Theory of exciton-vibrational dynamics in molecular dimers

The exciton transfer in a molecular dimer embedded in a condensed medium is investigated theoretically. In order to include coherent vibrational dynamics a single effective mode per monomer is split off from the whole set of vibrational degrees of freedom of the dimer and the environment. The remaining modes are treated as a heat bath. To study the dissipative exciton transfer dynamics the density matrix formalism is applied. Choosing an appropriate exciton-vibrational basis set with respect to the two effective modes the theory is formulated in the related state representation. The general approach is applied to the exciton motion in a chlorophyll a/b dimer of the light-harvesting complex of the photosystem II of higher plants, The complex dynamic behavior following an ultrafast optical excitation in the absorption region of the chlorophyll b monomer is studied in detail.