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 triad 2, we find a multiphased quenching of the Pc singlet excited state on the picosecond and nanosecond time scales. Upon intersystem crossing from the singlet excited state of Pc to the triplet state in triad 1, triplet-triplet energy transfer to either of the carotenoids takes place on a time scale significantly shorter than 5 ns. When dissolved in polar solvents, triads 1 and 2 exhibit light-induced electron transfer from either of the carotenoid moieties to the excited singlet Pc species with a time constant of about 2 ps. Charge recombination to the singlet ground state occurs in 10 ps in triad 1 and 17 ps in triad 2.