Conductive Charge-transfer Complexes Research Articles

Design of Novel Chalcogen-Containing Organic Metals: Extensively Conjugated Electron Donors and Acceptors With Reduced On-Site Coulomb Repulsion

Abstract This review describes the design of extensively conjugated electron donors and acceptors based on structural modifications of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ), respectively. These molecules have the advantage of reduced on-site Coulomb repulsion which is one of the prerequisites for components of organic metals. The construction of such large compounds is currently accomplished by insertion of an extensively π-conjugated system between two 1,3-dithiolylidene or dicyanomethylene groups. The usefulness of a heteroquinoid system as such a building block is demonstrated by examples of heteroatom-incorporating TCNQ acceptors (hetero-TCNQs), which form a variety of highly conductive charge-transfer complexes. In addition, recent advances concerning quino-2,2′-bis(1,3-dithioles) and TTF vinylogs as novel electron donors are introduced.

Conductive Charge-Transfer Complexes of Alkoxy Substituted Tetrathiafulvalene, BEDO-TTF

Abstract BEDO-TTF which is one of the oxygen substituted BEDT-TTF analogues, gave conductive organic charge transfer complexes in contrast to BEDT-TTF and other BEDT-TTF derivatives. Especially many complexes exhibit metallic temperature dependences even with the compressed disks. These facts suggest the remarkable tendency of BEDO-TTF to aggregate themselves.

Phenanthro[1,10-cd:8,9-c′d′]bis[1,2]-dithiole and -diselenole as novel electron donors

The preparation of the title compounds is reported along with their formation of electrically highly conductive charge-transfer complexes with the common electron acceptors TCNQ, TCNQF4, and DDQ.

Novel extended donors: Poly(peri-naphthylenes), poly(tetrathiafulvalenes), poly(arylenevinylenes)

Novel conjugated polymers are synthesized and the correlation between structure and physical properties is elucidated by comparison with suitable model oligomers. The electronic behavior is carefully tuned by creating linear, fulvalene-type and band-type π-conjugation. Although the emphasis is on synthesis, the role of the extended π-systems as (i) donors for conducting charge-transfer complexes, (ii) substrates for charge storage, and (iii) chromophores is also described.

Dimethyl and tetramethyl derivatives of anthra[1,9-cd:4, 10-c'd']- and naphthaceno[5,6-cd:11,12-c'd']-bis[1,2]dichalcogenoles

The title compounds, involving all chalcogen heterocycles for the anthracene series and the sulfur and selenium heterocycles for the napthacene series, formed a series of highly conductive charge-transfer complexes with typical electron acceptors such as TCNQ, DMTCNQ, TNAP, and DBBS.

Atomic scale surface studies of conductive organic compounds: 1. Scanning tunneling microscopy of tetrathiofulvalene-7,7′,8,8′-tetracyanoquinodimethane (TTF-TCNQ) and triethylammonium-TCNQ 2 (TEA-TCNQ 2)

The results of STM studies under ambient conditions of two conductive organic charge transfer complexes — tetracyanoquinodimethane (TCNQ) with tetrathiofulvalene (TTF) and triethylammonium (TEA) — are presented. The nature of STM images and their variations are discussed. Large As the atomic level the STM images consist of well-ordered patterns which permit detailed studies of electronic and molecular surface structures. For a monocrystal of TTF-TCNQ, the correspondence of STM patterns (observed on one of the monocrystal surfaces) with the atomic surface structure simulated from the crystallographic data is shown. For a TEA-TCNQ 2 monocrystal, the correlation between real surface and crystallographic axes was verified by X-ray diffraction. Subsequently, this surface was imaged by STM, and the correspondence between charge density patterns and surface atomic distribution was confirmed. The peculiarities of STM imaging of charge transfer systems and an interpretation of STM images are discussed.

