CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201300915

Radiation-Induced Synthesis of Nanostructured Conjugated Polymers in Aqueous Solution: Fundamental Effect of Oxidizing Species Youssef Lattach,[a] Cecilia Coletta,[a] Srabanti Ghosh,[a] and Samy Remita*[a, b] Synthesis of conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) polymers is achieved through the radiolysis of N2O-saturated aqueous solutions of 3,4-ethylenedioxythiophene by using two different oxidizing species: HOC (hydroxyl) and N3C (azide) radicals. Both oxidative species lead to self-assembled polymers that are evidenced in solution by cryotransmission electron microscopy and UV/Vis absorption spectroscopy and, after centrifugation and deposition, by scanning electron mi-

croscopy and attenuated total reflectance FTIR techniques. Whereas HOC radicals lead to PEDOT-OH globular nanostructures with hydrophilic properties, N3C radicals enable the formation of amphiphilic PEDOT-N3 fibrillar nanostructures. These results, which highlight the differences in the intermolecular interaction behaviors of the two kinds of PEDOT polymers, are discussed in terms of polymerization mechanisms.

1. Introduction Poly(3,4-ethylenedioxythiophene), a polythiophene derivative known as PEDOT (Scheme 1), is one of the most famous p-conjugated conducting polymers.[1] Indeed, conducting PEDOT films are currently being investigated for use as antistatic materials,[2] field-effect transistors (OFET),[3] electroluminescent diodes (OLED),[4] photovoltaic cells,[5] as well as sensors.[6] PEDOT polymers are usually prepared by oxidation of 3,4ethylenedioxythiophene (EDOT) monomers (Scheme 1), which are soluble in water,[7] and they present with some specific chemical properties that make them interesting building blocks for the synthesis of functional p-conjugated systems.[1] In fact, due to their strong electron-donor effects, the ether groups at the b,b’ positions of the thiophene rings, which confer high reactivity to the free a,a’ positions, prevent the formation of parasite a,b’ linkages during PEDOT polymerization (Scheme 1). Starting from EDOT oxidation, PEDOT is prepared according to two main routes: wet chemical synthesis[2, 8] and electropolymerization.[2, 9] Other protocols are also used, such as interfacial or vapor-phase polymerizations,[10–12] whereas some alternative methodologies, such as sonochemistry[13] and photochemistry,[14] remain rarely proposed. Despite all of these synthesis procedures, significant efforts are underway to explore alternative techniques intended to be used to synthesize new conjugated polymers. Besides, intensive research efforts with the

aim to optimize the preparation of nanostructured PEDOT in a cost-effective way, to control their morphology (shape and size), to increase their thermal and mechanical stabilities, to optimize their hydrophilic properties, and to adjust their performances in terms of optical and electrical properties still remain.[15] We recently, and for the first time, used g-radiolysis as an original, simple alternative way to synthesize conjugated PEDOT polymers in solution.[16] g-Rays and X-rays are indeed very often used at ambient temperature and pressure to initiate, in the absence of any external chemical initiators, oxidation[17, 18] or reduction[19, 20] reactions through the control of the nature of the initiator radicals formed during water radiolysis; these radicals are dependent on the medium (atmosphere and solute).[21, 22] Moreover, radiolysis is known as an interesting method for the synthesis of nonconjugated polymers.[23] Starting from EDOT monomers, radiolysis enabled us to prepare PEDOT under very soft conditions: in water in the absence of any solutes, at ambient temperature, and in open air.[16] We demonstrated that radiation-induced PEDOT polymerization proceeds through a recurrent step-by-step mechanism that involves a recurrent oxidation process by hydroxyl radicals formed during water radiolysis. After deposition onto a sub-

[a] Dr. Y. Lattach, C. Coletta, Dr. S. Ghosh, Prof. S. Remita Laboratoire de Chimie Physique, LCP, UMR 8000, CNRS Universit Paris-Sud 11, Bt. 349, Campus d’Orsay 15 Avenue Jean Perrin, 91405 Orsay Cedex (France) E-mail: [email protected] [b] Prof. S. Remita Dpartement CASER, Ecole SITI Conservatoire National des Arts et Mtiers, CNAM 292 rue Saint-Martin, 75141 Paris Cedex 03 (France)

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Scheme 1. Chemical structures of 3,4-ethylenedioxythiophene (EDOT) and poly(3,4-ethylenedioxythiophene) (PEDOT). The free a,a’ positions at which polymerization occurs is shown in the EDOT structure.

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CHEMPHYSCHEM ARTICLES strate, radiosynthesized PEDOT-containing films were shown to present optical and electrical properties close to those of homologous PEDOT materials we had already electrosynthesized.[6, 9, 24–26] If compared with the usual electrochemical methods, this new g-ray-based radiolytic method presents some advantages concerning the easy preparation of the samples, for which no supporting electrolyte is needed, and the ability to work under atmospheric conditions without purging solutions with an inert gas. Besides, contrarily to the electrochemical method, which leads to the direct deposition of conducting polymers onto conducting substrates, the radiolysis method leads to the formation of conducting polymers dispersed in water, which enables their further deposition over conducting and even nonconducting surfaces. In this work, we extend our original radiolytic methodology to the synthesis of PEDOT polymers in N2O-saturated aqueous solutions of EDOT. Under these conditions, and depending on the presence of sodium azide, two different oxidizing species are formed, namely, HOC (hydroxyl radical) and N3C(azide radical). Given the redox potentials of these two oxidative radicals (which are higher than that of EDOT monomers) and given that their well-known oxidation mechanisms are quiet different, one can expect the formation of two kinds of PEDOT polymers with tuned characteristics. Thus, the aim of this work is the study of the influence of the oxidation mechanism on the structure and properties of PEDOT polymers. As the role of the polymerization mechanism has been poorly investigated in the field of conducting materials, the present study is certainly important to optimize the general preparation of nanostructured conducting polymers and to enhance their physicochemical properties.

