TCNQ complexation

We saw in Section 12.2.3.1 that the presence of additional chalcogen atoms in BEDT-TTF/TCNQ promotes interstack interactions, suppressing the Peierls distortion and imparting upon the salt increased dimensionality compared to TTF/TCNQ. The result of including a different chalcogen into the TTF/TCNQ structure is shown in Table 2. Despite losing donor efficiency compared to TTF (Table 1) the TCNQ complexes of m/trans-diselenadithiafulvalene (DSDTF, 55/56) and TSF show an improvement in conductivity when two or four selenium atoms are incorporated. The reduced metal-insulator transition suggests that this effect is also caused by a suppression of the Peierls distortion. Increased Se-Se interstack contacts add dimensionality to the structure and limit the co-facial dimerisation typical of Peierls distortion. Wider conduction bands are afforded from the improved overlap of diffuse orbitals. 

Morita Y, Maki S, Ohmoto M, Kitagawa H, Okubo T, Mitani T, Nakasuji K (2002) Hydrogen-bonded charge-transfer complexes of TTF containing a uracil moiety crystal structures and electronic properties of the hydrogen cyananilate and TCNQ complexes. Org Lett 4 2185-2188... 

The diffuse reflectance spectra reported in Table I show that it is possible to assign a CN stretching frequency to both neutral and radical-anion TCNQ in crystalline samples of metal-TCNQ complexes because the reflectance peak for neutral TCNQ is shifted 20 cm higher in frequency than for radical-anion TCNQ". Specifically, the reflectance data for Cu-TCNQ when compared to other metal-TCNQ salts of known composition strongly suggests that neutral TCNQ is not present in the unswitched Cu-TCNQ films. On the other hand, the additional peak that appears in the spectra of Cu-TCNQ subjected to an applied field shows a peak superimpos-able with the peak recorded for neutral TCNQ in Cs2(TCNQ" )3. 

There are several interesting families of inorganic mixed-valence compounds that we have not discussed here (see Yvon, 1979 McCarley, 1982). For example, there are metal-cluster compounds such as the Chevrel phases, M,jMo6X8(X = S or Se) and condensed metal-cluster chain compounds such as TlMojScj, TijTe, NaMo O and M PtjO. TTF halides and TTF-TCNQ complexes (Section 1.9) constitute molecular mixed-valent systems in which the mixed valency is associated with an entire molecule the charge on TTF in such compounds is nonintegral. The structure of TTF-Br(, 79 and... 

The Cp2Ti(CO)(TCNE) and Cp2Ti(CO)(TCNQ) complexes were characterized in the oxidized and reduced forms through cyclic voltammetry, EPR, IR, and UV-visible spectroelectrochemistry. While oxidation at rather low potentials yields labile carbonyltitanium(IV) species of the TCNE and TCNQ ligands, the reduction occurs stepwise at unusually negative potentials, first on the ligand (to yield coordinated TCNE and TCNQ ) and then on the metal, to form Ti(II).i ... 

Cp2Ti(CO)2 and Cp2Ti(CO)2 react instantaneously with TCNE (tetracyanoethene) and TCNQ (7,7,8,8-tetracyano-j9-quinodimethane) to give the corresponding highly air-sensitive monocarbonyl TCNE (41) and TCNQ complexes. 

TTF TCNQ complex is, again, the existence of segregated stacks of donors and acceptors. 

The factors which influence the amount of charge transfer in complexes, e.g., of TCNQ have for long been a matter of dispute [263,329-331,335,336]. In one approach one has considered the ionic binding of the solid materials. Thereby one assumes that the electrons in the crystal of a TCNQ complex with a donor are in equilibrium between the neutral and anionic electronic structure according to Scheme 12a [337]. 

The face-toface contact between aromatic hydrocarbons such as that which occurs in the (C,oHg)2PFs structure (loc. cit.) of 3.2 A and that in TCNQ complexes of 3.24 A (Fritchie, C. J., Jr. Ada Crystallogr. 1966, 20, 892-898. Kobayachi, H. Bull. Chem. Soc., Jpn. 1975, 48, 1373-1377) is comparable with the sheet-to-sheet separation in graphite of 3.3 A. 

