Unusual Electrochemical Properties of the Electropolymerized Thin

Article pubs.acs.org/JPCC

Unusual Electrochemical Properties of the Electropolymerized Thin Layer Based on a s‑Tetrazine-Triphenylamine Monomer Sandra Pluczyk,† Pawel Zassowski,† Cassandre Quinton, and Mieczyslaw Lapkowski*,§,†

‡

Pierre Audebert,

‡

Valérie Alain-Rizzo,

‡

†

Faculty of Chemistry, Silesian University of Technology, Strzody 9, Gliwice, 44-100, Poland PPSM, CNRS UMR8531, ENS Cachan, 61 Avenue du President Wilson, Cachan, 94235, France § Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowska 34, Zabrze, 41-819, Poland ‡

S Supporting Information *

ABSTRACT: In this work the electrochemical and spectroelectrochemical characterization of a s-tetrazine-triphenylamine derivative with a donor−acceptor−donor-type structure is reported, which in addition undergoes electrochemical polymerization. The investigations of the polymerized layers have revealed unusual characteristics. Thin polymeric film exhibits chargetrapping properties with great storage and memory effect. Moreover, the polymer displays reversible switching of fluorescence in oxidation mode and emission of light during reduction.

1. INTRODUCTION The significance of organic compounds in optoelectronics applications has been growing rapidly since the electrical conductivity of such materials had been discovered.1−4 The high requirements of modern technology have resulted in the continued search for new materials with improved properties. sTetrazine ring has been known for a very long time, since it was prepared for the first time at the end of the 19th century.5 Although its chemistry has been well-explored since,6 there still exist several new and original properties, including especially fluorescence, the investigation of which has only recently started. s-Tetrazine is also a promising building block for ambipolar and n-type materials7 because of its very high electron affinity and strong electron-deficient character.8 It has a low-lying LUMO (lowest unoccupied molecular orbital), which makes s-tetrazine derivatives good candidates for airstable n-type semiconductors.9 Moreover, the energy value of n−π* transition in s-tetrazine derivatives is weakly allowed in the visible-light range; therefore, they are colored and often fluorescent, which can be noticed even by the naked eye.10,11 The functionalization of the s-tetrazine ring with the electrondonating group gives an opportunity to design an ambipolar structure with electrofluorochromic properties−the ability of reversible switching of fluorescence by the application of an electrochemical potential.12−14 There are only a few reports about bipolar polymer based on s-tetrazine ring. Those investigations demonstrate ambipolar character,15 low band gap,16,17 and a huge potential of such kind of polymers in photovoltaics applications.8 Nevertheless, none of those © XXXX American Chemical Society

polymers were obtained by electrochemical polymerization. In fact, only rare s-tetrazine monomers undergo electropolymerization.18,19 Electrochemical polymerization is a very useful method for the preparation of active layer on conductive surface. This alternative route of conductive layer synthesis offers some advantages such as cheaper and faster preparation of pure product, and also enables an alternative approach for fundamental characterization.20,21 Taking into consideration all those facts, our aim was to prepare and characterize stable, electroactive layers by electrochemical polymerization of bipolar s-tetrazine derivatives, which could be used in electrofluorochromic windows. As a donor group, the triphenylamine moiety was chosen due to the fact it is a well-known electron-rich fluorophore capable of electropolymerization22−25 and in parallel employed in many organic electronic and optoelectronic applications.26−28 Our study has brought unexpected results on the polymers.

2. EXPERIMENTAL SECTION Materials. The chemical structure of the investigated stetrazine-triphenylamine derivative, the monomer on which bears this study, is shown in Figure 1. The synthesis was described earlier on.12 Electrochemical, as well as spectroelectrochemical, measurements were conducted in 0.1 mol dm−3 tetrabutylammonium tetrafluoroborate (Bu4NBF4) (SigmaReceived: November 26, 2015 Revised: January 26, 2016

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DOI: 10.1021/acs.jpcc.5b11555 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Density functional theory/time-dependent density functional theory (DFT/TDDFT) calculations were carried out with B3LYP29−31 combined with 6-31G(d)32,33 basis set. For investigated compounds ground-state geometries were optimized with no symmetry constrains to a local minimum, which was followed by frequency calculations. In all cases no imaginary frequencies were found. Optical transitions were calculated using the TDDFT method on earlier optimized geometries. All calculations in this work were conducted with a polarizable continuum model (PCM) using dichloromethane as solvent as implemented in Gaussian 09 software.34 Input files and molecular orbital plots were prepared with Gabedit 2.4.7 software.35 All calculations had been carried out with Gaussian 09 software.

