Synthesis and Electrochemical Proprieties of Novel Unsymmetrical Bis-Tetrathiafulvalenes and Electrical Conductivity of Their Charge Transfer Complexes with Tetracyanoquinodimethane (TCNQ)

The synthesis and properties of a series of bis-tetrathiafulvalenes (bis-TTFs) containing nitrophenyl, aminophenyl or dimethylaminophenyl is reported. The synthesis was carried out by using routes involving Wittig-type, cross-coupling, reduction and alkylation reactions. The electron donor ability of these new compounds has been measured by cyclic voltammetry (CV). Charge transfer complexes with tetracyanoquinodimethane (TCNQ) were prepared by chemical redox reactions. The complexes have been proven to give conducting materials.


Introduction
Tetrathiafulvalene (TTF) and its derivatives have attracted attention for many years because of their electron donor ability and the electrical conductivity of their charge transfer salts, which was started in 1973 with the synthesis of the tetrathiafulvalene-tetracyanoquinodimethane complex (TTF-TCNQ) by Cowan and coworkers [1]. Since then, the progress made in the synthesis of such molecules has been closely related to the discovery of new materials [2] exhibiting conducting [3][4][5][6], superconducting [7,8], magnetic [9][10][11], or optical properties [12,13].
The TTF core and its derivatives-due to their characteristics, in particular their stability and reversible redox character-have found a significant number of applications in materials chemistry [14] such as molecular switches rotaxanes and catenanes [15], conductive materials [16] and superconductors [17], complex with the C 60 [18], conductive polymers [19], materials for nonlinear optics [13], sponges cations [20], ferromagnetic organic magnets [9], liquid crystals [21], and dendrimers [22]. Our research is focused on the conducting and superconducting materials in the hope to improve results in the field and to aim to find an organic superconductor at room temperature.
Dimeric tetrathiafulvalenes are currently of interest to chemists for their applications in organic molecular materials. They can be divides into two types: one is linked through conjugated π-systems and the other through non-conjugated σ-chains [2].
In a continuation of previous work of our group [23][24][25], in this paper we now describe the synthesis and properties of some new unsymmetrically π-donors of bis-TTFs which linked directly by σ-bond with alkyl chains of different lengths, containing nitrophenyl, aminophenyl or dimethylaminophenyl groups, synthesized via cross-coupling, reduction and alkylation methods. The redox behavior of such precursors has been studied by cyclic voltammetry and finally the electrical conductivity of charge transfer complexes was measured.
The syntheses of bis-TTFs 11-14, also based on a multi-step procedure, were carried out as shown in Scheme 3. The electrodonors 11a and 11b containing two TTF units were obtained in a mixture of cis/trans isomers in 40% and 38% yield, respectively, by using the reaction of 10a or 10b with 2,3-bis(2-cyanoethylthio)-6,7-di(methyl)tetrathiafulvalene 8 [30] using cesium hydroxide in DMF at room temperature. In this reaction, the risk of formation of a polymer is high, and to limit the polymerization it is necessary to work at high dilution (use a syringe pump). Thus, compounds 12a and 12b were obtained in 88% and 94% yield, respectively, by treatment of compounds 11a or 11b by cesium hydroxide in DMF at room temperature.
After, the nitro group of bis-TTFs 12a or 12b was reduced into an amino group in ethanol at reflux in the presence of tin and hydrochloric acid. The amino substituted bis-TTFs derivatives 13a or 13b were obtained after purification by column chromatography in 52% and 48% yield, respectively.
Finally the alkylation of amino bis-TTFs 13a or 13b was effected by treatment with K 2 CO 3 (2 equiv.) and with 4 equivalents of iodomethane at room temperature, the dimethylamino bis-TTFs 14a or 14b were isolated by filtration and then extracted with CH 2 Cl 2 and washed with water. 14a and 14b were obtained in 76% and 75% yields, respectively.
In the 1 H NMR spectra of compound 9, protons of CH=C exhibit a singlet at δ 6.82 ppm, in addition, protons of CH 2 CN showed a triplet at 2.78 ppm and 2.83 ppm, with coupling constants of 7.07 Hz and 6.00 Hz, respectively. Thus, the protons SCH 2 showed a triplet at 3.14 ppm and 3.21 ppm, with coupling constants of 7.07 Hz and 6.00 Hz, respectively. The 1 H NMR spectra of the 10a,b revealed the absence of the CH 2 CH 2 CN group protons and the presence of (CH 2 ) n Br protons. 10a showed two triplets at 3.30 ppm and at 3.85 ppm with coupling constants of 6.72 Hz and 6.33 Hz, respectively. 10b showed a multiplet at 2.15 ppm and two triplets at 2.93 ppm and at 3.55 ppm, with coupling constants of 6.78 Hz and 6.36 Hz, respectively. Further confirmation for the structure of 12a,b was obtained from their mass spectral data, where they showed ion peaks at [M + H] + 739 and [M + H] + 767, respectively, and by their 1 H RMN spectra, the aromatic protons for 12a as two doublets at 7.55 ppm and at 8.25 ppm, with the same coupling constants of 9.37 Hz, as well as another characteristic triplet at 3.59 ppm for the two (CH 2 ) 2 groups protons. On the other hand, the spectrum of 12b exhibited a multiplet at 2.47 ppm for the two CH 2 CH 2 CH 2 groups. Moreover, the spectra of 13a,b showed amino group protons as multiplet around 3.50-3.85 ppm. The final products 14a,b showed the absence of the amino group proton signals and the presence of dimethylamino group proton signals as singlets at 3.35 ppm and at 3.20 ppm, respectively.

