Synthesis and Properties of Novel Unsymmetrical Donor Molecules Containing P-acetoxy-or P-hydroxyphenyl Units

We report the synthesis and properties of eight new tetrathiafulvalene (TTF) derivatives containing two different functionalities, prepared with the aim of obtaining stable organic materials. The four acetoxyphenyl-and four hydroxyphenyl TTFs were synthesized via Wittig-type condensations. The electrochemical properties of these redox-active molecules were studied by cyclic voltammetry. Charge transfer complexes with tetracyanoquinodimethane (TCNQ) were prepared by chemical redox reactions. The complexes have been proven to give conducting materials. The UV-VIS and IR spectra of the TCNQ salts were recorded and used to characterize and estimate the degree of charge transfer of these complexes.


Introduction
Because of their exciting electroactive properties, organic tetrathiafulvalene (TTF) compounds are the subject of much interest [1][2][3].The most interesting of these systems is the salt formed between TTF and tetracyanoquinodimethane (TCNQ), which displays metallic-like behavior over a wide temperature range and exceptional electrical conductivity (σ max, 58 k = 10 4 S•cm -1 ) [4][5][6].Most of the prior work, however, has been devoted to the search for new TTF derivatives capable of giving salts with higher electrical conductivity.Consequently, many changes were made to the parent TTF donor leading, for example, to the synthesis of a wide variety of selenium derivatives, asymmetrically substituted compounds, polychalcogenated TTFs, etc.In order to consolidate the asymmetrical character of the donors, several tetrathiafulvalene derivatives containing different groups such as nitronylnitroxide [7], 4,5-diformyl [8], aminomethyl [9] and hydroxymethyl [10] have been described.They were found to be versatile starting new donors for the synthesis of new organic materials.
Recently, and proceeding from accessible conducting organic materials, we synthesized a series of TTF derivatives containing peripheral selenium atoms and demonstrated their ability to form charge transfer complexes and radical cation salts [11,12].Within the framework of our interest in organic materials we now describe in this paper the synthesis and properties of some new asymmetrical donors containing acetoxyphenyl and hydroxyphenyl groups.These compounds have been investigated by means of cyclic voltammetry.The electron donor properties of the various compounds obtained have been found to be lower to those of TTF.Conducting charge transfer complexes are also presented.The degree of charge transfer and molecular stacking were estimated by spectroscopic data.
The phosphonium salts 4a-d [16] were dissolved in dry acetonitrile together with one equivalent of selenoxonium salt 3 (Scheme 2).The mixtures were stirred under nitrogen and an excess of triethylamine was added slowly, followed by a further 3 h of stirring at room temperature.The solvent was evaporated and the crude products were dissolved in chloroform, washed with water and dried.Evaporation of chloroform, followed by purification using chromatography on silica with benzene as eluent and recrystallization from hexane gave pAcPhTrMeTTF 5a, pAcPhMeCpTTF 5b, pAcPhMeChTTF 5c and pAcPhMeBzTTF 5d as red crystals in 15-33 % yields.

Electrochemical studies
The oxidation potentials of the donor molecules were determined by cyclic voltammetry.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 and 1,2,2-trichloroethane, as supporting electrolyte.A scan rate of 100 mVs -1 was used.The results are reported in Table 1.
All TTFs showed two pairs of reversible waves.These results indicate that the presence of the acetoxy CH 3 CO-and hydroxy -OH functions relatively far away from the TTF core on the side C 6 H 4chain, has only a very weak influence on the redox potential values of the compounds 5a-d and 6a-d.It is also noteworthy that the benzo -C 6 H 4 -(buta-1,3-dien-1,4-diyl) grouping exerts withdrawing effects, as indicated by the observed E ox1 and E 1  1/2 values in acetonitrile solution for 5d (0.48, 0.45 V) and 6d (0.46, 0.43 V), respectively.Similar observations can be made about the other series of compounds 5a-c and 6a-c.
Due to the well known donating effect exerted by the methyl, propane-1,3-diyl and butane-1,4diyl substituents these TTF derivatives are better electron donors than the parent TTF taken as a reference.On the other hand, the ∆E 1/2 values, in ACN, of the majority of TTFs are smaller than those of the unsubstituted parent compound TTF (0.37 V), which suggests a decrease in the onsite coulombic repulsion in the dication by delocalisation of two positive charges over the whole molecule.In TCE as the solvent, the π-ability of all compounds appears exceptionally strong, since for all of them, the ∆ E OX is markedly higher than of the parent TTF (0.28 V).Those complexes have |E 1  1/2(D) -E 1  1/2(A) | values and favor the partial charge transfer.These results compare well with those of Wheland [17] and Torrance [18].
The thermodynamic stability of radical cations was determined from the difference in potentials of the corresponding radical cation and dication using the equation ∆E = E 1  1/2 -E 2 1/2 = 0.059 log K SEM (Table 1) where K SEM is the equilibrium constant in equation: D + D 2+  2 D .+ .For electron donor TTF compounds, the thermodynamic stability of the cation radical diminishes in the following order: 5b > 6a > 5c = 6d > 5a = 6c > 6b > 5d, due to a decrease in intermolecular coulombic repulsion.

