A critical assessment on the role of long-range interactions can be grasped when considering the same P3HT nanofiber stacks, but now based on very short polymer chains (including 6 monomer units, see Supplemental Figure ). Indeed, shortening the interacting chromophores partly removes the destructive interactions between the monomer transition dipoles at close distance and should thus be accompanied by a faster falloff of the excitonic couplings, as indeed confirmed by quantum-chemical calculations, Figure . One consequence is that the first, nearest-neighbor, interactions are about six times larger in 6-mer compared to 30-mer aggregates, which translates into a slight increase in interchain exciton delocalization (with thermalized 〈 〉 of in 6-mer versus in 30-mer). Yet, most remarkably, the diffusion coefficient obtained when retaining the full interaction range is about one order of magnitude smaller, with cm 2 /s in the 6-mer versus cm 2 /s in the 30-mer onedimensional stacks, see Figure . While being based on model systems, these results are representative of poorly ordered, solution-processed films of conjugated polymers featuring limited on-chain delocalization and conjugation length (here mimicked by using short physical lengths), for which the typical diffusion coefficients of cm 2 /s translate into singlet exciton diffusion lengths in the 10 nm range. They, of course, also hold for small molecules. In both cases, in contrast with the situation prevailing for extended polymer chains, long-range interactions are found to be (slightly) detrimental to exciton diffusion, with values lowered by roughly a factor two when going from nearest-neighbor to full-range interactions (Figure ). In the latter scenario, though the high-energy extended states are sporadically accessed, the exciton has higher probability to get trapped at the bottom of the EDOS (see Supplemental Figure ).