Spectroelectrochemical Approaches to Mechanistic Aspects of Charge

Article pubs.acs.org/JPCC

Spectroelectrochemical Approaches to Mechanistic Aspects of Charge Transport in meso-Nickel(II) Schiff Base Electrochromic Polymer Kamila Łępicka,† Piotr Pieta,*,† Aleksander Shkurenko,† Paweł Borowicz,† Marta Majewska,† Marco Rosenkranz,‡ Stanislav Avdoshenko,‡ Alexey A. Popov,*,‡ and Wlodzimierz Kutner†,§ †

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Leibniz Institute for Solid State and Materials Research, Helmholtzstrasse 20, D-01069 Dresden, Germany § Faculty of Mathematics and Natural Sciences, School of Sciences, Cardinal Stefan Wyszynski University in Warsaw, Wóycickiego 1/3, 01-815 Warsaw, Poland ‡

S Supporting Information *

ABSTRACT: A new redox conducting polymer, viz. poly[meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II)], poly[meso-Ni(II)-SaldMe], belonging to the Schiff base polymer family, was electrochemically synthesized. The charge transfer and polymerization mechanism were unraveled by simultaneous cyclic voltammetry (CV) and in situ UV−vis, FTIR-ATR, and ex situ low-temperature ESR spectroscopy. With the latter, a short-living paramagnetic transient form of electro-oxidized poly[mesoNi(II)-SaldMe] was detected. This form was identified as the bisphenolic radical cation. In situ UV−vis and FTIR-ATR spectroelectrochemistry measurements revealed that the charge transfer of the polymer involved bisphenolic radical cation formation at the potential lower than 0.80 V vs Ag/Ag+ and then dication formation at the potential exceeding 0.80 V. The proposed mechanism of electropolymerization of meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II), meso-Ni(II)SaldMe, involves two steps. First, electro-oxidation of the monomer results in bisphenolic radical cation generation, and then mutual binding of these radicals at the para positions of aromatic rings is activated by electron-donating phenol moieties. In this electropolymerization, the Ni(II) metal center played the role of a template providing planarity to the monomer molecule. Structures responsible for the charge transfer in the polymer and formed during electropolymerization were modeled with quantum chemistry calculations using the DFT method at the PBE level. The resulting polymer film was highly conducting and stable with respect to potential multicycling under cyclic voltammetry conditions, from 0 to 1.3 V vs Ag/Ag+. Under these conditions, it changes color from yellow through orange to russet for its neutral, bisphenolic radical cation, and bisphenolic dication form, respectively. High electrochemical stability and a wide potential range of electroactivity (0.40−1.30 V vs Ag/Ag+) of the polymer are very promising for its application as a new electrochromic electrode material for supercapacitors. That is, an anode composed of poly[meso-Ni(II)-SaldMe] can serve as an internal charging−discharging indicator in these supercapacitors.

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INTRODUCTION

doping. Charge accommodation during electro-oxidation causes conformational and structural changes of the polymer.2 Moreover, intermediary energy levels within the electronic band gap region of the polymer are then formed.3

The charge transfer mechanism within a conducting polymer involves its electrochemical doping and dedoping. During redox processes, electrons are withdrawn from or injected into this polymer. The resulting charge of the polymer is neutralized by oppositely charged counterions of a supporting electrolyte entering the polymer and/or releasing entrapped charged coions.1 These processes are considered as electrochemical © 2017 American Chemical Society

Received: May 16, 2017 Revised: July 10, 2017 Published: July 12, 2017 16710

DOI: 10.1021/acs.jpcc.7b04700 J. Phys. Chem. C 2017, 121, 16710−16720

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The Journal of Physical Chemistry C

Scheme 1. (a) Structural Formula and (b) X-ray Molecular Structure of the meso-N,N′-Bis(salicylidene)-2,3butanediaminonickel(II) Monomer, meso-Ni(II)-SaldMe, Presented as an ORTEP Diagram with the Atom Numbering Scheme Shown with 50% Probability with van der Waals Radii Taken into Account

structures were identified by modeling with the density functional theory (DFT) method and calculating for them excited states correlated with results of the in situ UV−vis spectroelectrochemical experiments. On the basis of that, we unraveled the electropolymerization mechanism of meso-N,N′bis(salicylidene)-2,3-butanediaminonickel(II), meso-Ni(II)SaldMe (Scheme 1). Therefore, our work is focused on characterizing different species generated during polymer oxidation and that way explaining the polymerization mechanism. Moreover, we herein show possible application of poly[meso-Ni(II)-SaldMe] as an electroactive material for supercapacitors. Several salen-type metal complexes capable of forming electroactive polymers have already been described.10−13 Apparently, electroactive films of these polymers are formed during anodic electropolymerization of Schiff base transition metal complexes in moderately electron-donating solvents.14−16 In a highly electron-donor solvent, e.g., DMSO, the Ni(II) complexes are oxidized to Ni(III), and no electropolymerization occurs (Figure S2 and discussion in the Supporting Information).17 The Schiff base transition metal polymers form multiredox systems where metal ions are linked through redoxactive π-conjugated ligand sites.18 In order to locate the site where electro-oxidation of the poly[metal-Salen] occurs, it has been sorted out whether the metal ion center or the ligand is the primary target of the electron release. Evidently, the charge is predominantly transferred along the poly[Ni(II)-Salen] chain.9 The polymer behaves like a polyphenylene with the metal ion acting as a bridge between biphenylene moieties. Importantly, no metal-centered oxidation, such as Ni(II)/ Ni(III), was observed.4,19,20 Instead, electron is transferred on the phenolate moiety of the salicylidene ligand forming a phenoxyl radical, e.g., like in poly[Cu(II)-salen].9 Moreover, there was no ESR signal for poly[Cu(II)-salen] at room temperature. That was because diamagnetic ESR silent species were formed by antiparallel spin coupling of an unpaired Cu(II) electron spin and the spin of the unpaired electron on the ligand.21 However, the charge transfer and polymerization mechanism were not fully explained. Importantly, the mechanism of charge transfer through the -Ph-O-Ni-O-Phcenters without changing the oxidation state of nickel, Ni(II)/ Ni(III), was not described. Moreover, the key role of the metal center in the polymerization of salen metal complexes was not discussed, and different redox centers participating in the charge transfer were not identified. These mechanistic details, however, are important because salen ligands alone are unable to polymerize, in contrast to their transition metal complexes. We believe that characterization of the polymerization and the

