Determination of excitation energy transfer efficiency in individual artificial light- harvesting complexes mimicking chlorosomes

Artificial light-harvesting antennas inspired by chlorosomes, the main light-harvesting complexes of green photosynthetic bacteria, represent self-assembling model systems for applications in solar energy conversion. They contain bacteriochlorophyll aggregates with remarkable light-harvesting properties. The pigments are purified from chlorosomes, thus avoiding an expensive chemical synthesis. Here we present results obtained on individual artificial light-harvesting complexes containing bacteriochlorophyll c as the main light-harvesting pigment, β-carotene as an aggregation-inducing agent, and bacteriochlorophyll a as an acceptor of the harvested energy. We combined multiple microscopic and macroscopic techniques to study free-floating and surface-deposited individual antennas which revealed their structural and functional properties on a single-complex level. The main output is a method which allows the assessment of excitation energy transfer efficiency in individual particles from a small number of detected photons (less than 20 in this work). The data allowed the correlation of the efficiency with the content of β-carotene. Additionally, we determined the size and arrangement of the self-assembled structures, which proved to form rather monodisperse aggregates clustering on the surface. Our method is generally applicable to other types of donor-acceptor systems, from inorganic nanoparticles to molecular assemblies, allowing the study of exciton dynamics in real time at the single-particle level.