To understand the origin of the deep red emission signal when introducing pyrene groups, we performed quantum chemical calculations of an NDI-pyrene heterodimer. DFT/MRCI results suggest that the experimentally observed emission signal at ~480 nm stems from transitions from the nπ* state of the NDI molecule, while the emission peak at ~640 nm cannot be attributed to transitions from either molecule (Table ). Starting with a center-of-mass intermolecular distance of 3.5 Å (Figure ), we performed a CT analysis of the excited-state wavefunctions of the heterodimer based on the one-particle transition density matrix, with the results presented in Table . For the NDI-pyrene heterodimer, the first singlet state (S1) is found at 1.85 eV (670 nm) with 95% of the exciton attributed to the CT state, which matches the experimental value precisely. The second excited state (S2) at 2.70 eV (459 nm)