Conducting materials prepared by the oxidation of p ... - Springer Link

J Solid State Electrochem (2015) 19:2653–2664 DOI 10.1007/s10008-015-2838-3

ORIGINAL PAPER

Conducting materials prepared by the oxidation of p-phenylenediamine with p-benzoquinone Jaroslav Stejskal 1 & Miroslava Trchová 1 & Zuzana Morávková 1 & Patrycja Bober 1 & Michal Bláha 1 & Jiří Pfleger 1 & Przemysław Magdziarz 1 & Jan Prokeš 2 & Marek Havlicek 3 & Niyazi Serdar Sariciftci 3 & Andreas Sperlich 4 & Vladimir Dyakonov 4 & Zoran Zujovic 5

Received: 20 January 2015 / Revised: 23 March 2015 / Accepted: 26 March 2015 / Published online: 11 April 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract p-Phenylenediamine was oxidized with p-benzoquinone in the aqueous solutions of methanesulfonic acid (MSA). The conductivity of the products increased with increasing concentration of MSA from 1.5×10−12 S cm−1 in 0.1 M MSA up to 3.4×10−4 S cm−1 in 5 M MSA. The lowmolecular-weight products are basically composed of one pbenzoquinone and two p-phenylenediamine molecules. Their molecular structure is discussed on the basis of mass, Fouriertransform infrared, Raman, NMR and electron paramagnetic resonance (EPR) spectroscopies. The formation of 2,5-di(pphenylenediamine)-p-benzoquinone protonated with methanesulfonic acid best complies with the information provided by spectroscopic techniques. Its conversion to hydroquinone tautomer explains the formation of unpaired spins observed by EPR and their potential contribution to the conduction.

Keywords p-Benzoquinone . p-Phenylenediamine . Conducting material . Conductivity

* Jaroslav Stejskal [email protected] 1

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic

2

Faculty of Mathematics and Physics, Charles University in Prague, 182 00 Prague 8, Czech Republic

3

Linz Institute for Organic Solar Cells/Institute of Physical Chemistry, Johannes Kepler University of Linz, 4040 Linz, Austria

4

University of Würzburg and Bavarian Centre for Applied Energy Research (ZAE Bayern), 97074 Würzburg, Germany

5

Polymer Electronics Research Centre and NMR Centre, School of Chemical Sciences, The University of Auckland, Auckland 1142, New Zealand

Introduction Polyaniline ranks among the most studied conducting polymers due to its electrical conductivity, redox activity and responsivity to external stimuli [1]. It is also available in various morphologies, such as nanofibres, nanotubes and thin films [2]. When analysing possible reaction routes in the oxidation of aniline to polyaniline, the reaction between aniline and p-benzoquinone is of interest, because p-benzoquinone may be a by-product of aniline oxidation with currently used peroxydisulfate [3, 4]. From the application point of view, the oxidation products comprising amino and quinone moieties are attractive in corrosion protection of metals [5] as this combination of groups promotes the adhesion to metals [6–8]. Redox properties [9] and conductivity may further improve corrosion protection [10]. This applies both to low-molecular-weight compounds and polymer analogues, polyaminoquinones. Electrorheology is another field where materials of this type are applicable [11, 12]. A relatively low conductivity and high polarizability in electric field promote the organization of particles suspended in nonconducting carrier liquid. The increase in the viscosity after application of electric field is exploited in dampers, robotics and electromechanical devices. Polyaniline itself is able to interact with p-benzoquinone and improve the stability of supercapacitor electrodes [13]. The conductivity of polyaniline was improved by the introduction of non-conducting aniline oligomers containing quinone moieties, thus confirming a strong electronic interaction between both components [14]. For that reason, the systems comprising both the constitutional units derived from aniline and p-benzoquinone are of interest. The reaction between aniline and p-benzoquinone in aqueous acidic medium at room temperature, i.e. under the

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conditions used for the preparation of polyaniline, led to 2,5dianilino-p-benzoquinone (Fig. 1). Such process can be regarded as a substitution on a quinonoid ring, or as Michael addition followed by the oxidation of produced 2,5-dianilino-1,4-dihydroxybenzene to 2,5dianilino-p-benzoquinone. The latter product forms salts with strong acids. Such salts are conducting and, depending on the degree of protonation, the conductivity varied between 10−13– 10−4 S cm−1. They have been characterized by dielectric spectroscopy [15] and successfully applied in electrorheology [11]. The similar scenario can be also proposed for the reaction of p-benzoquinone with p-phenylenediamine. In such case, 2, 5-di(p-phenylenediamine)-p-benzoquinone would be the product (Fig. 2). The Michael addition has frequently been used in macromolecular chemistry for the preparation of polymers [5, 10]. The reaction between aromatic diamines, viz. substituted benzidine, and p-benzoquinone produced a polymer [10]. Analogous reaction between p-benzoquinone and pphenylenediamine leading to a polymeric product can also be proposed (Fig. 3). The reaction between terminal amino groups of aniline oligomers and p-benzoquinone produced by hydrolysis of oxidized p-phenylenediamine was similarly considered [16]. The polycondensation leading to polyaniline in pernigraniline oxidation state has also been considered in the literature (Fig. 4) [17]. Its feasibility was demonstrated on anthraquinones [18], where the Michael addition is not possible. The similar reaction between p-benzoquinone and ophenylenediamine in acetic acid was also associated with the elimination of water molecule and led to 2-hydroxyphenazine [19]. Such processes, however, are unlikely to proceed with pbenzoquinone in aqueous media [17]. For the sake of completeness, the oxidative polymerization of p-phenylenediamine using p-benzoquinone as oxidant has also to be considered. Poly(p-phenylenediamine) is typically prepared by the oxidation of the corresponding monomer with ammonium peroxydisulfate [20]. The replacement of peroxydisulfate with p-benzoquinone may still result in the polymerization of p-phenylenediamine. In practice, this means that the potential chain structure does not need to be strictly alternating (Fig. 3), but longer sequences of pphenylenediamine constitutional can be separated with individual p-benzoquinone units. The oxidations leading to conducting polymers invariably proceed in acidic media [3, 14], and the products are obtained Fig. 1 The oxidation of aniline with p-benzoquinone yields 2,5dianilino-p-benzoquinone. Hydroquinone is a by-product

