Molecular Conductors Research Articles

Pressure-induced phase switching of Shubnikov–de Haas oscillations in the molecular Dirac fermion system α−(BETS)2I3

We report on the Shubnikov--de Haas (SdH) oscillations in the quasi-two-dimensional molecular conductor $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{I}}_{3}$ [BETS: bis(ethylenedithio)tetraselenafulvalene] laminated on polyimide films at 1.7 K. From the SdH phase factor we verified experimentally that the material is in the Dirac fermion phase under pressure. $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{I}}_{3}$ is in the vicinity of the phase transition between strongly correlated insulating and Dirac fermion phases, and is a possible candidate for an ambient-pressure molecular Dirac fermion system. However, the SdH oscillations indicate that the Berry phase is zero at ambient pressure. Under pressure, a $\ensuremath{\pi}$ Berry phase emerges when the metal-insulator crossover is almost suppressed at $\ensuremath{\sim}0.5$ GPa. The results contrast with those for the pioneering molecular Dirac fermion system $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{I}}_{3}$ [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] in which Dirac fermions and semiconducting behavior are simultaneously observed.

Charge transfer in self-assembled monolayers of molecular conductors containing tripodal anchor and terpyridine-metal redox switching element

This work presents charge transfer characteristics of molecular switches based on [M(terpyridine)2]2+/3+ (M = Ru, Os) electroactive element functionalized by a tripodal thiol-based anchoring platform allowing the formation of self-assembled monolayers (SAMs). It is demonstrated that the Os-tripod compound forms more compact film with less pronounced repulsive interactions among individual [M(terpyridine)2]2+/3+ units compared to Ru-tripod compound. The electron transfer rate constants were determined from cyclic voltammograms and electrochemical impedance spectra by three independent methods. The value of 1.4 × 103 s–1 was obtained for Os-tripod SAM and 1.6 × 103 s–1 for that of Ru-tripod SAM. These values are almost one order of magnitude higher than those reported in the literature for osmium and ruthenium dipodal thiol-based anchors and more than two orders of magnitude higher than those reported for single monopodal aliphatic anchoring groups.

Old Donors for New Molecular Conductors: Combining TMTSF and BEDT-TTF with Anionic (TaF6)1−x/(PF6)x Alloys

Tetramethyl-tetraselenafulvalene (TMTSF) and bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) are flagship precursors in the field of molecular (super)conductors. The electrocrystallization of these donors in the presence of (n-Bu4N)TaF6 or mixtures of (n-Bu4N)TaF6 and (n-Bu4N)PF6 provided Bechgaard salts formulated as (TMTSF)2(TaF6)0.84(PF6)0.16, (TMTSF)2(TaF6)0.56(PF6)0.44, (TMTSF)2(TaF6)0.44(PF6)0.56 and (TMTSF)2(TaF6)0.12(PF6)0.88, together with the monoclinic and orthorhombic phases δm-(BEDT-TTF)2(TaF6)0.94(PF6)0.06 and δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57, respectively. The use of BEDT-TTF and a mixture of (n-Bu4N)TaF6/TaF5 afforded the 1:1 phase (BEDT-TTF)2(TaF6)2·CH2Cl2. The precise Ta/P ratio in the alloys has been determined by an accurate single crystal X-ray data analysis and was corroborated with solution 19F NMR measurements. In the previously unknown crystalline phase (BEDT-TTF)2(TaF6)2·CH2Cl2 the donors organize in dimers interacting laterally yet no organic-inorganic segregation is observed. Single crystal resistivity measurements on the TMTSF based materials show typical behavior of the Bechgaard phases with room temperature conductivity σ ≈ 100 S/cm and localization below 12 K indicative of a spin density wave transition. The orthorhombic phase δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57 is semiconducting with the room temperature conductivity estimated to be σ ≈ 0.16–0.5 S/cm while the compound (BEDT-TTF)2(TaF6)2·CH2Cl2 is also a semiconductor, yet with a much lower room temperature conductivity value of 0.001 to 0.0025 S/cm, in agreement with the +1 oxidation state and strong dimerization of the donors.

