullerene radical anion) steps (Briggs et al. 2005 Kintigh et al. 2007). At or near room temperature, aliphatic amines and polyamines are known to hydroaminate [60]fullerene (Miller 2006), likely also involving preliminary electron transfer - proton transfer steps followed by free radical coupling of C and N based radicals (Fig. 6.8). At elevated temperatures in polyamine solution, however, this latter free radical coupling step becomes uncompetitive with... 

Time profiles of the formation of fullerene radical anions in polar solvents as well as the decay of 3C o obey pseudo first-order kinetics due to high concentrations of the donor molecule [120,125,127,146,159], By changing to nonpolar solvents the rise kinetics of Go changes to second-order as well as the decay kinetics for 3C o [120,125,133,148], The analysis of the decay kinetics of the fullerene radical anions confirm this suggestion as well. In the case of polar solvents, the decay of the radical ion absorptions obey second-order kinetics, while changing to nonpolar solvents the decay obey first-order kinetics [120,125,127,133,147]. This can be explained by radical ion pairs of the C o and the donor radical cation in less polar and nonpolar solvents, which do not dissociate. The back-electron transfer takes place within the ion pair. This is also the reason for the fast back-electron transfer in comparison to the slower back-electron transfer in polar solvents, where the radical ions are solvated as free ions or solvent-separated ion pairs [120,125,147]. However, back-electron transfer is suppressed when using mixtures of fullerene and borates as donors in o-dichlorobenzene (less polar solvent), since the borate radicals immediately dissociate into Ph3B and Bu /Ph" [Eq. (2)][156],... 

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],... 

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],... 

The rate constant for photoinduced electron transfer k4 and the charge recombination rate constant ks are directly observed experimentally. The reciprocal of the 3-ps time constant detected in the transient absorption experiments, equals kn, 3 X 10 s. This assignment is verified by the results for a model P-C6o dyad, where the same value was obtained for the rate constant for photoinduced electron transfer. The charge recombination of (Pzp)3-Pzc-P -C6o is associated with the 1330-ps decay component observed in transient absorption, as demonstrated by the spectral signature of the fullerene radical anion with absorption in the 1000-nm region. This lifetime is within a factor of 2.5 of the lifetime observed for the P" -C6o in a model dyad (480 ps). 

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. 

The radical anion 60 can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines vide supra), ferrocenes, tetrathiafulvalenes, and thiophenes. In homogeneous systems, back electron transfer to the reactant pair plays a dominant role resulting in an extremely short hfetime of Qo In these cases, no net formation of is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors. - Selective one-electron reduction of Qq to Cjo takes place by the irradiation of Qq in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicotinamide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 10). The formation of Qq confirmed by the observation of the absorption band at 1080 nm in the NIR spectrum assigned to the fullerene radical anion. 

Again, photoinduced electron transfer to the triplet excited state Cgo is the initial step to yield t-BuBNAH and Upon C(4)-C bond cleavage of f-BuBNAH +, the f-butyl radical is formed, which readily adds to the fullerene radical anion yielding f-BuCga In competition, back electron transfer takes place. Similar results are obtained in the photochemical reaction of C70 with the NADH analogues mentioned above. The reaction mechanism remains the same in the case of f-BuBNAH. However, there are some differences in the reaction mechanism for [ (BNAlj] and BNAH. After the photoinduced electron... 

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