DCNQI anion radical salts

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. 

The synthesis of new anion radical salts from DCNQI-type molecules has been presented recently [54]. Several of these salts have metallic behavior down to liquid He temperature it has been suggested that one of them, Cu(DMDCNQI)2, may have an AF ordered state around 5 K [55], but further experiments are necessary to confirm this point. 

Recently, a new class of electron acceptors based on N,N -dicyanoquinonedi-imine (DCNQI), and a number of their charge-transfer complexes and anion radical salts have been prepared (1-5). Among them the compound (2,5-DM-DCNQI)2Cu has attracted special interest,because it exhibits very high electrical conductivity (up to 5x10 S cm below 10 K) and retains its metallic behavior down to 1.3 K without metal-insulator transition (3). Anion radical salts with other counterions exhibit lower conductivities and are semiconductors, or undergo a Peierls transition in the temperature range 100-150 K (4). 

DM-DCNQI was synthesized according to the published procedure (1). The anion radical salts of Cu and Na were prepared upon reduction of the acceptor by the corresponding metal Iodide, while silver powder was used as the reducing agent for the preparation of the Ag-salt (4). These reactions were carried out in CH3CN, and resulted in the powders of the anion radical salts. For spectral measurements, free-standing pellets were pressed from the prepared compounds. 

It was demonstrated for TCNQ compounds that the reduction of the acceptor molecule causes significant vibrational frequency shifts, which are mostly pronounced for the totally symmetric rtMxles (21-23). Thus, the Raman spectra of the three anion radical salts of 2,5-DM-DCNQI have been measured and are presented in Figure 3. The spectrum of the neutral acceptor is also included for comparison. Even though no vibrational analysis for the neutral or charged acceptor has been reported so far, assignments of the main Raman bands can be proposed on the basis of the work on TCNQ compounds (21,22). 

Fig. 1.12 The temperature dependence of the specific electric conductivity cr of some Cu (DCNQI)2 radical-anion salts with different substituents of the two Me groups on the DCNQI molecules (cf Fig. 1.7). Me refers to a methyl group, I and Brto an iodine or bromine atom compare the image of the crystal...

In the Cu(DCNQI)2 radical-anion salt in the neighbourhood of the Peierls phase-transition temperature, it was found that the insulating state can be switched optically on a time scale of less than 20 ps into the conducting state (F. O. Karutz, H. C. Wolf et al, Phys. Rev. Lett. 81,140 (1998)). This optical switching process was termed a reversed Peierls transition . From the current transients, it was found that the switched volume must be at least 100 times larger than the directly photo-... 

There are a large number of strong CT complexes or radical-ion salts. Many organic compounds can be oxidised to radical cations by a suitable partner (e.g. perylene) or reduced to radical anions (e.g. TCNQ ".) Figure 9.12 shows a variety of such molecules. The complexes can consist of two organic molecules, as in the case of TTF-TCNQ. One of the partners can however also be an inorganic ion, e.g. Cu+ in (2,5-Dimethyl-DCNQI)2 Cu or also Ag", Li+, Cs+, Na", K+, Rb+ (see Sect. 9.5) or PF, as in the radical-cation salt (fluoranthene)J PF (see Sect. 9.6). 

Numerous 2,5-disubstituted N,N -dicyanoquinone-diimine molecules, Ri, R2-DCNQI (Ri, R2 = CH3, CH3O, Cl, Br, I) (compare Fig. 1.12) form 2 1 radical-anion salts with different monovalent metal ions (Cu, Ag+, Ii+, Na, K, Rb, Tl, and NHJ). The crystals, which are usually black, grow preferentially in the form... 

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