Exciton-exciton annihilation in a disordered molecular system by direct and multistep Forster transfer

Exciton annihilation dynamics in a disordered organic model system is investigated by ultrafast absorption spectroscopy. We show that the temporal evolution of the exciton density can be quantitatively understood by applying Forster energy transfer theory to describe the diffusion of the excitons as well as the annihilation step itself. To this end, previous formulations of Forster theory are extended to account for the inhomogeneous distribution of the S-0-S-1 transition energies resulting in an effective exciton diffusion constant. Two annihilation pathways are considered, the direct transfer of an exciton between two excited molecules and diffusive motion by multiple transfer steps towards a second exciton preceding the annihilation event. One pathway can be emphasized with respect to the other by tuning the exciton diffusion constant via the chromophore concentration. The investigated system allows one to extract all relevant parameters for the description and provides in this way a proof that the annihilation dynamics can be entirely understood and modeled by Forster energy transfer.