Electron-phonon and exciton-phonon coupling in light harvesting, insights from line-narrowing spectroscopies

Abstract

In photosynthetic antenna complexes, apart from defining the positions and orientations of the pigment molecules, the protein matrix plays an important role in excitation energy transfer dynamics. The low-frequency protein vibrations- often referred to as phonons-serve as acceptor modes in nonadiabatic excitation energy transfer between energetically inequivalent electronic or excitonic energy states, assuring a spatially and energetically directed flow of excitation energy within an antenna complex. Due to electron-phonon and electron-vibrational coupling, the purely electronic or excitonic transitions of pigment molecules are usually accompanied by a broad and asymmetric low-frequency sideband spreading a few hundred wavenumbers and, in addition, by a number of distinct lines covering the frequency range between ~200 and 1,700 cm-1. The low-frequency sideband peaking at 20-30 cm-1 is typically identified with a continuous distribution of widely delocalized protein vibrations. The narrow lines at higher frequencies can mostly be attributed to localized pigment vibrations of (bacterio-) chlorophyll molecules, which are only slightly modified by the surrounding protein matrix. In conventional absorption or fluorescence spectra measured at cryogenic temperatures this substructure is usually hidden by significant inhomogeneous broadening due to the heterogeneity of the amorphous protein matrix. Line-narrowing spectroscopies like spectral hole burning and (difference) fluorescence line-narrowing have been proved to be powerful experimental tools for unraveling the hidden homogeneous spectral features from the inhomogeneously broadened spectra. This contribution focuses on electron-phonon and exciton-phonon coupling in photosynthetic pigment-protein complexes. It also discusses the underlying concepts of electron-phonon and electron-vibrational coupling starting from the Franck-Condon principle and the general composition of homogeneously and inhomogeneously broadened spectra of pigment-protein complexes. The advantages and limitations of different spectroscopic techniques in revealing the electron-phonon and electron-vibrational coupling parameters are discussed based on model calculations. The chapter concludes with a discussion of recent results on electron-phonon and electron-vibrational coupling for isolated (bacterio-) chlorophyll molecules as well as for selected photosynthetic pigment-protein complexes.