Polymerization of tetracyanoethylene under pressure

Based on first-principles calculations, possible phase transformations in the molecular crystals of tetracyanoethyelene (TCNE) were investigated at high pressures. Six new carbon nitride systems with one-, two-, and three-dimensional structures were found. The predicted polymeric and graphitic structures are semiconductors with energy gaps of 0.3-1.7 eV. The predicted three-dimensional solids are also semiconductors with larger energy gaps between 1.8 and 2.2 eV with large bulk moduli >188 GPa. The calculation results suggest two polymerization mechanisms. Furthermore, inspired by the chemical vapor deposition experimentally obtained results, the possibility of folding of the predicted one-dimensional polymeric system into cylindrical molecular structures is considered. Results show that TCNE polymers longer than 18.0 Å can form TCNE-cylinders. Upon hydrogenation of the predicted TCNE-cylinders, they form very stable TCNE-cucurbituril-like structures with energy gaps larger than 2.67 eV.