Donor-Type Nickel–Dithiolene Complexes Fused with Bulky Cycloalkane Substituents and Their Application in Molecular Conductors

The effects of substituents on the arrangement of metal–dithiolene complexes based on π-conjugated systems, which are extensively used to synthesize various functional materials, have not been studied adequately. New donor-type nickel–dithiolene complexes fused with bulky cycloalkane substituents [Ni(Cn-dddt)2] (C5-dddt = 4a,5,6,6a-pentahydro-1,4-benzodithiin-2,3-dithiolate; C6-dddt = 4a,5,6,7,8,8a-hexahydro-1,4-benzodithiin-2,3-dithiolate; C7-dddt = 4a,5,6,7,8,9,9a-heptahydro-1,4-benzodithiin-2,3-dithiolate; and C8-dddt = 4a,5,6,7,8,9,10,10a-octahydro-1,4-benzodithiin-2,3-dithiolate) were synthesized in this study. All the complexes were crystallized in cis-[Ni(cis-Cn-dddt)2] conformations with cis-oriented (R,S) conformations around the cycloalkylene groups in the neutral state. Unique molecular arrangements with a three-dimensional network, a one-dimensional column, and a helical molecular arrangement were formed in the crystals owing to the flexible cycloalkane moieties. New 2:1 cation radical crystals of [Ni(C5-dddt)2]2(X) (X = ClO4− or PF6−), obtained by electrochemical crystallization, exhibited semiconducting behaviors (ρrt = 0.8 Ω cm, Ea = 0.09 eV for the ClO4− crystal; 4.0 Ω cm, 0.13 eV for the PF6− crystal) under ambient pressure due to spin-singlet states between the dimers of the donor, which were in accordance with the conducting behaviors under hydrostatic pressure (ρrt = 0.2 Ω cm, Ea = 0.07 eV for the ClO4− crystal; 1.0 Ω cm, 0.12 eV for the PF6− crystal at 2.0 GPa).

Electric Double Layer Doping of Charge-Ordered Insulators α-(BEDT-TTF)2I3 and α-(BETS)2I3

Field-effect transistors based on strongly correlated insulators are an excellent platform for studying the electronic phase transition and simultaneously developing phase transition transistors. Molecular conductors are suitable for phase transition transistors owing to the high tunability of the electronic states. Molecular Mott transistors show field-induced phase transitions including superconducting transitions. However, their application to charge-ordered insulators is limited. In this study, we fabricated electric double layer transistors based on quarter-filled charge-ordered insulators α-(BEDT-TTF)2I3 and α-(BETS)2I3. We observed ambipolar field effects in both compounds where both electron and hole doping (up to the order of 1013 cm−2) reduces the resistance by the band filling shift from the commensurate value. The maximum field-effect mobilities are approximately 10 and 55 cm2/Vs, and the gate-induced conductivities are 0.96 and 3.6 e2/h in α-(BEDT-TTF)2I3 and α-(BETS)2I3, respectively. However, gate-induced metallic conduction does not emerge. The gate voltage dependence of the activation energy in α-(BEDT-TTF)2I3 and the Hall resistance in α-(BETS)2I3 imply that the electric double layer doping in the present experimental setup induces hopping transport rather than band-like two-dimensional transport.

Chiral Radical Cation Salts of Me-EDT-TTF and DM-EDT-TTF with Octahedral, Linear and Tetrahedral Monoanions

Methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF (1) and dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF (2) are valuable precursors for chiral molecular conductors, which are generally obtained by electrocrystallization in the presence of various counter-ions. The number of the stereogenic centers, their relative location on the molecule, the nature of the counter-ion and the electrocrystallization conditions play a paramount role in the crystal structures and conducting properties of the resulting materials. Here, we report the preparation and detailed structural characterization of the following series of radical cation salts: (i) mixed valence (1)2AsF6 as racemic, and (S) and (R) enantiomers; (ii) [(S)-1]AsF6·C4H8O and [(R)-1]AsF6·C4H8O where a strong dimerization of the donors is observed; (iii) (1)I3 and (2)I3 as racemic and enantiopure forms and (iv) [(meso)-2]PF6 and [(meso)-2]XO4 (X = Cl, Re), based on the new donor (meso)-2. In the latter, the two methyl substituents necessarily adopt axial and equatorial conformations, thus leading to a completely different packing of the donors when compared to the chiral form (S,S)/(R,R) of 2 in its radical cation salts. Single crystal resistivity measurements, complemented by thermoelectric power measurements in the case of (1)2AsF6, suggest quasi-metallic conductivity for the latter in the high temperature regime, with σRT ≈ 1–10 S cm–1, while semiconducting behavior is observed for the (meso)-2 based salts.

A single atom change turns insulating saturated wires into molecular conductors

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements show tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. Here, we demonstrate experimentally that $$\beta \propto 1/\sqrt{{\varepsilon }_{r}}$$ β ∝ 1 / ε r , suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.