Gold(III) bisdithiolate complexes: molecular conductors that also exhibit anticancer and antimicrobial activities

Introduction Au(III) complexes with dithiolene ligands have been used as molecular conductors and magnetic materials [1]. These compounds feature square planar geometries, as that for cisplatin the Pt(II)-based compound in clinical use as chemotherapeutic drug. Based on the molecular structure, a similar mechanism of action, e.g. antitumor activity and interaction with DNA should be expected for these compounds. Reports on the biological activity of similar gold(III) compounds are relatively scarce in the literature. Only recently, a Au(III) 1,2-dithiolene cyclometalated complex had proved its potential against Gram-positive bacteria [2]. In this study we evaluated two related Au(III) complexes containing N-alkyl-1,3-thiazoline-2-thione dithiolate ligand, [Au(R-thiazdt)2]−1 (R = ethyl-1; R = hydroxyethyl-2) [3a,b] as antitumor and antimicrobial agents. The compounds were assessed in vitro towards cisplatin sensitive ovarian cancer cells (A2780), bacteria and fungus of clinical importance such as Staphylococcus aureus and Candida. Spectroscopic studies were also performed to evaluate the interaction with DNA. Materials and methods The gold complexes were synthesised as previously described [3a,b]. The cytotoxic activity against the A2780 ovarian cancer cells was assessed by the IC50 determined by the MTT assay. The antimicrobial activities of complexes 1 and 2 were assessed by the MIC values towards the Gram+ S. aureus, and the fungal strains C. glabrata and C. albicans, using reported methods [4,5]. The ability of compounds to bind to DNA was assessed by fluorescence spectroscopy using ethidium bromide (EB) as the fluorescence probe. Results Complexes 1 and 2 presented high cytotoxic activity in the cisplatin sensitive A2780 cells even superior than cisplatin. Complex 1 was able to inhibit the growth of S. aureus and both Candida strains, while 2 was much less active. The fluorescence studies revealed a weak interaction of compounds with CT-DNA, in contrast with that found for cisplatin, although the interaction of 2 is somewhat stronger than that found for 1. Discussion and conclusions Results evidenced the importance in what way minor modifications of the Au dithiolate structure can result in loss of activity in particular the antimicrobial activity. In contrast with cisplatin, DNA is not the main target involved in their mode of action. Further studies are needed to explore other potential targets and the mechanism of action.

Structures and Conducting Properties of Molecular Conductors Based on Dimethyl-Substituted DTDA-TTP and DTDH-TTP

Abstract Dimethyl substituted donors, 2-(4,5-dimethyl-1,3-dithiol-2-ylidene)-5-(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (DMDT-DA-TTP) and 2-(4,5-dimethyl-1,3-dithiol-2-ylidene)-5-(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene (DMDT-DH-TTP), were successfully synthesized, and (DMDT-DA-TTP)2X (X− = PF6−, AsF6−, and SbF6−) and (DMDT-DH-TTP)2X (X− = PF6− and AsF6−) were prepared. All the DMDT-DA-TTP and DMDT-DH-TTP salts adopt β-type molecular packing with a head-to-tail molecular stacking. Although the overlap modes of donor molecules in the DMDT-DA-TTP and DMDT-DH-TTP salts are similar, the degree of dimerization of donor molecules is quite different. That is, electronically strong dimerization of the DMDT-DA-TTP molecules was assessed in the salts, while the DMDT-DH-TTP molecules have electronically almost uniform stacking in the salts. A tight-binding band calculation suggested that all the DMDT-DA-TTP salts have effectively half-filled energy bands with narrow bandwidths of 0.39–0.40 eV, while the bandwidths of DMDT-DH-TTP salts (0.96 eV) are relatively wide due to the lack of an energy gap. The calculated Fermi surfaces of all the salts are quasi-one-dimensional. In correspondence with the different band structures, all the DMDT-DA-TTP salts exhibited semiconducting behavior derived from the Mott-insulating nature. In contrast, both the DMDT-DH-TTP salts showed metallic temperature dependence down to 5–7 K.

