Hole-Burning in Biological Systems

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 (ΓI). 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 Qy-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 ΓI and homogeneous broadening (ΓH) contributions to the bandwidth. Two important contributors to ΓH are the linear electron-phonon coupling (Γep) and exciton level structure accompanied by ultra-fast inter-exciton level relaxation (Γex). It turns out that Γex is most important for antenna protein complexes because their unit cells (of a 2-dimensional array) often contain several strongly interacting Chls.