Mimicry of carotenoid photoprotection in artificial photosynthetic reaction centers: Triplet-triplet energy transfer by a relay mechanism

Two artificial photosynthetic reaction centers consisting of a porphyrin (P) covalently linked to both a carotenoid polyene (C) and a fullerene derivative (C₆₀) have been prepared and found to transfer triplet excitation energy from the fullerene moiety of C-P-³C₆₀ to the carotenoid polyene, yielding ³C-P-C₆₀. The transfer has been studied both in toluene at ambient temperatures and in 2-methyltetrahydrofuran at lower temperatures. The energy transfer is an activated process, with E(a) = 0.17 eV. This is consistent with transfer by a triplet energy transfer relay, whereby energy first migrates from C-P-³C₆₀ to the porphyrin, yielding C-³P-C₆₀ in a slow, thermally activated step. Rapid energy transfer from the porphyrin triplet to the carotenoid gives the final state. Triplet relays of this sort have been observed in photosynthetic reaction centers, and are part of the system that protects the organism from damage by singlet oxygen, whose production is sensitized by chlorophyll triplet states. The fullerene-containing triads can also demonstrate stepwise photoinduced electron transfer to yield long-lived C^.+-P-C₆₀^.- charge-separated states. Electron transfer occurs even at 8 K. Charge recombination of C^.+-P-C₆₀^.- yields ³C-P-C₆₀, rather than the molecular ground state. These photochemical events are reminiscent of photoinduced electron transfer in photosynthetic reaction centers.