10] Synthesis of carotenoporphyrin models for photosynthetic energy and electron transfer

Abstract

Molecular dyads consisting of carotenoid polyenes covalently linked to porphyrin or chlorophyll derivatives serve as components of useful model systems for studying photosynthetic energy and electron transfer. The carotenoporphyrins mimic three important processes occurring in photosynthetic antennae and reaction centers. One of these is carotenoid antenna function, wherein carotenes absorb light in regions of the spectrum where chlorophylls have small extinction coefficients and transfer the resulting excitation to chlorophyll via a singlet-singlet energy transfer process. Second, carotenoids quench the triplet states of chlorophylls by triplet–triplet energy transfer, and help prevent chlorophyll-sensitized production of singlet oxygen. In this way, they provide photoprotection from the deleterious effects of this highly reactive oxygen species. The third process incorporates carotenoporphyrins into a number of more complex molecular devices that mimic the photoinitiated electron transfer cascades lying at the heart of the photosynthetic conversion of light energy into useful chemical potential in the form of long-lived charge separation. Carotenoporphyrins are also potentially useful for a variety of other studies in which it is desirable to sensitize the production of carotenoid triplet states, control the lifetime of porphyrin or chlorophyll triplet states, control singlet oxygen sensitization, or sensitize the production of porphyrin or chlorophyll singlet states with light of wavelengths not readily absorbed by these chromophores.