Electron DCNQI

Kanoda K (2006) Metal-insulator transition in k-(ET)2X and (DCNQI)2M two contrasting manifestation of electron correlation. J Phys Soc Jpn 75 051007/1-16... 

Ion radicals play a role as mediators in these two-electron transfers. Each one-electron step achieves a maximal rate, and both rate constants become close. Coulombic repulsion of positive (or negative) charges makes the double-charged ion formation difficult. Therefore, donors (or acceptors) are preferable for which some possibility exists to disperse the charge. Extension of the 77-system reduces intramolecular coulombic repulsion in the dianion state. Electron-donor (or electron-acceptor) substituents should be located at diametrically opposite sites of the molecule. Examples are ll,ll,12,12-tetracyano-9, 10-an-thraquinodimethane, TCNQ, DCNQI, and tetracyanobenzene. 

The crystal structure has tetragonal symmetry with the space group /4i/a. The planar DMe-DCNQI anions are stacked in ID columns along the tetragonal c axis. The LUMO consists of orbitals and forms a wide ID conduction band which is partly filled with electrons donated by the cations. The Cu ions are in a mixed valence state, namely [Cu ] [Cu" "] = 1 2... 

At birth, the molecular metal was the one-dimensional metal. KCP and TTF-TCNQ are typical examples. The one-dimensional metal, however, is not a metal in the low temperature region due to the instability of the planar Fermi surface. Of course, this instability has provided rich physics [3], but is not favorable to the superconductivity. Therefore, chemists made efforts to increase the dimensionality of the electronic structure by the chemical modification with great success. The first organic superconducting system, the Bechgaard salt, is a quasi-one-dimensional system [4]. BEDT-TTF salts, the second-generation organic superconductors, have typical two-dimensional Fermi surfaces [5]. Three-dimensional Fermi surface has been found in the DCNQI-Cu salt [6]. 

We have mentioned that molecular conductors exhibit simple and clear electronic structures where the simple tight-binding method is a good approximation. In most molecular metals, the conduction band originates from only one frontier moleeular orbital (HOMO for donor, LUMO for acceptor). This is because the inter-molecular transfer energy is smaller than energy differences among moleeular orbitals. However, it is possible to locate two bands with different characters near the Fermi level. In some cases, interplay of these two bands provides unique physical properties. The typical example is the (R, R2-DCNQI)2Cu system. 

As mentioned in 1.1, the conduction band of the DCNQI-Cu system is composed of the d (d y) orbital of Cu and the LUMO of DCNQI. This system exhibits a variety of physical properties depending on the chemical modification or pressure. Figure 10 is a schematie phase diagram for the DCNQI-Cu system. The electronic states of this system are classified into three types according to transport properties. The type I state is metallic down to the lowest temperature. In the metallie state, the Cu ion is in the mixed-valence state and the valenee of Cu is elose to 4/3 -1-. Therefore, the one-dimensional organic pir band interacts with the Cu 3d orbital. An applieation of pressure transforms the type I state into the type II state. The type II state exhibits a sharp first-order metal-insulator (M-I) transition. The M-I transition of this system is accompanied by a CDW formation... 

Some Cu NMR studies have been carried out for the organic conductors (DMe-DCNQIjaCu, (DMeO-DCNQI)2Cu, (DBr-DCNQI)2Cu and partiaUy deuterated (DMe-DCNQI-d2[l, l 0])2Cu. The results show that the metallic and insulating states each possess similar electronic and magnetic properties. A systematic measurement of (l/Ti) for Cu failed to identify any specific effect of d-electron correlations near the metal-insulator (Cu-I) boundary. 

