Structural and Physical Properties of a Series of 7,7,8,8-Tetracyanoquinodimethane Salts with Dications Bearing Two Terminal Pyridinium Rings

A series
of radical anion salts composed of 7,7,8,8-tetracyanoquinodimethane
(TCNQ) and dications with -CnH2n- (n = 1–6) chains between
two pyridinium rings are synthesized. The dication and TCNQ formed
either 1:4 or 1:3 stoichiometries with no correlation between the
composition and n. The 1:4 stoichiometry salts were
obtained when n = 1, 3, 4, 5, 6, where TCNQ formed
a one-dimensional column with either dimerization or tetramerization.
In these salts, the individual dication length varied considerably,
but the effective dication length (the distance between two adjacent
dications in the lattice) was independent of n. For n = 1, solvent molecules were involved. However, the 1:4
stoichiometry could not be formed for n = 2 since
an appropriate effective dication length could not be attained. The
1:3 stoichiometry salts were obtained when n = 2
and 6. In both crystals, TCNQ assembled into a trimer. For both stoichiometries,
the crystals were moderately conducting (resistivity at room temperature:
101–104 Ω cm) and had a semiconducting
temperature dependence, demonstrating nonuniform TCNQ stacking. The
charge-localized feature was revealed by the magnetic properties.
All of the salts showed antiferromagnetic interactions between the
localized spins with varying exchange interaction parameters, reflecting
the structural features.