Materials and Methods Chemical Products 3,4-Ethylenedioxythiophene (EDOT, Scheme 1), used as the monomer, and sodium azide, NaN3, used to produce N3C radicals in irradiated N2O-saturated aqueous solutions, were both purchased from Sigma–Aldrich. Ultrapure water (Millipore system 18.2 MW cm) was used as the solvent. Dichloromethane (CH2Cl2, density = 1.3) and cyclohexane (C6H12, density = 0.8), both obtained from Sigma–Aldrich, were used as organic solvents for checking the hydrophilic properties of radiosynthesized PEDOT.

Solutions Preparation and Irradiation Aqueous solutions containing 1 or 10 mm EDOT were prepared in the presence of or in the absence of NaN3 (10 mm for 1 mm in EDOT or 100 mm for 10 mm in EDOT), under stirring, at room temperature and in the dark to prevent any photochemical reaction. The chosen EDOT concentration, which is lower than its solubility in water,[7] was always checked by UV/Vis absorption spectroscopy, as the spectrum of EDOT had earlier been determined [e235 = 5650 L mol1 cm1, e255 = (7048  200) L mol1 cm1].[16] The pH of the solutions measured before irradiation, in the presence of or in the absence of NaN3, was found to be close to 7. Note that no drastic change in the pH was observed after irradiation. All of the solutions were deaerated by bubbling with nitrous oxide, N2O, and then irradiated with increasing doses up to 70 kGy to radiosynthe 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org size the PEDOT polymers. Note that 1 Gy corresponds to 1 J L1. The g-ray dose rate was 1.4 kGy h1 in all cases. Irradiations were performed with a 60Co panoramic g-ray source of 3000 Curie at LCP laboratory of Paris-Sud University. Note also that due to the relatively low concentrations of EDOT and NaN3, no direct ionization could occur during radiolysis.

Radiolysis of N2O-Saturated Solutions Within the nanosecond timescale, g-irradiation of deoxygenated diluted aqueous solutions, at neutral pH, generates the following radiolytic species from the water solvent [Eq. (1)]:[21, 22, 27] H2 O ! HOC þ HC þ eaq  þ H3 Oþ þ H2 O2 þ H2

ð1Þ

Under these experimental conditions, the radiolytic yields for the formation of the different radical species are well known [Eqs. (2)– (4)]:[21, 22, 27, 28] GHO ¼ 2:8  107 mol J1 C

ð2Þ

Geaq ¼ 2:8  107 mol J1

ð3Þ

GH ¼ 0:6  107 mol J1 C

ð4Þ

In air at neutral pH, although HOC remains unchanged with the same radiolytic yield of formation, HC radicals and eaq (solvated electrons) are scavenged by molecular oxygen O2 to produce O2C superoxide radicals.[27] In aqueous solutions saturated with nitrous oxide (25 mm N2O), eaq are quantitatively and immediately converted into hydroxyl radicals according to the following reaction [Eq. (5)]:[21, 22] eaq  þ N2 O þ H2 O ! HOC þ HO þ N2

ð5Þ

As a consequence, upon irradiating N2O-saturated aqueous solutions at neutral pH, as in the case of our EDOT solutions, only two short-lived transient species are formed: HOC (hydroxyl radicals) and HC (hydrogen atoms); the radiolytic yield of formation of HOC then becomes [Eq. (6)]: GðHOC Þ ¼ GHO þ Geaq ¼ 5:6  107 mol J1 C

ð6Þ

Under these conditions, HOC radicals, which are known to be strong oxidizing species, constitute 90 % of the total amount of reactive radicals that should oxidize the EDOT molecules. This enables us to neglect the presence of hydrogen atoms, the reactivity of which is not really known. In aqueous solutions containing sodium azide (NaN3 at 10 or 100 mm, dissociated into Na + and N3 ions) and saturated by nitrous oxide, HOC radicals are very quickly scavenged by N3 ions; this leads to the quantitative formation of N3C azide radicals according to [Eq. (7)]:[29, 30] HOC þ N3  ! HO þ N3 C

ð7Þ

Under our experimental conditions in the presence of NaN3, the concentration of EDOT monomers was always ten times lower than that of the N3 ions, and the direct oxidation of EDOT by hydroxyl radicals was neglected. Thus, upon irradiating N2O-saturated aqueous solutions at neutral pH in the presence of NaN3, as in the case of our EDOT solutions, the radiolytic yield for the formation of ChemPhysChem 2014, 15, 208 – 218

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www.chemphyschem.org piloting program. The functioning pressure was 105 Torr (1 Torr = 133 Pa) and the implicated voltage was 20 kV.

N3C amounts to [Eq. (8)]: GðN3 C Þ ¼ GHO þ Geaq ¼ 5:6  107 mol J1 C

ð8Þ

Under these conditions, the N3C radicals, which are known to be soft and selective oxidizing species, constitute 90 % of the total amount of reactive radicals in the medium. Note that hydrogen atoms can also react with N3 ions to give NH2C radicals.[31] Nevertheless, the reactivity of these minority species, such as that of hydrogen atoms, has not been well established. This will allow us to neglect their presence. In summary, depending on the presence of NaN3 in the irradiated N2O-saturated aqueous solutions of EDOT, two different oxidizing species can be generated: HOC or N3C radicals, both of which were used to oxidize EDOT monomers into PEDOT polymers.

For melting points measurements, the aqueous solutions were centrifuged after irradiation for 2 h at 4000 rpm (Fischer Bioblock Scientific, Sigma). The obtained powders were washed with an excess amount of distilled water to remove any residual monomers. The melting point of the obtained dried solid phase was then measured. For attenuated total reflectance (ATR)-FTIR experiments, irradiated samples were centrifuged and then lyophilized. Freeze-dried samples as well as pure nonirradiated EDOT (for comparison) were subjected to ATR-FTIR spectroscopy by using a Bruker Vertex 70 FTIR spectrometer with a diamond ATR attachment (PIKE MIRACLE crystal plate diamond/ZnSe) and mercury cadmium telluride (MCT) detector with a liquid nitrogen cooling system. Scanning was conducted from 4000 to 400 cm1 with a 4 cm1 spectral resolution and with 100 repetitious scans averaged for each spectrum.

