Carotenoid and Pheophytin on Semiconductor Surface: Self-Assembly and Photoinduced Electron Transfer

摘要

Carotenoids play several important roles in natural photosynthesis. They act as antenna by transferring singlet energy to chlorophylls and as photoprotective pigments by transferring triplet energy out of the chlorophyll system, thereby preventing chlorophyll-sensitized singlet oxygen production. Carotenoids also quench chlorophyll fluorescence under high light conditions and participate in electron-transfer processes in the reaction center. To mimic the functions of carotenoids in natural systems, synthetic carotenoid-tetrapyrrole dyads have been studied and shown to exhibit efficient carotenoid energy transfer and tetrapyrrole fluorescence quenching via electron transfer. However, in natural photosynthetic membranes, carotenoids and chlorophylls are not covalently linked but are held in suitably close proximity by proteins via noncovalent interactions. Herein we report that under certain condition, carotenoid and pheophytin a can self-assemble into a supramolecular system on the surface of nanocrystalline TiO_2. Excitation of the carotenoid initiates electron injection, leading to the formation of a carotenoid radical cation, while excitation of the pheophytin moiety results in ultrafast electron transfer from the carotenoid to the excited pheophytin, leading to the formation of a long-lived charge-separated state. The triplet carotenoid is also produced via primary charge recombination within 30 ns.