The concentration of this dopant molecule in the crystal is low (undetectable by single crystal X-ray diffractometry) and controllable by varying the molar ratio of TCNQ and melamine in the precursor solution, that leads to the colour changes as shown in Fig. . The mass spectrometry reveals the presence of DCTC in the precursor solutions (i.e. TCNQ in aueous THF) in which coloured melamine crystals are formed.