Stereodynamics of Intramolecular Triplet Energy Transfer in Carotenoporphyrins

Carotenoporphyrins, consisting of carotenoid polyenes linked covalently to porphyrins, are known to mimic both the antenna function and the photoprotection from singlet oxygen damage provided by carotenoids in photosynthetic organisms. A series of carotenoporphyrins whose conformations, as determined from ¹H NMR studies, range from folded (with the carotenoid π-electron system stacked over that of the porphyrin) to extended (with the chromophores widely separated) has been prepared. Time-resolved spectroscopic studies have revealed intramolecular triplet energy transfer from porphyrin to carotenoid. Two distinct pathways for such transfer (presumably occurring via an electron-exchange mechanism) were observed: static transfer which does not require significant intramolecular motions; (b) dynamic transfer mediated by intramolecular motions. The relative importance of these pathways is a function of molecular structure and dynamics. The results for this series of carotenoporphyrins help define the photochemical and photophysical requirements for protection from singlet oxygen damage both in photosynthetic organisms and in other biological systems.