Artificial photosynthetic reaction centers with porphyrins as primary electron acceptors

A triad consisting of a carotenoid (C), a dimesitylporphyrin (P), and a tris(heptafluoropropyl)porphyrin (P _f), C-P-P _F, has been synthesized and found to undergo rapid singlet-singlet energy transfer between the porphyrin moieties so that their excited states are in equilibrium. Photoinduced electron transfer from the first excited singlet state of P, or hole transfer from the first excited singlet state of P _F, yields C-P• ⁺-P _F• ^- Electron transfer from C then yields the final charge-separated state C• ⁺-P-P _F• ^- with a quantum yield of 0.73 and a lifetime of 500 ns in tetrahydrofuran solution at ambient temperature. The final charge-separated state decays to form primarily a triplet excited state localized on the carotenoid, ³C-P-P _F, rather than the ground state. A second triad in which P is metalated (P _zn) has also been synthesized. In this system, the excited singlet states of the porphyrins are no longer in equilibrium; fast electron transfer from excited P _zn to P _F to form C-P _zn• ⁺-P _F• ^- and also fast energy transfer from P _zn to P _F with subsequent hole transfer from P _F to P _zn converge to give the same C-P _zn• ⁺-P _F• _- species, which evolves to C• ⁺-P _zn-P _F• _- with a quantum yield of 0.14. This state decays to ³C-P _zn-P _F with a quantum yield of 0.06 in tetrahydrofuran at room temperature. The charge recombination reaction follows a single-step mechanism for C• ⁺-P-P _F• ^- from room temperature to 77 K and for C• ⁺-P _zn-P _F• ^- below 250 K. Above 250 K, a two-step pathway is accessed for the recombination reaction of C• ⁺-P _zn-P _F• ^- in addition to the direct recombination mechanism. This new pathway involves an endergonic step to populate C-P _zn• ⁺-P _F• ^- having an E _a of ∼0.23 eV. Certain photophysical characteristics of these triads, in particular the recombination to the triplet state, are reminiscent of those of artificial reaction centers with C ₆₀ as the primary electron acceptor.