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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 AH* = 2.7 kcal/mol and AS* = -20 cal/(deg-mol). The transient absorption measurements also reveal that the quantum yield of the C"+-P-Q*~s tate 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.

Thermodynamic and kinetic aspects of intramolecular electron-transfer reactions in carotenoporphyrin dyads and carotenoid-porphyrin-quinone triads have been studied by using pulse radiolysis and cyclic voltammetry. Rapid (<1 µß) electron transfer from carotenoid radical anions to attached porphyrins has been inferred. Carotenoid cations, on the other hand, do not readily accept electrons from attached porphyrins or pyropheophorbides. Electrochemical studies provide the

A series of tripartite molecules (I)‐(III) and porphyrin‐quinone analogues (IV) has been synthesized (described in schemes, no details or yields given) in order to investigate the effect of structure on the yield and lifetime of the charge‐separated state.

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