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Electrochemical behaviour of some BEDT-TTF and TTF derivatives
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 570 (2004) 101–105 www.elsevier.com/locate/jelechem
Electrochemical behaviour of some BEDT-TTF and TTF derivatives Esma Sezer b
a,*
, Figen Turksoy b, Umit Tunca a, Turan Ozturk
a,*
a Chemistry Department, Istanbul Technical University, Maslak, Istanbul, Turkey Chemistry Department, TUBITAK Marmara Research Center, Gebze, Kocaeli, Turkey
Received 28 July 2003; received in revised form 6 January 2004; accepted 9 March 2004 Available online 4 June 2004
Abstract The electrochemical behavior of eight bis(ethylenedithio)tetrathiafulvalene and three tetrathiafulvalene derivatives has been investigated to explore the influence of electron withdrawing and donating groups attached to the peripheral dithiin and thiophene rings. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Tetrathiafulvalene; Bis(ethylenedithio)tetrathiafulvalene; Dithiin; Thiophene; Half wave potentials; Diffusion coefficient; Substitution effect
1. Introduction Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) 1 derivatives have been studied extensively due to their ability to form both stable cation and dication radicals with mono anions and electron acceptors such as tetracyanoquinodimethane (TCNQ), which makes the molecule acquire metallic behaviour. Such cations exhibit superconducting, conducting and semiconducting properties [1–6], and can form Lagmuir–Blodgett (LB) films, which can be utilized in the field of organic conductors as well as in the production of sensors and electrochemical storage devices [7–14]. This wide range of application possibilities has led research groups to synthesize the derivatives of BEDT-TTF. Particular attention has been devoted to the preparation of systems having increased conjugation and functional groups for intermolecular hydrogen bonding, which would allow an increase in the stability of the cation radical and control the intermolecular architecture in the material, respectively [2,7]. Although the effects of various peripheral functional groups on the electrochemical behaviour of the tetrathiafulvalene 2 systems have been
*
Corresponding authors. Fax: +90-262-646-3929. E-mail address: [email protected] (T. Ozturk).
0022-0728/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.03.018
widely investigated [7,15–17], there are limited examples for BEDT-TTF systems. The most recent example available in the literature is a report of the effects of amphiphilic and non-amphiphilic long chains at the peripheral sulfur atoms of BEDT-TTF on its redox properties [18].
Here, we present a detailed electrochemical study of some BEDT-TTF and TTF derivatives, containing various electron donating or withdrawing peripheral groups, which were prepared by employing a recently developed 1,8-diketone ring closure reaction using Lawesson’s reagent (LR) or P4 S10 (Tables 1 and 2) [19– 21]. Using such a reaction as a tool, substituted dithiin or thiophene rings were easily introduced into the TTF system of compounds 3-13, including the fully unsaturated BEDT-TTF derivatives, 7–10 (see Scheme 1). It is obvious that having an additional double bond at the ethylene bridge, in most cases in conjugation with a phenyl ring, could be an advantage in stabilizing the radical cations in charge transfer salts. The same advantage could be seen for the systems where fused thiophene rings are incorporated in conjugation with
102
E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
Table 1
The electrochemical measurements of the donors 3– 13 were carried out in a standard three electrode cell, containing Pt working (area 0.16 cm2 ) and counter electrodes and AgjAgCl as a quasi reference electrode, using a POS Wenking 73 Model potentiostat connected to a Kipp and Zonen X–Y recorder. The reference electrode was calibrated with a ferrocene solution and the values were converted to the SCE scale. Dichloromethane (DCM) from Carlo Erba (HPLC grade) and tetrabutylammonium tetrafluoroborate from Fluka were used as solvent and electrolyte, respectively.
3. Results and discussion
Table 2
substituent groups. In this study, a detailed investigation of the effects of both electron donating and withdrawing units at the peripherals on the redox properties of these new and important systems is reported. Correlations between formal potential, diffusion coefficient and molar mass are also reported.