Characterization of Radiosynthesized PEDOT in Solution The UV/Vis absorption spectra of EDOT (before irradiation) and radiosynthesized PEDOT (after irradiation) solutions were recorded by using a HP 8543 spectrophotometer in quartz cells with an optical path length of 0.1 cm. The reference was always water. The different solutions were observed with transmission electron microscopy in a cryogenic environment (cryo-TEM) known to be adapted to low-density contrasts. A drop of each solution was deposited on “quantifoil” (Quantifoil Micro Tools GmbH, Germany) 200 mesh holey-carbon-coated grids. After blotting with filter paper, the grids were quench-frozen by rapid plunging into liquid ethane to form a thin ice film and thus avoiding water crystallization. The grids were then transferred into the microscope by using a side-entry Gatan 626 cryoholder cooled at 180 8C with liquid nitrogen. Images were recorder with an Ultrascan 2k CCD camera (Gatan, USA) by using a LaB6 JEOL JEM 2100 (JEOL, Japan) cryomicroscope operating at 200 kV with a low-dose system (Minimum Dose System, MDS) to protect the thin ice film from any irradiation before imaging and to reduce the irradiation during image capture. By freezing the system as is, cryo-TEM ensures the observation of soft nanoobjects in equilibrium in solution, because it avoids phase transition and possible PEDOT aggregation resulting from the drying procedures. The hydrophilic and hydrophobic properties of the radiosynthesized PEDOT polymers were checked by mixing the irradiated aqueous solutions with cyclohexane or dichloromethane, both of which are very less polar than water.

Characterization of Radiosynthesized PEDOT after Extraction/Deposition After irradiation, drops of the aqueous solutions were deposited and spread onto indium tin oxide (ITO) surfaces or onto gold substrates (Analytical Systems) and then dried at ambient temperature under atmospheric pressure. The drying procedure onto gold substrates assured, in all cases, the same surface coverage (200 mg cm2). Before deposition, all of the wafers were rinsed with distilled water and ethanol and then ultrasonically cleaned in distilled water. The UV/Vis absorption spectra of the layers deposited onto ITO were recorded by using a HP 8543 spectrophotometer. A nude, cleaned and dried ITO substrate was chosen as reference. The covered gold substrates were imaged by using scanning electron microscopy (SEM). The SEM observations were performed by using an EVO MA 10 ZEISS microscope supplied with SMRT SEM as  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

2. Results and Discussion In previous work, we demonstrated that g-irradiation (which produces both HOC and O2C radicals) of aerated aqueous solutions containing EDOT molecules led to the oxidation of these latter molecules by hydroxyl radicals and to the synthesis of PEDOT polymers.[16] Note that although HOC radicals oxidize EDOT monomers, O2C species remain unreactive. This was explained in terms of redox potentials. Indeed, in contrast to the O2C radical, HOC is a strong oxidizing species with a redox potential that amounts to E8’NHE (HOC/H2O) = 2.2 VNHE[27] at pH 7, (NHE = normal hydrogen electrode) which enables the oxidation of EDOT (the redox potential of which is about 1.4 VAg/AgCl[26]). We also previously demonstrated that the yield of EDOT monomers consumption, which is strictly equal to the yield of HOC formation during the irradiation of aerated aqueous solutions of EDOT, does not depend on the initial concentration of the monomer.[16] This demonstrates that PEDOT-radioinduced synthesis does not proceed through a chain reaction.[27] In fact, polymerization proceeds through a recurrent step-by-step mechanism that involves a repeated oxidation process by hydroxyl radicals formed during water radiolysis: HOC species react with EDOT monomers, then with dimers, then with oligomers… As a consequence, the quantitative synthesis of PEDOT polymers throughout such a step-by-step mechanism implies the use of two HOC radicals per EDOT molecule. This is consistent with the fact that, in a polymer, all the monomers apart from the terminal ones are bound to two neighbors in the a and a’ positions. Thus, the theoretical irradiation dose (Dmax) that should lead to the quantitative formation of PEDOT in aerated aqueous solutions is twice the dose necessary for the total oxidation of EDOT monomers and can be calculated as follows [Eq. (9)]:

Dmax ðGyÞ ¼

2½EDOT0 ðmolL1 Þ GHO ðmolJ1 Þ ChemPhysChem 2014, 15, 208 – 218

ð9Þ

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2.1. HOC-Induced EDOT Oxidation In the present work, we decided to extend our methodology to the irradiation of N2O-saturated aqueous solutions of EDOT in the presence of and in the absence of sodium azide. As already explained and justified, under N2O in the absence of NaN3, HOC radicals can be considered as the only oxidizing species, and their radiolytic yield of formation [G(HOC) = 5.6  107 mol J1] is higher than that obtained under aerated conditions (GHOC = 2.8  107 mol J1). In the absence of NaN3, the total irradiation dose (Dmax) that should enable the quantitative synthesis of PEDOT in N2O-saturated aqueous solutions is thus twice the dose necessary for the total oxidation of EDOT monomers and, as a consequence, can be calculated as previously explained, but by using G(HOC) instead of GHOC. In a N2O-saturated aqueous solution containing 1 and 10 mm EDOT, the dose that is necessary for the total oxidation of EDOT amounts to 1.8 and 18 kGy, respectively, and the dose Dmax that is necessary for the complete synthesis of PEDOT amounts to 3.6 and 36 kGy, respectively. In the absence of sodium azide, a N2O-saturated aqueous solution containing 1 mm EDOT was irradiated at increasing doses up to 4 kGy. The evolution of the UV/Vis absorption spectrum of this solution as a function of the irradiation dose, up to 2 kGy, displays a continuous decrease in the absorption of EDOT at 235 and 255 nm (Figure 1), which parallels an increase in the absorbance at both 290 and 350 nm. The presence of an isosbestic point indicates that a single process occurs up to 2 kGy, which implies EDOT oxidation by HOC radicals and the formation of a product that absorbs at both 290 and 350 nm. Given that thiophene dimers are known to absorb at both 246 and 301 nm,[32] the observed two absorption bands can be attributed to EDOT dimers. Indeed, auxo-

Figure 1. Absorption spectra of a N2O-saturated aqueous solution containing 1 mm EDOT irradiated in the absence of NaN3 with increasing doses: 0, 0.5, 1, 2, and 4 kGy. The optical path length was 0.1 cm. The reference was water. An isosbestic point is observed at lower doses.

