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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39251696
71842
10.1038/s41598-024-71842-0
Article
Investigating perimidine precursors for the synthesis of new multiredox polymers
Janasik Patryk 12
Chulkin Pavel 12
Czichy Malgorzata 12
Lapkowski Mieczyslaw mieczyslaw.lapkowski@polsl.pl

123
1 https://ror.org/02dyjk442 grid.6979.1 0000 0001 2335 3149 Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland
2 https://ror.org/02dyjk442 grid.6979.1 0000 0001 2335 3149 Centre for Organic and Nanohybrid Electronics, Silesian University of Technology, Konarskiego 22B, 44-100 Gliwice, Poland
3 grid.413454.3 0000 0001 1958 0162 Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 Curie-Sklodowska Str., 41-819 Zabrze, Poland
9 9 2024
9 9 2024
2024
14 2102714 5 2024
31 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
We present a new simple approach for electrochemical synthesis of semi-condensed ambipolar perinone polymers with phthaloperine (p1) or phenanthroline (p2) skeleton from available and cheap perimidine precursors. Polymerization of perimidine derivatives varies in efficiency depending on the monomer, but overall is highly efficient, especially when electropolymerization is used. Electrooxidation is well controllable and provides a certain characteristic share of new bonds in the structure of perimidine polymers: semi-ladder bis-perimidine unit, ladder bis-perimidine unit, and protonated bis-perimidine unit. Polymer p2 obtained with higher efficiency was put through broader analysis (UV–Vis, IR, ESR and quantum-chemical calculations). As indicated, donor–acceptor structure and specific intermolecular interactions of p2 assure its electrical conductivity and complex redox activity. Although protonated bonds break π-conjugation in the structure of the macromolecule, there is also a diradical state that favors intermolecular interactions and intermolecular π-conjugation channels within bis-perimidine segments. It has been proven that there is a diradical state which appears as an intermediate state between the oxidized and reduced states of the protonated polymer unit. This work positions perimidine polymers as a versatile ambipolar multiredox p- and n-type conductor, indicating a potential for expanding perinone-based perylene-diperimidine polymers for innovative electronics and (bio)sensors.

Keywords

Electrochemistry
Spectroelectrochemistry
Multiredox polymer
Conductive polymer
Perinone
Subject terms

Chemistry
Electrochemistry
the Silesian University of Technology04/040/BKM21/0161 Janasik Patryk Polish National Science Centre2021/41/B/ST5/03221 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The demand for innovative electronic organic materials is crucial for advancing technologies in the field of optoelectronic devices. It is not only essential to discover new materials, but also to develop new methods for processing existing ones. This dual approach drives progress in creating more efficient, cost-effective, and sustainable organic optoelectronic devices1–4. Organic materials with multiredox systems have gained significant interest for their potential applications in electrochemical devices such as high-energy density batteries5,6, capacitors7, and sensors8–10. These multiredox materials are characterized by their ability to undergo multiple redox reactions, making them versatile candidates for energy storage and conversion technologies. Their organic nature offers advantages, positioning them as promising alternatives to traditional inorganic materials in the development of advanced electrochemical devices11–13.

The coupling mechanism of electrochemical aromatic compounds with a perimidine unit in their structure through (electro)oxidation has recently been studied by our research group. It was found that the perimidine segment of perimidine-isoindolone14,15, perimidine-pyrrole16, and perimidine-naphthalene-carbazole17 monomers (Scheme 1) show evidence of a coupling process between the radical cations. This reaction creates stable dicationic bis-perimidine segments in the polymer that do not deprotonate under deep cathodic polarization. Additionally, our studies discovered the feasibility of creating two bonds between perimidine units to produce structures such as BBL (poly(17-oxo-7,10-benz[de]imidazo[4',5':5,6]-benzimidazo[2,1-a]isoquinoline-3,4:10,11-tetrayl-10-carbonyl))18,19 using electrochemical techniques, for example, cyclic voltammetry (CV), differential pulse voltammetry (DPV), and chronoamperometry (CA).Scheme 1 Perimidine monomers as a novel block for oxidative electropolymerization. (a) Difunctional diperimidine fused-ring monomers 1 and 2 investigated in this work (A) and perimidine-compounds described in the previous works (B–D)15–17.

