A. S. Perel

N-channel field effect transistors with excellent device characteristics have been fabricated by utilizing C60 as the active element. Measurements on C60 thin films in ultrahigh vacuum show on-off ratios as high as 106 and field effect mobilities up to 0.08 cm2/V s.

We have used the magneto-oscillatory behavior of the quasi-two-dimensional orbits in the charge-transfer organic salt (BEDT-TTF${)}_{2}$KHg(SCN${)}_{4}$ to probe its underlying magnetic ground states, which arise from the coexisting quasi-one-dimensional nature of the material. The complex nature of the oscillations results from the field dependence of these states. Our results allow comparison between several competing models which have attempted to describe the unusual properties of this material.

Condensation of oxamidrazone with sulfur dichloride in acetonitrile affords 4,4‘-bis(1,2,3,5-dithiadiazolium) dichloride in moderate yield. Reduction of this salt with triphenylantimony yields the diradical 4,4‘-bis(1,2,3,5-dithiadiazolyl) [S2N2C−CN2S2], which has been isolated and characterized in the solid state as its dimer [S2N2C−CN2S2]2. The diradical is disjoint, and ab initio molecular orbital methods confirm a very small energy gap (<0.5 kcal/mol) between the triplet and diradical singlet states, regardless of the torsion angle about the central C−C bond. In accord with these theoretical predictions the ESR spectrum of the diradical consists (in CHCl3 at 273 K) of a simple five-line pattern (aN = 0.50 mT, g = 2.011), i.e., there is no observable exchange coupling between the two centers. In the solid state, the dimer [S2N2C−CN2S2]2 forms a slipped stack structure, with a mean intradimer S−S distance of 3.078 Å and mean interdimer S- - -S contact of 3.761 Å. Cosublimation of the diradical with iodine produces the charge-transfer salt [S2N2C−CN2S2][I], orthorhombic space group Ccmm, a = 11.909(3) Å, b = 3.271(2) Å, c = 19.860(6) Å, Z = 4 (at 293 K). In this structure the heterocyclic rings form perfectly superimposed and evenly spaced stacks along the y direction, with channels of disordered iodines. The iodine-doped material is metallic at ambient temperatures, with a single-crystal conductivity of 460 S cm-1 at 300 K; variable temperature conductivity and magnetic measurements reveal a phase transition near 270 K, with the onset of semiconducting behavior. Transport data for the neutral and doped materials are discussed in the light of Extended Hückel band calculations.

Cosublimation of 1,3- and 1,4-benzene-bis(1,2,3,5-dithiadiazolyl) and iodine/bromine affords crystals of the mixed valence salts 1,3- and 1,4-[(S2N2C)C6H4(CN2S2)][X] (X = I, Br). The crystal structures of the two iodide salts consist of perfectly superimposed stacks of molecular units with interannular spacing along the stacks of 3.487(3) and 3.415(2) Å, for the 1,3- and 1,4-derivatives. In both compounds the iodines are disordered along the stacking direction. The 1,3-derivative has a highly one-dimensional structure; there are no short intercolumnar S-S interactions. In the 1,4-derivative, however, lateral S-S contacts of 3.911 Å, afford some measure of three-dimensionality. The bromide salt of the 1,4-derivative consists of ribbons of alternating 1,4-[(S2N2C)C6H4(CN2S2)]+ units and bromide ions. Within each molecule one heterocyclic ring is closed shell, i.e., a [CN2S2]+ cation, while the other is a discrete radical. The ribbons are layered in zigzag fashion that maximizes ion pairing and isolates the radical centers. The bromide salt of the 1,3-derivative also forms ribbon-like arrays, but the unit cell repeat consists of four layers of ribbons. Within these layers the [CN2S2] rings are approximately stacked. The four rings within the repeat unit along each stack consists of three rings clustered into a trimeric [CN2S2]3+ cation, while the remaining ring is a discrete [CN2S2]+ cation. Magnetic susceptibility and conductivity measurements on the two iodide salts indicate weakly metallic behavior at room temperature, with a charge density wave (CDW) driven metal-insulator phase transition occurring near 270 and 190 K for the 1,3- and 1,4-derivatives, respectively. For the 1,4-derivative, analysis of the CDW wavevector associated with the transition affords a degree of charge transfer of 1/4 of electron per radical, i.e., an overall formulation of [(S2N2C)C6H4(CN2S2)]0.5+[I]0.5-. The bromide salt of the 1,3-derivative is a closed shell insulator, while in the 1,4-bromide the isolated radical centers are antiferromagnetically coupled.