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...