N. Iwasawa

Switching effect in organic charge transfer complex crystals

A reproducible current switching effect has been observed in various mixed‐stack organic charge transfer crystals whose ionicities are close to the neutral‐ionic phase boundary. This effect appears to be related to the intrinsic nonlinear conductivity, which has recently been reported in these crystals. This switching has a reasonably large delay time and our results indicate that it is a bulk property of the crystals. Discussion is given on the possible mechanism of this effect.

Organic radical crystals, α-nitronyl nitroxide family: High-field magnetization study

Abstract Studies of intermolecular antiferromagnetic interactions in the crystals of several derivatives of α -nitronyl nitroxide radical by magnetization (up to 400 kOe) and susceptibility (1.8–300 K) measurements are presented. The estimated exchange couplings are attributed to the specific molecular arrangements, by making reference to the crystal structures.

Preparation, structure, and physical properties of a series of uncapped C6X4Y4 (X, Y = Sulfur, selenium, tellurium) compounds

Abstract The syntheses and redox properties of a series of uncapped C 6 X 4 Y 4 compounds (tetrakis(n-alkylchalcogeno)tetrachalcogenafulvalenes, TYC n −TXF: X = sulfur, rmY = sulfur, selenium, tellurium, n = 1–18; X = selenium, Y = sulfur, selenium, n = 1) are described. Many characteristic transport and structural features depending on the kinds of X, Y and the length of side alkyl chains of the uncapped C 6 X 4 Y 4 compounds are written. TYC n -TTF with long alkyl chains shows a high conductivity as a single component organic semiconductor due to the specific packing of molecules in a crystal (molecular fastener). On the other hand TTeC 1 -TTF having the shortest alkyl chains shows a high mobility together with a high conductivity with infinite chains constructed by quasi-covalent Te··Te atomic contacts The TTeC 1 -TTF·TCNQ complex is a quasi three-dimensional electrical conductor owing to its unique crystal structure including the short Te·N contacts. The similarity of the crystal structures not only as a single component but also as a charge transfer complex of TYC n -TXF series such as TTC 1 -TTF and TTC 1 -TSeF, (TTC 1 -TTF)I 2.468 and (TTC 1 -TSeF)I 2.477 , and (TTC 1 -TTF)TCNQ adn (TSeC 1 -TTF)TCNQ is discussed. The structures and physical properties of the TYC 1 -TTF·TCNQ series are compared especially from the view points of the replacement of outer chalcogen atoms Y.

Magnetic properties and crystal structures of some nitronyl nitroxide derivatives

Abstract Single crystals of four organic radicals, p -trifluoromethylphenyl ( 1 ), pentafluorophenyl ( 2 ), 2-thienyl ( 3 ), and biphenyl ( 4 ) derivatives of nitronyl nitroxide, were prepared. The paramagnetic susceptibilities, χ p , of the crystals of 1 , 2 , and 3 exhibit maxima at low temperatures. The temperature dependence of the susceptibilities were analyzed in terms of the one-dimensional (1D) alternating antiferromagnetic Heisenberg model. The crystal structure of compound 1 supports the alternating antiferromagnetic chain arrangement of the molecules. Although the magnetic susceptibility of biphenyl derivative 4 shows no maximum down to 1.8 K, χ p T decreases and seems to approach a constant value below about 4 K. This behavior was well reproduced by a symmetric four-spin cluster model with antiferromagnetic intermolecular interactions, J 1 / κ B = −1.5 K and J 2 / κ B = 6.6 K.

Thermal Properties of Tetrakis(Alkyltelluro)Tetrathiafulvalene (TTeCn-TTF)

Abstract Thermal properties of tetrakis(alkyltelluro)tetrathiafulvalenes (TTeCn-TTF) are investigated using DSC technique. It is shown that combining tellurium substituted TTF skeleton with the alkyl chains of different length we got a variety of thermal properties due to a competition between the alkyl chains and the TTF moiety. This competition leads to a polymorphism and to an irregular dependence of the melting point as well as enthalpy and entropy of melting on the alkyl chain length between the carbon number n = 4 and 7. For the compounds with long alkyl chains (n > 6) the analysis of thermodynamic properties confirms the existence of the molecular fastener effect, i.e. fastening of the TTF skeletons and decreasing of the distances between them in the crystal structure by crystallization of the long alkyl chains.

One-dimensional ferroelectricity and dynamics of domain-walls in organic compounds

Abstract Taking advantage of strong electron-lattice interaction characteristic in quasi-one-dimensional systems, the prototypes of new organic molecular solids with one-dimensional ferroelectricity are presented; the ionic charge-transfer (CT) compounds with the donor-acceptor mixed stacks and the hydrogen-bonded (H-bonded) solids with enol-form π-molecular chains. Dynamical properties of the kink-type domain-walls or solitons in these 1D solids are discussed.

Nonlinear soliton transport in charge transfer compounds

Abstract Nonlinear conductivities and anomalous current switching effect have been widely observed in single crystals of semiconducting organic charge transfer compounds. These features are found to be strongly dependent on the molecular ionicity and the stack structures, indicating that the switching effect is attributed to the breakdown of bond-ordered-wave (BOW) state induced by soliton current.

Electrical Switching Effect in Organic Charge Transfer Compounds

Abstract A current switching effect in charge transfer compounds has been investigated by probing photoconductivity. It has been found that the life time of photocurrent is suddenly elongated when the crystal is switched to the on-state of current.