2. Experimental The TTF derivatives 3–13 were synthesized, employing a method described previously [19–21].
It has been reported that the electrochemical process of electron transfer involves a reversible two-stage one electron–oxidation [17,22,23]. As is indicated in Scheme 2, these two electrons are subtracted from the dithiole units of the molecule during the reduction process and returned to the same moiety as oxidation takes place. The CVs of the compounds in Group I, II and III (Tables 1 and 2) were recorded at different scan rates, and the CVs of the compounds 10, 3, 7, 11, 12, and 13 are given in Figs. 1–3. All the voltammograms showed diffusion controlled behavior and the intensities of the peaks increased with the increase in scan rates. From the CV measurements, separation between the maximum intensities of the first E1=2 (I) and second E1=2 (II) oxidations (DE) and diffusion coefficients (D) are summarized in Table 3. The intensities of the peak currents are found to be similar in the oxidation and reduction processes, and DE values of oxidations and corresponding reductions were observed to be close to 60 mV, which is the value corresponding to a reversible process. Although the first and second formal potentials of the compounds in Group I and II are slightly higher than the potentials of ET, under similar measurement conditions, the formal potentials of the compounds in Group III were found to be lower than those of ET (Table 3 and Fig. 3), and in all three groups, while the addition of phenyl, p-BrPh or Me to the peripheral makes the redox process difficult, the presence of a methoxy group in the para position of the phenyl ring resulted in a decrease of the redox potential. Such a change could be due to the electron donating and withdrawing properties of the attached groups (Tables 1–3 and Fig. 3). As expected, when the compound undergoing electron transfer has two substituents, 7–10, the formal potentials shift to a higher positive potential and the diffusion coefficients become smaller. The compounds in Group III (Table 2), which have a substituted thiophene moiety, displayed significantly lower redox potentials, compared with the compounds
E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
103
Scheme 1.
Scheme 2.
Fig. 2. CV of compounds 3, 7 and 11 in 0.1 M TBABF4 containing CH2 Cl2 ; scan rate 125 mV s1 .
Fig. 1. Scan rate dependence of 1 106 mol cm3 compound 10 in 0.1 M TBABF4 containing CH2 Cl2 .
in Groups I and II. A possible explanation might be the easier participation of the aromatic thiophene moiety in the charge delocalization than the non-aromatic 1,4dithiin units of the molecules in Groups I and II. Not
Fig. 3. CV of compounds 11, 12 and 13 in 0.1 M TBABF4 containing CH2 Cl2 ; scan rate 125 mV s1 .
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E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
Table 3 Electrochemical and spectrophotometric properties Sample
3 4 5 6 7 8 9 10 11 12 13 ET
E1=2 (mV ) vs. SCE
DEpðII–IÞ (mV)
I
II
460 500 540 470 575 535 585 565 350 320 430 450
810 850 860 770 885 825 825 845 650 640 750 670
350 350 320 300 310 290 240 280 300 320 320 330
106 D (cm2 s1 )
Eg (eV)
A
B
8.0 5.3 4.1 3.2 2.1 1.4 0.2 0.6 3.0 3.1 15.0 –
7.5 5.0 4.3 3.0 2.5 1.2 0.6 0.7 2.6 2.9 12.0
3.22 3.28 3.30 3.23 3.27 3.20 3.30 3.22 3.02 2.56 2.80 2.95
A: Randles–Sevcik equation. B: Cottrell equation.