The aim of this work is to introduce a new electrochemical method for the synthesis of new materials, specifically targeting compounds of the structures similar to BBL. The BBL polymer possesses valuable properties such as photoconductivity and high electron mobility20,21. However, its conventional synthesis is challenging, primarily due to the instability of tetraamine and the polymer's poor solubility, necessitating the use of advanced technologies22–24. Therefore, we present in this work the electrochemical preparation of new materials based on the perimidine unit, which exhibited a multi-redox nature. The monomers were prepared through a simple, one-step synthesis, and their subsequent electropolymerization in our study may present a novel approach to obtaining BBL-like polymers, offering an advantage over the more complex synthesis involved in chemical polymerization.

Results

Electrochemistry

Electrochemical studies of monomers 1 and 2 showed that they undergo multiple redox processes. However, it was possible to conduct the appropriate studies on the anodic and anodic–cathodic modes, in which the electro-oxidation reaction was used to obtain layers of electroactive materials.

The study found that the oxidation of the monomers was linked to the oxidation of the perimidine unit, which is described in previous studies as coupling of perimidine radical cations15–17,25. The oxidation of monomers 1 and 2 was irreversible with current peak maxima at +0.93 V and +0.86 V, respectively. The electroactive response of products can differ depending on the deposition in an anodic (Fig. S3) or anodic–cathodic (Fig. 1) cycle due to the distinct organization of molecules on the electrode during the cathodic cycle, which involves n-type doping and can rearrange the material’s structure3. Table S1 shows the potentials of materials obtained during the electro-oxidation of monomers 1 and 2 in the anodic and anodic–cathodic cycles. In the anodic mode, three types of units were observed for both polymer 1 (p1) and polymer 2 (p2). The products were divided into three main fractions. Fig. S3 and Fig. 1 show the potential ranges of the different units, from − 0.25 to 0.19 V for 1ox, 0.20 to 0.64 V for 2ox, and 0.65 to 1.15 V for 3ox.Fig. 1 (a) and (b) CVs recorded during the electropolymerization of monomers 1 and 2 (black CV cycles) on Pt electrode in 28 and 23 μM solution, in 0.1 M Bu4NBF4/DCM at the scan rate of 0.1 V s-1, potential calibrated versus ferrocene/ferrocenium redox couple (Fc/Fc+). CVs for deposited product p1 and p2 in 0.1 M Bu4NBF4/DCM (red cycles).

Chronoamperometry was employed at the monomer-oxidizing potentials to determine that the formation of 2ox required an alternating potential. Figure S4 shows that at a constant potential, the current intensity for the product polarization was comparable to the background (Fig. S4c), hence, 2ox product was not formed. However, during the electrodeposition process, by successive switching of the potential from 0.9 to 0.7 V, 2ox unit was obtained (Fig. S4d). Therefore, 2ox is a combination of deprotonated monomer units,

During the electro-oxidation in the anodic–cathodic mode, a widening of the two reduction peaks for each material was observed (for p1 at − 1.52 and − 2.00 V; for p2 at − 0.98 and − 1.44 V). These peaks are marked in Fig. 1 as red1 and red2. The presence of these two reduction peaks proved that the redox-active moieties from the monomers were maintained in the polymer (Fig. S2). The reduction of both materials and monomers was comparable to the electroactivity of BBL polymer20,26, which consists of isomeric units of compound 1. The redox process in BBL is reversible and contains two single electron transfer steps at the potentials of − 1.14 and − 1.48 V vs. Fc/Fc+27. A two-stage reduction was also observed with pyrrolo[3,4-m]phthaloperinedione (for p1) and [3, 8]phenanthrolinedione (for p2) cores. 1red and 2red peaks that corresponded to these processes were broadened due to the development of various types of products and non-covalent interactions between the material segments in the solid.

