Highly conductive polyporphyrin films obtained by superoxide-assisted electropolymerization of para – aminophenyl porphyrin

Highly conductive polyporphyrin films obtained by superoxide-assisted electropolymerization of para – aminophenyl porphyrin

Journal Pre-proof Highly conductive polyporphyrin films obtained by superoxide-assisted electropolymerization of para – aminophenyl porphyrin Sergey M...

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Journal Pre-proof Highly conductive polyporphyrin films obtained by superoxide-assisted electropolymerization of para – aminophenyl porphyrin Sergey M. Kuzmin, Svetlana A. Chulovskaya, Vladimir I. Parfenyuk PII:

S0254-0584(19)31209-X

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122394

Reference:

MAC 122394

To appear in:

Materials Chemistry and Physics

Received Date: 5 August 2019 Revised Date:

30 October 2019

Accepted Date: 1 November 2019

Please cite this article as: S.M. Kuzmin, S.A. Chulovskaya, V.I. Parfenyuk, Highly conductive polyporphyrin films obtained by superoxide-assisted electropolymerization of para – aminophenyl porphyrin, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122394. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

HIGHLY CONDUCTIVE POLYPORPHYRIN FILMS OBTAINED BY SUPEROXIDEASSISTED ELECTROPOLYMERIZATION OF PARA – AMINOPHENYL PORPHYRIN Sergey M. Kuzmina,b,*, Svetlana A. Chulovskayaa, Vladimir I. Parfenyuka,c a

G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, Ivanovo 153045, Russia b Ivanovo State Power Engineering University, Ivanovo 153003, Russia c Ivanovo State University of Chemistry and Technology, Ivanovo 153000, Russia

ABSTRACT:

The process of deposition of 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (H2T(4NH2Ph)P) electopolymer films induced by superoxide anion radical in dimethyl sulfoxide (DMSO) has been studied. By the methods of electrode impedance spectroscopy and atomic-force microscopy it has, for the first time, been proved that it is possible to form a poly-H2T(4-NH2Ph)P

film

in

DMSO in

potentiostatic conditions

at

oxygen

electroreduction potentials. Charge transfer through the poly-H2T(4-NH2Ph)P film has been characterized by modeling electrochemical impedance spectroscopy (EIS) data. It has been shown that in potentiostatic conditions, the formation of a poly-H2T(4-NH2Ph)P film induced by superoxide anion radical consists of at least two stages. At the first stage, the resistance of the surface film changes insignificantly, and at the second one it increases approximately linearly over time. Using the vibrational spectroscopy we have determined the type of binding of monomer fragments in the polyporphyrin film. The morphology and thickness of the deposited polyporphyrin films have been studied by the methods of atomic force microscopy. Based on the resistance and thickness estimates, we have determined the value of specific conductivity of the obtained film (about 4.2•10-4 S cm-1). The metal-free aminophenyl porphyrin electopolymer was also successfully tested as an electrocatalyst for the oxygen reduction reaction.

KEYWORDS: tetrakis(para-aminophenyl)porphyrin, superoxide-assisted electrochemical deposition, poly-porphyrin films, electrical conductivity, metal-free electrocatalyst.

*Correspondence to: Sergey M. Kuzmin, e-mail: [email protected], tel: +7 915-824-9554

HIGHLY CONDUCTIVE POLYPORPHYRIN FILMS OBTAINED BY SUPEROXIDEASSISTED ELECTROPOLYMERIZATION OF PARA – AMINOPHENYL PORPHYRIN Sergey M. Kuzmina,b,*, Svetlana A. Chulovskayaa, Vladimir I. Parfenyuka,c a

G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, Ivanovo 153045, Russia b Ivanovo State Power Engineering University, Ivanovo 153003, Russia c Ivanovo State University of Chemistry and Technology, Ivanovo 153000, Russia

ABSTRACT:

The process of deposition of 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (H2T(4NH2Ph)P) electopolymer films induced by superoxide anion radical in dimethyl sulfoxide (DMSO) has been studied. By the methods of electrode impedance spectroscopy and atomic-force microscopy it has, for the first time, been proved that it is possible to form a poly-H2T(4-NH2Ph)P film in DMSO in potentiostatic conditions at oxygen electroreduction potentials. Charge transfer through the poly-H2T(4-NH2Ph)P film has been characterized by modeling electrochemical impedance spectroscopy (EIS) data. It has been shown that in potentiostatic conditions, the formation of a poly-H2T(4-NH2Ph)P film induced by superoxide anion radical consists of at least two stages. At the first stage, the resistance of the surface film changes insignificantly, and at the second one it increases approximately linearly over time. Using the vibrational spectroscopy we have determined the type of binding of monomer fragments in the polyporphyrin film. The morphology and thickness of the deposited polyporphyrin films have been studied by the methods of atomic force microscopy. Based on the resistance and thickness estimates, we have determined the value of specific conductivity of the obtained film (about 4.2•10-4 S cm-1). The metal-free aminophenyl porphyrin electopolymer was also successfully tested as an electrocatalyst for the oxygen reduction reaction.

KEYWORDS: tetrakis(para-aminophenyl)porphyrin, superoxide-assisted electrochemical deposition, poly-porphyrin films, electrical conductivity, metal-free electrocatalyst.

*Correspondence to: Sergey M. Kuzmin, e-mail: [email protected], tel: +7 915-824-9554

1. Introduction At the modern stage of development of the chemistry of macroheterocyclic compounds we are witnessing achievements in the field of porphyrin synthesis [1-5], growing interest in the studies of their physical and chemical properties [6-8] as well as in the design and practical applications of porphyrin-based materials [9, 10]. Specific features of the porphyrins, such as high thermal and chemical stability of the macrocycle, ability to form stable complexes with different metals including those with different and unusual oxidation states of the central metal atom, excellent chromophore properties of the π-conjugated system of the macrocycle, various supramolecular behaviour, high biocompatibility, etc., make this class of compounds attractive in terms of solving a lot of applied problems. For example, high biocompatibility of porphyrins allows considering tetraphenylporphyrin as a promising drug for photodynamic therapy of cancer [11, 12] and as a possible antioxidant agent [13-18]. Developing methods of formation of polyfunctional porphyrinbased film materials [19] is of great importance for creating catalysts [20-22], sensors [23-25], solar cells [26], nonlinear optics devices [27, 28], electronic devices [29, 30], etc. The formation of the active layer for such devices by using electrochemical methods seems quite attractive as their implementation is quite simple and it allows obtaining well-reproducible films of controlled thickness. And in case of aminophenylporphyrins, film formation can be achieved both by polymerization induced by electrochemical oxidation of the precursor [31-40] and by porphyrin interaction with the electrochemically synthesized superoxide anion radical (O2•−) [41-44]. And metal-free H2T(4-NH2Ph)P can be considered not only as an elementary member of the aminophenylporphyrin series but also as a metal-free catalyst for enectrocatalytic reduction of a nitrate ion [45] and for photosynthesis of hydrogen peroxide via two-electron reduction of oxygen [46]; as an electrochromic material [47]; as a component of the active layer of solar cells [48]; as methanol barriers in direct methanol fuel cells [49], etc. It is worth noting that several transition metals have been inserted into the poly-aminophenylporphyrin films after deposition [50]. As a rule, poly aminophenyl porphyrin films obtained via porphyrin electrochemical oxidation have a large specific surface due to their nanoscale structure[45, 47, 51-53]. In contrast, applying the superoxide-assisted electrochemical deposition method to form an aminophenylporphyrin electropolymer allows obtaining smooth and compact films [39, 41]. Besides, inclusion of dimethyl sulfoxide (DMSO) in the number of solvents used for coating deposition increases the variability of electrochemical approaches. Additionally, the low toxicity of DMSO brings us to sufficiently safe technological procedures. That is why a detailed analysis of the electropolymerization process of this porphyrin seems important both for developing the method of film electrodeposition induced by O2•− and for understanding the effect of the precursor nature on the formation of films and their properties.

2. Experimental H2T(4-NH2Ph)P has been synthesized by a two-stage method [54]. The chromatographically purified H2T(4-NH2Ph)P shows that the characteristics of the synthesized aminophenylporphyrin (Fig. 1) agree with the literature data [55]: Rf = 0.36; (1:2 hexane:acetone); 1H NMR (500 MHz; DMSO-d6; Me4Si, Bruker AVANCE500): δH, ppm -2.75 (2H, s), 5.58 (8H, s), 7.01, (8H, d, J = 7.5 Hz), 7.86, (8H, d, J = 7.5 Hz), 8.88 ppm (8H, s); UV-VIS (DMSO, Cary 50) λmax, nm (logε): 437(5.25), 529(4.24), 578(4.41), 667(4.21).

