Spectroscopic studies of the radical cation salt (BEDT-TTF)2Pt(NO2)4

We report on spectroscopic studies of the charge transfer salt formed by the organic donor bis(ethelenedithio)tetrathiafulvalene (BEDT-TTF) and [Pt(NO₂)₄]²⁻ anions. In this salt BEDT-TTF molecules are arranged in well-isolated (BEDT-TTF⁺)₂ dimers. Polarized reflectance spectra of single crystals were measured in the IR region (650–6500 cm⁻¹). Moreover, absorption spectra (400–15,000 cm⁻¹) and FT-NIR Raman spectra of powdered crystals dispersed in KBr pellets were recorded. Vibrational and electronic features are discussed. We suggest that the frequency of the CC stretching ν₃(a_g) mode depends non-linearly on the degree of ionization of BEDT-TTF.     

New organic metal (BEDO-TTF)4Pt(CN)4·H2O

A new charge transfer salt of (BEDO-TTF)₄Pt(CN)₄·H₂O (BEDO-TTF = bis(ethylenedioxo)tetrathiafulvalene) composition has been prepared. X-ray analysis shows that the conducting donor layers of new θ″-type packing alternate with anion sheets composed of [Pt(CN)₄]⁻² anions, which are associated with H₂O molecules via hydrogen bonds. Temperature and magnetic field dependences of the resistance are presented.     

Crystal and molecular structure of the new anion-radical salts—(N-Me-2-NH2-Pz)(TCNQ)2 and (N-Me-Tetra-Me-Pz)(TCNQ)2 (Pz is pyrazine)

Two anion-radical salts (ARS) of 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) – (N-Me-2-NH₂-Pz)(TCNQ)₂ and (N-Me-Tetra-Me-Pz)(TCNQ)₂ (N-Me-2-NH₂-Pz, N-methyl-2-amino-pyrazinium-; N-Me-Tetra-Me-Pz, N-methyl-tetra-methyl-pyrazinium-ions) – were synthesized and characterized. Both salts (ARS) were found to be semiconductors with a room-temperature conductivity of 3.8 × 10⁻⁵ and 1.93 × 10⁻² Ω⁻¹ cm⁻¹, respectively. For both salts a layered structure of cations and anion-radicals was discovered, where layers composed of cations alternate along the b-axis with the layers containing TCNQ anion-radicals. The cations in the (N-Me-2-NH₂-Pz)(TCNQ)₂ form pairs bonded by strongly shortened 1.97 (2) Å intermolecular hydrogen N(1)…H(3a) links.     

A new highly-conjugated TTF analogue: Synthesis,electrochemistry and a conducting TCNQ complex of 9,10-anthracenediylidene-2,2′-bis(4,5-dimethyl-1,3-dithiole)

The TTF analogue (4) has been isolated as the neutral and the dication species. Cyclic voltammetry of (4) reveals a single, two-electron redox couple. Compound (4) is a promising new electron donor and a complex formed with TCNQ is conducting, σ_rt = 10⁻² S cm⁻¹.     

SEM/EDX investigations of conducting organic composites formed by BEDO-TTF and iodides

Scanning electron microscopy (SEM) investigation and X-ray microanalysis (EDX) of the highly conducting organic composites of general formulae (BEDO-TTF)_x/I and (BEDO-TTF)_x/(AuI), where BEDO-TTF is bis(ethylenedioxy)tetrathiafulvalene, obtained by direct charge-transfer (CT) reaction in the solid state have been performed. The structure of the composites has been compared with the structure of ones based on the donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF). Differences in electrical properties of these two types of composites are discussed, basing on this comparison.     

Effects of dopants and polymer structures on electrical conductivity of organosilicon polymers

A series of organosilicon polymers comprising disilanylene-phenylene, disilanylene-ethynylene and disilanylene-(bisethynylene-phenylene) chains carrying dibutyl or phenyl and methyl groups as side chains, and polysilanes carrying alkyl, aromatic or heteroaromatic side chains were synthesized. The electrical conductivities of their polymers doped with electron acceptors such as I₂, FeCl₃ and SbF₅ were measured by using vapor-phase doping. The electrical conductivities of organosilicon polymers doped with I₂ showed a good correlation with the ionization potentials of the original polymers. In contrast, in the case of SbF₅ doping, the degradation of polymers occurred and every polymer gave the same order of conductivity, 10⁻⁴−10⁻³ S/cm. The polymers carrying heteroaromatic substituents gave the higher conductivity by doping with I₂, while in the case of the polymers carrying phenyl groups, the conductivity was enhanced by doping with FeCl₃.     