Sweeping the potential in a wide range (Fig. S5) did not destroy the materials proving their high stability. In the case of p2, the boundary between the reduction and oxidation regions was invisible (Fig. S3), which was considered as the electrochemical energy gap of the material close to zero. The same gap for p1 was equal to 0.44 V. During the calculation of the energy gap, also known as the band gap, we have to consider that the redox systems involved may not necessarily be neutral. Moreover, the gap is derived from the redox processes occurring in different parts of the molecule. Therefore, we recommend using UV–Vis spectroscopy to determine the band gap, as this technique provides a direct and precise assessment of the electronic transitions within the molecular structure.

Main issues arose during the development of further structural studies due to the low concentration of monomers 1 and 2 in the solution, making it impossible to obtain a sufficient amount of p1 and p2 materials to apply other research techniques. Therefore, to explain the structure of the obtained materials, we decided to use quantum chemical calculations, and UV–Vis, IR, ESR (Electron Spin Resonance) spectroelectrochemistry, which will be presented below.

Quantum chemical calculations

Quantum chemical calculations were utilized to determine the energies and localization of molecular orbitals for the syn- and anti-isomers of 1 and 2. Density Functional Theory (DFT) was employed to obtain information regarding the expected redox potentials and the preferred reactivity sites of these molecules (Table S2). These calculations were supplemented with localizations of spin densities for the appropriate radical ionic states (Table S3). HOMO (Highest Occupied Molecular Orbital) was localized mainly on the perimidine units for all calculated monomers and the energies values were − 5.33 eV for monomer 1 and − 5.11 eV for monomer 2. The energy and localization of orbitals are the same for syn- and anti-isomer. The energy value of HOMO-1 was lower than that of HOMO ( − 5.47 eV for monomer 1 and − 5.40 eV for monomer 2). This suggested the ‘electronic communication’ of the two terminal perimidine units through phenylene or naphtalene aromatic core. Additionally, it led to partial delocalization of the orbitals and extension of the charge throughout the molecule. LUMO (Lowest Unoccupied Molecular Orbital) was localized mainly on the aromatic core units (pyrrolo[3,4-m]phthaloperinedione and [3, 8]phenanthrolinedione) and partially delocalized over the oxygen atoms in the molecule. The energy values of these orbitals were − 2.94 eV for monomer 1 and − 3.06 eV for monomer 2.

Based on the calculated localization of electron spin density for excited forms, we proposed the position of the covalent bonding in the electro-oxidation products. All calculated radical cations showed that the spin was localized mainly on the four carbon positions 1, 3, 4, and 6 and nitrogen atoms (Table S3). The entire spin density of both radical cations and diradical dications was localized on the perimidine units. On the contrary, the spin of the radical anions and diradical dianions was localized on the aromatic central unit e.g., pyrrolo[3,4-m]phthaloperinedione and [3, 8]phenanthrolinedione.

To facilitate the calculations for the polymerization products, some assumptions and simplifications were used. The syn- and anti-isomers of monomer 2 were only calculated. Three main groups of products were categorized (Fig. 2)—protonated bis-perimidine (Table S4), semi-ladder bis-perimidine (Table S5); and ladder bis-perimidine (Table S6). Additional chains (n = 4 or 8) were computed with only 4 possible structures of ladder bis-perimidine products (Table S8 and S9). Intermolecular interactions and supporting electrolyte ions were ignored. The calculated HOMO value for monomer 2 was − 5.11 eV, which was compared with the calculated energies of the orbitals for electro-oxidation products. Just to remind, to compare the results of quantum chemical calculations with the electrochemical data, one has to remember that a difference of 1 eV in energy scale corresponds to a difference of 1 V in potential scale28.Fig. 2 Structures of the expected electropolymerization products for monomers 1 and 2.

