Polyaniline Emeraldine Salt in the Amorphous Solid State: Polaron

Article pubs.acs.org/JPCB

Polyaniline Emeraldine Salt in the Amorphous Solid State: Polaron versus Bipolaron Manel Canales,*,† Juan Torras,*,‡ Georgina Fabregat,§,∥ Alvaro Meneguzzi,§,⊥ and Carlos Alemán*,§,∥ †

Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, Campus Nord-Edifici B4−B5, Jordi Girona 1-3, Barcelona E-08034, Spain ‡ Department of Chemical Engineering, Escola d’Enginyeria de Igualada (EEI), Universitat Politècnica de Catalunya, Av. Pla de la Massa 8, Igualada 08700, Spain § Department of Chemical Engineering, Barcelona School of Industrial Engineering, Universitat Politècnica de Catalunya, Diagonal 647, Barcelona E-08028, Spain ∥ Centre for Research in Nano-Engineering, Universitat Politècnica de Catalunya, Campus Sud, Edifici C′, C/Pasqual i Vila s/n, Barcelona E-08028, Spain ⊥ Departamento de Materiais, Escola de Engenharia, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves, 9500, 91509-900, Porto Alegre, RS Brazil S Supporting Information *

ABSTRACT: The polaronic and bipolaronic forms of polyaniline emeraldine salt (PAni-ES) in the amorphous solid state have been simulated using classical molecular dynamics (MD) and hybrid quantum mechanical/molecular mechanical-molecular dynamics (QM/MM-MD) approaches. It should be remarked that the electronic state of PAni-ES has been theoretically investigated in the gas phase, solution phase, and crystalline state, but this is the first study in the amorphous solid state, which is the most typical for this conducting polymer. MD simulations were carried out using force-field parametrizations explicitly developed for polaronic and bipolaronic models. QM/MM-MD calculations were performed using a quantum mechanical zone defined by four repeat units. In addition of the structural and electronic characteristics of the two forms of PAni-ES, MD and QM/MM-MD simulations indicate that the bipolaronic is the most stable state of amorphous PAniES. Complementary studies have been carried out using different experimental techniques. Although the morphology and topography of doped and undoped PAni are very similar, comparison of their UV−vis spectra supports the preference toward the bipolaronic form of PAni-ES.

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INTRODUCTION Among conducting polymers, polyaniline (PAni) is under a high expansion on its technological use. This phenomenon should be mainly attributed to its excellent properties (e.g., high chemical and environmental stabilities, electrical conductivity, and easy processability) and low cost.1,2 The potential applications of PAni go from biosensors,3 capacitors,4 microelectronics, and up to optical displays.2 As synthesized, PAni consists of alternating reduced (amine) and oxidized (imine) repeat units, as known as the emeraldine base (EB) form (Scheme 1a). In the presence of an acid dopant, the half oxidized PAni-EB becomes protonated and transform into the conductive emeraldine salt (ES) form, which can present a polaronic (Scheme 1b) or bipolaronic (Scheme 1c) structure. Thus, protonation of PAni-EB provokes a considerable increase of the conductivity, up to 10 orders of magnitude, without change in the number of chain electrons.5,6 In a seminal work, Stafström et al.7 proposed that the protonation of PAni-EB with HCl leads to a spinless structure (bipolaron) that finally rearranges into a polaronic structure. However, the polaron/bipolaron structure and the nature of the © 2014 American Chemical Society

conducting state of PAni-EB have been the subject of controversy during the last two decades, the whole process of conduction being not well understood.8,9 Although most of experimental information date back to the 80s and 90s and they are not very clear in this regard,10 some studies suggest the greater stability of polaronic structures,11,12 while other cases show that the most stable structure is bipolaronic.13,14 More recently, it has been reported that the predominant structure of PAni-ES obtained by doping depends on the preparation procedure.15,16 On the other hand, theoretical studies based on first principle calculations, which are more prevalent in the past decade, have tried to bring some light on the electronic structure of PAni-ES but, as occurs for experimental studies, there are disparities on the final conclusions.17 Early studies on this issue were addressed by means of semiempirical18−21 approaches, while more recently, ab initio computational techniques have also Received: July 7, 2014 Revised: September 5, 2014 Published: September 19, 2014 11552

dx.doi.org/10.1021/jp5067583 | J. Phys. Chem. B 2014, 118, 11552−11562

The Journal of Physical Chemistry B

Article

PAni-ES, which is reached in the first stages of the acidic doping of PAni-EB, rapidly evolves toward the polaronic form through the almost flat potential energy surface connecting the two states. More recently, bipolaronic defects in oligomers of PAni-ES were studied using different computational approaches.33 Authors concluded that the HF method does not describe adequately the protonation in PAni-ES, while the B3LYP approximation fits the experimental data. Based on these results, the authors conducted a study using oligomers of different lengths, the bipolaronic forms being energetically more favored than the polaronic ones within the B3LYP/631G*/PCM framewok (where PCM refers to the Polarizable Continuun Model used to consider solvent effects).17 As mentioned above, theoretical studies on the relative stability of the polaron and bipolaron states of PAni-ES were performed mainly by considering oligomers in vacuum or applying PBC to the crystal structure. However, Epstein and co-workers showed that PAni is essentially amorphous, with diffuse halos in the X-ray diffraction pattern, even though a fraction of crystallinity of up 0.5 can be obtained when the polymer is washed with solvents.34−36 In recent years, computer simulation methods based on atomistic molecular mechanics and coarse-graining potentials have been used to get detailed information about structure and properties of conducting conjugated polymers.37−40 Lee et al. used multiscale simulations based on molecular dynamics (MD) to examine both single chain and aggregation properties of PAni-EB.37 More recently, we used atomistic MD simulation to investigate the amorphous structure of PAni-EB,38 as well as the absorption of water in this half-oxidized polymeric matrix.39,40 In this work, we investigate the molecular organization of both polaronic and bipolaronic PAni-ES in the amorphous phase using a multiscale approach based on atomistic MD simulations with classical potentials and hybrid quantum mechanical/molecular mechanical-molecular dynamics (QM/ MM-MD) approximations. Although the main aim of this work is to determine and to characterize the two main defects in amorphous PAni-ES, additional information have been obtained by considering different charge distributions on the repeat units during the simulations. Results have been compared with experimental data obtained by UV−vis spectroscopy and cyclic voltammetry (CV). The article is organized as follows. In the next section, we present both the molecular models studied in this work and the generation of reliable starting microstructures of amorphous PAni-ES. Next, the force fields proposed for the different forms of PAni-ES are described and, subsequently, tested by comparing the calculated

