Reiji Kumai

Charge-cluster glass in an organic conductor

Geometrically frustrated spin-systems do not order magnetically even at absolute zero, forming instead a spin liquid or a glassy state. An organic conductor in which the charges, rather than spins, are frustrated now shows a similar absence of long-range order, resulting in a charge-cluster glass at low temperature.

Modulation of a Molecular π‐Electron System in a Purely Organic Conductor that Shows Hydrogen‐Bond‐Dynamics‐Based Switching of Conductivity and Magnetism

New important aspects of the hydrogen-bond (H-bond)-dynamics-based switching of electrical conductivity and magnetism in an H-bonded, purely organic conductor crystal have been discovered by modulating its tetrathiafulvalene (TTF)-based molecular π-electron system by means of partial sulfur/selenium substitution. The prepared selenium analogue also showed a similar type of phase transition, induced by H-bonded deuterium transfer followed by electron transfer between the H-bonded TTF skeletons, and the resulting switching of the physical properties; however, subtle but critical differences due to sulfur/selenium substitution were detected in the electronic structure, phase transition nature, and switching function. A molecular-level discussion based on the crystal structures shows that this chemical modification of the TTF skeleton influences not only its own π-electronic structure and π-π interactions within the conducting layer, but also the H-bond dynamics between the TTF π skeletons in the neighboring layers, which enables modulation of the interplay between the H-bond and π electrons to cause such differences.

Crystal structure of asymmetric organic semiconductor 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene

A full X-ray crystal structure analysis of 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), reported to be a promising semiconducting component for high-performance organic thin-film transistors, has been carried out. The analysis revealed the formation of a bilayer-type crystal structure composed of alternating antiparallel polar monomolecular layers that are formed by the herringbone packing of asymmetric molecules. The large and highly isotropic calculated intermolecular interactions within the two-dimensional semiconducting layers indicate the potentially excellent semiconducting performance of Ph-BTBT-C10.

Few‐Volt Operation of Printed Organic Ferroelectric Capacitor

The fabrication of single-crystalline thin-film arrays for an organic ferroelectric small molecule is achieved by a simple solution process without additional thermal annealing. Based on a cooperative proton tautomerism through a hydrogen-bonding network, films show the polarity switching with an operating voltage of less than 5 V at room temperature. This approach provides a low-cost and eco-friendly fabrication of ferroelectric devices.

Protonation of Pyridyl-Substituted TTF Derivatives: Substituent Effects in Solution and in the Proton–Electron Correlated Charge-Transfer Complexes†

Protonated pyridyl-substituted tetrathiafulvalene electron-donor molecules (PyH(+)-TTF) showed significant changes in the electron-donating ability and HOMO-LUMO energy gap compared to the neutral analogues and gave a unique N(+)-H⋅⋅⋅N hydrogen-bonded (H-bonded) dimer unit in the proton-electron correlated charge-transfer (CT) complex crystals. We have evaluated these features from the viewpoint of the molecular structure of the PyH(+)-TTF derivatives, that is, the substitution position of the Py group and/or the presence or absence of the ethylenedithio (EDT) group. Among 2-PyH(+)-TTF (1 oH(+)), 3-PyH(+)-TTF (1 mH(+)), 4-PyH(+)-TTF (1 pH(+)), and 4-PyH(+)-EDT-TTF (2 pH(+)) systems, the para-pyridyl-substituted donors 1 pH(+) and 2 pH(+) exhibit more marked changes upon protonation in solution; a larger redshift in the intramolecular CT absorption band and a larger decrease in the electron-donating ability. Furthermore, the EDT system 2 pH(+) has the smallest intramolecular Coulombic repulsion energy. These differences are reasonably interpreted by considering the energy levels and distributions of the HOMO and LUMO obtained by quantum chemical calculations. Such substituent effects related to protonation were also examined by comparing the structure and properties of a new H-bonded CT complex crystal based on 2 pH(+) with those of its 1 pH(+) analogue recently prepared by us: Both of them form a similar type of H-bonded dimer unit, however, its charge distribution as well as the overall molecular arrangement, electronic structure, and conductivity were significantly modulated by the introduction of the EDT group. These results provide a new insight into the structural and electronic features of the PyH(+)-TTF-based proton-electron correlated molecular conductors.

