Electropolymerization of crown-annelated bithiophenes

Electrochemistry Communications 9 (2007) 1587–1591 www.elsevier.com/locate/elecom

Electropolymerization of crown-annelated bithiophenes Dora Demeter a

a,b

, Philippe Blanchard

a,*

, Ion Grosu b, Jean Roncali

a,*

Groupe Syste`mes Conjugue´s Line´aires, CIMMA, CNRS UMR 6200, Universite´ d’Angers, 2 Boulevard Lavoisier, F-49045 Angers, France b Organic Chemistry Department and CCOCCAN, ‘‘Babes-Bolyai’’ University, 11 Arany Janos Street, 400028 Cluj-Napoca, Romania Received 16 February 2007; received in revised form 5 March 2007; accepted 5 March 2007 Available online 12 March 2007

Abstract Two crown-annelated bithiophenes in which the external b-positions are linked together by a polyether chain connected via a sulphide linkage have been synthesized. Preliminary investigations of their electrochemical properties shows that they can be straightforwardly electropolymerized into electroactive functional polythiophenes. A first test of their cation-dependent electrochemical behaviour in the presence of lithium cations is also presented. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Electropolymerization; Conducting polymers; Polythiophenes; Crown ether

1. Introduction Functional poly(thiophenes) (PTs) possessing specific electrochemical properties have been a subject of continuous interest for two decades [1,2]. Besides applications in energy storage or electrochromic systems, a large amount of work has been focused on the design of modified electrodes usable as electrochemical or bioelectrochemical sensors [1,2]. PTs possessing cation recognition properties have been investigated by a number of groups [3–14]. Following initial work on cations sensitive PTs derived from thiophene 3-substituted by polyether chains [3], a large variety of systems have been developed. Thus, Ba¨uerle and coworkers have synthesized PTs with macrocyclic crown-ethers covalently attached at a b-position of a thiophene monomer or oligomer by a flexible alkyl linker and analyzed their cation-dependent redox properties [5]. Sannicolo` et al. have described crown ether derivatized poly(cyclopentabithiophenes) [6]. An alternative strategy consists in the fusion of the macrocyclic cavity with the p-conjugated system. This approach presents the advan-

*

Corresponding authors. Fax: +33 2 41 735405 (J. Roncali). E-mail address: [email protected] (J. Roncali).

1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.03.004

tage to maximize electronic interactions between the complexing cavity and the p-conjugated chain and hence to optimize signal transduction. Initial attempts in this direction involved the polymerization of precursors structures in which two polymerizable groups are linked by a polyether chain [7–10]. Swager and Marsella first reported the chemical polymerization of bithiophenes linked at their 3,3 0 positions by oligooxyethylene loops that exhibit large changes in their optical spectrum in the presence of alkali cations [11]. Ba¨uerle and Scheib have investigated the electropolymerization and cation complexing properties of oligo- and polythiophenes with crown ether attached between the 3 and 4 positions of a thiophene ring [12]. More recently we have synthesized crown-annelated oligothiophenes in which cation complexation produces large conformational changes in the conjugated chain [14]. Although these various approaches have led to several classes of PTs functionalized by polypodants or macrocyclic crown-ethers, examples of electrogenerated functional PTs derived from crown-annelated precursors remains scarce due to a combination of steric and reactivity problems [12,13]. We report here the synthesis of two examples of crownannelated bithiophenes in which the external b-positions are linked together by a polyether chain connected via a

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sulphide linkage (1, 2) and preliminary investigations on their electropolymerization and electrochemical properties. 2. Experimental H NMR and 13C NMR spectra were recorded on a Bruker AVANCE DRX 500 spectrometer operating at 500.13 and 125.7 MHz; d are given in ppm (relative to TMS) and coupling constants (J) in Hz. Mass spectra were recorded under EI mode on a VG-Autospec mass spectrometer or under MALDI-TOF mode on a MALDITOF-MS BIFLEX III Bruker Daltonics spectrometer. UV–vis optical data were recorded with a Perkin–Elmer lambda 19 spectrophotometer. IR spectra were recorded on a Perkin–Elmer model 841 spectrophotometer, samples being embedded in KBr discs or thin films between NaCl plates. Melting points were obtained from a Reichert–Jung Thermovar hot-stage microscope apparatus and are uncorrected. Column chromatography purifications were carried out on Merck silica gel Si 60 (40–63 lm). Electrochemical experiments were performed with a EG&G 273 potentiostat in a standard three-electrode cell using a saturated calomel reference electrode. The solutions were degassed by argon bubbling and experiments were carried under an argon blanket. The working electrode was a 2 mm Pt disk sealed in glass. Tetrabutylammonium hexafluorophosphate (Fluka puriss) was used as received.