as polymer salts with acids constituting the reaction medium. Also, in the case of p-benzoquinone unit incorporated in the polymer chain or in low-molecular-weight analogue, the rearrangement of keto-enol type allows for the formation of imine nitrogen atoms that become readily protonated with acid (Fig. 5), similarly like with polyaniline [2]. This type of material containing both nitrogen and oxygen moieties is applicable in corrosion protection due to good adhesion to iron [5, 8, 10]. The electrical properties enabling the potential applications in organic electronics, however, have not yet been addressed and are reported in the present communication.

Experimental Preparation p-Phenylenediamine (0.2 M, Sigma Aldrich) was oxidized with p-benzoquinone (0.5 M, Sigma Aldrich), at mole ratio [BzQ]/[PDA]=2.5 in aqueous solutions of methanesulfonic acid of various concentrations. This acid is well suited for the development of conducting systems as it does not precipitate silver ions that are often used for the enhancement of conductivity by incorporation of silver nanoparticles [21, 22]. After mixing the solutions of p-phenylenediamine and p-benzoquinone, the reaction mixture was left at room temperature for 24 h. The solids were isolated by filtration on paper filter, rinsed with corresponding acid solution, then with ethanol and dried in air at room temperature, and then over silica gel. Characterization Fourier-transform infrared (FTIR) spectra of the powders dispersed in potassium bromide pellets have been recorded with a Thermo Nicolet NEXUS 870 FTIR Spectrometer with a DTGS TEC detector in the 400–4000 cm−1 wave number region. Raman spectra of the powders were measured with a Renishaw InVia Reflex Raman microspectrometer. The spectra were excited with an Ar-ion 514 nm, a HeNe 633 nm or a diode 785 nm lasers. A research-grade Leica DM LM microscope with an objective magnification×50 was used to focus the laser beam on the sample placed on an X-Y motorized sample stage. The scattered light was analysed by the O

O

O

O


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O

O

O

O

Fig. 2 The oxidation of p-phenylenediamine with p-benzoquinone is expected to give 2,5-di(p-phenylenediamine)-p-benzoquinone. A part of p-benzoquinone is reduced to p-hydroquinone in this process

spectrograph with holographic gratings 2400, 1800 and 1200 lines mm–1 for corresponding lasers, respectively. A Peltiercooled CCD detector (576×384 pixels) registered the dispersed light. Gel permeation chromatography (GPC) analyses were performed on a Calc 100 (Labio, Czech Republic) chromatograph equipped with a PLgel mixed-C column (Polymer Laboratories, UK) using N-methylpyrrolidone containing 0.005 g cm−3 of lithium bromide (to prevent aggregation) as the eluent at the flow rate of 0.7 mL min−1. Samples were dissolved in mobile phase containing 0.005 g cm −3 of triethanolamine for sample deprotonation. The system was calibrated with polystyrene standards. Samples were detected spectrophotometrically at 340 nm. The conductivity of powders compressed with a manual hydraulic press at 540 MPa to pellets of 13 mm diameter and 1 mm thick was measured by a two-point method using a Keithley 6517 electrometer after deposition of gold electrodes (thickness 50 nm) on both sides of pellets. Samples of the same configuration were used for the broadband impedance spectroscopy measurements in the frequency range 6 mHz to 10 MHz using Alpha A high performance frequency analyser equipped with a ZGS Active Sample Holder and BDS1100 Cryostat (Novocontrol Technologies, Germany) in nitrogen environment. The samples were contacted using a spring loaded top contact to avoid changes in contact quality due to the volume changes during temperature cycling. Before the measurements, the samples were left to settle for about 60 min to equilibrate with the environment. The complex permittivity was calculated from the impedance and known geometry of the sample. Laser desorption/ionization (LDI) mass spectra were recorded with a New UltrafleXtreme mass spectrometer (Bruker). X-band electron paramagnetic resonance (EPR) spectra were recorded with a Bruker EMX X-band spectrometer equipped with TE102 cavity and Oxford 900 He continuous flow cryostat. Q-band EPR spectra were recorded with a Fig. 3 The oxidation of pphenylenediamine with p-benzoquinone may yield a polymer

