Electronic structure of thin film iron-tetracyanoethylene: Fe(TCNE)x

Thin film iron-tetracyanoethylene Fe(TCNE)x, x ~ 2, as determined by photoelectron spectroscopy, was grown in situ under ultra-high vacuum conditions using a recently developed physical vapor deposition-based technique for fabrication of oxygen- and precursor-free organicbased molecular magnets. Photoelectron spectroscopy results show no spurious trace elements in the films, and the iron is of Fe²+ valency. The highest occupied molecular orbital of Fe(TCNE)x is located at ~1.7 eV vs. Fermi level and is derived mainly from the TCNE− singly occupied molecular orbital according to photoelectron spectroscopy and resonant photoelectron spectroscopy results. The Fe(3d)- derived states appear at higher binding energy, ~4.5 eV, which is in contrast to V(TCNE)2 where the highest occupied molecular orbital is mainly derived from V(3d) states. Fitting ligand field multiplet and charge transfer multiplet calculations to the Fe L-edge near edge X-ray absorption fine structure spectrum yields a high-spin Fe²+ (3d6) configuration with a crystal field parameter 10Dq ~ 0.6 eV for the Fe(TCNE)x system. We propose that the significantly weaker Fe-TCNE ligand interaction as compared to the room temperature magnet V(TCNE)2 (10Dq ~ 2.3 eV) is a strongly contributing factor to the substantially lower magnetic ordering temperature (TC) seen for Fe(TCNE)x - type magnets.