Incorporating Spacer Molecules into the Tetrathiafulvalene-p-Chloranil Charge-Transfer Framework: Modulating the Neutral-Ionic Phase Transition.

The charge-transfer (CT) tetrathiafulvalene-p-chloranil (TTF-CA) crystal, a representative functional organic electronic material, has been the subject of both basic and applied research. This material shows a neutral-ionic phase transition (NIPT) that induces drastic changes in its physical properties. Here, we use this crystal as a framework and demonstrate a method for modulating physical properties of TTF-CA. A number of multicomponent (ternary) CT crystals were obtained by crystallizing TTF-CA with a third molecular species. These complexes all contain molecular sheets formed with TTF-CA; however, the third molecules were differently inserted between these sheets as spacers to induce a variety of physical properties in the CT crystals. Some showed spacer-modified NIPT, while the transition to the ionic state was suppressed in one complex despite the presence of TTF-CA sheets, which indicates that spacer molecules can modulate the physical properties or functions of CT crystals.

Band-Like Carrier Transport at the Single-Crystal Contact Interfaces between 2,5-Difluoro-7,7,8,8-tetracyanoquinodimethane and Electron Donors.

Some heterojunction interfaces formed with molecular solids show metal-like transport behavior. In order to clarify the requirement, interfaces are fabricated by lamination of single-crystal electron-accepting 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2TCNQ) and electron-donating molecules with a wide range of ionization potentials. Carrier injection between the acceptor and donor crystals leads to highly conducting interfaces, some of which exhibited band-like charge transport behaviors. Combinations with weak donors also resulted in interfaces with band-like transport properties. Accordingly, band-like conduction was achieved for interfaces where the donor and acceptor crystals do not have well-matched band energies. The results indicate that the wide range of candidates have great potential for modification of the electronic structure of organic crystals. The present method is expected to enable control of the electronic properties of the interface.

Ferroelectricity and Piezoelectricity in Free-Standing Polycrystalline Films of Plastic Crystals.

Plastic crystals represent a unique compound class that is often encountered in molecules with globular structures. The highly symmetric cubic crystal structure of plastic crystals endows these materials with multiaxial ferroelectricity that allows a three-dimensional realignment of the polarization axes of the crystals, which cannot be achieved using conventional molecular ferroelectric crystals with low crystal symmetry. In this work, we focused our attention on malleability as another characteristic feature of plastic crystals. We have synthesized the new plastic/ferroelectric ionic crystals tetramethylammonium tetrachloroferrate(III) and tetramethylammonium bromotrichloroferrate(III), and discovered that free-standing translucent films can be easily prepared by pressing powdered samples of these compounds. The thus obtained polycrystalline films exhibit ferroelectric polarization switching and a relatively large piezoelectric response at room temperature. The ready availability of functional films demonstrates the practical utility of such plastic/ferroelectric crystals, and considering the vast variety of possible constituent cations and anions, a wide range of applications should be expected for these unique and attractive functional materials.

Structural and transport properties of neutral radical crystals of CoIII(tmp)(CN)2 (tmp = 5,10,15,20-tetramethylporphyrinato) and the CN-bridged polymer [CoIII(tmp)(CN)]n.

An axially ligated Co(tmp) (tmp = 5,10,15,20-tetramethylporphyrinato) anion, [CoIII(tmp)(CN)2]-, has been prepared and subjected to electrochemical oxidation to obtain the open shell tmp π-ligand. With acetone as the solvent, solvent-free neutral radical crystals of CoIII(tmp)(CN)2 are obtained, whereas solvent-inclusive crystals of CoIII(tmp)(CN)2·2CH3CN are obtained with CH3CN as the solvent. In both crystals, the open shell tmp ring deforms into a ruffled form, which makes the π-π interactions in the crystal weak. Thus, the electrical conductivity is low, and the crystals behave as semiconductors with a room temperature resistivity of 106 Ω cm and an activation energy of about 0.3 eV. When the solvent is acetone, non-oxidized crystals of the CN-bridged polymer {[CoIII(tmp)(CN)](acetone)}n are obtained as a byproduct. The closed shell tmp ring deforms into a ruffled form, and there π-π interactions in the crystal are negligible. The room temperature resistivity is rather high at about 108 Ω cm.