In situ spectroelectrochemical measurements are convenient for both probing different oxidation states of redox conducting polymers and locating redox centers. One of the possible approaches to gaining closer insight into mechanistic aspects of the redox conducting polymer charging and discharging is to follow changes in electronic properties of the polymer upon electrode reactions. This is readily accomplished by in situ UV−vis absorption spectroscopy measurements because subband-gap transitions are formed as a result of charging of the polymer. These new electronic energy levels provide intermediate channels for promotion of electrons from the polymer valence band (VB) to the conduction band (CB). In effect, the energy required for electronic transition (band gap) decreases upon polymer electro-oxidation, thus leading to a band red-shift in the UV−vis spectrum.2 Another approach is to follow changes in vibrational energy levels of a polymer during electrochemical reaction with measurements of vibrational spectroscopy techniques, e.g., Fourier transform infrared (FTIR) and Raman spectroscopy. That way structural and conformational changes in the studied polymer can be monitored. Upon polymer electro-oxidation or electroreduction, new bands, sensitive to a change in the dipole moment during the polymer chain vibration in the direction of charge delocalization, are dominant in the FTIR spectrum.4 These bands are termed infrared activated vibrational (IRAV) bands. The intensity of the IRAV bands increases with the increase of the doping level.5 These bands are important because their presence unambiguously evidences the presence of charged excitations.6 Furthermore, the doping-induced infrared absorption depends upon geometric orientation and direction of charge conduction of the polymer. One more important experimental step of studying the mechanism of the polymer charge transfer involves measurements of electron spin resonance (ESR) spectroscopy.4,7,8 The ESR spectroelectrochemistry can provide very sensitive information on the presence of paramagnetic species in electroactive polymers formed during charge injection, thus improving the understanding of electrode redox processes of the polymer.9 Therefore, ESR spectroscopy allows detecting different oxidized polymer species featuring an unpaired electron spin. The research goal of the present work was to prepare by electropolymerization a new redox conducting film of poly[meso-Ni(II)-SaldMe] and then unravel, first, the mechanism of charge transfer within the polymer by combining experimental results of in situ UV−vis, FTIR-ATR, and ex situ low-temperature ESR spectroscopy experiments. This combination allowed recognizing different polymer structures responsible for directional charge transfer. Then, the polymer 16711


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theory with the GPAW code.28,29 A repeat unit comprised two monomer fragments; the unit cell parameter along the periodic axis was set to 24.648 Å to reproduce optimized C−C distance between monomers in DFT-optimized oligomers. The electron density was converged with k-mesh 4 × 1 × 1 in all cases. The Harris calculation for the bands was performed with a resolution of 100 grid points along each symmetry line, which for the 1 D chain consists of the X-Γ segment.

charge transfer mechanism in particular to a greater detail will, presumably, make application of metal salen polymers broader.

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EXPERIMENTAL SECTION Chemicals. All supporting electrolyte salts and anhydrous solvents (HPLC grade) were purchased from Sigma-Aldrich or ABCR. Herein, a new meso-[Ni(II)-Salen]-type complex monomer, viz. meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II) 1 (Scheme 1), was synthesized in three steps, and its crystal structure was solved.22 Electrochemical Synthesis of Poly[meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II)]. A film of poly[meso-Ni(II)-SaldMe] was grown by oxidative electropolymerization from an acetonitrile solution of 0.1 M mesoNi(II)-SaldMe and 0.1 M (TBA)ClO4 under conditions of potentiodynamic multicycling of the potential between 0 and 1.30 V at the scan rate of 10 mV s−1 under an argon atmosphere. A three-electrode electrochemical cell was used for the electrochemical synthesis as well as electrochemical and spectroelectrochemical measurements. A platinum disk or mesh electrode, a laminated indium−tin oxide (ITO) film coated glass slide with 0.13 cm2 electrochemically active area, and a ZnSe-ART crystal coated with an Au transparent layer were used as the working electrode in electrochemical, ESR/UV−vis, and FTIR-ATR spectroscopy measurements, respectively. A Pt mesh or a Pt foil and an Ag/Ag+ pseudoreference electrode were used as the counter and reference electrode, respectively. After deposition of the film of the polymer of 1, the resulting modified electrode was rinsed with abundant acetonitrile before further use. Spectroelectrochemical Measurements. In situ UV− vis−NIR and ex situ ESR spectroscopy experiments were performed with the working electrode mounted in an optical ESR cavity (ER 4104OR, Bruker). The ESR spectra were recorded using an EMX plus X-band CW spectrometer (Bruker). For the UV−vis−NIR spectra recording, an AvaSpec-2048x14-USB2 spectrometer with the CCD detector and an AvaSpec-NIR256-2.2 spectrometer with the InGaAs detector were used. Both spectrometers were controlled with AvaSoft 7.5 software. All UV−vis−NIR and ESR spectrometers were connected to a PG 390 potentiostat/galvanostat of an HEKA Elektronik. A three-electrode spectroelectrochemical flat glass cell was used,23−25 and measurements were triggered; then experimental data were acquired with the PotMaster v2x73 software package (HEKA Elektronik). In situ FTIR-ATR spectroscopy experiments were performed with a dedicated three-electrode PTFE cell. The FTIR-ATR spectra were recorded using a VERTEX 80v spectrometer of Bruker controlled by the OPUS 7 software package of the same manufacturer. Electrochemical measurements and the time synchronized triggering of the FTIR spectrometer hardware (PotMaster v2x73) were controlled by the PG 390 potentiostat/galvanostat. Data Processing and Calculations. Molecular structures of neutral and charged oligomers as well as their vibrational spectra were calculated at the GGA-PBE level of theory using Priroda package.26,27 Implemented TZ2P-quality basis set comprising full electron {3,1}/(5s,1p) basis for H atoms, {6,3,2}/(11s,6p,2d) basis for C, N, and O atoms, and SBJKtype effective core potential with the {5,5,4}/(9s,9p,8d) valence part for Ni was used throughout these calculations. Single-point band-structure calculations for the neutral and charged polymer were then performed at the PBE/DZ level of

