Daniel J. Sandman

Photoemission studies of TCNQ compounds

Abstract The densities of occupied states of a series of TNCQ compounds have been determined by u.v. photoemission. A clear difference between TCNQ weak charge transfer complexes and anion radical salts including TTF-TCNQ is found. Due to strong electron-molecule coupling narrow delocalized electron bands are not observable.

Inelastic electron scattering lengths and charge transfer in tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ) films

Abstract Inelastic scattering lenghts of hot electrons in TTF and TCNQ have been determined. The electronic structure of a 1 : 1 mixture of TTF and TCNQ, obtained by ultraviolet photoemission spectroscopy, shows that at least 40% of the surface molecules are charge transferred. Implications for crystalline TTF-TCNQ are discussed.

Studies of The Soltd State Reactivity and Properties of Diacetylene Polymers and Acetylenes

Abstract The physical considerations which motivate our interest in fully crystalline polymers derived from acetylenic and diacetylenic monomers are presented. The interaction of the polymer derived from 1,6-di-N-carbazolyl-2,4-hexadiyne (DCH) with antimony pentahalides (SbX5, X=F, Cl) has been studied as a function of polymerization method. It was found that thermally polymerized DCH takes up SbX5, and the chemical, spectral, crystallographic, and electrical properties of the resultant material are reported. The reactivity in the solid state of the simple hydrogen bonded acetylenes propiolamide and the oxime and semicarbazone of propiolaldehyde using heat and 60Co gamma radiation as stimuli has been studied. While the above acetylenes are unreactive to gamma radiation in comparison to diacetylenes and metal propiolates, their reactivity was enhanced when they were crystallized from chlorinated solvents. Propiolaldehyde semicarbazone is monoclinic, space group P2/n, a = 10.582(3), b = 3.801(1), c = 14.065...

Synthesis of a new class of highly conducting organic ion–radical salts

The organosulphur donors Δ4,4′-bithiopyran and 2,2′,6,6′-tetramethyl-Δ4,4′-bithiopyran react with 7,7,8,8-tetracyanoquinodimethane to give a new class of ionradical salts whose conductivities, measured on compressed pellets at room temperature, are comparable to those of the highest conducting classes of organic solids.

Preparative, structural, and electrical studies of the 1 : 1 and 2 : 3 tetracyanoquinodimethane salts of 4,4′-bithiopyranylidene (BTP)

The preparation, single crystal growth, and single crystal conductivity of the 1 : 1 and 2 : 3 7,7,8,8-tetracyanoquinodimethane (TCNQ) salts of 4,4′-bithiopyranylidene are reported. The crystal structure of the 2 : 3 salt, the first structure of a 2 : 3 TCNQ salt from a neutral planar closed-shell donor, has been determined by X-ray diffraction at room temperature. (BTP)2(TCNQ)3 crystallizes in the triclinic space group P, a= 8.351(5), b= 17.445(4), c= 8.020(4), α= 97.35(3), β= 103.10(4), γ= 88.15(4), U= 1 128.6(8)A3, Z= 1, Dcalc.= 1.467 g cm–3, Dobs.= 1.46(1) g cm–3. The structure was solved by Patterson and Fourier methods to a final R of 0.046 for 1 490 unique reflections with F0 > 3σ(F0). (BTP)2(TCNQ)3 has a mixed stack crystal structure which alternates a pair of BTP cation-radicals and three TCNQ molecular species. There are two crystallographically distinct types of TCNQ : a centrosymmetric (c-TCNQ) and a non-centrosymmetric (n-TCNQ) form in the ratio 1 : 2. The bond lengths of centrosymmetric TCNQ closely resemble those of neutral TCNQ while those of non-centrosymmetric TCNQ are similar to those of TCNQ–1, a situation found in other 2 : 3 TCNQ salts. The bridging double bond of the BTP cation-radical is quite long, 1.429(6)A, ca. 0.05 A longer than that of a neutral BTP derivative. As found in the 1 : 1 heterafulvalene–TCNQ segregated stack conductors, there are short contact distances between the sulphur atoms on the BTP cation and the cyano nitrogens on TCNQ. However, in the present structure this mode of interaction is qualitatively different in that each BTP sulphur atoms contacts (‘plugs into’) two cyano groups, while in the heterafulvalene–TCNQ salts, each chalcogen contacts only one cyano nitrogen. While BTP is iso-π-electronic to tetrathiafulvalene (TTF), which interacts with TCNQ to give a metallic 1 : 1 salt, BTP–TCNQ is a semiconductor with a room temperature conductivity (σ) of 30 ohm–1 cm–1. The temperature dependence of σ for BTP–TCNQ between room temperature and 80 K is well described by σ(T)=AT–αexp(–Δ/T), an expression representing the product of an activated charge carrier concentration with a system specific activation energy, Δ and a temperature dependent mobility, with α 2.64 and Δ 1 480 K (0.13 eV). As expected from its mixed stack crystal structure, (BTP)2(TCNQ)3 is a semiconductor with a room temperature single crystal conductivity of 0.10, ohm–1 cm–1 and an activation energy of 0.16 and 0.18 eV measured from conductivity and thermoelectric power respectively.