The magnetoresistance effect in a conducting molecular crystal consisting of dicyano(phthalocyaninato)manganese(iii).

A conducting molecular crystal TPP[MnIII(Pc)(CN)2]2 (MnIII: d4, S = 1, TPP = tetraphenylphosphonium and Pc = phthalocyanine) was fabricated. In its crystal structure, the [MnIII(Pc)(CN)2] units formed a one-dimensional regular chain along the c-axis with an overlap integral value of 8.6 × 10-3. TPP[MnIII(Pc)(CN)2]2 showed a semiconducting behaviour that also has been observed for isostructural TPP[CoIII(Pc)(CN)2]2 (CoIII: d6, S = 0) and TPP[FeIII(Pc)(CN)2]2 (FeIII: d5, S = 1/2) whose ground states are charge-ordered states. In spite of the local magnetic moment of the MnIII ion (S = 1) at the centre of the Pc ligand, TPP[MnIII(Pc)(CN)2]2 exhibited an almost isotropic and small negative magnetoresistance (MR) effect (the MR ratio was -8.7% under 8 T at 10.7 K), contrarily to the anisotropic giant negative MR effect of TPP[FeIII(Pc)(CN)2]2. The isotropy was found to be due to a (dxz)1(dyz)1 electronic configuration, and the smaller MR effect was explained by a weaker antiferromagnetic interaction between MnIII ions than that between FeIII ions, as suggested by a Weiss temperature Θ of -3.1 K (|Jdd|/kB = 1.2 K).

Directionally tunable and mechanically deformable ferroelectric crystals from rotating polar globular ionic molecules.

Ferroelectrics are used in a wide range of applications, including memory elements, capacitors and sensors. Recently, molecular ferroelectric crystals have attracted interest as viable alternatives to conventional ceramic ferroelectrics because of their solution processability and lack of toxicity. Here we show that a class of molecular compounds-known as plastic crystals-can exhibit ferroelectricity if the constituents are judiciously chosen from polar ionic molecules. The intrinsic features of plastic crystals, for example, the rotational motion of molecules and phase transitions with lattice-symmetry changes, provide the crystals with unique ferroelectric properties relative to those of conventional molecular crystals. This allows a flexible alteration of the polarization axis direction in a grown crystal by applying an electric field. Owing to the tunable nature of the crystal orientation, together with mechanical deformability, this type of molecular crystal represents an attractive functional material that could find use in a diverse range of applications.

Transport Characteristics of the Organic Field-Effect Transistors Based on Charge Transfer Complex as Semiconductors.

The n-type organic field-effect transistors are fabricated with using four kinds of charge transfer (CT) complexes with PXX (peri-xanthenoxanthene) as a donor component. The CT complexes with four kinds of acceptors form mixed-stack type one-dimentional columns with different PXX-acceptor overlaps. Comparison of the field-effect properties reveals the correlation between the device performance and intermolecular interaction in the semiconducting CT complexes.

Molecular motion, dielectric response, and phase transition of charge-transfer crystals: acquired dynamic and dielectric properties of polar molecules in crystals.

Molecules in crystals often suffer from severe limitations on their dynamic processes, especially on those involving large structural changes. Crystalline compounds, therefore, usually fail to realize their potential as dielectric materials even when they have large dipole moments. To enable polar molecules to undergo dynamic processes and to provide their crystals with dielectric properties, weakly bound charge-transfer (CT) complex crystals have been exploited as a molecular architecture where the constituent polar molecules have some freedom of dynamic processes, which contribute to the dielectric properties of the crystals. Several CT crystals of polar tetrabromophthalic anhydride (TBPA) molecules were prepared using TBPA as an electron acceptor and aromatic hydrocarbons, such as coronene and perylene, as electron donors. The crystal structures and dielectric properties of the CT crystals as well as the single-component crystal of TBPA were investigated at various temperatures. Molecular reorientation of TBPA molecules did not occur in the single-component crystal, and the crystal did not show a dielectric response due to orientational polarization. We have found that the CT crystal formation provides a simple and versatile method to develop molecular dielectrics, revealing that the molecular dynamics of the TBPA molecules and the dielectric property of their crystals were greatly changed in CT crystals. The TBPA molecules underwent rapid in-plane reorientations in their CT crystals, which exhibited marked dielectric responses arising from the molecular motion. An order-disorder phase transition was observed for one of the CT crystals, which resulted in an abrupt change in the dielectric constant at the transition temperature.