Space) is divided as usual into Brillouin zones (BZ). The 1st BZ extends from q = -7t Iau to +7TIau (Fig. 9.8a). When the amplitude V of the potential is weak and can be treated as a perturbation, the band splits at the boundary of the 1st BZ. The band gap is then 2 V. When the one-dimensional system has two electrons per unit cell, then n = Ijau- Then, kp = 7t jau, the band is thus filled and the onedimensional system is an insulator. When however n < Ifau, e.g. n = Ijau and therefore kp <7t ja, for the example kp = OSnIum, the system becomes metallic. This is the simplest picture of an intrinsic semiconductor or a metal. N < 2/um corresponds to a partial and n=llau full CT degree 8 per unit cell in an organic metal (CT complex, class 3), independently of how many molecules are in the unit cell. If it contains e.g. two acceptors and one donor (as in (DCNQI)jCu ), or two donors and one acceptor (as in (Fa)2 PF ), then for n = l/aM the CT degree of the... 

The relatively small changes in the geometrical and electronic configuration of the copper compound shown in the uppermost curves in Fig. 1.12 and in the two upper curves a and b in Fig. 9.15 are apparently sufficient to partially lift its one-dimensionality by increasing the interstack interactions via the bonds between the central Cu ion and the CN groups of the DCNQI. Here, the Cu " d states (d ) medi-... 

The charge-carrier mobility is based on the overlap of the n orbitals of the DCNQI molecules in the stacks. The high conductivity is due as in every conductor to the partial filling of the conduction band. Conduction along the Cu chains is not possible, because the Cu ions have closed electronic shells and because their spacing is too large. In the Cu salts of DCNQI, the Cu-Cu distance is about 50% greater than in Cu metal. 

Detection of the conduction electrons by ESR is not feasible in the conductive Cu salts. Owing to the spin-orbit coupling and their pseudo-three-dimensional character (see below), the relaxation times are too short and therefore the linewidths too large [17]. There is thus an anticoincidence here between high conductivity and ESR [18]. In the one-dimensional conducting state of the li salts of DCNQI, with a smaller spin-orbit interaction, the electron-spin resonance can, in contrast, indeed be observed. 

Fig. 3 gives the structures of (i) six classes of good one-electron donors [TTF and friends HMTTF and friends TTT and friends BEDT-TTF perylene, and MPc (metal phthalocyanines)] and (ii) three classes of good one-electron acceptors (TCNQ, TNAP, and DCNQI). All these donors and acceptors are flat, or almost perfectly flat, molecules this requirement seems almost self-evident if one wants to form a tight crystal lattice with good intermolecular overlap. It is conceivable that this requirement could be relaxed, but a good example of this has not been found. It is known that there are small non-planarities (e.g. in BEDT-TTF) but it is not clear whether this is critical for the solid-state properties. 

Recently, a new class of electron acceptors based on N,N -dicyanoquinonedi-imine (DCNQI), and a number of their charge-transfer complexes and anion radical salts have been prepared (1-5). Among them the compound (2,5-DM-DCNQI)2Cu has attracted special interest,because it exhibits very high electrical conductivity (up to 5x10 S cm below 10 K) and retains its metallic behavior down to 1.3 K without metal-insulator transition (3). Anion radical salts with other counterions exhibit lower conductivities and are semiconductors, or undergo a Peierls transition in the temperature range 100-150 K (4). 

Taking into account the chemical analogy between =0 and =C(CN)2 and between =C(CN)2 and =N-CN, Hiinig reported the DCNQIs (14) as a new class of efficient electron acceptor molecules [28] in the preparation of CT complexes and especially in highly conducting CT salts. These novel acceptors are readily available from the corresponding quinones in a one-step procedure by reaction with bis(trimethylsilyl)carbodii-mide (BTC) (13) in the presence of titanium tetrachloride. 

LUMO of DCNQI. Although the symmetries and atom coefficients of the LUMO orbitals of TCNQ and DCNQI are almost identical, the electron density on the nitrogen atom of the cyano group is larger in DCNQI, thus increasing the solvation energy (Figure 1.5). This has been used to explain the similar values found for the reduction potentials of both TCNQ and DCNQI molecules [30]. 

The unique properties exhibited by the DCNQI molecule are responsible for the excellent behaviour as electron acceptor component in the preparation of CT complexes and CT salts. Considering that the preparation of these electrically conducting materials will be... 