home-modified Bruker E300 spectrometer equipped with a QTW 5106 cavity and Oxford CF935 He continuous flow cryostat. All measurements were done at room temperature under same conditions. Parameters were set as follows: modulation frequency 100 kHz in both cases. X-band attenuation was set to 30 dB (0.2 mW) and modulation amplitude was 1 G. Q-band attenuation was set to 8–50 dB (32 mW–2 μW) and modulation amplitude was 2 G. Special care was taken to detect and prevent any signal saturation and possible signal broadening due to extensive microwave power and high modulation amplitude. Solid-state NMR experiments were carried out using a Bruker Avance III spectrometer operating at a 1H Larmor frequency of 300.13 MHz, using a double-resonance 1H/13C 7 mm probe. Cross-polarization magic-angle spinning (CPMAS) experiments were carried out with 4000–30,000 scans at the ambient temperature using samples enclosed in the rotors. All spectra were obtained at a MAS frequency of 7000±1 Hz except for the spectrum of the sample with the 5.0 M methanesulfonic acid which was obtained at a 4000± 1 Hz rotation frequency. In all experiments, 90° pulse lengths of 4.2 μs (1H) and the contact time of 1.5 ms were used. Continuous wave decoupling was applied during an acquisition time. The recycle delay was 1.5 s and the spectral width was 40 kHz. The magic angle was adjusted by maximizing the sidebands of the 79Br signal of a KBr sample. Isotropic 13C chemical shifts were indirectly referenced to tetramethylsilane using the CH3 resonance of adamantane as a secondary reference (low field signal of adamantane, diso(13C)=38.48 ppm).

Results and discussion Yield The reaction between p-phenylenediamine and p-benzoquinone proceeds smoothly at room temperature. The mole ratio [BzQ]/[PDA]=2.5 was used, as typical for the oxidation O

O

O

O

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J Solid State Electrochem (2015) 19:2653–2664 O

Fig. 4 Hypothetical polycondensation of pphenylenediamine and p-benzoquinone

−

O

of p-phenylenediamine with one-electron inorganic oxidants [20]. The stoichiometry expected in the present case, [BzQ]/ [PDA]=1.5 (Fig. 2), suggests the formation of 1.48 g of 2,5di(p-phenylenediamine)-p-benzoquinone per 1 g of pphenylenediamine. This corresponds to 2.37 g if imine nitrogens were protonated with methanesulfonic acid (Fig. 5) or 3.37 g, if all four nitrogens participated in the formation of salt. The experimental yields generally vary within these limits (Fig. 6). The higher yields reflect the increasing degree of protonation of the product with acid.

Conductivity The conductivity of the samples is clearly dependent on the concentration of the methanesulfonic acid in the reaction medium (Fig. 6). It increases with increasing concentration of acid, as expected for increasing degree of protonation. The data can be fitted by a power law with coefficient 2.76× 10−7 S cm–1 mol–1 and exponent 5.1 mol–1. Similarly, like in O

the case of polyaniline, only a salt form is able to generate the polarons that are needed for the conduction. Molecular weights It is of crucial importance if the low-molecular-weight products (Fig. 2), i.e. oligomers, or a polymer (Fig. 3), are produced in the reaction. Gel permeation chromatography clearly rules out the presence of any polymer fraction (Fig. 7). The samples in this experiment are in base form. According to experimental setup, suitable for high molecular weight compounds, are the concrete molecular weights of a limited significance. In the present communication, GPC is used only to demonstrate that there is no polymer fraction. Laser desorption/ionization mass spectrum Mass spectroscopy identifies the main product to have a molecular weight 320 (Fig. 8), which corresponds to 2,5-di(pphenylenediamine)-p-benzoquinone (Fig. 2). The base form, i.e. the product with removed methanesulfonic acid, has been used in these experiments. FTIR spectroscopy

O

Infrared spectra of the samples obtained at mole ratio [BzQ]/ [PDA]=2.5 and prepared in 0.1 and 0.2 M methanesulfonic acid

Fig. 5 After Bketo-enol^ rearrangement, imine nitrogen atoms are protonated with an acid (HA) to produce a corresponding salt

Fig. 6 The yield (mass of the product per 1 g of p-phenylenediamine) and conductivity of the samples prepared at p-benzoquinone/pphenylenediamine mole ratio, [BzQ]/[PDA]=2.5, in the solutions of methanesulfonic acid of various molar concentrations, [MSA]. The conductivity follows a power law as shown by the fit to the data


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[MSA] = 0.1

Mw = 2 200; D = 1.8

0.2

Mw = 3 000; D = 2.3

0.5

Mw = 1 900; D = 2.4

1

Mw = 1 700; D = 2.0

2

Mw = 1 300; D = 1.9

5 2

Mw = 1 000; D = 1.5

3

4

5

6

log M Fig. 7 Molecular weight distributions obtained by gel permeation chromatography for samples prepared at p-benzoquinone/pphenylenediamine mole ratio [BzQ]/[PDA] = 2.5 at various molar concentrations of methanesulfonic acid, [MSA]. The traces are shifted for clarity. Weight-average molecular weights, Mw, and dispersity, D= Mw/Mn, are given at the individual curves

