  Intermolecular interactions are crucial in determining the morphology of
solution-processed semiconducting polymer thin films. However, these random
interactions often lead to disordered or short-range ordered structures.
Achieving long-range order in these films has been a challenge due to limited
control over microscopic interactions in current techniques. Here, we present a
molecular-level methodology that leverages spatial matching of intermolecular
dynamics among solutes, solvents, and substrates to induce directional
molecular assembly in weakly bonded polymers. Within the optimized dynamic
scale of 2.5 \r{A} between polymer side chains and self-assembled monolayers
(SAMs) on nanogrooved substrates, our approach transforms random aggregates
into unidirectional fibers with a remarkable increase in the anisotropic
stacking ratio from 1 to 11. The Flory-Huggins-based molecular stacking model
accurately predicts the transitioning order on various SAMs, validated by
morphologic and spectroscopic observations. The enhanced structural ordering
spans over 3 orders of magnitude in length, raising from the smallest 7.3 nm
random crystallites to >14 um unidirectional fibers on sub-millimeter areas.
Overall, this study provides insights into the control of complex
intermolecular interactions and offers enhanced molecular-level controllability
in solution-based processes.
