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Synthesis, electrochemical and electroluminescent properties of oligothiophene-based conjugated polymers
Synthetic Metals 109 Ž2000. 277–280 www.elsevier.comrlocatersynmet
Synthesis, electrochemical and electroluminescent properties of oligothiophene-based conjugated polymers L. Trouillet a , M. Lapkowski a
c
b,)
, O. Stephan c , S. Guillerez
a
Departement de Recherche Fondamentale sur la Matiere ´ ` Condensee, ´ SI3Mr EMSI, CEA Grenoble, 17, rue des Martyrs, 38054 Grenoble Cedex 09, France b Department of Chemistry, Silesian UniÕersity of Technology, Ks. M. Strzody 9, 44100 Gliwice, Poland Laboratoire de Spectrometrie Physique, UniÕersite Joseph Fourier, Domaine UniÕersitaire Saint-Martin-d’Heres-Gieres, 38041 Grenoble Cedex 9, France Received 26 June 1999; received in revised form 30 June 1999; accepted 10 September 1999
Abstract The paper deals with a new conjugated polymer having the possibility to form a complex with transition metals. The conjugated backbone is constituted by the alternation of regioregular alkylated oligothiophene of variable size and of 2,2X-bipyridine as the chelating unit. Electrochemical studies were conducted on both the non-metalated and RuŽII. complex forms together with the in-situ conductance measurement showing that p- and n-doping occur. The non-metalated form was showed to exhibit electroluminescent properties. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PdŽ0. catalyzed cross-coupling; Conjugated polymer; Metal-containing conjugated polymer; Conductivity; n-Doping; Electroluminescence
1. Introduction Metal-containing conjugated polymers constitute an emerging class of materials designed with the aim of combining as intimately as possible the electronic properties of transition metal complexes with those of conducting polymers w1–16x. From this strong interaction, new tunable electrochemical, photophysical, magnetic or transport properties are expected to evolve, leading to materials with potential interest in various fields of research such as electrocatalysis, molecular recognition, photoconductivity and molecular electronics. As shown in Scheme 1, we developed a chemical synthesis of novel all conjugated polymers based on the alternation of oligothiophene and bipyridine units. The oligothiophene sequence is regioregularly alkylated in order to provide the final polymer with reproducible properties and to enhance its solubility in usual organic solvents. The complexing unit 2,2X-bipyridine is inserted into the conjugated backbone by its 5,5X positions in order to allow
)
Corresponding author. E-mail: [email protected]
formal electronic delocalization along the polymer chain and to allow the formation of octahedral complexes of the type MŽbipy. 3nq ŽM s Fe, Ru, Os, etc...
2. Results and discussion Bis-stanylated regioregular alkylated oligothiophenes 2 4 and 2 6 were obtained from the corresponding a-chlorosubstituted oligothiophenes 1 4 and 1 6 w17x by reductive dehalogenation with Bu 3 SnH under radical conditions, giving up to 95% of the free-end oligomers. By lithiation with 2.5 equivalents of n-BuLi and quenching of the dianions with trimethyltin chloride, we obtained the bisfunctionalized oligomers with nearly quantitative yields. A PdŽ0. catalyzed Stille cross-coupling reaction between 5,5X-dibromo-2,2X-bipyridine 3 w18x, and 2 4 and 2 6 conducted to polymers P4 and P6, respectively. The crude polymers were extracted successively with boiling ethanol, hexane, 1,2-dimethoxyethane to remove inorganic impurities and low molecular weight oligomers. Finally, an extraction with chloroform allowed the isolation of pure polymers P4 and P6 with yields of 55% and 61%, respec-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 4 5 - 3
278
L. Trouillet et al.r Synthetic Metals 109 (2000) 277–280
Scheme 1.
tively. Both polymers were soluble enough to allow their full characterization by NMR spectroscopy and Gel Permeation Chromatography. 1 Our efforts to fully metalate polymers P4 and P6 remained unsuccessful until now partly because the formation of the RuŽbipy. 32q core is difficult to monitor due to the superposition of the p–pU band of the conjugated backbone and the MLCT band of the metallic complex. However, the Stille reaction could be conducted on the pre-synthesized bifunctional complex 4 to lead to the metalated polymer P4-Ru. Analysis by NMR showed that all the bipyridine units on the polymer backbone are involved in the complexation of ruthenium ions, demonstrating the efficiency and the versatility of the Stille cross-coupling reaction. Pure P4-Ru was isolated, after removal of impurities by successive washings with boiling methanol and ethanol, in its hexafluorophosphate form by precipitation from water–acetonitrile mixtures.