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RESULTS AND DISCUSSION In the present work, first, electropolymerization of the mesoNi(II)-SaldMe monomer and electrochemical characterization of the resulting polymer film are discussed. Next, ex situ ESR and in situ UV−vis spectroelectrochemical experiments are described to unravel the charge transfer mechanism within the polymer and possible mechanism of meso-Ni(II)-SaldMe electropolymerization. Then, modeling with DFT quantum chemistry calculations is employed to identify polymer structures of different oxidation states. Finally, in situ FTIRATR spectroelectrochemical results combined with calculations are presented. We synthesized the meso-Ni(II)-SaldMe monomer and determined its crystal structure by single-crystal X-ray diffraction analysis (SCXRD). This double substituted by methyl groups Ni(II) salen complex is a square-planar meso diastereoisomer. The presence of the Ni(II) metal center governs complex planarity. The meso-SaldMe ligand does not polymerize itself (Figure S1 in the Supporting Information) because two phenol moieties are labile in the supporting electrolyte solution, and thus, they repel each other. The meso-Ni(II)-SaldMe monomer was specially designed to form, via oxidative electropolymerization, a polymer film electroactive in the positive potential range. Figure 1a shows oxidative electropolymerization of the meso-Ni(II)-SaldMe monomer performed under potentiodynamic multicyclic conditions in the potential range of 0.0−1.30 V vs Ag/Ag+. The polymer film deposition is manifested by the current increase in consecutive potential cycles. Moreover, this increase indicates that the deposited polymer film is conducting. After deposition, and then rinsing with acetonitrile, the polymercoated electrode was transferred to a monomer-free acetonitrile solution of 0.1 M (TBA)ClO4, and then the cyclic voltammetry (CV) curve (Figure 1b) was recorded. The current−potential curves provide electrochemical information on redox processes occurring in the polymer. Two partially overlapping anodic peaks in this figure, centered at ∼0.60 and ∼0.85 V vs Ag/Ag+, recorded in the anodic potential sweep indicated that two redox processes might occur. This CV behavior may suggest that the charge transfer occurring involves at least two faradaic reactions leading to the electron exchange between the polymer and the electrode. Presumably, the first, less positive, faradaic reaction involves one-electron oxidation of the polymer with the charge centered on the phenolic oxygen atom, resulting in formation of a bisphenolic radical cation (Scheme 2a).10 Removal of one electron from the polymer chain, neutralized by the anion ingress, can be regarded as an electrochemical doping. Because of this doping, electronic defects are formed and new energy levels are generated within the VB−CB gap of the polymer.3,30 Because of generation of these new energy levels, the number of intragap transitions is also changed. The presence of the second, more positive anodic peak at ∼0.85 V vs Ag/Ag+ (Figure 1b) may indicate that the generated bisphenolic radical cation is subsequently electro-oxidized, by 16712


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during the electron withdrawal. Importantly, no ESR signal was detected in measurements performed at room temperature. Therefore, subsequent experiments were performed in such a way that the desired potential was applied for 30 s to the polymer coated working electrode, and then the temperature was abruptly lowered to T = 150 K. Figure 2 shows ex situ low-

Figure 2. Low-temperature (T = 150 K) ex situ ESR spectra for the poly[meso-Ni(II)-SaldMe] film deposited on a laminated Pt mesh (with 0.13 cm2 electroactive area) electrode in an acetonitrile solution of 0.1 M (TBA)ClO4. Parameters of the ESR spectroscopy measurements were as follows: microwave power 5 mW, modulation amplitude 5 × 10−4 T (at 100 kHz), and g factor 2.011. The electrode was kept for 30 s at the potential of either (1) 0.80 or (2) 1.30 V vs Ag/Ag+ and then quickly cooled.

Figure 1. (a) Multicyclic potentiodynamic curves of oxidative electropolymerization of 1 mM meso-Ni(II)-SaldMe in an acetonitrile solution of 0.1 M (TBA)ClO4. The potential scan rate was 10 mV s−1. (b) Cyclic voltammogram for poly[meso-Ni(II)-SaldMe] in the monomer-free acetonitrile solution of 0.1 M (TBA)ClO4 recorded at the potential scan rate of 10 mV s−1.

