Semiconducting behavior in simple and complex charge-transfer salts of tetracyanoquinodimethane with substituted pyridinium donors

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Abstract
A series of eight anion–radical salts has been prepared, each with tetracyano-p-quinodimethane (TCNQ) as the acceptor and a substituted pyridinium ion as the donor. Four species of donor substitution were made as follows: placement of a CN group at either the 3 or 4 position on the pyridinium ring, and use of either N-methyl or N-benzyl substituents. The simple (1:1) and complex (1:2) salts were crystallized for each species. Compaction conductivity measured between 100 and 300 K showed that the simple salts are weakly conducting semiconductors, of activation energy ≳0.3 eV. Single crystal conductivity data for two of these showed essentially isotropic conduction, compatible with alternating stack structures. The four 1:2 salts are all semiconductors, with considerably enhanced conductivity and with activation energies ?0.15 eV. Single crystal data for one of these showed the large conductivity anisotropy characteristic of structures with segregated TCNQ stacks.