Linkage and Solvent Dependence of Photoinduced Electron Transfer in Zincporphyrin-C₆₀ Dyads

Four different kinds of C60-linked zincporphyrins have been prepared by changing systematically the linking position at meso-phenyl ring from ortho to para and their photophysical properties have been investigated. Regardless of the linkage between the two chromophores, photoinduced charge separation (CS) and subsequent charge recombination (CR) were observed in a series of zincporphyrin-C60 dyads by picosecond fluorescence lifetime measurements and time-resolved transient absorption spectroscopy. In THF the CS occurs from both the excited singlet state of the porphyrin and the C60 moieties, implying that the increase of the absorption cross section by both the chromophores results in the efficient formation of the ion pair (IP) state. On the other hand, in benzene the IP state generated by the photoinduced CS from the excited singlet state of the porphyrin to the C60 produces or energetically equilibrates with the locally excited singlet state of the C60. Both the CS and CR rates for the meta isomer are much slower than those for the other porphyrin-linked C60. Linkage dependence of the electron transfer (ET) rates can be explained by superexchange mechanism via spacer. These results demonstrate that C60 is a new promising building block as an acceptor in artificial photosynthetic models.