Ultrafast photodynamics of exciplex formation and photoinduced electron transfer in porphyrin-fullerene dyads linked at close proximity

The ultrafast photodynamics of porphyrin-fullerene dyads in which the distance between the porphyrin and C₆₀ moieties is varied systematically at close proximity has been examined using fluorescence up-conversion and pump-probe transient absorption techniques with time resolutions of ca. 100 fs. The porphyrin-fullerene dyads examined are MP-D-C₆₀ (M = Zn and 2H) in which the C₆₀ moiety is directly connected with the porphyrin ring at the meso position and MP-O-C₆₀, MP-M-C₆₀, and MP-P-C₆₀ in which the C₆₀ moiety is linked with porphyrin moieties through the benzene ring at the ortho, meta, and para positions, respectively. The charge transfer (CT) bands are observed for MP-D-C₆₀ and MP-O-C₆₀, whereas no CT band is seen or MP-M-C₆₀ and MP-P-C₆₀. Time-resolved absorption spectral measurements indicate that the photoexcitation of ZnP-D-C₆₀ in benzonitrile (PhCN) results in formation of the exciplex, which decays to the ground state without forming the charge-separated state. The strong interaction between the ZnP and the C₆₀ moieties due to the short linkage distance in ZnP-D-C₆₀ as indicated by the observation of the strongest CT band at the ground state results in formation of the exciplex. The energy of the exciplex is lower than that of the charge-separated state even in a polar solvent such as PhCN. In contrast, the photoexcitation of the dyad with longer linkage, ZnP-O-C₆₀, in PhCN results in formation of the charge-separated state via the exciplex formation, which is higher in energy than the charge-separated state. The photodynamics of exciplex formation of porphyrin-C₆₀-linked dyads with a short linkage is characterized by the extremely fast formation rate from the singlet excited states of porphyrins involving both the second and first excited states due to the interaction between the porphyrin and C₆₀ moieties, which are placed at close proximity. In the case of MP-D-C₆₀, the exciplex formation from the first singlet excited state of MP occurs at an ultrafast time scale with a time constant of 160 fs and that from the second singlet excited state occurs faster with a time constant less than 50 fs.