Light Harvesting and Photoprotective Functions of Carotenoids in Compact Artificial Photosynthetic Antenna Designs

Artificial light-harvesting constructs were synthesized by covalently linking two carotenoids to the central silicon atom of a phthalocyanine (Pc) derivative. Triad 1 binds two carotenoids having nine conjugated double bonds, whereas triad 2 binds two carotenoids having 10 carbon−carbon double bonds in conjugation. Fluorescence excitation experiments indicated that, in triad 1 dissolved in n-hexane, the carotenoid to Pc singlet energy transfer efficiency is ca. 92%, whereas in triad 2, it is 30%. Results from ultrafast laser spectroscopy indicate that upon population of the optically allowed S2 state of the carotenoid the optically forbidden states S1 and S* are rapidly generated in both triad 1 and triad 2. In triad 1, S2, S1, and S* all contribute singlet electronic energy to Pc. In triad 2, singlet electronic energy transfer to Pc occurs primarily from the optically allowed S2 state with little energy transfer to Pc via the S1 state, and there is no evidence for energy transfer via S*. Instead, in tria...