Excitonic effects in the light-harvesting Chl a/b-protein complex of higher plants

Exciton states and their coupling to long-wavelength protein vibrations are investigated both, experimentally and theoretically, in Chl a/b pigment-protein complexes acting as light-harvesting antennae in higher plants. It is shown that the 12 chlorophyll molecules located within the involved protein-monomers form extended exciton states and act cooperatively in the light absorption process. Besides one-exciton excitations also two-exciton excitations are demonstrated at higher photon densities incident on the monomer clusters resulting in nonlinear transmission and emission effects. The distribution of excitons over the exciton energy levels follows a Boltzmann law as shown by emission and nonlinear transmission experiments. The exciton-vibration coupling is weak as indicated by a resonance-like zero-phonon emission line which shows a strong reabsorption. Conclusions are drawn about the structure-function relationship of this antenna type. Interesting similarities between the biological structure investigated and semiconductors or semiconductor structures are discussed.