Allan E. Underhill

A molecular metal with ion-conducting channels

Metallic behaviour is well known in charge-transfer complexes that contain stacks of planar, partially oxidized (or reduced) π-conjugated molecules. Electronic conduction occurs in the partially occupied, delocalized π bands formed by intermolecular orbital overlap, and some of these materials exhibit superconductivity,. Counter-ions, present to achieve charge neutrality, usually play a passive role, although in some cases they couple to the electronic structure, for example by imposing a new structural periodicity (a superlattice) by orientational ordering. The development of molecular solids that can simultaneously support the transport of both electrons and ions is important for several fields, including the development of solid-state batteries,, electroluminescent devices and biomimetic systems,. Crown ethers are promising components for such systems, as they provide cavities through which ion motion might occur. Here we report that the charge-transfer salt Li0.6(15-crown-5-ether)[Ni(dmit)2]2·H2O exhibits both electron and ion conductivity: the stacks of the nickel complex (dmit is an organic molecule) provide a pathway for electron conduction, and stacks of the crown ethers provide channels for lithium-ion motion. Evidence for the latter above 250 K is provided by NMR and conductivity studies. We also see evidence for coupling of the electron and ion motions. This compound might serve as a model for the development of other hybrid electronic/ionic conducting materials.

Effect of oxygen on the electrical characteristics of field effect transistors formed from electrochemically deposited films of poly(3-methylthiophene)

Field effect devices have been formed in which the active layer is a thin film of poly(3-methylthiophene) grown electrochemically onto preformed source and drain electrodes. Although a field effect is present after electrochemical undoping, stable device characteristics with a high modulation ratio are obtained only after vacuum annealing at an elevated temperature, and only then if the devices are held in vacuo. The polymer is shown to be p type and the devices operate in accumulation only. The hole mobility in devices thermally annealed under vacuum is around 10-3 cm2 V-1 s-1. On exposure to ambient laboratory air, the device conductance increases by several orders of magnitude. This increase may be reversed by subjecting the device to a further high-temperature anneal under vacuum. Subsidiary experiments show that these effects are caused by the reversible doping of the polymer by gaseous oxygen.

Synthesis, structure and properties of [ethylpyridinium][Ni(mnt)2]: evidence for an unusual magnetically ordered ground state

[Ethylpyridinium][Ni(mnt) 2 ] has been prepared by electrochemical oxidation of the corresponding dianionic salt. The X-ray structure has been determined. Three different environments were observed for the paramagnetic anions with two in a stacking mode and one approximately orthogonal to the stack. The stacking showed an ACA-type repeat unit with an inversion centre on complex C. Magnetic susceptibility measurements show Curie-Weiss behaviour with a change in parameters at around 50 K. Above this temperature a Curie constant of 0.3 emu K mol –1 and a Weiss constant of –150 K were observed and below 50 K these values were 0.08 emu K mol –1 and +2 K respectively. A maximum in the susceptibility was observed at 285 mK which suggests low temperature ordering to a phase with a non-cancelled alignment of the spins. Intermolecular interactions and the effect of temperature were assessed in terms of transfer integral calculations.

Energy level inversion in strongly dimerized [Pd(dmit)2] salts

Abstract Polarized reflectance spectra of single crystals of (CH 3 ) 4 As[Pd(dmit) 2 ] 2 and Cs[Pd(dmit) 2 ] 2 were measured and compared with those of (CH 3 ) 4 N[Ni(dmit) 2 ] 2 . Prominent absorption bands, which do not exist in the reflectance spectra of (CH 3 ) 4 N[Ni(dmit) 2 ] 2 were observed around 10000cm −1 in the spectra of [Pd(dmit) 2 ] salts. Analysis of the reflectance spectra reveals the anomalous electronic structure in these salts where the energy levels derived from HOMO and LUMO of one molecule are partly inverted due to their strongly dimerized structure.

Non-linear optical studies of nickel dithiolene complexes

The third-order optical non-linearity, χ(3), linear absorption coefficient, α, and the ratio of the real to imaginary parts of χ(3)have been measured at 1.06µm for a range of nickel dithiolene complexes with absorption bands between 0.77 and 1.06µm. From these data the non-linear retractive index, n2, and the device-based figure of merit, W(=Δnsat/αλ), were calculated. W was found to exceed 2 for materials with absorption maxima below 0.8–0.85µm. The non-linear refractive index of a poly(methyl methacrylate) host doped with 8 × 1019 molecules cm–3 of a nickel dithiolene was also measured and found to be nearly 2 × 10–9cm2 kW–1.