temperature ESR spectra recorded after applying the potential of 0.80 or 1.30 V vs Ag/Ag+ (curves 1 and 2 in Figure 2, respectively), and then fast cooling. At each potential, a sharp single-line ESR spectrum characteristic of bisphenolic radical cation31 and not of the oxidized, Ni(III), form was recorded.32 As expected, a paramagnetic bisphenolic radical cation (Scheme 2a) is formed at 0.80 V vs Ag/Ag+. No hyperfine splitting of protons at para positions was observed because polymer units were mutually bound at this position. Surprisingly, an ESR signal was also detected at 1.30 V, suggesting that radicals are still present in the polymer at higher potentials. However, this latter signal is weaker than that at 0.80 V. Most likely, not all radicals generated at the lower potential were oxidized to diamagnetic dications (Scheme 2b). The temperature decrease significantly lowered the energy of the vibrational states of the bisphenolic radical cation and therefore inhibited the electron transfer between two redox centers, i.e., the bisphenolic radical cation and the bisphenolic dication. As a consequence, the electron was localized and the ESR signal could be detected. A similar result for different poly[Ni(II)salen)] was explained by insufficient delocalization of radicals to accommodate more than one positive charge.4 Moreover, coexistence of radical cations and dications at intermediate oxidation states was detected in PPy.33 Linear charge mobility significantly contributes to the charge transfer within the polymer chain. If electrons are unable to move linearly via rearrangement of single and double bonds along the phenolic oxygen atoms in an external electric field, and then tunnel through the Ni(II) barrier, these atoms cannot be electrooxidized to dications. Our present results show that, indeed, bisphenolic cation radical is formed at the first anodic peak at ∼0.60 V vs Ag/Ag+ (Figure 1b). Lower intensity of the ESR signal at higher oxidation potential may indicate that the next

Scheme 2. Proposed Structural Formula of (a) a Paramagnetic Bisphenolic Radical Cation and (b) a Diamagnetic Bisphenolic Dication

removal of the second electron from the phenolate oxygen moiety of the polymer, to form a bisphenolic dication (Scheme 2b). The distance between two phenolic oxygen atoms decreased as a consequence of Ni(II) coordination. Therefore, electrons can tunnel between phenolic oxygen atoms, and therefore no Ni(II)/Ni(III) oxidation occurs. Ex situ ESR spectroelectrochemistry was herein employed to detect and identify paramagnetic species in the polymer formed 16713


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The Journal of Physical Chemistry C step of the charge transfer within the polymer involves oxidation of these particular bisphenolic radical cations, which can linearly tunnel electrons through the Ni(II) barrier. If this process is absent, then a weak ESR signal is observed. To verify this hypothesis, in situ UV−vis and FTIR-ATR spectroelectrochemical measurements were performed. The UV−vis absorption spectra measurements allowed following the changes in electronic properties of the polymer upon its electro-oxidation and subsequent electroreduction. The spectra measured in the potential range of 0.0−0.40 V vs Ag/Ag+ (Figure S3) do not change significantly, indicating no change in the electronic structure of the polymer. Indeed, current is not growing in this potential range (Figure 1b), suggesting that the polymer can be regarded as undoped under these redox conditions. The spectrum of the undoped poly[meso-Ni(II)-SaldMe] exhibits two bands with maxima located at ∼323 and ∼430 nm (Figures 3a). With the subsequent potential increase, the resulting spectra show remarkable evolution. That is, the first band at ∼323 nm decreases at potentials above 0.50 V vs Ag/Ag+. Simultaneously, the second band at ∼430 nm increases and its maximum shifts to ∼415 nm. Moreover, a new broad band appears at ∼950 nm. The absorption intensity of bands at ∼415 and ∼950 nm increases with the potential increase until the potential reaches 0.80 V vs Ag/Ag+ (Figure 3a), indicating occurrence of an electronic transition characteristic of a bisphenolic radical cation.4 This behavior is in accord with the electrochemical and ex situ ESR spectroscopy results shown in Figures 1b and 2, respectively. As expected, the first anodic peak at ∼0.60 V (Figure 1b) was associated with electrogeneration of the bisphenolic radical cation (Scheme 2a). In effect, new electronic states, localized close to the lower energy state, were generated. These states contained unpaired electron spins and, therefore, they were paramagnetic. Both the ESR and UV−vis spectroscopy measurements confirmed that the bisphenolic radical cation was generated in the potential range of 0.50−0.80 V vs Ag/Ag+. The energy level associated with this radical cation represents that of a destabilized binding orbital and, therefore, its energy is higher than that of the valence band.34 At E ≥ 0.40 V vs Ag/ Ag+, poly[meso-Ni(II)-SaldMe] is oxidized (doped state), and hence, it is conducting. When the potential reaches 0.80 V, the band at ∼323 nm decreases and broadens, and its maximum shifts to ∼331 nm (Figure 3a). Moreover, bands located at ∼415 and ∼950 nm increase and broaden. This behavior indicates farther generating of the bisphenolic radical cations and accumulating them in the polymer. At 0.80 V vs Ag/Ag+, the intensity of the broad band at ∼950 nm is the highest (Figure 3c). Presumably, the concentration of the radical cation capable to be oxidized to the dication is the highest at this potential. The presence of the isosbestic point at ∼355 nm in the potential range 0−0.80 V (Figure 3a) indicates that two species are at equilibrium in the system, i.e., an undoped polymer and a polymer in its bisphenolic radical cation form. This equilibrium is attained at ∼0.70 V. Upon further electro-oxidation, the subsequent faradaic electron transfer is manifested by a new more positive anodic peak at ∼0.85 V vs Ag/Ag+ in the CV curve (Figure 1b). Electron removal from linearly stabilized in the π-conjugated direction bisphenolic radical cations (Scheme 2a) leads to generation of a bisphenolic dication (Scheme 2b) rather than a radical pair.

Figure 3. UV−vis absorption spectra for poly[meso-Ni(II)-SaldMe] in the acetonitrile solution of 0.1 M (TBA)ClO4 recorded during anodic potential scanning under CV conditions. (a) Spectra recorded in the potential range 0−0.90 V. (b) Spectra recorded in the potential rage 0.90−1.30 V vs Ag/Ag+. (c) Changes in the normalized intensity of UV−vis absorption bands at (1) 323, (2) 480, and (3) >900 nm as a function of the potential applied.