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www.chemphyschem.org chromic substitution by the ethylenedioxy group in EDOT is responsible for the shift in the absorption maxima relative to the position of these bands in thiophene.[16] In the same way, auxochromic substitution could explain the difference in the absorption behavior between EDOT dimers and bithiophene molecules. At 2 kGy, the residual absorption at 235 and 255 nm (Figure 1) indicates the quasitotal disappearance of EDOT, the initial concentration of which was 1 mm. Now, 2 kGy is close to the dose (1.8 kGy) that enables the formation of 1 mm hydroxyl radical thanks to water radiolysis. We can then conclude that as long as EDOT monomers remain present in aqueous solution, hydroxyl radicals react exclusively with them. Yet, the oxidation potential of the dimers is always lower than that of the monomers, which enables their further oxidation,[33] in particular by hydroxyl radicals. Thus, the fact that hydroxyl radicals do not react with dimers at doses lower than 2 kGy could be explained by kinetic considerations: dimer oxidation should be very much slower than EDOT oxidation. At doses higher than 2 kGy, the spectral evolution appears different (Figure 1): the isosbestic point is no longer observed and a progressive shift in the absorption bands occurs. Given that EDOT is not present at these doses, such a spectral evolution indicates that hydroxyl radicals oxidize EDOT dimers, and this progressively leads to PEDOT oligomers. Such an interpretation is plausible, as it was shown that the wavelength values of the absorption maxima of a-thiophene oligomers increase with the number of thiophene units.[34] Whereas a concentration of 1 mm EDOT monomers was needed for the successful follow-up of their oxidation mechanism, 10 mm EDOT was necessary for the characterization of PEDOT polymers either in aqueous solution or after their deposition onto the substrate. In the absence of the sodium azide salt, N2O-saturated aqueous solutions containing 10 mm EDOT were irradiated at increasing doses up to 70 kGy. This latter dose is high enough to ensure the total polymerization of EDOT (at 10 mm). Figure 2 displays the evolution of the UV/Vis absorption spectrum of this solution as a function of the dose. We can note that, in the present case (10 mm in EDOT) and in contrast to the previous case (1 mm in EDOT), the disappearance of EDOT as well as the isosbestic point (observed in Figure 1) could not be observed due to the relatively high values of the molar extinction coefficients at 235 and 255 nm.[16] Nevertheless, as previously observed for 1 mm EDOT, at low irradiation doses, up to about 20 kGy (the dose that produces 10 mm hydroxyl radicals), two absorption bands appear at 290 and 350 nm (Figure 2), which demonstrates the formation of dimers. Above 20 kGy, no absorption band is observed around 250 nm. This clearly indicates that EDOT has completely disappeared. If the dose is higher than the total irradiation dose, as observed in the case of the 70 kGy irradiation in Figure 2, the absorption bands of the dimers disappear and an intense absorption band appears around 370 nm together with continuous scattering, the intensity of which increases with the irradiation dose. This should result from the lengthening of the EDOT oligomers. In fact, at doses higher than 36 kGy, the appearance of a continuous scattering comChemPhysChem 2014, 15, 208 – 218

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absorption spectrum of this solution as a function of the irradiation dose (results not shown) is similar to that obtained in the absence of the azide salt, even if the intensities of the spectra are somewhat different. Indeed, a continuous decrease in the absorption of EDOT parallels an increase in the absorbance at both 290 and 350 nm. Once again, the presence of an isosbestic point indicates that EDOT monomers are oxidized into dimers. This also proves that in addition to hydroxyl radicals, N3C species Figure 2. Absorption spectra of a N2O-saturated aqueous solution containing 10 mm EDOT irradiated in the abare also able to oxidize EDOT sence of NaN3 with increasing doses: 0, 1.4, 5.6, 14, and 70 kGy. The optical path length was 0.1 cm. The reference into dimers. In addition, as long was water. Inset) Image of an aqueous solution irradiated at 70 kGy after the addition of dichloromethane as as EDOT monomers remain presa denser organic phase (v/v): PEDOT-OH suspension remains in the upper aqueous phase. ent in aqueous solution, N3C reacts exclusively with them. The ponent, in the extinction spectrum of the irradiated solutions, further formation of EDOT oligomers and polymers could be is due to the formation of a brown-yellow suspension in the achieved only at higher doses. By analogy with the case of the bulk solution. This suspension becomes denser and the soluhydroxyl radicals, in the presence of NaN3, the total irradiation tion appears more turbid as the irradiation dose increases. dose (Dmax) that should enable the quantitative synthesis of PEDOT in N2O-saturated aqueous solutions is twice the dose After the deposition of this suspension onto the ITO substrate, necessary for the total oxidation of EDOT monomers and can the resulting film displays an intense absorption band at be calculated as previously explained, but by using G(N3C) in550 nm. As it will be further demonstrated, this indicates that the suspension is made up of polymers, denoted PEDOT-OH, stead of GHOC. In a N2O-saturated aqueous solution containing C formed at high doses thanks to the recurrent step-by-step HO 10 mm EDOT in the presence of the azide salt, the dose that oxidation process. enables the total oxidation of EDOT is 18 kGy, and the dose After irradiation at high doses (from 36 to 70 kGy), very slow Dmax that is necessary for the complete production of PEDOT is precipitation of the suspension was observed. This precipitathen about 36 kGy. tion corresponds to a sedimentation process of the PEDOT-OH In the presence of sodium azide, N2O-saturated aqueous solpolymers of relatively high molecular weights. This precipitautions containing a higher concentration of EDOT (10 mm) tion cannot be assigned to the hydrophobia of the radiosynwere irradiated at increasing doses up to 70 kGy. Figure 3 disthesized polymers. Indeed, after adding either cyclohexane or plays the evolution of the UV/Vis absorption spectrum as dichloromethane to the aqueous suspensions (v/v) and after a function of the dose. At low irradiation doses, up to about stirring and decantation, the polymer particles always remain 20 kGy (the dose that produces 10 mm azide radicals), two in the aqueous phase, whereas the organic phase appears colshoulders appear at 290 and 350 nm, which demonstrates the orless, as illustrated in the case of dichloromethane (Figure 2, formation of dimers. As the dose becomes higher than the inset). This result proves the hydrophilic properties of PEDOTtotal irradiation dose, as observed in the case of the 70 kGy irOH polymers. radiation in Figure 3, the absorption bands of the dimers disappear and a more intense absorption band appears at around 370 nm. Again, this should result from lengthening of the 2.2. N3C-Induced EDOT Oxidation EDOT oligomers. Moreover, a brown-colored suspension was observable within the bulk solution, which quickly deposited As previously explained, under N2O and in the presence of at the bottom of the solution. Note that the fast precipitation NaN3, HOC radicals are scavenged by N3 , which leads to N3C process, observed here, explains the absence of any scattering radicals that can then be considered as the only oxidizing specomponent in the extinction spectra of Figure 3. After the depcies able to react with EDOT. N3C is known as an alternative oxiosition of this suspension onto the ITO substrate, the resulting dizing system that is more selective than HOC. It is a soft, onefilm displays a broad absorption between 500 and 600 nm. As electron oxidant[35] with a redox potential E8’NHE (N3C/N3) = it will be further demonstrated, this indicates that the suspen1.33 VNHe at pH 7,[35–38] and it is used in this work for oxidizing sion is made up of polymers, denoted PEDOT-N3, formed at EDOT. In the presence of sodium azide, a N2O-saturated aqueous high doses thanks to the N3C-induced oxidation process. solution containing 1 mm EDOT was irradiated at increasing After irradiation at high doses (from 36 to 70 kGy), a precipidoses up to 4 kGy. At low doses, the evolution of the UV/Vis tate was systematically observed. This precipitation was due to  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemphyschem.org hydrophilic properties of the products formed in the presence of NaN3 lead to their deposition at the bottom of the solution and their adsorption onto the walls of the samples. 2.3. Chemical Characterization of PEDOT-OH and PEDOT-N3

