TSeF

The following compounds are the most popular donors for organic metals (see Scheme 8.9) TTF, tetraselenafulvalene (TSeF), tetrathiatetracene (TTT), tetraselenatetracene (TSeT), tetra-methyl tetraselenafulvalene (TMeTSeF), bis(thiadimethylene) tetrathiafulvalene (BTDM-TTF), bis(ethylenedithia) tetrathiafulvalene (BEDT TTF or ET), tetrakis(methyltelluro) tetrathiafulvalene (TTeCj-TTF), tetramethylbis(ethylenedithia) tetrathiafulvalene (TMET), 2,2 -(2,6-naphthalenediy-lydene) bis(l,3-dithiole) (NBDT). All the abbreviations are those used in the current literature. 

Takimiya et al. (2001, 2003, 2006) have synthesized methylenedithiotetraselenafulvalene (MDT-TSeF) and ethylenethiotetraselenafulvalene (ET-TSeF), which were found to be soluble in organic solvents. Using anodic electrolysis supported by BU4NAUI2 in ethanol-chlorobenzene solution, the authors have obtained diiodoaurate cation-radical salts of the donors. The formulas of the salts are depicted in Scheme 8.11. At room temperature, both salts showed very high conductivities of 2 X 10 and 1.6 X 10 cm , respectively. However, these salts differ with respect to... 

Increasing the polarizability of components facilitates the collective shift of electrons and the stabilization of the material s metallic state. Thus, substituting selenium for sulfur (changing from TTF to TSeF) allows one to obtain organic metals that do not transform into dielectrics up to very low temperatures. Chloride and bromide of tetraselenatetracene, (TSeT)2(Cl)i and (TSeT)2(Br)j have the same conductivity at room or low temperatures. 

The theoretical design of donor oligomers that gives parallel spins upon electron transfer is reported (Mizonchi et al. 1995). TTF and TSeF were nsed as the donor units in the corresponding ion-radical salts. Reviews by Enoki and Miyazaki (2004) as well as Turner and Day (2005) consider and explained magnetic properties of these systems from physical background. 

DMEDO-TSeF afforded eight superconductors. Six of them are k-(DMEDO-TSeF)[Au(CN)2](solvent) and their T s (1.7-5.3 K) are tuned by the use of cyclic ethers as solvent of crystallization [262]. 

Saito G, Yoshida Y, Murofushi H, Iwasawa N, Hiramatsu T, Otsuka A, Yamochi H, Isa K, Mineo-Ota E, Konno M, Mori T, Imaeda K, Inokuchi H (2010) Preparation, structures, and physical properties of tetrakis(aLkyIthio)-tetraseIenafuIvaiene (TTC -TSeF, n = 1-15). Bull Chem Soc Jpn 83 335-344... 

Shirahata T, Kibune M, Imakubo T (2006) New ambient pressure organic superconductors kh- and /CL-(DMEDO-TSeF)2[Au(CN)4](THF). Chem Commun 1592-1594... 

Kato R, Aonuma S, Okano Y, Sawa H, Tamura M, Kinoshita M, Oshima K, Kobayashi A, Bun K, Kobayashi H (1993) Metallic and supCTConducting salts based on an unsymmetrical Tt-donor dimethyl(ethylenedithio)tetraselenafulvalene (DMET-TSeF). Synth Met 61 199-206... 

The tetrabutylammonium salts of the bis chelates are semiconductors but exhibit enhanced conducting properties after doping with iodine. The corresponding salts which are formed when Bi N is replaced by TTF+ or TSeF+ have higher conductivities than the analogous dithiolene complexes with magnitudes similar to that of TTF-TCNQ (TTF = tetrathiafulvalene, TSeF = tetraselenofulvalene, TCNQ = tetracyano-p-quinodimethane). 

Fig. 31 The logarithmic derivative of the resistivity with respect to T 1 versus temperature as a function of impurity doping for samples of TSeF, TTF/TCNQ 0> x = 0 , x = 0.003 A, x = 0.0125 0, x = 0.025. The maxima at ca. 28 K and ca. 36 K correspond to the phase transitions. (After Craven etal., 1977)...

Chemical alloying is simply replacing parts of the molecules by isostructural and isoelectronic parts. For example, alloys such as (TTF) . .(TSeF TCNQ [163], (TMTSF), t(TMTTF),C104 [164], and (TMTSF)2(C104), (ReO,), [165] have been obtained. 

Figure 8 Room-temperature conductivity

Figure 9 Longitudinal resistivity ph versus temperature for (TSeT)2Cl at ambient pressure (from Ref. 18), TTF [Pd(dmit)2]2 at 24 kbar (from Ref. 70), and TSeF-TCNQ at 32 kbar (from unpublished results of J. R. Cooper, D. Jerome, and E. M. Engler, 1978).