The ESR spectroscopy measurements showed that intensity of the signal characteristic of the radical is lower at ∼1.30 V. This signal decrease suggests that the radical concentration decreases. At 0.90 V vs Ag/Ag+, a new broad band at ∼470 nm appears in the UV−vis spectrum (Figure 3b). This band can be assigned to the diamagnetic bisphenolic dication formed (Scheme 2b), thus confirming that the second anodic CV peak (Figure 1b) corresponds to oxidation of the bisphenolic radical cation to the bisphenolic dication. In the UV−vis spectra recorded in the potential range 0.90−1.30 V, there is the second isosbestic point at ∼405 nm (Figure 3b). Apparently, 16714


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enable constructing a supercapacitor with an internal charging−discharging indicator. It allows evaluating whether the supercapacitor is charged or discharged by observing the color of the anode of the supercapacitor. Similar multielectrochromic behavior was observed for the poly[Ni(Salens)] films based on Ni(II)−Salene complexes with the methyl group in the 3-position of each salicylaldehyde moiety.35 Those polymers exhibited yellow, green, and russet colors according to their oxidation states in the LiClO4 propylene carbonate supporting electrolyte solution. Moreover, one of those polymers, named poly[Ni(3-Mesalen)], was electrochemically stable with respect to 9000 cycles of pulsed chronoamperometry. For better understanding electronic properties of poly[mesoNi(II)-SaldMe], we performed a series of DFT calculations at the PBE level. To model the polymer structure, molecular structures of oligomers with different chain lengths and of different oxidation states were optimized. Structural optimization of the dimers (Scheme S1 and S2) shows that trans and cis isomers are almost isoenergetic (the difference is merely 2 kJ/ mol in favor of the trans dimer). The intermonomer bond length is 1.480 and 1.481 Å in the trans and cis isomer, respectively. Planes of connected benzene rings from different monomer units are twisted by 34° and 37° in the trans and cis form, respectively. Almost identical local geometry parameters were found for longer oligomers (trimers and tetramers), thus indicating that the oligomer structure reasonably represents that of the undoped polymer. Lengths of all C−O bonds in the tetramer were 1.303−1.304 Å. These values are intermediate between those typical for lengths of the single and double C−O bond (∼1.40 and ∼1.20 Å, respectively). Imine C−N bond lengths were ∼1.309 Å, whereas average Ni−O and Ni−N bonds were 1.864 and 1.851 Å long, respectively. The intermonomer bond lengths and nonzero twist (dihedral) angles show the presence of certain electronic communication between the monomer units in the undoped polymer. This inference is further justified by the band structure of the polymer. As a repeat unit in these calculations, we have chosen a trans isomer of the dimer and used the same intermonomer bond lengths and twist angles for the whole polymer. Figure 5 shows the computed band structure and electronic density of states. Whereas lowest-energy conductance bands are very flat, several high-energy valence bands exhibit non-negligible energy dispersion signaling considerable electronic communication in the polymer. In particular, the top valence band has major contribution coming from the polymer π-system with some contribution from Ni atoms and exhibits dispersion of 0.14 eV. The next valence band of similar origin has even higher dispersion of 0.22 eV (both bands are almost degenerated at the X-point, but their energy difference is 0.39 eV at the Γ-point). Positive charging clearly but not dramatically affects intermonomer interactions. In the dimer, the intermonomer bond length is 1.469 and 1.458 Å in the mono- and dication, respectively, and the intermonomer dihedral twist angle is decreased from 30° (neutral) to 30° (cation) and 21° (dication). Moreover, the C−C bond lengths in the phenyl rings are changed because of charging, in accordance with the pattern expected for the quinoid structure. Thus, there is a certain tendency of increasing the quinoid contribution to the structure (such as that shown in Scheme 2) at a higher charge, but the true double-bond nature of the intermonomer bond is still not reached. In longer oligomers, the effect of the positive

dication formation is energetically more favorable than formation of two radical cations. At this state, the bisphenolic radical cation and bisphenolic dication coexist in the poly[meso-Ni(II)-SaldMe] film (curves 2 and 3 in Figure 3c). This coexistence is evidenced by the UV−vis spectrum recorded at 0.90 V (Figures 3a) where two peaks merge to form one broad band extending from ∼400 to ∼600 nm. Electro-oxidation of the bisphenolic radical cation is confirmed by the decrease of absorbance of the band at ∼415 nm, characteristic of the radical cation, in the 0.90−1.30 V potential range and the increase of intensity of the dication band at ∼470 nm in the same range (Figure 3b,c). Above 1.0 V, bands at ∼415 and ∼950 nm decrease while a new band at ∼470 nm increases until the potential reaches the value of 1.30 V, i.e., the potential range end of the anodic polarization direction (Figure 3b). Presumably, this behavior is due to further oxidation of poly[meso-Ni(II)-SaldMe] to the bisphenolic dication. The results obtained clearly show a transition from the radical cation state to the dication state of the polymer. However, not all of the bisphenolic radical cations are oxidized to bisphenolic cations presumably because some electrons transferred through bisphenolic radical cations are unable to tunnel via the Ni(II) barrier. The changes observed in the absorption spectra of the poly[meso-Ni(II)-SaldMe] with the increase of the positive potential are reversible. That is, the absorption spectrum of the pristine polymer can be recovered by back (cathodic) potential excursion from 1.30 to 0.0 V (Figure S4). The poly[meso-Ni(II)-SaldMe] film exhibits reversible color change (electrochromism) depending on the potential applied. In the neutral state, the polymer film is yellow (Figure 4). There are two charged states of the polymer, namely, bisphenolic radical cation and bisphenolic dication. In these states, the polymer film changes color from yellow to orange and from orange to russet, respectively. Electrochromic and capacitive features of the poly[meso-Ni(II)-SaldMe] film