Complexes of the donor-acceptor type have been obtained by treatment of thietes with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and other electron acceptors. The TCNQ complex with 3-phenylthiete is blue-black and a 1 1 complex. 

Fits. 6.27 Change in (A) absorbance at 356 nm and (B) conductivi of azobenzene amphiphite/ TCNQ complex 45 LBK film (adapted from reference 123 with permission). 

Single crystals of the (TTT)(TCNQ) complex have been studied between 2°K and room temperature." At room temperature, the conductivity of this complex is 1 cm ... 

K, but then drops again as the temperature is reduced still further. These conductivities are comparable with those of (TTF)(TCNQ) complexes. The temperature dependence of the electrical properties of (TTT)2l3 has been studied in detail. 

Dithiotetracene (DTT) has been found as a by-product of some TTT syntheses,and its most likely structure is given in Table V although quinonoid structures may also perhaps be considered." A better synthesis of DTT has been given more recently and the (DTT)(TCNQ) complex has a single crystal conductivity of 3 10 D cm ... 

For Ar = phenyl, the single crystal conductivity of the charge-transfer salt is about 0.17 10 H cm , much less than for the (TTF)(TCNQ) complex, a fact which has been ascribed to a different type of crystal packing. ... 

After the discovery of the interesting electrical properties of tetrathia-fulvalene (TTF)-tetracyanoquinodimethane (TCNQ) complexes, many TTF derivatives have been prepared by reaction of 1,3-dithiolium salts with tertiary amines. This reaction has been interpreted as proceding by deprotonation of a 1,3-dithiolium cation to the corresponding carbene which in turn reacts as a nucleophile on the C-2 of another 1,3-dithiolium cation. This topic having been recently reviewed, we refer in Table 309,310 Qjjjy jQ papers subsequent to this review. ... 

The interatomic distances and angles of TTF and of its radical-cation in the TTF-TCNQ complex were measured. There are no significant differences in the angles between the neutral and ionized species. However, in the cation the sulfur-carbon distances are nearly all equivalent, while in TTF the sulfur to bridging-carbon distances are longer than the other S—C bonds, and C—C distances are both longer in the cation. 

Several HRP-TCNQ and RR P-TCNQ complexes illustrate mixed dimerized stacking, as confirmed by their triplet spin excitons (TSE) spectraSuch dimerized CT complexes are apparently common, but previous examples had essentially neutral ground states and required optically excited triplets. The partial ionicity of phenazine-TCNQ complexes results in a lower, thermally accessible triplet state. 

The sharp separation of CT solids in Fig. 2 into neutral (...DADA...) and ionic (...D A"D A"...) is not satisfied by several phenazine-TCNQ complexes whose ground states are best characterized as partly-ionic, with fractional q in mixed stacks. Partial ionicity raises both theoretical and experimental questions. The main theoretical problem for weakly-overlapping sites is that any one-electron treatment, even in the Hartree-Fock limit, yields a minimum energy for integral, rather than fractional q. Perturbing the individual molecular sites is not an attractive solution for small overlap. 

Table 4. Structural data for RP-TCNQ complexes with n-n dimers ...

The 5.10-dihydro-5-methyl-10-ethylphenazine (MEP) complex of TCNQ has been prepared but not characterized in detail. We anticipate the MEP-TCNQ structure and physical properties to resemble MjP-TCNQ and E2P-TCNQ. The 5.10-dihydro-5.10-diethylphenazine (E2P)-TCNQ complex s structure has been determined. The E2P moeities are folded along the N-N axis, as shown in Fig. 11 for MjP-TCNQ the two benzene halves have a dihedral angle of 12.7°. The mixed stack is again dimerized. Both ethyls are bent to one side of the average phenazine plane, a configuration that reinforces the dimerization or is reinforced by the dimerization. 

These caveats are borne out in Table 9, where selected bond lengths and angles, including the dihedral angle for N-N folding, are listed for phenazine-TCNQ complexes whose structures and estimated ionicities were mentioned individually in Sections 3-5. The selected features are the most sensitive to ionicity variations. The change from planar to bent geometry, and possibly the reduced symmetry of RP donors, are probably responsible for the limited structure-ionicity relations in Table 9. 

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