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The effect of charge transfer (CT) in complexes of the donors tetra- and
hexamethoxyprene (TMP and HMP) with the classical acceptor
tetracyanoquinodimethane (TCNQ) was studied using hard X-ray
photoemission (HAXPES). Microcrystals of the complex were grown via
vapour diffusion from donor-acceptor mixtures. The bulk sensitivity of
HAXPES at a photon energy of 6 key completely eliminates the problem of
surface contamination for such delicate organic materials grown from
solution. The donor molecules were produced using a novel synthesis
route functionalizing polycyclic aromatic hydrocarbons at their
periphery. For comparison, spectra were also taken from thin-film
samples of the same compounds produced via co-deposition in UHV. Upon
complex formation, the oxygen 1s core-level spectra (being a fingerprint
of the methoxy-group of the donors) change from the single-line spectrum
of pure HMP (TMP) to a spectrum with two distinct lines shifted by 1.4
(0.9) eV and 2.6 (2.3) eV with respect to the position of the oxygen 1s
line of the pure donors. The nitrogen 1s spectra (being a fingerprint of
the cyano-group in the acceptor) show two peaks as well with a
corresponding shift of 0.9 eV and 2.0 eV in comparison with the leading
line of pure TCNQ in opposite direction to the oxygen is spectra. These
values are substantially larger than shifts in near edge X-ray
absorption fine structure (NEXAFS) and ultraviolet photoelectron
spectroscopy (UPS) spectra of the same complexes. The changes in the
spectra are discussed in terms of the CT in the complexes. Residues of
pure donor and acceptor materials in the microcrystal fractions of the
complexes are evident from the presence of non-shifted lines. Peak-area
analysis reveals that charge is transferred to a fraction of 60% of the
molecules in the complexes. (C) 2012 Elsevier B.V. All rights reserved.