Solvent-induced on/off switching of intramolecular electron transfer in a cyanide-bridged trigonal bipyramidal complex.

A cyanide-bridged [Co3Fe2] cluster with trigonal bipyramidal geometry shows solvent-driven reversible on/off switching of its thermally induced electron-transfer-coupled spin transition (ETCST) behaviour.

The effect of a highly twisted CC double bond on the electronic structures of 9,9′-bifluorenylidene derivatives in the ground and excited states

We synthesized methyl-substituted 9,9′-bifluorenylidene (9,9′-BF) derivatives in which two planar fluorene units are connected through a CC double bond. The CC double bond is twisted owing to the steric crowding between the fluorene units, and by introducing substituents at 1,1′-positions (inner space of 9,9′-BF) it becomes more twisted. Indeed, single crystal X-ray structural analysis and theoretical calculation reveal that the dihedral angle between two fluorene π-planes of a 1,1′-dimethyl-substituted 9,9′-BF is 56°, which is clearly larger than those of pristine 9,9′-BF (42°) and 1-methyl-substituted 9,9′-BF (50°). The twisted conformation of 1,1′-dimethyl-substituted 9,9′-BF facilitates the cis–trans isomerization process which we assessed quantitatively by variable-temperature NMR measurements. The 9,9′-BF derivatives with different numbers of methyl groups also exhibit remarkable changes in optoelectronic properties, primarily because of the change in the twisting angle of the central CC double bond. Theoretical calculation further indicates that the electronic structures of methyl-substituted 9,9′-BF derivatives in the excited states are considerably different from those of pristine 9,9′-BF.

Anion substitution in hydrogen-bonded organic conductors: the chemical pressure effect on hydrogen-bond-mediated phase transition

We have synthesized three kinds of novel hydrogen-bonded (H-bonded) organic conductor, β′-[H3(Cat-EDO-TTF)2]X (X = ClO4, PF6, AsF6; Cat-EDO-TTF = catechol-fused ethylenedioxytetrathiafulvalene), as anion-substituted analogues of the parent BF4 salt (X = BF4). These salts are all isostructural, however, only the BF4 salt shows a phase transition, upon which the molecular arrangement and physical properties are drastically changed in cooperation with bending of the H-bond. A systematic comparison of their crystal and electronic structures disclosed that a significant chemical pressure is generated by the present anion substitution, causing differences in their physical properties and phase transition nature. Interestingly, this chemical pressure is highly anisotropic, which exclusively changes the side-by-side interactions between the Cat-EDO-TTF skeletons in the π-stacking layer. Therefore, we conclude that this unique phase transition is caused by a cooperation of the intercolumnar side-by-side interactions with the π–π intra-dimer interactions and H-bond bending.

Pressure-induced coherent sliding-layer transition in the excitonic insulator Ta2NiSe5

Ta2NiSe5 has recently attracted interest as an excitonic insulator. In this paper, crystallographic parameters of three non-ambient phases are reported from across the P–T phase diagram. A reversible pressure-induced structural transition above 3 GPa is associated with the coherent sliding of weakly coupled layers.

Thickness-dependent physical properties of La1/3Sr2/3FeO3 thin films grown on SrTiO3 (001) and (111) substrates

We have investigated the thickness-dependent transport properties of La1/3Sr2/3FeO3 thin films grown on SrTiO3 (001) and (111) substrates. At a thickness of ∼40 nm, both films show a clear transition in resistivity associated with the characteristic charge disproportionation at approximately 190 K. The transition temperature of the charge disproportionation is nearly unchanged with decreasing film thickness down to a certain thickness of ∼13 nm for both orientations, while the change in resistivity gradually decreases. Below this thickness, the transition becomes unclear, strongly suggesting the suppression of the charge disproportionation at the critical thickness of ∼13 nm. Furthermore, there is no significant difference in the thickness dependence of La1/3Sr2/3FeO3 thin films between the (001) and (111) orientations. The negligible crystallographic-orientation dependence may reflect the isotropic nature for the domain of charge disproportionation states in La1/3Sr2/3FeO3.