Fluorescence from pentacyanopropenide in melamine

Aggregation-induced optical phenomena are at the forefront of modern materials science. In this work, tetracyanoethylene (TCNE) is reacted and encapsulated within melamine. Crystallization from aqueous tetrahydrofuran solutions containing melamine and TCNE at varying concentrations yields colorful crystals exhibiting multi-wavelength fluorescence emission. Combined infrared spectroscopy and mass spectrometry reveal that the crystals are melamine doped with trace amounts of 1,1,2,3,3-pentacyanopropenide. Fluorescence excitation-emission spectral mapping elucidates the concentration dependence of fluorescence emission in both the precursor solutions and the resulting crystals. Density functional theory calculations attribute the observed multi-wavelength emission to dimers of the pentacyanopropenide. Encapsulating reactive molecules within crystalline melamine, as demonstrated with 1,1,2,3,3-pentacyanopropenide and its dimer, offers a versatile strategy for stabilizing a wide range of otherwise unstable species.