1
Detailed results will be published elsewhere.
Electrochemical studies were conducted on polymers P4 and P4-Ru. The Fig. 1 illustrates the behavior of films of P4 deposited on the platinum electrode surface, and of P4-Ru in solution in acetonitrile. Compared to a structurally homogeneous polyŽalkylthiophene., the main point consists in the easy attainable reduction process at E1r2 s y2.15 V vs. AgrAgq 10y2 M, which is close to the value of E1r2 s y2.21 V reported by Yamamoto et al. w1x for the reduction of polyŽ2,2X-bipyridine.. This reduction process is associated to the n-doping of the polymer as demonstrated by the in-situ measurement of resistance ŽFig. 2.. Although the level of attained conductance is less than those associated with p-doping, it corresponds to an increase of conductivity greater than 3 orders of magnitude. This demonstrates that our polymer, because of the alternation of electron-rich Žoligothiophene. and electronpoor Žbipyridine. units, exhibit a readily accessible dual doping process. However, the fact that in the resistance profile no plateau can be reached and that hysteresis is almost non-existing, suggests that the conductivity have a redox origin more likely than an electronic origin. The
L. Trouillet et al.r Synthetic Metals 109 (2000) 277–280
Fig. 1. Cyclic voltametry curves of -a: film of P4 in acetonitrile. -b: film of P4 in dichloromethane. -c: P4-Ru in solution in acetonitrile. Working electrode: Pt disk Ž Ss 0.071 cm2 .. Counter electrode: Pt foil. Reference electrode: AgrAgq 10y2 M in acetonitrile. Supporting electrolyte: tetrabutylammonium hexafluorophosphate.
conductivity is more likely due to the hopping of localized charges by a self-exchange process and is probably not due
279
Fig. 2. In-situ measurement of resistance of a film of P4 deposited on the electrode surface. Working electrode: Pt disk Ž Ss 0.071 cm2 .. Counter electrode: Pt foil. Reference electrode: AgrAgq 10y2 M in acetonitrile. Supporting electrolyte: tetrabutylammonium hexafluorophosphate.
to an extended delocalization of charges. P4-Ru exhibits a more complicated electrochemical behavior as illustrated by Fig. 1c. By comparison with the behavior of P4 and RuŽbipy. 32q, the two first oxidation waves at E1r2 s q0.71 and q0.86 V were attributed to processes localized on the
Fig. 3. I s f Ž V . characteristics of a ITOrP4rAl. LED. Polymer layer thicknesss 33 nm. Ža. Iel s f Ž V .. Žb. I lum s f Ž V ..
280
L. Trouillet et al.r Synthetic Metals 109 (2000) 277–280
polymer conjugated backbone, whereas the third oxidation wave at E1r2 s q1.12 V was associated to the Ru3qr2q redox couple. In the reduction side, three waves corresponding to formal Ru2qrq, Ruqr0 and Ru0ry1 couples at, respectively, y1.32, y1.61 and y2.13 V were observed. As these electronic processes involve the LUMO of the complex that is mainly pU of the ligands in character, the E1r2 values strongly depend on the substitution of bipyridines. The first reduction wave was then attributed to the reduction of the bipyridine included in the conjugated backbone, whereas the second and third waves were localized on the two remaining unsubstituted ligands. Due to the electron-accepting character of the conjugated backbone, the electrochemical gap in P4 is considerably reduced compared to polyŽalkylthiophenes.. This may be of interest when considering the electroluminescent properties of such conjugated materials, and the offered possibility to decrease the emission onset. Some preliminary experiments were conducted with this aim in a classical ITOrpolymerrAl. sandwich geometry, and typical Iel s f Ž V . and I lum s f Ž V . curves are shown in Fig. 3. The light emission onset is just above 10 V, which constitutes an encouraging result despite the relatively low intensity recorded for the emitted light.
3. Conclusion Soluble conjugated polymers alternating regioregular oligoŽ3-octylthiophene. and either 2,2X-bipyridine or its rutheniumŽII. complex have been synthesized by the PdŽ0. catalyzed Stille cross-coupling reaction. Electrochemical studies of one Ru-metalated polymer showed that this type of material is either p- or n-dopable. Preliminary results concerning the electroluminescence properties of this fam-
ily of polymers are encouraging us to pursue this study, and to evaluate the possibility of controlling the electronic properties of the conjugated backbone via the metalation of the bipyridine units.