Scheme 1. (a) PAni-EB and (b) polaronic and (c) bipolaronic forms of PAni-ES

been applied.22−25 Most of these studies examined the electronic structure of oligomers in the gas phase, even though no unanimous consensus about polaron−bipolaron predominance was reached.17 On the other hand, additional studies using periodical boundary conditions (PBC) were performed at different levels of approximation. Cavazzoni et al. used Car− Parrinello molecular dynamics (CP-MD) to study the structural, electronic, and optical properties of 1D PAni-EB and PAni-ES,26 as well as of 3D crystalline Pani-EB.27 Later, CP-MD was used to study the 3D crystal structure of PAni-ES, the bipolaronic form being proposed as the most stable form of PAni-ES.28 The same results were obtained by Varela-Á lvarez et al.,29 who applied density functional theory (DFT) methods to 1D PBC models of PAni-ES. In opposition to previous studies, more recently Cavazzoni et al.9 used the same CP-MD methodology to conclude that the polaronic is the most stable form of crystalline PAni-ES, the bipolaronic form being unfavored by about 11 kcal/mol. Indeed, optical properties calculated for the polaronic form of PAni-ES were found to be in good agreement with available experimental data. 30 According to these features, Varela-Á lvarez et al.31 reformulated the conduction mechanism early proposed by Stafström et al.7 by introducing a new thermodynamic equilibrium between bipolaronic and polaronic forms, the most stable structure in this equilibrium being the trication single-radical polaronic lattice by only 0.3 kcal/mol. The possible coexistence of the two forms of PAni-ES was also explored by Cavazzoni et al.32 using CP-MD. They concluded that the bipolaronic form of

Scheme 2. Repeat Units and Atom Types of (a) P1 and (b) B1 Models

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all models in order to impose ring planarity in the way that all carbon atoms and the exocyclic nitrogen atom were located in the same plane, while hydrogen atoms were allowed to move out of the plane. Electrostatic parameters were obtained by fitting the quantum mechanical molecular electrostatic potential, which was computed at the HF/6-31G(d) level, to the Coulomb electrostatic potential through a Levenberg− Marquardt nonlinear optimization procedure. Figures S1 and S2, Supporting Information, show the electrostatic parameters obtained for the three developed force-fields. Moreover, the Lennard-Jones chloride ion parameters (σ = 4.0447 Å and ε = 0.6276 kJ/mol) were obtained by Patra et al.50 Molecular Dynamics Simulation. Identical simulation protocols have been used to model the different states of amorphous PAni-ES. More specifically, using as a starting point the microstructures generated for the models P1 and B1, several MD runs in the NPT ensemble were performed (temperature 298 K and 1 atm of pressure). All classical MD simulations were carried out using the GROMACS 4.5 program.51−53 The integration step was set to 1 fs. Atom pair distance cutoffs were applied at 10.0 Å to compute the van der Waals interactions. Electrostatic interactions were extensively computed by means of the Ewald summations approach. The real space term was defined by using the same cutoff applied to compute van der Waals interactions, while the reciprocal space was computed by interpolation into an infinite grid of points (Particle Mesh Ewald).54 Previous to the production MD trajectory, an equilibration protocol was followed. Initially, 10 4 steps of energy minimization were carried out to relax conformational and structural tensions. Next, 20 ns of MD simulation in a NPT ensemble was used to equilibrate the molecular chains in the simulation box. Thus, the weak coupling method of Berendsen thermo-barostat55 was used to control the temperature and pressure of the system, the time constant for the heat bath coupling, and the pressure relaxation time being of 1 ps. Finally, production NPT-MD trajectories were obtained for 30 ns, coordinates being saved at 1 ps intervals for analyses. Multiscale Approach (Hybrid QM/MM-MD Calculations). Although torsional relaxation of PAni-ES chains embedded in the amorphous environment is satisfactorily considered in classical MD simulations, charge distribution is kept fixed. Furthermore, bond lengths and bond angles remain no far away from the equilibrium point assigned by the force field during the whole trajectory, the coupling of these geometric parameters and the charge distribution being omitted. In order to take into account the latter aspect, the coupling between charge and geometry relaxations in amorphous PAni-ES was specifically examined by applying a multiscale approach (hybrid QM/MM-MD methodology). Thus, explicit inclusion of the environment to study the effect of the bulk in the electronic structure of amorphous PAni-ES was considered. In this methodology, atomic motions are handled by MD, energies and forces being calculated by dividing the system into two different regions. The QM region is constituted by a short PAni-ES oligomer made of four repeat units, while a classical potential energy function is applied to the rest of the system. Thus, the molecular mechanics (MM) region corresponds to the amorphous embedding material. All hybrid QM/MM-MD simulations were run using a Gaussian03-PUPIL-DL_POLY scheme of interface between QM and MD programs. Thus, classical MD calculations were performed on DL_POLY Classic44 program, while QM

density with the experimental value. After this, structural and electronic properties of amorphous PAni-ES have been evaluated using classical MD and hybrid QM/MM-MD simulations. In order to carry out the latter multiscale simulations, an update of the electrostatic embedding scheme on the interface Gaussian0326-PUPIL27-DL_POLY28 was built. Finally, the properties of PAni-EB and PAni-ES have been compared and discussed. The conclusions are outlined in the last section.