Based on the calculations performed for the possible electro-oxidation products, we proposed a series of products obtained during the electropolymerization of monomer 2 (Fig. 3). The recombination of two perimidine radical cations can lead to formation of a single bond in the 1-position closer to the oxygen atom. The calculated LUMO value of this type of connection was between − 4.44 and − 4.65 eV. Based on the potential value of monomer oxidation and the obtained calculated data (Table S4), i.e. the difference between the LUMO of the dication state and HOMO of the monomer, one may expect the 1ox reduction peak at about 0.17 V. The electrochemical response for 1ox was in the range of − 0.2 to 0.2 V for DPV measurement and it is most probably a set of redox reactions involving bis-perimidine junctions in the form of dications. During the previous calculations of bis-perimidine dications, it was found that some bonds, such as 1,1’ or 1,6’, were more stable due to the possibility of possessing hydrogen bond interactions inside the molecule. The products stabilized in this way did not undergo any further oxidation reaction15,17.Fig. 3 Mechanism of perimidines electropolymerization. Differential Pulse Voltammetry (DPV) for p2 layer and the assignment of material current responses to specific electrochemical reactions. The polarization of p2 material layer was carried out on Pt in a pure electrolyte—0.1 M Bu4NBF4/DCM. For the calculations, the frontier orbitals values and exemplary visualizations of these orbitals were presented, and calculated for individual bonds, using B3LYP 6-31G(d)/CPCM(DCM) function.

2ox and 3ox units were products of deprotonation of bis-perimidine dications to neutral bonds of semi-ladder and ladder bis-perimidines, respectively. According to the calculations, 2ox and 3ox should undergo oxidation at potentials 0.36 and 0.91 V, respectively. In this case, the calculations were consistent with DPV measurement results. To test our theory of the generation of ladder connections in 2ox, electrochemical analysis of perylene was conducted due to ladder dimers being derivatives of perylene (Fig. S6). The oxidation potential of perylene was 0.55 eV, which confirmed the presence of a perylene unit in p2 material, found in 2ox. Additionally, the current response assigned to 3ox may be caused by the oxidation of perimidine terminal units and monomers included in the material structure.

In the reduction region (below 0 V) current responses on DPV curve starting from -0.4 V were observed. These were generated during the reduction of neutral dimers and larger oligomers with semi-ladder and ladder perimidines. During the calculations, it was found that the reduction of longer oligomers should occur at higher potentials compared to the monomer, owing to the energy value of LUMO orbital larger oligomers becoming lower. Additional signals below − 1 V originated from the reduction of the monomer occluded in the polymer structure, the terminal units, and from further stages of the reduction of the oligomers.

The calculations showed that with each additional monomer unit in the polymer chain, the band gap decreases to a limit of 0.93 eV, as shown by fitting the trend function for the calculated oligomers (Fig. S7). The geometry and values of the boundary orbitals were calculated in a pure solvent, ignoring intermolecular interactions. The geometry and values of the boundary orbitals were calculated in a pure solvent, ignoring intermolecular interactions. In a condensed system, hydrogen or π-electron interactions (such as π-stacking) may occur, which stabilized the structure and reduced the band gap of the material29–31.

UV–Vis-NIR spectroelectrochemistry

During the electro-oxidation process, when the central naphthalene unit undergoes π-π* transitions, there are two distinct bands observed at wavelengths of 332 nm and 380 nm (Fig. 4a). The fact that these bands remain unchanged suggests that the electro-oxidation process does not significantly alter the electronic structure of the naphthalene unit and naphthalene unit does not actively participate in this process. In contrast, the charge-transfer (CT) transition bands associated with the perimidine units are affected during electro-oxidation32. Specifically, bands at 611 nm and 654 nm disappear, indicating that the electronic structure of the perimidine unit changes significantly. This proves that the perimidine unit is actively involved in the electro-oxidation process. A newly formed broad band observed at 921 nm indicates the formation of charged species. The appearance of this band proves the formation of radical cations in investigated process. The broadening of the band in the 700–900 nm region is attributed to the formation of dicationic products. These dicationic species are formed as a result of coupling reactions involving the radical cations generated during electro-oxidation, as it was observed in our previous work14,17,25.Fig. 4 UV–vis–NIR spectra of electrochemical reactions for 2 and p2 recorded in situ in 0.1 M Bu4NBF4/DCM. (a) Spectra recorded during electro-oxidation of monomer 2. (b) Electro-reduction of p2 deposited on ITO electrode in anodic–cathodic mode. (c) Electro-oxidation of p2 deposited on ITO electrode in anodic–cathodic mode.