The turbid solutions obtained after high-dose irradiation (36 and 70 kGy) in the absence of or in the presence of azide salt were centrifuged to recuperate a solid phase. The isolated brown powders were lyophilized Figure 3. Absorption spectra of a N2O-saturated aqueous solution containing 10 mm EDOT irradiated in the presence of NaN3 with increasing doses: 0, 1.4, 5.6, 14, and 70 kGy. The optical path length was 0.1 cm. The reference to eliminate any residual water was water. Inset) Image of an aqueous solution irradiated at 70 kGy after the addition of cyclohexane as a lessmolecules that could be trapped dense organic phase (v/v): PEDOT-N3 polymers remains at the interface between the two liquid phases. in the polymer-containing solid phase. The solid samples were a sedimentation process of the PEDOT-N3 polymers of relatively then characterized by ATR-FTIR spectroscopy to investigate the high molecular weight, but it was also the result of the poor chemical nature of the obtained polymers. hydrophilic properties of these systems. Indeed, after adding The ATR-FTIR spectra of the PEDOT-OH and PEDOT-N3 polyeither cyclohexane or dichloromethane to the aqueous suspenmers, radiosynthesized at 70 kGy and similar to those obtained sions (v/v) and after stirring and decantation, the polymer parat 36 kGy, are presented in the upper part of Figure 4 in the ticles always remained at the organic solvent–water interface, wavenumber region from 4000 to 700 cm1 together with the as illustrated in the case of cyclohexane (Figure 3, inset). This spectrum of pure nonirradiated EDOT (Figure 4, bottom). Beresult proves the amphiphilic properties of the PEDOT-N3 polytween 1600 and 700 cm1, the three obtained spectra are in mers. Note that at the interface, these polymers appear as clusgood agreement with those previously reported for PEDOT tered fibers. and EDOT.[16, 39–42] The vibrations at 1483, 1446, and 1360 cm1 One can note that for the same initial concentration of are attributed to C=C and CC stretching modes in the thioEDOT (10 mm), the spectral evolution in the presence of NaN3 phene ring, whereas those observed at 1280, 1236, 1160, and (Figure 3) is similar to that observed in the absence of this salt 1065 cm1 are assigned to the stretching modes of the ethyle(Figure 2). However, upon comparing both evolutions, one can nedioxy group (CC and COROC). The vibration modes note that for a given irradiation dose, the maximal absorption of the CS bond that are present in the thiophene ring can be is always lower in the presence of NaN3. Even if hydroxyl radiobserved at 940, 902, and 817 cm1. All of these bands are cals and azide radicals oxidize EDOT and induce polymerization present in all of the spectra in Figure 4 even if they appear relthrough the formation of EDOT dimers, it seems that the two atively slightly displaced. implied mechanisms are somewhat different. Maybe the kinetic The CH stretching band at 754 cm1 that is observed in the growth of dimers, oligomers, and polymers is slower in the spectrum of EDOT[43] is clearly absent in the ATR-FTIR spectra C case of N3 . This would induce a lower polymerization yield and lower values of the maximum absorbancies. However, such an interpretation is not in agreement with the observed dose effect. Besides, the products of EDOT oxidation could be chemically and structurally different depending on the nature of the N3C and HOC oxidizing radicals. This could explain the different spectral properties of the products obtained either in the presFigure 4. ATR-FTIR spectra of pure EDOT (bottom spectrum) and the PEDOT polymers radiosynthesized at 70 kGy ence of or in the absence of the under a N2O atmosphere both in the presence (PEDOT-N3) of and in the absence (PEDOT-OH) of NaN3, isolated azide salt. In addition, the poor after centrifugation and lyophilization steps.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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of both radiosynthesized PEDOT polymers (Figure 4). This demonstrates, without any ambiguity, that N3C- and HOC-induced EDOT polymerizations have both taken place thanks to a,a’coupling reactions and that the resulting solid powders obtained after centrifugation are effectively composed of PEDOT polymers. Nevertheless, as the powders were centrifuged before their characterization, the absence of the CH stretching band at 754 cm1 cannot prove the total consumption of EDOT after irradiation at 70 kGy. The PEDOT nature of the radiosynthesized polymers, which comprise the obtained solid phases, was also proven by their melting points, which were obtained after centrifugation and a washing procedure. Indeed, we found for both PEDOT-OH and PEDOT-N3 a melting point of 145 8C, which is very close to the reported melting point of PEDOT (146 8C).[44] This result demonstrates the high purity of our products and, thus, the efficiency of our procedure. Contrarily to the 4000–700 cm1 wavenumber region in which the spectra of PEDOT-OH and PEDOT-N3 are very much alike, significant differences can be observed above 2000 cm1. Indeed, the spectrum of PEDOT-OH exhibits a very large band between 3200 and 3600 cm1, whereas the spectrum of PEDOT-N3 is characterized by an absorption in two different infrared regions: between 2110 and 2065 cm1 and between 3565 and 3525 cm1. To understand these spectral differences and to propose a reasonable interpretation, we needed to have a look at the differences that exist between the chemical reactivity of the hydroxyl/azide radicals and the aromatic compounds, such as our EDOT monomers. In general, hydroxyl radicals add onto double bonds between two carbon atoms in aromatic molecules, whereas azide radicals lead to the formation of radical cations through electron abstraction, as demonstrated in the case of oxidized purines[45] and in the case of tryptamine.[46] Note that depending on the pH, the OH adducts may undergo a dehydration reaction to give the one-electron oxidized radicals.[45] Also, the reactivity of hydroxyl radicals towards thiophene molecules and of azide radicals towards pyrrole compounds was studied by pulsed radiolysis.[32] In agreement with all of the previous results, it was found that HOC radicals add preferentially to the a position in thiophene (Th) to give (Th OH)C adducts.[32] Differently, N3C radicals abstract electrons of pyrrole derivatives, which leads to radical cations.[35] In our case and according to the literature, EDOT monomers should then be oxidized by HOC and N3C radicals according to the two following pathways [Eqs. (10) and (11)]: EDOT þ HOC ! ðEDOTOHÞC

