Siegfried Hünig

Radical anion salts of N,N′-dicyanoquinonediimine (DCNQI): Conductivity and magnetic properties

Abstract DC-conductivity (σ), static susceptibility (χ), ESR-intensity, resonance position and linewidth of (R 1 ,R 2 -N,N′ -DCNQI) 2 Me ( R 1 = CH 3 ; R 2 = CH 3 , Cl , Br ; Me = Ag , Cu ) between 300 K and 1.2 K are reported. All copper salts have a high χ of about 5 – 6.10 −4 emu/mole in the conducting temperature range. This can be explained by strong electron-electron repulsion. In two salts, a CDW driven phase transition is seen and antiferromagnetic resonance (AFMR) is found below 10 K. The lack of any ESR signal in the conducting state is due to strong spin-orbit coupling (SOC) given by the nitrogen(p)-Cu(d) admixture which implies some electronic transport across the stacks. The silver salts do not exhibit these features. They behave as quasi-one dimensional conductors with reduced SOC effects. The conductivity is limited solely to the anion stack. Consequently, 1D-localization effects dominate the temperature dependence of the conductivity.

The pressure-temperature phase diagram of the organic conductor (2,5 DM-DCNQI)2Cu

The authors show that it is possible to suppress the ambient-pressure metallic phase in (2,5 DM-DCNQI)2Cu by applying a pressure-even one as low as 100 bar. On increasing the pressure up to 8.5 kbar, the metal-insulator transition temperature increases and its strong first-order character becomes weaker, whereas the activation energy stays constant. In addition, a re-entrance of the metallic phase is observed at very low temperatures up to pressures of about 300 bar.

Violene/cyanine hybrids as electrochromics part 2: tetrakis(4-dimethylaminophenyl)ethene and its derivatives

The general structure of violene/cyanine hybrids (see below) is exemplified by tetrakis(4-dimethylaminophenyl)ethene 1(RED) its vinylogue 2(RED) and its diazavinylogue 3(RED). As judged from their cyclic voltammograms and spectroelectrograms, oxidation occurs perfectly reversible by loss of two electrons creating closed shell systems 1-3(OX)+2 with strong bathochromic shifts (Michlers hydrol blue moieties). ESR spectra indicate only minor amounts of radical cations. At much higher potentials by another reversible loss of two electrons (-->1-3(OX)+4) the long wavelengths absorptions are replaced by shorter ones. In system 4, containing two 4-dimethylaminophenyl units only, the violene character is better preserved since oxidation occurs stepwise by single electron transfer up to 4(OX)+4. These results are backed by theoretical calculations for 1-4, demonstrating the strong geometrical differences between the various oxidation levels. Besides, new types of cyclic structures for 1-4(OX)+4 are indicated by these calculations: For systems 1-3 cyclic structures for tetracations have been found to be more stable by 3-20 kcalmol(-1) than acyclic structures, whereas for system 4 the acyclic structure is more stable by about 22 kcalmol(-1). The redox behavior of systems 1-4 is of general importance for electrochromic systems.

Violene/Cyanine Hybrids: A General Structure for Electrochromic Systems

Reversible redox systems for electrochromics with closed-shell colored species are provided by a combination of violene and cyanine structure types according to a general scheme shown in the figure.

N,N′-dicyanoquinone diimines (DCNQIs): unique acceptors for conducting materials

Replacement of the classical acceptor TCNQ with (substituted) DCNQIs opens new routes to organic conducting materials. The radical anion salts [2,5-R1,R2-DCNQI]2M (R1,R2= Hal, Me, MeO; M = Li, Na, K, NH4, Cu, Ag, Tl) all belong to the same space group l41/a or related ones (C2/c or P4/n); therefore, the correlation between molecular size and electronic effects of the substituents and bulk properties (conductivity, susceptibility, etc.) can be evaluated. Only the copper salts display metallic conductivity, some of which undergo a phase transition (TM → 1) to a semiconductor on cooling. For this transition a threshold coordination angle N–Cu–N is found, by changing which an unprecedentedly large isotope effect upon deuteriation of [2,5-Me2DCNQI]2 can be induced. Upon the transition TM → 1 the conductivity decreased by a factor of 107–108.

Synthesen mit Enaminen, V. Kettenverlängerung von Mono- und Dicarbonsäuren um 5 bzw. 10 C-Atome

Die uber die acylierten 1-Morpholino-cyclopentene-(1) erhaltlichen 2-Acyl-cyclopentanone lassen sich durch Alkali zu δ-Acyl-valeriansauren aufspalten, die glatt durch Hydrazinhydrat reduziert werden. Auf diesem Wege werden Carbonsaurechloride mit 50–60% d. Th. um 5 C-Atome, Dicarbonsaure-ester-chloride mit etwa 40% d. Th. um 5 C-Atome und Dicarbonsaure-dichloride mit mindestens 8 C-Atomen mit etwa 40% d. Th. um 10 C-Atome verlangert.

Synthesis and structure of new anion radical salts from DCNQIs

Abstract Anion radical salts [2-R 1 ,5-R 3 -DCNQI] m M n (M=metal or organic cation) are described, which are derived from the newly introduced class of acceptors, N,N′-dicyanoquinonediimines. Single crystals were easily prepared from DCNQIs and metal wires. The salts are discussed in terms of structural properties and (temperature dependent) conductivities together with the behaviour of alloys.

Pressure-Induced Metal-to-Insulator Phase Transitions in the Organic Conductor (2,5 DM-DCNQI)2Cu

The electrical resistivity along the high-conductivity axis has been measured at pressures up to 9 kbar in the novel organic conductor (2,5 DM-DCNQI)2Cu. Deviations from the usual linear behaviour of the conductivity under pressure at room temperature have been observed. Moreover, at lower temperatures metal-to-insulator phase transitions have been induced with values of transition temperatures depending on the applied pressure. The obtained results are discussed and compared with corresponding data for other organic compounds. A tentative interpretation of this novel and unusual phase diagram is proposed in terms of a mixed valence state for copper ions.

The conducting salts of N,N′-dicyanoquinonediimine (DCNQI)

Abstract New results on a series of X,Y-N,N′-DCNQI salts with metallic counterions are presented (X,Y = H, CH3 ≡ Me, Cl, Br, I). The comparison of conductivity, ESR (linewidth and susceptibility) and NMR (Proton spin relaxation) of these radical anions salts indicates that all salts can be separated into two groups: salts containing copper as counterion, which can be characterised as organic metals (at least at high temperatures); and salts with other counterions, whose physical properties are strongly influenced by low dimensionality. The metallic nature of (DMe-DCNQI)2Cu has been demonstrated directly by UPS-photoelectron spectroscopy, which reveals a high density of states near the Fermi energy.

The electronic structure of (2-X-5-M-DCNQI)2Cu

Abstract Using XPS and UPS photoelectron spectroscopy, we have studied the electronic structure of single-crystalline needles of 2-Cl-5-M-, 2-Br-5-M- and 2,5-DM-substituted (DCNQI) 2 Cu charge-transfer salts, prepared in situ. The results indicate that the oxidation state of the counterion is Cu + . The electronic structure of the valence band contains contributions from C 2p and N 2p states, but also from Cu 3d atomic levels. There is a metallic density of states near the Fermi energy for (DM-DCNQI) 2 Cu. This is the first experimental evidence for the existence of an organic metal with an admixture of Cu 3d states. The admixture from Cu states near the Fermi energy is reduced by substitution of Cl and disappears in amorphous films, demonstrating the strong influence of local structure. The implication of these spectroscopic results on the conductivity mechanism is discussed.