Based on the TCNQ and DCNQI molecules, during the last years a variety of novel acceptors have been reported. We will present some of the most significant modifications carried out on the quinoid skeleton and will discuss separately the rr-extended systems. N,7,7-tricyanoquinomethanimines (20) (Scheme 1.8), which can be considered as hybrids of the TCNQ and DCNQI systems, were reported by Bryce and co-workers in 1989 [45], These new electron acceptors were prepared from the corresponding quinones by reaction with malononitrile in the presence of titanium tetrachloride and pyridine (Lehnert s reagent) to form the dicyano-quinomethides (18), which on reaction with BTC afforded the hybrid tricyano derivatives (20). 

The CV measurements carried out in acetonitrile at room temperature are collected in Table 1.11. All the DCNQI derivatives showed two one-electron reversible reduction waves to the radical anions and dianions. These compounds displayed better reduction potentials than those reported for other DCNQI derivatives bearing two laterally fused rr-systems [130]. 

Although the CV data indicate that these 7t-extended DCNQI derivatives are slightly poorer electron acceptors than DCNQI, the smaller difference between the midpoint potentials for the first and second reductions, compared with that for the parent DCNQI, suggest a reduction of the intramolecular Coulomb repulsion due to the rr-system extension. 

Although it has not yet been studied by quantum-chemical calculations, preliminary results indicate that these A, A -dicyano-l,4-anthraquinonediimines should present an interesting photoinduced electron transfer from the donor naphthalene moiety to the acceptor unit located on the DCNQI ring in agreement with that observed for the TCNQ analogues recently reported [24]. 

The cyclic voltammetry measurements of these compounds (70 and 71) indicate the presence of two one-electron reduction waves to the radical anion and dianion at reduction potential values similar to those of TCNQ and DCNQI, as shown in Table 1.12. The voltammograms did not show the presence of an oxidation wave corresponding to the radical cation, thus suggesting that these compounds also behave in... 

Photoinduced electron transfer in DCNQI based systems electrochemical, structural and theoretical aspects... 

The electronic structures of compounds 79a and 80a were calculated using the PM3-optimized geometries and the VEH method. Figure 1.62 displays the energy, symmetry, and atomic orbital (AO) composition of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbital (LUMO) calculated for 79a. These orbitals are correlated with those of the parent DCNQI and those of benzo-DCNQI. The LUMO has an energy of -5.98 eV and is destabilized with respect to DCNQI (—6.67 eV) by 0.69 eV This destabilization already appears for benzo-DCNQI (—6.29 eV) and is due to the lateral extension of the TT-system which reduces the bonding interactions in the... 

The reduction process mainly affects the DCNQI moiety because the LUMO, i.e. the orbital where the extra electrons are placed, is located on this moiety. This is illustrated in Figure 1.63, where the PM3-optimized geometry obtained for the dianion of 79a is displayed together with that of DCNQI. ... 

The molecular structures obtained for the anions and dianions of the DCNQI derivatives 79a, 80a, and DCAQI, together with those previously reported for the TCNQ analogues, explain the different electrochemical behaviour these compounds show upon reduction. For the DCNQI derivatives, the acceptor moiety in both the anions and dianions is planar and achieves a degree of aromaticity similar to that of DCNQI " and DCNQI ", respectively. The cyclic voltammograms of compounds 79a, 80a, and DCAQI [32] therefore present two-well separated one-electron reduction waves that respectively correspond to the formation of the radical anion and dianion. For the anions of TCNQ derivatives, the acceptor moiety is by contrast highly distorted from planarity. The lack of planarity leads to less aromatic, i.e. less stable anions, and, as a consequence, the first reduction potential shifts to more negative values and collapses under a two-electron reduction wave with the second reduction potential [26,32]. 

We tried to improve the donor-acceptor properties obtained for previously reported compounds [26,162] by combining a strong electron-acceptor moiety such as benzo-TCNQ or benzo-DCNQI with the 1,4-benzoxazine system as the donor fragment. The synthesis of compounds 92 and 94 was carried out according to Scheme 1.17 by reaction of 2,3-dichloro-1,4-naphthoquinone with 2-methylaminophenol in pyridine yr) at 100°C to yield the novel A(-methylben-zo[6]phenoxazin-6,11-dione (91) in good yield. 

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