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

Devens Gust,* Thomas A. Moore,* Ana L. Moore,* Darius Kuciauskas, Paul A. Liddell, and Brian D. Halbert

 Center for the Study of Early Events in Photosynthesis,   Department of Chemistry and Biochemistry,    Arizona State University, Tempe, AZ 85287-1604

 Abstract

     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. These triads are unique in that they also demonstrate stepwise photoinduced electron transfer to yield long-lived C^{· +} -P-C₆₀^{· -} charge-separated states. Electron transfer occurs even at 10 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.