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Photodriven electron transfer in triad molecules a two step charge recombination reaction

, : Photodriven electron transfer in triad molecules a two step charge recombination reaction. Journal of the American Chemical Society 108(25): 8028-8031

Excitation of carotenoid-porphyrin-quinone (C-P-Q) triad molecules initiates a two-step electron transfer to yield a final charge separated state of the form C.+-P-Q.-. This state has a very long lifetime in solution (10-7-10-6 s), and the nature of the ultimate charge recombination reaction has not previously been investigated. Nanosecond transient absorption spectroscopic studies have been performed on a series of such triad molecules wherein the nature of the linkages joining the porphyrin to the quinone and/or carotenoid moieties is varied systematically. The results reveal that charge recombination in C.+-P-Q.- does not occur in a single step but rather via an unusual two-step electron transfer involving an intermediate C-P.+-Q.- species. The temperature-dependence is consistent with the formation of C-P.+-Q.- via a thermally activated process, and measurements over the range 221-296 K yield .DELTA.H.dbldag. = 2.7 kcal/mol and .DELTA.S.dbldag. = 20 cal/(deg .cntdot. mol). The transient absorption measurements also reveal that the quantum yield of the C.+-P-Q.- state is a function of three electron transfer rate constants and that it can therefore be maximized by controlling the ratios of these rate constants to one another and to the rates of other pathways which depopulate the relevant excited states.

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