Fig. 1. Structural formula of H2T(4-NH2Ph)P.

In this work, we used 10-3 М solutions of H2T(4-NH2Ph)P in dimethyl sulfoxide (DMSO > 99.5%, Aldrich) containing 0.02M tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte. The solutions were prepared by the gravimetric method («Sartorius» ME215S). The electrochemical experiment was made in a three-electrode electrochemical cell. The mirror polished face of a glassy carbon rod (Ø 4 mm) pressed into a fluoroplastic shell was used as the working electrode. The reference electrode was a calomel electrode Hg/Hg2Cl2, Cl− (1 М LiCl) with a Luggin capillary, the auxiliary electrode was a disc (∅ 25 mm) of platinized platinum. Before each experiment, the active surface of the working electrode was mirror-polished, degreased with alcohol, carefully washed with distilled water and then rinsed with the solution under study. Before the experiments, the solutions were saturated with oxygen by passing oxygen through a bubbling tube for 30 minutes. During the experiment, the oxygen was passed over the solution. The working electrode potential was set with a SP-150 potentiostat (Bio-Logic Science Instruments, France). The film deposition experiments by the method of electrochemical quartz microbalance (EQMB) were made with a QCM922A apparatus (SEIKO EG & G, Japan) in conditions of natural aeration. And

the working electrode was one of the quartz crystal sides covered with Pt (∅ 5 mm); the reference electrode was a saturated chloride-silver electrode, the auxiliary electrode was a Pt wire. The electrode impedance spectroscopy was carried out on an impedance analyzer Solartron 1260A with an electrochemical interface Solartron 1287А. The amplitude of sinusoidal measuring voltage was 10 mV, the frequency ranged from 10-1 to 105 Hz. The kinetics of changes in the interface parameters was measured by alternating measurements of impedance characteristics at the given potential of the working electrode and potentiostatic polarization of the working electrode at the same potential. The results were analysed by using programs ZView2 and CView3.10. The images of the surface of the poly-H2T(4-NH2Ph)P films were obtained by the method of atomic-force microscopy (AFM) with a SolverP47-PRO (MDT, Russia) employing Nova RC 1.0.26.578 software. UV-Vis spectra were obtained using a Cary 50 spectrometer (Varian, USA). The spectra of diluted DMSO solution of porphyrin were recorded in a 1 cm quartz optical cell; the spectra of the films on the ITO plate electrode were recorded in a 1 cm quartz optical cell using DMSO as the immersion medium. The IR-spectra were recorded by a Fourier transform IR spectrometer Bruker Vertex 80 in the region of 4000−400 cm−1 with a resolution of 2 cm−1. The spectra of the initial H2T(4-NH2Ph)P were obtained in KBr tablets; of the films – by the method of attenuated total reflectance (ATR) with a Harrick MVP2 SeriesTM attachment (with a diamond prism). To compare the ATR spectrum with the transmission spectra, we carried out its automatic correction in Opus 6.5 software (Bruker). The SEM micrographs were obtained using a microscope «VEGA 3 SBH» (Tescan, Czech Republic). The microscope resolution did not exceed 3 nm at the accelerating voltage of 30 kV (secondary-electron micrograph), the accelerating voltage ranged between 0.2 and 30 kV, the increase was from 3 to 1000000, the electron beam current did not exceed 2 µA. The micrographs of the polyporphyrin films surface were obtained at the accelerating voltage of 5 kV, and the field-of-view ranged from 10 to 150 µm. 3. Results and discussion As the data obtained show (Fig. 2a, 3a), the following processes are observed in the H2T(4NH2Ph)P solutions in DMSO in the presence of oxygen: 1. porphyrin electrooxidation 2. oxygen electroreduction