Figure 10 Normalized room-temperature conductivities versus pressure for single crystals of TSeF-TCNQ. At ambient pressure ob = 800 100 (fl-cm)-1, and cra and (tc are 4 2 (fl-cm)"1. (From Ref. 27.)...

One of the initial motivations for pressure studies was to suppress the CDW transitions in TTF-TCNQ and its derivatives and thereby stabilize a metallic, and possibly superconducting, state at low temperatures [2]. Experiments on TTF-TCNQ and TSeF-TCNQ [27] showed an increase in the CDW or Peierls transition temperatures (Tp) with pressure, as shown in Fig. 12 [80], Later work on materials such as HMTTF-TCNQ showed that the transitions could be suppressed by pressure, but a true metallic state was not obtained up to about 30 kbar [81]. Instead, the ground state was very reminiscent of the semimetallic behavior observed for HMTSF-TCNQ, as shown by the resistivity data in Fig. 13. One possible mechanism for the formation of a semimetallic state is that, as proposed by Weger [82], it arises simply from hybridization of donor and acceptor wave functions. However, diffuse x-ray scattering lines [34] and reasonably sharp conductivity anomalies are often observed, so in many cases incommensurate lattice distortions must play a role. In other words, a semimetallic state can also arise when the Q vector of the CDW does not destroy the whole Fermi surface (FS) but leaves small pockets of holes and electrons. Such a situation is particularly likely in two-chain materials, where the direction of Q is determined not just by the FS nesting properties but by the Coulomb interaction between CDWs on the two chains [10]. 

Other organic superconductors are based on DMET (dimethyl-ethylene-dithio-diselenedithiafulvalene), an asymmetric molecule hybridized from ET and TMTSF [17]. According to its origin the physical properties are somewhere between one and two dimensional. Also based on asymmetric donors are the ambient-pressure superconductors (MDT-TTF)2Aul2 with Tc 3.5 K [18] and (DMET-TSeF)2A with X = A11I2 and I3 (7[ below 1K), where MDT-TTF stands for methylenedithio-TTF and DMET-TSeF for dimethyl-ethylenedithio-tetraselenafulvalene [19]. 

The organic molecule BETS [bis(ethylenedithio)tetraselenafulvalene, BEDT-TSeF, Figure 4.8] is one of the key molecules of this type. ... 

Diphenyl-l,2-diselenol-3-one and thiophosgene reacted with the formation of 3-chloro-4,5-diphenyl-1,2-diselenolylium chloride which was further reduced by Zn to 3,3 -bis(4,5-diphenyl-l,2-diselenol-ylidene), (1,2-TSeF) according to Scheme 5 <88BSF101>, 1,2-TSeF is one of the few known TSeFs <93SR245>. 

A Model for the Metal-Insulator Transition in TSeF-TCNQ (Abstract only) 301... 

X-Ray Scattering of TSeF-TCNQ and HMTSel -TCNQ (Abstract only) 361... 

Anisotropy in the Critical Behaviour of TTF-TCNQ and TSeF-TCNQ 469... 

The inverted band picture with hybridized bands appears to apply to TSeF-TCNQ, with both bands ordering at the same temperature, 29°K (Tl). This may be due to a larger tj. than in TTF-TCNQ or to longer correlation lengths and larger distortions on the donor chains (S3) or both. In HMTSF-TCNQ, the value of tj. is even larger, so that it behaves at low temperatures much more like a 3D semimetal (INV 3, S5, Jl). Pressures of the order of a few kbar are sufficient to increase tj. and 3D effects significantly in a number of TCNQ compounds (INV 10). 

A MODEL FOR THE METAL-INSULATOR TRANSITION IN TSeF-TCNQ... 

TSeF-TCNQ differs from its isostructural analog, TTF-TCNQ, in several important ways 1) It has only one... 

Some results of a X-ray diffuse scattering investigation on the charge transfer compounds TSeF-TCNQ and HMTSeF-TCNQ are presented. In the metallic state the results show the formation of a 1-D distortion or a Kohn anomaly in the phonon spectrum. For TSeF-TCNQ this is observaibelow T 238 K and corresponds to a periodicity of (3.15+ 0.05) Ibl along the chain axis. For HMTSeF-TCNQ the distortion was detected even at room temperature and has a period of (2-7+ 0,1) Icl In both materials the distortion has both transverse components. In the insulating state of TSeF-TCNQ below 29 K the 1-D distortion on each stack are correlated to font a 3-D super-lattice. 

In the last few years a large amount of work has been devoted to the search for better organic charge transfer conductors. Most of the efforts have been concentrated on the study of the TTF-TCNQ family /TQ/ and of the selenium analogue family, TSeF-TCNQ /TSe-Q/. 

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