Radical-Anion Salts of DCNQI

In this chapter, we will not discuss the weak CT complexes further. In these complexes, at least in the ground state, the charge transfer S is not large. They in general crystallise in mixed, alternating stacks. We have already mentioned the example systems anthracene-tetracyanobenzene or A-TCNB (cf Fig. 1.6) and anthracene-pyromellitic acid-dianydride or A-PMDA (cf Fig. 6.14). 

The conductivities of these crystals lie typically in the range of 1(T to 10 Q cm clearly higher than those of pure anthracene crystals, for example. Their ground states, as shown by photoelectron spectra, are similar to anthracene, with 5-10% CT character. The triplet states Ti are likewise similar to anthracene, as seen in their optical spectra. The Si - So absorption is, however, very different it is a typical CT absorption. As its photoconductivity shows, the charge separation in anthracene-PMDA takes place in the Si state with an activation energy of 0.14 eV. 

In the two next sections, we wish to enter into more detail for two groups of radical-ion salts, because they demonstrate in a particularly impressive way the variety of interesting physical properties of organic metals and highly-conductive organic radical-ion salts these are the radical-ion salts with dicyanoquinone-diimine (DCNQI), already mentioned in Chap. 1 (Sect. 9.5), and the likewise previously mentioned radical-cation salts with fluoranthene (Fa) (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 

These radical-anion salts also provide a nice ejample of the way in which organic materials offer the possibility of broad variation of their physical properties through the multiphcity of compounds obtainable from organic chemistry. A good survey of this group of substances is given by the review article of Hiinig et al. [13] and, somewhat older, by Kato [14]. 

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