(MSA) are close to the products of oxidation of pphenylenediamine with other oxidants, such as ammonium peroxydisulfate or silver nitrate [23] (Fig. 9). We can distinguish a broad maximum at 3410 cm−1 and a sharp peak at 3223 cm−1 in the high wave number region of the spectra. The last sharp peak is also observed in the spectrum of the product of reaction between aniline and p-benzoquinone in aqueous acidic medium at room temperature, 2,5-dianilino-p-benzoquinone (Fig. 1) [4]. These peaks correspond to the hydrogen-bonded N–H stretching vibrations, most probably with C=O group. The presence of hydroquinone is possible. The peaks situated at 1573, 1510/1468, 1348 and 1284 cm–1 in the spectra of the products are present also in the spectrum of 2,5-dianilino-p-benzoquinone. The peaks situated at 1573 and 1510/1468 cm–1 belong to the various quinonoid and benzenoid ring-stretching vibrations. The peak situated at 1348 cm−1 corresponds to C–N

2

Raman spectroscopy Raman spectra of the samples obtained at mole ratio [BzQ]/ [PDA]=2.5 when the concentration of MSA increases are shown in Fig. 10. The Raman spectra recorded with three different laser excitation wavelengths (514, 633 and 785 nm) contain the peaks practically at the same positions; only their relative intensities are different due to resonance effect. They are close to the products of oxidation of p-phenylenediamine with other oxidants, such as ammonium peroxydisulfate [26] or silver nitrate [23]. We can distinguish two broad maxima at about 1600 cm–1 (C=C and C~C stretching of quinonoid and semi-quinonoid rings, respectively, and ‘~’ denotes the bond intermediate between the single and double bond) and 1530 cm–1 (C=N stretching of quinonoid segments, ν(C= N)Q, mixed with N−H and/or N+ −H in-plane bending), and small peak at 1415 cm–1 (cross-linked structures) (Fig. 10b). A broad band with maxima at 1324 cm–1 (C~N+• stretching vibrations), 1248 cm–1 (C−N stretching in benzenoid units) and 1184 cm–1 (C–H bending on benzenoid or quinonoid ring vibrations) is present in the spectra obtained with 514-nm excitation line (Fig. 10a). Weak peaks at 838, 755, 640, 567, 500, 453 and 413 cm–1 are connected with skeletal deformations [26]. The peaks at 1352 and 575 cm–1 may be connected with phenazine-like or cross-linked structures [23]. They are well distinguished with 785-nm excitation line (Fig. 10c). We can conclude that vibrations expected for 2,5-di(pphenylenediamine)-p-benzoquinone are detected in the Raman spectra. Electron paramagnetic resonance spectroscopy

408

1

305

Intensity, a.u.

320

3

bonding of variously substituted aromatic ring. The presence of the C–N stretching vibrations of a primary aromatic amine is connected with the sharp peak at 1284 cm−1 [15, 24, 25]. Besides these peaks, we detect the peaks of methane sulfonate ions at about 1206, 1038, 760 and 551 cm–1, which are also present in the spectrum of ammonium methane sulfonate. In the spectra of samples prepared with higher content of MSA, we observe a sharp peak at about 1630 cm−1 of N–H scissoring vibrations of aromatic amines with the contribution of benzoquinones, which is expected in the spectrum of 2,5-di(pphenylenediamine)-p-benzoquinone (Fig. 2).

0

0

200

400

600

m/z Fig. 8 LDI mass spectrum of the sample prepared at [BzQ]/[PDA]=2.5 in 5 M MSA and converted to a base form

This method can provide a valuable insight into the conduction mechanism of investigated materials [27–29]. It has been frequently used to investigate the nature of conductivity of conducting polymers, charge-carrier separation in bulkheterojunction-based solar cells, doping effects on conductivity upon, e.g., iodine doping, and the investigation of various quasi-particles with the spin ½. In the present contribution, 0.2 M p-phenylenediamine oxidized with 0.5 M p-benzoquinone in aqueous media of

2658 [BzQ]/[PDA] = 2.5 3410

FTIR in KBr

3223

[MSA] = 0.1

Absorbance

Fig. 9 FTIR spectra of the samples prepared at pbenzoquinone/pphenylenediamine mole ratio [BzQ]/[PDA]=2.5 at various concentrations of MSA


1510 1468 1148 1284 1573 1415 1206 551 1630 1348 1038 818760

0.2 3030

871

0.5 1

2876

2 2646

5

4000

3500

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Wavenumbers, cm

various acidity has been studied in X-band and Q-band EPR. All investigated samples exhibit excellent stability in air. No obvious qualitative or quantitative differences in the EPR spectra have been observed after one-month storage in air at room temperature. The EPR spectra correspond to the first derivative of the absorption signal due to the use of magnetic field modulation, which enables lock-in detection of the EPR signal. Figure 11 shows a X-band EPR spectrum of a sample prepared with 0.1 M MSA. The spectra measured for samples prepared with different MSA concentrations show almost identical absorption peaks. For further analysis, EPR spectra were fitted with the first derivative of the Lorentzian line in the form of ðB−B0 Þ=w Y ¼ y0 þ A 2 1 þ ðB−B0 =wÞ2

