To evaluate the PDI packing configuration within the copolymer structure, molecular dynamics (MD) simulations were again applied to visualize the geometry of PDI-oligomer aggregates. A copolymer with 36 styrenic units and 4 vinylpyrene units (PDI-tetrapoly(St36-co-VPy4)) was selected as the model molecule, and an aggregate structure consisting of 8 copolymers was simulated to explore the configuration of the PDI core and the monomeric units. As shown in Figure , 3 PDI-pyrene pairs with face-to-face stacking structure were found in this 8-copolymer aggregate after a simulation time of 100 ns, with a donor-acceptor distance of 3.20, 3.37, and 3.40 Å, respectively. This result indicated that charge transfer may easily occur between pyrene groups and PDI core when the pyrene-based monomer loading is about 10%, even when the polymer chain length is relatively longer. Indeed, as shown in Table , even one single pyrene unit in the PDI copolymers (i.e., PDI-coPyMA and PDI-coVPy) can significantly quench the aggregate emission, resulting in polymer film PLQY of only 3%-7%. In parallel, face-to-face PDI-pyrene pairs were also observed in the MD simulated configurations of the single copolymer in solution (Figure ). This is in line with the fact that PDI-coPyMA-15% and PDI-coVPy-5% showed reduced emission intensity in DMF by >2 folds as compared to that of PDI-tetraPS (Figure and Figure ). All of these results support the aggregate-state TSCT processes between the donor monomers and PDI core especially upon a strong electron donating group existing in the polymer chain. To provide a more in-depth understanding of the charge transfer processes in PDI polymers, we selected a PDI-pyrene heterodimer (Figure ) as a simplified model structure of PDI-coPyMA or PDI-coVPy. The vertical excitation energies for the PDI-pyrene heterodimer are shown in Table , calculated using the density functional theory-based multireference configuration interaction DFT/MRCI method, with corresponding molecular orbitals visualized in Figure . As per the standard Kasha model, the first singlet state (S1) is shown to be dark, and the second singlet state (S2) is bright across both DFT/MRCI and CC2 levels, with oscillator strengths calculated using DFT/MRCI of < 10 -3 and 0.667 respectively. The S1 state is shown to be of strong charge transfer quality, with a near unity contribution due to a pyrene → PDI charge transfer matrix element. This correlates well with the behavior observed by Kaletaş and coworkers. Conversely, the S2 state is the localized excitation of the PDI scaffold; a Frenkel exciton. At the electronic ground state, the vertical energy gap between the dark and bright states is less than 0.3 eV for either DFT/MRCI or CC2 treatments. This implies an efficient S2 → S1 internal conversion mechanism, in addition to no obvious level inversion points along any reaction coordinate.