Stoichiometry, TCNQ salt

The intermolecular (interionic) distances must be regular. This "mixed valency" requires that there be only one crystallographically unique molecular site, which must share its partial valency with the nearest neighbor sites along the stack. The many "complex stoichiometry" TCNQ salts—for example, Cs2(TCNQ)32 or triethylammonium(TCNQ)2-, which exhibit "trimeric" or "tetrameric" units of several crystallographically distinct TCNQ molecules and TCNQ- anions held at van der Waals separations—do not conduct well. 

As mentioned at the beginning of Section III, the conducting properties of organic charge-transfer materials are critically dependent on (1) their stoichiometry (i.e., the number of donor to acceptor molecules), and (2) the degree of charge transfer p per acceptor molecule. In particular, as a result of inherently strong Coulomb repulsions, all 1 1 TCNQ salts with complete ionization, or p = 1, are insulators [45]. 

Additional structural possibilities in Sections 3-5 include complex salts, with 2 3 or 1 2 stoichiometry for phenazine and TCNQ, and many polyiodides. Phenazines thus hardly fit into convenient isostructural families that facilitate systematic investigations. The organic donors related to tetrathiafulvalene (TTF) and macrocyclic organometallic donors involving porphyrins and phthalocyanines have been notably successful in producing isostructural series. The more numerous TCNQ salts and substituted phenazines display instead structural variety, although isostructural pairs undoubtedly exist. Now differences rather than similarities become interesting. 

Structure, Physical Properties and Phase Transition of a TCNQ Salt of 1 2 Stoichiometry DECA (TNCQ)2 499... 

Chemical oxidation of the TTF groups in compounds 34 and 35 has been achieved by reaction with an excess of iodine in dichloromethane solution, leading to new low-energy absorptions in the UV/visible spectra which are diagnostic of TTF cation radicals the broad absorption at = 830 nm for the iodide salt of 35 suggests the formation of aggregated TTF species. A charge transfer complex formed by 35 and tetracyano-p-quinodimethane (TCNQ) has been isolated as an insoluble black powder. The stoichiometry is (35), (TCNQ)3 (i.e. 8 TTF units 3... 

It Is postulated that mixed-valence species or complex salts (12) formed as a result of this field Induced redox reaction control the semiconducting behavior of these films and these complex salts exist In a solid-state equilibrium with the simple 1 1 salt. Since non-integral oxidation states are common In solids, It Is difficult to predict exact stoichiometry In the equilibrium equation, but a likely equation for switching In Cu-TCNQ, for example, may Involve... 

In the case of a mixed-valence salt containing neutral TCNQ there are more TCNQ molecules than there are unpaired electrons and, therefore, electrostatic repulsion of charge carriers Is kept at a minimum by allowing conduction electrons to occupy the empty molecular orbitals of TCNQ . This Is a lower energy pathway compared to putting more than one electron on the seime TCNQ site and It may explain how mixed-valence semiconducting salts like CS2 (TCNQ )s and the "switched" form of Cu-TCNQ can exhibit greater conductivity than similar salts with 1 1 stoichiometry. 

Complex" stoichiometry (MP)2(TCNQ) and (PP)2(TCNQ)i. A few complex salts of 5-alkyl- or 5-arylphenazines and TCNQ have been reported. The (MP)2(TCNQ)3 and (PPljfTCNQls structures have been determined. Struc-187... 

The donor properties of Pt(PTT)2 are sufficiently pronounced to allow the direct formation of a TCNQ complex of 1 1 stoichiometry by reaction with an acetonitrile solution of the acceptor. It was noted with initial satisfaction that, in accord with our earlier prediction concerning the distortion of mononuclear species, the TCNQ complex is insulating its main crystal axis conductivity at 300 °K is 5 10 0 cm However, preliminary x-ray diffraction data for Pt(PTT)2-TCNQ indicate that the crystal consists not of independent but of alternating D-A-D-A stacks. Therefore, there is still no proof for the distortion that we postulated in charge transfer salts of monouclear complexes. 

The highest conductivities thus far observed in organic molecules are found among the donor-acceptor complexes. The activation energies for conduction are small, on the order of 0.1 eV ( 2 kcal) or less. Examples are the complexes of aromatic hydrocarbons with iodine and complexes of amines with chloranil or related quinones. The best conductors have a high free-radical content and are usually not of simple stoichiometry. The complex salts of TCNQ ° (I) are the best of the known organic conductors. The... 

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