Polymeric 2,3-naphthalocyaninatoiron (II) complexes with bidentate isocyanides as intrinsic semiconductors

2,3-Naphthalocyaninatoiron(II) was reacted with monodentate and bidentate isocyanides to form monomeric and bridged polymeric complexes, respectively. The products were characterized by physical and spectrochemical methods. The bridged complexes show electrical conductivities in the range of 10⁻³–10⁻⁶ S/cm without additional doping.     

Optical spectroscopy and scanning tunneling microscopy of thin films of the metal-tetracyanoquinodimethane derivatives

Semiconducting thin films of charge-transfer complexes which exhibit a novel field-induced phase change have been examined by optical spectroscopy and scanning tunneling microscopy (STM). We have demonstrated a field-induced, charge-transfer reaction driven by the electric field at the microscope tip when the field generated by the microscope exceeds the switching threshold of the organic charge-transfer complex: CuTCNQCl₂ (TCNQ = tetracyanoquinodimethane). The threshold of phase transition induced by the scanning tunneling microscope for CuTCNQCl₂ is 7.5 ×10⁵ V/cm, whereas for the parent compound CuTCNQ it is 1.0 × 10⁵ V/cm. These threshold values are proportional to the strength of the acceptor, i.e., the first half-wave reduction potential E₁ of the acceptor molecule. We used an acetonitrile-vapor-assisted film preparation method to deposit weak donor-acceptor complexes (such as 2,5-dialkyl- and 2,5-dialkoxy-substituted TCNQ derivatives). These complexes do not react with copper metal by the conventional post-heating method. These deposited films have been analyzed by UV-Vis and FT-IR spectroscopy and the surfaces have been imaged using STM. This STM-induced solid-state chemical reaction is being explored for possible development as a high-density molecular-based information storage media.     

Study of new conducting organic composites based on BEDO-TTF molecule

Preparation of new organic composites by direct solid–solid charge-transfer reaction between bis(ethylenedioxy)tetrathiafulvalene (BO) and iodine is described. We present also dc electrical conductivity investigation of composites: (BO)_x/I, where 0.5<x<1.5. In the best case, corresponding to x=1, a metallic behaviour is observed down to 200 K prior to any annealing stage with a room temperature conductivity as high as 12.8 S cm⁻¹.     

Infrared spectra of tetramerized organic semiconductors PrPht(TCNQ)2 and PrQuin(TCNQ)2 — the problem of ag phonon mode splitting

Polarized infrared reflectance spectra from single crystals of two organic charge-transfer salts PrPht(TCNQ)₂ and PrQuin(TCNQ)₂ (where TCNQ = tetracyano-pquinodimethane, PrPht = N-propylphtalazinium, PrQuin = N-propylquinoxalinium) are studied over the frequency range 400–7000 cm⁻¹ at room temperature (in both compounds the TCNQ molecules are arranged to form linear stacks with the tetramer repeat unit). Kramers-Kronig analysis of the reflectance is used to determine the optical properties. Some vibrational lines related to the coupling of a_g phonon modes of the TCNQ molecule with electrons are split. The origin of a_g mode splitting is discussed. Moreover, the electronic spectra (up to 20 000 cm⁻¹) are measured at room temperature using the KBr pellet technique. The charge-transfer bands in tetramerized TCNQ salts are analysed.     

Electrical conductivity of organosilicon polymers II: Synthesis and electrical conductivities of iodine-doped polysilanes containing amino groups

A series of polysilanes containing an N,N-dimethylaminophenyl group and N-pyrrolyalkyl group as in the side chain were prepared by the Wurtz coupling polymerization. The polymerization of N-pyrrolylbutyl or N-pyrrolylhexyl-substituted dichlorosilane with sodium gave successfully the polysilanes with higher molecular weight than 10⁵. The electrical conductivities of I₂-doped polysilanes were dependent on the chemical structure of the amino side chain. Moreover, by use of excess iodine, the conductivity of the polymer was enhanced. The electrical conductivity of poly[bis(6-(N-pyrrolyl)hexyl]silane] (7a) increased to a maximum value of 8 × 10⁻² S/cm.     

