Solvent dependence of charge separation and charge recombination rates in porphyrin-fullerene dyad

Photoinduced processes in zinc porphyrin-C₆₀ dyad (ZnP-C₆₀) in different organic solvents have been investigated by fluorescence lifetime measurements and pico- and nanosecond time-resolved transient absorption spectroscopies. Irrespective of the solvent polarity, the charge-separated state (ZnP⁺-C₆₀2^-) is formed via photoinduced electron transfer from the excited singlet state of the porphyrin to the C₆₀ moiety. However, the resulting charge-separated state decays to different energy states depending on the energy level of the charge-separated state relative to the singlet and triplet excited states of the C₆₀ moiety. In nonpolar solvents such as benzene (ε_s = 2.28), the charge-separated state undergoes charge recombination to yield the C₆₀ singlet excited state, followed by intersystem crossing to the C₆₀ triplet excited state, since the energy level of the chargeseparated state is higher than that of the C₆₀ singlet excited state (1.75 eV). More polar solvents such as anisole (ε_s = 4.33) render the energy level of the charge-separated state lower than the C₆₀ singlet excited state, resulting in the direct formation of the C₆₀ triplet excited state (1.50 eV) from the charge-separated state, formed by the photoinduced charge separation from the porphyrin to the C₆₀ singlet excited state as well as from the porphyrin excited singlet state to the C₆₀. In polar solvents such as benzonitrile (ε_s = 25.2), where the energy level of the charge-separated state (1.38 eV) is low compared with the C₆₀ triplet excited state, the charge-separated state, produced upon excitation of the both chromophores, decays directly to the ground state. Such solvent dependence of charge recombination processes in ZnP-C₆₀ can be rationalized by small reorganization energies of porphyrins and fullerenes in electron-transfer processes.