To estimate the optical band gap of each co-crystal, the diffuse reflectance spectra shown in Figures were first transformed using the Kubelka-Munk transformation. The resulting F(R) spectra were treated as a pseudo-absorption function, assuming the scattering coefficient is constant for small spectral windows near the onset of the lowest energy feature. In this approximation, the linear portion of plots of F(R) 1/γ vs. photon energy was extrapolated to the x-axis to determine the optical band gap. For direct band gap materials, such plots will appear linear for γ = 1/2 whereas indirect band gap materials will show good linearity for γ = 2. F(R) plots for both materials, along with linear extrapolations, are presented in ESI, Figures . As none of these F(R) plots show obvious improvements in linearity, we report the direct band gap. Due to the vibronic structure observed in Npe:TCNQ, multiple linear fits were performed up to the lowest energy shoulder at ~862 nm.