Disorder-induced exciton scattering in the light-harvesting systems of purple bacteria: Influence on the anisotropy of emission and band→band transitions

Abstract

A time-domain analysis of the effects of pigment inhomogeneity upon the dynamics of optical excitations within bacterial light-harvesting complexes is presented. We focus upon examining the manifestation of disorder scattering in polarized femtosecond spectroscopy and the degree to which exciton delocalization is revealed in emission and transient absorption anisotropy measurements. The time evolution of states prepared by impulsive excitation of a statically disordered circular aggregate model for LH2 antenna complexes have been calculated exactly for varying degrees of pigment inhomogeneity. For a Gaussian distribution of site energies, the dynamics of coherence-loss (scattering) is explored as a function of the ratio of the standard deviation (σ) of the distribution to the intersite interaction energy (β). It is found that modest degrees of disorder (σ/β∼0.4) are sufficient to cause scattering on a sub-100 fs time scale. Results from model calculations of the pump–probe anisotropy strongly suggest that the initial ultrafast emission depolarization component reported for LH1 and LH2 antenna complexes by several groups represents the decay of an initially delocalized exciton, prepared by coherent excitation of eigenstates that become partially localized due to inhomogeneity. A novel approach to studying exciton coherence is proposed based on measurement of the anisotropy of band→band transient absorption transitions, which exhibit a pronounced sensitivity to exciton delocalization.