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METHODS Molecular Models Description. Two different molecular models, each one with four chains of identical molecular weight, have been considered for PAni-ES. The first one corresponds to the complete polaronic form (hereafter, P1) with 50 repeat units per chain, each repeat unit involving two benzene rings (Scheme 2a). The whole charge of the P1 model is neutralized with 200 chloride ions. The second model describes the bipolaronic form (hereafter, B1), each chain containing 25 repeat units of charge +2 and four benzene rings (Scheme 2b). As for P1, the system neutrality is achieved by a total of 200 chloride ions. In all cases chains were blocked with an amino group (−NH2) at one end and an additional hydrogen atom on the −NH moiety at the other end. Atomic microstructures of the two main models of amorphous PAni-ES under study, which have been used as starting points of MD simulations, were produced using the SuSi (Structure Simulation) computer strategy.45 This method was recently developed to generate and relax atomistic microstructures of amorphous polycyclic materials. The method is based on a previously reported procedure for study of linear amorphous polymers,46,47 which consists of a generation algorithm that eliminates the torsion strain and a simple Monte Carlo Metropolis method to relax nonbonding interactions. Force-Field Parametrization. Scheme 2 shows the labels used to identify the atom types of the repeat units for each model of amorphous PAni-ES, that is, P1 (Scheme 2a) and B1 (Scheme 2b). Labeling of atoms in Scheme 2 has been carried out according to the following nomenclature: [atom type][ring number][position number]. It is worth noting that, for computational efficiency reasons, the C6H4 rings were represented using a pseudoatom model for the C−H groups (united atom approximation). Two different sets of parameters were derived, FF-P1 and FF-B1, which correspond to the polaronic (P1) and bipolaronic (B1) forms of PAni-ES. In all cases, the GROMOS 53A6 force-field parametrization48 was modified to describe the different forms of amorphous PAniES. All force constants and Lennard-Jones pair potential parameters were taken from GROMOS 53A6 libraries, whereas equilibrium bond lengths (l0), bond angles (θ0), and atomic point charges were specifically derived in this work. Equilibrium geometric parameters were extrapolated from quantum mechanical calculations on model compounds emulating the two studied forms of PAni-ES. Figures S1 and S2 display the equilibrium geometry parameters for FF-P1 and FF-B1, respectively. These parameters were taken from the molecular geometries of PAni-ES model oligomers that were minimized using the ab initio Hartree-Fock (HF) method combined with the 6-31G(d) basis set49 (i.e., HF/6-31G(d) level). Torsional parameters were extracted from previous quantum mechanical results24 and listed in Table S1 of the Supporting Information. Improper torsion angles were used in 11554



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calculations were carried out using the Gaussian 0341 program. The PUPIL43,56 package (Program for User Package Interfacing and Linking) was responsible for conduct and manage the whole QM/MM-MD simulation between both programs. In this work the PUPIL-DL_POLY side of the interface was updated in order to include the electrostatic interaction between QM and MM regions by means of the electrostatic embedding approach.57 The initial version of the PUPILDL_POLY interface was programmed implementing mechanical embedding only.42 Accordingly, QM calculations, which were carried out in vacuum, were coupled classically to MM via nonbonding interactions using van der Waals and point charges at the QM nuclear sites. In this work the electrostatic embedding has been reimplemented in the PUPIL-DL_POLY interface by including the electrostatic contribution to QM Hamiltonian generated by the MM atoms and the reaction force over the surrounding point charges. The initial structures selected for Gaussian03-PUPILDL_POLY calculations on amorphous PAni-ES were extracted from equilibrated MD simulations of models P1 and B1. In order to run hybrid QM/MM-MD simulations, these structures were converted into all-atom systems by adding hydrogen atoms to the united-atom pseudoatoms used to describe the benzene rings at the classical level. The influence of an amorphous environment is an important part of this work. However, this kind of modelization has to deal with the heterogeneity problem than can be easily solved by statistical average when classical MD is used, but is not so easy when QM/MM MD approach is used, instead. The latter suffers the well-known sampling problem that makes difficult to obtain a sufficient number of configurations to average a general behavior, especially by their high computational cost by averaging among a large number of quantum zones. In spite of that, the interest in the study of an amorphous environment influence at the quantum level instead of the classical MD leads to propose a simple study of general tendencies because of the environment. Thus, a short oligomer made of four aromatic rings was treated at the QM level for both P1 and B1, which correspond to the polaronic and bipolaronic forms of PAni-ES, respectively, while the rest of the system was treated classically. On the boundaries between regions, the chemical bond crossing between QM and MM regions was treated by means of the link-pair atom approach. All models were described by using the general Amber force field (GAFF),58 whereas the DL_POLY program was used for all classical calculations.44 MD input files were built by means of the DL_FIELD application.59 Initially, the starting structures were equilibrated using classical MD. For this purpose, after energy minimization, the two structures were subjected to NVT-MD equilibration at 298 K. Subsequently, four repeat units were changed to a QM description for the treatment at the B3LYP/6-31+G* level, whereas the rest of the amorphous system remained within the MM framework. After that, the whole QM/MM systems were allowed to relax for 0.5 ps with a production run of 1.5 ps (4000 steps, 0.5 fs time step) in the NVT ensemble at 298 K and using a Nosé−Hoover thermostat. Periodic boundary conditions were applied in the preparation of the Gaussian03 input so as to wrap neighboring point charges around the quantum region. The QM region comprised a total of 576 basis functions and 48 atoms.

