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Mechanistic Aspects of Radiation-Induced Oligomerization of 3,4-Ethylenedioxythiophene in Ionic Liquids Radosław Michalski, Adam Sikora, Jan Adamus, and Andrzej Marcinek* Institute of Applied Radiation Chemistry, Technical UniVersity of Lodz, Zeromskiego 116, 90-924 Lodz, Poland ReceiVed: July 20, 2010; ReVised Manuscript ReceiVed: September 20, 2010
Thiophene and its disubstituted derivatives, such as 3,4-ethylenedioxythiophene (EDOT), 3,4-dimethoxythiophene (DMT), 3,4-propylenedioxythiophene (PDOT), and 3,4-butylenedioxythiophene (BuDOT) were oxidized in organic solvents and in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+ PF6-) at RT and under cryogenic conditions. Their radical cations were spectrally characterized at 77 K. Annealing of the irradiated matrix, which triggers the diffusion processes, led to spontaneous oligomerization. The oxidative coupling between a radical cation and a neutral monomer was identified as the first step of the oligomerization process. The scale of oligomerization could be extended by the addition of chloroform, which acts as a dissociative electron scavenger, whereas the dichloromethylperoxyl radicals formed in the reaction with the dissolved oxygen act as secondary oxidizing agents. Introduction Poly-(3,4-ethylenedioxythiophene) PEDOT, one of the most successful conducting polymers of excellent stability in its highly conducting cationic “doped” form, is generally obtained by chemical or electrochemical polymerization.1 The conducting properties of the polymer depend on several factors, among others the methods and conditions of polymerization, and oxidants used.2,3 Room temperature ionic liquids (RTILs) are a well-known class of compounds that have found application in organic synthesis,4 catalysis,5,6 analytical chemistry,7,8 and separation processes.9 One of the potential applications of ionic liquids is their use as a medium for polymerization processes. In the past few years, several papers concerning this subject have been published.10-13 Because of its wide electrochemical window and high ionic conductivity, RTILs have been applied to electrochemical polymerization,11-13 including the electrochemical polymerization of PEDOT.14-18 On the other hand, it has been shown that radicals, and radical ions generated during the radiolysis of RTILs19-29 can be used as oxidants, initializing free radical polymerization in ionic liquids.30-33 In this work, we analyze the mechanistic aspects of radiationinduced polymerization of thiophenes in ionic liquid 1-butyl3-methylimidazolium hexafluorophosphate (BMIM+PF6-), and its mixture with chloroform and in typical organic matrix solvents, like 2-chlorobutane. The imidazolium ionic liquid was chosen as the polymerization medium because this type of RTIL possesses good radiation stability. Such ionic liquids are even considered as possible solvents within the nuclear fuel cycle, where they are exposed to extremely high radiation doses.34-40 The radiolysis of low temperature matrices has been used to investigate the mechanism of oligomerization of 3,4-ethylenedioxythiophene (EDOT) and related derivatives, that is, 3,4dimethoxythiophene (DMT), 3,4-propylenedioxythiophene (PDOT), and 3,4-butylenedioxythiophene (BuDOT) (Scheme 1). The results are compared to those obtained for the parent thiophene. * To whom correspondence should be addressed. Phone: 042 631 30 96. Fax: 042 636 50 08. E-mail:
[email protected].