Figure 4. Color change of poly[meso-Ni(II)-SaldMe] film in propylene carbonate solution of 0.1 M (TBA)PF6 upon potential cycling under CV conditions. The potential scan rate was 20 mV s−1. 16715


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positive charge per a monomer unit) and, most likely, is not attained experimentally. Spin density distribution in the singly charged repeat unit shows predominant free spin localization on benzene rings and oxygen atoms with non-negligible contribution from nitrogen and nickel atoms (Figure 5d). Partial localization of the spin density on the Ni atom is in accord with experimental observation of noticeable deviation of the g factor of the radical cation state from that of the freeelectron value. Typically, such deviations are caused by a nonquenched spin−orbit coupling typical for transition metals. Non-negligible dispersion of the polymer bands means that the use of the oligomer for prediction of electronic excitation may encounter some difficulties (because it relies on the local nature of excitations). We performed TD-DFT calculations of electron excitations in the dimer (0, +1, and +2 states) and trimer (0 and +2 states) (Figures S6 and S7). For the neutral forms of oligomers, theory predicts the lowest energy excitations at 2.5 and 2.1 eV for the dimer and trimer, respectively. High-intensity transitions are predicted at somewhat higher energies (3.3−3.7 and 2.7−3.3 eV in the dimer and trimer, respectively). If systematic overestimation of transition energies by hybrid functionals is taken into account, then these data are in accord with those experimental provided by absorption spectra of the undoped polymer showing relatively large bandgap (Figure 3a). For the charged oligomers, the analysis of the TD-DFT computed spectra revealed deficiency of the oligomer approach with respect to model structure of the polymer electro-oxidized at E ≥ 1.0 V (see discussion in the Supporting Information). Presumably, the accuracy would be higher if calculations were performed for longer oligomers. However, already computations for the trimer are rather resource demanding, and hence, computations for longer chains are not feasible. In situ spectroelectrochemical UV−vis measurements were performed to unravel the mechanism of electropolymerization of the meso-Ni(II)-SaldMe monomer. Figure 6a shows the first potential cycle of the multiscan potentiodynamic electropolymerization leading to deposition of the poly[meso-Ni(II)SaldMe] film from an acetonitrile solution of 1 mM mesoNi(II)-SaldMe and 0.1 M (TBA)ClO4 on the ITO laminated electrode, between 0.0 and 1.30 V. Figure 6b presents corresponding UV−vis absorption spectra in situ recorded for selected potentials during the potentiodynamic measurements. A well-pronounced anodic peak at ∼0.90 V (Figure 6a) corresponds to electro-oxidation of the monomer to the bisphenolic radical cation. The anodic hump at ∼1.10 V presumably arises from oxidation of the resulting polymer to its dicationic form. At 0.90 V, first changes in the UV−vis spectra are observed (curve 2′ in Figure 6b). That is, a low-intensity broad band centered at ∼480 nm appears. The higher the potential, the higher is this band, and moreover, new bands at ∼340−380, ∼630, ∼670, and >900 nm appear and then increase. The positive charge generated is mobile at the experiment time scale but localized within the polymer (most likely, along the linear pathway via electron-donating oxygen atoms in the ligand moiety linked with the transition metal cation and πcoupled by aromatic rings in para positions). Thus, our experimental results indicate that the mechanism of electropolymerization proceeds through electro-oxidation of the monomer. In effect, the bisphenolic radical cation is formed. Then, double and single bonds of the bisphenolic radical cations are rearranged in the electric field leading to electron

Figure 5. (a) Molecular structure of poly[meso-Ni(II)-SaldMe] used in DFT calculations; the repeat unit (composed of the dimer) is shown in a rectangular frame. (b) The DFT-computed band structure of the polymer. (c) Angular-momentum resolved density of states. (d) Spin density distribution in the dimeric radical cation (one positive charge per repeat unit).

charge is weaker and its nature is more complex. Namely, there is a tendency to “localize” the charge on dimer fragments, especially clear for dications. This effect is reminiscent of the Peierls’ distortion of one-dimensional lattice and may be a deficiency of the oligomer approach to the periodic system with electronic communication. Aside from C−C bonds in phenyl rings and intermonomer bonds, other structural parameters of oligomers experience only minor changes upon charging. For instance, average lengths of CN, CO, and Ni−N bonds in mono- and dication in the tetramer are almost identical to those in the neutral molecule, and the Ni−O bond is shortened from 1.864 Å (neutral) to 1.857 and 1.851 Å in the mono- and dication, respectively. To summarize, from optimization of geometry parameters it follows that charging does not lead to dramatic changes in intermonomer bond length, although there is some tendency for bond shortening and structure planarization, as might be expected for the system with a pronounced quinoid nature. Presumably, the charge is distributed over the whole π-system with certain localization on Ni atoms. Similar conclusion can be drawn from the band structure calculations for the positively charged polymer (Figure S5). If electrons are removed from the topmost valence band, the polymer becomes metallic, especially in its dicationic state. Obviously, this state requires very high degree of doping (one 16716


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Figure 7. (a) FTIR-ATR spectrum of the poly[meso-Ni(II)-SaldMe] film deposited on the ATR crystal (ZnSe)/Au electrode, recorded immediately after polymer electrochemical synthesis. (b) In situ differential FTIR-ATR spectra of this film deposited on the ATR crystal (ZnSe)/Au electrode, recorded under CV conditions between 0 and 1.30 V vs Ag/Ag+ in the monomer-free acetonitrile solution of 0.1 M (TBA)ClO4, at the scan rate of 10 mV s−1.