3. RESULTS 3.1. Electrochemistry. The electrochemical characterization of s-tetrazine-triphenylamine (STTA) derivative (Figure 1) has been performed. The registered voltammogram (Figure 2) shows quasireversible reduction (peak separation is equal to ca. 0.122 V),

Figure 1. Chemical structure of the investigated STTA derivative.

Aldrich 98%)/dichloromethane (Sigma-Aldrich ≥99.9%) electrolyte. In the case of electrochemistry, measurements were carried out in 0.002 mol dm−3 solution of the investigated monomer. Spectroelectrochemical investigations were performed in 0.001 mol dm−3 s-tetrazine- triphenylamine solution. All spectroelectrochemical investigations of the polymeric film were conducted in monomer-free electrolyte with a working electrode covered with the film obtained by electrochemical polymerization. Instrumental Characterization. Electrochemical measurements, which also included electrochemical polymerization, were conducted using the Ecochemie AUTOLAB potentiostatgalvanostat model PGSTAT20 or M101. The obtained data were analyzed using the GPES program in the case of model PGSTAT20 or NOVA 1.8 in the case of AutolabM101. Cyclic voltammetry was used for electrochemical characterization. The typical three-electrode cell was comprised of the platinum disk electrode (diameter: 1 mm) as a working electrode, a platinum spiral as an auxiliary electrode, and a silver wire as a pseudo reference electrode which was calibrated versus the ferrocene redox couple. The solutions were purged with argon. Spectral measurements were carried out using a UV−vis Hewlett-Packard spectrophotometer 8453, Ocean Optics QE65000 and NIRQuest 512 diode array spectrometers, a JEOL JES-FA 200, X-band CW-EPR spectrometer operating at 100 kHz field modulation, an Hitachi 2500 fluorescence spectrometer, and a Renishaw InVia Raman microscope equipped with an 830 nm laser. UV−vis measurements during reduction processes were carried out using a thin-layer cell with an indium tin oxide (ITO) electrode as the working electrode, a platinum coil as the auxiliary electrode, and a silver wire as the pseudo reference electrode. Electron paramagnetic resonance (EPR) measurements were carried out in a cylindrical cell equipped with a platinum wire as the working electrode, a platinum spiral as an auxiliary electrode, and a silver wire as a pseudo reference electrode. Fluorescence spectroscopy was performed in a triangular quartz cuvette equipped with a set of electrodes like in UV−vis measurements. The Raman spectroelectrochemical investigations were conducted in a quartz cell with a set of electrodes similar to the one used for EPR measurements.

Figure 2. Voltammogram registered in 0.002 mol dm−3 solution of STTA monomer in 0.1 mol dm−3 Bu4NBF4/dichloromethane electrolyte. Scan rate 0.10 V s−1.

which is characteristic for a s-tetrazine ring and irreversible oxidation, classical for a triphenylamine with unsubstituted para-positions. The reduction process starts at −0.990 V and the peak maximum occurs at −1.014 V. The corresponding anodic peak appears at −1.136 V. The oxidation starts at 0.529 V and the maximum occurs at 0.658 V. Despite the presence of two triphenylamine groups, only one redox couple occurs in the positive potential range. However, this anodic peak current is significantly larger than the peak currents registered within the negative range of potential. Reduction of s-tetrazine ring is a one-electron process. The ratio of the peak current of oxidation and reduction process, which is equal to ca. 2.77, indicates that the oxidation process is multielectronic.19 Moreover, the presence of only one redox couple in the positive range of potential shows that the oxidation of both triphenylamine groups occurs concomitantly at the same potential.36 It also suggests the multielectron system involved in the oxidation of investigated molecules. Furthermore, electrochemical oxidation of investigated monomer leads to polymerization which is also a multielectron process. In the successive scans of cyclic voltammetry (CV) new peaks with maxima at 0.373 and 0.487 V appear in the anodic cycle, and minima at 0.353 and B