The bis(ethylenedithio)tetrathiafulvalene-based ionic charge-transfer complex with 2,3-dichloro-5,6-dicyano-p-benzoquinone.

In the ionic charge-transfer (CT) complex composed of bis(ethylenedithio)tetrathiafulvalene (ET) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), C10H8S8·C8Cl2N2O2, the donor and acceptor molecules both form centrosymmetric dimers associated by strong face-to-face π-π interactions. The disordered DDQ molecules form a one-dimensional π-stacked column, while the ET molecules form a two-leg ladder through additional short S···S contacts between adjacent π-π-bonded dimers. The crystal structure of ET-DDQ revealed in this study will provide a valuable example of the two-leg spin ladder system, which has rarely been reported for ET-based CT complexes.

Molecular photoconductor with simultaneously photocontrollable localized spins.

UV irradiation reversibly switches a new insulating and nonmagnetic molecular crystal, BPY[Ni(dmit)(2)](2) (BPY = N,N'-ethylene-2,2'-bipyridinium; Ni(dmit)(2) = bis(1,3-dithiole-2-thione-4,5-dithiolato)nickelate(III)), into a magnetic conductor. This is possible because the bipyridyl derivative cations (BPY(2+)) trigger a photochemical redox reaction in the crystal to produce a change of ∼10% in the filling of the Ni(dmit)(2) valence band, leaving localized spins on the BPY themselves. In the dark, almost all of the BPY molecules are closed-shell cations, and most of the Ni(dmit)(2) radical anions form spin-singlet pairs; thus, this material is a diamagnetic semiconductor. Under UV irradiation, a photocurrent is observed, which enhances the conductivity by 1 order of magnitude. Electron spin resonance measurements indicate that the UV irradiation reversibly generates carriers and localized spins on the Ni(dmit)(2) and the BPY, respectively. This high photoconductivity can be explained by charge transfer (CT) transitions between Ni(dmit)(2) and BPY in the UV region. In other words, the photoconduction and "photomagnetism" can be described as reversible optical control of the electronic states between an ionic salt (BPY(2+)/[Ni(dmit)(2)](-), nonmagnetic insulator) and a CT complex (BPY(2(1-δ)+)/[Ni(dmit)(2)]((1-δ)-) (δ ≈ 0.1), magnetic conductor) in the solid state.

Simultaneous control of carriers and localized spins with light in organic materials.

An organic insulating crystal reversibly becomes a magnetic conductor under UV irradiation. The rapid and qualitative change in the physical properties is wavelength selective and explained by charge transfer between donor and photochemically active acceptor molecules. The photochemical redox reaction in the crystal produces a partially filled band and localized spins simultaneously.

Charge-transport in tin-iodide perovskite CH3NH3SnI3: origin of high conductivity.

The structural and electrical properties of a metal-halide cubic perovskite, CH(3)NH(3)SnI(3), have been examined. The band structure, obtained using first-principles calculation, reveals a well-defined band gap at the Fermi level. However, the temperature dependence of the single-crystal electrical conductivity shows metallic behavior down to low temperatures. The temperature dependence of the thermoelectric power is also metallic over the whole temperature range, and the large positive value indicates that charge transport occurs with a low concentration of hole carriers. The metallic properties of this as-grown crystal are thus suggested to result from spontaneous hole-doping in the crystallization process, rather than the semi-metal electronic structure. The present study shows that artificial hole doping indeed enhances the conductivity.

Stable π-π dependent electron conduction band of TPP[M(Pc)L2]2 molecular conductors (TPP = tetraphenylphosphonium; M = Co, Fe; Pc = phthalocyaninato; L = CN, Cl, Br).

