Abstract

Understanding the excited state electronic structure, optical excitation (energy) transfer and primary charge separation in photosynthetic protein-chlorophyll complexes represents an important but formidable problem [1]. At the outset it is important to realize that the protein, as a host for the various pigments (cofactors), is glass-like, meaning that the optical absorption bands of pigments such as chlorophylls (Chl) suffer from significant site inhomogeneous broadening (Γ_I). For Chl and pheophytin cofactors the absorption band of primary interest is the origin or (0,0) band of the lowest excited singlet state which is commonly referred to as the Q_y-state, a ππ* state. Such bands carry widths from ~ 100-500 cm^-1, depending on the system. For several reasons it becomes important to unravel the Γ_I and homogeneous broadening (Γ_H) contributions to the bandwidth. Two important contributors to Γ_H are the linear electron-phonon coupling (Γ_ep) and exciton level structure accompanied by ultra-fast inter-exciton level relaxation (Γ_ex). It turns out that Γ_ex is most important for antenna protein complexes because their unit cells (of a 2-dimensional array) often contain several strongly interacting Chls.

© 1992 Optical Society of America