Porph ↔ Porph+ + е (the peak current is around +0.6 V), О2 + е ↔ O2•−

(the peak current is around -0.9 V),

3. porphyrin electroreduction Porph + е ↔ Porph− (the peak current is around -1.25 -1.25 V). In the first cycle, the H2T(4-NH2Ph)P film deposition proceeds both in the region of oxygen electroreduction potentials and in the region of porphyrin electrooxidation potentials (Fig. 2a). And the mass of the film formed in the region of oxygen electroreduction potentials is significantly bigger than that of the film formed as a result of direct porphyrin electrooxidation. In the second

cycle, the region of film formation potentials in electroreduction conditions becomes noticeably wider. And in the region of positive potentials, desorption of the deposited layer is observed; as a result, the film mass change for the cycle region in the range from 0 to +0.8 0 до +0.8 В is equal to zero. Starting from the third cycle, the film growth associated with the porphyrin electroreduction stops, and desorption is observed in the region of porphyrin electroreduction potentials. By the tenth cycle, desorption is observed when the potential changes from negative values to positive ones and proceeds in three stages: slow mass reduction in the range from -1.6 to -0.6 V; fast mass reduction in the range from -0.6 to -0.3 V and fast mass reduction in the range from +0.6 to +0.8 V. Film deposition is observed when the potential changes from positive to negative values in the range from -0.6 to -1.8 V. The appearance of several film deposition waves can be attributed both to the competition between the deposition and desorption processes and to the participation of anionic porphyrin forms or anionic associates of porphyrin and oxygen in the film formation. The dependence of the film mass on the amount of transmitted electricity (Fig. 2b) allows us to estimate the current efficiency (film formation efficiency). The most efficient film formation is observed in the first cycle in the potential range of oxygen electroreduction. Tangent line 1 to the dependence in this region (Fig. 2b) shows that to bind one H2T(4-NH2Ph)P molecule in the polyporphyrin film requires ∼ 20 elementary charges. Such amount of electricity proves that only a small part of the superoxide formed in the oxygen electroreduction reaction interacts with H2T(4-NH2Ph)P and leads to irreversible binding of the porphyrin with the surface. For the cycles numbered 5 to 10, it is possible to introduce the value of averaged efficiency of film formation (dashed line 2, Fig. 2b). The averaged efficiency of film formation takes into account desorption and electrochemical processes that do not lead to film formation. The calculations for the cycles numbered 5-10 show that the irreversible binding of one H2T(4-NH2Ph)P molecule in the polyporphyrin film results from transmitting ∼ 480 elementary charges through it.

Fig. 2. Process of poly-H2T(4-NH2Ph)P film formation in conditions of natural aeration of solution according to EQMB data.

а: Correlation of electrochemical responses and film mass changes on the quartz crystal resonator; b: Dependence of film mass on the quantity of electricity passed.

The experiment carried out by the chronoamperometric method in conditions of oxygen saturation of the solution at the working electrode potential of -0.9 V vs Hg/Hg2Cl (near the peak of electrochemical response of oxygen electroreduction, Fig. 3a) has shown that at short process times (up to 15 minutes), the porphyrin presence in the solution has negligible effect on the quantity of electricity passed. At longer process times, the quantity of electricity used for oxygen electroreduction in the presence of porphyrin is much smaller than in its absence (Fig. 3b). It can be supposed that at longer process times, the slow-down of the oxygen electroreduction process in the presence of porphyrin is caused by changes in the surface characteristics over time as a result of surface polyporphyrin film formation. The electrode impedance spectroscopy at the working electrode potential of -0.9 V vs Hg/Hg2Cl (Fig. 4) confirms this hypothesis.

Fig. 3. a: Electrochemical response of oxygen in DMSO in conditions of solution saturation with oxygen at 22 °С, scan rate of 20 mV/s; b: Dependence of the quantity of electricity passed on the time at potentiostatic polarization of the working electrode in the absence of porphyrin (1) and at the H2T(4-NH2Ph)P concentration of 10-3 mol/L (2).

The formation of a surface film changes the state of the interface electrode/solution over time, which leads to the corresponding changes in the Nyquist plots (Fig. 4 a, c). The Nyquist plots consist of three characteristic regions: I - a slanting line with the length decreasing over time in the region of low frequencies (Fig. 4 a); II - an element of the semicircle with the diameter increasing over time in the region of medium frequencies (Fig. 4 a); III - an element of the semicircle, with the diameter increasing over time, formed in the region of high frequencies (Fig. 4 c). Regions I and II on the Nyquist plots appear when there is a mixed diffusion- and kinetically controlled electrochemical process going on at the interface. The high frequency region on the

Nyquist and Bode plots (Fig. 4 c, d) characterizes charge transfer in the surface film [40, 56]. Modelling of the interface state was done with an equivalent circuit (Fig. 4 b) containing the following elements: solution bulk resistance (Rs), resistance of charge transfer at the interface (polarization resistance, Rp), resistance of charge transfer in the film (Rf), an element of the stationary phase describing the capacitive properties of the film (Qf) an element of the stationary phase describing the capacitive properties of the interface double layer (Qdl) and a Gerischer element (G). The circuit used approximates the experimental data with the statistical criterion χ2 of about 10-3 for all the times of the film deposition. Element Q is represented by relation (1) [57].

