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 poryphy rin (P) covalently linked to both a carotenoid polyene (C) and a fulle rene derivative (C-60) have been prepared and found to transfer triple t excitation energy from the fullerene moiety of C-P-C-3(60) to the ca rotenoid polyene, yielding C-3-P-C-60. The transfer has been studied b oth in toluene at ambient temperatures and in 2-methyltetrahydrofurane at lower temperatures. The energy transfer is an activated process, w ith E-a = 0.17 eV. This is consistent with transfer by a triplet energ y transfer relay, whereby energy first migrates from C-P-C-3(60) to th e poryphyrin, yielding C-P-3-C-60 in a slow, thermally activated step. Rapid energy transfers from the porphyrin triplet to the carotenoid g ives the final state. Triplet relays of this sort have been observed i n photosynthetic reaction centers, and are part of the system that pro tects the organism from damage by singlet oxygen, whose production is sensitized by chlorophyll triplet states. The fullerene-containing tri ads can also demonstrate stepwise photoinduced electron transfer to yi eld long-lived C.+-P-C-60(.-) charge-separated states. Electron transf er occurs even at 8 K. Charge recombination of C.+-P-C-60(.-) yields C -3-P-C-60, rather than the molecular ground state. These photochemical events are reminiscent of photoinduced electron transfer in photosynt hetic reaction centers. (C) 1998 Elsevier Science S.A. All rights rese rved.