DCNQI

If we now incorporate functional groups in the CeHe-based molecules we will have important molecules such as CA, DMe-DCNQI, TCNQ, TMPD, TNAP and ITT, but the really big jump is encountered when pentagons are allowed to be included. The possibilities to build new molecules become even more numerous than with only CeHe pieces, although we already had an immense number... 

Charge ordering below 220 K has also been found, in the organic conductor (DI-DCNQI)2Ag (Hiraki Kanoda, 1998). 

Kanoda K (2006) Metal-insulator transition in k-(ET)2X and (DCNQI)2M two contrasting manifestation of electron correlation. J Phys Soc Jpn 75 051007/1-16... 

The electrochemical properties of the imidazo[4,5-,7 [l,3,2]diazaborolidines 56, especially the AgEM constants (10 ), are close to the boron-bridged violenes indeed, the values obtained for 56 are on a par with those of the acceptor part in organic metals derived from TCNQ or -dicyanochinodiimine (DCNQI) (AgEM 10 -10 ) <2004CC1860>. 

Ion radicals play a role as mediators in these two-electron transfers. Each one-electron step achieves a maximal rate, and both rate constants become close. Coulombic repulsion of positive (or negative) charges makes the double-charged ion formation difficult. Therefore, donors (or acceptors) are preferable for which some possibility exists to disperse the charge. Extension of the 77-system reduces intramolecular coulombic repulsion in the dianion state. Electron-donor (or electron-acceptor) substituents should be located at diametrically opposite sites of the molecule. Examples are ll,ll,12,12-tetracyano-9, 10-an-thraquinodimethane, TCNQ, DCNQI, and tetracyanobenzene. 

The chemistry and physics DCNQI acceptors (45) were pioneered by Hunig. 

DCNQI salts represent an extremely interesting class of molecular con-... 

One of the most investigated salts based on this acceptor molecule is (DMe-DCNQI)2Cu, where DMe is dimethyl (=( 113)2). This compound is especially interesting because a metal-insulator transition, sometimes even with a reentrance to metallic behavior, can be induced either by pressure or by the substitution of hydrogen with deuterium in the organic complex... 

The crystal structure has tetragonal symmetry with the space group /4i/a. The planar DMe-DCNQI anions are stacked in ID columns along the tetragonal c axis. The LUMO consists of orbitals and forms a wide ID conduction band which is partly filled with electrons donated by the cations. The Cu ions are in a mixed valence state, namely [Cu ] [Cu" "] = 1 2... 

They interconnect four DCNQI molecules in a tetrahedral coordination. 

Table 4.3. Experimental and calculated values of dHvA frequencies and effective cyclotron masses in undeuterated (DMe-DCNQI)2Cu. For the e orbits no effective masses have been calculated. From [388]...

Fig. 4.45. (a) Schematic projection of the 3D FS of (DMe--DCNQI)2Cu in the repeated zone scheme, (b) Cut of the FS through the ab plane. The proposed dHvA orbits are shown by the solid lines. From [388]... 

At birth, the molecular metal was the one-dimensional metal. KCP and TTF-TCNQ are typical examples. The one-dimensional metal, however, is not a metal in the low temperature region due to the instability of the planar Fermi surface. Of course, this instability has provided rich physics [3], but is not favorable to the superconductivity. Therefore, chemists made efforts to increase the dimensionality of the electronic structure by the chemical modification with great success. The first organic superconducting system, the Bechgaard salt, is a quasi-one-dimensional system [4]. BEDT-TTF salts, the second-generation organic superconductors, have typical two-dimensional Fermi surfaces [5]. Three-dimensional Fermi surface has been found in the DCNQI-Cu salt [6]. 

The DCNQI-Cu salt indicates another way to the higher dimensional system with the use of the coordination bond [30]. The crystal structure of the anion radical salt (DCNQI)2Cu is shown in Fig. 8. Planar DCNQI molecules stack to form one-dimensional columns. These DCNQI columns are interconnected to each other through tetrahedrally coordinated Cu ions to form the three-dimensional DCNQI-Cu network. If there is no interaction between Cu and DCNQI, this structure gives only one-dimensional tt (LUMO) band, as is the case of ordinary molecular conductors. But, in this case, the Cu is in the mixed valence state [31] and provides three-dimensional band structure. 

Molecular conductors at the first stage were single component systems. For example, the conduction band in KCP is a one-dimensional dz band. TTF-TCNQ has a HOMO band of TTF and a LUMO band of TCNQ, but both of them are one-dimensional pure -ir bands. Recently, however, increasing number of interesting systems which have two bands with different characters near the Fermi level have been reported for example, the DCNQI-Cu salt with tt and (itinerant) d bands, Pd(dmit)2 salts with a two-dimensional HOMO band and a one-dimensional LUMO band, the organic superconductor (TMET-STF)2BF4... 

We have mentioned that molecular conductors exhibit simple and clear electronic structures where the simple tight-binding method is a good approximation. In most molecular metals, the conduction band originates from only one frontier moleeular orbital (HOMO for donor, LUMO for acceptor). This is because the inter-molecular transfer energy is smaller than energy differences among moleeular orbitals. However, it is possible to locate two bands with different characters near the Fermi level. In some cases, interplay of these two bands provides unique physical properties. The typical example is the (R, R2-DCNQI)2Cu system. 

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