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-BQ(2)) has been synthesized and its photochemistry has been investigated using time-resolved spectroscopic techniques. Excitation of the porphyrin chromophore of the P-NQ-BQ(2) tetrad in benzonitrile solution is followed by photoinduced electron transfer with a rate constant of 8.3 x 10(10) s(-1) to produce an initial P.+-NQ(.-)-BQ(2) 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 x 10(11) 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.