Figure 6. (a) First cycle of potentiodynamic electropolymerization of 1 mM meso-Ni(II)-SaldMe in an acetonitrile solution of 0.1 M (TBA)ClO4 on the ITO electrode at the potential scan rate of 5 mV s−1. Points 1−5 represent the potential triggers of spectra sent from the potentiostat to the UV−vis spectrophotometer. (b) UV−vis absorption spectra in situ recorded at (1′) 0 to 0.80, (2′) 0.90, (3′) 1.00, (4′) 1.20, and (5′) 1.30 V Ag/Ag+ during the electropolymerization shown in (a).

presented above. At E ≥ 0.40 V, there were significant changes, mostly in the region of 1000−1700 cm−1. The increase of the band centered at ∼1090 cm−1 in this potential range evidently convinces that more perchlorate anions enter the film upon further polymer electro-oxidation in order to maintain film electroneutrality. Electronic charge moves along the polymer chain, and as a consequence of this charge delocalization, these spectral features are progressively enhanced with the extent of electro-oxidation. Vibrational spectra calculated for oligomers of different lengths were used to guide the assignment of the experimental data (Figures S8 and S9). The spectra for the neutral dimer, trimer, and tetramer were almost identical, thus confirming that the majority of vibrations have local nature and, hence, the oligomer reasonably describes the spectra. According to PBE/ TZ2P calculations, the high-intensity band at 1618 cm−1 in the experimental spectrum corresponds to the overlap of two intense modes, i.e., C−C stretching vibrations in phenyl rings (at ∼1600 cm−1) and stretching modes of imine CN bonds (at ∼1570 cm−1) (Scheme S3). In the charged states, the frequency of ν(CN) modes remains almost constant at ∼1570 cm−1, whereas phenyl modes experience not only noticeable red-shifts (∼20−40 cm−1) but also scattering in a broader frequency range (hence broadening of the bands). In the experimental spectra of the doped polymer, these changes correspond to the appearance of the negative peak at the position of the ν(C−C)Ph vibrations of the neutral polymer and increase of the intensity at ∼1570−1600 cm−1. Moreover, the

paring of aromatic rings at para positions. In effect, the polymer is linearly elongated. Furthermore, FTIR-ATR spectroelectrochemistry was used as the third technique directly confirming the presence of different poly[meso-Ni(II)-SaldMe] forms responsible for the charge transfer during electro-oxidation. FTIR spectroscopy is sensitive to polymer conformational and structural changes under electrochemical conditions.5 Figure 7a shows the FTIRATR spectrum of the poly[meso-Ni(II)-SaldMe] film in its initial state, i.e., a freshly prepared polymer. The presence of the band centered at 1090 cm−1 (Figure 7a), assigned to vibrations of the perchlorate anion bonds,9 indicates that this supporting electrolyte anion is partially entrapped in the polymer. By subtracting the reference spectrum (Figure 7a) from the spectra recorded at selected potentials, we were left with spectral changes, which occurred during poly[mesoNi(II)-SaldMe] doping at different potentials (Figure 7b). Electro-oxidation of the polymer results in the appearance, and subsequent growth, with the increase of the potential applied, of bands associated with vibrations of some bonds of the charge carriers. The bands that do not change with the potential change clearly indicate which part of the polymer does not directly participate in the charge transfer. Expectedly, there were no significant changes in the FTIR-ATR spectra in the potential range 0.0− 0.40 V vs Ag/Ag+ (Figure 7b). This result corresponds well to the UV−vis spectroelectrochemical results 16717


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Figure 8. (a) CV curves for the poly[meso-Ni(II)-SaldMe] film in 0.1 M (TBA)ClO4, in propylene carbonate, recorded for different potential scan rates. (b) Dependence of SC on the potential scan rate calculated from the CV depicted in (a). (c) Normalized specific capacitance vs the number of the CV cycle. CV measurements were performed for the potential range 0−1.30 V vs Ag/Ag+ in the monomer-free acetonitrile solution of 0.1 M (TBA)ClO4, at the scan rate of 10 mV s−1.

band located at ∼1650 cm−1 significantly increases. It confirms the presence of vibrations characteristic of α,β-unsaturated carbonyl groups (CPhCPh−CO) in the oxidized polymer fragments.36 Furthermore, strong involvement of the phenyl rings in the charge distribution is reflected by a significant increase in the intensity of C−H bending modes in phenyl groups upon charging. The bands in the 1200−1500 cm−1 range correspond to complex vibrations involving C−O and C−N stretches as well as vibrations in the phenyl rings. The doping results in the changes in the frequency and intensity distribution in this frequency range, thus indicating that positive charges in the doped states are delocalized over the whole Ph− O−Ni backbone of the polymer, in agreement with the spin density distribution depicted in Figure 5d. Therefore, the charge is most likely transferred along the −Ph−O−Ni−O− Ph− chain with the nickel atom serving as a charge tunneling barrier. This behavior is consistent with that of poly[Ni(II)salen].4 Hence, the polymer electro-oxidation in the present study is predominantly ligand centered, i.e., electrons are removed from the ligand where bisphenolic oxygen atoms play a role of redox centers. Furthermore, the potential range of poly[meso-Ni(II)-SaldMe] electroactivity is similar to that of poly[Ni(II)-salen] analogues.4,34 This similarity indicates that two electrodonating methyl substituents on the imine bridge do not significantly affect electrochemical activity of the resulting polymer and, moreover, imine moieties are not involved in the linear transfer of electrons. However, these methyl substituents