night. M.p. 118–119 °C, IR 2247 cm1 (CN). 1H NMR (CDCl3) 7.27 (s, 2H), 7.13 (s, 2H) 3.05 (t, 4H, 3 J = 7.5 Hz), 2.62 (t, 4H, 3J = 7.5 Hz). 13C NMR (CDCl3) 137.8, 129.5, 127.3, 127.2, 117.8, 31.13, 18.4. MS EI m/z (I%) 336 [M+] (100); 283 (45); 230 (39).

1

2.3. 4,4 0 -Bis(1,10-dithia-4,7-dioxadecyl-1,10-diyl)-2,2 0 bithiophene (1) A solution of CsOH Æ H2O (0.25 g 1.5 mmol) in 5 mL of degassed methanol is added dropwise to a solution of 4,4 0 bis(2-cyanoethylsulfanyl)-2,2 0 -bithiophene (3) (0.2 g, 0.6 mmol) in 15 mL of degassed DMF. After 1 h of stirring at 20 °C, the reaction mixture and a solution of diiodo compound 6 (0.55 g, 0.6 mmol) in 20 mL of DMF are added simultaneously at a rate of 6 mL/h using perfusor pumps to 100 mL of degassed DMF. After 72 h stirring at 20 °C the mixture is diluted with CH2Cl2, washed with water and dried over Na2SO4. After solvent removal, the residue is chromatographed (silica gel eluting with CH2Cl2) to give 0.13 g (39%) of a yellow oil which crystallizes. M.p. 139–140 °C. 1H NMR (CDCl3) 7.84 (s, 2H), 6.87 (s, 2H), 3.81 (t, 4H, 3J = 5 Hz), 3.70 (s, 4H), 3.17 (t, 4H, 3 J = 5 Hz). 13C NMR (CDCl3) 136.9, 133.5, 126.6, 118.3, 73.0, 70.9, 33.9. MS MALDI 343.94 [M+]. 2.4. 4,4 0 -Bis(1,12-dithia-4,7,10-trioxatridecyl-1,13-diyl)2,2 0 -bithiophene (2)

2.1. 3-(Tributylstannylsulfanyl)propanenitrile (5) 3-Mercaptopropionitrile was synthesized in 47% yield from 3-bromopropionitrile using a known procedure [15]. The crude product was used without further purification. To a stirred solution of 3-mercaptopropionitrile (3.5 g 40.9 mmol), triethylamine, (4.84 g, 1.17 eq) in 200 mL of dry ether, tributyltin chloride (13.3 g, 1 eq) is added dropwise under a nitrogen atmosphere. A white precipitate forms and the mixture is stirred 2 h at room temperature. The mixture is filtrated and the solid is washed with 5% acetic acid and water. Purification of the compound by Kugelrho¨r distillation (78 mbar, 70 °C) gives 8 g (53%) of an oil which is directly used for the next step. 1H NMR (CDCl3) 2.82 (t, 2H, 3J = 5 Hz), 2.61 (t, 2H, 3J = 5 Hz), 1.63–1.57 (m, 6H), 1.41–1.34 (m, 6H), 1.32–1.19 (m, 6H), 0.94 (t, 9H, 3J = 5 Hz). 2.2. 4,4 0 -Bis(2-cyanoethylsulfanyl)-2,2 0 -bithiophene (3) A mixture of dibromobithiophene 13 [11] (1 g, 3.1 mmol), Stille reagent 5 (4.66 g, 12.3 mmol,) and Pd(PPh3)4 (0.550 g, 7 mol%) in 50 mL of anhydrous toluene is refluxed for 12 h. After concentration, the residue is taken in CH2Cl2 and the organic phase is washed twice with an aqueous solution of NaHCO3 then with water. After drying over MgSO4 and solvent removal, the product is chromatographed on silica gel eluting with CH2Cl2 to give 0.62 g (62%) of an orange oil which crystallizes over-

