Ultrafast energy relaxation in single LH2 complexes: Dependence on excitation wavelength

We have recently reported observation of ultrafast energy relaxation in individual LH2 complexes of purple bacterium Rps. Acidophila ^[1]. Here we present a detailed investigation of the dependence of the relaxation time on the excitation wavelength and its slow fluctuations in time. We find that the relaxation time distribution changes with the excitation wavelength in a non-monotonous way. Using a Frenkel-exciton model with combined generalized Förster and Redfield dissipative dynamics, we connect the observed relaxation to energy transfer within the LH2 electronic excited state manifold. By comparison of the theoretical and experimental relaxation time distributions, we show that the relaxation time fluctuations can be explained by dynamic sampling of the pigment energetic disorder. This highlights the dynamic, disordered nature of these light-harvesting complexes.