Electron‐ and X‐Ray Spectroscopies of Organic Charge‐Transfer Complexes

Recent work aiming at a deeper understanding of the electronic properties of classical and novel charge-transfer (CT) complexes is reviewed herein. From a variety of studied CT salts, the prototypical examples TTF-TCNQ, (TMTTF)(2)SbF6, (TMTSF)(2)PF6, and TMP/HMP-TCNQ have been selected. Three different types of electron and X-ray spectroscopies were applied, namely ultraviolet photoelectron spectroscopy (UPS), hard X-ray photoelectron spectroscopy (HAXPES), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Using UPS, the development of the valence band structure during co-deposition of the donor and acceptor moieties TMP/HMP and TCNQ has been studied. The spectra reveal characteristic level shifts and a strong reduction of the HOMO-LUMO gap upon formation of the complex. HAXPES probes the core levels and reveals characteristic shifts of the binding energies in (TMTTF)(2)SbF6 at the phase transition to the low-temperature charge-ordered state. NEXAFS is a local, element-specific probe for the unoccupied bands above the Fermi level and reveals changes in the population of specific orbitals when the CT complex is formed. As an outlook, the novel method of photoelectron momentum microscopy is presented, giving access to the valence bands and core-levels and in particular to high-resolution X-ray photoelectron diffraction patterns.