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The Journal of Physical Chemistry C 0.455 V in the cathodic cycle are observed. The emergence and the growth of new peaks for repetitive CVs indicate the deposition of an active layer on the surface of the working electrode. Therefore, the investigated monomer undergoes electrochemical polymerization. During the polymerization process, maxima of the new peaks are shifted to 0.388 and 0.496 (anodic cycle) and 0.339 and 0.437 (cathodic cycle), respectively. Moreover, peaks appearing at negative potentials also are shifted. In the last scan of polymerization, the minimum of the peak in the cathodic cycle occurred at −1.108 and the maximum of the anodic cycle is located at −1.038 V. The behavior of the polymeric layer obtained during the electrochemical oxidation of the STTA derivative was also investigated by cyclic voltammetry. Two reversible redox couples at positive potentials can be distinguished on the CV curves of the polymeric film (Figure 3).

Figure 4. CV voltammogram of the poly(STTA) film registered in the negative range of potential after previous “charge” to the layer. Scan rate 0.10 V s−1.

Figure 3. Voltammogram of the poly(STTA) registered in monomer free electrolyte. The arrow indicates the direction of the first scan. Scan rate 0.10 V s−1.

Figure 5. Voltammogram of the poly(STTA) layer registered in monomer free electrolyte. The arrow indicates the direction of the first scan. Scan rate 0.10 V s−1.

The voltammograms show two well-defined surface waves at positive potentials characteristic of the symmetric oxidation and reduction peaks of the poly(STTA). Peak maxima occur at 0.381 and 0.497 V for the anodic scan and at 0.366 and 0.476 V, respectively, for the cathodic scan. Peaks are sharp and clearly separated. The area under curves is almost the same in the anodic cycle as in the cathodic cycle, demonstrating the chemically reversible character of the process. The thin film is also active in the negative range of potential. One reversible redox couple is observed peaking at −1.056 V for the cathodic cycle and at −1.035 V for the anodic cycle. Additionally, a small peak with a minimum at −0.883 is observed in the cathodic cycle. The second scan was different from the first. After the scanning of the film at negative potentials, a new peak at 0.129 V appears which is stable in the successive scans. It is an effect of release of the trapped negative charges. The area under reduction peak (peak with minimum at −1.056 V) is almost equal to the sum of areas under the oxidation peak in the negative range of potential (peak with maximum at −1.035 V) and the new peak at 0.129 V. All of the redox systems are stable over a wide range of potential (Figure SI.1, Supporting Information). However, scanning the film only at the negative potentials results in a fast decrease in intensity until the entire disappearance of the redox system (Figure 4), but after oxidation of the investigated layer, the redox couple is completely recreated (Figure 5). 3.2. Spectroelectrochemistry of Monomer. The changes in absorption properties were traced during the

reduction, as well as the oxidation of the studied monomer. STTA monomer exhibits strong absorption with a maximum at 300 nm. As a result of the reduction, the absorption band related to the neutral form of the monomer (300 nm) progressively loses its intensity and broadens (Figure 6a). Additionally, a slight band grows at ca. 470 nm. The isosbestic point at 328 nm might be noticed on the set of the UV−vis spectra, which means that both the neutral and reduced form of monomer are in the solution. The changes in UV−vis spectra recorded during oxidation of monomer are shown in Figure 6b. First, the absorption band connected with the neutral state of the monomer (located at 300 nm) gradually loses its intensity and simultaneously is shifted to a shorter wavelength. In the same time new absorption bands appear: a sharp band with a maximum of absorption at 488 nm and a broad absorption band with a maximum at 706, 1300, and 1480 nm. Next, when the peak at 488 nm achieves its maximum, it turns into a broad band without clearly outlined maximum. Concomitantly, broad bands at 1300 and 1480 nm disappear, peak at 706 nm still grows, and the absorption band at around 300 nm stops decreasing. It is stabilized and created a maximum at 330 nm. The oxidation of monomer leads to deposition of the polymeric layer on the working electrode. Decreasing the working electrode potential to 0 V results in the recording of the neutral film spectra (Figure 6b: the green curve with maximum at 348 nm). C


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Figure 6. Set of UV−vis spectra recorded during: (a) reduction; (b) oxidation of the STTA monomer (the green curve corresponds to spectra of film deposited on the working electrode).

