Special Issue "Molecular Conductors"
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A special issue of Crystals (ISSN 2073-4352).
Deadline for manuscript submissions: closed (15 March 2012)
Special Issue Editor
Guest Editor
Prof. Dr. Reizo Kato
Chief Scientist, Director of Condensed Molecular Materials Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1, Hirosawa, Wako-shi, Saitama, 351-0198, Japan
Website: http://www.riken.jp/lab-www/molecule/index-e.html
E-Mail: reizo@riken.jp
Phone: 81 48 467 9408
Fax: 81 48 467 9408
Interests: molecular conductors; tetrathiafulvalene (TTF) derivatives; metal dithiolene complexes; N,N'-dicyanobenzoquinonediimines (DCNQI); π-d systems; crystal and electronic structures; transport and magnetic properties
Special Issue Information
Submission
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Keywords
- synthesis and crystal growth of new molecular conductors
- molecular superconductors
- single component molecular metals
- magnetic molecular conductors
- zero-gap molecular conductors
- fullerene-based molecular conductors
- organic FET and other molecular devices
- strongly correlated electron systems
- metal-insulator transition
- Mott transition
- charge order
- geometrical frustration
- photo-induced phenomena
- pressure effects
- non-linear phenomena
- X-ray diffraction
- transport and magnetic properties
- dielectric properties
- thermodynamic properties
- NMR and ESR
- vibrational spectroscopy
- electronic state calculations
- theoretical models
Published Papers (18 papers)
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Received: 6 December 2011; in revised form: 21 December 2011 / Accepted: 22 December 2011 / Published: 6 January 2012
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Abstract: New radical cation salts (BEDT-TTF)[8,8',(7)-Cl2(Cl0.09)-3,3'-Co(1,2-C2B9H9.91)(1',2'-C2B9H10)] (1), (BEDT-TTF)[8,8'-Br0.75Cl1.25-3,3'-Co(1,2-C2B9H10)2] (2), and (BMDT-TTF)4[8,8'-Br1.16(OH)0.72-3,3'-Co(1,2-C2B9H10.06)2] (3) were synthesized, and their crystal structures and electrical conductivities were determined. All the radical cation salts are semiconductors. Compounds 1 and 2 were found to be isostructural, however their electrical conductivities strongly differ (s293 = 2 Ω−1cm−1 and 10−5 Ω−1cm−1, respectively).
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Received: 1 December 2011; in revised form: 26 December 2011 / Accepted: 31 December 2011 / Published: 6 January 2012
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Abstract: Dynamics of photoinduced phase transitions in molecular conductors are reviewed from the perspective of interplay between correlated electrons and phonons. (1) The charge-transfer complex TTF-CA shows a transition from a neutral paraelectric phase to an ionic ferroelectric phase. Lattice phonons promote this photoinduced transition by preparing short-range lattice dimerization as a precursor. Molecular vibrations stabilize the neutral phase so that the ionic phase, when realized, possesses a large ionicity and the Mott character; (2) The organic salts θ-(BEDT-TTF)2RbZn(SCN)4 and α-(BEDT-TTF)2I3 show transitions from a charge-ordered insulator to a metal. Lattice phonons make this photoinduced transition hard for the former salt only. Molecular vibrations interfere with intermolecular transfers of correlated electrons at an early stage; (3) The organic salt κ-(d-BEDT-TTF)2Cu[N(CN)2]Br shows a transition from a Mott insulator to a metal. Lattice phonons modulating intradimer transfer integrals enable photoexcitation-energy-dependent transition pathways through weakening of effective interaction and through introduction of carriers.
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Received: 21 February 2012; in revised form: 27 March 2012 / Accepted: 28 March 2012 / Published: 10 April 2012
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Abstract: A novel organic dianion, N,N'-Disulfo-1,4-benzoquinonediimine (1) has been prepared, which is a strong electron acceptor. The reduction potential of the PPh4 salt indicates that 1 is a stronger acceptor than DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone). The dianionic acceptor provided a BEDT-TTF salt, (BEDT-TTF)41·3H2O, the structures and physical properties of which are reported.