Article

EXPERIMENTAL SECTION

Synthesis. PAni-ES was prepared according to the classical procedure, with amounts adjusted to allow the polymerization in a 5 L capacity double-walled reactor under controlled temperature and stirring. A solution consisting of oxidizing agent [(NH4)2S2O8] (0.4 mol·L−1) in HCl (1.5 mol·L−1) was added slowly under constant stirring at −5 °C to a second HCl solution (1.5 mol·L−1) containing aniline (0.4 mol·L−1). The green powder, PAni-ES with density of 1.421 g·cm−3, was filtered and exhaustively rinsed with distilled water in order to eliminate the excess of HCl and finally dried in an oven at 60 °C for 24 h. PAni-EB, a dark blue powder was obtained after treatment of PAni-ES with a 0.5 mol·L−1 NH4OH aqueous solution. The resulting emulsion was maintained at pH = 10 and under stirring for 6 h, the solid being finally dried in an oven at 60 °C for 24 h. Scanning Electron Microscopy (SEM). SEM studies were performed to examine the effect of the oxidation on the surface morphology. Dried samples of PAni-ES and PAni-EB were placed in a Focused Ion Beam Zeiss Neon 40 scanning electron microscope operating at 5 kV, equipped with an EDX spectroscopy system. Samples were mounted on a doublesided adhesive carbon disc. Atomic Force Microscopy (AFM). Topographic AFM images were obtained with a Molecular Imaging PicoSPM using a NanoScope IV controller under ambient conditions. The tapping mode AFM was operated at constant deflection. The row scanning frequency was set to 1 Hz and the physical tip− sample motion speed was 10 μm·s−1. The root-mean-square roughness (Rq) was determined using the statistical application of the Nanoscope software, which calculates the average considering all the values recorded in the topographic image with exception of the maximum and the minimum. AFM measurements were performed on various parts of the powder, which produced reproducible images similar to those displayed in this work. The scan window sizes used in this work were 5 × 5 μm2. UV−Vis Spectroscopy. UV−vis absorption spectra of PAni-ES and PAni-ES films were registered using a UV−visNIR Shimadzu 3600 spectrophotometer equipped with a tungsten halogen visible source, a deuterium arc UV source, a photomultiplier tube UV−vis detector, and an InGaAs photodiode and cooled PbS photocell NIR detectors. Spectra were recorded in the absorbance mode using the integrating sphere accessory (model ISR-3100), the wavelength range being 185−3300 nm. The interior of the integrating sphere was coated with highly diffuse BaSO4 reflectance standard. Uncoated ITO glass was used as reference. Samples were deposited onto ITO-glass electrodes for measurements. Single-scan spectra were recorded at a scan speed of 60 nm/ min. Measurements, data collection, and data evaluation were controlled by the computer software UVProbe version 2.31. Cyclic Voltammetry (CV). Electrochemical assays were performed with an Autolab PGSTAT302N (Ecochimie, The Netherlands) potentiostat/galvanostat equipped with ECD module for measuring very low current densities (100 μA− 100 pA) as well as GPES and FRA software. All assays were carried out using a conventional three-electrode system. Platinum wires, which were arranged circularly forming rings (diameter: 2 mm), were used as working and counter electrodes. PAni-ES and PAni-EB samples, shaped as pills, 11555



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Cl− ions is better defined in the profile obtained for the bipolaronic form, which a marked peak at the position of 7 Å, than for the polaronic one. More specifically from Figure 1, since bipolaronic structures present two adjacent protonated imine moieties (see Scheme 2b), the expected distance between two Cl− ions should be around 8.3 Å that is relatively close to the main peak position observed on the B1 profile. Figure 2 displays the rdf of H···Cl pairs, gH−Cl(r), for the P1 and B1 models of PAni-ES. Both profiles show peaks at around

were put inside the platinum rings for assays. The reference electrode was an Ag|AgCl electrode containing a KCl saturated aqueous solution (E0 = 0.222 V vs standard hydrogen electrode at 25 °C), which was connected to the working compartment through a salt bridge containing the electrolyte solution. All CV assays were performed using a 0.1 M HCl electrolytic solution. PAni samples were immersed in these solutions 1 min before registering the assays.

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RESULTS AND DISCUSSION Polaron/Bipolaron Comparison, a MD Study. NPT-MD simulations were carried out using the FF-P1 and FF-B1 forcefields of amorphous PAni-ES. Table 1 lists the averaged values Table 1. Averaged Density (ρ) and Energy (E) Obtained for the Bipolaronic (B1) and Polaronic (P1) Forms of Amorphous PAni-ES from Classical MD Simulations

a

model

ρ (g/cm3)

E (kJ/mol)

B1 P1 expa

1.324 ± 0.003 1.279 ± 0.003 1.329 ± 0.027

−116530 ± 34 −94199 ± 34

Ref 41.

of the density and the energy derived from 30 ns simulations of each model. As it can be seen, the density determined experimentally for PAni hydrochloride (ρ = 1.329 ± 0.027 g/ cm3)60 is perfectly reproduced by the B1 model. This suggests a predominance of the bipolaronic form for the amorphous material. In order to get more information about the origin of the differences between the two models, more deep analyses of the chemical structure have been carried out, special emphasis being placed in the role played by the chloride counterions. The three-dimensional organization of the molecular chains in amorphous PAni-ES has been examined by calculating the radial distribution functions (rdf) between atomic species. Figure 1 displays the rdf of Cl···Cl pairs, gCl−Cl(r), for the P1 and B1 models of PAni-ES. As can be seen, the distribution of

Figure 2. Radial distribution functions of H···Cl pairs, gH−Cl(r), calculated for B1 (black solid line) and P1 (black dotted line) models of amorphous PAni-ES.

1.9, 5.5, and 7.5 Å, which can be correlated with the expected distances between adjacent repeat units (scheme in Figure 2). Besides the common peaks on both profiles located at 1.9 and 7.5 Å, we want to notice about the peak located at 5.5 Å. This peak reflects the main differences between bipolaronic and polaronic forms of PAni-ES, being slightly more pronounced in the bipolaronic distribution. This has been attributed to the interchain distance around the chloride ion, which is closer for the bipolaronic form than for the polaronic one, leading to an increase in the final density of the B1 model, as being observed in Table 1. In order to reinforce the main differences between bipolaronic and polaronic forms, Figure 3 represents the rdf involving N···Cl pairs (scheme in Figure 3), gN−Cl(r), for the P1 and B1 forms of PAni-ES. Similarly to previous gH−Cl(r) profiles, the main peaks located at around 3.1 and 7 Å have been attributed to interaction with nitrogen belonging to different chains. More specifically, the sharp and narrow peak at 3.1 Å corresponds to the chloride ions and the closest nitrogen atom (i.e., that located at the repeat unit just in front of the chloride). The broad and small peak at around 7 Å should be attributed to chloride ions and nitrogen atoms separated by a benzene ring from the closest one (i.e., adjacent repeat unit with respect to the nitrogen atom identified for the peak at 3.1 Å). On the other hand, the small but clear peak located at around 4.2 Å only appears in the B1 profile. This peak has been attributed to interchain interactions, reinforcing the densification of the bipolaronic form of PAni-ES with respect to the polaronic one.