Atomic scale surface studies of conductive organic compounds 2. Scanning tunneling microscopy and crystal structure of the charge transfer complex of 4-ethylpyridine-TCNQ 2 (4-EP-TCNQ 2)

The scanning tunneling microscopy (STM) studies of the conductive charge transfer complex of 4-ethylpyridine (4EP) with tetracyanoquinodimethane (TCNQ) were combined with X-ray diffraction measurements. The existence of two different crystalline modifications of the complex was revealed. The block-type monocrystals have a triclinic crystallographic structure with the following parameters: a=7.112 2 A ̊ , b=7.862 7 A ̊ , c=13.954 4 A ̊ , a=72.95 4°, β=78.40 2° and γ=69.92 3° . The existence of a columnar structure of TCNQ molecules with diadic repeat unit is shown. The interplanar distances between neighboring TCNQ molecules in stack is 3.21 Å. The STM images of one of the surfaces of such monocrystals correspond well with the distribution of TCNQ atoms in the surface layer constructed from the crystallographic ( a, b) plane. The quality of needle-shaped monocrystals does not permit detailed crystallographic measurements. Only the triclinic crystal system and the lattice constants were determined: a=3.865 2 A ̊ , b=6.658 9 A ̊ , c=26.386 7 A ̊ , α=92.63 5°, β=94.12 3° and γ=106.80 8° . STM images provide additional structural information about this crystal modification. The formation of stacks of TCNQ molecules with the average parameters of 4.1 Å (periodicity in stack) and 13.1 Å (distance between neighboring stacks) seems to be the characteristic feature of this structure. Observed structures are compared with the known crystallographic data of other charge transfer systems. The peculiarities of STM imaging in charge transfer systems are discussed in due course.

Conductivity of organic composites with 3- and 2- dimensional crystalline networks I. Continuity of the conducting phase

Several reticulate doped polymers (i.e. conducting composites consisting of an insulating polymer matrix and of a minute amount of crystalline conducting charge-transfer complex organized in a 3- or quasi-2-dimensional network) are investigated in terms of the contunuity of the network. The analysis of the temperature dependences on the conductivity for different concentrations of the conducting phase, the discussion of the probability of continuous linkages formation and the comparison of the experimentally achieved very high conductivities with theoretically allowed maximal values demonstrate that, already at 0.3% content of the conducting phase, continuous pathways are formed. The effectiveness of the network formation approaches that predicted by the Clausius-Mossotti approximation. A mechanism yielding highly connective networks is proposed, based on the specific conditions of the complex crystallization allowing the formation of very thin filamentary connections.

Structure analysis of conductive polymer systems: Poly-4-vinylpyridine and poly(butadiene-b-4-vinylpyridine) with 7,7′,8,8′-tetracyanoquinodimethane

The structure analysis of two conductive polymer systems-poly-4-vinylpyridine and poly(butadiene-b-4-vinylpyridine) with 7,7′,8,8′-tetracyanoquinodimethane (TCNQ)-was done by X-ray diffraction, scanning tunneling microscopy (STM) and FTIR. The charge transfer complex formed between the pyridine group and the known electron acceptor, TCNQ, is supposed to be the conductive element in these systems. In order to understand the structure of this complex, a model compound, the complex of 4-ethylpyridine (4EP) with TCNQ, 4EP/TCNQ2, was studied by the mentioned methods. It appears that there are two crystalline modifications of the model compound with different type of stacks of the TCNQ molecules. In polymer systems only one type of the complex is dominant as revealed by joint analysis of X-ray diffraction diagrams, STM and FTIR data. In the STM image of the polymer surface one can distinguish that molecular stacks with periodicities of 4.1 A in a row are separated (12.5A) from each other. Such organization is similiar to the one observed in conductive charge transfer complexes such as tetrathiofulvalene (TTF) with TCNQ. The ordered molecular domains are scattered on the polymer surface and take part in the formation of the conductive network.

4,7‐Dimethyl‐4,7‐dihydro[1,2,5]thiadiazolo‐[3,4‐b]pyrazine, a Novel Electron Donor with a 12π‐Electron Ring System

A very low first oxidation potential (0.15 V vs SCE) characterizes the 12π-electron title compound 1, which is formed by reduction of the corresponding 14π-electron system with potassium and trapping of the dianion with methyl iodide. Reaction of 1 with the electron acceptor 2 affords conducting charge-transfer complexes (σ = 5.6 × 10−2 S cm−1).

