originates from the nπ* state that resides on NDI. How the CT transition varied as a function of heterodimer configuration was further explored by computing the RI-CC2 potential energy scans along different intermolecular coordinates (Figure to 5F). The calculated energy bandgaps are higher than DFT/MRCI results in the initial configuration (Figure ) but with a similar trend in the singlet wavefunctions observed (Table ). At close NDI-pyrene distances, the CT state appears lower compared to its counterpart. However, it undergoes rapid destabilization in energy when increasing the separation, which eventually favors the nπ* emission of the NDI monomer. Because the twisting coordinate does not introduce substantial energy changes, we can conclude that ~6.5-7 Å is a critical radius for realizing the CT emission component of the NDI-pyrene pair. Altogether, these results indicate that the emission spectrum is dominated by nπ* transitions at ~480 nm from the NDI core at low pyrene concentrations, while the increasing peak at ~640 nm is due to CT transitions from pyrene to NDI, where pyrene is within ~7 Å of NDI. Accordingly, the variation of donor-acceptor spatial configuration in the NDI-based polymers via copolymerization or end group engineering led to the fluctuating combination of nπ* transitions and CT transitions, which explains the widely tunable emission colors observed in polymer aggregates.