Carotenoid and pheophytin on semiconductor surface: self-assembly and photoinduced electron transfer.

Self-assembling of a carotenoid and pheophytin a into a supramolecular system was observed on the surface of nanocrystalline TiO2, and the photoinduced electron-transfer reactions within the system were studied by means of femtosecond transient absorption and laser flash photolysis techniques. Excitation of the pheophytin moiety results in ultrafast electron transfer from carotenoid to the excited pheophytin, creating a long-lived charge-separated state. Two decay pathways of the formed pheophytin a anion radical are proposed. The first is a direct back electron recombination forming a carotenoid triplet state on the nanosecond time scale, while the other is suggested to occur via electron injection to the TiO2 nanoparticle. These results demonstrate that a self-assembled carotenoid-pheophytin system leads to an efficient reductive quenching of the pheophytin moiety, suggesting that a similar mechanism can operate also in natural photosynthetic systems. Moreover, the formation of a long-lived charge-separated state indicates that such self-assembling strategy may be also considered for novel dye-sensitized solar cell constructions and other artificial systems aiming to mimic the electron-transfer chain in natural photosynthesis.