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

Abstract

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)2SbF6, (TMTSF)2PF6, 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)2SbF6 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.

Photoelectron spectroscopy on single crystals of organic semiconductors: experimental electronic band structure for optoelectronic properties

“…For the photoemission works on these materials, one may consult a recent review paper. 28 Instead, photoemission works on recently emerging semiconductor materials, organic-inorganic hybrid perovskites, are discussed (Section III.6) because this class of materials shares some common characteristics with the organic semiconductors. In addition to the ARUPS works on the single crystal samples, some advanced topics being enabled by the developments of such experimental methodologies on the organic semiconductor single crystal samples are also introduced in this article.…”

Charge transfer in mixed and segregated stacks of tetrathiafulvalene, tetrathianaphthalene and naphthalene diimide: a structural, spectroscopic and computational study

“…The bountiful literature on organic donor-acceptor (D-A) charge transfer (CT) complexes, or 'organic metals,' has revealed their unprecedented electronic properties. [1][2][3][4] The discovery of the CT complex tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) in 1973, which exhibited metallic conductivity comparable to copper (1.47 Â 10 4 S cm À1 vs. 6 Â 10 5 S cm À1 ), 5 inspired a movement towards the development of organic conductors and superconductors. 6 Organic CT complexes hold significant promise in the development of high performance devices by virtue of their excellent electrical properties, low contact resistance, and tunable metal Fermi energies.…”

“…10 This theory enabled engineering of CT complexes with desired electronic properties by judicious choice of donors and acceptors by means of their redox properties. 4 TTF is of further interest due to its facile incorporation into organic CT complexes mechanochemically, 11 which is favourable for large scale syntheses and applications. It is well documented that TTF forms CT complexes when mechanochemically combined with chloranil, [12][13][14] iodide salts 15 and TCNO; 16 however, mechanochemical syntheses with DPNI-based ligands are less well studied.…”

Charge-transfer chemistry of azithromycin, the antibiotic used worldwide to treat the coronavirus disease (COVID-19). Part I: Complexation with iodine in different solvents

“…Given the constant increase in the number and types of applications involving CT complexation and the concomitant increase in relevance and attention, extensive research efforts have been dedicated to characterizing the crystallographic, thermodynamic, kinetic, photophysical, and spectral characteristics of CT complexation [ [67] , [68] , [69] , [70] , [71] , [72] , [73] , [74] , [75] , [76] , [77] , [78] , [79] , [80] , [81] , [82] , [83] , [84] , [85] , [86] , [87] , [88] , [89] , [90] , [91] , [92] , [93] , [94] , [95] , [96] , [97] , [98] , [99] , [100] , [101] , [102] , [103] , [104] , [105] , [106] , [107] , [108] , [109] , [110] , [111] , [112] , [113] , [114] , [115] , [116] , [117] , [118] , [119] ]. New CT complexations between π-conjugated organic ring molecules or π-electron-deficient acceptors with π-electron-rich donors represent an important approach to developing optoelectronic, light-emitting superconductor and conductor devices with valuable properties such as long-persisting luminescence, superconductivity, and high electrical conductivity [ [120] , [121] , [122] , [123] , [124] ,…”