Acknowledgements Ones of us, M.L., thanks Committee for Scientific Research of Poland ŽKBN. for financial support ŽGrant No. 3T09A 012 16..
References w1x T. Yamamoto, T. Maruyama, Z. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takezoe, A. Fukuda, K. Kubota, J. Am. Chem. Soc. 116 Ž1994. 4832. w2x T. Maruyama, T. Yamamoto, Inorg. Chim. Acta 238 Ž1995. 9. w3x O. Wolf, M.S. Wrighton, Chem. Mater. 6 Ž1994. 1526. w4x S.S. Zhu, T.M. Swager, Adv. Mater. 8 Ž1996. 497. w5x S.S. Zhu, T.M. Swager, J. Am. Chem. Soc. 119 Ž1997. 12568. w6x C.G. Cameron, P.G. Pickup, Chem. Commun. Ž1997. 303. w7x K.D. Ley, C.E. Whittle, M.D. Bartkerger, K.S. Schanze, J. Am. Chem. Soc. 119 Ž1997. 3423. w8x K.D. Ley, K.S. Schanze, Coord. Chem. Rev. 171 Ž1998. 287. w9x B. Wang, M.R. Wasielewski, J. Am. Chem. Soc. 119 Ž1997. 12. w10x Z. Peng, A.R. Gharavi, L. Yu, J. Am. Chem. Soc. 119 Ž1997. 4622–4632. w11x Q. Wang, L. Wang, L. Yu, J. Am. Chem. Soc. 120 Ž1998. 12860. w12x P.L. Vidal, M. Billon, B. Divisia-Blohorn, G. Bidan, J.M. Kern, J.P. Sauvage, Chem. Commun. Ž1998. 629. w13x J.L. Reddinger, J.R. Reynolds, Chem. Mater. 10 Ž1998. 1236–1243. w14x J.L. Reddinger, J.R. Reynolds, Chem. Mater. 10 Ž1998. 3. w15x S.C. Yu, X. Gong, W.K. Chan, Macromolecules 31 Ž1998. 5639. w16x H. Plenio, J. Herman, J. Leukel, Eur. J. Inorg. Chem. Ž1998. 2063. w17x G. Bidan, A. De Nicola, V. Enee, ´ S. Guillerez, Chem. Mater. 10 Ž1998. 1052. w18x F.M. Romero, R. Ziessel, Tetrahedron Lett. 36 Ž1995. 6471.
Synthesis, electrochemical and electroluminescent properties of oligothiophene-based conjugated polymers L. Trouillet a , M. Lapkowski a
c
b,)
, O. Stephan c , S. Guillerez
a
Departement de Recherche Fondamentale sur la Matiere ´ ` Condensee, ´ SI3Mr EMSI, CEA Grenoble, 17, rue des Martyrs, 38054 Grenoble Cedex 09, France b Department of Chemistry, Silesian UniÕersity of Technology, Ks. M. Strzody 9, 44100 Gliwice, Poland Laboratoire de Spectrometrie Physique, UniÕersite Joseph Fourier, Domaine UniÕersitaire Saint-Martin-d’Heres-Gieres, 38041 Grenoble Cedex 9, France Received 26 June 1999; received in revised form 30 June 1999; accepted 10 September 1999
Abstract The paper deals with a new conjugated polymer having the possibility to form a complex with transition metals. The conjugated backbone is constituted by the alternation of regioregular alkylated oligothiophene of variable size and of 2,2X-bipyridine as the chelating unit. Electrochemical studies were conducted on both the non-metalated and RuŽII. complex forms together with the in-situ conductance measurement showing that p- and n-doping occur. The non-metalated form was showed to exhibit electroluminescent properties. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PdŽ0. catalyzed cross-coupling; Conjugated polymer; Metal-containing conjugated polymer; Conductivity; n-Doping; Electroluminescence
1. Introduction Metal-containing conjugated polymers constitute an emerging class of materials designed with the aim of combining as intimately as possible the electronic properties of transition metal complexes with those of conducting polymers w1–16x. From this strong interaction, new tunable electrochemical, photophysical, magnetic or transport properties are expected to evolve, leading to materials with potential interest in various fields of research such as electrocatalysis, molecular recognition, photoconductivity and molecular electronics. As shown in Scheme 1, we developed a chemical synthesis of novel all conjugated polymers based on the alternation of oligothiophene and bipyridine units. The oligothiophene sequence is regioregularly alkylated in order to provide the final polymer with reproducible properties and to enhance its solubility in usual organic solvents. The complexing unit 2,2X-bipyridine is inserted into the conjugated backbone by its 5,5X positions in order to allow
)
Corresponding author. E-mail: [email protected]
formal electronic delocalization along the polymer chain and to allow the formation of octahedral complexes of the type MŽbipy. 3nq ŽM s Fe, Ru, Os, etc...