ð1Þ

where A, B0, w and y0 are the signal amplitude, resonance field of the main peak, peak-to-peak linewidth and baseline offset, respectively. Samples prepared with low MSA concentration exhibit nearly perfect fits at the resonance magnetic field B0, and high accuracy at the inflection points, which enables an accurate estimation of the g-factor to g=2.0043 and peak-topeak linewidth of≈1 mT. The g-factor value is close to that of the free electron (2.0023), which is common for electrons delocalized within the π-system of conjugated organic molecules. The small changes of the linewidth in dependence of MSA concentration are shown in the inset of Fig. 11. This is in line with the signal origin being mostly unaffected by remaining MSA. As the concentration of MSA increases, additional poorly resolved shoulders appear in the spectrum. In order to investigate the origin

of these additional features, the sample with the highest MSA concentration (5 M) and the most pronounced shoulders was analysed in the Q-band in more detail. Figure 12 shows X-band and Q-band spectra for the same sample prepared with 5 M MSA. The spectra are shifted from their respective resonance magnetic field B0 to overlap at B=0 mT. They exhibit the same linewidth, despite being recorded at different microwave frequencies (9.4 vs. 34 GHz), which is usually due to an isotropic g-factor. In that case, the linewidth is then defined by (unresolved) hyperfine and dipolar interactions instead of an anisotropic g-tensor or a g-strain mechanism. This is also evident when plotting the spectra over the gfactor as shown in the inset to Fig. 12. The spectrum is wider for X-band than for Q-band because the linewidth is not caused by g-factor anisotropy. For the Q-band sample, several spectra with different microwave power attenuation (8–50 dB) were recorded that differ only by signal amplitude and are almost identical to the one shown in Fig. 12 after normalization. The comparison of EPR spectra recorded at different microwave power settings can often be used to determine whether there is more than one overlapping signal contribution or spin centre involved with the observed EPR signature. Different spin centres typically show different saturation behaviour and can thus be discerned by comparing EPR spectra recorded at various microwave powers. The observed invariance of the normalized spectra suggests that there is only one spin centre involved. Additionally, the signal exhibits only very little saturation for high microwave powers, which is caused by rapid spin-lattice relaxation times. This is unusual for spins, localized on conjugated organic systems, but is in line with the presumed hyperfine or dipolar interactions as a relaxation channel.


a

1597

Absorbance

Raman, exc. 514 nm

[Bzq]/[PDA] = 2.5 [MSA] =

1642

1324 1530 1248 1495 1184 1350 1450 1166 1420 1125

838 755 822 870

640

500 413 520 453 567

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0.5

490

1 2 1038

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b

Absorbance

Raman, exc. 633 nm

[BzQ]/[PDA] = 2.5 [MSA] = 0.1

0.2

1530 1545 1600 1513 1370 1350 1252 1415 1328 1238 1642 1445 1300 1184 1166 1140 1045 1654

494 415 522 450 575

0.5 1 2 5

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−1

800

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400

Wavenumbers, cm

c

Raman, exc. 785 nm

[BzQ]/[PDA] = 2.5 [MSA] =

Absorbance

Fig. 10 Raman spectra of the samples prepared at pbenzoquinone/pphenylenediamine mole ratio, [BzQ]/[PDA]=2.5, in the solutions of methanesulfonic acid of various molar concentrations, [MSA]. Laser excitation: a 514, b 633 or c 785 nm

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1422 1530 1410 1600 1510 1352 1630 1450 1328 1197 1680 1235 1172

0.1 805 712

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412 510 455

0.5 1 2 5 873

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From EPR, we can draw the following conclusions: The observed EPR signature at g=2.0043 is caused by unpaired electrons within [BzQ]/[PDA] and unresolved hyperfine interactions are responsible for the broad linewidth of ≈1 mT. The various unequal nitrogen atoms in the conjugated molecule are most likely involved. At high MSA concentration (5 M), a salt may be formed that gives rise to additional shoulders due to dipolar interaction. Solid-state NMR spectroscopy 13

Fig. 11 X-band EPR spectrum (solid line) of [BzQ]/[PDA] with 0.1 M MSA together with a Lorentz fit (dotted line). Inset: fitted X-band EPR signal linewidth versus molar concentration of MSA

The shoulders observed at approximately ±2 mT (Fig. 12) with a separation of ΔB=4.1±0.2 mT can be explained by a dipolar interaction D(r)=ΔB/2. This can be due to a salt, which is produced when the MSA concentration increases. In this case, the salt is distributed within the sample volume and forms randomly oriented crystalline structures with a defined anion-cation distance. Due to the direct analytical relation [30] of dipolar coupling D and interaction radius, sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2:785 3 ð2Þ rab ðDÞ½nm ¼ DðrÞ½mT we can estimate this distance to rab =1.1±0.02 nm. If we consider this as a Wigner-Seitz radius, we arrive at an approximate spin concentration of n=(1.8±0.01)×1020 cm–3.

C CPMAS (cross-polarization magic-angle spinning) NMR spectra of samples prepared at the mole ratio [BzQ]/[PDA]= 2.5 using the methanesulfonic acid of various concentrations are shown in Fig. 13. The assignment of resonance peaks in the 13C CPMAS spectrum of the sample obtained with 0.1 M MSA (Fig. 13) is based on data reported elsewhere [31–34]. Site assignments correspond to those given in Fig. 14. The most deshielded peak at 180.2 ppm can be attributed to C= O group (carbon C-1) from p-benzoquinone [31, 32]. The peaks at 96.7 and 149.5 ppm can be assigned to carbons C-3 and C-2, respectively [31]. The peaks at 126.1 and 118.2 ppm can be attributed to carbons C-5 and C-6 while the shoulder at 138.2 and the peak at 134.8 ppm can be assigned to carbons C-4 and C-7 [31]. These carbons belong to a part originating from p-phenylenediamine. The peak at 40.8 ppm (C-8) can be attributed to CH3 group from MSA. This implies the presence of the acid in the final product. An overall shape of the spectrum and a consequent assignment indicate the prevalence of trimer, 2,5-di(p-phenylenediamine)-p-benzoquinone (Fig. 14). However, due to lower spectral resolution, the presence of hydroquinone to some extent (possible peaks at ~148 and in the spectral region from 115 to 119 ppm) [33, 35] cannot be excluded. NMR Spectral evolution