A new BEDT-TTF-based organic metal with an anionic weak acceptor 2-sulfo-1,4-benzoquinone

An anionic weak acceptor, 2-sulfo-1,4-benzoquinone, has provided a new BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) salt (BEDT-TTF)₂(2-sulfo-1,4-benzoquinone)·H₂O. Single crystal X-ray analysis indicates that the unit cell has two crystallographically independent donor layers, one of which has a β″-type packing motif and the other being disordered. Population analysis and SQUID magnetometry suggests that non-β″-layer has an α?-type packing motif. The dual-layered salt shows metallic behavior with a metal–insulator transition at 90 K.     

Conductive complexes of [Ni(dmid)2] with TTF,TMTTF, and ET

Three charge-transfer salts of [Ni(dmid)₂]⁻ (dmid: 1,3-dithiol-2-one-4,5-dithiolate)—TMTTF[Ni(dmid)₂], TTF_x[Ni(dmid)₂] and ET_x[Ni(dmid)₂] (TMTTF: tetramethyl-tetrathifulvalene, TTF: tetrathifulvalene, ET: bis(ethylenedithio)-tetrathiafulvalene) are prepared and characterized. The TMTTF[Ni(dmid)₂] complex has a structure with mixed packs is formed by cations and anions, which alternate each other with subsequent shift. This compound is semiconductor with the room-temperature conductivity of 2.2 × 10⁻³ Ω⁻¹ cm⁻¹. The TTF_x[Ni(dmid)₂] and ET_x[Ni(dmid)₂] complexes have a highest conductivity: σ_RT 1.86 and 0.54 Ω⁻¹ cm⁻¹, respectively.     

Optical study on bis(propylenedithio)tetrathiafulvalenium (BPDT-TTF) salts

Polarized reflectance spectra of (BPDT-TTF)₃(PF₆)₂ and (BPDT-TTF)₂I₃ were measured at room temperature over the spectral region from 720 cm⁻¹ to 25 000 cm⁻¹. Both of these salts exhibited one-dimensional behaviour in contrast to the BEDT-TTF (bis-(ethylenedithio)tetrathiafulvalenium) salts. The transfer integrals and the on-site Coulomb energy of these materials were estimated by analysing the optical spectra, and the on-site Coulomb energy of (BPDT-TTF)₃(PF₆)₂ was found to be significantly larger than that of TCNQ salts.     

Conducting polymers synthesized by dopant-induced polymerization of insulating charge-transfer crystals

Conductive polymer complexes are formed by a one-step process resulting in successive doping and dopant-induced solid-state polymerization of insulating molecular complexes involving the donor biphenyl or terphenyl and acceptors such as TCNQ (7,7,8,8-tetracyano-p-quinodimethane) and DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone). The conductor resulting from the gas-phase doping of the terphenyl/TCNQ complex with AsF₅ contains low molecular weight poly(p-phenylene) chains complexed with AsF₆^? ions. Results are consistent with the presence of neutral TCNQ molecules as ‘molecular spacers’ within the conducting phase. The solid-state reaction and resulting conducting polymer are examined using spectroscopic and diffraction techniques, as well as anisotropic conductivity measurements.     

On the spectral properties of the family of complex TCNQ salts with substituted pyrazines

The group of six TCNQ salts with N-methyl derivatives of pyrazine, bearing up to four methyl groups in various pyrazine sites, was selected for the spectral studies. The influence of the spatial structure of the donors on the charge distribution on the acceptor as well as spectral features of adequate TCNQ salts were deduced. The stretching vibrations of the CC bonds (mode ν₄) were used for the evaluation of the charge density on the TCNQ^? anions but the stretching vibrations of the CN bonds (mode ν₂) indicate a diversity of the neighborhood of TCNQ species. It was shown that for the salts containing nonequivalent TCNQ anions the band ν₂ is splitted; each component of the multiplet seen in the IR and Raman spectra represents various environment of the anion. On the other hand the splitted mode ν₄ indicates an existence of TCNQ^? anions differently charged. Thus, it was shown that the spectral methods are suitable for indicating nonequivalence of the TCNQ in their salts.     

