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

Photoinduced processes in zinc porphyrin-C-60 dyad (ZnP-C-60) 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.+-C60(.-)) is formed via photoinduced electron transfer from the excited singlet state of the porphyrin to the C-60 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 stares of the C-60 moiety. In nonpolar solvents: such as benzene (epsilon (s) = 2.28), the charge-separated state undergoes charge recombination to yield the C-60 Singlet excited state, followed by intersystem crossing to the C-60 triplet excited state, since the energy level of the charge-separated state is higher than that of the C-60 singlet excited state (1.75 eV), More polar solvents such as anisole (epsilon (s) = 4.33) render the energy level of the charge-separated state lower than the C-60 Singlet excited state, resulting in the direct formation of the C-60 triplet excited state (1.50 eV) from the charge-separated state, formed by the photoinduced charge separation from the porphyrin to the C-60 singlet excited state as well as from the porphyrin excited singlet slate to the C-60. In polar solvents such as benzonitrile (epsilon (s) = 25.2), where the energy level of the charge-separated state (1.38 eV) is low compared with the C-60 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-C-60 can be rationalized by small It organization energies of porphyrins and fullerenes in electron-transfer processes.