Segregated stack π-molecular complexes

Abstract

The phenomenon of high electrical conductivity in some organic crystals appears to be associated with stacked arrangements of radical ions of electron donors and/or acceptors. A variety of chemical types has been developed, principally based on the parent donor tetrathiafulvalene (TTF) and the parent acceptor tetracycanoquinodimethane (TCNQ). TTF cation radical salts and TCNQ anion radical salts have stacked structures with marked anisotropy of arrangement and physical properties, and hence are termed ‘quasi-one dimensional’. However, lateral interactions between stacks lead to a measure of two-dimensional character in some salts, and this is taking on increasing importance with the development of some of the newer types of donor and acceptor. The degree of stacking ranges from π-dimeric pairs to stacks of infinite length — the longer stacks often contain dimeric or tetradic sub-groupings, particularly in the TCNQ radical anion salts. In the cation radical-anion radical salts, there are segregated stacks of cation and anion radicals. {[TTF][TCNQ]} itself has monad stacks in its averaged structure, but there are also examples of diad stacks. The phase transitions in {[TTF][TCNQ]} below 54 K are unusual, the drastic drop in stack axis conductivity on cooling below 54 K not being accompanied by appreciable changes in average moiety arrangement. Studies of the very weak diffuse scattering above 54 K and the weak satellite reflections below 54 K lead to a model in which the high resistivity below 54 K is accounted for by pinned charge density waves (CDW), which become mobile above the phase transitions. In the temperature region from 54 K up to 300 K, the conductivity can be explained semi-quantitatively by a combination of CDW and single phonon scattering.