Time-Resolved Ultrafast Excitation Dynamics in the B850 Light-Harvesting Antenna from Density Functional Theory

Antenna complexes absorb sunlight and transfer the harvested energy with remarkable quantum efficiency. In spectroscopic experiments, they are typically excited with laser pulses that differ substantially from sunlight. Using density functional theory calculations in real time, we reveal the excitation dynamics that results in the B850 antenna ring of the purple bacterium Rhodoblastus acidophilus upon excitation by a short, strong pulse as typically used in experiments. The pulse dominantly triggers the exciton modes that are also the most relevant ones in the natural process. Quantum mechanical interference patterns noticeably influence the electronic density distribution after about 40 fs, and on the same time scale, the effects of nuclear motion start to have a noticeable influence on the excitation dynamics. About 20 fs after the laser peak, the B850 ring transitions into dynamics in which the excitation energy is mostly localized on segments that comprise just a few bacteriochlorophyll molecules.