Molecular dynamics (MD) simulations were first conducted to better understand the molecular stacking mode in the aggregate state. 20 molecules of TPE-BBT and TPA-BBT were placed in a cubic box with a length of 10 nm and relaxed to obtain the amorphous aggregate conformation. The aggregates were then solvated by water molecules to study their behaviors in water (Figures 3a and Figure ). The properties of single molecules in different environments were also investigated, and the details of these simulations are shown in the supporting information. To investigate the molecular motions of TPE-BBT and TPA-BBT under different conditions, we calculated the dihedral angle between the thiophene and BBT core of the innermost molecule, which represented a major rotor. Dihedral angles of the innermost molecules in TPE-BBT and TPA-BBT aggregates showed a wide distribution in the single-molecule state, which suggested their free rotations (Figure ). However, the distributions became narrow when they formed under vacuum, indicating the effect of RIM. Interestingly, when the aggregates were solvated by water, the distribution of the dihedral angles for TPE-BBT was further narrowed, while that of TPA-BBT became broader (Figure ). To explain the change of dihedral angles, the inner part of TPE-BBT and TPA-BBT aggregates were studied (Figure ). Water molecules could hardly be found inside TPE-BBT aggregates. However, water molecules appeared close to the core part of the innermost TPA-BBT.