KINETICS OF MULTISTEP PHOTOINITIATED ELECTRON-TRANSFER REACTIONS IN AMOLECULAR TRIAD

Time-resolved fluorescence and subpicosecond transient absorption expe riments have been carried out on a triad molecular device (C-P-Q) comp rising a porphyrin (P) covalently linked to an electron-donating carot enoid pigment (C) and a quinone (Q), which acts as an electron accepto r. Rate constants governing the intramolecular electron transfer proce sses originating from the excited singlet state of the porphyrin have been determined. Excitation of the porphyrin moiety of the triad in be nzonitrile solution results in photoinduced electron transfer to give C-P.+-Q.- (k, =2.5 x 10(9) s-1) followed by rapid non-photochemical el ectron transfer to yield C.+-P-Q.- (k3 almost-equal-to 1 X 10(11) s-1) . The intermediate C-P.+-Q.-, also decays by charge recombination to t he ground state with a rate constant k2 of about 6 x 10(11) s-1. Nanos econd laser flash experiments were used to measure a quantum yield of 0.13 for the long-lived (370 ns) charge-separated species C.+-P-Q.-. E xcitation of the carotenoid moiety of the triad results in singlet ene rgy transfer to the attached porphyrin with a quantum yield of about 0 .07.