TCNQ molecule

Single-Stack Acceptor. Simple charge-transfer salts formed from the planar acceptor TCNQ have a stacked arrangement with the TCNQ units facing each other (intermolecular distances of ca 0.3 nm (- 3). Complex salts of TCNQ such as TEA(TCNQ)2 consist of stacks of parallel TCNQ molecules, with cation sites between the stacks (17). The interatomic distance between TCNQ units is not always uniform in these salts, and formation of TCNQ dimers (as in TEA(TCNQ)2) and trimers (as in Cs2(TCNQ)Q can lead to complex crystal stmctures for the chainlike salts. 

The TCNQ molecule in [TR(bzim)]2-TCNQ is sandwiched between two units of [ J,-N, C -bzimAu]3 in a face-to-face manner so that it is best represented by the formula (7t-[ J,-N, C -bzimAu]3)( j,-TCNQ)(7t-[p-N, C -bzimAu]3). The cyanide groups clearly are not coordinated to the gold atoms. The distance between the centroid of TCNQ to the centroid of the AU3 unit is 3.964 A. The packing of [TR(bzim)]2-TCNQ shows a stacked linear-chain structure with a repeat pattern of-(Au3)(Au3)(p-TCNQ) (Au3)(Au3)(p-TCNQ)- an ABBABB repeat The complex [TR(bzim)]2-TCNQ contains two very short intermolecular Au Au distances of 3.152 A (identical for the two aurophilic bonds). The intermolecular Au Au distance is even shorter than the intramolecular distances in the starting compound, which are 3.475, 3.471, and 3.534 A. The adjacent AU3 units in [TR(bzim)]2-TCNQ form a chair-type structure rather than the face-to-face (nearly eclipsed) pattern reported in Balch s studies of the nitro-9-fluorenones adducts with the trinuclear Au(I) alkyl-substituted carbeniate complexes. 

The donor cations H2[pz(iV-Me2)8]+ and acceptor anions TCNQ are each situated about a crystallographic inversion center. The pz is essentially planar and the pz and TCNQ molecules form a stepped stack that extends in the crystallographic b direction. 

Since the molecular volumes of neither the TTF nor the TCNQ molecule are easily described by a regular volume of integration, the parallelepiped subunit method was used and integration of the valence density was performed using Eqs. 

Figure 6.16 shows the NEXAFS spectra as functions of the incidence angle 9e for the oriented TTF-TCNQ hlms. For 0e = 0° and Fe close to 90° E lies parallel and perpendicular to the molecular ai>-plane, respectively. In this case the planar geometry of both TTF and TCNQ molecules is a clear advantage strongly simplifying the analysis. 

Figure 6.17. Schematic representation of the relevant TCNQ unoccupied MOs for the analysis of the NEXAFS spectra of TCNQ and TTF-TCNQ. The symmetry labels are those appropriate for >2 symmetry. The energy values given (in eV) are relative to those of the LUMO of TCNQ in the isolated TCNQ molecule. Reprinted with permission from J. Fraxedas, Y. J. Lee, I. Jimdnez, R. Gago, R. M. Nieminen, R Ordejon and E. Canadell, Physical Review B, 68, 195115 (2003). Copyright (2003) by the American Physical Society.

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. 

FIGURE 6.7 Structures of (a) TTF and TCNQ and (b) solid TTF-TCNQ, showing alternate stacks of TTF and TCNQ molecules. 

Construction of LB films having lateral d.c. conductivities is a burgeoning activity. Results of published work are summarized in Table 9 [726-772]. The first formation of a conducting LB film was reported by French workers in 1985 [726]. Non-conducting LB films were formed from N-docosylpyridinium TCNQ. Subsequent exposure to iodine vapor resulted, however, in the lateral conductivities in the order of 0.1 S cm-1 [726]. The initially formed LB film was shown to consist of (TCNQ. )2 dimers whose molecular planes were almost parallel to the substrate. Iodination resulted in the development of a brown-purple color, the partial oxidization erf the radical anions to TCNQ° and, most importantly, a dramatic rearrangement of the LB film. In iodine-doped films, the TCNQ molecules have been shown to assume a position almost perpendicular to the substrate [721, 773],... 

In the case of the trinuclear [ t-N1,C2-bzimAu]3 (bzim = benzylimidazolate), in addition to the extended structures that form with other metals (see Section 6.3), it also forms supramolecular networks, acting as an electron donor with small organic acids [48]. For example, it reacts with TCNQ (tetracyanoquinodimethane) giving rise to a columnar structure in which each TCNQ molecule is sandwiched between two units of the trinuclear complex in a face-to-face manner. Thus, the repetition of this pattern leads to a stacking of the type (Au3)(Au3)( t-TCNQ)(Au3)... 

Fig. 23 The structure of the complex salt Cs2(TCNQ)3, an example of the parallel face-to-face stacking of planar TCNQ molecules in one direction which is characteristic of strong charge-transfer complexes. (After Chesnut and Arthur, 1962)...

Since their early studies Eley et al. (1959) (see Eley, 1967) have largely confined their attentions to the study of the electronic and structural properties of bipyridinium2+ (TCNQ)2" and related complexes (Ashwell et al., 1975a, b, c Ashwell et al., 1977a, b, c Eley et al., 1977). Most complexes, such as 4,4-bipyridyl (TCNQ)2, five (N,N-dialkyl-4,4 -bipyridylium)2+ (TCNQ)J+ salts (alkyl = methyl, ethyl, propyl, isopropyl and benzyl ), and l,2-di(N-ethyl-4-pyridinium) ethylene2+ (TCNQ), are low gap semiconductors except one form of the last compound which exhibited metallic behaviour. The asterisked complexes comprise planar sheets of TCNQ molecules grouped in tetrads. 

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. 

In order to ensure a metallic state, partial filling of the conduction band is required. This may be obtained either through partial oxidation, as in nonintegral oxidation state (NIOS) salts such as KCP (14, 48-52), or through partial charge transfer, as in donor-acceptor (D-A) adducts such as (TTF) -(TCNQ) involving donor (such as TTF) and acceptor (such as TCNQ) molecules (11-13). 

We should note here that the bond length versus charge-transfer relationship in TCNQ molecules has been established by Flandrois and Chasseau [72] from x-ray data and is thus valid for x-ray data only. In fact, x-rays and neutrons provide atom positions which generally do not coincide. The former radiation measures the charge centroid location, while the latter measures the nuclear position. As a consequence, a different parametri-zation must be used for neutron data [36]. 

Figure 28 Room-temperature STM image of a (001) face of a TTF-TCNQ crystal. The image is 18 A wide. The white lines are the unit cell in the (a,b) plane. The one-dimensional conduction band is formed by the wave functions of p electrons on TTF and TCNQ molecules. It is suggested that the double-row features (small arrows) can be identified as the CN radicals at the extremes of the TCNQ molecules, and the row between (bigger arrow) as the ir-bond electron clouds of the TTF molecules. (From Ref. 209.)...

This type of interaction determines above all the optical properties of lowdimensional organic conductors in the middle infrared region. It leads to activation of modes that are normally nonactive in the IR. For the tetra-cyano-p-quinodimethane (TCNQ), tetrathiafulvalene (TTF), and other symmetrical molecules, tt molecular orbital occupied by the radical electron are nondegenerate, so linear e-mv coupling is possible only for the totally symmetric (ag) modes. For example, TCNQ molecule has 54 normal modes, among which only 10 are the ag modes. They cover a range of frequencies from about 130 cm-1 to about 3050 cm-1 [16], while TTF molecules has seven totally symmetric modes of the range 250 cm-1 < toa < 3100 cm-1... 

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