The partially-oxidized TPP[M(Pc)L(2)](2) molecular conductors exhibit variable electronic and magnetic transport bulk materials properties due to central metal and axial ligand molecular modifications. The controllable electrical conductivity and giant negative magnetoresistance can be mainly attributable to the varying ligand field energy and physical bulkiness of the axial ligands which cause modulation in the intra-molecular π-d (Pc-M) and inter-molecular π-π (Pc-Pc) interactions in the TPP[M(Pc)L(2)](2) system, respectively. Characterization of the electronic conduction band utilizing one-dimensional (1-D) tight-binding approximation from infrared reflectance and thermoelectric power profile reveal consistent band widths of 0.43 eV-0.62 eV for the Co series (L = Br < Cl < CN) and 0.44-0.56 eV for the Fe series (L = Br < Cl < CN). The fixed band width suggests that stable electron conduction bands (transport pathway) can be constructed which can withstand the molecular π-d interaction modifications that severely alter the bulk electronic and magnetic materials properties of the TPP[M(Pc)L(2)](2) molecular conductors.

Dramatic effect of dispersed carbon nanotubes on the mechanical and electroconductive properties of polymers derived from ionic liquids.

Free-radical polymerization of an imidazolium ion-based ionic liquid bearing a methacrylate group, gelling with single-walled carbon nanotubes (SWNTs), allows fabrication of a mechanically reinforced, electroconductive soft material (bucky plastic). A film sample of this material displays an excellent conductivity of 1 S cm(-1) and a 120-fold enhancement of the Young's modulus at a 7 wt % content of SWNTs. The conductivity is temperature-dependent in the range 5-300 K, suggesting that the conductive process involves carrier hopping. Scanning electron and atomic force micrographs of a bucky plastic film display the presence of crosslinked networks consisting of finely dispersed SWNTs. Such nanotube networks, developed in the polymer matrix, likely suppress slipping of entrapped polymer molecules via a strong interfacial interaction and also facilitate intertubular carrier transport. Although a bucky plastic derived from a vinylimidazolium ion-based ionic liquid monomer shows a comparable conductivity to that of the methacrylate version, the film is brittle irrespective of the presence or absence of SWNTs.

Synthesis, isolation, and structural characterization of the C4h isomer of metal(1,2-naphthalocyanine) and its one-dimensional conductor of the axially substituted species.

The C4h isomer of the 1,2-Nc (1,2-naphthalocyaninato) ligand has been efficiently isolated as a hydrated magnesium complex by the fractional crystallization from the benzene/acetone solution after treating the crude mixture of the four isomers (C4h, C(s), C2v, D2h) with benzene. The C4h symmetry has been confirmed by X-ray structure analysis. The central metal ion has been demetalated and subsequently converted to the Li2 complex followed by conversion to the cobalt(II) complex. Electrochemical oxidation of the Co(III)(1,2-Nc)(CN)2 anion prepared from the cobalt(II) complex with TPP (tetraphenylphosphonium) has yielded a partially oxidized salt, TPP[Co(III)(1,2-Nc-C4h(CN)2]2. The crystal comprises slipped stacked Co(III)(1,2-Nc-C4h)(CN)2 one-dimensional chains and one-dimensional arrays of TPPs. The conductivity at room temperature is 0.1 S cm(-1), and the temperature dependence is semiconducting with a small activation energy of about 0.05 eV. The positive temperature-independent value of about 60 microV deg(-1) observed in the thermoelectric power measurements suggests that the salt is in the correlated hopping regime.

Conformational analysis of chiral helical perfluoroalkyl chains by VCD.

Vibrational circular dichroism (VCD) technique successfully revealed the absolute configuration of the biased helix of perfluoroalkyl chains in solution with the aid of theoretical calculations, which was supported by an X-ray crystallographic study.

Coupled protonic and electronic conduction in the molecular conductor [2-(2-1H-Benzimidazolyl)-1H-benzimidazolium]-TCNQ.

