The absorption spectra of our model Npe:TCNQ dimer were calculated, shown in Figure . The simulated spectrum of Npe:TCNQ shows a peak at 660 nm, associated with the S1 excited state. This state corresponds to CT from the Npe to the TCNQ with oscillator strength, f = 0.0299 a.u. and a HOMO→LUMO orbital transition, as shown in the orbital isosurfaces in Figure . This peak is observed experimentally within the vibronic structure region between 600 and 900 nm. Two additional peaks in the calculated spectrum are located at 440 and 380 nm, associated with excited states S2 and S4, respectively. These two states have higher oscillator strengths, f = 0.310 a.u. and 0.280 a.u. for S2 and S4, respectively, with mixed character between locally excited and CT transitions, originating mainly from the TCNQ π → π * transition. These peaks likely correspond to the less intense dip observed in the experimental spectrum at 420 nm. A final strong peak at 320 nm originates from the S8 excited state with strong oscillator strength, f = 0.860 a.u., aligning with the observed experimental dip at 340 nm. This excitation is characterized by the intramolecular Npe π→π * transition; see Figure . Similar vibronic structure observed between 600 and 900 nm in other co-crystals has been attributed to formation of radical anion pairs of TCNQ and donor. However, this assignment has recently come under scrutiny with evidence indicating that donor:TCNQ cocrystals can exhibit a small degree of CT in the S0 state and a large degree of CT in the S1 state, indicating formation of a radical anion pair only in the S1 state. To explain this, the authors refer to the foundational work of Mulliken on D-A pairs and the emergence of CT. Mulliken's two-state system model describes a D-A pair as a linear combination of neutral and ionic states, wherein systems with ground states that exhibit predominantly neutral character must necessarily exhibit predominant ionic character in an excited state due to orthogonality requirements. In our calculations of CT in the S0 and S1 states of Npe:TCNQ, we also find dominantly neutral character in S0 and ionic character in S1, supporting the assignment of Monezi et al. The UV-Vis diffuse reflectance spectra were analyzed to determine the band gap of Npe:TCNQ crystals, as described in Sections 2.2.6 and 3.2. A direct band gap is determined as 1.38 eV. Our PBC DFT calculations determine the direct band gap to be 0.79 eV. In the case of Npe:TCNQ, PBC DFT and experimental band gaps do not agree as nicely as in the case of Npe:TCNB, but still fall within typical deviations for hybrid functionals.