Organic charge-transfer conductors

Examination of the structure of the organic charge transfer conductors and the oxide superconductors reveals that, in general, they are anisotropic In each case the structures consist of two parts. In the organic radical cation salts, the parts are ... 

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/. 

The most efficient organic conductor material consists of co-crystals of tetracyano-Jt-quinodimethane, an electron-poor quinone analog, and tetrathia-fulvalen, an extremely potent electron donor. The crystals are green and have a conductivity of o = 1.5 x 10 Siemens cm Vat 66 K as compared to metallic copper with a a = 6 x 10 Siemens cm at 298 K. In order to obtain such high conductivity, organic charge transfer complexes must not appear as face-to-face dimers in crystals. In such cases, the acceptor takes up an electron and... 

In condensed-phase molecular systems, and particularly insulating molecular crystals, infrared and Raman spectroscopies are well-established tools for investigating molecular and crystalline structural properties. On the other hand, in inorganic semiconductors and metals the same spectroscopic methods are widely used to evaluate electronic band structure parameters. Organic charge transfer (CT) crystals, as molecular conductors and semiconductors, share some properties of both the above classes of solids, and, therefore, new spectroscopic phenomena are expected, and indeed observed for these materials. ... 

Carrier generators in molecular conductors have been associated for a long time to a partial charge transfer between the HOMO (or LUMO) electronic band and other chemical species. These systems are known as two-component molecular conductors. Tetrathiofulvalene derivatives are versatile systems for the formation of molecular organic conductors due to their electron donor capacity by transferring one u-electron from the HOMO orbital, and to their planar shape that promotes their stacking as a consequence of the n-n orbital overlap. The electronic properties of these salts are essentially determined by the packing pattern of the donor molecules which, in turn, depends on the counter-ion. 

Since the discovery of the first organic conductors based on TTF, [TTF]C1 in 1972 [38] and TTF - TCNQ in 1973 [39], TTF has been the elementary building block of hundreds of conducting salts [40] (1) charge-transfer salts if an electron acceptor such as TCNQ is used, and (2) cation radical salts when an innocent anion is introduced by electrocrystallization [41]. In both cases, a mixed-valence state of the TTF is required to allow for a metallic conductivity (Scheme 5), as the fully oxidized salts of TTF+ cation radicals most often either behave as Mott insulators (weakly interacting spins) or associate into... 

Bloch AN (1974) Design and study of one-dimensional organic conductors I the role of structural disorder. In Masuda K, Silver M (eds) Energy and charge transfer in organic semiconductors. Plenum, New York... 

Murata T, Nishimura K, Saito G (2007) Organic conductor based on nucleobase structural and electronic properties of a charge transfer solid composed of TCNQ anion radical and hemiprotonated cytosine. Mol Cryst Liq Cryst 466 101-112... 

The prospective applications ofmolecular assemblies seem so wide that their limits are difficult to set. The sizes of electronic devices in the computer industry are close to their lower limits. One simply cannot fit many more electronic elements into a cell since the walls between the elements in the cell would become too thin to insulate them effectively. Thus further miniaturization of today s devices will soon be virtually impossible. Therefore, another approach from bottom up was proposed. It consists in the creation of electronic devices of the size of a single molecule or of a well-defined molecular aggregate. This is an enormous technological task and only the first steps in this direction have been taken. In the future, organic compounds and supramolecular complexes will serve as conductors, as well as semi- and superconductors, since they can be easily obtained with sufficient, controllable purity and their properties can be fine tuned by minor adjustments of their structures. For instance, the charge-transfer complex of tetrathiafulvalene 21 with tetramethylquinodimethane 22 exhibits room- temperature conductivity [30] close to that of metals. Therefore it could be called an organic metal. Several systems which could serve as molecular devices have been proposed. One example of such a system which can also act as a sensor consists of a basic solution of phenolophthalein dye 10b with P-cyciodextrin 11. The purple solution of the dye not only loses its colour upon the complexation but the colour comes back when the solution is heated [31]. 

A particular subgroup of these low-dimensional conductors are the charge transfer complexes between dithiolenes and organic donor species such as TTF and related compounds. Here, nonintegral charge transfer is often seen as the reason for high conductivity down to low temperatures. 

---