D. Gust et al., MIMICRY OF CAROTENOID PHOTOPROTECTION IN ARTIFICIAL PHOTOSYNTHETIC REACTION CENTERS - TRIPLET-TRIPLET ENERGY-TRANSFER BY A RELAY MECHANISM, Journal of photochemistry and photobiology.B, Biology, 43(3), 1998, pp. 209-216
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.