ð10Þ

EDOT þ N3 C ! EDOTþ C þ N3 

ð11Þ

The (EDOT-OH)C hydroxyl adduct as well as the EDOT + C radical cation subsequently undergo a second-order radical–radical reaction with themselves to produce EDOT dimers, as observed experimentally in this work and as reported in the case of both thiophene systems[32] and pyrrole derivatives.[35]  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

In the field of conducting polymers, experiments concerning the photodegradation of polythiophene-based polymers demonstrated in the same way that HOC radicals lead to hydroxyl adducts, whereas N3C species give cation radicals of the polymers.[47] Given that radiation-induced polymerization proceeds through a recurrent step-by-step oxidation mechanism, both HOC and N3C radicals should react successively on EDOT dimers and then on oligomers to produce continuously hydroxyl adducts in the case of HOC and radical cations in the case of N3C. Then, one can expect the exclusive presence of hydroxyl groups in the HOC-induced radiosynthesized PEDOT polymers. Contrarily to PEDOT-N3, PEDOT-OH should then contain alcohol functionalities. As observed in Figure 4, the spectrum of PEDOT-OH polymers exhibits a very large band between 3600 and 3200 cm1. Such a broad absorption in this spectral region is characteristic of OH bonds[43] and is traditionally observed in samples containing water molecules that are known to absorb between 3560 and 3520 cm1. However, as our samples were conscientiously freeze dried, the possible presence of water molecules must be ruled out. Besides, the absence of such a band in the spectrum of PEDOT-N3 validates our assumption. However, it is well known that free hydroxyl groups (OH) in alcohols and phenols intensively absorb between 3650 and 3584 cm1.[43] Nevertheless, in the presence of intermolecular H-bonds between the solutes, it was established that a very large band appears between 3550 and 3200 cm1 at the expense of the band of free hydroxyl groups.[43] Then, the presence of a band in this exact region of the FTIR spectrum in the case of PEDOTOH (Figure 4) first proves the presence of hydroxyl groups all along the polymers, as predicted from the growth mechanism and second demonstrates the existence of strong intermolecular interactions between the functionalized polymers through hydrogen bonds. The presence of hydroxyl groups also confers hydrophilic properties upon the PEDOT-OH polymers, and this explains their poor solubility into organic solvents as previously discussed. Contrarily to PEDOT-OH, PEDOT-N3 polymers, which result from N3C-induced EDOT oxidation, should not contain alcohol functionalities. This was proven by the absence of a large OH band in the ATR-FTIR spectrum of PEDOT-N3 (Figure 4). The absence of such a polar group in the PEDOT-N3 polymer impacts its solubility in water and explains its poor hydrophilic properties. After irradiation, for ATR-FTIR measurements, the samples were centrifuged and then freeze dried without any washing. As a consequence, in the case of PEDOT-N3 that had been synthesized in the presence of dissolved sodium azide, Na + and N3 ions remained in the samples and elimination of water molecules induced the precipitation of NaN3 as an ionic solid together with the solid polymers. In the spectrum of PEDOTN3, two bands are observed at 3565 and 3525 cm1 and two others are present at 2110 and 2065 cm1 (Figure 4). These bands are attributable to the presence of azide anions. Indeed, the azido group is generally characterized by an asymmetric stretching frequency in the 2160–2090 cm1 region, which often appears as a doublet.[43] In the solid state, due to the ChemPhysChem 2014, 15, 208 – 218