To investigate the nature of charge carriers generated in the reduction process, EPR spectroelectrochemistry measurements were conducted. In the neutral state, no EPR signal is observed. When the working electrode reaches reduction potential adequate of the investigated compound, a multiline, wellresolved spectrum is obtained (Figure 7). The observed spectra

Figure 8. Set of UV−vis spectra recorded during electrochemical oxidation of the poly(STTA) film deposited on the ITO electrode via electrochemical polymerization.

polymer. The changes in spectra are fully reversible. In the reduction mode no change in the UV−vis−NIR spectra was observed, even if the polymer layer has been oxidized previously (Figure SI.2). EPR spectroelectrochemistry measurements show that the oxidation of the polymeric film leads to the cation-radicals generation (Figure 9). The first step of the layer oxidation (onset: ca. 0.30 V) results in the emergence of the EPR signal, which starts to disappear when scanning farther than the maximum potential of the first oxidation peak (ca. 0.42 V). When the working electrode potential is scanned back, the reproduction of the EPR signal was observed in the same potential range. Further lowering of potential causes loss of the signal. The reduction of polymeric layer further on does not generate charge carriers detectable by EPR spectroscopy. To study the emission properties of the polymeric film layer, fluorescence spectroelectrochemistry (electrofluorochromism) measurement was performed. During scanning the film in the range of positive potentials, oscillations of the fluorescence response (excitation wavelength, 400 nm; emission wavelength, 485 nm) were observed (Figure 10). The oxidation of the polymeric layer leads to fluorescence quenching, which is a reversible process. In the return cycle, the regeneration of fluorescence is detected. This trend is recurrent throughout the consecutive CV scans. The excitation (350 nm) during reduction of the investigated layer shows a very interesting effect. After scanning at positive potentials, the reduction of the layer results in an intense emission at 460 nm

Figure 7. Experimental (black dashed line) and simulated (red line) EPR spectra of radical anion generated during electrochemical reduction of STTA derivative (modulation width 0.050 mT, microwave power 1 mW, Hffc constants used for simulating spectra aN = 0.530 mT, line width 0.023 mT).

can be simulated assuming isotropic hyperfine interactions of the unpaired electron with the four nuclei of nitrogen atoms of the s-tetrazine ring (hyperfine constants is equal to 0.53 mT). 3.3. Spectroelectrochemistry of the Polymeric Film. The polymeric film deposited on the ITO-coated quartz electrode surface via electrochemical oxidation was investigated by UV−vis−NIR spectroelectrochemistry during the oxidation (Figure 8) as well as reduction process (Figure SI.2). One sharp peak at 348 nm can be distinguished in the UV−vis spectra of the neutral film, which gradually decreases in intensity during the whole oxidation process (Figure 8). However, a two-step oxidation of the polymeric layer is observed. In the first step, a sharp absorption band located at 480 nm and a broad band with a maximum at ca. 1400 nm start to appear. The increase of the working electrode potential up to the potential corresponding to the second step of oxidation results in bleaching of those new peaks and a progressive increase of the peak at 725 nm. After the direction of the working electrode polarity changes, the UV−vis−NIR spectra returns to the neutral form of the D


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Figure 9. (a) EPR spectra of a radical cation generated during electrochemical oxidation (E = 0.40 V) of the poly(STTA) layer deposited on the platinum electrode via electrochemical polymerization; (b) changes in the relative concentration of spins generated during oxidation of polymeric film.

To characterize the chemical structure of the generated cation radicals by the electrochemical oxidation of the obtained layer, in situ Raman spectra were registered (Figure 12). The

Figure 10. Fluorescence spectroelectrochemistry at excitation wavelength 400 nm and emission wavelength 485 nm; spectra were registered during cyclic voltammetry within the potential range from −0.25 to 0.45 V; scan rate 0.05 V s−1, 8 cycles. Figure 12. Changes in the Raman spectra during oxidation of the poly(STTA) film.