surprisingly, when the same groups, Ph, 4-BrPh and 4MeOPh, were attached to the thiophene parts of the compounds in Group III, an oxidation potential trend similar to that seen in the compounds in Groups I and II was observed (Fig. 3, Table 3). Compound 11 with a 4MeOPh group in Group III has the lowest oxidation potential, while 13 with Ph has the highest value. A calculation of the difference between the first and second oxidation potentials (DEpðIIIÞ ) showed that the compounds in Group II have lower DEpðIIIÞ values. This could be a good indication of lower coulombic forces in the Group II compounds, because of a better delocalization of the charges over the molecules. The band gaps of substances were estimated by extrapolation of the low energy edge of the absorption spectra to the baseline and are collected in Table 3. The UV–Vis spectra and electronic band gaps between the HOMO and LUMO indicated that the substituents have the same effects as in the case of the formal potentials (Table 3), namely, the band gaps for the compounds in Group III are significantly lower than those of the compounds in Groups I and II. A possible explanation might be that the changes in the band gaps result from the electron donating and withdrawing properties of the attached groups, since electron-donating groups increase the HOMO and electron-withdrawing groups decrease the LUMO levels. Such substituents can affect the electronic properties of the substances and these changes are also related to changes in the formal potential. Diffusion coefficients were calculated according to the Randles–Sevcik equation [16], Ip ¼ 2:69105 n3=2 AcD1=2 t1=2 , where Ip is the maximum current in A, n is the number of electrons involved in the electrochemical process, A is the electrode area in cm2 , D is the diffusion coefficient in cm2 s1 , c is the concentration in mol cm3 and t is the scan rate in V s1 . The diffusion coefficient was obtained from the plot of log Ip versus log t, which
indicated a linear relationship with a regression coefficient higher than 0.997. Chronoamperometric methods were also used for the determination of the diffusion coefficient, using the Cottrell equation I ¼ nFAc0 D1=2 p1=2 t1=2 ; where the symbols are as above and t is the time in s. The diffusion coefficient (D) was obtained from a plot of I versus t1=2 , which indicated a linear relationship with a regression coefficient higher than 0.997. The plot of It1=2 versus t also gave a horizontal line. Although no direct physico–chemical relation between the diffusion coefficient (D) and the formal potential (E1=2 ) is established in the literature, our study indicated an empirical and qualitative relationship between the diffusion coefficients (Table 3), the formal potentials and the molar masses (Fig. 4). The diffusion coefficient versus formal potential plot shows a parallel trend with the diffusion coefficient versus molar mass
Fig. 4. Empirical and qualitative relationship between (j) first oxidation potential and () molar mass versus diffusion coefficient for several compounds.
E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
105
respectively. They have even lower oxidation potentials and band gaps than ET itself. In conclusion, we have demonstrated the importance of the substituent groups on the oxidation potentials and band gaps of new BEDT-TTF and TTF systems, which could have interesting applications in materials chemistry.
References
Fig. 5. Correlation between the electrochemical oxidation potential E1=2 and the Hammett rm parameters.
(MW ) plot, and as expected, the diffusion coefficient decreases with an increase in MW , although there is a small deviation, which could be due to some interactions of the electroactive species such as: (i) S S interactions, (ii) resonance interactions of the p-electron system, (iii) the Van der Waals interactions and (iv) interactions between polar groups [18]. Furthermore, a possible explanation for the relationship between the diffusion coefficient and formal potential could be the presence of electron donating groups, which makes electron transfer reactions easier and decreases the formal potentials. The application of a correlation analysis to the compounds revealed a satisfactory correlation with the Hammett parameters, which supports our conclusions on the substituent groups (Fig. 5).
4. Conclusion The formal potentials and diffusion coefficients of some BEDT-TTF derivatives 3–13 fused with CH3 , Ph, 4-MeOPh and 4-BrPh substituted thiophene and 1,4dithiin rings have been examined. The effects of the substituent groups demonstrated that improved charge delocalization over the molecule lowers the oxidation potential and the band gap. This effect can be observed better with the TTF derivatives 11–13, which are fused with 4-MeOPh, 4-BrPh and Ph substituted thiophenes,