Conjugated ‘bis’-tetrathiafulvalenes: Donors for a new class of conducting radical cation salts

Tetrathiafulvalene units coupled via conjugated π systems constitute a new class of electron-rich donor molecules. Electrochemical oxidation of the title compounds 1 leads to the formation of tetracationic species. The influence of the extended π-conjugation on the redox properties of 1c is discussed. Electrically conducting charge-transfer complexes are prepared from 1a and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

Syntheses and Properties of 2,2′-Binaphtho[1,8-de]-1,3-dithiinylidene and Its Selenium Analog, 2-(1,3-Dithiol-2-ylidene)naphtho[1,8-de]-1,3-dithiin, and 2-(4H-Thiopyran-4-ylidene)naphtho[1,8-de]-1,3-dithiin

Abstract In order to discover new electron donors for conducting charge-transfer complexes, the four title compouds 1–4 composed of two heterocycles have been prepared via 1,8-dichalcogen-bridged naphthalene. The cyclic voltammetery indicates that symmetrical 1 and 2 are poor donors, but unsymmetrical 3 and 4 possess considerable donor abilities. The latter compounds are thus capable of forming some crystalline charge-transfer complexes with strong acceptors such as TCNQ, TCNQF4, and DDQ, which are, however, semi-conducting.

Connectivity of conducting crystalline networks in reticulate doped polymers

Measurements of temperature dependences of conductivity for morphologically different samples of reticulate doped polymers containing dentrite-like microcrystals of a conducting charge-transfer (CT) complex demonstrate that there is no difference in the mechanism of charge carrier transport along dendrite branches, perpendicular to them or across interdendrite boundaries. Comparison of the conductivity behaviour, as measured along polymer films and perpendicular to the surfaces ('sandwich' samples), suggest that the same mechanism of conductivity operates in both directions. It is also demonstrated that lowering of the CT complex content, even to 0.003 vol. fraction, leads to some decrease of the conductivity but does not change its temperature dependence. The results demonstrate the high connectivity of the crystalline CT complex network and support the hypothesis that continuity of this network is due to submicroscopic dendritic structures scaling from microns, probably down to tens of angstroms.

Electrocrystallization of poorly conducting charge-transfer complexes

Donnees sur la preparation de complexes par transfert de charge constitues de decanethylferrocenium et de tetracyanoquinodimethane

ChemInform Abstract: Tetrakis(methylthio)‐Δ2,2′‐dithieno[3,4‐d]‐1,3‐dithiole. A New TTF‐Type Donor.

AbstractAuf dem im Formelschema angegebenen Weg wird aus Tetrabromthiophen (I) das Dithieno‐dithiol (V) hergestellt, das als Donor für leitfähige Charge‐Transfer‐ Komplexe eingesetzt werden soll.

Thiophene-fused tetracyanoquinodimethanes as electron acceptors for conducting charge-transfer salts

Four isomeric benzodithiophene analogues of 11,11,12,12-tetracyano-9,10-anthraquinodimethane (1)–(4) have been prepared, in which (1) and (2) exhibited excellent reduction potentials as electron acceptors and formed highly conducting charge-transfer complexes with tetrathiafulvalene.

TETRAKIS(METHYLTHIO)-Δ2,2′-DITHIENO[3,4-d]-1,3-DITHIOLE. A NEW TTF-TYPE DONOR

Abstract The title compound, which is designed to increase the interstack interaction in the solid state, was synthesized as a new donor for conductive charge-transfer complexes.

Benzo-1,3,2-Dithiazol-2-YL and its Derivatives: A New Class of Donor Molecules for Highly Conducting Charge Transfer Complexes

Abstract Benzo-1,3,2-dithiazol-2-yl and its derivatives form charge transfer complexes with tetracyanoquinodimethane; their powder conductivities are as high as 3 Ω−1 cm−1.

Chemical Oxidation of Polyacetylene by Cupric Chloride

Abstract Polyacetylene, (CH)X, has been oxidized (doped) by CuCl2 to give highly conducting charge-transfer complexes. Conductivities of highly doped films range from 30 to 80 Scm−1 corresponding to y = 0.06 – 0.12. In lightly doped samples a rapid decrease in the activation energy of conduction is observed with increasing dopant concentration as well as the appearence of the typical dopant-associated IR-absorptions. Doping is believed to proceed via a partial reduction of CuCl2 and the insertion of stabilizing anions such as CuCl− 3.

Highly conducting charge transfer complexes of benzo-1,3,2-dithiazol -2yl and its derivatives with teracyanoquinodimethane

Benzo-1,3,2-dithiazol-2yl and its derivatives form charge transfer complexes with tetracyanoquinodimethane; their powder conductivities are as high as 3Ω–1/cm.