U. Langohr

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.

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.

2,5‐Disubstituted N,N′‐Dicyanobenzoquinonediimines (DCNQIs): Charge‐Transfer Complexes and Radical‐Anion Salts and Copper Salts with Ligand Alloys: Syntheses, Structures and Conductivities

The new members of the series of 2,5-disubstituted DCNQIs, 1d (Cl/OMe), 1e (Br/OMe), 1j (Cl/I), 1k (Br/I), 1l (I/I), form conducting charge-transfer complexes with TTF (tetrathiofulvalene) which are comparable to known DCNQI/TTFs. From these DCNQIs highly conducting radical-anion salts [2-X, 5-Y-DCNQI]2M (M = Li, Na, K, NH4, Tl, Rb, Ag, Cu) can also be prepared either from the DCNQIs and MI (not AgI), on a metal wire (Ag, Cu), or by electrocrystallization (M = Tl, Ag,Cu). For better crystals a method using periodical switching between reduction and partial oxidation has been developed. With CF3 (large, strongly electron-attracting) as the substituent in DCNQIs 1m (OMe/CF3) and 1n (Me/CF3), conducting TTF complexes remain whereas only 1n yields an insulating copper salt. DCNQI–Cu salts with high conductivities are obtained with alloys containing two or three different DCNQIs. The temperature-dependent conductivities of DCNQI–M salts (other than copper) are similar to those of metal-like semiconductors. All new DCNQI–Cu salts are metallic [M] down to low temperatures, except [1d (Cl/OMe)]2Cu which undergoes a sharp phase transition to an insulating state[M I]. By variation of the ligands or their ratios in conducting alloys of DCNQI–Cu salts temperature-dependent conductivities can be tuned from M I to M. In addition, alloying three ligands produced for the first time a radical salt with temperature-independent conductivity from 5 to 300 K. Most remarkably, alloys of the type [(2,5-Me2DCNQI)m] Cu/[{2,5-(CD3)2-DCNQI}n]2Cu which exhibit a sharp M I phase transition on further cooling reenter the conducting state by an I M transition, with changes of ca. 108 Scm−1 both ways. For the first time in the field of organic metals crystal structures of DCNQI–copper salts have been determined by X-ray powder diffraction methods and refined by Rietveld analysis. Unit cell data, coordination angles and distances of the π planes are in excellent agreement with the single-crystal X-ray data. However, bond lengths and angles of the ligands are to be less accurate. This powder method proves to be most valuable if only microcrystalline material is available.

A comparison of 3-D Cu-DCNQI with 1-D Tl-DCNQI by ESR and NMR

Abstract Among the large variety of highly conductive DCNQI-salts with metallic counterions, only (Me,Me-DCNQI)2Cu and (Me,Me-DCNQI)2Tl exhibit NMR behavior like the metals Cu and Tl, respectively, i.e. considerable Knight shifts and longitudinal relaxation rates of Cu and Tl nuclei. These seem point to a bonding between the DCNQIs and the counterions in both systems. Whereas in the copper salts there is no frequency dependence of the proton spin-relaxation rate. The T1H−1 of Tl-DCNQI is proportional to ω − 1 2 pointing to a preferential 1-d electron spin-motion. In contrast to the copper salt, which exhibits NCN-Cu-NCN bridges, there is no charge- / spin carrier transportation by the counterion in the Tl-salt. This is deduced by narrow ESR lines and a conductivity, lower by an order of magnitude. The Knight shift is due to a core polarization of the 6s orbital at the Tl nuclei.

Tethered 1,4-Benzoquinones and Their DCNQI Derivatives: Syntheses, Electronic Interactions, Redox Properties, Charge-Transfer Complexes, and Copper Salts

Bisquinones 16–20, in which the quinoid moieties are tethered by 2-6 methylene units, have been synthesized via the bis(dimethoxybenzenes) 2, 6, 9, 14, and 15. Compounds 16–20 were transformed into the corresponding bis(DCNQIs) 21–25 using standard procedures. Interactions between the two quinoid units, which fade with the length of the tethering chain, have been observed by 13C-NMR spectroscopy, backed by calculated charge densities (Table 1) and by their cyclovoltammograms which indicate two independent two-step electron transfers. Similar results arise from ESR investigations of the bis(quinone) radical ions. All DCNQIs 21–25 form charge-transfer complexes with TTF and copper radical anion salts with reasonable powder conductivities. Since single crystals could not be grown, the structure of these materials remains unknown.

Radical anion salts and charge-transfer complexes of bridged DCNQIS

Abstract New acceptors of the DCNQI-type were synthesized, in which two acceptor units are connected by a tethering chain of different length. From these dimers both conducting radical anion salts (cation Cu + ) and charge-transfer complexes (donor TTF) were obtained, which are compared with their monomeric counterparts, especially with respect to stochiometry and conductivity.

DCNQI anion radical salts with bisammonium cations

Abstract DCNQI anion radical salts with various bisammonium cations have been synthesized as black powders or crystals with metallic luster and semiconducting behavior with an activation energy of 0.11 eV at room temperature. X-ray structure determination of one compound shows segregated stacks of dication and acceptor molecules. The DCNQI molecules form trimers with an typical “ring over double bond” and “ring to ring pattern”.