2. Results and discussion Bis-stanylated regioregular alkylated oligothiophenes 2 4 and 2 6 were obtained from the corresponding a-chlorosubstituted oligothiophenes 1 4 and 1 6 w17x by reductive dehalogenation with Bu 3 SnH under radical conditions, giving up to 95% of the free-end oligomers. By lithiation with 2.5 equivalents of n-BuLi and quenching of the dianions with trimethyltin chloride, we obtained the bisfunctionalized oligomers with nearly quantitative yields. A PdŽ0. catalyzed Stille cross-coupling reaction between 5,5X-dibromo-2,2X-bipyridine 3 w18x, and 2 4 and 2 6 conducted to polymers P4 and P6, respectively. The crude polymers were extracted successively with boiling ethanol, hexane, 1,2-dimethoxyethane to remove inorganic impurities and low molecular weight oligomers. Finally, an extraction with chloroform allowed the isolation of pure polymers P4 and P6 with yields of 55% and 61%, respec-
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 4 5 - 3
278
L. Trouillet et al.r Synthetic Metals 109 (2000) 277–280
Scheme 1.
tively. Both polymers were soluble enough to allow their full characterization by NMR spectroscopy and Gel Permeation Chromatography. 1 Our efforts to fully metalate polymers P4 and P6 remained unsuccessful until now partly because the formation of the RuŽbipy. 32q core is difficult to monitor due to the superposition of the p–pU band of the conjugated backbone and the MLCT band of the metallic complex. However, the Stille reaction could be conducted on the pre-synthesized bifunctional complex 4 to lead to the metalated polymer P4-Ru. Analysis by NMR showed that all the bipyridine units on the polymer backbone are involved in the complexation of ruthenium ions, demonstrating the efficiency and the versatility of the Stille cross-coupling reaction. Pure P4-Ru was isolated, after removal of impurities by successive washings with boiling methanol and ethanol, in its hexafluorophosphate form by precipitation from water–acetonitrile mixtures.
1
Detailed results will be published elsewhere.
Electrochemical studies were conducted on polymers P4 and P4-Ru. The Fig. 1 illustrates the behavior of films of P4 deposited on the platinum electrode surface, and of P4-Ru in solution in acetonitrile. Compared to a structurally homogeneous polyŽalkylthiophene., the main point consists in the easy attainable reduction process at E1r2 s y2.15 V vs. AgrAgq 10y2 M, which is close to the value of E1r2 s y2.21 V reported by Yamamoto et al. w1x for the reduction of polyŽ2,2X-bipyridine.. This reduction process is associated to the n-doping of the polymer as demonstrated by the in-situ measurement of resistance ŽFig. 2.. Although the level of attained conductance is less than those associated with p-doping, it corresponds to an increase of conductivity greater than 3 orders of magnitude. This demonstrates that our polymer, because of the alternation of electron-rich Žoligothiophene. and electronpoor Žbipyridine. units, exhibit a readily accessible dual doping process. However, the fact that in the resistance profile no plateau can be reached and that hysteresis is almost non-existing, suggests that the conductivity have a redox origin more likely than an electronic origin. The
L. Trouillet et al.r Synthetic Metals 109 (2000) 277–280
Fig. 1. Cyclic voltametry curves of -a: film of P4 in acetonitrile. -b: film of P4 in dichloromethane. -c: P4-Ru in solution in acetonitrile. Working electrode: Pt disk Ž Ss 0.071 cm2 .. Counter electrode: Pt foil. Reference electrode: AgrAgq 10y2 M in acetonitrile. Supporting electrolyte: tetrabutylammonium hexafluorophosphate.
conductivity is more likely due to the hopping of localized charges by a self-exchange process and is probably not due
279
Fig. 2. In-situ measurement of resistance of a film of P4 deposited on the electrode surface. Working electrode: Pt disk Ž Ss 0.071 cm2 .. Counter electrode: Pt foil. Reference electrode: AgrAgq 10y2 M in acetonitrile. Supporting electrolyte: tetrabutylammonium hexafluorophosphate.
to an extended delocalization of charges. P4-Ru exhibits a more complicated electrochemical behavior as illustrated by Fig. 1c. By comparison with the behavior of P4 and RuŽbipy. 32q, the two first oxidation waves at E1r2 s q0.71 and q0.86 V were attributed to processes localized on the
Fig. 3. I s f Ž V . characteristics of a ITOrP4rAl. LED. Polymer layer thicknesss 33 nm. Ža. Iel s f Ž V .. Žb. I lum s f Ž V ..
