MESOMERIC FUSED BETAINIC RADICALS AS ORGANIC CONDUCTORS

G. Saito, K. Balodis, Y. Yoshida, M. Maesato, H. Yamochi, S. Khasanov, and T. Murata

1 INTRODUCTION

Varieties of one- to two-dimensional (ID, 2D) metallic and superconducting organic solids have been developed based on charge transfer (CT) complexes composed of donor and acceptor molecules, e.g. TTF, BEDO-TTF (BO), BEDT-TTF (ET), TCNQ, p-chloranil etc. (Scheme 1). They are multi-component conductors. As for the solids of uni-component, such features have been achieved only by specific physical methodology, namely high pressure, except for a few rare cases which include transition metals. Their transport properties have essentially been treated by band theory. For uni-molecule, on the other hand, metallic features have not yet been realized, where the transport phenomena are considered theoretically as either tunneling, ballistic or loop current. In the latter two cases the scattering events are of negligible probability, and the elastic mean free path λ of the carriers within a molecule limits such transport regime.

Studies of the electron transport in CT and uni-component solids have given two important issues which may be related with the transport in a uni-molecular conducting wire (i.e. molecular wire): I) the electron correlation U is of crucial importance for the organic metal, and 2) electron mean free path is not much longer than the lattice constant. As for the latter issue, the estimated intermolecular mean free path λinter is ~3Å at room temperature (RT) for an organic metal TTF·TCNQ, where dissipation events are mainly originated from defect, electron-phonon interaction, electron-molecular vibration coupling, conformational change of molecule and electron-electron interaction (U). In a molecular wire, the last three events will be the main sources of dissipation. Since the λ is proportional to the square of transfer integral t, the intramolecular mean free path λintra of TTF and TCNQ type molecules is estimated as 24-39 Å (λintra/λinter) = 0.7-0.9 eV/0.25-0.3 eV), which is not longer than the molecular length of the molecular wire proposed by many authors. Therefore, electron migration will be dissipated within a molecular wire before reacting to anodic electrode and ballistic transport is hardly achieved. That is a remarkable contrast between molecular and atomic wires (cf. λ, of Cu at RT ~300 Å). Consequently, the transport via the organic molecules should take into account the interaction of the charges within each molecule, which is usually described in terms of U. The development of molecules having small U is indispensable to widen the dissipationless regime of molecular wires.

2. BRIEF REVIEW OF UNI-COMPONENT ORGANIC CONDUCTORS

As is described in the preceding section, the control of U is necessary for not only bulk studies of (super)conductors and magnets but also nanoscience and technology of organic materials. It is noteworthy that U has been well examined quantitatively for bulk molecular conductors. This paper discusses the control of U by employing the conventional methodology of solid state science. Prior to describe our recent results, we briefly review the uni-component solids that exhibit metallic or highly conductive properties, in order to find a good candidate for small U system.

Applying pressure can induce the high conductivity for several molecular solids composed of closed shell molecules. Pentacene is the first example to exhibit metallic phase reported by Drickamer et al. in 1964 followed by Pt(dimethylglyoxime)2, Pt(benzoquinonedioxime)2, p-iodanil and hexaiodobenzene (Scheme 2). The latter two solids exhibit even superconductivity, suggesting that heavy atoms enhance not only the intermolecular interactions but also electron-phonon coupling.

Uni- or nearly uni-component conducting solids at ambient condition are classified into four groups: I) Organometallic coordination complexes, II) Neutral closed-shell molecules, III) Neutral π-radicals, and IV) Betainic π-radicals. The advantages of Group I stem from the characteristics of the transition metals; 1) mixed valence, as demonstrated by cytochrome-c3, 2) mixing of π-d orbitals to give rise to a small HOMO-LUMO gap (ΔE), as observed in the metallic dithiolate complexes, and 3) 3D crystal architecture as found in the phthalocyanine (Pc) solids. The metallic behavior down to low temperatures was reported on the Tl2Pc compound for the first time, having a resistivity at RT (ρRT) of 10-4 Ω cm. A specific 3D packing of the TL2Pc molecules is anticipated to form a 3D semimetallic band, typical of the assembly of the molecules with small ΔE. However, the reproducibility to obtain metallic Tl2Pc is poor.

All purely organic solids of uni-component so far prepared are semiconductors under ambient pressure with the following minimum ρRT; Group II (Scheme 4, 103-106 Ω cm), III (Schemes 5, 101-103 Ω cm), and IV (Scheme 6, 101-103 Ω cm).

