Photoinduced electron transfer in a symmetrical diporphyrin-fullerene triad

Two triad molecules consisting of either two zinc, or two free-base porphyrins symmetrically joined to a fullerene via phenyleneethynylene- containing linkages have been synthesized, and their photochemistry investigated. In the zinc form of the triad. P_Zn-C ₆₀-P_Zn, excitation of a zinc porphyrin in 2-methyltetrahydrofuran solution is followed by photoinduced electron transfer to the fullerene with a time constant of 20 ps. The resulting P _Zn^.+-C₆₀^.--P_Zn charge-separated state is formed with a quantum yield of 98% and has a lifetime of 820 ps. The first excited singlet state of the free-base analog gives the P_2H^.+-C₆₀^.--P_2H charge-separated state with a time constant of 200 ps and a yield of 98%. The charge-separated state decays with a lifetime of 2.8 ns. The difference in the rates of photoinduced electron transfer is consistent with reaction in the normal region of the Marcus-Hush relationship of transfer rate and driving force, and charge recombination is consistent with Marcus-Hush inverted behavior. The presence of the two porphyrin electron donors in these triads enhances the absorption cross section for light collection, and the molecular framework employed could be used to prepare molecules with enhanced energy conversion or optoelectronic properties.