steadily grows with interaction range reaching only saturation when up to the first 15 neighbors are included, corresponding to interaction distances (15 times the intermolecular separation, ) of nm. Interestingly, applying a nearestneighbor model where excitonic interactions are renormalized to reproduce the full-range excitonic bandwidth (which boils down to multiplying the close contact excitonic couplings by a factor ) leads to a relatively modest increase in from cm 2 /s to cm 2 /s, revealing that what matters is the range of the interactions rather than their magnitude. The longrange nature of the excitonic interactions also makes the exciton diffusion process immune to dislocation defects in the stack, as shown in Supplemental Figure , where the transient delocalization mechanism is still in play, even when of the chains in the stack are missing. Not surprisingly, the long-range dependence of directly follows from the Coulombic interactions and can be understood from a simple qualitative analysis. At interchain distances, , large with respect to the molecular size, , these interactions can be reasonably well described using the point dipole approximation (PDA) and evolve as . At shorter distances, however, the PDA breaks down and largely overshoots the through-space Coulomb couplings . In fact, when expanding the molecular transition moments in the more accurate line dipole approximation (assigning for instance a local transition dipole to each monomer unit), it has been shown that the nearest excitonic interactions should decay inversely with molecular size, with scaling relationships ranging from to depending on the model used for onchain delocalization . This reflects the destructive interferences between positive (H-like) and negative (J-like) local dipole-dipole interactions at short distances. Because this cancellation effect weakens as interchain separation increases (with the PDA becoming valid at very large ), the excitonic couplings falloff slowly with distance, with the decay being best fit by a double exponential function, see Figure . We note that the Coulombic interactions used so far are based on gas-phase calculations; however, screening the long-range interactions to reproduce the Förster dependence , with , here taken to be 4 for organics, only slightly reduces the diffusion coefficient (by less than 10%, see Supplemental Figure ).