280
L. Trouillet et al.r Synthetic Metals 109 (2000) 277–280
polymer conjugated backbone, whereas the third oxidation wave at E1r2 s q1.12 V was associated to the Ru3qr2q redox couple. In the reduction side, three waves corresponding to formal Ru2qrq, Ruqr0 and Ru0ry1 couples at, respectively, y1.32, y1.61 and y2.13 V were observed. As these electronic processes involve the LUMO of the complex that is mainly pU of the ligands in character, the E1r2 values strongly depend on the substitution of bipyridines. The first reduction wave was then attributed to the reduction of the bipyridine included in the conjugated backbone, whereas the second and third waves were localized on the two remaining unsubstituted ligands. Due to the electron-accepting character of the conjugated backbone, the electrochemical gap in P4 is considerably reduced compared to polyŽalkylthiophenes.. This may be of interest when considering the electroluminescent properties of such conjugated materials, and the offered possibility to decrease the emission onset. Some preliminary experiments were conducted with this aim in a classical ITOrpolymerrAl. sandwich geometry, and typical Iel s f Ž V . and I lum s f Ž V . curves are shown in Fig. 3. The light emission onset is just above 10 V, which constitutes an encouraging result despite the relatively low intensity recorded for the emitted light.
3. Conclusion Soluble conjugated polymers alternating regioregular oligoŽ3-octylthiophene. and either 2,2X-bipyridine or its rutheniumŽII. complex have been synthesized by the PdŽ0. catalyzed Stille cross-coupling reaction. Electrochemical studies of one Ru-metalated polymer showed that this type of material is either p- or n-dopable. Preliminary results concerning the electroluminescence properties of this fam-
ily of polymers are encouraging us to pursue this study, and to evaluate the possibility of controlling the electronic properties of the conjugated backbone via the metalation of the bipyridine units.
Acknowledgements Ones of us, M.L., thanks Committee for Scientific Research of Poland ŽKBN. for financial support ŽGrant No. 3T09A 012 16..
References w1x T. Yamamoto, T. Maruyama, Z. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takezoe, A. Fukuda, K. Kubota, J. Am. Chem. Soc. 116 Ž1994. 4832. w2x T. Maruyama, T. Yamamoto, Inorg. Chim. Acta 238 Ž1995. 9. w3x O. Wolf, M.S. Wrighton, Chem. Mater. 6 Ž1994. 1526. w4x S.S. Zhu, T.M. Swager, Adv. Mater. 8 Ž1996. 497. w5x S.S. Zhu, T.M. Swager, J. Am. Chem. Soc. 119 Ž1997. 12568. w6x C.G. Cameron, P.G. Pickup, Chem. Commun. Ž1997. 303. w7x K.D. Ley, C.E. Whittle, M.D. Bartkerger, K.S. Schanze, J. Am. Chem. Soc. 119 Ž1997. 3423. w8x K.D. Ley, K.S. Schanze, Coord. Chem. Rev. 171 Ž1998. 287. w9x B. Wang, M.R. Wasielewski, J. Am. Chem. Soc. 119 Ž1997. 12. w10x Z. Peng, A.R. Gharavi, L. Yu, J. Am. Chem. Soc. 119 Ž1997. 4622–4632. w11x Q. Wang, L. Wang, L. Yu, J. Am. Chem. Soc. 120 Ž1998. 12860. w12x P.L. Vidal, M. Billon, B. Divisia-Blohorn, G. Bidan, J.M. Kern, J.P. Sauvage, Chem. Commun. Ž1998. 629. w13x J.L. Reddinger, J.R. Reynolds, Chem. Mater. 10 Ž1998. 1236–1243. w14x J.L. Reddinger, J.R. Reynolds, Chem. Mater. 10 Ž1998. 3. w15x S.C. Yu, X. Gong, W.K. Chan, Macromolecules 31 Ž1998. 5639. w16x H. Plenio, J. Herman, J. Leukel, Eur. J. Inorg. Chem. Ž1998. 2063. w17x G. Bidan, A. De Nicola, V. Enee, ´ S. Guillerez, Chem. Mater. 10 Ž1998. 1052. w18x F.M. Romero, R. Ziessel, Tetrahedron Lett. 36 Ž1995. 6471.