[1] J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini, M.-H. Whangbo, Organic Superconductors (Including Fullerenes), Prentice Hall, Englewood Cliffs, 1992. [2] M.R. Bryce, J. Mater. Chem. 5 (1995) 1481. [3] M.R. Bryce, J. Mater. Chem. 10 (2000) 589. [4] M.C. Petty, M.R. Bryce, D. Bloor, An Introduction to Molecular Electronics, Oxford University Press, New York, 1995, p.168. [5] G. Schukat, A.M. Richter, E. Fanghanel, Sulfur Rep. 7 (1987) 186. [6] G. Schukat, E. Fanghanel, Sulfur Rep. 18 (1996) 80. [7] P. Frere, K. Boubekeur, M. Jubault, P. Batail, A. Gorgues, Eur. J. Org. Chem. (2001) 3741. [8] T. Bjørnholm, T. Hassenkam, N. Reitzel, J. Mater. Chem. 9 (1999) 1975. [9] L.M. Goldenberg, J.Y. Becker, O.P.-T. Levi, V.Y. Khodorkovsky, L.M. Shapiro, M.R. Bryce, J.P. Cresswell, M.C. Petty, J. Mater. Chem. 7 (1997) 901. [10] C. Pearson, Z.A. Ibrahim, A.S. Dhindsa, A.S. Batsanov, M.R. Bryce, J.A.K. Howard, M.C. Petty, A.E. Underhill, J. Mater. Chem. 8 (1998) 387. [11] L.M. Goldenberg, M.R. Bryce, S. Wegener, M.C. Petty, J.P. Cresswell, I.K. Lednev, R.E. Hester, J.N. Moore, J. Mater. Chem. 7 (1997) 2033. [12] J. Razna, P. Hodge, D. West, S. Kucharski, J. Mater. Chem. 9 (1999) 1693. [13] M.A. Herranz, N. Martin, L. Sanchez, J. Garin, J. Orduna, R. Alcala, B. Villacampa, C. Sanchez, Tetrahedron 54 (1998) 11651. [14] M.R. Bryce, A.J. Moore, A.S. Batsanov, J.A.K. Koward, M.C. Petty, G. Williams, V. Rotello, A. Cuello, J. Mater. Chem. 9 (1999) 2973. [15] R. Carlier, P. Hapiot, D. Lorcy, A. Robert, A. Tallec, Electrochim. Acta 46 (2001) 3269. [16] V. Khodorkovsky, A. Edzifna, O. Neilands, J. Mol. Electron. 5 (1989) 33. [17] T. Suzuki, M. Yamada, M. Ohkita, T. Tsuji, Heterocycles 54 (2001) 387. [18] P. Cea, J.I. Pardo, J.S. Urieta, M.C. Lopez, F.M. Royo, Electrochim. Acta 46 (2000) 589. [19] T. Ozturk, Tetrahedron Lett. 37 (1996) 2821. [20] T. Ozturk, F. Turksoy, E. Ertas, Phosphorus Sulfur Silicon 153– 154 (1999) 417. [21] E. Etras, T. Ozturk, Chem. Commun. (2000) 2039. [22] M. Mizuno, M.P. Cava, A.F. Garito, J. Chem. Soc., Chem. Commun. (1978) 18. [23] H. Muller, H.P. Fritz, A. Lerf, J.O. Besenhard, Z. Naturforsch 44 (1989) 1199.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 570 (2004) 101–105 www.elsevier.com/locate/jelechem
Electrochemical behaviour of some BEDT-TTF and TTF derivatives Esma Sezer b
a,*
, Figen Turksoy b, Umit Tunca a, Turan Ozturk
a,*
a Chemistry Department, Istanbul Technical University, Maslak, Istanbul, Turkey Chemistry Department, TUBITAK Marmara Research Center, Gebze, Kocaeli, Turkey
Received 28 July 2003; received in revised form 6 January 2004; accepted 9 March 2004 Available online 4 June 2004
Abstract The electrochemical behavior of eight bis(ethylenedithio)tetrathiafulvalene and three tetrathiafulvalene derivatives has been investigated to explore the influence of electron withdrawing and donating groups attached to the peripheral dithiin and thiophene rings. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Tetrathiafulvalene; Bis(ethylenedithio)tetrathiafulvalene; Dithiin; Thiophene; Half wave potentials; Diffusion coefficient; Substitution effect
1. Introduction Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) 1 derivatives have been studied extensively due to their ability to form both stable cation and dication radicals with mono anions and electron acceptors such as tetracyanoquinodimethane (TCNQ), which makes the molecule acquire metallic behaviour. Such cations exhibit superconducting, conducting and semiconducting properties [1–6], and can form Lagmuir–Blodgett (LB) films, which can be utilized in the field of organic conductors as well as in the production of sensors and electrochemical storage devices [7–14]. This wide range of application possibilities has led research groups to synthesize the derivatives of BEDT-TTF. Particular attention has been devoted to the preparation of systems having increased conjugation and functional groups for intermolecular hydrogen bonding, which would allow an increase in the stability of the cation radical and control the intermolecular architecture in the material, respectively [2,7]. Although the effects of various peripheral functional groups on the electrochemical behaviour of the tetrathiafulvalene 2 systems have been
*
Corresponding authors. Fax: +90-262-646-3929. E-mail address: [email protected] (T. Ozturk).
0022-0728/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.03.018
widely investigated [7,15–17], there are limited examples for BEDT-TTF systems. The most recent example available in the literature is a report of the effects of amphiphilic and non-amphiphilic long chains at the peripheral sulfur atoms of BEDT-TTF on its redox properties [18].