This compound is prepared using the same procedure from CsOH Æ H2O (0.199 g, 1.18 mmol) in 5 mL of methanol, 0.16 g (0.48 mmol) of 4,4 0 -bis(2-cyanoethylsulfanyl)2,2 0 -bithiophene (3) in 15 mL of DMF and (0.197 g 0.48 mmol) of diiodo compound 7 in 20 mL of DMF. Usual work-up and column chromatography (silica gel, eluting with CH2Cl2) gave 70 mg (38%) of a brown oil. 1 H NMR (CDCl3) 7.55 (s, 2H), 7.14 (s, 2H), 3.65–3.59 (m, 12H), 2.98 (t, 4H, 3J = 5 Hz). 13C NMR 137.4, 131.9, 128.3, 125.3, 70.53, 70.4, 70.1, 35.3. MS MALDI 388.89 [M+]. 3. Results and discussion The target compounds have been synthesized from bithiophene 3 bearing two thiolate functions protected by 2-cyanoethyl groups at the external b-positions. Attempts to synthesize compound 3 by halogen–lithium exchange from dibromo bithiophene 4 [16] followed by addition of sulfur and 3-bromopropionitrile [17] gave the desired compound only in rather low yields (9%). The key compound 3 was then synthesized in 62% yield from dibromothiophene 4 by Stille coupling using the new tributylstannyl reagent 5. This one was obtained in three steps from 3-bromopropionitrile by modification of a known procedure [18]. The target compounds 1 and 2 were then obtained in 39% and 38% yield, respectively by deprotection of the two thiolate functions by cesium hydroxide followed by reaction with the

D. Demeter et al. / Electrochemistry Communications 9 (2007) 1587–1591

bithiophene into its cation radical at 1.14 and 1.16 V vs SCE for compounds 1 and 2, respectively. The higher value of the oxidation potential observed for compound 2 is in agreement with the more twisted structure already indicated by UV–vis data. Fig. 1 shows the cyclic voltammograms (CV) resulting from the application of recurrent potential scans between 0.0 and +1.30 V. In both cases, application of repetitive potential scan leads to the progressive development of a broad redox system in the 0.50–1.00 V region corresponding to the electrodeposition of an electroactive material on the anode surface. In spite of the sterically twisted structure of compound 2, electropolymerization proceeds straightforwardly in both cases. This result thus confirms that the electron-donating sulfide groups at the external b-positions confer a high reactivity on the neighboring coupling a-positions [19,20].

appropriate diiodooligooxyethylene according to the already reported procedures (Scheme 1) [17]. The UV–vis absorption spectra of compounds 1 and 2 recorded in methylene chloride solutions show absorption maxima at 333 and 306 nm, respectively. Since the electron-releasing electronic effect of the sulfide group is the same in both compounds, this difference must be related to a larger dihedral twisted angle between the two thiophene units caused by larger steric hindrance to planarity due to the larger size of the polyether loop in compound 2. The electrochemical properties of the two compounds have been analyzed in acetonitrile containing 5 mM of the precursor and 0.10 M tetrabutylammonium hexafluorophosphate. Application of a single potential scan to a millimolar solution of the two compounds shows and irreversible anodic peak corresponding to the oxidation of the

Br

H2N

S

CN

+ H2N

NaOH

S

CN

CN

HS

NNH2+Br-

NH2

Et3N Et2O

Bu3SnCl NC Br

CN

Bu3Sn-S

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Pd(PPh3)4 Br 4

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Scheme 1. Synthesis of the macrocyclic bithiophenes 1 and 2.

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Fig. 1. Potentiodynamic electropolymerization of compounds 1 (left) and 2 (right) (5 mM in 0.10 M Bu4NPF6/MeCN, scan rate 100 mV s1).