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Received: 7 March 2012; in revised form: 30 March 2012 / Accepted: 31 March 2012 / Published: 12 April 2012
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Abstract: Properties of the spin systems of the localized 3d Mn2+ ions and the conduction π electrons in quasi-two-dimensional organic conductor κ-(BETS)2Mn[N(CN)2]3 were accessed using 1H and 13C NMR in order to find their relation to the metal-insulator transition which occurs at ∼23 K. The transition of the system into the insulating state is shown to be followed by localization of the π spins into a long-range ordered staggered structure of AF type. In contrast, the 3d Mn2+ electron spin moments form a disordered tilted structure, which may signify their trend to AF order, frustrated geometrically by the triangular arrangement of Mn in the anion layer. This result suggests that the MI transition in κ-(BETS)2Mn[N(CN)2]3 is not the consequence of the interactions within the Mn2+ spins but due to the interactions within the π-electron system itself. Vice versa, it is more likelythat the disordered tilted structure of the Mn2+ spins is induced by the ordered π-spins via the π-d interaction.
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Received: 8 March 2012; in revised form: 2 April 2012 / Accepted: 9 April 2012 / Published: 18 April 2012
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Abstract: Calculating electron-vibration (vibronic) interaction constants is computationally expensive. For molecules containing N nuclei it involves solving the Schrödinger equation for Ο(3N) nuclear configurations in addition to the cost of determining the vibrational modes. We show that quantum vibronic interactions are proportional to the classical atomic forces induced when the total charge of the system is varied. This enables the calculation of vibronic interaction constants from O(1) solutions of the Schrödinger equation. We demonstrate that the O(1) approach produces numerically accurate results by calculating the vibronic interaction constants for several molecules. We investigate the role of molecular vibrations in the Mott transition in κ-(BEDT-TTF)2Cu[N(CN)2]Br.
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Received: 6 March 2012; in revised form: 14 April 2012 / Accepted: 16 April 2012 / Published: 19 April 2012
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Abstract: In this short review, I will give an overview on the current understanding of the superconductivity in quasi-two-dimensional organic metals. Thereby, I will focus on charge-transfer salts based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET for short). In these materials, strong electronic correlations are clearly evident, resulting in unique phase diagrams. The layered crystallographic structure leads to highly anisotropic electronic as well as superconducting properties. The corresponding very high orbital critical field for in-plane magnetic-field alignment allows for the occurrence of the Fulde–Ferrell– Larkin–Ovchinnikov state as evidenced by thermodynamic measurements. The experimental picture on the nature of the superconducting state is still controversial with evidence both for unconventional as well as for BCS-like superconductivity.
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Received: 26 March 2012; in revised form: 10 April 2012 / Accepted: 10 April 2012 / Published: 20 April 2012
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Abstract: The dynamical property of electrons with the tilted Dirac cone was examined using the tilted Weyl equation. The polarization function exhibits cusps and nonmonotonic structures by varying both the frequency and the momentum. A pair of tilted Dirac cones exhibits a new plasmon for the intermediate magnitude of momentum owing to the combined effects of two tilted cones. Dirac electrons with the zero-gap state (ZGS) in organic conductor α-(BEDT-TTF)2I3 are examined by calculating the Berry curvature, which displays the peak structure for a pair of Dirac particles between the conduction band and the valence band. The ZGS is theoretically predicted for α-(BEDT-TTF)2NH4Hg(SCN)4 under uniaxial pressure. Examining the band structure of the stripe charge ordered state of α-(BEDT-TTF)2I3 under pressure, we have found a topological transition from a conventional insulator to a new phase of a pair of Dirac electrons with a finite mass. Further, investigating the zero-energy (N = 0) Landau level under a strong magnetic field, we propose ferromagnetism breaking the SU(2) valley-pseudo-spin symmetry, and the phase fluctuations of the order parameters leading to Kosterlitz-Thouless transition at lower temperatures.