Fig. 12 X-band (solid line) and Q-band (dashed line) EPR spectra of the [BzQ]/[PDA] with 5 M MSA. The spectra are shifted to 0 mT for better comparability. Dashed arrows point to the shoulders on both sides of the spectrum with separation of 4.1 mT. Inset: the same spectra plotted versus the g-factor

The main spectral features (a presence/absence of certain resonances, line shapes, line widths and chemical shifts) are clearly dependent on the acid concentration (Fig. 13). Three peaks centred at ~149 ppm and a relative increase in intensity of the peak at ~118 ppm in a spectrum of the sample obtained with 0.2 M MSA imply the rising presence of hydroquinone units [35, 36]. A further increase in acid concentrations (0.5 and 1 M) results in significantly simplified spectra, which closely resemble a spectrum of the mixture of two polymorphic forms of hydroquinone [33, 35]. It is interesting to note the missing peak at ~180 ppm (C=O) from a p-benzoquinone part and the very low intensity of the CH3 peak (~40 ppm) in both spectra. With an additional increase of MSA concentration (2 M), intensity of the CH3 peak significantly increases (indicates the greater presence of MSA) and three new peaks show up: 158.1, 131.1 and 126.3 ppm. This could be due to


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obtained with 5 M acid was used for NMR measurements. Paramagnetic centres, i.e. polarons, may extensively broaden linewidths of certain resonances beyond the detection limits and significantly change the spectral information. At this point, further work is needed to unambiguously assign the peaks in the spectrum of the sample obtained with 5 M acid. In summary, it seems that there exists a complex interplay between various structures (which includes p-benzoquinones and hydroquinones) at different levels of acidity in the reaction solution. This type of samples can be quite complicated and could result in complex NMR spectra in the solid state. This is because of a possible presence of various complexes, polymorphs, positive charges/polarons and strong directional forces, e.g. hydrogen-bonded solids and salts. For instance, only the crystal structure effect can introduce significantly large chemical shift differences (3–4 ppm) [35]. On the other hand, CPMAS experiments are very sensitive to the presence of polarons because of paramagnetic effects and shortening of T1ρH relaxation [38]. This could slow down or even block a magnetization transfer to 13C nuclei (especially to nonprotonated ones) which may ultimately prevent their detection. Impedance spectroscopy The results of the impedance spectroscopy measured on pellets with two MSA concentrations in the temperature range from 20 to 110 °C are shown in Fig. 15. Here we present the impedance spectra as an imaginary part of the electrical modulus, M 00 ¼ Fig. 13 13C CPMAS NMR spectra of the samples obtained with different molar concentrations (0.1–5 M) of the methanesulfonic acid (MSA). Sidebands are denoted with asterisks

the rearrangement of a keto-enol type and the formation of imine nitrogens (the peak at 158.1 ppm [37]). A further increase in MSA concentration up to 5 M results in a very simple spectrum that consists of three peaks at 131.1, 125.8 and 40.6 ppm. Due to the presence of charges, it was difficult to tune the probe and only a very small amount of the sample

Fig. 14 Chemical structures of 2,5-di(p-phenylenediamine)-pbenzoquinone and methanesulfonic acid

ε00 ε0 2 þ ε00 2

ð3Þ

This representation of the electrical relaxation processes is particularly advantageous for conductive materials since it suppresses the contribution of the electric conductivity and of the electrode polarization effects making the relaxation processes more visible. The spectra for low MSA concentration (below 0.5 M) are characterized with a single relaxation peak, position of which shifts towards higher frequency with increasing temperature and increasing MSA concentration. The peak is asymmetric; the low

2662


Fig. 15 Representative plots of the dependences of the imaginary part of the modulus on frequency and temperature for two concentrations of MSA: a 0.2 M and b 2 M

frequency side follows the power law ωα with the exponent α= 1, which corresponds to the dependence of the imaginary part of dielectric constant ε″~ω−1 and could be assigned to the frequency dependence of the conductivity. At higher MSA concentrations, the relaxation peak (more than 0.5 M) was shifted out of frequency window of our observations. Only the increase in M″ scaled with frequency as ωα was seen. In order to explain the impedance characteristics, a rather complex composition has to be considered. There could be several contributions to the overall impedance: contribution of mobile polarons or ions, dipolar relaxation that stems from the anion-cation pairs or interfacial polarization originated on in the inhomogeneity of the samples. In order to obtain the characteristic relaxation times τ, the frequency dependencies were fitted using a single Havriliak-Negami function: ΔM ½1 þ ðiωτ Þα

β

ð4Þ

where ΔM and M∞ mean the strength of the relaxation process and high frequency limit of the electrical modulus, respectively, and parameters α and β represent the width and asymmetry of the relaxation peak, respectively. The dependences of the characteristic relaxation times on reciprocal temperatures are shown in Fig. 16 as an Arrhenius plot, giving the activation energies 1.3 and 0.78 eV for the samples with MSA content of 0.1 and 0.2 M, respectively. It can be seen that the relaxation time saturates at higher temperature. However, this saturation could be only apparent since there were irreversible changes observed in both the M″ and τ values when the sample was left at temperature close to 100 °C, probably due to the desorption of trapped water

