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

Abstract

Abstract 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.