TCNQ compound, transition metal

The four systems listed in Table 6 consist of stacks of planar organic molecules surrounded by transition-metal complexes that are not closely associated with each other. All but the [Pt(bipy)2][TCNQ]3 system contain planar organic cations and metal dithiolene anions. The properties of these compounds vary considerably, so each system will be described briefly. 

The transition-metal component of [Pt(bipy)2][TCNQ]3 is also diamagnetic. The structure86 consists of stacks of trimers, [T CNQ], surrounded by non-interacting Pt(bipy)2+ cations. Adjacent trimers are closely spaced with an interplanar separation of only 3.33 A as compared with 3.23 A within the trimer. The magnetic properties of this system have been studied much less extensively than those of [TMPD]2[Ni(mnt)2]. Single crystal EPR spectra do show, however, that spins are interacting along the TCNQ stacks. Structurally, this compound is very different from the 1 2 compound [Pt(bipy)2][TCNQ]258> (Sect. 2) which forms Da stacks with the two TCNQ moieties <7-bonded to each other.. ... 

Figure 1 also illustrates the fact that many of these anisotropic compounds undergo metal-semiconductor transitions below about 50 K [2]. For TTF-TCNQ, which was investigated intensively in the 1970s, diffuse x-ray scattering studies first showed conclusively that this was a Peierls transition [28]. There are, in fact, three phase transitions, at 53, 48, and 38 K. 

Thus, the interest of TCNE and TCNQ is more concentrated on their use as precursors of such materials than on their use as redox tools to obtain oxidized transition-metal compounds. Nevertheless, both aspects are somewhat connected, and transition-metal derivatives of TCNE and TCNQ have also been reviewed [281]. The most interesting aspect probably is the ability of TCNE to both oxidize and bind metal centers, but an important related problem is that TCNE is toxic. 

The temperature of the metal-to-insulator transition in TTF—TCNQ is 53 K. For systems with increased interchain coupling, the transition temperature for the onset of metallic conduction increases roughly as the square of the interaction between the chains. This behavior is tme as long as the coupling between chains remains relatively weak. For compounds with strong interactions between stacks, the material loses its quasi-ID behavior. Thus, the Peieds distortion does not occur even at low temperatures, and the materials remain conductive. 

Pal et al. [89] have reported on substituted ferrocenyl compounds, where one of the cyclopentadienyl rings is linked to an aromatic Schiff base, that were synthesized and analyzed for their second-order nonlinearity ((3). Their results indicate that the metal to ligand charge transfer (MLCT) transition dominates their second-order response. These compounds form charge transfer (CT) complexes with acceptors such as Ij, p-chloranil (CA), 2,3-dichloro-5,6-dicyano-l,4-bcnzoquinonc (DDQ), tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The CT complexes exhibit much higher second-order response. Bisferrocenyl complexes where two ferrocene moieties are linked through the same aromatic Schiff base... 

The synthesis of the first organic metal TTF-TCNQ was reported in 1973 by Coleman etal. and Ferraris et alP In this compound, TTF and TCNQ molecules are in 1 1 ratio and form separate stacked donor (TTF) columns and stacked acceptor (TCNQ) columns. A partial charge transfer between TTF and TCNQ transforms the molecular stacks into one-dimensional conductive paths via the formation of partially filled bands. Although TTF-TCNQ shows metallic conductivity down to around 60 K, it abruptly transfers to an insulator below 54 K, which was explained by a charge-density wave (CDW) phase-locking (Peierls transition ) due to its one dimensionality. 

Since the TCNQ stack was shown in the previous discussion to drive the metal-insulator transition of TTF-TCNQ it is quite puzzling that the corresponding temperature in TSeF-TCNQ having the ame kind of stack, is lowered. Moreover the relationship between the existence of several phase transitions and the presence of 2 kinds of stacks in TTF-TCNQ makes the existence of a single phase transition in TSeF-TCNQ quite surprising. In the following I will show that the difference between the 2 compounds is caused by a different hybridization between the donor and acceptor electronic wave functions. 

We should also mention the domain model suggested by J. C. Phillips which describes the metal-nonmetal transition in TTF-TCNQ and related compounds as a percolation transition in a system composed of metallic and insulating domains. In the present context, the essential feature of such a model is that the critical behavior would be isotropic as illustrated in Table 1. 

An important consideration,in deciding what compounds to synthesise,in order to stabilise the superconducting state versus the CDW or SDW in molecular lattices, is the effective dimensionality. A first priority is to make the lattice less purely one-dimensional, so as to make it stable to Peierls distortion. In that context, the compound (HMTSF)(TCNQ) was a significant early example.HMTSF (hexa-methylene-tetraselenofulvalene) was synthesised to explore the consequence of introducing more bulky substitution of the TSF moieties,with the expectation that the system would be rendered more purely one-dimensional. Yet, it was found that, unlike TMTSF-TCNQ, which like TTF-TCNQ has a major metal-insulator transition near... 

---