Narrow-gap semiconductors with high conductivity and mobility are an important class of materials for various applications, especially for thermoelectric and optical device applications. Herein, we designed and synthesized novel organic narrow-gap semiconductors, which are modified forms of the main skeleton of a single-component pure organic metal, tetrathiafulvalene-extended dicarboxylate (TED). Molecular design of the TED derivatives with substituent groups on the skeleton led to highly-conducting semiconductors even when powder crystalline samples were used. Their thermopower is greater than that of metallic TED without a substituent group, demonstrating the successful tuning of carrier concentration in the TED system by molecular design. Near-/middle-infrared (IR) diffuse reflectance measurements revealed each band gap, and optical parameters extracted from the spectra evaluated the carrier concentration and mobility of the TED derivatives with fitting calculations on the basis of a Drude–Lorentz dielectric function.

Molecular packing arrangement is a very important factor in the charge carrier mobility of organic semiconductors, but its rational design has not been established as yet. Two-dimensional (2D) lamellar packing is an advantageous arrangement for high charge mobility, but few examples have been reported thus far. Herein we show crystal structures and the electronic properties of newly designed bis-fused tetrathiafulvalene (TTF) semiconductors with hetero substituent groups with distinct electronic effects. Unprecedented 2D lamellar alignment is achieved in a single crystal, where the bis-fused TTF rings interact three dimensionally with face-to-face and side-by-side intermolecular S···S contacts up to a total of 20 sites per π molecule and form graphitelike “brickblock” structure. The charge mobility of a single crystal is as high as 0.78 cm2 V–1 s–1. Systematic investigations of the semiconductors reveal a key role of intramolecular S···O interaction between a bis-fused TTF ring and a methoxycarbonyl group in controlling efficient arrangement, leading to high mobility.

Ammonium proton in a solid ionic semiconductor, TTFCOONH4, is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND4 exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH4. 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 TTFCOONH4 and TTFCOOND4 is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH4 shows 260 μV K−1, which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND4, with 138 μV K−1. 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.

A single crystal of anilinium tetrathiafulvalene-2-carboxylate exhibits a characteristic electrical conduction; it is a semiconductor with activation-type transport above 200 K; σrt = 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.

•Negative magnetoresistance is observed in single crystal of TTFCOONH3Ph at room temperature.•Magnetization curve of roughly oriented single crystals exhibits a hysteresis loop at room temperature.•TOF-SIMS analysis verifies trace amount of magnetic impurity in a single crystal of TTFCOONH3Ph.•Ferromagnetic resonance detects magnetically ordered spins at room temperature.•An isotope-substituted TTFCOOND3C6D5 shows a role of ionic part in the magnetic properties.TTFCOONH3Ph is a recently synthesized open-shell ionic semiconductor, the electronic state of which differs from that of typical organic closed-shell semiconductors. Magnetotransport properties were examined using a single-crystal sample, and found to exhibit small negative magnetoresistance (∼0.2%) for 9 T at room temperature (rt). The magnetization curve verifies the existence of a ferromagnetic (35%) and a paramagnetic (65%) component at rt, which is very similar to that of diluted magnetic semiconductors, despite the absence of any ferromagnetic metal elements. Electron spin resonance reveals weak localization of paramagnetic molecular spins, and moreover, ferromagnetic resonance confirms the existence of magnetically ordered spins in addition to the paramagnetic ones. The origin of the spin-polarized transport is discussed.

Bis-fused tetrathiafulvalenes with mono- and dicarboxylic acids, 2-{5-(1,3-dithiol-2-ylidene)-[1,3]dithiolo[4,5-d][1,3]dithiol-2-ylidene}-1,3-dithiole-4-carboxylic acid (1) and 2-{5-(1,3-dithiol-2-ylidene)-[1,3]dithiolo[4,5-d][1,3]dithiol-2-ylidene}-1,3-dithiole-4,5-dicarboxylic acid (2) have been synthesized. The electronic structure of 1 and 2 was examined from their optical absorption spectra and using density-functional calculations.

The origin of carrier doping in TTFCOO−NH4+ has been verified to include protonic defect in salt bridge by means of X-ray photoelectron spectroscopy (XPS), for the first time. The emergence of spin in TTFCOO−NH4+ is tunable over quite a wide range (9–33%) only by selecting a suitable solvent for the salt crystallization. The spin concentration of the solvent-dependent salts weakly correlates with intensity of optical absorption in near-infrared region, values of g-tensor and dc conductivity at rt. The solvents determining doping level of the salt are classified into three categories by self-dissociation ability (pKSH) of solvent, which likely controls inclusion of protonic defect in the salts.Highlights► The origin of the carrier doping in TTFCOO−NH4+ has been revealed to include protonic defects in the salt bridge by XPS. ► Doping level of TTFCOO−NH4+ is tunable by selecting suitable solvent over the range, 9–33%. ► Criterion determining doping level of TTFCOO−NH4+ correlates with self-dissociation ability of solvent, controlling inclusion of protonic defect in the salt.

A single crystal of anilinium tetrathiafulvalene-2-carboxylate exhibits a characteristic electrical conduction; it is a semiconductor with activation-type transport above 200 K; σrt = 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.

Ammonium proton in a solid ionic semiconductor, TTFCOONH4, is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND4 exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH4. 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 TTFCOONH4 and TTFCOOND4 is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH4 shows 260 μV K−1, which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND4, with 138 μV K−1. 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 1H NMR strongly indicates a stable metallic state of this compound down to 4 K.