Ambient-pressure Dirac electron system in the quasi-two-dimensional molecular conductor α−(BETS)2I3

We investigated the precise crystal structures and electronic states of a quasi-two-dimensional molecular conductor $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{I}}_{3}$ at ambient pressure. The electronic resistivity of this molecular solid shows metal-to-insulator (MI) crossover behavior at ${T}_{\mathrm{MI}}=50\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Our x-ray diffraction and $^{13}\mathrm{C}$ nuclear magnetic resonance experiments revealed that $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{I}}_{3}$ maintains the inversion symmetry below ${T}_{\mathrm{MI}}$. First-principles calculations found a pair of anisotropic Dirac cones at a general $k$ point, with the degenerate contact points at the Fermi level. The origin of the insulating state in this system is a small energy gap of $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ opened by the spin-orbit interaction. The ${Z}_{2}$ topological invariants indicate that this system is a weak topological insulator. Our results suggest that $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{I}}_{3}$ is a promising material for studying the bulk Dirac electron system in two dimensions.

Investigation of Superconductivity in Molecular Conductors Using Strain-Controlled Field-Effect Transistors

Investigation of Superconductivity in Molecular Conductors Using Strain-Controlled Field-Effect Transistors

Solid-solution (alloying) strategies in crystalline molecular conductors

Solid solutions in molecular conductors are key tools for investigating their conducting and magnetic properties, addressing phase transitions, chemical pressure effects and band filling manipulation.

First-principles study of the effective Hamiltonian for Dirac fermions with spin-orbit coupling in two-dimensional molecular conductor $$\alpha $$-(BETS)$$_2 \hbox {I}_3$$

We employed first-principles density-functional theory (DFT) calculations to characterize Dirac electrons in quasi-two-dimensional molecular conductor $$\alpha $$ -(BETS) $$_2 \hbox {I}_3$$ [= $$\alpha $$ -(BEDT–TSeF) $$_2 \hbox {I}_3$$ ] at a low temperature of 30 K. We provide a tight-binding model with intermolecular transfer energies evaluated from maximally localized Wannier functions, where the number of relevant transfer integrals is relatively large due to the delocalized character of Se p orbitals. The spin–orbit coupling gives rise to an exotic insulating state with an indirect band gap of about 2 meV. We analyzed the energy spectrum with a Dirac cone close to the Fermi level to develop an effective Hamiltonian with site potentials, which reproduces the spectrum obtained by the DFT band structure.

Doping and coupling strength in molecular conductors: polyacetylene as a case study.

The doping mechanisms responsible for elevating the currents up to eleven orders of magnitude in semiconducting polymer films are today well characterized. Doping can also improve the performance of nanoscale devices or single molecule conductors, but the mechanism in this case appears to be different, with theoretical studies suggesting that the dopant affects the electronic properties of the junctions. In the present report, multiscale time-dependent DFT transport simulations help clarify the way in which n-type doping can raise the current flowing through a polymer chain connected to a pair of electrodes, with the focus on polyacetylene. In particular, our multiscale methodology offers control over the magnitude of the chemical coupling between the molecule and the electrodes, which allows us to analyze the effect of doping in low and strong coupling regimes. Interestingly, our results establish that the impact of dopants is the highest in weakly coupled devices, while their presence tends to be irrelevant in low-resistance junctions. Our calculations point out that both the equalization of the frontier orbitals with the Fermi level and a small gap between the HOMO and the LUMO must result from doping in order to observe any significant increase of the currents.

A TTF–TCNQ complex: an organic charge-transfer system with extraordinary electromagnetic response behavior

Charge-transfer based molecular conductors have attracted great attention in recent years, but have been rarely explored in the field of electromagnetic materials.

Phenalenyls as tunable excellent molecular conductors and switchable spin filters.

Phenalenyl-based radicals are stable radicals whose electronic properties can be tuned readily by heteroatom substitution. We employ density functional theory-based non-equilibrium Green's function (NEGF-DFT) calculations to show that this class of molecules exhibits tunable spin- and charge-transport properties in single molecule junctions. Our simulations identify the design principles and interplay between unusually high conductivity and strong spin-filtering.

