β-Carotene to bacteriochlorophyll c energy transfer in self-assembled aggregates mimicking chlorosomes

Carotenoids are together with bacteriochlorophylls important constituents of chlorosomes, the light-harvesting antennae of green photosynthetic bacteria. Majority of bacteriochlorophyll molecules form self-assembling aggregates inside the chlorosomes. Aggregates of bacteriochlorophylls with optical properties similar to those of chlorosomes can also be prepared in non-polar organic solvents or in aqueous environments when a suitable non-polar molecule is added. In this work, the ability of β-carotene to induce aggregation of bacteriochlorophyll c in aqueous buffer was studied. Excitation relaxation and energy transfer in the carotenoid-bacteriochlorophyll assemblies were measured using femtosecond and nanosecond transient absorption spectroscopy. A fast, ∼100-fs energy transfer from the S2 state of β-carotene to bacteriochlorophyll c was revealed, while no evidence for significant energy transfer from the S1 state was found. Picosecond formation of the carotenoid triplet state (T1) was observed, which was likely generated by singlet homo-fission from the S1 state of β-carotene.