Artificial photosynthetic reaction centers with carotenoid antennas

Two reaction center-antenna models based on a purpurin macrocycle linked to a C60 and to a carotenoid polyene have been synthesized. In these systems the C60 moiety is the primary electron acceptor, the purpurin is the primary electron donor and the carotenoid moiety acts both as an antenna and secondary electron donor. Formation of the initial charge separated state, C–Purradical dot+–C60radical dot−, following excitation with light absorbed by either the purpurin or C60 takes place on the 10 ps time scale. The final charge separated state, Cradical dot+–Pur–C60radical dot−, is formed in one of the compounds with a quantum yield of 32% based upon light absorbed by the carotenoid. In order to function as an antenna, the carotenoid pigment must be electronically coupled to the purpurin. The purpurin C ring provides an excellent framework for locating a carotenoid polyene in partial conjugation with the macrocycle, leading to a relatively strong electronic communication between the chromophores; functionalization of a meso position of the purpurin provides a site for the covalent attachment of C60.