Photoinduced Intra- and Intermolecular Energy Transfer in Chlorophyll a Dimer

Applying nonadiabatic excited:state molecular dynamics, we investigate excitation energy transfer and exciton localiiation dynamics in a chlorophyll a (Chla) dimer system at the interface of two monomers of light-harvesting complex II trimer After its optical excitation at the red edge of the Soret (B) band, the Chla dimes experiences an ultrafast intra- and intermolecular nonradiative relaxation process to the lowest band (Q(y)). The energy relaxation is found to-run faster in the Chla dimes than in the Chia monomer:Once,the molecular system reaches the lowest Qy band composed of two lowest excited states S-1 and S-2, the concluding relaxation step involves the S-2 -> S-1 population transfer, resulting in a relatively slower relaxation-rate. The strength of-thermal fluctuations exceeds intraband electronic coupling-between the states belonging to a certain band (B, Q(x,) and Q(y)), producing localized states on individual chromophores. Therefore, time-evolution of spatial electronic localization during internal conversion reveals transient trapping on one of the Chia monomers participating in the events of intermonomeric energy exchange. In the phase space domains where electronic states are strongly coupled these states become neatly degenerate promoting Frerikel-excitonon-like delocalization and interchroinophore energy transfer. As energy relaxation,occurs, redistribution of the transition density on two Chia monomers leads to nearly equal distribution of the excitors among the molecules. For a single Chla, our analysis of excitonic dynamics reveals wave function amplitude transfer from nitrogen and outer carbon atoms to inner carbon atoms during nonradiative relaxation.