(Figure 11). Fluorescence starts to appear at the beginning of the anodic peak during the return scan, and gradually grows in the intensity up to the potential corresponding to the charge trapping peak at the positive range of potential.

neutral state of the film does not show any visible bands in the Raman spectra. The bands at 284, 698, 1149, and 1416 cm−1 are associated with the background. As a result of the first step oxidation, new bands at 390, 536, 738, 901, 1157, 1189, 1325, 1369, 1517, and 1558 appear. Further oxidation through the second step leads to an intensity decrease of those bands. The changes are reversible.

4. DISCUSSION Electrochemical data make it possible to estimate some crucial parameters for (opto)electronic application. On the basis of the onset potential of reduction (Ered), the electron affinity (EA) was calculated. Analogously, on the basis of the onset potential of oxidation (Eox), the ionization energy (EI) was evaluated. According to The Koopmans’ Theorem, the amount of energy required to remove an electron from an orbital (the ionization energy) is approximately equal to the energy of this orbital assuming that the orbitals of the ion are identical to those of the neutral molecule. Hence, the first ionization energy could be correlated with the energy values of the highest occupied molecular orbital (HOMO). Similarly, the electron affinity is correlated with the energy of the lowest unoccupied molecular orbital (LUMO). However, in this case the approximation is

Figure 11. Fluorescence spectroelectrochemistry at excitation wavelength 350 nm and emission wavelength 460 nm; spectra were registered during cyclic voltammetry within the potential range from −1.25 to 0.50 V; the first scan in a positive direction; scan rate 0.05 V s−1, 2 cycles. E


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The Journal of Physical Chemistry C worse than in the case of HOMO energy. Some differences appear between the values obtained from electrochemical data and DFT calculations (Table 1). However, the value of the electron affinity energy is higher than −4.00 eV in both cases, which limits the use of the investigated monomer in air-stable devices.37

Table 2. Raman Shifts of the Oxidized Polymeric Film wave number (cm−1) 390 536 738 901 1157 1189 1325 1369 1517 1558

Table 1. Electrochemical Data Compared with DFT Calculations electrochemical data DFT calculation a b

EIa (eV)

EAb (eV)

Eg (eV)

−5.33 −5.16

−3.81 −3.08

1.52 2.08

a

−Eox + 4.8. Eox and Ered: potential onset of corresponding processes. −Ered + 4.8. Eox and Ered: potential onset of corresponding processes.

assignmentsa inter-ring deformation

C−H bending (Q) C−H bending (B) C−N stretching (cationic) C−C stretching (Q) CC stretching

Q = quinoid, B = benzenoid.

the registered voltammogram was investigated. The differences between the voltammograms observed while scanning in the different potential ranges indicate the occurrence of chargetrapping phenomena in the film. This is suggested by the appearance of prepeaks which precede the main peak when the film is scanning over a wide range of potential (Figures 3, 4, and 6).45,46 Those peaks do not occur in the first CV scan without previous reduction (Figure 3) or oxidation (Figure 5). This is a well-known phenomenon, but in our case, the layer displays great storage properties. During the positive potential scan, the layer is “charged”. The reduction of the “charged” layer results in the gradual disappearance of the redox couple in the negative range of potentials (Figure 4). Subsequent oxidation of investigated film leads to the recovery of this redox system. Such a property could be useful for example in the building of memory devices.47−49 A huge effort has been made to develop charge-trapping layers to get as large as possible memory window. Moreover, the employment of an ambipolar layer could lead to achieving a large window of multibit flash memories.50 Nonconjugated redox polymer layers of poly(STTA) can be used also as charge-transport materials in various wet-type devices for energy conversion and storage, such as secondary batteries. The capability of radical polymers of acting as both cathode and anode active materials by tuning the redox potentials allows the fabrication of an organic battery.51−53 Most of the known materials used in radical organic batteries exhibit only one type of electrode activity. They can be applied only as an anode or cathode material, whereas poly(STTA) displays both types of properties. Furthermore, excitation of the active layer during reduction results in the emission of a blue light (Figure 11). To the best of our knowledge, no material with such properties has been described in the literature. Moreover, there is certainly no report of such a stable material obtained by electrochemical methods. To conduct more precise characteristics of the investigated monomer as well as electrochemically obtained polymer, the nature of charge carries generated during electrode processes were studied by UV−vis and EPR in situ spectroscopy. It is obvious that the reduction of monomer generates an anion radical which is located on the s-tetrazine ring in the central part of the molecule. This was confirmed by EPR spectroscopy measurements and simulation (Figure 7) as well as by DFT calculations (Figure SI.3). DFT calculations allow prediction of properties of new materials. Those methods provide also explanations for unexpected behavior of those materials and help in better understanding of the mechanism of how they work.54 DFT calculations have shown also good agreement