Figure 1. Radial distribution functions of Cl···Cl pairs, gCl−Cl(r), calculated for B1 (black solid line) and P1 (black dotted line) models of amorphous PAni-ES. 11556



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runs of 1.5 ps lead to the average values listed in Tables 2 and 3, which display the impact of the electronic structure in the geometric parameters and electronic charge distribution for the bipolaronic and polaroric forms of amorphous PAni-ES, respectively. In the case of the B1 model (Table 2), analysis on the geometric parameters obtained for the QM rings reveals a progressive transformation toward a quinoid-like structure along the structure. More specifically, the first ring, which exhibits a benzenoid-like structure (i.e., C−C and CC bond lengths of 1.405 and 1.392 Å, respectively) evolves toward the quinoid-like structure of the last ring (i.e., C−C and CC bond lengths of 1.429 and 1.363 Å, respectively). Similarly, a recent study based 1D-DFT periodical structure of PAni-ES29 also showed a quinoid-like structure localized in the fourth ring (i.e., C−C and CC bond lengths of 1.440 and 1.356 Å, respectively) from a unit cell made of four repeat units, whereas the first ring exhibited a benzenoid-like structure remains with C−C bond lengths of 1.398 Å. Interestingly, the P1 model (Table 3) presents a more benzenoid-like structure with homogeneous values for the bond lengths located at the edge rings (C−C and CC bond lengths of 1.409 and 1.390 Å, respectively). Thus, the largest distortion associated with the formation of a quinoid-like structure is located at the central rings. More specifically, the C−C and CC bond lengths at the third ring are 1.425 and 1.375 Å, respectively. Furthermore, comparison of the electronic charges at the second and third ring evidence a significant reduction (from 0.76 to 0.20 au). In order to illustrate the distortion of the atomic bond lengths for both B1 and P1 models, the bond length alternation (BLA) pattern was examined. Figure 4 compares the BLA patterns, expressed as the modulus of the difference between the lengths of two adjacent bonds, for the two models. Values were obtained by considering all C−C and N−C bond lengths located at the oligomer used to define the QM zone and averaging for all snapshots recorded during QM/MM-MD simulations. The calculated BLA patterns reveal that the first

Figure 3. Radial distribution functions of N···Cl pairs, gN−Cl(r), calculated for B1 (black solid line) and P1 (black dotted line) models of amorphous PAni-ES.

Polaron/Bipolaron Comparison, a Hybrid QM/MM-MD Study. In order to get a deeper insight into the electronic structure of PAni-ES system, additional calculations were carried out on models B1 and P1 using QM/MM-MD methodology. Initially, fully equilibrated systems extracted from classical MD simulations with united-atom models were transformed into all-atom systems to deal with the QM/MMMD approximation. For this purpose, hydrogen atoms were added to the phenyl rings, transforming each CH pseudoatom into two explicit atoms. After a short MD equilibration, four repeating units embedded into the amorphous system were selected to be described at the QM level, whereas the rest of the system was considered at the MM level. Next, a new QM/MMMD equilibration was carried out for 0.5 ps. Finally, production

Table 2. Average Values of Selected Bond Lengths (Å), Bond Angles (deg), Dihedrals (deg), and Charges (au) at Each Repeat Unit (RU#) of the Bipolaronic Form Described at the QM Levela RU1 ± ± ± ± ±

CA-CB CB-CB CB-CA CA-N N-CA

1.405 1.392 1.407 1.404 1.389

CA-N-CA

128.5 ± 3.8

CB-CA-CA-CBb CA-H-CA-Nc CB-CA-N-CAd CA-N-CA-CBe

23.9 ± 5.8 10.0 ± 8.1 −127.5 ± 8.8 155.4 ± 8.6

RU2

RU3

Bond Lengths 1.423 ± 0.031 1.425 ± 0.028 1.390 ± 0.024 1.373 ± 0.023 1.407 ± 0.025 1.422 ± 0.024 1.407 ± 0.027 1.376 ± 0.027 1.376 ± 0.029 1.359 ± 0.024 Bond Angles 121.4 ± 3.9 131.8 ± 2.7 Dihedrals 61.6 ± 5.6 −32.4 ± 5.8 −6.7 ± 8.6 −4.3 ± 5.4 44.1 ± 8.8 −164.1 ± 6.6 −154.5 ± 6.2 162.6 ± 8.1 Charges 0.58 ± 0.11 0.43 ± 0.10

0.029 0.021 0.022 0.029 0.029

0.10 ± 0.08

RU4 1.429 1.363 1.437 1.332 1.358

± ± ± ± ±

0.023 0.022 0.028 0.024 0.026b

127.6 ± 2.4b 57.6 ± 5.9b −6.1 ± 6.1b −166.1 ± 7.6b −52.0 ± 8.9b 0.90 ± 0.10

a

Results were derived from hybrid QM/MM-MD simulations on the bipolaronic model (B1) of amorphous PAni-ES. Standard deviations are shown. bVirtual dihedral angle defined by the CB−CA bonds of consecutive aromatic rings (i.e., omitting the N−H group). cImproper dihedral angle used to measure the N atom pyramidalization. dDihedral angle defined by involving the last CB-CA atoms of one aromatic ring and the first CA atom of the next aromatic ring. eDihedral defined by the last CA atom of one aromatic ring and the CA-CB atoms of the next ring. 11557



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Table 3. Average Values of Selected Bond Lengths (Å), Bond Angles (deg), Dihedrals (deg), and Charges (au) at Each Repeat Unit (RU#) of the Polaronic Form Described at the QM Levela RU1

