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

摘要

Photoinduced electron transfer processes in zincporphyrin-C_(60) dyad (ZnP_C_(60)) in different organic solvents have been examined by fluorescence lifetime measurements and pico- and nanosecond time-resolved transient absorption spectroscopies. Irrespective of the solvent polarity, photoinduced electron transfer takes place from the porphyrin singlet excited state to the C_(60) moiety to produce a charge-separated state (ZnP~(·-)).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 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). In more polar solvents such as anisole (ε_s = 4.33) the energy level of the charge-separated state is located between those of the C_(60) singlet and triplet excited states. As a result photoinduced charge separation occurs from the porphyrin to the C_(60) singlet excited state as well as from the porphyrin singlet excited state to the C_(60). In addition, the resulting charge-separated state decays to the C_(60) triplet excited state (1.50 eV) rather than the ground state. In polar solvents such as THF (ε_s = 7.58), benzonitrile (ε_s = 25.2), and DMF (ε_s = 36.7), the energy level of the charge-separated state (1.21-1.42 eV) is lower than that of the C_(60) triplet excited state. Thus, all the excited states (i.e. the porphyrin and C_(60) excited singlet and triplet states) lead to the production of the charge-separated state, which slowly decays 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.