Electrochemical Studies
The redox behavior of these new functional mono-and bis-TTF was studied in solution by cyclic voltammetry (CV) and by square wave voltammetry (SQW). Measurements were performed under nitrogen at room temperature using a glassy carbon working electrode, a Pt counter electrode and a standard calomel electrode (SCE) as reference, with tetrabutylammonium perchlorate (n-Bu 4 NClO 4 , 0.1 M) in dry acetonitrile, as supporting electrolyte. A scan rate of 100 m·Vs −1 was used. The CV measurements showed reversible redox waves for all the compounds studied and the corresponding oxidation potentials Eox were determined by the SQW technique. The results are summarized in Table 1.
The type I (a) SQW curve shows two sharp oxidation waves each with one electron process for the mono-TTF (Figure 1a). This kind of voltammogramm is observed for compounds 9, 10a and 10b. The type II (b) SQW curve is observed for compounds 11a,b, 12a,b, 13a,b and 14a,b (Figure 1b). We can clearly see three oxidation peaks with respectively a 1, 1 and 2 electron process. The real distinction of the two first oxidation waves is clearly due to the difference of the two TTF units of the bis-TTFs.  The oxidation potentials are almost identical for each pair of bis-donors (11a, 11b), (12a, 12b), (13a, 13b) and (14a, 14b). The first oxidation potentials in each pair are shifted cathodically by 20 mV. This may be due to the alkyl linked group. The oxidation potentials of compounds 14a,b are slightly higher than that of compounds 13a,b, on the other hand, the compounds 12a,b are slightly higher than that of compounds 14a,b. This should be attributable to the electronic properties of the nitrophenyl, aminophenyl and dimethylaminophenyl groups. All the oxidation potential values measured for mono-TTFs were found higher than the oxidation potentials of bis-TTFs. These results showed the good donor ability of this new series of mono-TTFs derivatives, which consequently should lead to conducting materials.

Preparation and Electrical Conductivity of Charge Transfer Complexes
The first report on the electrical conductivity in an organic solid appeared in 1954 [31], namely, a perylene-bromine complex, which has a room-temperature conductivity of 0.1 S cm −1 . In 1960, the organic acceptor TCNQ [32] was synthesized as well as a great number of its conducting charge-transfer complexes and radical ion salts.
In the 1970s, the organic donor TTF [33] led to the first organic metal TTF-TCNQ [1]. Its room-temperature conductivity (500 S cm −1 ) increases with a decrease of the temperature to the value of 6000 S cm −1 at 60 K where a metal-insulator transition occurs. Since then, great interest has been devoted to this type of material, and a great number of new organic donors and acceptors have been synthesized as well as their charge-transfer salts. Therefore, the complexation of the donors 9-14 with 7,7,8,8-tetracyanoquinodimethane (TCNQ) in hot acetonitrile solution gave the corresponding charge transfer complexes (CTC). Most of the solids were isolated in powder forms. Electrical conductivity was only measured on compressed pellets at room temperature using a two probe technique. The results are reported in Table 2.
For this family of materials, only CTC 9-TCNQ, 10a-TCNQ and 10b-TCNQ resulting from mono-TTFs, can be classified in the area of conductors. In fact, they have a conductivity measured on powder compressed pellets of 8.60 10 −1 to 1.30 10 −2 S cm −1 , that allows a conductivity ten times greater on single crystal.
Other, CTC resulting from bis-TTFs can be classified in the category of semi-conductors materials with conductivities from 10 −3 to 10 −6 S cm −1 . This can be due to a structural disorder and/or a full charge transfer of an electron for each molecule.