The low-dimensional solids in Group II have an energy gap Δε of ca. ΔE - 2(t + t'), here t and t' are the intermolecular transfer energies of HOMOs and LUMOs, respectively. Although a high carrier mobility of 9-28 cm2 V-1 sec-1 for solids a and b in Scheme 4 was achieved by the aid of heteroatomic contacts and/or van der Waals interactions, t and t' were not yet enhanced enough at ambient pressure to afford a semimetal.

Within the band theory, Groups III and IV have a high potential for the creation of metals. In the Group III molecules, a, d and f in Scheme 5 are mono-radicals and b, c and e are bi-radicals. Molecules a, b and e may have degenerate SOMOs. For the group III solids, the large Ueff (= Δε + 4t", t": transfer energy of SOMOs), which is expressed as (U - V) where V is the nearest-neighbor Coulomb repulsion, dominates over the band width, resulting in the localization of carriers and hence the formation of Mott insulators. Solid of f shows a weak ferromagnetism due to spin-canting antiferromagnetic interactions with Tc = 36 K.

The betainic (zwitterionic) character of Group IV, where the donor (D) and acceptor (A) moieties are bound by π-bond forming D+-π-A-, is expected to decrease the Ueff according to the LeBlanc's theory as eq. 1

Ueff(Group IV solid) = (1 - α/r3)(U - V) (1)

where α is the molecular polarizability, and r is the average distance between the radical electron and the polarizable media. So far several betainic radicals or precursors have been prepared as shown in Scheme 6 (Molecule a in Scheme 5 may also have a betainic character).

In betainic form, they simultaneously have positive and negative charges located in close proximity. Among them, a and b have a large r value and hence their Ueff values are not much reduced resulting in poor conductivity, which may also be caused by inadequate stacking of the molecules. Even though the betainic radical molecule of c have not been prepared yet, a reduction of Ueff is expected compared to a and b. Furthermore, much effective reduction of Ueff is expected in the fused mesomeric betaine d, keeping the molecular size small, that is critical owing to the short λ value of organic materials.

Our classification of the uni-component organic conductors also evaluates the potential of the molecules for molecular wire. Even if the component molecules in Group I and II afford a semimetal, the molecule itself has a closed shell electronic structure that may cause a high bias for transport as a molecular wire. The dithiolate complexes in Group I (Scheme 3) may be too long for ballistic transport at RT though their exact Ueff and λintra are unknown. In case of Group III, the radical electrons bring about the competition among the itinerancy, localization and bond-formation by themselves. In fact, dimerization or polymerization has been observed in some cases, e.g. molecules c-e in Scheme 5. Although the competition is an interesting subject to study switching phenomena, this feature reduces the opportunity to provide ballistic molecular wires. The molecular wire consisting of the molecules in Group IV is of special interest since the structural feature, in which each ion radical part is located in the vicinity of the polarizable counter ion, has an analogous aspect to that of the polymer superconductor designed by Little.

In the following, we describe the preparation and electric resistivity, magnetic and optical properties on betaines 2 and 4-6 (Scheme 7) including the structural analysis of the TBA (tetrabutylammonium) salts of 1 and 3. We aimed 1) the deprotonation at pyrimido N atom and oxidation of TTF moiety to afford fully betainic system, 2) the introduction of ethylenedioxy group in 1 and 2 to provide moderate solubility like BO, 3) the N-Me group in 4 to prevent the formation of complementary hydrogen bonds (HBs) that were observed in the TBA salt of 1 (vide infra) and may be formed in 2, 5 and 6, and 4) the selenium analogue of 5 to improve the transport property of 6.

2 RESULTS AND DISCUSSION

2.1 Synthesis

1-6 were synthesized using the modified methods developed by Neilands' group. Figure 1 shows the synthetic procedures of neutral species of 5 and 6. The neutral species were converted into the TBA salts by treating with TBA·OH in acetonitrile. The betainic radicals were prepared either by electrooxidation of the TBA salts in acetonitrile, methanol or DMF, or chemical oxidation by TCNQ or I2 in acetonitrile. The latter procedure using h sometimes afforded a betainic radical contaminated with cation radical salt with I3, as has been observed for the first report on the highly conductive betaine 6 by Neilands et al. Since the betainic radicals, in general are extremely insoluble, the materials were purified by repeating the processes in Figure 2: (TBA salt of anion species) [??] (neutral species) [??] (betainic radical species). Namely the betainic radicals were converted to neutral species by the reduction with hydrazine hydrate and TBA·OH. Also the TBA salts can be returned back to the neutral species by treating with acetic acid in alcohol. The characteristics and preparative methods of the betainic radicals are summarized in Table1.

2.2 Crystal structures of the tetrabutylammonium salts of 1 and 3

Although the TBA salts of anion species of 5 and 6 or their analogues so far prepared (Scheme 8 shows other pyrimido fused TTF species, the physical properties of their betainic radicals will be reported elsewhere) have not afforded single ciystals, single crystals of 1 and 3 were obtained by recrystallization.

