M.D. Mays

Structural and electronic properties of TXF-TCNQ (X = S, Se, Te)

Abstract We report the crystal structure and physical properties of TTeF-TCNQ, compare the electrical conductivities of TXF-TCNQ (X ˭ S, Se, Te) by performing tight-binding band calculations on TXF-TCNQ, and examine the differences in the packing patterns of TXF-TCNQ (X ˭ S, Se, Te) by carrying out ab initio SCF-MO/MP2 calculations on appropriate model systems. The electrical conductivity of TXF-TCNQ increases as X varies from S to Se to Te. This increase is primarily caused by the conductivity enhancement in the donor stacks. In TXF-TCNQ (X ˭ S, Se) the C-H bonds of both the donor and the acceptor molecules make intermolecular contacts shorter than the van der Waals radii sums, but in TTeF-TCNQ only the C-H bonds of the donor molecules do. This difference in the packing patterns was rationalized on the basis of the C-H…X and N(sp)…X (X ˭ S, Se, Te) contact interaction energies estimated by ab initio SCF-MO/MP2 calculations on H3C-H…XH2 and H-C≡N…XH2.

The importance of interfaces and phases in switching and memory systems containing semiconducting charge-transfer complexes

Abstract Electrical measurements made on Cu-TCNQ thin films prepared in the manner originally described by Potember, Poehler and Cowan [1] strongly suggests that conduction channel formation can dominate the transport properties. This process might be attributed to a nonuniform Cu-TCNQ film which could arise as a consequence of a nonuniform oxide film formed on polycrystalline copper. Using a spherical single copper crystal, it was found that Cu-TCNQ grew at the Cu faces with the thinner oxide films.

The synthesis and study of new selenium and tellurium heterocyclic π-donors

Abstract Since the first reported synthesis of a tetratellurafulvalene π-donor [1], the number of compounds of this kind has slowly increased. We wish to report here the synthesis of two new tetratellurafulvalenes, tetramethylhexamethylene-tetratellurafulvalene (TMHM-TTeF) and dicyclopropylhexamethylenetetratellurafulvalene (CpHM-TTeF), together with their cyclic voltametry data and the conductivities of a number of their charge-transfer salts.

Initial studies on a new tellurium containing organic metal: Tetratellurafulvalene-tetracyanoquinodimethane (TTeF-TCNQ)

Abstract Experimental conditions for the growth of acicular crystals of the new organic conductor tetratellurafulvalene-tetracyanoquinodimethane (TTeF-TCNQ) are reported. The room temperature conductivity along the needle axis was found to be 1800 ± 300 (omega-cm)−1. dc Conductivity studies down to 77 K displayed metallic behavior and microwave conductivity studies showed the compound to still be highly conducting at 5 K. EPR studies down to 8 K were performed but no measurable resonances were recorded. Preliminary x-ray scattering studies indicate a segregated stack structure with a stacking distance of 3.95 ± 0.01 A. The charge transfer was found to be 0.71 ± 0.03 e- from infrared spectroscopy data. In most respects the physical properties of TTeF-TCNQ are similar to those of HMTSF-TCNQ.

Tellurium Containing Organic Metals

Abstract The electrical conductivity as a function of temperature is reported for single crystals of HMTTeF-TCNQ and HMTTeF-DMTCNQ. The crystal structure for one form of the DMTCNQ salt of HMTTeF is described and a procedure is outlined that could ultimately produce TMTTeF.

Organic Metals Containing Tellurium

Abstract Examination of the structural and electronic trends among organic metals and organic superconductors, which are composed of complex salts of heterocyclic π-donors, has been greatly facilitated by the preparation and study of a number of new tellurium containing organic π-donors. These studies required that a number of new synthetic strategies be developed. A brief review of these methods leading up to and including the recent synthesis of tetratellurafulvalene (TTeF) is provided. The increase in electrical conductivity on going from S to Se to Te in the series (TXF-TCNQ) (where X[dbnd]S, Se, Te) is shown to be consistent with the square of the bandwidths or to an increase in intrachain interaction (σ∞WT 2). While TTeF-TCNQ has not been found to undergo a transition to a superconducting state as the temperature is lowered at ambient pressure neither does it undergo a metal to insulator transition as is observed for TTF-TCNQ (Tmax = 59K) or for TSF-TCNQ (Tmax = 40K). This change in behavior is like...

New organic π-donors: Analogues of HMTSF and HMTTeF

Abstract In light of the interesting electrical properties in hexamethylenetetraselenafulvalene (HMTSF, 1a) and, most recently, hexamethylenetetratellurafulvalene (HMTTeF, 1b) based organic charge-transfer (C-T) salts, we wish to report here the synthesis of three new π-donors: β,β′-dimethylhexamethylenetetraselenafulvalene (DMHM-TSF, 2a); β, β′-dimethylhexamethylenetetratellurafulvalene (DMHM-TTeF, 2b); and β, β′-dicylopropylhexamethylenetetraselenafulvalene (CpHM-TSF, 3a), together with their cyclic voltammetry data and the conductivities of their TCNQ C-T salts.

An improved synthesis and purification of tetratellurafulvalene (TTeF)

Abstract The synthesis of tetratellurafulvalene (TTeF) was accomplished in a five-step, one-pot reaction from (Z)-1,2-bis-(trimethylstannyl)ethylene (1). Compound 1 was allowed to undergo two consecutive transmetallation/tellurium insertion steps. The resultant 1,2-dilithioditelluraethylene was treated with tetrachloroethylene which provided a 36% crude yield (5% purified). We have found that the use of tetrabromoethylene increases the yield (after rigorous purified). We have found that the use of tetrabromoethylene increases the yeild accomplished with the platinum reagent, cis-Pt(PPh3)2Cl2, and using this method we have shown that the tetrachloroethylene step, in the original TTeF synthesis, was a relatively low yield step in the reaction.

Electrical and magnetic studies on some new organic conductors made with tetratellurafulvalene (TTeF)

Abstract Experimental conditions for the growth of acicular crystals with the new organic donor tetratellurafulvalene (TTeF) are reported. dc Conductivity studies down to 77 K displayed metallic behavior for the tetracyanoquinodimethane (TCNQ) compound and an activated behavior for the perchlorate (ClO4). Microwave conductivity studies showed the TCNQ compound to still be highly conducting at 5 K. EPR studies down to 8 K were performed on the TCNQ salt but no measurable resonances were recorded. Preliminary x-ray scattering studies on the TCNQ compound indicate a segregated stack structure. The magnetic susceptibility of TTeF-TCNQ was measured and the charge transfer for the TCNQ salt was calculated from infrared spectroscopy.

(BTDM-TTF)-TCNQ complex, a new organic metal

Abstract Single crystals of the charge-transfer complex (BTDM-TTF)-TCNQ, showing metallic character down to 26K, have been obtained. The X-Ray structure reveals segregated stacks of donors and acceptors in a very similar packing with the related HMTSF-TCNQ complex. Electronic structure calculations using a tight-binding model indicate that (BTDM-TTF)-TCNQ is an almost pure ID metal. The electronic and magnetic properties are in accordance with its electronic and structural characteristics. This complex shows the lowest transition temperature reported up to date for charge transfer complexes of TTF derivatives.