  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.