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Received: 8 March 2012; in revised form: 13 April 2012 / Accepted: 13 April 2012 / Published: 23 April 2012
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Abstract: A new mixed-anion crystal composed of BEDT-TTF radical cation salt [BEDT-TTF]2(CuBr2)0.4(CuCl2)0.6 with an α'-type donor arrangement with a formal charge of +0.5 per BEDT-TTF was prepared by using a chemical oxidation method and characterized by using X-ray diffraction, four-probe electrical resistivity measurements (semiconductor: ρrt = 2 × 102 Ω cm, Ea = 0.2 eV), and energy band calculations. The results showed that this system had a quasi-one dimensional Fermi surface.
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Received: 27 March 2012; in revised form: 12 April 2012 / Accepted: 13 April 2012 / Published: 23 April 2012
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Abstract: Up to five different crystalline radical salts have been prepared with the organic donor BEDT-TTF and three different polynitrile anions. With the polynitrile dianion tcpd2− (=C[C(CN)2]32−), two closely related radical salts: α'-(ET)4tcpd·THF (1) (THF = tetrahydrofurane) and α'-(ET)4tcpd·H2O (2) have been prepared, depending on the solvent used in the synthesis. With the mono-anion tcnoetOH− (=[(NC)2CC(OCH2CH2OH)C(CN)2]−) two polymorphs with similar physical properties but different crystal packings have been synthesized: θ-(ET)2(tcnoetOH) (3) and β''-(ET)2(tcnoetOH) (4). Finally, with the mono-anion tcnoprOH− (=[(NC)2CC(OCH2CH2CH2OH)C(CN)2]−) we have prepared a metallic radical salt: β''-(ET)2(tcnoprOH)(CH2Cl2CH3Cl)0.5 (5). Salts 1‑4 are semiconductors with high room temperature conductivities and activation energies in the range 0.1–0.5 eV, whereas salt 5 is metallic down to 0.4 K although it does not show any superconducting transition above this temperature.
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Received: 22 March 2012; in revised form: 11 April 2012 / Accepted: 11 April 2012 / Published: 24 April 2012
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Abstract: Oxidation of 3,4-ethylenedithio-3'-iodo-tetrathiafulvalene (EDT-TTF-I) and 3,4-ethylenedithio-3',4'-diiodo-tetrathiafulvalene (EDT-TTF-I2) with DDQ afforded two different salts formulated as (EDT-TTF-I)(DDQ) and (EDT-TTF-I2)2(DDQ)·(CH3CN), both characterized with a full charge transfer to the DDQ acceptor moiety and by short and linear halogen bonding interactions between the iodine atom as halogen bond donor, and the carbonyl oxygen or the nitrile nitrogen atoms of reduced DDQ.
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Received: 22 March 2012; in revised form: 6 April 2012 / Accepted: 9 April 2012 / Published: 24 April 2012
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Abstract: The condensation reaction of (4-(6,7-dimethyldithio-tetrathiafulvalene)-aniline) with 2,6-diformylpyridine afforded an electroactive Schiff base (N,N,N) pincer (3). This pincer was reacted with Zn(II) cation to yield the corresponding Zinc chloride complex (4). The crystal structure of the newly prepared electroactive zinc complex reveals that the tetrathiafulvalene (TTF) is neutral and the zinc cation is pentacoordinated. The two chlorines are involved in a set of hydrogen bonds giving rise to a 2D supramolecular grid arrangement. The electronic absorption properties and the electrochemical behavior have been elucidated. These two compounds are promising for the construction of crystalline radical cation salts.