On the basis of cyclic voltammetry measurements, it could also be concluded that the investigated STTA derivative undergoes electrochemical polymerization easily. Although literature mentions the possibility of triphenylamine (TPA) in electropolymerization,23 it is not very common. TPA is known to dimerize through electrochemical oxidation to give N,N,N′,N′-tetraphenyl-4,4′-diaminobiphenyl (TPB);24 TPB does not effectively couple to give polymers.38 Introduction of a blocking group between the amino units would prevent the cation radicals formed in the first oxidation step from delocalization, resuming their reactivity toward electropolymerization.39 Electrochemically obtained poly(STTA) layer exhibits unique electrochemical properties. Analyzing the shape of voltammogram recorded in the free-monomer electrolyte with the working electrode covered with polymerized film it could be concluded that the obtained polymer exhibits redox properties.40 The registered peaks are narrow and sharp (Figures 3 and 4); the peak-to-peak separation of the forward and reversed waves is equal to 0.015 V in the case of the first oxidation step and 0.021 V in the case of the second oxidation step and reduction. Additionally, no capacitive current is observed on those voltammograms, which also confirmed that received polymer backbone is probably not a conjugated one.41 The conjugation here is broken as an effect of the connection of the s-tetrazine units with tetraphenylbenzidine units by the oxygen atom. Taking this into account, this polymer can be considered as a copolymer consisting of donor and acceptor units which do not interact directly with each other, but via electron hopping. Considering the structure of polymeric layers, it is probably branched (not linear). It means that some of the monomers could undergo polymerization at not only one para position in a triphenylamine moiety but also at both of them. It is also possible that cross-linking occurs in the polymeric structure of poly(STTA) during electrochemical oxidation. Unfortunately, on the basis of reported data, it is not possible to determine the structure precisely. On the basis of Raman spectroelectrochemical results, it can be concluded that the oxidized form of the polymer consists of both quinoid and benzenoid units. The bands at 1157 and 1188 cm−1 are assigned to C−H bending, the first in the quinoid form and the second in benzenoid form (Table 2).42−44 Taking into account the aforementioned remarks, the schemes of electrode reactions occurring in polymeric film could be proposed (Schemes 1 and 2). Additional interesting electrochemical behavior of obtained film was observed when the influence of the range potential on F


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The Journal of Physical Chemistry C Scheme 1. Anodic (Oxidation) Reaction Occurring in Polymeric Film of Poly(STTA)a

a

Tz is s-tetrazine.

ature.56,57 Some STTA compounds also were investigated during chemical oxidation.12,58 Our results are similar to this presented in ref 56 with the difference that in our case both triphenylamine units are oxidized concomitantly at the same potential. Hence, new bands formed during oxidation of monomer (488, 706, 1300, and 480 nm) start to increase at the same time. During the electrochemical oxidation, the monomer undergoes polymerization. Thereby, changes shown in Figure 6b are the results of the oxidation of the monomer as well as the polymeric film which is deposited on the working electrode. This can be seen clearly by comparing the results presented in Figure 6b (UV−vis spectroelectrochemistry during oxidation of monomer) and Figure 8 (UV−vis spectroelectrochemistry during oxidation of polymeric film). In the second case, the two-step oxidation of polymeric film can be observed (Figure 8). Together with EPR spectroelectrochemistry (Figure 9b) it can be concluded that the first oxidation step is related to generation of cation radicals with a g-value of 2.0034 (Figure 9a), and which absorb at 480, 1400 nm. The second oxidation step is associated with the transition of the cation radicals to dications which absorb at 725 nm and do not provide any EPR signal. This behavior is typical of N,N′-diphenyl-N,N′-bis(4phenyl)benzidine-containing polymers.59−61 During the oxidation of the polymeric film, changes in the emission properties were observed. We demonstrate the reversible electrofluorochromism of the polymeric film (Figure 10).

