The electronic structure of the light-harvesting LH2 complex in the light of single-molecule optical spectroscopy

Abstract

The three-dimensional structure of LH2 is characterized by a high symmetry which imposes particular selection rules on the optical transitions involving the exciton states that are associated with the interacting bacteriochlorophyll a pigments. Therefore, this system offers a unique case for a detailed study of parameters that determine the electronic structure of light-harvesting complexes. Of particular interest are the actual interaction strengths between the pigments, the effect of intrinsic disorder, and the contribution of electron-phonon interactions. Many experiments have been designed to determine these parameters, but the supposed exciton structure of LH2 is masked by inhomogeneous broadening of the absorption lines due to heterogeneity of the sample. Here we report on the results obtained by single-molecule optical spectroscopy at low temperature, by which effects of inhomogenous broadening are largely eliminated. This allows us to obtain optical spectra with well-resolved structure, which offer a critical test for theoretical models of the LH2 complex. We will present the results that were obtained sofar, and discuss the implications for the electronic structure of LH2.