Solvent Dependence of Charge Separation and Charge Recombination Rates in Porphyrin−Fullerene Dyad

Abstract

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•+−C60•-) 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 (εs = 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 charge-separated state is higher than that of the C60 singlet excited state (1.75 eV). More polar solvents such as anisole (εs = 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 (εs = 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.