Nonadiabatic Simulations of Exciton Dissociation in Poly-p-phenylenevinylene Oligomers

We present simulations of exciton dissociation and charge separation processes in the prototypical conjugated polymer, poly-p-phenylenevinylene. Our mixed quantum/classical simulations focus on the nonadiabatic excited state dynamics of single and pi-stacked oligomers of varying length. By applying a constant external electric field, our simulations reveal the details and time scale for exciton dissociation and fluorescence quenching and suggest how those processes relate to charge carrier (polaron) formation in polymer systems. We find that, in such a polarizing environment, sufficiently long chromophores (either single or interacting chains) can form polaron pairs via a delayed exciton dissociation mechanism or nearly instantaneously following photoexcitation. However, we find that these processes are mechanically essentially the same, being highly nonadiabatic in character and requiring transitions through "gateway" states to reach the completely charge separated electronic states. Finally, we observe thermally driven polaron hopping dynamics between chains, similar to the energy transfer dynamics we had described previously (J. Phys. Chem. A 2009, 113 (15), 3427). Our results are consistent with a range of apparently conflicting experiments, resolving some controversies regarding the molecular mechanism for charge carrier photogeneration in conjugated polymers.