Solvent Polarity Dependence of Photoinduced Charge Separation and Recombination Processes of Ferrocene−C₆₀ Dyads

Photoinduced charge-separation and charge-recombination processes of the three ferrocene−C₆₀ dyads have been investigated by using laser flash photolysis methods. From molecular orbital calculations, the HOMO states are delocalized over the ferrocene and linking phenyl-ethenyl group connecting ferrocene and C₆₀, while the LUMO states are on C₆₀. From short fluorescence lifetimes (≤ 120 ps) of the fullerene moieties of the dyads, charge separations were suggested. Charge-separated states in the dyads were observed upon femtosecond laser irradiation. The rate constants for the charge separation were on the order of 10¹⁰ s^-1, and the quantum yields were close to unity in various solvents, such as benzonitrile, anisole, and toluene. It was confirmed that the absorption bands of the radical cation moiety shifted to longer wavelength side in polar solvents, probably due to structural change in the charge-separated states. The lifetimes of the charge-separated states of the dyads with longer linkage became longer, up to several 10s of nanoseconds in less polar solvents, while the lifetimes in polar solvents were shorter, since the charge-recombination processes are in the Marcus “inverted region”. On the other hand, the lifetime of the charge-separated state in the dyad with an acetyl linking group was on the order of nanoseconds in all of the solvents because of fast charge recombination of shorter distance. It was confirmed that the charge-recombination process generated the ground state of the dyads predominantly.