General
NMR spectra were recorded on a Brucker AC 250 instrument. Microanalyses were performed in the Microanalysis Laboratory of ENSCM (Montpellier). FAB mass spectra were recorded on a JOEL JMS-DX 300 spectrometer. Uncorrected melting points were measured on a 510 Buchi apparatus. Cyclic voltammetry measurements were carried out on a PAR-273 potentiostat/galvanostat. All solvents were dried by standard methods and all commercial reagents used without purification. All reactions were performed under an inert atmosphere of nitrogen.

Synthesis of Mono-TTFs 10a and 10b
Cesium hydroxide monohydrate (0.62 g, 3.72 mmol) in dry methanol (10 mL) was added to tetrathiafulvalene dicyano derivative 9 (0.5 g, 1.69 mmol) dissolved in dry and degassed DMF (30 mL). The reaction mixture was stirred for 10 min, the colour becoming dark violet. Then, an excess of 1,2-dibromoethane or 1,3-dibromopropane (10 equiv.) was added in one portion. The colour of the reaction mixture became clear, and the reaction mixture was stirred at room temperature for 1 h. The solvent was removed in vacuo, the residue was dissolved in dichloromethane (100 mL), washed three times with water and dried over magnesium sulphate. The mixture was concentrated in vacuo and the residue was purified by chromatography on a silica gel column (eluent: dichloromethane).

Synthesis of Bis-TTFs 12a and 12b
Compounds 12a and 12b were synthesized by employing the same experimental process as 10 from 1 equiv. of 11a or 11b and 1 equiv. cesium hydroxide.

Synthesis of Bis-TTFs 13a and 13b
A stirred mixture of 4-p-nitrophenyl-bis-TTFs derivatives 12a or 12b (4 mmol), tin (0.94 g, 8 mmol), and aqueous solution of HCl (35%) (1.8 mL, 20 mmol) in ethanol (30 mL) was refluxed for 4 h under nitrogen. During this time the initial black solution turned light yellow. The solution was then concentrated in vacuo and treated with an aqueous solution (100 mL) of sodium hydroxide (0.1 M) and extracted with ether. The organic phase was washed with water, dried (MgSO 4 ), and concentrated in vacuo. The product was subjected to column chromatography on silica gel (CH 2 Cl 2 ), affording the expected compounds 13a and 13b as powder.

Synthesis of Bis-TTFs 14a and 14b
To a stirred solution of 4-aminophenyl-bis-TTF 13a or 13b (3 mmol) and of iodomethane (0.75 mL, 12 mmol) in acetone (15 mL) under nitrogen, K 2 CO 3 (0.83 g, 6 mmol) was added. After 4 days of stirring at room temperature, the precipitate obtained was filtered, washed with acetone, and then extracted with CH 2 Cl 2 . The organic phase was dried (MgSO 4 ) and concentrated in vacuo, providing the expected compounds 14a and 14b as dark orange powder.

Conclusions
In summary, we have successfully prepared via Wittig-type, cross-coupling, reduction and alkylation synthetic strategies some new mono-TTFs and bis-TTFs containing nitrophenyl, aminophenyl or dimethylaminophenyl groups. All donors synthesized during the course of this work have been characterized by cyclic voltammetry and their oxidation potentials were determined by cyclic voltammetry. Charge transfer complexes of the donors with TCNQ were prepared and the electrical conductivity of these materials was measured, some CTC are conductive.