Here, we present a detailed electrochemical study of some BEDT-TTF and TTF derivatives, containing various electron donating or withdrawing peripheral groups, which were prepared by employing a recently developed 1,8-diketone ring closure reaction using Lawesson’s reagent (LR) or P4 S10 (Tables 1 and 2) [19– 21]. Using such a reaction as a tool, substituted dithiin or thiophene rings were easily introduced into the TTF system of compounds 3-13, including the fully unsaturated BEDT-TTF derivatives, 7–10 (see Scheme 1). It is obvious that having an additional double bond at the ethylene bridge, in most cases in conjugation with a phenyl ring, could be an advantage in stabilizing the radical cations in charge transfer salts. The same advantage could be seen for the systems where fused thiophene rings are incorporated in conjugation with
102
E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
Table 1
The electrochemical measurements of the donors 3– 13 were carried out in a standard three electrode cell, containing Pt working (area 0.16 cm2 ) and counter electrodes and AgjAgCl as a quasi reference electrode, using a POS Wenking 73 Model potentiostat connected to a Kipp and Zonen X–Y recorder. The reference electrode was calibrated with a ferrocene solution and the values were converted to the SCE scale. Dichloromethane (DCM) from Carlo Erba (HPLC grade) and tetrabutylammonium tetrafluoroborate from Fluka were used as solvent and electrolyte, respectively.
3. Results and discussion
Table 2
substituent groups. In this study, a detailed investigation of the effects of both electron donating and withdrawing units at the peripherals on the redox properties of these new and important systems is reported. Correlations between formal potential, diffusion coefficient and molar mass are also reported.
2. Experimental The TTF derivatives 3–13 were synthesized, employing a method described previously [19–21].
It has been reported that the electrochemical process of electron transfer involves a reversible two-stage one electron–oxidation [17,22,23]. As is indicated in Scheme 2, these two electrons are subtracted from the dithiole units of the molecule during the reduction process and returned to the same moiety as oxidation takes place. The CVs of the compounds in Group I, II and III (Tables 1 and 2) were recorded at different scan rates, and the CVs of the compounds 10, 3, 7, 11, 12, and 13 are given in Figs. 1–3. All the voltammograms showed diffusion controlled behavior and the intensities of the peaks increased with the increase in scan rates. From the CV measurements, separation between the maximum intensities of the first E1=2 (I) and second E1=2 (II) oxidations (DE) and diffusion coefficients (D) are summarized in Table 3. The intensities of the peak currents are found to be similar in the oxidation and reduction processes, and DE values of oxidations and corresponding reductions were observed to be close to 60 mV, which is the value corresponding to a reversible process. Although the first and second formal potentials of the compounds in Group I and II are slightly higher than the potentials of ET, under similar measurement conditions, the formal potentials of the compounds in Group III were found to be lower than those of ET (Table 3 and Fig. 3), and in all three groups, while the addition of phenyl, p-BrPh or Me to the peripheral makes the redox process difficult, the presence of a methoxy group in the para position of the phenyl ring resulted in a decrease of the redox potential. Such a change could be due to the electron donating and withdrawing properties of the attached groups (Tables 1–3 and Fig. 3). As expected, when the compound undergoing electron transfer has two substituents, 7–10, the formal potentials shift to a higher positive potential and the diffusion coefficients become smaller. The compounds in Group III (Table 2), which have a substituted thiophene moiety, displayed significantly lower redox potentials, compared with the compounds
E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
103
Scheme 1.
Scheme 2.
Fig. 2. CV of compounds 3, 7 and 11 in 0.1 M TBABF4 containing CH2 Cl2 ; scan rate 125 mV s1 .
Fig. 1. Scan rate dependence of 1 106 mol cm3 compound 10 in 0.1 M TBABF4 containing CH2 Cl2 .
in Groups I and II. A possible explanation might be the easier participation of the aromatic thiophene moiety in the charge delocalization than the non-aromatic 1,4dithiin units of the molecules in Groups I and II. Not
Fig. 3. CV of compounds 11, 12 and 13 in 0.1 M TBABF4 containing CH2 Cl2 ; scan rate 125 mV s1 .