RU2

RU3

RU4

Bond Lengths ± ± ± ± ±

0.025 0.021 0.028 0.031 0.026

1.420 1.390 1.408 1.430 1.375

± ± ± ± ±

CA-CB CB-CB CB-CA CA-N N-CA

1.409 1.390 1.422 1.405 1.386

0.025 0.022 0.028 0.030 0.028

CA-N-CA

131.9 ± 3.4

123.0 ± 3.4

CB-CA-CA-CBb CA-H-CA-Nc CB-CA-N-CAd CA-N-CA-CBe

23.6 ± 6.7 −6.8 ± 6.1 51.4 ± 10.6 −67.3 ± 10.1

−64.2 ± 5.6 11.4 ± 6.6 157.6 ± 9.7 −151.6 ± 9.2

0.69 ± 0.08

0.76 ± 0.11

1.425 1.375 1.428 1.367 1.426

± ± ± ± ±

0.025 0.022 0.023 0.025 0.033

1.403 1.386 1.424 1.380 1.373

± ± ± ± ±

0.030 0.026 0.028 0.024 0.027b

Bond Angles 121.9 ± 2.7

122.8 ± 3.0b

−93.0 ± 12.2 −0.9 ± 6.1 −5.3 ± 4.9 −114.2 ± 9.8

−51.7 ± 5.5b 12.9 ± 7.1b 3.1 ± 11.3b −53.0 ± 7.2b

0.20 ± 0.09

0.35 ± 0.07

Dihedrals

Charges a

Results were derived from hybrid QM/MM-MD simulations on the polaronic model (P1) of amorphous PAni-ES. Standard deviations are shown. b Virtual dihedral angle defined by the CB−CA bonds of consecutive aromatic rings (i.e., omitting the N−H group). cImproper dihedral angle used to measure the N atom pyramidalization. dDihedral angle defined by involving the last CB-CA atoms of one aromatic ring and the first CA atom of the next aromatic ring. eDihedral defined by the last CA atom of one aromatic ring and the CA-CB atoms of the next ring.

However, the N atom hybridization results in a substantial pyramidalization evidenced by the CA−H−CA−N dihedral angle, which ranges from 1° up to 12°. On the other hand, the tilt angle between adjacent aromatic rings is mainly localized in those repeat units with clear N-imine character, which is reflected by CB−CA−CA−CB dihedral angles larger than those observed in the crystal structure of PAni-ES (30−40°).29 These results are in good agreement with those obtained by Xray scattering data of amorphous PAni-ES.61 In that work, the ∠C−N−C bond angles were found to exhibit significant inequivalency between odd and even N sites, the resulting values being of ∼132 and ∼124°, respectively. Also, the conformational entanglements and the flexibility across the nitrogen linkages were pointed as the main contributory factors to the origin of the nonplanarity, which in turn were suggested to hinder charge transport through the polymer matrix.61 Figure 5 represents the charge evolution on each repeat unit (referred as a ring fragment of charge) along QM/MM-MD simulation. The charge at each ring fragment was obtained by summing the atomic Mulliken charges of all atoms in the repeat unit. In both models charge evolves from uniform distributions, which corresponds to those used in the classical MD study (Figures S1 and S2, Supporting Information) to more clearly localize distributions. More specifically, after relaxing the electronic density because of the inclusion of the amorphous environment, the electronic charge flows to the edge in the following way. Most of the two positive charges on the P1 model (Figure 5a) are located on the two first repeating units (∼1.45 au), whereas the rest of the charge is distributed through the second half of the QM zone, the gap between the third and the fourth repeat unit being relatively important. In the B1 model (Figure 5b) charge accumulates in the last repeat unit (∼0.9 au), whereas the rest of the electronic charge (∼1.1 au) is distributed in the middle of the QM zone, the charge located in the first repeat unit being very small. Results presented in this subsection indicate that the conformational entanglements associated with the amorphous solid state provoke a significant tilt between adjacent benzene rings. This loss of coplanarity obstructs charge transfer along

Figure 4. Bond length alternation pattern for the bipolaronic (B1) and polaronic (P1) forms of amorphous PAni-ES. Repeat units are separated by vertical thin lines.

and the second repeat units of B1 and P1 behave similarly while remarkable differences are detected at the third and fourth repeat unit. Thus, the fourth repeat unit of B1 presents a significant degree of quinoid-like structure as well as of N-imine bond behavior, whereas the largest distortion from the benzenoid-like structure in P1 occurs at the third repeat unit. Interestingly, at central region of the oligomer described at the QM level the N-imine character is clearer for P1 than for B1 (i.e., C−N bond lengths identified as numbers 9 and 14 in Figure 4). This pattern is not consistent with what is expected for a polaronic structure (Scheme 1b) but it is closer to that of a bipolaronic structure (Scheme 1c). Analysis of both the ∠C−N−C bond angle and the dihedral angle between adjacent aromatic rings, which is defined by the CB−CA−CA−CB sequence, also deserves consideration. Thus, the ∠C−N−C angle enables the differentiation between the amine and imine bonds (i.e., values of about 130 and 123°, respectively, has been reported in the solid state29), while the dihedral between adjacent aromatic rings is related with the tilt deformation. Values derived from QM/MM-MD simulations on B1 and P1, which are included in Tables 2 and 3, reveal an important presence of N-imine bond character on both models. 11558



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Figure 6. Comparison of the HOMO for the quantum zone of amorphous PAni-ES. Results were obtained from hybrid QM/MMMD simulations on (a) the bipolaronic (B1) and (b) polaronic models (P1) of amorphous PAni-ES.

Figure 5. Time evolution of the ring charge on the four repeat units (r1 up to r4) of the quantum zone involving four repeat units (r1 up to r4) of amorphous PAni-ES. Results were obtained from hybrid QM/ MM-MD simulations on (a) the bipolaronic (B1) and (b) polaronic models (P1) of amorphous PAni-ES.