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D. Demeter et al. / Electrochemistry Communications 9 (2007) 1587–1591 0.0006

0.0002 0.0004

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Fig. 2. Cyclic voltammograms of poly(1) (left) and poly(2) (right) in 0.10 M Bu4NPF6/MeCN, scan rate 100 mV s1.

site involves the direct grafting of an oxygen or sulphur atom on the thiophene ring, cation complexation induces a decrease of the inductive electron-releasing effect of the chalcogen which also contributes to increase the oxidation potential of the PT backbone [12]. On the other hand, a negative shift of anodic peak potential has been observed in some specific cases. Thus, in the case of poly(dioxaheptylthiophene) in which no direct interaction between the complexing site and the conjugated PT backbone is possible, a 200 mV negative shift of the anodic peak potential was observed when replacing Bu4N+ by Li+. Spectroelectrochemistry and UV–vis data have demonstrated that this unexpected behaviour was due to a planarization of the conjugated backbone induced by metal cation complexation by the polyether side chains [3,4]. Recently, more complex interplay between electronic and conformational processes have been reported for crown-annelated oligothiophenes [14]. In this context, the electrochemical behaviour of poly(2) in the presence of Li+ can be interpreted as the result of

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0.0001

I (A)

Fig. 2 shows the CV of the two polymers recorded in a monomer-free electrolytic medium. The CV of poly(1) shows a broad anodic wave with two discernible sub-components at 0.70 and 0.90 V and a cathodic wave peaking at 0.65 V. In the case of poly(2), the CV exhibits two components but the anodic peak potentials are shifted toward more positive potential to ca 0.80 and 1.00 V. This result can be attributed to the already discussed larger dihedral angle between the two thiophene rings which reduces the effective conjugation length in poly(2). Compounds 1 and 2 have also been electropolymerized in potentiostatic conditions at an applied potential of 1.10 V vs SCE. Measurement of the ratio of the amount of charge reversibly exchanged upon redox cycling to the deposition charge gave values of 27% and 15% for poly(1) and poly(2), respectively. The lower value observed for poly(2) which suggests a lower doping level, could be attributed to a shorter effective conjugated length due to the more twisted structure. A preliminary analysis of the cation complexing properties of poly(1) and poly(2) has been carried out by recording the cyclic voltammetric response of the polymers in the presence of different metal cations. In the case of poly(1) no change in the electrochemical response was observed when replacing Bu4N+ by cations such as: Li+, Ag+, Ba2+, Pb2+. In contrast, for poly(2) replacing Bu4N+ by Li+ produces a ca 100 mV negative shift of the main anodic peak potential (Fig. 3.). However no change in the CV was observed with other metal cations. The electrochemical behaviour of PTs functionalized by polyether or crown ether has been analyzed on oligomeric or polymeric conjugated systems of very different structures [3–17]. When the complexing site can directly interact with the p-conjugated backbone, metal cation complexation produces a positive shift of the anodic peak potential of the polymer caused by the coulombic repulsion between the positive charge of the metal cation and that of the polaronic (or bipolaronic) state of the oxidized p-conjugated backbone [5,6,9,12,13]. Additionally, when the complexing

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E (V/SCE) Fig. 3. Cyclic voltammograms of poly(2). Dotted line: in 0.10 M TBAPF6/MeCN, solid line: in 0.10 M LiClO4/MeCN, scan rate 100 mV s1.

D. Demeter et al. / Electrochemistry Communications 9 (2007) 1587–1591

O

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O O

S

O

S S

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O Scheme 2.

two counteracting effects namely purely coulombic and inductive electronic effects which should increase the oxidation potential and a conformational effect in which cation complexation induces a planarization of the p-conjugated system and hence a decrease of the oxidation potential (Scheme 2). Further spectroelectrochemical studies are needed to analyze theses processes in more detail. To summarize two examples of crown-annelated bithiophenic precursors have been synthesized and electropolymerized into the corresponding functional electroactive polymers. The analysis of the electrochemical behaviour of these polymers in the presence of various metal cations has revealed some sensitivity towards lithium cations. In the light of the excellent polymerizability of these precursors, this still modest cation sensitivity provides a strong incitement to further work aiming at the design of more sensitive and selective electrogenerated polymers based on this class of precursor structure. References [1] J. Roncali, J. Mater. Chem. 9 (1999) 1875. [2] D.T. McQuade, A.E. Pullen, T.M. Swager, Chem. Rev. 100 (2000) 2537. [3] J. Roncali, R. Garreau, D. Delabouglise, F. Garnier, M. Lemaire, J. Chem. Soc. Chem. Commun. (1989) 679. [4] J. Roncali, L.H. Shi, F. Garnier, J. Phys. Chem. 95 (1991) 8983.

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