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CHEMPHYSCHEM ARTICLES presence of cations that perturb the internal vibrations of the N3 anions, azide salts are known to generally absorb in two different infrared regions: 3400–3150 and 2150–2025 cm1.[48] In particular, the spectrum of sodium azide displays an absorption band centered at 2106 cm1 in addition to two other bands[49] around 3500 cm1 in agreement with our results. This definitely confirms the presence of NaN3 in our solid sample, as expected from samples preparation. Note that functionalization of PEDOT-N3 polymers by azido groups should be ruled out, as azide radicals are known to only abstract electrons without any possible addition. We can note that in solution amine molecules also absorb between 3500 and 3300 cm1. In particular, the NH bonds of primary amines are characterized in this spectral region by two bands (a doublet) that are always narrower than the OH stretching bands.[43] These bands may sometimes be displaced to higher wavenumbers.[43] In addition, in the case of primary amines, these two peaks are associated to a shoulder that appears at lower wavenumbers and that corresponds to the angular deformation of the NH bond. Thus, the two bands observed in Figure 4 at 3565 and 3525 cm1 as well as the shoulder present around 3450 cm1 in the case of PEDOT-N3 in our sample could also be due to the presence of hydrazoic acid, HN3, which is the acidic form of the azide ion [pKa(HN3/ N3) = 4.6 in aqueous solution][50] and which contains NH bond. 2.4. Structural Characterization of PEDOT-OH and PEDOT-N3 Aqueous solutions containing 10 mm in EDOT and irradiated at 36 and 70 kGy in the absence of NaN3 were observed by cryo-

www.chemphyschem.org transmission electron microscopy just after irradiation and before any sedimentation or centrifugation. Representative images show the presence of low-density globular structures forming polydisperse spherical nanoparticles with a diameter between 100 and 300 nm as observed in Figure 5 (mean diameter of 200 nm). These observations are consistent with our previous results concerning the hydroxyl-induced radiosynthesis of PEDOT under an air atmosphere.[16] Furthermore, the same mean size value was found by using dynamic light scattering (results not shown). Indeed, a mean value of 100 nm was obtained for the hydrodynamic radius of the scatterers. Given that no other low-density objects were observed during our cryo-TEM experiments, we deduce that these spherical nanoparticles are made up of PEDOT-OH polymers. Note that the size and the shape of our polymer nanostructures are the same at doses of 36 and 70 kGy. This means that the irradiation dose has no noticeable influence on the structure of PEDOT-OH above the total irradiation dose (Dmax). Each observed nanoparticle has a complex structure and seems to be composed of interdigitated polymer chains (see in particular Figure 5 c). As no parasite a,b’ linkages could occur during polymerization, radiosynthesized PEDOT-OH nanostructures must be composed of linear chain polymers that are not branched or networked. Thus, each globular structure observed in Figure 5 should correspond to the self-assembly of independent amorphous PEDOT-OH chain polymers as schematically represented (Figure 5, right). The presence of hydroxyl groups, which was previously demonstrated, should explain not only the hydrosolubility of the PEDOT-OH polymers but also the as-observed packing and nanostructuration of the spherical supramolecular PEDOT-OH self-assemblies (Figure 5).

Figure 5. Cryo-TEM images of aqueous samples containing 10 mm EDOT irradiated at 36 (a and b) and 70 kGy (c and d) under a N2O atmosphere in the absence of NaN3. They exhibit spherical nanoobjects with a mean diameter of 200 nm attributed to self-assembled hydrophilic PEDOT-OH polymers. A schematic representation of a single spherical polymer nanoparticle highlights H-bonding interactions between the chains.

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CHEMPHYSCHEM ARTICLES Note that ethylenedioxy groups (H-bond acceptors) present all along the polymer chains could also be involved in H-bonds with the hydroxyl groups. Aqueous solutions containing 10 mm EDOT irradiated at 36 and 70 kGy in the presence of NaN3 were also observed by cryo-TEM just after irradiation and before any sedimentation or centrifugation. Instead of the compact spheroid surface morphology that was found in the case of PEDOT-OH, the obtained images clearly display nanoobjects forming fibrillar or lamellar (platelike) nanostructures, more or less folded, with a maximal fiber thickness of 10 nm and a length that can reach several microns (Figure 6). Given that no other low-density objects were observed during our cryo-TEM experiments, we deduce that these nanostructures are made up of PEDOTN3 polymers. Note that above the total irradiation dose (Dmax), no effect of the dose is noticeable on the size and the shape of the polymer nanostructures. As no parasite a,b’ linkages could occur during polymerization, the nanofibers are not branched or bonded to each other. Then, the nodes observed in Figure 6 correspond to the crossing between independent nanofibers. Due to the thickness of the fibers, each of them should correspond to a self-assembly of linear PEDOT-N3 polymer chains as schematically represented (Figure 6, right). Given that hydroxyl groups are not present in PEDOT-N3 polymers, the packing of these polymers cannot result from H-bonds. This self-assembling should come from p-stacking interactions between conjugated polymers (and aromatic monomers), which keep the polymer chains parallel to each other and which can maybe confer a lamellar structure to the polymers. This self-assembling could explain the macroscopic observation of PEDOT-N3 polymers as clustered fibers at the interface between water and an organic solvent. Also, the fact that PEDOT-N3 polymers can either inter-

www.chemphyschem.org act by van der Waals interactions (p-stacking interactions) or by hydrogen bonds, thanks to the presence of ethylenedioxy groups, could explain the amphiphilic properties of these polymers. To characterize the morphology of the polymers after a deposition procedure, drops of the aqueous solution containing the PEDOT-OH suspension obtained after irradiation at 70 kGy (polymer self-assemblies of Figure 5) were deposited onto a gold substrate and dried. The surface was then imaged by SEM. The images indicate the presence of very close-packed spheroid polymeric particles (Figure 7 a). These structures should come from the globular nanostructures already observed in aqueous solution by cryo-TEM (Figure 5). The particles observed by SEM after deposition are polydisperse in size with a diameter between 200 nm and 1 mm. Their size is then slightly bigger than that of the globular nanostructures observed by cryo-TEM in solution. The flattening of the particles upon deposition onto the substrate and their aggregation upon drying due to phase transition should explain the increase and the polydispersity in size observed in Figure 7 a. To characterize the morphology of PEDOT-N3 in the solid state, drops of the aqueous solution containing the suspension obtained after irradiation at 70 kGy (polymer self-assemblies of Figure 6) were deposited onto a gold substrate and then dried. The SEM images demonstrate the presence of close-packed lamellar (platelike) structures characterized by a maximal thickness of about 1 mm (Figure 7 b). These structures should come from the aggregation of the fibrillar nanostructures that had previously been observed in aqueous solution by cryo-TEM (Figure 6). We found that substantial differences exist between the structural properties of the PEDOT-OH and PEDOT-N3 polymers after deposition. It is generally admitted that the morphology