Chemical tuning of molecular quantum materials κ-[(BEDT-TTF)1−x(BEDT-STF)x]2Cu2(CN)3: from the Mott-insulating quantum spin liquid to metallic Fermi liquid

The electronic properties of molecular conductors are varied by substituting ions with extended wave functions to enlarge the bandwidth W. This enables them to cross the Mott insulator-to-metal phase transition by reducing electronic correlations U/W.

Electronic Structure of a Single-Component Molecular Conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) under High Pressure

We examined high-pressure electronic structure of a single-component molecular conductor [Pd(dddt)$_2$] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) at room temperature, based on the crystal structure determined by single crystal synchrotron X-ray diffraction measurements at 5.9 GPa. The monoclinic unit cell contains four molecules that form two crystallographically independent molecular layers. A tight-binding model of 8 $\times$ 8 matrix Hamiltonian gives an electronic structure as a Dirac electron system. The Dirac point describes a loop within the first Brillouin zone, and a nodal line semimetal is obtained. The noticeable property of the Dirac cone with a linear dispersion is shown by calculating density of states (DOS). The Dirac cone in this system is associated with the crossing of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) bands, which originates from the direct interaction between different molecular layers. This is a newly found mechanism in addition to the indirect one [J. Phys. Soc. Jpn., {\bf 86}, 064705 (2017)]. The Dirac points emerge as a line, when the HOMO and LUMO bands meet on the surface and the HOMO-LUMO couplings are absent. Such a mechanism is verified using a reduced model of 4 $\times$ 4 matrix Hamiltonian. The deviation of the band energy ($\delta E$) at the Dirac point from the Fermi level is very small ($\delta E < $ 0.4meV). The nodal line is examined by calculating the parity of the occupied band eigen states at TRIM (Time Reversal Invariant Momentum) showing that the topological number is 1.

Synthesis, Structure and Physical Properties of (trans-TTF-py2)1.5(PF6)·EtOH: A Molecular Conductor with Weak CH∙∙∙N Hydrogen Bondings

The studies of crystal structures with hydrogen bonds have been actively pursued because of their moderate stabilization energy for constructing unique structures. In this study, we synthesized a molecular conductor based on 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (trans-TTF-py2). Two pyridyl groups were introduced into the TTF skeleton toward the structural exploration in TTF-based molecular conductors involved by hydrogen bonds. In the obtained molecular conductor, (trans-TTF-py2)1.5(PF6)·EtOH, short contacts between the pyridyl group and the hydrogen atom of the TTF skeleton were observed, indicating that hydrogen bonding interactions were introduced in the crystal structure. Spectroscopic measurements and conductivity measurement revealed semiconducting behavior derived from π-stacked trans-TTF-py2 radical in the crystal structure. Finally, these results are discussed with the quantified hydrogen bonding stabilization energy, and the band calculation of the crystal obtained from density functional theory calculation.

Chiral Conducting Me-EDT-TTF and Et-EDT-TTF-Based Radical Cation Salts with the Perchlorate Anion

Introduction of chirality in the field of molecular conductors has received increasing interest in recent years in the frame of modulation of the crystal packing, and hence conducting properties, with the number of stereogenic centers and absolute configuration, e.g., racemic or enantiopure forms. Here, we describe the preparation by electrocrystallization of chiral radical cation salts, based on the donors methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF) 1 and ethyl-ethylenedithio-tetrathiafulvalene (Et-EDT-TTF) 2 containing one stereogenic center, with the perchlorate anion. Donor 1 provided the series of crystalline materials [(rac)-1]ClO4, [(S)-1]2ClO4 and [(R)-1]2ClO4, while for donor 2 only the 1:1 salts [(rac)-2]ClO4 and [(R)-2]ClO4 could be prepared as suitable single crystals for X-ray analysis. The enantiopure salts of 1 show β-type packing and metallic conductivity in the high temperature regime, with room temperature conductivity values of 5–10 S cm−1, whereas compound [(rac)-2]ClO4 is a very poor semiconductor. Tight-binding extended Hückel band structure calculations support the metallic conductivity of the enantiopure salts of 1 and suggest that small structural changes, possibly induced by thermal contraction or pressure, could lead to a pseudo-elliptic closed Fermi surface, typical for a 2D metal.