Conducting radical salts of the new electron donor bis(1.3-cyclobutylenedithio)tetrathiafulvalene

The new π-electron donor bis(1.3-cylobutylenedithio)tetrathiafulvalene (1.3-CBDT-TTF), which is only slightly different in the periphery of the molecule with respect to the well-known donor BEDT-TTF, was synthesized. The molecular structure has been characterized and the electrochemical behavior of this new donor was investigated. New cation radical salts were prepared by electrocrystallization methods. The structures of (1.3-CBDT-TTF)I₂, (1.3-CBDT-TTF)₂[AuI₂], (1.3-CBDT-TTF)_x(ReO₄)_y and (1.3-CBDT-TTF)₂PF₆·THF were characterized by X-ray analyses. The room temperature electrical resistivities of the radical salts are low and range between 0.5 and 5 Ω cm.     

Polymer charge-transfer complexes for opto-electronic applications

The formation of charge-transfer (CT) complex to increase the conductivity has been the subject of intense research activity for the past decades. Those CT complexes have been used as organic semiconductors in field effect transistors (FETs), charge injection and transport materials in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells. In this paper, a serials of new CT complexes with polymers as donor and TCNQ as acceptor were prepared. The polymers are polycarbazoles with various content of carbazole moiety in the back chain. The X-ray crystal structure of the model compound 4,4′-bis (N-carbazolyl)-1,1′-biphenyl(CBP)/TCNQ complex showed the formation of 2:1 stack structure (with 1:1 carbazole moiety: TCNQ ratio). The polycarbazole/TCNQ complexes form uniform films by spin-coating. Devices with the structure of ITO/polycarbazole:TCNQ complex/Mg:Ag were fabricated. The current–voltage characteristics showed that the devices exhibit much higher conductivity compared to their analogy ITO/polycarbazole/Mg:Ag structure devices. Devices with different polycarbazole:TCNQ ratios were fabricated and the current–voltage results showed that the conductivity increases as the ratio of polycarbazole:TCNQ increases. The conductivity reaches the maximum at the ratio of 1:1. These polymer complexes can be low-temperature processed on large area flexible substrates and are of potential use for low-cost printed electronics.     

Molecular rectification: Confirmation of its molecular origin by chemical suppression of the electrical asymmetry

Bis[N-(ω-decyl)-5-(4-dimethylaminonaphthalen-1-yl-methylene)]-5,6,7,8-tetrahydroisoquin-olinium]-disulfide diiodide forms self-assembled monolayers (SAMs) on gold substrates in which the acceptor–(π-bridge)–donor moieties are aligned: Au|Au–S–C₁₀H₂₀–A⁺–π–D (I^?). These diode-like molecules exhibit rectification ratios of 30–80 at ±1 V but display symmetrical current–voltage (I–V) characteristics when exposed to HCl and rectify again when exposed to NH₃. This reversible switching is explained by protonation/deprotonation of the amino group which disrupts/restores the donor–acceptor combination and provides evidence of the molecular origin of the electrical asymmetry.     

Low-dimensional magnetic behaviour of new radical-ion salts: (BEDT-TTF)2[AuIII(i-mnt)2] and (BEDT-TTF)2[BiBr4]

Electrical and magnetic properties of two new radicalion salts of BEDT-TTF (BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene) are reported. (BEDT-TTF)₂[Au^III(i-mnt)₂] (i-mnt is 1,1-dithio-2,2-dicyano-ethylene) is a semiconductor, σ_r.t. = 2.0 × 10⁻¹−5.5 × 10⁻² S cm⁻¹ with a temperature dependent activation energy (0.14–19 eV). The powder magnetic susceptibility (10–300 K) has been fitted according to a uniform antiferromagnetic (S = 1/2) Heisenberg model by using the experimental g value of 2.0067 and a magnetic exchange constant J/k_B of −110 K. (BEDT-TTF)₂ [BiBr₄] shows similar electrical and magnetic behaviour with σ_r.t. = 5 × 10⁻² S cm⁻¹ on a powdered sample and J/k_B = − 79 K. At low temperature impurities are responsible for the Curie-like behaviour.