The optical properties of the CTF series were investigated by diffuse reflectance UV-Vis spectroscopy (Figure ). Following the increase in the bipyridine content within the series, and consequently increased n-doping, the light absorbance shifts towards higher wavelengths, in line with predictions obtained from DFT calculations by Van Speybroeck and co-workers. In the series, the direct band-gap depends on the molecular composition, e.g. CTF-02 without any bipyridine moieties holds the largest direct bandgap with 3.05 eV, while CTF-02-Bpy possesses the narrowest direct bandgap with 2.87 eV (Table ).