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

Abstract

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 •+ −C 60 •- ) 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 states of the C 60 moiety. In nonpolar solvents such as benzene ( ε 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 ( ε 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 state to the C 60 . 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 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 reorganization energies of porphyrins and fullerenes in electron-transfer processes.