104
E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
Table 3 Electrochemical and spectrophotometric properties Sample
3 4 5 6 7 8 9 10 11 12 13 ET
E1=2 (mV ) vs. SCE
DEpðII–IÞ (mV)
I
II
460 500 540 470 575 535 585 565 350 320 430 450
810 850 860 770 885 825 825 845 650 640 750 670
350 350 320 300 310 290 240 280 300 320 320 330
106 D (cm2 s1 )
Eg (eV)
A
B
8.0 5.3 4.1 3.2 2.1 1.4 0.2 0.6 3.0 3.1 15.0 –
7.5 5.0 4.3 3.0 2.5 1.2 0.6 0.7 2.6 2.9 12.0
3.22 3.28 3.30 3.23 3.27 3.20 3.30 3.22 3.02 2.56 2.80 2.95
A: Randles–Sevcik equation. B: Cottrell equation.
surprisingly, when the same groups, Ph, 4-BrPh and 4MeOPh, were attached to the thiophene parts of the compounds in Group III, an oxidation potential trend similar to that seen in the compounds in Groups I and II was observed (Fig. 3, Table 3). Compound 11 with a 4MeOPh group in Group III has the lowest oxidation potential, while 13 with Ph has the highest value. A calculation of the difference between the first and second oxidation potentials (DEpðIIIÞ ) showed that the compounds in Group II have lower DEpðIIIÞ values. This could be a good indication of lower coulombic forces in the Group II compounds, because of a better delocalization of the charges over the molecules. The band gaps of substances were estimated by extrapolation of the low energy edge of the absorption spectra to the baseline and are collected in Table 3. The UV–Vis spectra and electronic band gaps between the HOMO and LUMO indicated that the substituents have the same effects as in the case of the formal potentials (Table 3), namely, the band gaps for the compounds in Group III are significantly lower than those of the compounds in Groups I and II. A possible explanation might be that the changes in the band gaps result from the electron donating and withdrawing properties of the attached groups, since electron-donating groups increase the HOMO and electron-withdrawing groups decrease the LUMO levels. Such substituents can affect the electronic properties of the substances and these changes are also related to changes in the formal potential. Diffusion coefficients were calculated according to the Randles–Sevcik equation [16], Ip ¼ 2:69105 n3=2 AcD1=2 t1=2 , where Ip is the maximum current in A, n is the number of electrons involved in the electrochemical process, A is the electrode area in cm2 , D is the diffusion coefficient in cm2 s1 , c is the concentration in mol cm3 and t is the scan rate in V s1 . The diffusion coefficient was obtained from the plot of log Ip versus log t, which
indicated a linear relationship with a regression coefficient higher than 0.997. Chronoamperometric methods were also used for the determination of the diffusion coefficient, using the Cottrell equation I ¼ nFAc0 D1=2 p1=2 t1=2 ; where the symbols are as above and t is the time in s. The diffusion coefficient (D) was obtained from a plot of I versus t1=2 , which indicated a linear relationship with a regression coefficient higher than 0.997. The plot of It1=2 versus t also gave a horizontal line. Although no direct physico–chemical relation between the diffusion coefficient (D) and the formal potential (E1=2 ) is established in the literature, our study indicated an empirical and qualitative relationship between the diffusion coefficients (Table 3), the formal potentials and the molar masses (Fig. 4). The diffusion coefficient versus formal potential plot shows a parallel trend with the diffusion coefficient versus molar mass
Fig. 4. Empirical and qualitative relationship between (j) first oxidation potential and () molar mass versus diffusion coefficient for several compounds.
E. Sezer et al. / Journal of Electroanalytical Chemistry 570 (2004) 101–105
105
respectively. They have even lower oxidation potentials and band gaps than ET itself. In conclusion, we have demonstrated the importance of the substituent groups on the oxidation potentials and band gaps of new BEDT-TTF and TTF systems, which could have interesting applications in materials chemistry.
References
Fig. 5. Correlation between the electrochemical oxidation potential E1=2 and the Hammett rm parameters.