Scheme 2. Cathodic (Reduction) Reaction Occuring in Polymeric Film of Poly(STTA)a

a

Bz is N,N,N′,N′-tetraphenylbenzidine.

with experimental data.55 The anion radical possesses similar absorption spectra as the neutral form of the monomer. The UV−vis spectra of the monomer is the result of the superposition of the absorption spectra of the triphenylamine substituent36 and s-tetrazine ring.7 The low band gap estimated from the CV measurements is not consistent with absorption at short wavelength (300 nm) which is related to high binding energies. On the basis of DFT calculations, it was found that this peak is not connected with a HOMO−LUMO transition (Figure 13a) The value of the oscillator strength (f) of the HOMO−LUMO transition is very low, so it could not be observed on experimental UV−vis spectra. The DFT calculations also confirmed the experimental UV−vis spectroelectrochemical results. The simulated spectra of the anion radical (Figure 13b) is almost the same spectrum as the neutral form of our monomer. However, a slight peak at 464 nm appears additionally on the experimental spectra (Figure 6a). The UV−vis spectroelectrochemistry in oxidation of triphenylamine derivatives is well-documented in the liter-

Figure 13. Simulated UV−vis spectra of STTA in (a) neutral state and (b) reduced form. G


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P.Z. are scholars supported by the “Doktorisscholarship program for an innovative Silesia”, cofinanced by European Union within European Social Fund. This research was supported in part by PL-Grid Infrastructure.

The reduction of the polymeric film does not lead to any changes in UV−vis spectra (Figure SI.2). Although the reduction of s-tetrazine ring generates the anion radical (Figure 7), the EPR spectra is not observed during reduction of poly(STTA). This could be the result of the coupling of the opposite spins in dimers of tetrazine units, which form in the solid state of the polymeric layer. Such types of phenomena have been observed for molecules containing four nitrogen atoms, as in the case of the formation the diamagnetic π[tetracyanoethylene]22− dimers62 and for several other compounds, such as 1,2,4,6-thiatriazinyl radical dimer,63,64 2,3-dicyano-5,6-dichlorosemiquinone radical anion dimers,65 and conjugated oligomers.66

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5. CONCLUSION In summary, we have studied the electrochemical and spectroelectrochemical behavior of the STTA derivative with a donor−acceptor−donor type of structure. The investigated monomer exhibits bipolar properties. It undergoes quasireversible electrochemical reduction which leads to the generation of stable anion radicals located at the s-tetrazine ring. Electrochemical oxidation results in the deposition of electroactive polymeric film on the surface of the working electrode. The film obtained exhibits very interesting and unique electrochemical and emission properties: the differences between the voltammograms registered during scanning in different potential ranges indicate the occurrence of chargetrapping phenomena, which suggest great storage properties. Upon scanning of the film in the positive potential range, the layer is “charged”. The reduction of “charged” layer results in the complete disappearance of the redox couple in the negative range. Subsequent oxidation of the “discharged” film restores this redox system. Materials with this type of properties have been proposed for applications in memory devices. Moreover, film excitation during reduction results in the emission of a blue light. To the best of our knowledge, no materials with such properties have been described in the literature.

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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.5b11555. CV voltammogram of poly(STTA) in the wide range of potential, UV−vis spectra recorded during reduction of poly(STTA) film, and distribution of spin density in radical anion (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: (+48) 32 237 15 09. Fax: (+48) 32 237 15 09. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by research project 2013/11/N/ST5/ 02005 funded by the Polish National Science Centre. S.P. and H


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