the chain, the opposite situation being predicted from PCMDFT calculations on isolated PAni-ES oligomers in solution.17 The structural distortions associated with the disordered amorphous phase are accompanied by charge distributions different from those expected for the polaronic and bipolaronic forms. More specifically, alternating ring charges between consecutive repeat units and a distribution of positive charges with the maximum located at the third ring were expected for the polaronic and bipolaronic rings, respectively.17 However, results displayed in Figure 5 reflect that the important steric hindrance observed in amorphous PAni-ES leads to a nonuniform charge delocalization through the unsaturated backbone of the polymer chain. This situation is probably different from that occurring on both crystalline and in solution phases. In Figure 6 we compare the highest occupied molecular orbital (HOMO) of the quantum zones of both B1 and P1 models of amorphous PAni-ES. These orbitals reflect the differences in the charge distribution between the bipolaronic and polaronic forms. Polaron/Bipolaron Comparison, an Experimental Study. The morphology and topography of PAni-ES and PAni-EB are compared in Figure 7. The two systems are very similar, even though detailed inspections of high resolution SEM micrographs reveal that the granular aggregates onto the disordered sheets is considerably more abundant for PAni-ES than for PAni-EB. This feature is corroborated by both height and phase AFM images. As a consequence, the roughness of PAni-ES (Rq = 223 ± 30 nm) is ∼40% higher than that of PAni-EB (Rq = 162 ± 18 nm), as is clearly illustrated in the representative cross-section profiles included in Figure 7. It should be mentioned that, in order to avoid the discontinuities typically associated with powder-like materials, Rq values were

Figure 7. For (a) PAni-ES and (b) PAni-EB: high and low resolution SEM micrographs (Left), height and phase 2D AFM images (Right) and representative cross-section profile from the region indicated by an arrow in the phase image.

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calculated averaging 10 cross-profiles from which a horizontal distance of only 2.5 μm was taken from each one. The UV−vis spectrum of PAni-EB (Figure 8, inset) show two bands with λmax = 326 and 650 nm, which are assigned to

Figure 8. UV−vis spectra of PAni-EB obtained by deprotonation of the as-prepared PAni-ES in NH4OH aqueous solution (inset) and after application of a constant potential (0.1, 0.5, and 0.7 V) during 10 s. The bands associated to the doped state are indicated by arrows in the spectrum of the sample treated at 0.7 V.

the π−π* transition of the benzenoid rings and the exciton absorption of the quinoid rings, respectively.62 These results indicate the undoped state of PAni-EB,62,63 which was obtained by deprotonation of the as-prepared PAni-ES in NH4OH aqueous solution. A constant potential (chronoamperometry) of 0.1, 0.3, 0.5, or 0.7 V was applied to PAni-EB during during 10 s, the UV−vis spectrum of each sample being subsequently recorded. As it can be seen in Figure 8, potentials ≤0.5 V only provoke small shifts in the above-discussed bands. However, two new bands appear in the spectrum recorded for the sample treated with a constant potential of 0.7 V. More specifically, a small shoulder detected at λmax = 438 nm is attributed to the formation of cation radicals (polarons), while the very broad band centered at λ ≈ 1000 nm is typically attributed to the formation of polaron dications, indicating the doped state of the sample. Cyclic voltammograms of both PAni-ES and PAni-EB in 0.1 M HCl, which are displayed in Figure 9a, show two main oxidation peaks, labeled as A and B. The first maximum (A), at 0.25 and 0.30 V for PAni-ES and PAni-EB, respectively, corresponds to the oxidation of leucoemeraldine to emeraldine (Figure 9b). The second maximum (B) at the higher potential of 0.86 and 0.78 V for PAni-ES and PAni-EB, respectively, is attributed to the oxidization of emeraldine to pernigraniline (Figure 9b). The detection of the corresponding reduction peaks, labeled as A′ and B′ in Figure 9a, indicate that these deelectronation and deprotonation processes are reversible. Although the behavior of PAni-ES and PAni-EB is relatively similar because of the doping of the latter in the HCl medium, some differences are observed. For example, the ratio between the anodic and cathodic peak current densities for A and A′ is 0.84 and 1.15 for PAni-ES and PAni-EB, respectively. This indicates that the reduction and oxidation processes are favored for the former and latter specie, respectively. In contrast, the ratio between anodic and cathodic peak current densities for the emeraldine ↔ pernigraniline process (i.e., B ↔ B′) is 1.36 and 1.13 for PAni-ES and PAni-EB, respectively, indicating that the oxidation is favored for both species.

Figure 9. (a) Cyclic voltammograms recorded for PAni-ES and PAniEB in 0.1 M HCl. The displayed voltammograms correspond to those obtained after 2 and 100 consecutive oxidation−reduction cycles. Oxidation (A and B) and reduction (A′ and B′) processes are labeled. (b) Oxidation and reduction processes associated to the voltammograms displayed in (a).

Figure 9a suggests that the transformation from emeraldine to pernigraniline (process B) occurs simultaneously with the formation of PAni charge carriers, which can consists of polarons (radical cations) and bipolarons (dications) forms delocalized on PAni chains or on a mixture of both. Unfortunately, the nature of these charges species is not clear from the voltammograms displayed in Figure 9a. Moreover, voltammograms recorded using smaller potential windows, which were focused on each of the above-mentioned processes, and voltammograms recorded in 0.1 M LiClO4 solutions rather than on 0.1 M HCl did not provide any additional information. On the other hand, voltammograms recorded after consecutive oxidation−reduction cycles indicated that these oxidation and reduction processes are detected until approximately 30 cycles, no significant variation in the potentials being observed during the first five cycles. PAni-ES and PAni-EB voltammograms recorded after 100 consecutive redox cycles are very similar (Figure 9a) with oxidation peaks at 0.59 and 0.56 V, respectively. The loss of electrochemical activity (i.e., the loss of ability to exchange charge reversibly) after 100 redox cycles is of 49 and 45% for PAni-ES and PAni-EB, respectively.

