Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for the V (TCNE)x room-temperature molecular magnet

V (TCNE)x, with TCNE=tetracyanoethylene and x∼2, is an organic-based molecular magnet with potential to be used in spintronic devices. With the aim of shedding light on the unoccupied frontier electronic structure of V (TCNE)x we have studied pristine TCNE and sodium-intercalated TCNE by near edge x-ray absorption fine structure (NEXAFS) spectroscopy as well as with theoretical calculations. Sodium-intercalated TCNE was used as a model system of the more complex V (TCNE)x and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of V (TCNE)x. By comparing the experimental and theoretical C K -edge of pristine TCNE, the contributions from the various carbon species (cyano and vinyl) could be disentangled. Upon fully sodium intercalation, TCNE is n doped with one electron per molecule and the features in the C and N K -edge spectra of pristine TCNE undergo strong modification caused by partially filling the TCNE lowest unoccupied molecular orbital (LUMO). When comparing the C and N K -edge NEXAFS spectra of fully sodium-doped TCNE with V (TCNE)x, the spectra are similar except for broadening of the features which originates from structural disorder of the V (TCNE)x films. The combined results from the model system and V (TCNE)x suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in V (TCNE)x has no significant hybridization with vanadium and is similar to the so-called singly occupied molecular orbital of the TCNE anion. This suggests that the LUMO of V (TCNE)x is TCNE- or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between V (3d) and cyano carbons. The completely different nature of the unoccupied and occupied frontier electronic structure of the material will most likely affect both charge injection and transport properties of a spintronic device featuring V (TCNE)x.