Tetracyanoethylene $$\hbox {Na}^+$$/$$\hbox {K}^+$$ radical anion coordination sites unveiled via electronic and vibrational fingerprints

Tetracyanoethylene (TCNE) is a very effective electron acceptor and can form stable complexes with various chemical species and is known for its interesting electronic and optical properties. The formation of the monoanion gives TCNE unique electronic and chemical properties, making it a material of interest for several technological applications including solar cells, organic battery electrodes, sodium/potassium-ion batteries and information storage devices. The chemico-physical characterization of Na+ and K+ coordination of TCNE in solution is not available so far, although it would be valuable for understanding the molecular mechanisms of charging and discharging processes in innovative battery devices. The present theoretical-computational study characterizes the vibrational and electronic properties of ion-coordinated TCNE in solution. We analyzed the ground electronic state through equilibrium structures, focusing on structural features, stability, optical properties and Raman vibrational modes. The Na+ and K+ cations, coordinated at two distinct sites, significantly alter the geometry, and modulate electronic and vibrational properties. Using ab initio molecular dynamics simulations, we computed vibrational spectra, finding that TCNE complexes serve as effective probes for detecting environmental variations, with key vibrational modes on CC and CN moieties being sensitive to cation interactions. Additionally, a correlation between C=C bond distances and vibrational frequencies was observed. In conclusion, this study provides further insight into the role of TCNE in optoelectronic applications and nonaqueous battery systems.