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CONCLUSIONS Force-field parameters to describe the polaronic and bipolaronic forms of PAni-ES in the amorphous solid state have been 11560



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(3) Dhand, C.; Das, M.; Datta, M.; Malhotra, B. D. Recent Advances in Polyaniline Based Biosensors. Biosens. Bioelectron. 2011, 26, 2811− 2821. (4) Long, Y.-Z.; Li, M.-M.; Gu, C.; Wan, M.; Duvail, J.-L.; Liu, Z.; Fan, Z. Recent Advances in Synthesis, Physical Properties and Applications of Conducting Polymer Nanotubes and Nanofibers. Prog. Polym. Sci. 2011, 36, 1415−1442. (5) Macdiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Polyaniline: a New Concept in Conducting Polymers. Synth. Met. 1987, 18, 285−290. (6) MacDiarmid, A. G. Synthetic Metals: a Novel Role for Organic Polymers. Synth. Met. 2001, 125, 11−22. (7) Stafström, S.; Brédas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; MacDiarmid, A. G. Polaron Lattice in Highly Conducting Polyaniline: Theoretical and Optical Studies. Phys. Rev. Lett. 1987, 59, 1464−1467. (8) Heeger, A. J. Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials. J. Phys. Chem. B 2001, 105, 8475− 8491. (9) Cavazzoni, C.; Colle, R.; Farchioni, R.; Grosso, G. HCl-Doped Conducting Emeraldine Polymer Studied by Ab Initio Car-Parrinello Molecular Dynamics. Phys. Rev. B 2006, 74, 033103. (10) Gospodinova, N.; Terlemezyan, L. Conducting Polymers Prepared by Oxidative Polymerization: Polyaniline. Prog. Polym. Sci. 1998, 23, 1443−1484. (11) Krinichnyi, V. I.; Chemerisov, S. D.; Lebedev, Y. S. EPR and Charge Transport Studies of Polyaniline. Phys. Rev. B 1997, 55, 16233−16244. (12) Konkin, A. L.; Shtyrlin, V. G.; Garipov, R. R.; Aganov, A. V.; Zakharov, A. V.; Krinichnyi, V. I.; Adams, P. N.; Monkman, A. P. EPR, Charge Transport, and Spin Dynamics in Doped Polyanilines. Phys. Rev. B 2002, 66, 075203. (13) Lippe, J.; Holze, R. Electrochemical In-Situ Conductivity and Polaron Concentration Measurements at Selected Conducting Polymers. Synth. Met. 1991, 43, 2927−2930. (14) Tang, J.; Allendoerfer, R. D.; Osteryoung, R. A. Simultaneous EPR and Electrochemical Measurements on Polyaniline in Ambient Temperature Molten Salts. J. Phys. Chem. 1992, 96, 3531−3536. (15) Long, Y.; Chen, Z.; Shen, J.; Zhang, Z.; Zhang, L.; Xiao, H.; Wan, M.; Duvail, J. L. Magnetic Properties of Conducting Polymer Nanostructures. J. Phys. Chem. B 2006, 110, 23228−23233. (16) Bhadra, S.; Singha, N. K.; Khastgir, D. Polyaniline by New Miniemulsion Polymerization and the Effect of Reducing Agent on Conductivity. Synth. Met. 2006, 156, 1148−1154. (17) Petrova, J. N.; Romanova, J. R.; Madjarova, G. K.; Ivanova, A. N.; Tadjer, A. V. Fully Doped Oligomers of Emeraldine Salt: Polaronic versus Bipolaronic Configuration. J. Phys. Chem. B 2011, 115, 3765− 3776. (18) Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W. P. Solitons in Conducting Polymers. Rev. Mod. Phys. 1988, 60, 781−850. (19) Ginder, J. M.; Epstein, A. J. Role of Ring Torsion Angle in Polyaniline: Electronic Structure and Defect States. Phys. Rev. B 1990, 41, 10674−10685. (20) Libert, J.; Cornil, J.; dos Santos, D. A.; Brédas, J. L. From Neutral Oligoanilines to Polyanilines: A Theoretical Investigation of the Chain-Length Dependence of the Electronic and Optical Properties. Phys. Rev. B 1997, 56, 8638−8650. (21) de Oliveira, Z. T., Jr; dos Santos, M. C. Relative Stability of Polarons and Bipolarons in Emeraldine Oligomers: a Quantum Chemical Study. Solid State Commun. 2000, 114, 49−53. (22) Kwon, O.; McKee, M. L. Calculations of Band Gaps in Polyaniline from Theoretical Studies of Oligomers. J. Phys. Chem. B 2000, 104, 1686−1694. (23) Lim, S. L.; Tan, K. L.; Kang, E. T.; Chin, W. S. A Comparative Ab Initio and DFT Study of Neutral Aniline Oligomers. J. Chem. Phys. 2000, 112, 10648−10658. (24) Alemán, C.; Ferreira, C. A.; Torras, J.; Meneguzzi, A.; Canales, M.; Rodrigues, M. A. S.; Casanovas, J. On the Molecular Properties of

developed and, subsequently, applied to model a molecular system made of 100 repeat units distributed in four chains of identical molecular weight. Both classical MD and hybrid QM/ MM-MD simulations have been carried out with periodic boundary conditions, allowing to consider environmental effects in the relative stability and properties of the two examined electronic forms. Classical MD simulations indicate that the energy is lower for the bipolaronic form than for the polaronic one. Moreover, the density calculated for the bipolaronic model is in excellent agreement with that experimentally reported for amorphous PAni-ES. Indeed, the bipolaronic form has been found to be denser than the polaronic one, which has been attributed to the fact that interchain interactions are stronger in the former than in the latter. The preference toward the bipolaronic forms has been also supported by hybrid QM/MM-MD simulations on a simulation box of amorphous PAni-ES in which a fragment of four repeat units was used to define the quantum zone. Indeed, properties derived from hybrid simulations on the bipolaronic and polaronic forms indicate some inconsistencies, as for example in the bond length alternation pattern, with respect to the expected behavior for the latter electronic state. The bipolaronic form has been also observed in PAni-ES by UV−vis spectroscopy, which also agrees with the theoretical conclusions reached from classical MD and hybrid QM/MM-MD simulations. In summary, results presented in this work indicate that the bipolaronic form is responsible of the properties exhibited by proton-doped PAni in the amorphous solid state.

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ASSOCIATED CONTENT

S Supporting Information *

Constituents of equilibrium bond length, bond angles, and punctual charges on classical particles of modeled system for the polaronic (Figure S1) and bipolaronic (Figure S2) forms of amorphous PAni-ES; constituents of the torsion angles in the polaron and bipolaron forms of PAni-ES (Table T1). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work has been supported by MINECO and FEDER funds (MAT2012-34498), MINECO (FIS2012-39443-CO2-01), and the DIUE of the Generalitat de Catalunya (2009SGR925, 2009SGR1003, and XRQTC). Support for the research of C.A. was received through the prize “ICREA Academia” for excellence in research funded by the Generalitat de Catalunya.

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