Factors controlling lifetimes of photoinduced charge-separated states of fullerene-donor molecular systems

Lifetimes of the photoinduced charge-separated states for composite molecular systems of covalently bonded fullerenes with electron donors are usually very long compared with those of the flat electron-acceptor molecules with functional groups such as keton and cyano-groups. In order to confirm such long-lived charge-separated states, it is very important to carefully identify the transient radical ion pairs by observing both the radical anions and the radical cations in the same time. However, in general, assignments of the transient species are not easy, because the absorption bands overlap with those of other species such as short lived S1-states and long-lived T1-states. In this review, we selected reliable data of the dyads studied mainly by the transient absorption spectral methods in the wide wavelength regions (UV–vis–NIR) and wide time regions (picosecond, nanosecond, microsecond, and millisecond). The lifetimes of the charge-separated states evaluated at room temperature are summarized in order to reveal the factors controlling the lifetimes of photoinduced charge-separated states of fullerene-donor molecular systems. In most cases, the rate parameters and efficiencies for photoinduced charge-separation and charge-recombination processes can be reasonably interpreted by the concepts based on the Marcus theory; some Marcus parameters were experimentally evaluated by temperature dependency of the rate parameters. In addition, spin-multiplicity of the charge-separation precursors and generated radical ion pair may play important roles. As a whole, selections of the kinds of the electron-donors, lengths of the bridges, solvent polarities, which strongly affect the photoinduced electron transfer processes, are all important to achieve the long lifetimes of the charge-separated states.