Fluorescence line narrowing and Δ-FLN spectra in the presence of excitation energy transfer between weakly coupled chromophores.

Abstract

The results of simulations of fluorescence line narrowed (FLN) and Δ-FLN spectra of weakly coupled chromophore dimers and trimers, connected by slow (hundreds of picoseconds) excitation energy transfer and embedded in amorphous solids such as proteins, are presented. The model includes wavelength-dependent excitation energy transfer (EET) rate distributions converted to nonidentical sub- site distribution function (sub-SDF) (one for each EET rate), as well as spectral hole burning in pigments excited via EET. Emission of the donor molecules and realistic EET probability dependence on the donor-acceptor energy gap are also included. The shapes of the FLN and Δ-FLN spectra are very sensitive to even small variations of interpigment coupling in this weak-coupling regime, and proper modeling is essential for extracting correct parameters from the experimental spectra. Applications of the model to the trimeric FMO complex and the dimeric cytochrome b6f are demonstrated.