10-1

Relaxation time, τ (s)

M* ¼ M∞ þ

molecules. No distinguished relaxations were observed for samples with higher MSA content within the frequency and temperature window of our experiment. Similar shape of the relaxation peaks together with the decrease of the characteristic relaxation time was observed with increase of the conductive filler fraction in the conducting polymer composites [39] and assigned to the Maxwell-Wagner relaxation. It shows that our samples are not doped homogeneously. Contrary to the recently published results on aniline-based oligomers with MSA [15], there was no clear percolation threshold observed either in the DC and low frequency ε″ or the relaxation time. Instead, both the relaxation time and conductivity increase continuously with the increased MSA concentration. It suggests that although the sample is not well completely homogeneous, both domains contribute to the overall conductivity.

10-2

Ea = 1.3 eV

10-3

Ea = 0.78 eV

10-4

10-5

10-6

0.0026

0.0028

0.0030

0.0032

0.0034

Reciprocal temperature, 1/T (K-1)

Fig. 16 Dependences of the relaxation times on reciprocal temperature for samples with MSA concentration of 0.1 M (square) and 0.2 M (circle)


2663 2.

3. 4.

5.

6.

7.

8.

Fig. 17 2,5-Di(p-phenylenediamine)-1,4-benzoquinone tetramethanesulfonate. The redistribution of the electrons within the molecule may produce electrons with unpaired spins. A− stands for the methanesulfonate counter-ion, CH3SO3−

9. 10. 11.

Conclusions 12.

The reaction between p-phenylenediamine and p-benzoquinone at strongly acidic media constituted by 5 M methanesulfonic acid yields a material with conductivity as high as 3.4× 10−4 S cm−1. Gel permeation chromatography suggests and laser desorption mass spectroscopy confirms that the product is not a polymer but a ‘mixed trimer’, 2,5-di(p-phenylenediamine)-p-benzoquinone (Fig. 2). Its primary amino groups form a salt with methanesulfonic acid, 2,5-di(p-phenylenediamine)-p-benzoquinone methanesulfonate. This is supported by the analysis of FTIR, Raman and NMR spectra. The by-product of the reaction, hydroquinone, was detected in NMR spectra. In addition, its tautomer form, 2,5-di(p-phenylenediamine)1,4-hydroquinone, can be protonated both on primary amino groups and imine ones (Fig. 17). Such molecular structure allows for the redistribution of electrons, and consequent generation of unpaired spins that are detected by EPR.

13.

14.

15.

16.

17.

18.

19. Acknowledgments The authors thank the Czech Science Foundation (P205/12/0911) and the Czech–Austrian mobility project (7AMB14AT005) for financial support. LDI mass spectra has kindly been provided by Z. Walterová from IMC in Prague.

References 1.

Ćirić-Marjanović G (2013) Recent advances in polyaniline research: Polymerization mechanisms, structural aspects, properties and applications. Synth Met 177:1–47

20. 21.

22. 23.

Stejskal J, Sapurina I, Trchová M (2010) Polyaniline nanostructures and the role of aniline oligomers in their formation. Prog Polym Sci 35:1420–1481 Stejskal J, Trchová M (2012) Aniline oligomers versus polyaniline. Polym Int 61:240–251 Stejskal J, Bober P, Trchová M, Horský J, Pilař J, Walterová Z (2014) The oxidation of aniline with p-benzoquinone and its impact on the preparation of a conducting polymer, polyaniline. Synth Met 192:66–73 Mather BD, Viswanathan K, Miller KM, Long TE (2006) Michael addition reactions in macromolecular design for emerging technologies. Prog Polym Sci 31:487–531 Phani KLN, Pitchumani S, Muralidharan S, Ravichandran S, Iyer SVK (1993) Electrosynthesis of polyamino-benzoquinone (PAQ) polymers. J Electroanal Chem 353:315–322 Muralidharan S, Phani KLN, Pitchumani S, Ravichandran S, Iyer SVK (1995) Polyamino-benzoquinone polymers: a new class of corrosion inhibitors for mild steel. J Electrochem Soc 142:1478– 1483 Vaccaro E, Simone CD, Scola DA (2000) Amino-p-benzoquinone adducts and polymers as adhesion promoters for steel. J Adhes 72: 157–176 Grgur BN (2014) On the role of aniline oligomers on the corrosion protection of mild steel. Synth Met 187:57–60 Va c c a r o E , S c o l a D A ( 1 9 9 9 ) N e w a p p l i c a t i o n s o f polyaminoquinones. Chemtech 29:15–23 Mrlik M, Sedlacik M, Pavlinek V, Bober P, Trchová M, Stejskal J, Saha P (2013) Electrorheology of aniline oligomers. Colloid Polym Sci 291:2079–2086 Mrlik M, Pavlinek V, Alajdalawi S, Saha P, Bober P, Stejskal J (2013) Electrorheology of aniline-oligomer suspensions under oscillatory shear. J Phys Conf Ser 412:012007 (1–6) Vonlanthen D, Lazarev P, See KA, Wudl F, Heeger AJ (2014) A stable polyaniline-benzoquinone-hydroquinone supercapacitor. Adv Mater 26:5095–5100 Dennany L, Innis PC, McGovern ST, Wallace GG, Forster RJ (2014) Electronic interactions within composites of polyanilines formed under acidic and alkaline conditions. Conductivity, ESR, Raman, UV-vis and fluorescence studies. Phys Chem Chem Phys 13:3303–3310 Mrlik M, Moucka R, Ilčíková M, Bober P, Kazantseva N, Špitálský Z, Trchová M, Stejskal J (2014) Electrical transport in aniline-based oligomers. Synth Met 192:37–42 Durgaryan AA, Arakelyan RA, Durgaryan NA (2014) Oxidative polymerization of p-phenylenediamine. Russ J Gen Chem 84: 1095–1100 Hall HK Jr, Padias AB, Boone HW (2007) Condensation routes to polyaniline and its analogs. J Polym Sci A Polym Chem 45:4751– 4763 Evaerts A, Roberts S, Hall HK Jr (1986) Synthesis and properties of semiconducting aromatic polyquinonediimines. J Polym Sci A Polym Chem 24:1703–1716 Kour H, Paul S, Singh PP, Gupta R (2014) A mild and simple method for the synthesis of substituted phenazines. Synlett 25: 495–500 Stejskal J (2015) Polymers of phenylenediamines. Prog Polym Sci 41:1–31 Bober P, Stejskal J, Trchová M, Prokeš J (2011) Polyaniline–silver composites prepared by the oxidation of aniline with mixed oxidants, silver nitrate and ammonium peroxydisulfate: the control of silver content. Polymer 52:5947–5952 Stejskal J (2013) Conducting polymer–silver composites. Chem Pap 67:814–848 Ćirić-Marjanović G, Marjanović B, Bober P, Rozlívková Z, Stejskal J, Trchová M, Prokeš J (2011) The oxidative polymerization of pphenylenediamine with silver nitrate: Toward highly conducting