Ferrimagnetism Induced by Off-Site Coulomb Interaction in an Itinerant Electron System

Motivated by weak ferromagnetism (FM) in a $\tau$-type molecular conductor ($\tau$-MC), we examine its mechanism using a two-band extended Hubbard model. Applying the random phase approximation, we elucidate the uniform spin and charge fluctuations between unit cells in the presence of on-site and off-site interactions. Applying the mean-field approximation, we find the ordered state mixing with antiferromagnetism (AFM), weak FM, and charge ordering (CO) components in each unit cell: we classify this state as ferrimagnetism (FIM). We reveal the phase diagrams in the interaction and interaction-temperature spaces. The former shows that the off-site interaction induces FIM from pure AFM and the latter shows that lowering the temperature stabilizes FIM. To clarify the stabilization mechanism of the phases, we focus on the microscopic nature of the ordered states, including the band structure, Fermi surface, and density of states. We find that the FIM state is obtained from mixing features of AFM, CO, and FM; therefore, the emergence of FIM requires both the on-site and off-site interactions. Then, we discuss the effect of lowering the temperature and predict that the AFM gap assists the emergence of FIM based on AFM. This FIM state is possibly related to the observation of the weak FM in $\tau$-MC.

Syntheses, Structures, and Physical Properties of Neutral Gold Dithiolate Complex, [Au(etdt)2]·THF

In order to develop new types of single-component molecular conductors with novel electronic structures and physical properties, the neutral gold dithiolate complex with an etdt (= ethylenedithiotetrathiafulvalenedithiolate) ligand, [Au(etdt)2] was prepared. However, unlike the reported single-component molecular metals, the neutral gold complex [Au(etdt)2]·THF (2) contains a solvent molecule of tetrahydrofuran (THF). The crystals of 2 form a two-dimensional conducting layer structure, which are separated by the terminal ethylene groups and THF molecules. The fairly high room-temperature conductivity of 0.2 S/cm and semiconducting behavior with a low activation energy of 0.1 eV of 2, is consistent with the result of the density functional theory band structure calculations. The observed non-magnetic behavior of 2 is caused from the dimeric structure of [Au(etdt)2] molecules.

Molecular conductors composed from Organic-Transistor Materials

Assistant Professor Tomofumi Kadoya is part of a team within the Graduate School of Material Science at the University of Hyogo in Japan. He is engaged with a range of different investigations related to conductive organic materials. One of the main focuses of Kadoya's research is organic transistors and organic charge-transfer (CT) complexes. CT complexes achieve conductivity by chemical doping but in organic transistors, conduction carriers are generated by field effect, where an electric field is used to control the flow of current. Among the many goals of the research, Kadoya and his team want to increase the methods and types of organic doping.

Electric Transport of Nodal Line Semimetals in Single-Component Molecular Conductors

We examine an effect of acoustic phonon scattering on the electric conductivity of a single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) with a half-filled band by applying the previous calculation in a two-dimensional model with Dirac cone [Phys. Rev. B. 98, 161205 (2018)], wherethe electric transport by the impurity scattering exhibits a noticeable interplay of the Dirac cone and the phonon scattering, resulting in maximum of the conductivity with increasing temperature. The conductor shows a nodal line semimetal, where the band crossing of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) provides a loop of Dirac points located close to the Fermi energy followed by the density of states (DOS) similar to that of a two-dimensional Dirac cone. Using a tight-binding (TB) model [arXiv:2008.09277], which was obtained using the crystal structure observed from a recent X ray diffraction experiment under pressure, it is shown that the obtained conductivity explains reasonably the anomalous behavior in [Pd(dddt)2] exhibiting temperature-independent resistivity at finite temperatures. This paper demonstrates a crucial role of the acoustic phonon scattering at finite temperatures in the electric conductivity of Dirac electrons. The present theoretical results of conductivity are compared with those of the experiments.