Fluorescence-Detected Wavepacket Interferometry Reveals Time-Varying Exciton Relaxation Pathways in Single Light-Harvesting Complexes

Photosynthesis relies on efficient energy relaxation within the excited-state manifold of pigment-protein complexes. Since the protein scaffold is rather flexible, the resulting energetic and structural disorder gives rise to a complex excited-state energy level structure that fluctuates on all time scales. Although the impact of such fluctuations on relaxation processes is known, the precise exciton states involved in relaxation as well as the nature of the vibrational modes driving relaxation are under debate. Here, single pigment-protein complexes from a photosynthetic purple bacterium are excited with two identical ultrashort phase-locked pulses, producing two exciton wave packets that can interfere. This leads to a modulation of the emission intensity as a function of the delay time between the pulses that fades out within about ≈100 fs due to fluctuating environments on those time scales. For several single complexes, we find variations in the interference patterns on a time scale of several tens of seconds that reveal fluctuations in the energy relaxation pathways toward the lowest-energy exciton states. This relaxation is driven by temporal variations in the coupling between electronic excitations and low-frequency vibrational modes.