| Carotenoids serve as antenna pigments in photosynthesis, absorbing light in the blue-green spectral region and efficiently transferring energy to neighboring chlorophylls. Model photosynthetic antenna systems consisting of carotenoid moieties covalently linked to phthalocyanines have been prepared to study the mechanism of singlet-singlet energy transfer between the carotenoid moiety and the tetrapyrrole macrocycles. Two types of models have been prepared. The first type, consisting of zinc carotenophthalocyanine dyads where the carotenoids are attached to the periphery of the phthalocyanines, models the carotenoid-bacteriochlorophyll B850 arrangement in natural systems. In these compounds, the two moieties are essentially on the same plane. The second type consists of silicon dicarotenophthalocyanine triads; in these models, the carotenoids are attached axially to the central silicon atom, where the carotenoids and the phthalocyanine are in virtually perpendicular planes, modeling the carotenoid-bacteriochlorophyll B800 arrangement in natural systems. Despite the completely different geometries and types of linkage in the models, singlet-singlet energy transfer from the carotenoid to the phthalocyanine is observed, resulting in highly efficient artificial photosynthetic antennas. |
