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It is demonstrated that the near-edge X-ray absorption fine structure(NEXAFS) provides a powerful local probe of functional groups in novel chargetransfer (CT) compounds. Microcrystals of tetra- and hexamethoxypyrene asdonors 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 andnitrogen K-edge spectra are direct spectroscopic fingerprints of the donatingand accepting moieties. The orbital selectivity of the NEXAFS resonances allowsto precisely elucidate the participation of specific orbitals in thecharge-transfer process. In the present case charge is transferred frommethoxy-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 2ereflects the anionic character of the methoxy groups. Surprisingly, the chargetransfer increases with increasing HMP content of the complex. As additionalindirect signature, all spectral features of the donor and acceptor are shiftedto higher and lower photon energies, respectively. Providing quantitativeaccess to the relative occupation of specific orbitals, the approachconstitutes the most direct probe of the charge-transfer mechanism in organicsalts found so far. Although demonstrated for the specific example ofpyrene-derived donors with the classical acceptor TCNQ, the method is veryversatile and can serve as routine probe for novel CT-complexes on the basis offunctionalized polycyclic aromatic hydrocarbons.

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