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Received: 12 February 2012; in revised form: 17 April 2012 / Accepted: 27 April 2012 / Published: 3 May 2012
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Abstract: A polyoxotungstate-surfactant hybrid layered compound was synthesized as a single phase by using decatungstate ([W10O32]4−, W10) and hexadecylpyridinium (C16py). The X-ray structure analysis combined with infrared spectroscopy and elemental analysis revealed the formula to be (C16py)4[W10O32] (C16py-W10). The layered structure consisted of alternative stacking of W10 inorganic monolayers and interdigitated C16py bilayers with layered periodicity of 23.3 Å. Each W10 anion in the W10 inorganic monolayers was isolated by the hydrophilic heads of C16py. The hybrid crystals of C16py-W10 decomposed at around 500 K. The conductivity of the hybrid layered crystal was estimated to be 4.8 × 10−6 S cm−1 at 423 K by alternating current (AC) impedance spectroscopy.
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Received: 12 March 2012; in revised form: 18 April 2012 / Accepted: 20 April 2012 / Published: 8 May 2012
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Abstract: The Mott-Anderson transition has been known as a metal-insulator (MI) transition due to both strong electron-electron interaction and randomness of the electrons. For example, the MI transition in doped semiconductors and transition metal oxides has been investigated up to now as a typical example of the Mott-Anderson transition for changing electron correlations by carrier number control in concurrence with inevitable randomness. On the other hand, molecular conductors have been known as typical strongly correlated electron systems with bandwidth controlled Mott transition. In this paper, we demonstrate our recent studies on the randomness effect of the strongly correlated electrons of the BEDT-TTF molecule based organic conductors. X-ray irradiation on the crystals introduces molecular defects in the insulating anion layer, which cause random potential modulation of the correlated electrons in the conductive BEDT-TTF layer. In combination with hydrostatic pressure, we are able to control the parameters for randomness and correlations for electrons approaching the Mott-Anderson transition.
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Received: 23 March 2012; in revised form: 24 April 2012 / Accepted: 24 April 2012 / Published: 9 May 2012
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Abstract: Tetrathiafulvalene derivatives condensed with 2-alkylidene-1,3-dithiole moiety, MeDTES (2-isopropylidene-1,3-dithiolo[4,5-d]-4,5-ethylenediselenotetrathiafulvalene), EtDTES (2-(pentan-3-ylidene)-1,3-dithiolo[4,5-d]-4,5-ethylenediselenotetrathiafulvalene), and CPDTES (2-cyclopentanylidene-1,3-dithiolo[4,5-d]-4,5-ethylenediselenotetrathiafulvalene) have been synthesized. Crystal structure analysis of MeDTES salts with Au(CN)4−, ReO4−, and I3− and a CPDTES salt with I3− reveals that the donor−anion ratios of all salts are 1:1. Band calculation of (MeDTES)[Au(CN)4] suggests a quasi-one-dimensional Fermi surface that could be the result of the uniform stack of donor molecules. In spite of this stacking, the salt is a Mott insulator because of a large on-site Coulomb interaction U. (MeDTES)(ReO4)(H2O)0.5 possesses Fermi points and exhibits semiconducting behavior with small activation energy (Ea = 0.058 eV). I3− ions form disordered infinite chain in (MeDTES)(I3)(DCE)0.25, but those in (CPDTES)(I3) exist as discrete ions. They show low conductivity (10−4−10−2 S cm−1) at room temperature and the band calculation suggests that they are band insulator.
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Received: 30 March 2012; in revised form: 20 April 2012 / Accepted: 2 May 2012 / Published: 14 May 2012
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Abstract: We report the preparation, crystallization and solid state characterization of a cyclohexanemethylamine substituted spirobiphenalenyl radical; in the solid state the compound is iso-structural with its dehydro-analog (benzylamine-substitued compound), and the molecules packed in a one-dimensional fashion that we refer to as a π-step stack. Neighboring molecules in the stack interact via the overlap of one pair of active (spin bearing) carbon atoms per phenalenyl unit. The magnetic susceptibility measurement indicates that in the solid state the radical remains paramagnetic and the fraction of Curie spins is 0.75 per molecule. We use the analytical form of the Bonner-Fisher model for the S = 1/2 antiferromagnetic Heisenberg chain of isotropically interacting spins with intrachain spin coupling constant J = 6.3 cm−1, to fit the experimentally observed paramagnetism [χp (T)] in the temperature range 4–330 K. The measured room temperature conductivity (σRT = 2.4 × 10–3 S/cm) is comparable with that of the iso-structural benzyl radical, even though the calculated band dispersions are smaller than that of the unsaturated analog.