(MW ) plot, and as expected, the diffusion coefficient decreases with an increase in MW , although there is a small deviation, which could be due to some interactions of the electroactive species such as: (i) S S interactions, (ii) resonance interactions of the p-electron system, (iii) the Van der Waals interactions and (iv) interactions between polar groups [18]. Furthermore, a possible explanation for the relationship between the diffusion coefficient and formal potential could be the presence of electron donating groups, which makes electron transfer reactions easier and decreases the formal potentials. The application of a correlation analysis to the compounds revealed a satisfactory correlation with the Hammett parameters, which supports our conclusions on the substituent groups (Fig. 5).
4. Conclusion The formal potentials and diffusion coefficients of some BEDT-TTF derivatives 3–13 fused with CH3 , Ph, 4-MeOPh and 4-BrPh substituted thiophene and 1,4dithiin rings have been examined. The effects of the substituent groups demonstrated that improved charge delocalization over the molecule lowers the oxidation potential and the band gap. This effect can be observed better with the TTF derivatives 11–13, which are fused with 4-MeOPh, 4-BrPh and Ph substituted thiophenes,
[1] J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini, M.-H. Whangbo, Organic Superconductors (Including Fullerenes), Prentice Hall, Englewood Cliffs, 1992. [2] M.R. Bryce, J. Mater. Chem. 5 (1995) 1481. [3] M.R. Bryce, J. Mater. Chem. 10 (2000) 589. [4] M.C. Petty, M.R. Bryce, D. Bloor, An Introduction to Molecular Electronics, Oxford University Press, New York, 1995, p.168. [5] G. Schukat, A.M. Richter, E. Fanghanel, Sulfur Rep. 7 (1987) 186. [6] G. Schukat, E. Fanghanel, Sulfur Rep. 18 (1996) 80. [7] P. Frere, K. Boubekeur, M. Jubault, P. Batail, A. Gorgues, Eur. J. Org. Chem. (2001) 3741. [8] T. Bjørnholm, T. Hassenkam, N. Reitzel, J. Mater. Chem. 9 (1999) 1975. [9] L.M. Goldenberg, J.Y. Becker, O.P.-T. Levi, V.Y. Khodorkovsky, L.M. Shapiro, M.R. Bryce, J.P. Cresswell, M.C. Petty, J. Mater. Chem. 7 (1997) 901. [10] C. Pearson, Z.A. Ibrahim, A.S. Dhindsa, A.S. Batsanov, M.R. Bryce, J.A.K. Howard, M.C. Petty, A.E. Underhill, J. Mater. Chem. 8 (1998) 387. [11] L.M. Goldenberg, M.R. Bryce, S. Wegener, M.C. Petty, J.P. Cresswell, I.K. Lednev, R.E. Hester, J.N. Moore, J. Mater. Chem. 7 (1997) 2033. [12] J. Razna, P. Hodge, D. West, S. Kucharski, J. Mater. Chem. 9 (1999) 1693. [13] M.A. Herranz, N. Martin, L. Sanchez, J. Garin, J. Orduna, R. Alcala, B. Villacampa, C. Sanchez, Tetrahedron 54 (1998) 11651. [14] M.R. Bryce, A.J. Moore, A.S. Batsanov, J.A.K. Koward, M.C. Petty, G. Williams, V. Rotello, A. Cuello, J. Mater. Chem. 9 (1999) 2973. [15] R. Carlier, P. Hapiot, D. Lorcy, A. Robert, A. Tallec, Electrochim. Acta 46 (2001) 3269. [16] V. Khodorkovsky, A. Edzifna, O. Neilands, J. Mol. Electron. 5 (1989) 33. [17] T. Suzuki, M. Yamada, M. Ohkita, T. Tsuji, Heterocycles 54 (2001) 387. [18] P. Cea, J.I. Pardo, J.S. Urieta, M.C. Lopez, F.M. Royo, Electrochim. Acta 46 (2000) 589. [19] T. Ozturk, Tetrahedron Lett. 37 (1996) 2821. [20] T. Ozturk, F. Turksoy, E. Ertas, Phosphorus Sulfur Silicon 153– 154 (1999) 417. [21] E. Etras, T. Ozturk, Chem. Commun. (2000) 2039. [22] M. Mizuno, M.P. Cava, A.F. Garito, J. Chem. Soc., Chem. Commun. (1978) 18. [23] H. Muller, H.P. Fritz, A. Lerf, J.O. Besenhard, Z. Naturforsch 44 (1989) 1199.