Charge Separation and Energy Transfer in Carotenopyropheophorbide-Quinone Triads

In natural photosynthetic membranes, chlorophyll molecules serve as the site of the initial photodriven charge separation. In addition, they play a role in subsequent electron-transfer steps, accept singlet excitation energy from carotenoid antenna molecules, and transfer triplet energy to carotenoid acceptors (thereby preventing sensitized singlet oxygen production and subsequent photodamage to the organism). We report herein the synthesis and study of chlorophyll-based carotenopyropheophorbide-quinone triad molecules which mimic all of these natural processes. Irradiation of 1 in solution initiates a two-step electron transfer leading to the formation of an energetic charge-separated state with a quantum yield of ca. 4% and a lifetime of 120 ns. Caro-tenopyropheophorbide 3 demonstrates singlet-singlet energy transfer from the carotenoid moiety to the pyropheophorbide with 50% efficiency. The carotenoid moiety of 3 also provides pho-toprotection from singlet oxygen formation by quenching the pyropheophorbide triplet state within 50 ns of its formation.