Phase diagram of quarter-filled band organic salts [EDT-TTF-CONMe2]2X, X=AsF6 and Br

Charge localization in 1D tetramerized organic conductors: the special case of (tTTF)2ClO4

We report a detailed structural and spectroscopic study of the 1D 2:1 cation radical salt (tTTF)₂ClO₄, where tTTF  =  trimethylenetetrathiafulvalene, which exhibits a semiconductor-semiconductor phase transition at ca. T  =  137 K. Crystal structures are determined above and below the transition; the tTTF molecules in stacks are grouped into weakly interacting tetramers. The reorganization of tTTF stacks is accompanied with an order-disorder transition in anion sublattice. Polarized infrared and Raman spectra of (tTTF)₂ClO₄ are measured in the broad frequency range as a function of the temperature (10-293 K). The structural and vibrational features are investigated to elucidate the origin of the semiconductor-semiconductor phase transition. We discuss the electron-intramolecular vibration coupling effects in the vibrational spectra of (tTTF)₂ClO₄ and identify signatures of high- and low-temperature states of charge localization in the tetramerized system. Both the C=C and C-S stretching modes of tTTF give evidence of strong charge distribution fluctuations in conducting stacks for T  >  137 K, which are responsible for the appearance of molecules with charge  +1e, and charge localization in tTTF tetramers for T  <  137 K. The uniqueness of the salt (tTTF)₂ClO₄ in comparison with other tetramerized 1D systems is discussed.

Correlating conduction properties with the molecular symmetry: segregation of Z and E isomers in the charge-assisted, halogen-bonded cocrystal [(Z,E)-Me2I2TTF]2Br

Co-crystallization of theZandEisomers of Me₂I₂TTF in a mixed-valence bromide salt leads to segregated stacks with two different charge order patterns and associated charge-assisted halogen bonding.

Unprecedented stacking of MV2+ dications and MV˙+ radical cations in the mixed-valence viologen salt (MV)2(BF4)3 (MV = methylviologen)

Using a slow liquid-gas diffusion method, the mixed-valence viologen salt (MV)2(BF4)3 (1) and the radical cation salt (MV)(BF4) (2) are crystallized. Both structures contain regular stacks of MV˙(+) radical cations (2) or alternating MV˙(+) and MV(2+) entities (1). A short intrastack intermolecular separation (3.23 Å) unprecedently reveals strong interactions between MV(2+) and MV˙(+) in 1.

Characterization of the quasi-one-dimensional compounds δ-(EDT-TTF-CONMe2)2X, X=AsF6 and Br by vibrational spectroscopy and density functional theory calculations

We have investigated the infrared spectra of the quarter-filled charge-ordered insulators δ-(EDT-TTF-CONMe2)2X (X= AsF6, Br) along all three crystallographic directions in the temperature range from 300 to 10 K. DFT-assisted normal mode analysis of the neutral and ionic EDT-TTF-CONMe2 molecule allows us to assign the experimentally observed intramolecular modes and to obtain relevant information on the charge ordering and intramolecular interactions. From frequencies of charge-sensitive vibrations we deduce that the charge-ordered state is already present at room temperature and does not change on cooling, in agreement with previous NMR measurements. The spectra taken along the stacking direction clearly show features of vibrational overtones excited due to the anharmonic electronic molecule potential caused by the large charge disproportionation between the molecular sites. The shift of certain vibrational modes indicates the onset of the structural transition below 200 K.

C—I...NC halogen bonding in two polymorphs of the mixed-valence 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4), with segregatedversusalternated stacks

Oxidation of diiodoethylenedithiotetrathiafulvalene (EDT-TTF-I2), C8H4I2S6, with the strong oxidizer tetrafluorotetracyanoquinodimethane (TCNQF4), C12F4N4, affords, depending on the crystallization solvent, two polymorphs of the 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4), represented asD2A. In both salts, the TCNQF4is reduced to the radical anion state, and is associated through short C—I...NC halogen bonds to two EDT-TTF-I2molecules. The two polymorphs differ in the solid-state association of these trimericD–A–Dmotifs. In polymorph (I) the trimeric motif is located on an inversion centre, and hence both EDT-TTF-I2molecules have +0.5 charge. Together with segregation of the TTF and TCNQ derivatives into stacks, this leads to a charge-transfer salt with high conductivity. In polymorph (II) two crystallographically independent EDT-TTF-I2molecules bear different charges, close to 0 and +1, as deduced from an established correlation between intramolecular bond lengths and charge. Overlap interactions between the halogen-bondedD0–A^{{-}{\bullet}}–D^{{+}{\bullet}} motifs give rise, in a perpendicular direction, to diamagneticA22−andD0–D22+–D0entities, where the radical species are paired into the bonding combination of respectively the acceptor LUMOs and donor HOMOs. The strikingly different solid-state organization of the halogen-bondedD–A–Dmotifs provides an illustrative example of two modes of face-to-face interaction between π-type radicals, into either delocalized, uniform chains with partial charge transfer and conducting behaviour, or localized association of radicals into face-to-faceA22−andD22+dyads.

A single-component molecular metal based on a thiazole dithiolate gold complex

A single component molecular conductor has been isolated from electrocrystallization of the monoanionic gold bis(dithiolene) complex based on the N-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate (Et-thiazdt) ligand. The crystal structure of the system exhibits layers built from parallel uniform one-dimensional stacks of the planar molecule. At room temperature and ambient pressure the system is semiconducting (0.33 S x cm(-1)) with a small activation energy. However, the single crystal conductivity is strongly pressure dependent reaching 1000 S x cm(-1) at 21 kbar. At 13 kbar there is a crossover between semiconducting and metallic regimes. Thus, the present system is the first well characterized single-component molecular metal without TTF dithiolate ligands. First-principles DFT calculations show that the ground state is antiferromagnetic with a very small band gap. A simulation of the effect of pressure on the electronic structure provides a rationale for the observed variations of the conductivity and gives insight on how to further stabilize the metallic state of the system.