(N-methylthiocarbamoyl )tetrathiafulvalene derivatives and their radical cations : synthetic and X-ray structural studies

Lithiation of 4,5-bis(methylsulfanyl)-TTF 9, 4,5-(ethylenedisulfanyl)-TTF 10, 4,5-dimethyl-TTF 11 and 4,5,5′-trimethyl-TTF 12 (TTF=tetrathiafulvalene) followed by reaction with methyl isothiocyanate affords the corresponding (N-methylthio-carbamoyl)-TTF derivatives 14–17, respectively, in 54–70% yields. These new TTF derivatives display a broad intramolecular charge-transfer band in their UV–VIS spectra arising from conjugation between the donor TTF ring and the acceptor N-methylthiocarbamoyl moiety. Steric hindrance between the adjacent N-methylthiocarbamoyl and methyl substituents in 17 causes a marked hyposchromic shift in this band (λmax 395 nm) compared to compounds 14–16 (lambda;max 435–467 nm). Consistent with the electron-withdrawing properties of the N-methylthiocarbamoyl substituent, its attachment to the TTF ring raises slightly the oxidation potential of the system. Charge transfer complexes of these donors and (N-methylthiocarbamoyl)-TTF 2 with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and salts with bromide anions are reported, some of which have high room temperature conductivity values. The X-ray crystal structures are presented for 16, 17 and the salts 2·Br, 14·TCNQ and (17)2·20. The structure of 16 comprises orthogonal dimers (kappa packing) while in the structure of 17 individual molecules are orthogonal to each other. There is weak intermolecular hydrogen bonding in both 16 and 17. In the structure of 2·Br, the radical cations 2+· are almost planar and they form an infinite stair-like stack of dimers, with bromide anions situated between the stacks, and linked with the cation by a strong N–H‥Br bond. The structure of 14·TCNQ comprises mixed ‥DDAADD‥ stacks; the N-methylthiocarbamoyl group engages in an interstack N–H‥N bond with a TCNQ anion. Analysis of the bond lengths in the structure suggests that there is partial charge transfer from 14 to TCNQ. In the structure of (17)2·20 molecules form mixed ‥DDADDA‥ stacks and analysis of bond lengths suggests that there is only a small degree of charge transfer from donor to acceptor. The geometries of compounds 2, 14, 16, 17 were optimised using the PM3 semi-empirical method and the results compare favourably with the X-ray structural data.

Feature article. Molecular metals and superconductors

The field of molecular metals and superconductors has developed rapidly since the first molecular metal was discovered in the late 1960s. The most extensive range of compounds are based on planar organic donor molecules containing sulfur or selenium atoms and complex metal anions containing multi-sulfur ligands. This review discusses the types of molecules involved and the relationship between the solid-state structures of these compounds and their conducting and superconducting properties.

A new one-dimensional ‘metal’ with conduction through bis (dicyanoethylenedithiolato) platinum anions

Black needles of Lix[Pt(S2C4N2)2].2H2O (x ca. 0.75) exhibit a room temperature conductivity of 30–200 Ω–1 cm–1 and a temperature dependence of the conductivity similar to that of the one-dimensional metallic tetracyanoplatinate complexes such as K2[Pt(CN)4]Br0·3.3H2O.

Structural and electronic properties of Cs(Pd(dmit)2)2

The authors report a series of measurements of the structural and electronic properties of the molecular conductor Cs(Pd(dmit)2)2 (dmit=isotrithionedithiolate). This material has a C2/c structure with stacks of the acceptor groups arranged in sheets separated by the cations, with appreciable dimerization of the acceptor groups along the stack direction. It shows metallic properties at room temperature. We have characterized this material with measurements of conductivity, thermopower, magnetic susceptibility and polarized optical reflectivity, and they find that the electronic structure is quite isotropic in the plane of the acceptor sheets. On cooling, there is a metal-insulator transition at 56.5 K, and this opens an energy gap at the Fermi energy. This transition is associated with a complex periodic lattice distortion comprising an incommensurate modulation and also a commensurate distortion which breaks the C centring symmetry. They present a calculation of the electronic band structure using an extended Huckel formalism, and show that, owing to the strong dimerization along the stack direction, the Fermi energy lies halfway in a band of highest occupied molecular orbital character. They consider also how the observed structural instability is able to produce an energy gap over the whole Fermi surface, and propose a model that requires coupling between the two types of modulation.

Crystal structure of NH4[Ni{S2C2(CN)2}2]·H2O, the first equidistant stack d7 monoanion metal dithiolene complex

The X-ray crystal structure of NH4[Ni{S2C2(CN)2}2]·H2O has revealed that the planar [Ni{S2C2(CN)2}2]– anions form an eclipsed equidistant stack structure with a repeat distance of 3.918(8)A. At 98 K the repeat distance in the stack direction is double that at room temperature indicating that the anions are now associated as dimers. The room-temperature structure and low-temperature transition are related to the unusual magnetic properties of this compound.