Figure 6. Cryo-TEM images of aqueous samples containing 10 mm EDOT irradiated at 36 (a and b) and 70 kGy (c and d) under a N2O atmosphere in the presence of NaN3. They exhibit fibrillar or lamellar (platelike) nanostructures attributed to self-assembled amphiphilic PEDOT-N3 polymers. A schematic representation of a single nanostructure highlights p-stacking interactions between the chains.

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www.chemphyschem.org droxyl pendant groups that induce H-bonding interactions, PEDOT-OH chain polymers cannot pack together regularly enough to form lamellar structures in the solid state. Differently, the chains coil, entangle, and randomly self-assemble into amorphous globular structures.

3. Conclusions

Figure 7. SEM images of PEDOT polymers after deposition onto gold substrates. Polymers were obtained after irradiation at 70 kGy: a) in the absence of NaN3 (PEDOT-OH) and b) in the presence of NaN3 (PEDOT-N3).

of polymers in the solid state results from the contributions of three macroconformations (Scheme 2):[51] a) random coils or irregularly folded molecules as found in the glassy state, b) folded chains, as found in lamellar structures, and c) extended chains. Fringed micelles (d) can be seen, in summary, as a mixture of the three previous conformations. Whereas the morphology of solid PEDOT-N3 polymers agrees with the folded-chain model found in lamellar structures, the morphology found for solid PEDOT-OH polymers is in good agreement with the random-coil model found in the amorphous state. In the solid state, as it was proposed in aqueous solution, PEDOTN3 lamellar structures should correspond to self-assemblies of organized PEDOT-N3 chain polymers that remain almost parallel owing to p-stacking interactions. Due to the presence of hy-

Scheme 2. Drawing of the different macroconformations in solid linear polymers. a) Random, glassy; b) folder chain, lamellar; c) extended chain; d) fringed micelle, mixture of a, b, and c (according to ref. [51]).

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We recently, and for the first time, used g-radiolysis as an original simple alternative way to synthesize conducting polymers in solution. In the present work, we successfully extended our original radiolytic methodology to the synthesis of PEDOT conjugated polymers in N2O-saturated aqueous solutions at ambient temperature and in the absence of any external oxidizing species. Starting from EDOT monomers and depending on the presence of sodium azide, two different oxidizing species were formed thanks to water radiolysis: HOC hydroxyl radicals and N3C azide radicals. Both species were shown to oxidize EDOT monomers into PEDOT polymers (called PEDOT-OH and PEDOT-N3). We demonstrated, thanks to ATR-FTIR experiments, that contrarily to PEDOT-N3, PEDOT-OH polymers contain OH functionalities. This noticeable difference between these two kinds of polymers was explained by the difference that exists between the well-known oxidizing behaviors of HOC and N3C : whereas HOC radical adds onto aromatic molecules to afford radical adducts, N3C azide radicals form radical cations through electron abstraction. As radioinduced PEDOT polymerization proceeds through a step-by-step mechanism that involves a recurrent oxidation process, as we already demonstrated, HOC radicals, in contrast to N3C species, lead to PEDOT-OH adducts. Whereas unfunctionalized PEDOT-N3 polymers are insoluble in water, PEDOT-OH polymers are highly soluble in aqueous solution thanks to the presence of hydroxyl groups. Much work has been conducted to modify the chemical structure of EDOT building blocks and thus to improve the solubility of the resulting functionalized PEDOT polymers.[1] Various soluble PEDOT derivatives bearing alkyl, oligooxyethylene, and alkylsulfonate chains were then synthesized with this aim.[52–57] In this work, without any prefunctionalization of the EDOT monomers, we succeeded in the synthesis of hydrophilic PEDOT polymers thanks to the use of hydroxyl radicals as oxidizing species. Then, we showed that the knowledge of the growth mechanism could easily help to tailor the hydrophilic properties of the polymers. Due to the presence of OH functionalities that enable Hbonding interactions, PEDOT-OH polymers self-assemble in aqueous solutions into globular nanostructures, as observed by cryo-TEM. Differently, unfunctionalized PEDOT-N3 polymers self-assemble in water, by p-stacking interactions, into fibrillar or lamellar nanostructures. Interestingly, the shapes of all of these structures remain unchanged after the deposition and drying procedure. Thus, the simple choice of the oxidizing species, which further determines the whole oxidation mechanism, has not only a clear influence on the chemical structure and the hydrophilic properties of synthesized polymers, but it ChemPhysChem 2014, 15, 208 – 218

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also plays a crucial role on the final morphology of the polymer nanostructures. Current research aims to develop new synthesis strategies and new conducting polymers with tuned morphologies and properties. However, the study of the influence of the polymerization mechanism remains poorly investigated in the field of conducting materials. We thus aim to make progress in this domain to determine a better way to optimize the preparation of nanostructured conducting polymers with adjusted morphologies and tuned properties. With this aim, we will study the influence of the nature of alternative oxidizing radiolytic species on the functionalities and the structural properties of radiosynthesized polymers. We will also study the influence of irradiation dose (concentration of oxidizing radicals and thus number of oxidizing steps) and the effect of dose rate (number of oxidizing species per unit of time and thus kinetics rate) on the conjugation length of the polymers. Finally, pulsed radiolysis studies coupled with structural and spectroscopic characterization are underway to identify the first steps of the polymerization mechanism and to better understand the growth process of PEDOT polymers in aqueous solution. Keywords: conducting polymers polymerization · radicals · radiolysis

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nanostructures

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Received: October 7, 2013 Revised: October 31, 2013 Published online on December 18, 2013

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