Chapter 8: Electron-Transfer Processes of Fullerenes Studied with Time-Resolved Spectroscopies

Photoinduced ET processes of fullerenes have been reviewed in this chapter on the basis of data collected from the optical time-resolved spectral methods such as fluorescence lifetime measurements and transient absorption spectra in wide wavelength and wide time regions. For mixture systems of fullerenes with electron-donor, the partitions of ¹C₆₀*/¹C₇₀* and ³C₆₀*/³C₇₀* depend on the donor concentration and solvent polarity as obtained by the transient spectra showing both the absorption bands of the reactants and the products without overlap. For many C₆₀–donor diads, the radical ion pairs (C₆₀˙⁻–donor˙⁺) have longer lifetimes compared with conventional acceptor–donor diads. This is reasonably interpreted by the inverted region of the Marcus parabola due to small reorganisation energy, in addition to the triplet-spin character of the radical ion pairs. When a second donor is attached to the original C₆₀–donor diads, the lifetimes of the radical ion pairs are further prolonged due to the distant cation radical from the anion radical centre. When C₆₀ and donor are connected with long-conjugated chains, distant charge-separation processes through the π-conjugated bridges become possible. Within the supramolecular assembly both through-bonds and through-space charge-separation processes take place. In the case of rotaxanes containing the fluctuating donor and acceptor, the through-space charge-separation process tends to take place via the triplet states. When C₆₀ molecules are covalently embedded in the hole-transporting polymers, long-range hole migration along the polymer chain takes place. With these fundamental studies of photoinduced electron-transfer processes of the fullerene derivatives, many new aspects for photochemistry and photophysics are developing as summarised in this chapter.