Q=

1 , A ⋅ (iω )n

(1),

where A is the frequency-independent pre-exponential factor; n is the exponent determining the character of the frequency dependence (–1 < n <1); i is the imaginary unit; ω = 2πf is the angular frequency. Element Q characterizes the capacitive impedance at the value of n close to 1 and the diffusion impedance at n equaling about 0.5. In all the calculations (Table 1), parameter n has the value close to 1, which allows considering the Аdl value to be equal to the electrical double layer capacitance. Electrochemical film deposition suggests that the chemical reaction proceeds near the electrode surface. According to [41-43], the superoxide anion radical interacts with the porphyrin, which leads to the formation of a radical porphyrin form and triggers the radical polymerization mechanism. Therefore, the diffusion mass transfer was modelled by Gerischer element G (Fig. 4 b), represented by relation (2) [58, 59]:

G=

1 Y k + iω

(2),

where Y is the frequency-independent pre-exponential factor, k is the constant of the chemical reaction rate.

Fig. 4. EIS data (a, c, d) and equivalent circuit (b). Kinetics of changes in the Nyquist plot (a), in the high-frequency part of the Nyquist plot (c), in the high-frequency part of the Bode plot (d). The potential of the poly-H2T(4-NH2Ph)P film formation is equal to -0.9 V.

Table 1. Results of EIS data modelling in the process of poly-H2T(p-NH2Ph)P film formation

t, min 0 12 24 36 48 60 72 84 96

Adl, Om-1sn 2.51E-07 2.00E-07 2.64E-07 2.70E-07 2.47E-07 2.56E-07 2.36E-07 2.40E-07 2.48E-07

n 1.02 1.05 1.02 1.01 1 0.99 1 1 0.99

Rp, Om 27662 46366 45950 54262 52596 59537 58938 60708 61837

Rf, Om 387.6 414.1 394.6 491.7 706.7 898.9 1154 1366 1546

k, s-1 0.53 0.16 0.27 0.17 0.18 0.15 0.15 0.13 0.17

Y, Om-1 3.57E-05 4.53E-05 4.03E-05 4.38E-05 4.74E-05 4.48E-05 4.80E-05 4.71E-05 4.37E-05

The results of the EIS data modelling (Table 1) show that at the initial stage of film formation (∼ 0-12 min.), resistance Rp grows considerably and the interface capacitance decreases. We suppose that at short deposition times, the active centers of the surface are blocked by oligomeric porphyrin. At the time of 12-36 minutes, the double electrical layer capacitance and resistance Rp tend to grow, which can be explained by the formation of an island polyporphyrin film. At the time over 36 minutes, the double electrical layer capacitance tends to decrease, whereas the resistance (Rp) and the surface film resistance (Rf) rise. Such changes can be associated with the increase in the polyporphyrin film thickness. According to the AFM data (Fig. 5), the surface of the poly-H2T(4-NH2Ph)P film is formed from rounded globules of a similar size with the lateral size of about 70 nm. The topographic image of the polyporphyrin film surface (Fig. 5a) coincides with the image obtained in the phase contrast mode (Fig. 5c), which indicates homogeneity of the film chemical composition. The surface structure contains aggregates with the lateral size of 200-700 nm rising above the average surface level by 10-35 nm (Fig. 5b). The presence of these aggregates on

the surface is shown on the height histogram (Fig. 5d) as a more gently sloping long tail in the region of high heights. The statistical processing of the topographic image (Table 2) shows that the average size of the film surface roughness is about 2.7 nm.

Fig. 5. AFM-images of the poly-H2T(4-NH2Ph)P film in topography (a) and phase contrast modes (c); surface cross section at the selected A-A’ line (b); height distribution histogram (d).

