Highly Luminescent TCNQ in Melamine

Optical properties of molecules change drastically as results of interactions with surrounding environments as observed in solutions, clusters and aggrergates. Here we make 7,7,8,8-tetracyanoquinodimethane (TCNQ) highly luminescent by encapsulating it in crystalline melamine. Coloured single crystals are synthesized by slow evaporation of a solution of melamine and TCNQ in water and tetrahydrofuran. Single crystal X-ray diffraction reveals the lattice structure of pure melamine, meaning that the colour is of impurities. Both mass spectrometry and UV-Vis spectroscopy combined with density-functional theory calculations elucidate that the impurity species are neutral TCNQ and its oxidation product, dicyano-$p$-toluoyl cyanide anion (DCTC$^-$), whose concentrations in a melamine crystal can be controlled by adjusting the molar ratio between melamine and TCNQ in the precursor solution. Fluorescence exciation-emission wavelength mappings on the precursor solutions illustrate dominant emissions from DCTC$^-$ while the emission from TCNQ is quenched by the resonance energy transfer to DCTC$^-$. On the contrary, TCNQ in crystalline melamine is a bright fluorophore whose emission wavelength centered at 450 nm and slow fluorescence lifetimes of about 2 ns. Our method of encapsulating molecules into transparent melamine would make many other molecules fluorescencent in solids.