Computational Modeling of Exciton Localization in Self-Assembled Perylene Helices: Effects of Thermal Motion and Aggregate Size

The effects of aggregation on the excited-state properties in a solution of perylene diimide self-assembled helix-like structures of different sizes are investigated by means of first-principles density functional theory (DFT), time-dependent DFT (TD-DFT), and classical molecular dynamics (MD) simulations. Excited-state analysis based on the one-particle transition density matrices is then used to study the exciton nature and its delocalization as a function of the thermal motion and aggregate size. Overall, the results point to a rather small delocalization of the Frenkel excitonic state even in large aggregates also related to a concerted motion of blocks of four monomers along the MD trajectories. Although dynamic effects do not remarkably affect the calculated position and shape of the absorption spectrum, they cause the appearance of several low-energy states of charge-transfer character and hence of weak intensity (dark states) that might be populated along the ultrafast exciton relaxation process potentially influencing the charge-separation processes in PDI-sensitized photoactive heterointerfaces.