2664 micro/nanostructured silver/conjugated polymer composites. J Polym Sci Polym Chem 49:3387–3403 24. Silva CHB, Ferreira DC, Ando RA, Temperini MLA (2012) Aniline-1,4-benzoquinone as a model system for the characterization of products from aniline oligomerization in low acidic media. Chem Phys Lett 551:130–133 25. Ferreira DC, Pires JR, Temperini MLA (2011) Spectroscopic characterization of oligoaniline microspheres obtained by an aniline– persulfate approach. J Phys Chem B 115:1368–1375 26. Plachy T, Sedlacik M, Pavlinek V, Morávková Z, Hajná M, Stejskal J (2013) An effect of carbonization on the electrorheology of poly(p-phenylenediamine). Carbon 63:187–195 27. Mizoguchi K, Nechtschein M, Travers JP (1991) Spin dynamics and conductivity in polyaniline: temperature dependence. Synth Met 41–43:113–116 28. Sariciftci NS, Heeger AJ, Cao Y (1994) Paramagnetic susceptibility of highly conducting polyaniline: Disordered metal with weak electronelectron interactions (Fermi glass). Phys Rev B 49:5988–5992 29. Dyakonov V, Frankevich E (1998) On the role played by polaron pairs in photophysical processes in semiconducting polymers. Chem Phys 227:203–217 30. Weil JA, Bolton JR (2008) Electron paramagnetic resonance. Elementary theory and practical applications, 2nd edn. WileyInterscience, New York 31. Lotina-Hennsen B, Achnine L, Ruvalcaba NM, Ortiz A, Hernandez J, Farfan N, Aguilar-Martınez M (1998) 2,5-Diamino-p-benzoquinone derivatives as photosystem I electron acceptors: synthesis and

J Solid State Electrochem (2015) 19:2653–2664 electrochemical and physicochemical properties. J Agric Food Chem 46:724–730 32. Joseph-Nathan P, Abramo-Bruno D, Ortega DA (1981) Carbon NMR studies of benzoquinones. Org Magn Reson 15:311–316 33. Scheffer JR, Wong YF, Patil AO, Curtin DY, Paul IC (1985) CPMA S 13CNMR spectra of quinones, hydroquinones, and their complexes. Use of CMR to follow a reaction in the solid state. J Am Chem Soc 107:4898–4904 34. Kolbert AC, Caldarelli S, Their KF, Sariciftci NS, Cao Y, Heeger AJ (1995) NMR evidence for the metallic nature of highly conducting polyaniline. Phys Rev B 51:1541–1545 35. Reepmester J (1980) Application of solid state 13C NMR to the study of polymorphs, clathrates and complexes. Chem Phys Lett 74:536–538 36. López C, Claramunt RM, Elguero J (2008) Oxalic acid/phenols and oxalic acid/cholesterol co-crystals: a solid state 13C CPMAS NMR study ARKIVOC (iv): 33–46 37. Kříž J, Starovoytova L, Trchová M, Konyushenko EN, Stejskal J (2009) NMR investigation of aniline oligomers produced in the early stages of oxidative polymerization of aniline. J Phys Chem B 113:6666–6673 38. Zujovic ZD, Bowmaker GA, Nieuwoudt M, Kilmartin PA (2013) Nanostructures obtained in the oxidative polymerization of aniline: effects of polarons. Polymer 54:6363–6372 39. Panda M, Srivinas V, Thakur AK (2014) Universal microstructure and conductivity relaxation of polymer-conductor composites across the percolation threshold. Curr Appl Phys 14:1596–1606