Near Edge X-ray Absorption Fine Structure Resonances of Quinoide Molecules

Abstract

We report on C 1s and N 1s near-edge X-ray absorption (NEXAFS) investigations of highly oriented ultrathin films of 2,5-dimethyl-N,N‘-dicyanoquinonediimine (DMe-DCNQI), tetracyano-quinonedimethane (TCNQ), and 2,5-dimethyl-1,4-benzoquinone (DMe-BQ) adsorbed on Ag(111). Several sharp resonances are resolved in the spectra indicating the localized electronic structure of these quinoide molecules. The comparison of the angle-dependent DMe-DCNQI results with those from other molecules and literature data on cyano-containing molecules leads to the conclusion that the common building-block approach is not sufficient, but that the interpretation of most valence orbitals being delocalized within the whole molecule appears to be more appropriate as confirmed by semiempirical INDO/S SDCI calculations. The NEXAFS spectra of the related DMe-DCNQI charge transfer salts also reflect strongly delocalized molecular orbitals very different from those of unreacted DCNQI.

Radiation chemistry of molecular compounds and polymers by soft X-ray spectroscopy and microscopy

“…2a →π* C≡ ≡ ≡ ≡N transition at 399.8 eV, and a rise of the pre-edge feature at 398.7 eV, which is attributed to the formation of C=N double bonds, similar to the N 1s →π* C=N transitions in polyaniline 41 and in quinoide molecules. 42 Similar N 1s spectroscopic changes were also observed in soft X-ray radiation damage to amide functional groups in amino acids 43 and proteins, 20 which attributed the created pre-edge features to the formation of C-N multiple bonds or C=N bonds with different local environments. Note that a resonant Auger spectroscopy study has assigned a close feature at 398.9 eV to N 1s →π* C=C transition in solid acrylonitrile.…”

Electronic States of Quinones for Organic Energy Devices: The Effect of Molecular Structure on Electrochemical Characteristics

“…Detailed peak assignments of the allowed unoccupied states in the C K-edge spectra are difficult to make using spectroscopic data; previous studies have only estimated spectroscopic results by comparing data with those of reference compounds. − Here we adopt overlap-population calculations to evaluate the energy levels of the unoccupied states and to associate them with the experimentally measured peak components. Molecular orbitals (Ψ = ∑ C i φ i ) were obtained by DFT calculations using GaussSum.…”

Symmetry, Shape, and Energy Variations in Frontier Molecular Orbitals at Organic/Metal Interfaces: The Case of F4TCNQ

“…37,38,41 By contrast, in the F 4 TCNQ/metal system the high electron affinity of the molecule drives a much larger charging, which not only converts the LUMO of the free molecule into the new n-HOMO, 11,42 but, as we show here, drastically alters the nature of all frontier levels with respect to the multilayer case. 43 In particular, the new n-LUMO appears defined by a combination of the pristine LUMO+1, LUMO+2 and LUMO+3 levels. As a consequence, we observe the spatial modulation of the orbital symmetry at the new n-LUMO edge, varying from π * at the quinone center to a more dominant σ * symmetry at the cyano end group of the molecule.…”

“…NEXAFS, XPS, and ARPES provide us with the full spectrum of interface molecular levels and, in combination with a local molecule/metal DFT model, allows us to understand in detail the chemistry of the molecule/surface contact. Partial LUMO filling and metal hybridization occur at other molecule/Ag(111) interfaces where the LUMO aligns close to E F , such as PTCDA ,,− or CuPc. ,, For these systems, NEXAFS spectra show that the unoccupied molecular levels at low energies maintain a π* character in both the multilayer and monolayer form. ,, By contrast, in the F 4 TCNQ/metal system the high electron affinity of the molecule drives a much larger charging, which not only converts the LUMO of the free molecule into the new n -HOMO, , but, as we show here, drastically alters the nature of all frontier levels with respect to the multilayer case . In particular, the new n -LUMO appears defined by a combination of the pristine LUMO+1, LUMO+2, and LUMO+3 levels.…”