Metallic conduction generally requires high carrier concentration and wide bandwidth derived from strong orbital interaction between atoms or molecules. These requisites are especially important in organic compounds because a molecule is fundamentally an insulator; only multi-component salts with strong intermolecular interaction-namely, only charge transfer complexes and conducting polymers-have demonstrated intrinsic metallic behaviour. Herein we report a single-component electroactive molecule, zwitterionic tetrathiafulvalene(TTF)-extended dicarboxylate radical (TED), exhibiting metallic conduction even at low temperatures. TED exhibits d.c. conductivities of 530 S cm at 300 K and 1,000 S cm at 50 K with copper-like electronic properties. Spectroscopic and theoretical investigations of the carrier-generation mechanism and the electronic states of this single molecular species reveal a unique electronic structure with a spin-density gradient in the extended TTF moieties that becomes, in itself, a metallic state.

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Ammonium proton in a solid ionic semiconductor, TTFCOONH 4 , is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND 4 exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH 4 . First-principles calculations reveal that an efficient proton-transfer pathway via low-barrier N/H + /N hydrogen bonds reduces the activation energy to 0.12 eV, which is quite small and comparable to that reported in a bulk water system. The ac conductivity of TTFCOONH 4 and TTFCOOND 4 is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH 4 shows 260 mV K À1 , which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND 4 , with 138 mV K À1 . The 1.9 H/D isotope effect in thermopower closely relates to the 2.2 H/D isotope effect in ion carrier mobility. Proton carriers in the temperature gradient enhance thermopower without cancelling out the effect of holes in the solid state owing to possession of the same positive charge.

Ammonium tetrathiapentalene carboxylate [(TTPCOO)2NH4] was prepared via protonic defect-induction doping without electrochemical oxidation. The high electric conductivity of 13 S cm(-1) and Pauli paramagnetic-like behavior of magnetic susceptibility in a wide temperature range exhibit a melting of the charge degrees of freedom induced by a mobile dopant in a salt bridge. Solid-state (1)H NMR strongly indicates a stable metallic state of this compound down to 4 K.

A single crystal of anilinium tetrathiafulvalene-2-carboxylate exhibits a characteristic electrical conduction; it is a semiconductor with activation-type transport above 200 K; σ(rt) = 0.16 S cm(-1) with an activation energy of 0.11 eV. On the other hand, below 200 K, it does not obey the Arrhenius relation but is conductive even at 4 K with 2.1 × 10(-4) S cm(-1) at a frequency of 2 MHz. Its behavior exhibits strong frequency dependence and suggests a particular conduction coupled with dielectric relaxation, reflecting its ionic nature. The crystal structure of the salt shows that conducting molecules are assembled supramolecularly with multiple nonbonding interactions, such as the hydrogen bond, and the π/π and CH/π interactions. The hydrogen bond and CH/π interactions have a short bond length, which is similar to the charge-assisted-type interaction observed in organometallics.

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