The right panels in Fig. are the fluorescence excitation-emission wavelength maps for single crystals prepared from the respective precursor solutions. Their integrated excitation-and emission-wavelength profiles are plotted in Fig. . The M 1 emission of a crystal of pure melamine is located at λ ex = 312 nm (M ex 1 ) and λ em = 358 nm (M em 1 ). This emission becomes weaker as the concentration of TCNE is increased, and disappears at a concentration of 20 %. For the doped melamine crystals, multiple emissions are observed for each crystal. At concentrations of 0.2 % and 2.0 %, the three emissions labelled as C em 1 , C em In order to explain the experimental absorption bands observed in the wavelength range from 350 to 450 nm, further DFT calculations have been carried out for a dimer of 1,1,2,3,3,-pentacyanopropenide. Figure shows the calculated absorption spectrum for the dimer of 1,1,2,3,3,-pentacyanopropenide in comparison with the spectrum for the monomer. The dimer exhibits the intense absorption corresponding to the HOMO-LUMO transition at 449 nm, which matches well with the sholder at 470 nm in the UV-Vis absorption in Figure , the S ex 2 peak in the excitation wavelength profiles for the 100 and 200 % aqueous THF solutions, and the C ex 3 peak observed in the excitation wavelength profiles for the 2 -100 % crystals. Hence, it can be concluded that the 1,1,2,3,3,-pentacyanopropenide dimer is responsible for the color of the doped melamine crystals. Insets are the HOMO and LUMO.