SCHEME 1: Chemical Structures of Investigated Compounds
Oxidation of monomer and small oligomers of thiophenes leads to the formation of longer oligomers and consequently to the polymer. This process involves the formation of radical cations and/or other radical or cationic intermediates. The identification of these primary products of the one electron oxidation and the observation of their reaction paths is usually complicated by their high reactivity. Therefore, for investigation of the initial steps of the oxidative polymerization of thiophenes, the pulse radiolysis of low temperature organic matrices was applied. This methodology, suitable for the generation and spectroscopic characterization of radical ions under stable matrix conditions, is also very convenient for monitoring the reactivity of those elusive species upon controlled annealing and softening of the matrix. Experimental Section Compounds. The ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+PF6-) was prepared by following the method of Huddleston et al.41 Other solvents used as matrices in low temperature measurements: 2-chlorobutane (sec-BuCl) and 2-methyltetrahydrofuran (MTHF) were purchased from Sigma Aldrich. Carbon tetrachloride and chloroform were obtained from POCH (Poland). The following thiophenes were used as received from Sigma Aldrich: thiophene, DMT, EDOT, PDOT, 2,2′-bithiophene (BT), and quaterthiophene (QT). All chemicals used were of the purest grade available. BuDOT42 and 3,4-(ethylenedioxy)-2,2′-bithiophene (BEDOT)43 were synthesized according to methods described previously. Pulse Radiolysis. Pulse radiolysis experiments with optical detection were performed by means of an ELU-6 linear electron
10.1021/jp1067389 2010 American Chemical Society Published on Web 10/08/2010
Radiation-Induced Oligomerization of EDOT
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accelerator. All measurements were performed at room temperature. Seventeen ns pulses (radiation dose 40-80 Gy) were applied to obtain spectra, and 7 ns pulses (radiation dose 15-20 Gy) were used for kinetic analysis. Fresh solutions were prepared before measurements. A fresh volume of solution was used for each electron pulse in kinetic measurements. Kinetic traces were monitored at selected wavelengths, and the absorption spectra were reconstructed from these traces. Second-order rate constants were determined from the linear dependence of pseudo first-order rate of the process, kobs versus compound concentration, using at least six different concentrations. The dose per pulse was estimated by thiocyanate dosimetry. The details of the pulse radiolysis system based on the ELU-6 linear accelerator are given elsewhere.44,45 Cryogenic Measurements. The glassy samples were prepared by immersing solutions in liquid nitrogen. The thickness of the glassy samples was within the range of 1-3 mm. The samples were placed in a temperature-controlled, nitrogencooled cryostat (Oxford Instruments - OptiStat DN) and were irradiated with 4 µs pulses from the accelerator. The temperature controlled annealing of the samples enabled observation of spectral changes. UV-vis-NIR absorption spectra were recorded on a Cary 5 (Varian) spectrophotometer. Radiolysis of the glassy sample of BMIM+PF6- leads to the production of solvated electrons and oxidizing holes represented by radical dication BMIM2+• and PF6• (reaction 1). Electrons are captured by BMIM+ cations yielding BMIM• radical (reaction 2) preventing geminate recombination of the primary species.19,21 2+ · BMIM+, PF, PF6· ,e6 f BMIM
(1)
BMIM+ + e- f BMIM ·
(2)
The positive charge is transferred to the solute molecules of lower ionization potential (in this case thiophene derivatives, T) to give the corresponding radical cations (reaction 3). ·+ BMIM2+ · , PF6· + 2T f BMIM+, PF6 + 2T
give peroxyl radicals that can be used in oxidative polymerization processes as secondary oxidizing species. Radical Anions of Thiophenes. To ensure that upon radiolysis of more concentrated samples no absorption of radical anions interfere with those of the radical cations we characterized spectrally the radical anions of thiophenes in the MTHF matrix. None of the characteristic absorption bands of radical anions were observed under present investigation of the radiation-induced oxidative oligomerization of thiophenes. However, contrary to radical cations, experimental data on thiophene radical anions are sparse, and these results are summarized in Table S1 and some of the spectra are presented in Figure S1 in the Supporting Information. Results and Discussion
(3)
We have shown previously that these novel solvents based on the imidazolium cation form transparent glasses at 77 K and that the glass quality does not change upon addition of an organic component to the ionic liquid.19,46,47 For example, 1:1 mixtures of the 1-butyl-3-methylimidazolium hexafluorophosphate and CHCl3 form an excellent transparent glass for optical measurements. The addition of chloroform or methylene chloride to the matrix improves the solubility of many precursors but also results in more effective scavenging of electrons by dissociative electron attachment (reaction 4) and thus leads to a higher yield of radical cations generated on radiolysis.
CHCl3 + e- f CHCl2· + Cl-
Figure 1. Electronic absorption spectra of radical cations of thiophene and EDOT in low temperature matrices at 77 K, optical path 1 mm: (A) 25 mM solution of thiophene in BMIM+PF6-/CHCl3 (1:1 v/v), radiation dose 2.3 kGy; (B) 0.1 M solution of EDOT in sec-BuCl, radiation dose 2.4 kGy. The spectra before irradiation were subtracted.
