Kinetics of multistep photoinitiated electron transfer reactions in a molecular triad

Time-resolved fluorescence and subpicosecond transient absorption experiments have been carried out on a triad molecular device (CPQ) comprising a porphyrin (P) covalently linked to an electron-donating carotenoid pigment (C) and a quinone (Q), which acts as an electron acceptor. Rate constants governing the intramolecular electron transfer processes originating from the excited singlet state of the porphyrin have been determined. Excitation of the porphyrin moiety of the triad in benzonitrile solution results in photoinduced electron transfer to give CP^.+Q^.- (k₁=2.5 × 10⁹s^-1) followed by rapid non-photochemical electron transfer to yield C^.+ PQ^.- (k₃ ≈ 1 × 10¹¹ s^-1). The intermediate CP^.+Q^.-, also decays by charge recombination to the ground state with a rate constant k₂ of about 6 × 10¹¹ s^-1. Nanosecond laser flash experiments were used to measure a quantum yield of 0.13 for the long-lived (370 ns) charge-separated species C^.+PQ^.-. Excitation of the carotenoid moiety of the triad results in singlet energy transfer to the attached porphyrin with a quantum yield of about 0.07.