Strong CT Complexes, Radical-ion Salts

In crystals which are composed of donors and acceptors, one can distinguish two limiting cases weak CT complexes and strong CT complexes or radical-ion salts. The boundary between the two groups is fuzzy. 

See in this connection the examples given in Chaps. 1 and 2, anthracene-TCNB (Fig. 1.6) and in Chap. 6, anthracene-PMDA (Fig. 6.14). 

In the compounds which are composed of donors and acceptors in a ratio other than 1 1, the charge is correspondingly distributed with a ratio of 2 1, two partners with (formally) one-half elementary charge each have a singly-charged partner. 

Typical and important for the strong CT and the radical-ion crystals is in many cases an arrangement of donors and acceptors in separate stacks, as in the compound TTF-TCNQ, Fig. 2.17. This is shown schematically in Fig. 9.1b. 

In many radical-ion salts with separate stacks, one finds conversely a high electrical dark conductivity, sometimes also with metallic character. One then refers to organic metals. We shall concern ourselves with these salts in more detail in the following sections of this chapter. There are, however, also numerous strong CT complexes with separate stacks in which the conductivity is lower. Table 9.1 lists the conductivities of some CT complexes and radical-ion salts of TCNQ. 

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