Characterization of the unoccupied and partially occupied states of TTF-TCNQ by XANES and first-principles calculations

Abstract

We report a combined experimental and theoretical study of the unoccupied electronic states of the neutral molecular organic materials TTF ͑tetrathiafulvalene͒ and TCNQ ͑7,7,8,8-tetracyano-p-quinodimethane͒ and of the one-dimensional metallic charge transfer salt TTF-TCNQ. The experimental density of states ͑DOS͒ is obtained by x-ray absorption near edge spectroscopy ͑XANES͒ with synchrotron light and the predicted DOS by means of first-principles density functional theory calculations. Most of the experimentally derived elementspecific XANES features can be associated to molecular orbitals of defined symmetry. Because of the planar geometry of the TTF and TCNQ molecules and the polarization of the synchrotron light, the energy dependent or character of the orbitals can be inferred from angular dependent XANES measurements. The present work represents the state of the art analysis of the XANES spectra of this type of materials and points out the need for additional work in order to elucidate the governing selection rules in the excitation process.

Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces

“…The simplest assignment would be to consider these states as LUMO, LUMO +1 and LUMO +2, ignoring the slight degeneracy of the individual molecular states. Indeed, we observe approximately correct, ~1 eV energy difference between LUMO +1 and LUMO +2 states that is very close to that in the gas-phase molecule [27]. From their position relative to E F , we also surmise a downshift of ~2.5 eV due to combined effects of screening and adsorption.…”

Spintronic effects at nickel-graphene interfaces

“…The carbon K-edge absorption shows a double main peak, whereas the neutral molecule shows a single peak (Fraxedas et al, 2003). This splitting is attributed to hybridisation of the cobalt surface with the different molecular orbitals that contribute to this peak.…”

“…The deposition of the TCNQ layer is confirmed by the appearance of a strong nitrogen 1s (K-edge) absorption peak (figure 6.1a). The features in the nitrogen K-edge XAS spectrum of TCNQ adsorbed on cobalt are distinctly different from those of the neutral molecule (Fraxedas et al, 2003), which is indicative of chemisorption and thus hybridisation of electronic states. The N(1s) → π * peak at the onset of the K-edge near 395 eV clearly exhibits two split components.…”

“…The N(1s) → π * peak at the onset of the K-edge near 395 eV clearly exhibits two split components. The corresponding feature of the neutral molecule is a single asymmetric peak, and has been attributed to excitations into the nearly degenerate a u and b 1u π * orbitals (where the symmetry labels refer to D 2h symmetry) (Fraxedas et al, 2003). For the neutral molecule, a small peak associated with the b 2g π * orbital is present on the high excitation energy side of the main N(1s)π → * resonance, which is absent for TCNQ on cobalt.…”

“…Both the presence of the nitrogen K-edge peak and the attenuation of the cobalt L-edge peak demonstrate the formation of a TCNQ film. Comparison of the recorded nitrogen K-edge with that of the neutral TCNQ molecule (Fraxedas et al, 2003) shows clear differences indicative of chemisorption.…”