Stepwise sequential and parallel photoinduced charge separation in a porphyrin-triquinone tetrad

A molecular tetrad consisting of a free base porphyrin (P) linked to a triquinone moiety consisting of a naphthoquinone bearing two benzoquinones (NQ-BQ₂) has been synthesized and its photochemistry has been investigated using time-resolved spectroscopic techniques. Excitation of the porphyrin chromophore of the P-NQ-BQ₂ tetrad in benzonitrile solution is followed by photoinduced electron transfer with a rate constant of 8.3 × 10¹⁰ s^-1 to produce an initial P^.+-NQ^.--BQ₂ state with a quantum yield of unity. A charge shift reaction gives a final P^.+-NQ-(BQ^.-BQ) charge-separated state, which decays to the ground state with a rate constant of 1.9 × 10¹⁰ s^-1. The tetrad features both sequential and parallel multistep electron-transfer pathways. No evidence was found for significant involvement of direct electron transfer from the porphyrin first excited singlet state to a benzoquinone.