Crystals of Molecules with Charge Transfer, Radical-ion Salts

Crystals of Molecules with Charge Transfer, Radical-ion Salts 

StiU more strongly influenced by dipolar and Coulomb forces are the structures of the strong donor-acceptor crystals and the radical-ion salts. 

The ground-state wavefunction I g of a charge-transfer complex is in the simplest case given by 

The organic molecules are in these cases usually planar and form stacks, which can be mixed (D A D A ) or separated and (A A A A ). 

Stacks in which the neighbouring molecules are equidistant are termed regular, while those with dimerisation are termed alternating. The different stackings are of vital importance for determining the character of these materials as insulators, semiconductors, or metallic conductors. Fig. 2.17 indicates schematically that the TTF-TCNQ crystal is built up from separate stacks. An example of mixed stacks is the insulator anthracene-PMDA (Fig. 6.14). 

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