Table 2. Statistical characteristics of poly-H2T(4-NH2Ph)P film obtained by superoxide-assisted electrochemical deposition in potentiostatic conditions

Amount of sampling Max Min Peak-to-peak, Sy Ten point height, Sz Average Average Roughness, Sa Second moment Root Mean Square, Sq Surface skewness, Ssk

65536 38.67 nm 0 nm 38.67 nm 18.42 nm 6.46 nm 1.71 nm 6.98 2.64 nm 2.97

Coefficient of kurtosis, Ska Entropy Redundance

15.93 6.52 -0.245

The poly-H2T(4-NH2Ph)P film deposited on the Ito surface has a yellow colour (fig. 6 а). The SEM microphotographs of the film on the ITO electrode (fig. 6b, c) show high transparency of the original film on the electrode to an electron beam for the chosen method of sample preparation and image recording. The image obtained for the original film on the electrode (fig. 6b) is similar to the images of pure ITO electrode (the upper left and lower right angles of the image in fig. 6c). Fig. 6c shows a fragment of the film removed from the substrate by a flow of distilled water due to capillary forces producing disjoining pressure [60-63] and hydrodynamic flow. The film fragment was transferred onto a pure ITO electrode and dried in natural conditions The fact that it is possible to transfer a film fragment from one electrode part to another indicates quite high mechanical strength of the obtained polymer. The homoheneous colour of the image over a large area allows us to suppose that the obtained film is smooth and has the same thickness.

Fig. 6. Poly-H2T(4-NH2Ph)P film obtained on ITO in an oxygen saturated porphyrin solution as a result of potential cycling (25 times) in the range from -2 to +1.2 V vs Hg/Hg2Cl2 at a scan rate of 20 mV/s. a – photo; b, c – SEM microphotographs (see comments in the text).

The mechanical characteristics of the poly-H2T(4-NH2Ph)P film are significantly different from those of the substrate (ITO), which allows measuring the thickness of the formed coating. For this purpose, we plotted a line with a thin metal stick on the surface of the film formed. The depth of the groove that formed was studied by the AFM method (Fig. 7 a, b). According to this test, the thickness of the poly-H2T(4-NH2Ph)P film deposited in potentiostatic conditions over the time of ∼ 100 minutes is about 80 nm.

Fig. 7. Thickness of the poly-H2T(4-NH2Ph)P film deposited in potentiostatic conditions over the times of ∼ 100 minutes according to the AFM-test data. Surface topology (a); surface cross section at the selected A-A’ line (b).

The film resistance (according to the EIS data modelling results), the geometric surface area of the electrode and the thickness of the poly-H2T(4-NH2Ph)P film allow estimating the value of specific conductivity of the obtained material (according to relation 3):

σ=

d RS

(3),

where d is the film thickness, R is the film resistance, S is the geometric area of the electrode. Value σ was equal to about 4.2•10-4 S cm-1. The obtained specific conductivity value was much higher than the one that is usually observed for materials based on metal-free porphyrins [64, 65]. The porphyrin state in the film was estimated by spectral methods (Fig. 8 a, b). The presence of the main spectral lines of the original monomer in the UV-vis spectra of the films (Fig. 8 a) shows that the porphyrin cycle has not been destroyed. After the poly-H2T(4-NH2Ph)P film formation, the Soret band around 435 nm and the Q-bands (the triplet in the range of 500-700 nm) become wider. The relative intensity of the Q-bands increases, and all the spectral lines become wider and shift to the blue region. The Soret band shifts by 3 nm (435 nm instead of 438 nm for H2T(4-NH2Ph)P in DMSO), the shift in the maxima of the Q-bands is even bigger (525, 570 and 662 nm instead of 529, 579 and 668 nm, respectively). The shift of the bands can be caused by changes in the structure of the aryl-substituent [66], changes in the macrocycle conformation [6769] and π-π interaction of the porphyrin fragments in the film [70]. The appearance of the absorption in the region of 700-800 nm in the film spectrum can be explained by protonation of the porphyrin fragments in the film [71].

Fig. 8. a: Normalized UV-vis absorption spectra of H2T(4-NH2Ph)P dissolved in DMSO (1) and poly-H2T(4-NH2Ph)P film (2). b: Fragments of FTIR spectra of H2T(4-NH2Ph)P (1) and ATR FTIR spectra of poly-H2T(4-NH2Ph)P film (2)

