A carotenoid-porphyrin-diquinone tetrad: synthesis, electrochemistry and photoinitiated electron transfer

A molecular tetrad (C-P-Q_A-Q_B) consisting of a carotenoid polyene, a porphyrin and a diquinone moiety has been synthesized. The quinone with the lower reduction potential, the naphthoquinone (Q_A), was linked directly to the porphyrin and the benzoquinone (Q_B) was attached in series by a rigid bicyclic bridge. This arrangement was designed to promote a biomimetic sequential electron transfer from the porphyrin to Q_A, and on to Q_B. Cyclic voltammetric measurements show two distinct reduction steps at-0.65 and -0.46 V (vs. SCE) for the diquinone moiety, indicating independence of the benzoquinone and the naphthoquinone components. The initial charge separated state, C-P^.+-Q_A^.--Q-_B is formed within 15 ps of excitation and lies about 1.6 eV above the ground state. The final charge separated state C^.+-P-Q_A-Q_B^.- is formed with a quantum yield of 0.23 at room temperature (0.5 at 240 K) and lies ca. 1.1 eV above the ground state. Both parallel and sequential mechanisms for the electron transfer processes are elucidated from studies with model triads which feature a carotenoid, a porphyrin and only one of the components of the diquinone moiety.