Exciton Self Trapping in Photosynthetic Pigment–Protein Complexes Studied by Single-Molecule Spectroscopy

Evidence for the formation of self-trapped exciton states in photosynthetic antenna complexes is provided by comparing single-molecule fluorescence–excitation and emission spectra that have been recorded from the same individual LH2 complex from Rhodopseudomonas acidophila. While the excitation spectra showed the signatures for the B800 and B850 bands as observed previously, two distinctively different types of emission spectra were found. One group of antenna complexes shows spectra with a relatively narrow spectral profile with a clear signature of a zero-phonon line, whereas the other group of complexes displays spectra that consist only of a broad featureless band. Analysis of these data reveals clear correlations between the spectral position of the emission, the width of the spectral profile, and the associated electron–phonon coupling strength.