Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane - a NEXAFS Study

Source: arXiv

ABSTRACT It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds. Microcrystals of tetra- and hexamethoxypyrene as donors with the strong acceptor tetracyanoquinodimethane (TMPx/HMPx - TCNQy) were grown from solution via vapour diffusion in different stoichiometries x:y = 1:1, 1:2 and 2:1. Owing to the element specificity of NEXAFS, the oxygen and nitrogen K-edge spectra are direct spectroscopic fingerprints of the donating and accepting moieties. The orbital selectivity of the NEXAFS resonances allows to precisely elucidate the participation of specific orbitals in the charge-transfer process. In the present case charge is transferred from methoxy-orbitals 2e (PI*) and 6a1 (SIGMA*) to the cyano-orbitals b3g and au (PI*) and - to a weaker extent - to b1g and b2u (SIGMA*). The occupation of 2e reflects the anionic character of the methoxy groups. Surprisingly, the charge transfer increases with increasing HMP content of the complex. As additional indirect signature, all spectral features of the donor and acceptor are shifted to higher and lower photon energies, respectively. Providing quantitative access to the relative occupation of specific orbitals, the approach constitutes the most direct probe of the charge-transfer mechanism in organic salts found so far. Although demonstrated for the specific example of pyrene-derived donors with the classical acceptor TCNQ, the method is very versatile and can serve as routine probe for novel CT-complexes on the basis of functionalized polycyclic aromatic hydrocarbons. Comment: 13 pages, 5 figures

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Available from: Martin Baumgarten, Jun 24, 2015
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    ABSTRACT: We examine how the electronic structure (via synchrotron radiation XPS, UPS, and NEXAFS) and the molecular orientation (via NEXAFS) of a strong acceptor molecule, tetracyanoquinodimethane (TCNQ), change as a function of thickness when it is deposited on the cyclopentene-covered Si(001)-2X1 substrate. XPS shows that the monomolecular cyclopentene layer acts as an efficient chemical protective barrier. All spectroscopies indicate that anionic TCNQ is formed at (sub)monolayer coverage. However, the transfer should only concern those CN moieties pointing toward the Silicon surface. At higher thicknesses, neutral TCNQ is observed. We do not observe the upward bending of the silicon bands associated with electron transfer from the substrate to the acceptor molecular that one would expect for an unpinned Fermi level interface. In fact, donor levels are likely created within the cyclopentene layer or at its interface with silicon. The formation of TCNQ is associated with a strong increase in the work function. The attained value (similar to 5.7 eV) is independent of the work function of the cyclopentene-modified Si(001) surface (that varies with Si doping), in agreement with the integral charge transfer model. Therefore, ultrathin layers of TCNQ can be used to improve the hole-injection properties of this alkene-modified silicon surface.
    The Journal of Physical Chemistry C 10/2014; 118(39):22499-22508. DOI:10.1021/jp502680b · 4.84 Impact Factor