(4)
Effective scavenging of the solvated electrons is very important for the simplification of the methodology targeting the characterization and identification of transient species formed in the oxidation processes. Not only do the radicals formed in reaction 4 remain a silent species under cryogenic matrix conditions (they lack absorption bands in the UV-vis-NIR spectral region), but additionally, in reaction with oxygen, they
Radical Cations of Thiophenes (T•+). The spectra of radical cations obtained by low-temperature matrix methodology agree well with the spectra already reported in literature, obtained through chemical or photoinduced oxidation, time-resolved methods such as pulse radiolysis or flash photolysis. For example, thiophene and its oligomers (up to 6 monomer units) have been very well characterized in solution by the pulse radiolysis method.48,49 The strongest absorption band of the thiophene radical cation is located below 300 nm for the monomer and shifts toward longer wavelengths on going to higher oligomers. In Figure 1 we present only two spectra of radical cations of monomeric thiophene itself and EDOT, previously not fully characterized. The UV-vis spectrum of the thiophene radical cation obtained under cryogenic conditions is presented in Figure 1A. It is composed of a strong absorption band peaking below 300 nm, two small peaks around 320 and 380 nm, and a broad long wavelength absorption band around 750 nm. The spectrum of the thiophene radical cation in the BMIM+PF6-/CHCl3 (1:1 v/v) matrix agrees well with the spectrum obtained previously in methylene chloride by the pulse radiolysis technique.48 The weak absorption band around 750 nm was not, however, observed in that earlier experiment, despite the fact that this band is predicted by computational methods.
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Michalski et al. TABLE 1: Maxima of the Absorption Bands of Oligothiophenes Radical Cations T•+ and Their Dimer Radical Cations (T2)•+ in Different Media compound
The absorption spectrum of the radical cation of EDOT is presented for the first time and is composed of two distinct structureless absorption bands in the UV range peaking at 293 and 395 nm (Figure 1B). Other radical cations of disubstituted thiophenes have similar absorption spectra to that of EDOT•+. Formation of Dimer Radical Cations (T2)•+. It is well recognized that, upon ionization, planar π systems form stable complexes consisting of two such species carrying a single charge, which are formed in reaction of radical cations with their neutral precursors.50 Apart from Coulombic and dispersive attractions, these so-called dimer radical cations are bound by a substantial valence contribution that arises from the interactions of two HOMOs occupied by three electrons. One of the methods for generating and observing such dimer species, which are usually characterized by broad NIR absorptions,51,52 is by controlled annealing of glassy matrices. Thereby, the two partner species can gradually attain their optimal orientation in the π-complex, and it is possible that its absorption changes in intensity and position or shape during the process of their formation and relaxation which is governed by the rigidity of the glassy environment. Upon annealing of the glassy samples, all investigated monomers formed dimer radical cations (T2)•+, and no steric limitation was found, even for BuDOT, which possesses the most bulky substituent. The process of the formation of the dimer radical cation from BuDOT is presented in Figure 2. Changes in the shape and intensity of the absorption bands are seen, characteristic for a radical cation, but the most prominent feature of the spectrum of the dimer radical cation is its absorption band in NIR. It is interesting to notice that the position of the dimer (T2)•+ absorption band in NIR, similarly to the band position observed for radical cations, shifts toward longer wavelengths with the increasing chain length of the oligomer (see Table 1). In the case of soft matrices, for example a Freon mixture, dimer radical cations could be observed immediately after irradiation, already at 77 K, because local annealing of the matrix upon irradiation is usually sufficient for the formation of such complexes for highly concentrated samples. In fact, for the concentration of 42 mM of thiophene in a Freon matrix, its long wavelength absorption band was observed at 830 nm.53 Dependence of the intensity of this band on the concentration of solute indicated probable formation of the dimer radical cation of thiophene, which has found support from EPR studies.54 We have found that at the high concentration of 0.1 M this band is seen also in more rigid matrices whereas, in contrast to thiophene itself, dimer radical cations of DMT, EDOT, PDOT, and BuDOT are not observed immediately after irradiation of the highly concentrated frozen samples.
T•+, λmax [nm]
bithiopheneb
250(s), 320-380(w), 750(vw) 430(vs), 595(vw)
terthiophenec quaterthiopheneb quinquethiophenec sexithiophenec EDOTb BEDOTb TerEDOTe DMTb PDOTb BuDOTa
545(vs) 655(vs), 1084(vw) 730(vs) 790(vs) 295(vs), 397(s) 429(vs), 628(vw) 549(vs), 897(vw)