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Received: 20 March 2012; in revised form: 19 April 2012 / Accepted: 20 April 2012 / Published: 21 May 2012
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Abstract: We review structural aspects of the Bechgaard and Fabre salts in relationship with their electronic, magnetic and superconducting properties. We emphasize the role of bond and charge modulations of the quarter filled organic stack in the various instabilities and ground states exhibited by these salts. A special consideration is also devoted to the influence of anions and methyl groups in these processes. In particular we point out the importance of the anions in achieving the inter-stack coupling by either direct or indirect (via the polarization of the methyl group cavities) interactions with the donors. In this framework we discuss the role of anions and methyl group disorders in the inhibition of the divergence of the high temperature bond order wave instability of the Bechgaard salts. We analyze the modulation in the magnetic ground states by considering explicitly the coupling of the magnetization with structural degrees of freedom. We consider the role of the anions and methyl groups in stabilizing the charge ordering pattern in the Fabre salts. We also discuss the spin-Peierls transition of the Fabre salts in relation with the charge ordering transition and the adiabaticity of the phonon field. We review the anion ordering transitions by considering more particularly the influence of the ordering process on the electronic structure and on the ground states which results. In this framework we show that the texture of the anion ordered structure has direct consequences on the superconducting properties of (TMTSF)2ClO4. Finally we conclude on the essential implication of the structural degrees of freedom on the generic phase diagram of the Bechgaard and Fabre salts.
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Received: 2 March 2012; in revised form: 30 April 2012 / Accepted: 1 May 2012 / Published: 23 May 2012
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Abstract: Charge ordering in the (TMTTF)2X salts with centrosymmetric anions (X = PF‾6 , AsF‾6 , SbF‾6 ) leads to a ferroelectric state around 100 K. For the first time and in great completeness, the intra- and intermolecular vibrational modes of (TMTTF)2X have been investigated by infrared and Raman spectroscopy as a function of temperature and pressure for different polarizations. In this original paper, we explore the development and amount of charge disproportionation and the coupling of the electronic degrees of freedom to the counterions and the underlying lattice. The methyl groups undergo changes with temperature that are crucial for the anion cage formed by them. We find that the coupling of the TMTTF molecules to the hexafluorine anions changes upon cooling and especially at the charge-order transition, indicating a distortion of the anion. Additional features are identified that are caused by the anharmonic potential. The spin-Peierls transition entails additional modifications in the charge distribution. To complete the discussion, we also add the vibrational frequencies and eigenvectors based on ab-initio quantum-chemical calculations.
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Received: 23 March 2012; in revised form: 25 April 2012 / Accepted: 11 May 2012 / Published: 24 May 2012
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Abstract: The interplane spin cross relaxation time Tx measured by high frequency ESR in X-ray irradiated κ-(BEDT-TTF)2Cu[N(CN)2]Cl is compared to the interplane resisitivity ρ⊥ and the in-plane resistivity ρII between 50 K and 250 K. The irradiation transforms the semiconductor behavior of the non-irradiated crystal into metallic. Irradiation decreases Tx, ρ⊥ and ρII but the ratio Tx/ρ⊥ and ρ⊥/ρII remain unchanged between 50 and 250 K. Models describing the unusual defect concentration dependence in κ-(BEDT-TTF)2Cu[N(CN)2]Cl are discussed.
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Last update: 2 May 2012