Synthesis of a new organic π-donor: 2-(thiopyran-4-ylidene)-1,3-dithiole and its conducting salts with 7,7,8,8-tetracyanoquinodimethane

The new organic π-donor, 2-(thiopyran-4-ylidene)-1,3-dithiole (3), iso-π-electronic to both tetrathiafulvalene (1) and Δ4,4′-bithiopyran (2) and obtained by a three-step synthesis in 15% overall yield, reacts with 7,7,8,8-tetracyanoquinodimethane (TCNQ) to give two semiconducting salts.

Spectroscopic studies of polydiacetylenes: Raman evidence for surface phases on single crystals

Abstract Approaches to the use of π donor-acceptor interactions to modify the linear electronic spectrum of polydiacetylene (PDA) crystals are discussed. Raman spectra of the chemically similar polydiacetylene-1,6-(di- N -carbazolyl)-2,4-hexadiyne (PDA-DCH) and polydiacetylene-1,1,6,6-tetraphenyl-2,4-hexadiynediamine (PDA-THD) were studied as a function of the excitation wavelength. Using 1064, 632.8, 514.5, 488.0 and 457.9 nm as excitation wavelengths, spectra are observed using light polarized in the chain axis direction. Spectra observed at 457.9 nm are clearly different to those obtained at the two longest wavelengths used, and it is deduced that these spectra are associated with surface phases of these PDA. The position of the dilute solution absorption maximum of a soluble PDA with p -chlorocinnamate side groups is dependent on the refractive index of the solvent, indicating the sensitivity of the conjugated backbone to the local solvent environment.

The Crystal Structure-Thermochromism Relationship for the Polydiacetylene ETCD

Abstract The polydiacetylene of the bis-ethylurethane of 5,7-dodecadiyn-1,12-diol (ETCD) is the best defined example of a thermochromic polymer at present. Thermochromism in poly-ETCD is associated with an endothermic first-order phase transition involving an increase in unit cell volume. The conformer population of the side-chain methylene groups changes on heating from 23 to 130°C. Hydrogen bonding and backbone planarity do not appear to be significantly disrupted during the transition. Solid state thermochromism in other polymers and examples of reversible thermochromism in inorganic and molecular organic solids are discussed in relationship to poly-ETCD.

An E.S.R. study of the ion pairs of the anion radicals of tetracyanoquinodimethan (TCNQ) and tetrafluorotetracyanoquinodimethan(TCNQF4) in solution

A comparative E.S.R. study of the potassium salts of the anion radicals of tetracyanoquinodimethan(TCNQ) and tetrafluorotetracyanoquinodimethan(TCNQF4) in dilute solutions of ether type solvents reveals that both anion radicals are ion paired. Careful measurements of the hyperfine splittings (hfs) as a function of temperature for both anions in solution show that the isotropic hfs are temperature and solvent dependent. Generally, at temperatures below room temperature, both anion radicals in solution display linewidth alternation phenomena which arise from modulation of the nitrogen and fluorine hfs in TCNQ and TCNQF4, respectively. In contrast, the isotropic g-values of both anion radicals in solution are independent of temperature and the solvents used in this study. The experimental evidence, molecular orbital calculations of the charge density distribution in the free anion radicals, calculations of the ion pair association energies for various K+/TCNQ- and K+/TCNQF4 structures and comparison of the c...

Electron energy-loss spectroscopy of tetracyanoquinodimethane, TCNQ, tetrathiafulvalene, TTF, and the salt TTF-TCNQ

Abstract Energy loss spectra of low energy electrons (ELS) have been obtained for thin solid films of tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF), and the charge transfer salt TTF-TCNQ as a function of incident energy (20 to 100 eV). The incident energy dependence of energy loss features allows the identification of triplet and symmetry forbidden excitations in TCNQ and TTF. The ELS spectrum of TTF-TCNQ is compared with high energy electron loss data to show that the surface of evaporated films of TTF-TCNQ contains 20–50% neutral TCNQ molecules.