are oriented in one plane and contribute to electrochemical stabilization of the polymer by affecting relative position (mutual repulsions) of polymer chains within the polymer network. Moreover, the present spectral analysis confirms linear type conductivity of poly[meso-Ni(II)-SaldMe]. Moreover, the poly[meso-Ni(II)-SaldMe] film was herein examined as a plausible electrode material for energy storage in supercapacitors. At an early stage of the present research we designed and fabricated different poly(Ni(II)Salen) films with respect to their durability under multiscan CV conditions. However, these investigations were abandoned because unsubstituted and tetramethyl-substitued analogues were unstable under these conditions. Figure 8a shows CV curves at different potential scan rates for the poly[meso-Ni(II)SaldMe] film. From this curves, specific capacitance (SC) of the polymer film was calculated. Figure 8b shows the dependence of SC on the potential scan rate for the polymer film. The SC only slightly decreases with the increase of the potential scan rate, thus indicating effective mass transfer during discharging of the polymer film. Moreover, CV measurements showed that the anodic-to-cathodic peak potential separation increased with the increase of the potential scan rate. Apparently, the mass transfer rate within the film is the limiting factor influencing capacitive properties of the polymer film. Finally, the stability of the polymer film was examined under CV conditions. Figure 8c shows the dependence of the change of normalized capacitance 16718



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CONCLUSIONS The present mechanistic investigations show that a new mesoNi(II)-SaldMe monomer is oxidatively electropolymerized with initial generation of the bisphenolic radical cation. Then, these radical cations are paired at para positions to form a linear path of charge conduction. The resulting poly[meso-Ni(II)-SaldMe] is electroactive in the potential range 0.40−1.30 V vs Ag/Ag+. The polymer is electrochromic. That is, it switches between three colors, confirming the presence of the neutral, radical cationic, and dicationic state. Further electro-oxidation of this polymer is ultimately ligand centered with the Ni(II) metal center templating the ligand molecule planarity. That is, upon electro-oxidation electrons are removed from the phenolic part of the ligand where phenolic oxygen atoms are considered as redox centers. The accumulated positive charge is delocalized along the linear −Ph−O−(Ni-barrier)−O−Ph− conduction path. Importantly, the nickel cation is not the main redox center in poly[meso-Ni(II)-SaldMe], but it is considered as the tunneling barrier for electrons during the charge conduction process. The resulting polymer film is very stable under CV conditions. Its normalized specific capacitance decreased merely by ∼10% after 100 CV cycles and then was constant for at least subsequent 900 cycles. High value of SC, excellent durability, and fast discharging are very promising for potential application of the polymer as an electroactive material for energy storage devices.

of the electrode coated with the poly[meso-Ni(II)-SaldMe] film on the number of charge−discharge CV cycles. Initially, normalized SC of the poly[meso-Ni(II)-SaldMe] film abruptly decreased, and then reached ∼91% of its original value after ∼100 CV cycles, equal to ∼155 F g−1 at the potential scan rate of 2 mV s−1. Advantageously, subsequent consecutive charging and discharging of the electrode (900 CV cycles) did not change this SC value. The observed ∼10% drop in SC is presumably associated with irreversible mass transfer into and out of the polymer film during its doping and dedoping. Figure 9 shows the change of the charge stored in

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04700. CV curve for meso-SaldMe ligand; low-temperature ex situ ESR spectra of one-electron Ni(II)/Ni(III) electrooxidation in Salen monomers; UV−vis absorption spectra for the neutral and oxidized poly[meso-Ni(II)saldMe] during backward potential scanning; computational studies: optimization of parameters, band structures calculations for the neutral and charged polymer, charged states TD-DFT calculations, computations of IR spectra; synthesis of meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II), meso-Ni(II)saldMe (PDF)

Figure 9. Plots of the potential dependence of the charge stored in poly[meso-Ni(II)-SaldMe] (curves 1 and 1′) and corresponding changes in the normalized intensity of (a) the UV−vis absorption band at 323 cm−1 (curve 2) characteristic of the dedoped polymer as well as (b) the FTIR-ATR band at 1090 cm−1 (curve 2′) characteristic of ClO4−. The UV−vis and FTIR spectroelectrochemical measurements were performed under CV conditions at the potential scan rate of 5 mV s−1 and the potential range 0−1.30 V vs Ag/Ag+ in the acetonitrile solution of 0.1 M TBAClO4.

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the polymer film (curves 1 and 1′ in Figure 9a and 9b, respectively) and corresponding changes in the normalized intensity of the UV−vis absorption band at 323 cm−1 (curve 2 in Figure 9a) characteristic of the undoped polymer as well as the FTIR-ATR band at 1090 cm−1 (curve 2 in Figure 9b) characteristic of ClO4− as a function of the potential applied. Both UV−vis and FTIR-ATR spectroscopy measurements showed that doping of poly[meso-Ni(II)-SaldMe] is associated with irreversible trapping of the counterion. A charge stored in the polymer did not reach baseline after electro-oxidation and subsequent electroreduction. Similar behavior was observed for the FTIR-ATR band at 1090 cm−1, characteristic of ClO4−. This behavior significantly influences the polymer electrochemical stability/durability. The initial negative drop in SC results from irreversible charge trapping, and most likely, it is attributed to the charge that is unable to move linearly via conduction pathway along the charge transmitting −Ph−O··· O−Ph− moiety coupled by aromatic rings at para positions.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.P.). *E-mail: [email protected] (A.A.P.). ORCID

Piotr Pieta: 0000-0003-0341-2310 Aleksander Shkurenko: 0000-0001-7136-2277 Alexey A. Popov: 0000-0002-7596-0378 Wlodzimierz Kutner: 0000-0003-3586-5170 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was financially supported by the National Science Center (Krakow, Poland) through Grant 2012/07/D/ ST5/02241 to P.P. We acknowledge National Science Center of Poland, NCN 2014/15/B/NZ/01011 to W.K., for financial support. 16719


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