In order to get further insight into the nature of the crystal colour, UV-Vis spectroscopy has been carried out. Panels c and d in Fig. show the UV-Vis spectra of TCNQ freshly disolved in THF and in aqueous THF (1:1 volume ratio), respectively, with molar concentrations of 0.04, 0.16 and 1.25 mM. All three spectra for TCNQ in THF show the instense visible absorption peak at a wavelength of ∼ 395 nm. The small peak at 485nm can be attributed to that of DCTC - , possibly due to a trace of water absorbed in THF.