Figure , Supporting Information). Although the resolution of the AFM measurements was not sufficient to fully characterize the local distribution of (S)-2PDI molecules on the (R)-1NDI nanofibers, each red-emitting spot was interestingly associated with multiple branching junctions in the nanofiber networks. This nanofiber network contained small spherical nano-objects or larger nanolayers, likely composed of (S)-2PDI aggregates (Figure , Supporting Information). The AFM images suggest that the presence of repeated branching and coiling of the thin nanofibers may act as packing defects, suppressing both the long-range exciton hopping along the (R)-1NDI nanofibers and energy transfer efficiency compared to the solution phase. Moreover, the (R)-1NDI and (S)-2PDI host-guest system deposited on glass is expected to be more rigid than in the solution phase. The random but less uniform distribution of (S)-2PDI guest on (R)-1NDI nanofibers during the solvent drying process upon drop-casting onto glass slides might result in a variety of fixed localized positions and orientations of some (S)-2PDI molecules on (R)-1NDI fibers. Some of these geometries may be less compatible for energy transfer from (R)-1NDI nanofibers to (S)-2PDI molecules, thus reducing the energy transfer efficiency, compared to the solution phase. Finally, the presence of partially self-aggregated (S)-2PDI molecules in the vicinity of (R)-1NDI nanofibers, which interact effectively with the (R)-1NDI donors, favors excitation energy transfer from nanofibers to (S)-2PDI aggregates ((S)-2PDIagg) acting as acceptors.