To find out the molecular structure of the obtained polymer, we recorded FTIR spectra of H2T(4-NH2Ph)P and ATR FTIR spectra of poly-H2T(4-NH2Ph)P film (Fig. 8 b). The IR-spectra were interpreted according to [72-79]. In the original porphyrin, the band at 803 cm-1 corresponds to the collective out-of-plane vibrations of the C-H bond in the phenyl substituent. This band disappears when a polyporphyrin film is formed, which indicates changes in the phenol ring substitution type. The band at 3452 cm-1 corresponding to the stretching vibrations of the N-H bonds in the phenyl substituent also disappears, whereas the bands of stretching N-H vibrations of the tetrapyrrole cycle remain. Such changes in the vibration spectrum confirm that the film formation is caused by detachment of the hydrogen atoms from the NH2 group of the phenyl substituent and recombination of the porphyrin-radicals formed. Bridges of the phenazine type are formed between the porphyrins, indicating the appearance of bands with frequencies of about 1117, 1365, 1411 and 1454 cm-1 in the polyporphyrin film spectrum [53, 74-77]. The change in the effective mass of the substituent in the process of film formation leads to the expected red shift of the tetrapyrrole cycle deformation vibrations [73, 78]. The presence of bands around 1000 and 1730 cm-1 in the poly- H2T(4-NH2Ph)P spectrum indicates the capture of the ClO4− [79] and O2•− ions by the film during its formation. The analysis of frequencies in the IR-spectra has allowed us to identify the main motive of binding in poly-H2T(4-NH2Ph)P which indicates that the mechanism of polyporphyrin film formation for the porphyrin ligand can be the same as the one earlier supposed for metal complexes [41, 42]. In our opinion, one of the attractive application fields for the obtained films can be metalfree electrocatalysis of oxygen reduction reaction (ORR) [80-83]. In alkali electrolytes, oxygen is reduced both due to the four-electron electrochemical reaction (reaction I) and due to tow-electron electrochemical reactions (reactions II and III) [84]. Two-electron reduction of oxygen usually takes

place in carbon materials, with the hydroperoxyl ion formed in reaction II being reduced at more negative potentials (reaction III) [85, 86]. The catalytic process on porphyrin metal complexes including both 2-electron and 4-electron oxygen reduction was discussed in [87, 88] O2+ 2H2O + 4e− → 4OH− O2 + H2O + 2e− → HO2− + OH− HO2− + H2O + 2e− → 3OH−

(I) (II) (III)

Fig. 9 shows CV for dissolved oxygen reduction on glassy carbon (GC) (curver 1) and on GC covered with a poly- H2T(4-NH2Ph)P film (curver 2). As the obtained data show, for the electrode modified with a polyporphyrin film, the onset of the ORR is shifted to the positive region by a value of about 60 mV, which indicates catalytic properties of the obtained film for this reaction. In contrast to O2 electroreduction on glassy carbon, the CV of ORR on a poly- H2T(4-NH2Ph)P film shows lower current density within the range from -0.6 to -0.8 V. This can be explained by inhibiting reaction III on the electrode covered with a poly- H2T(4-NH2Ph)P film and uninhibited reaction of this type on the original GC.

Fig. 9. CV of dissolved O2 electroreduction on a GC electrode as a blank (1) and on GC covered with a polyH2T(4-NH2Ph)P film (2)

Conclusion The paper is devoted to the development of the method of superoxide-assisted electrochemical deposition and, for the first time, shows the possibility of formation of a metal-free aminophenylporphyrin film from DMSO solutions in potentiostatic conditions at oxygen electroreduction potentials. The possibility of using DMSO for polyporphyrin film formation is a step towards “green organic electrochemistry” thanks to the low toxicity of the solvent. We have shown that in the conditions considered by us the process of film formation goes in two stages. We have also evaluated the morphology of the deposited films, determined the character of monomer

fragment binding in the polyporphyrin film and confirmed the mechanism of film formation proposed earlier for metal complexes. Applying the proposed approach, we have obtained a polyporphyrin film with a high value of specific conductivity, which allows considering the obtained material promising for different applications including ORR electrocatalysis. Acknowledgments The work was financially supported by the Russian Foundation for Basic Research (grant No. 1703-00678). We express our gratitude to the Center for Joint Use of Scientific Equipment “The Upper Volga Region Center of Physico-Chemical Research” (ISC of the RAS, Ivanovo) and Kostroma State University for the equipment provided for the research. References

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Highlights A superoxide-assisted method leads to metal-free aminophenyl porphyrin polymer; •



The developed method is a step towards “green organic electrochemistry”;

A homogeneous film with low roughness and high conluctivity has been obtained; •



The electopolymer can be used as a metal-free electrocatalyst for the ORR.