Electrical conductivity and activation energies of charge transfer complexes
Complexation of the donors 5a-d and 6a-d with 7,7,8,8-tetracyanoquinodimethane in hot acetonitrile solution gave the corresponding complexes.Most of the solids were isolated in polycrystalline or 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.All TCNQ complexes exhibited a conductivity higher than 0.5 Scm -1 ; the 5d-TCNQ sample displayed semiconducting behavior, with conductivity less than 10 -6 Scm -1 .The 6-TCNQ salts (0.5-5.5 Scm -1 ) exhibits higher conductivity than that of the other 5-TCNQ salts (0.5 -3.5 10 -6 Scm -1 ).
A p-hydroxyphenyl group attached to a TTF framework seems to increase the conductivity, compared with the corresponding p-acetoxyphenyl group.For the TCNQ complexes with comparatively higher electrical conductivities the E a values were small (0.09-0.15 eV).On the other hand, an insulating salt such as 5d-TCNQ had considerably higher activation energy (0.46 eV).

IR Spectroscopic studies
It is established that the complexes of type TTF-TCNQ have to satisfy two conditions to give a conductor.The first requires the presence of a crystal structure composed of regular segregated stacks …DDD-AAA… of donors and acceptors [19]; while the second is a partial electron transfer (0<ρ<1) between these two species.Any of mixed stack "sandwich" structures …DAD-DAD… of the salt or the absence of a charge transfer (molecular complex) or a complete charge transfer (ionic complex) lead to an insulating material [20,21].Frequency of vibration of the CN group / cm-1 degree of charge transfer / p Due to the lack of monocrystals, various authors were able to estimate the rate of charge transfer of this type of complex from different techniques [22] and in particular from the IR spectra [23,24].Based on the relation between the frequency of vibration of the CN group and the degree of charge transfer for reference compounds such as TCNQ 0 (ν CN = 2227 cm -1 , ρ = 0 e -/molecule), TCNQ -K + (2183 cm -1 , 1), TTF-TCNQ (2204 cm -1 , 0.59), HMTTF-TCNQ (2195 cm -1 , 0.72), TMTTF-TCNQ (2200 cm -1 , 0.65), TMTSF-TCNQ (red) (2217 cm -1 , 0.23), etc.We were able to draw a ρ = f (ν CN ) straight line, which corresponds to the correlation between the vibration frequencies of the CN group and the degree of charge transfer for different TTFs.The curve equation is y = -0.0228x+ 50.71.The coefficient of determination is R 2 = 0.9933 (Figure 1).
We recorded the IR spectra of all complexes prepared in the 2100 and 2250 cm -1 range and we obtained the vibration frequency of the CN group for each complex.The ν CN frequency was then used on the calibration ρ = f (ν CN ) curve to determine the approximate value of the degree of charge transfer (ρ).The results are reported in Table 3.It appears that the conducting materials: 5a-c-TCNQ, and 6a-d-TCNQ, present a partial degree of charge transfer in the 0.30 to 0.79 e -/molecule range.These values are comparable to those of known conductors, such as Cu(PC)I (0.33), TTF-TCNQ (0.59) and HMTTF-TCNQ (0.72) [24].Considering these results, it is reasonable to propose that the conducting charge transfer complexes studied here possess a segregated stacking structure.
As for the nonconducting 5d-TCNQ, the value of the degree of charge transfer is about 0.43 e -/ molecule.This value is in a range that would favor a metallic behavior for this salt.Since it is semiconducting, we are lead to believe that the requirement regarding the stacking of the molecules is not met.Several explanations for this behavior might be a mixed stack (sandwich) structure, a large separation between stacked donors, the presence of structural disorder [19], etc.When a comparison was made between the E 1  1/2 values of the hydroxy donors 6a-d and ρ values of the TCNQ complexes, it was found that the degree of electron transfer from the donor to TCNQ decreases in the TCNQ complex, as the E 1  1/2 value of the donor becomes smaller.The electrical results are consistent with those of the electronic spectra of the complexes in KBr pellets.The high conductivity of the conducting complexes was expected from both the black shiny crystals and the broad band observed in the FTIR spectrum at about 2500 cm -1 .This band is generally present in all the conducting charge transfer complexes of TTF-TCNQ [19].The other complex, which is insulator, is slightly colored and the band at 2500 cm -1 is not observed in the IR spectrum.The CT band is probably shifted to higher frequencies as it is typical in such semiconducting complexes [19].

Conclusions
Although the Wittig-type synthetic strategy described in this paper affords low overall yields we have prepared two series of new asymmetric TTF derivatives containing p-acetoxyphenyl and p-hydroxyphenyl groups.The side chains that we introduced preserve the electron donating character of these TTFs, as shown by cyclic votammetry.Several salts showing comparatively high room temperature conductivity were obtained.We have discussed the origin of the semiconducting behavior of some of the TTF derivatives and indicated that the semiconducting salts exhibited a charge transfer band at higher energy, suggesting stronger coulombic repulsions.The possibility of producing new cation-radical salts in these series and the X-ray structural details [12], are under investigation.

Experimental
General NMR spectra were recorded on a Bruker AC 250 instrument.FAB mass spectra were recorded on a JOEL JMS-DX 300 spectrometer.IR spectra were recorded at 0.5 cm -1 resolution on an Equinox 55 instrument.Melting points were measured on a Buchi apparatus.Cyclic voltammetry measurements were carried out on a PAR-273 potentiostat/galvanostat.All reagents were of commercial quality and solvents were dried, were necessary, using standard procedures.All reactions were performed under an inert atmosphere of nitrogen.THF was distilled from sodium/benzophenone immediately prior to use in pre-dried glassware.

Scheme 3 .
Scheme 3. Efficient Synthetic Route for the Desacetylisation of TTF Derivatives

Figure 1 .
Figure 1.The correlation between the ν CN and ρ.

Table 1 .
Redox potentials and K SEM data of 5a-d and 6a-d.

Table 2 .
Electrical conductivity and activation energies of charge transfer complexes.

Table 3 .
Degree of charge transfer of TCNQ complexes.