A novel molecular based proton-electron mixed conductor, (H3BBIM(+))(TCNQ)(Cl(-))(0.5)(H(2)O) (1), where H3BBIM(+) is 2-(2-1H-benzimidazolyl)-1H-benzimidazolium and TCNQ is 7,7,8,8-tetracyano-p-quinodimethane, was synthesized. The salt exhibited peculiar phase transitions as a result of proton-electron coupling phenomena within the crystal. Salt 1 is composed of a closed-shell H3BBIM(+) cation and an open-shell TCNQ anion radical, and was obtained by electrocrystallization in a buffered CH(3)CN solution. Crystal 1 was constructed from the segregated uniform stacks of H3BBIM(+) and TCNQ. The regular stack of partially electron-transferred TCNQ(-0.5) provided a one-dimensional electron-conducting column. Between the regular H3BBIM(+) columns, a channel-like sequence of holes was formed at the side-by-side space that is filled with disordered Cl(-) ions and H(2)O molecules, and which offer a proton-conducting path. The electrical conductivity at room temperature (10 S cm(-1)) was greater by a magnitude of four than the protonic conductivity (1x10(-3) S cm(-1)). Electronic conduction changed from metallic (T>250 K) to semiconducting (250>T>100 K), then insulating (T<100 K). Protonic conductivity was observed above 200 K. The continuous metal-semiconductor transition at 250 K is caused by the formation of the Cl(-) superstructure, whereas the disappearance of protonic conductivity at 200 K is related to the rearrangement of the [Cl(-)-(H(2)O)(2)] sublattice within the channel. The magnetic susceptibility continuously shifted from Pauli paramagnetism (T>250 K) to the one-dimensional linear Heisenberg antiferromagnetic chain (T<250 K). Lattice dimerization in regular TCNQ columns was confirmed by the appearance of vibrational a(g) mode at low temperatures. The strong localization of conduction electrons on each TCNQ dimer caused a Mott transition at 100 K. The melting and freezing of the [Cl(-)-(H(2)O)(2)] sublattice within the channel was correlated to the conduction electrons on the TCNQ stack and the protonic conductivity.

Supramolecular cation assemblies of hydrogen-bonded (NH4+/NH2NH3+)(crown ether) in [Ni(dmit)2]-based molecular conductors and magnets.

Hydrogen-bonded supramolecular cation assemblies of (NH4+/NH2-NH3+)(crown ether), where the crown ether is [12]crown-4, [15]crown-5, or [18]crown-6, were incorporated into electrically conducting [Ni(dmit)2] salts (dmit2- = 2-thioxo-1,3-dithiole-4,5-dithiolate). (NH4+)([12]crown-4)[Ni(dmit)2]3(CH3CN)2 had a pyramidal shape, while ionic channels were observed in (NH4+)(0.88)([15]crown-5)[Ni(dmit)(2)]2 and (NH4+)(0.70)([18]crown-6)[Ni(dmit)(2)]2. Both (NH4+)(0.88)([15]crown-5) and (NH4+)(0.70)([18]crown-6) contained regularly spaced [Ni(dmit)(2)] stacks formed by N-H.O hydrogen bonding between the oxygen atoms in crown ethers and the NH4+ ion. NH4+ occurred nonstoichiometrically; there were vacant ionic sites in the ionic channels. The ionic radius of NH4+ is larger than the cavity radius of [15]crown-5 and [18]crown-6. Therefore, NH4+ ions could not pass through the cavity and were distributed randomly in the ionic channels. The static disorder caused the conduction electrons to be randomly localized to the [Ni(dmit)2] stacks. Hydrazinium (NH2-NH3+) formed the supramolecular cations in (NH2-NH3+)([12]crown-4)2[Ni(dmit)2]4 and (NH2-NH3+)2([15]crown-5)3[Ni(dmit)2]6, possessing a sandwich and club-sandwich structure, respectively. To the best of our knowledge, these represent the first hydrazinium-crown ether assemblies to be identified in the solid. In the supramolecular cations, hydrogen bonding was detected between the ammonium or the amino protons of NH2-NH3+ and the oxygen atoms of crown ethers. The sandwich-type cations coexisted with the [Ni(dmit)2] dimer stacks. Although the assemblies were typically semiconducting, ferromagnetic interaction (Weiss temperature = +1 K) was detected in the case of (NH2-NH3+)2([15]crown-5)3[Ni(dmit)2]6. The (NH2-NH3+)0.8([18]crown-6)[Ni(dmit)2]2 and (NH4+)0.76([18]crown-6)[Ni(dmit)2]2 crystals were isomorphous. The large and flexible [18]crown-6 allowed for maintaining the same ionic channel structure through replacement of the NH4+ cation by NH2-NH3+.