The anion 1 and water molecules in the TBA salt form a layer (Figure 3a) which is sandwiched by the layers of TBA cations along the b-axis. Robust HBs, N-H···O (2.881(4), 2.891(4) Å vs. the sum of the van der Waals radii of N and 0 = 3.07 Å) connect two 1 molecules to form homo-dimer 1=1 constituting the building unit of the crystal (shaded part in Figure 3a). Similar self-complementarity has been observed in uracilbetaine dimer and in the neutral species, dimethyl(2,4-dioxo(1H,3H)pyrimido)TTF; it is expected to be present in 2, 5 and 6. Such HB is of importance for molecular recognition of nucleobases 31 and for crystal engineering in supramolecules. However, its role for the electron transport is not clearly understood yet, and it has often been described as a source of high conductivity. It is desirable that HBs are uniform not to create the variation of transfer energies causing dimerized lattice with narrow bands susceptible to undesirable perturbation to give insulating state. Furthermore, it is desirable that hydrogen-bonded structures should provide at least 2D electronic structure to suppress Peierls or Jahn-Teller instabilities typical for low-dimensional conductors. In addition to those HBs, short side-by-side S···S atomic contacts (3.426(1) Å vs. the sum of the van der Waals radii of 3.60 Å) connect neighboring TTF moieties and contribute to the formation of a stable 2D network (Figure 3a).

Contrary, the anion molecules in the TBA salt of 3, where two kinds of independent 3 molecules exist, do not exhibit self-complemental) HBs as shown in Figures 3b and 3c confirming that N-Me group in 3 is effective to protect the pyrimido group from the formation of self-complementary HBs.

2.3 Optical spectra of betainic radicals 2 and 4-6.

The UV-Vis-NIR spectra of the betainic radicals dispersed in KBr are compared with that of a CT salt of fully ionized EDO-TTF molecule, EDO-TTF·IBr2 in Figure 4. The betainic radicals exhibit a characteristic low energy band near 4.5-7.6×103 cm-1 which disappears in diluted solution. This band is known to arise from CT between radical molecules. Its energy is represented by eq. 2 in analogy with that of the dimer with two electrons, where td and td* are the intra- and inter-dimer transfer integrals, respectively.

h VCT = Ueff/2 + (Ueff2/4 + 4td2)1/2 - 4td* (2)

The CT energy (Table 1) is related with the electron-correlation Ueff and is considerably low compared with those of CT solids of TTF and its alkyl derivatives like TMTTF (10-12×103 cm-1), or even of the extended TTF systems like BO or ET (6-10×103 cm-1, demonstrating that these betaines belong to low-Ueff system. A comparison with the spectrum of EDO-TTF·IBr2 (Figure 4, curve a) or TMTTF·Br (h VCT = 10×103 cm-1 implies that, owing to the betainic nature, the Ueff in the solid 4 is reduced by nearly 25%, i.e. (1 - α/r3) [congruent to] 3/4. For the other betainic radicals, which are presumed to have complementary HBs in the solids, the CT band appears near or below 5.0×103 cm-1 indicating that a dimerization of betaine molecules by complementary HBs further reduces Ueff in the solid by nearly 25%.

2.4 Resistivity and magnetic properties of betainic radicals 2 and 4-6.

The resistivity at RT (ρRT) and activation energy for conduction (εa) of the pellet samples of these betainic radicals and EDO-TTF·IBr2 are summarized in Table 1. At first the preparation methods to afford pure betainic radicals were studied on 5, which was contaminated with I3 by 2 mol% and exhibited low resistivity of 5-10 Ω cm in the first report by Neilands et al. We have found that both methods, electrooxidation and chemical oxidation by TCNQ followed by a through washing with acetonitrile, afforded samples not contaminated with starting reagent(s) and cation radicals. Thus the betainic radicals in Table 1 were prepared by either method. 5 was obtained as green powder either by electrooxidation or chemical oxidation with TCNQ, where the latter method is more preferable to yield large amount of betainic radicals than the former one. The compaction pellet sample of 5 is highly conductive (ρRT = 7-10 Ω cm, εa = 0.12-0.23 eV). Similarly, the selenium analogue 6 was obtained as highly conductive dark green powder with smaller activation energy than that of 5 (ρRT = 20 Ω cm, εa = 0.09-0.12 eV). These are the most conductive uni-component solids so far prepared except transition metal complexes, and as far as the activation energy concerns, 6 is the best betainic radical. However, no structure-property relation has been elucidated since no single crystals of 5 and 6 were available.