Kinetic Models for the Formation of “Unconventional” TCNQ Ion-Radical Salts of 1 : 1 Stoichiometry

While there are many examples of salts of 7,7,8,8-tetracyanoquinodimethan (TCNQ) which form segregated stack crystal structures, in TCNQ salts of 1:1 stoichiometry, the only cations which always give stacked structures are the alkali metals. The TCNQ salts formed by reaction with tetrathiafulvalene (TTF) i.e., TTF-TCNQ-TCNQ, a quasi-one-dimensional metal with a uniform segregated stack crystal structure, is termed “unconventional” because, before its discovery, reactions of neutral closed-shell donor with a neutral closed-shell acceptor would have been expected to give an alternating cation-anion stacking arrangement. Prior to the discovery of (TTF-TCNQ), the best organic conductor was the 1:1 TCNQ salt of the N-methyl-phenazinium (NMP) cation which was found to have a uniform segregated stack crystal structure. This 1:1 salt is unusual because at least twenty-nine other 1:1 TCNQ salts of planar closed-shell nitrogen heterocyclic monocations are insulators with crystal structures that do not exhibit the “infinite chains” of the NMP-TCNQ structure in the three reported cases. The perspective adopted is that the twenty-nine insulating salts cited above constitute “normal” behavior for this subclass of 1:1 TCNQ salts and that an explanation for the formation of NMP-TCNQ would be desirable. The formation of phases such as TTF-TCNQ, NMP-TCNQ, and all other 1:1 segregated stack structures in TCNQ salts may be accounted for by kinetic models involving van der Waals interactions, Mulliken donor-acceptor interactions, specific interatomic contacts, hydrogen bonding and molecular aggregation. The limitations of several of the models will be discussed. For example, the addition of bulky alkyl groups to the TTF ring system brings about a change in crystal structure from uniform segregated stacks to a conventional alternating cation-anion stacking arrangement. Additionally the difference in morphology of single crystal growth in supersaturated acetonitrile vs. the vapor phase for TTF-TCNQ is accounted for in terms of competing mechanisms for b- and a-axis growth. The implications of the model for formation of NMP-TCNQ, which invokes donor—acceptor interactions between several donor “impurities” and the acceptor NMP+, for the low temperature heat capacity and magnetic susceptibility are discussed. The conductivity of the insulator N-ethylphenazinium-TCNQ is increased by more than 109 by the selective addition of phenazine and TCNQ in solution via formation of a new phase with a uniform segregated stack structure.