Polymer p2 deposited on ITO electrode using an anodic–cathodic electropolymerization mode (CV of electropolimerization and polarization of p2 were included in Fig. S8, a) results in exhibiting several changes during its electro-oxidation process. Starting from of − 0.1 V, there is a disappearance of a band at 659 nm, accompanied by the formation of a new band at 577 nm (Fig. 4c). Additionally, a broad band spanning the wavelength range of 700 to 900 nm becomes visible. These spectral alterations are attributed to the generation and subsequent evolution of dicationic species. This phenomenon aligns with observations made during the electro-oxidation of both the monomeric precursor 2 and other compounds that contain the perimidine unit14–17. Going higher with the potential to 1 V, a further transformation is detected in the form of a distinct band appearing at 889 nm. This band is attributable to the presence of radical cations originating from the terminal perimidine units of the polymer structure and it suggest that chains of newly formed material are rather short.

During the process of electro-reduction involving the polymer p2, we observed a distinct decrease in the intensity of the band at 364 nm (Fig. 4b). This is attributed to the involvement of the central naphthalene core in the reduction process. The initial step of this reduction at − 0.8 V process is marked by the appearance of a band at 1055 nm, which comes from the presence of a radical anion species. An interesting feature of this reduction process is the extension of the absorption band to wavelengths exceeding 1300 nm, as demonstrated in Fig. S8, b. This spectral behavior indicates the presence of a delocalized radical species at this stage. Notably, a similar pattern was identified in our previous research, reinforcing the significance of this observation25. Upon proceeding to the second stage of reduction at, a new absorption bands appear at 494 and 822 nm (Fig. S8c). This spectral change is an evidence for the formation of a stable diradical dianion species33.

Utilizing the UV–vis–NIR spectroscopic analyses, we have determined the optical energy gaps (Egopt) for synthesized p2 layers. The polymer film, deposited by the anodic–cathodic mode, showed an optical energy gap of 1.30 eV (Fig. S9). The observed value corresponds to the difference in energy levels between the HOMO and the LUMO. This energy difference pertains specifically to the donor and acceptor regions within the molecule and has been determined through computational calculations. It's important to note that these calculations are conducted on isolated molecules within a DCM solvent environment. The HOMO is consistently found to be localized on the bis-perimidine segments of the molecule, while the LUMO is predominantly localized on the [3, 8]phenanthrolinedione segments. This observation supports the conclusion that the polymer p2 possesses a donor–acceptor structure. However, it should be noted that under certain conditions, such as changes in solvent or when the polymer is in a redox-active state, intermolecular distances between segments can be significantly reduced. This reduction in intermolecular distances leads to increased interactions, particularly of the π-π type, which can in turn decrease the energy gap (Eg) value between the HOMO and LUMO.

IR spectroelectrochemistry

The polymer p2 deposited at the platinum electrode was investigated by IR spectroscopy to probe its molecular structure and dedoping characteristics. The IR spectra were acquired under various key polarization potentials corresponding to electrochemical reaction points of the p2 product (Fig. 5). Upon analysis of the p2 IR spectra we observed broadening of bands, and its indicating polymeric nature of deposited material.Fig. 5 Changes of the IR spectrum depending on the applied potential for p2. The p2 polymer was deposited on a platinum electrode in an anodic–cathodic mode. The spectra are shown for the key film polarization potentials.

We started the analysis by focusing on the doping mechanisms and our initial attention was directed on the spectral range of 3000–2800 cm−1, which are assignable to the stretching vibrations of C–H bonds stemming from alkyl chains introduced via cationic doping. These peaks dissapear only in the exceedingly positive polarization potentials, for the 3ox. This phenomenon suggests that the complete dedoping process after polymer reduction does not occur instantaneously upon the initiation of polymer oxidation. Rather, the dopant species remains trapped within the polymer matrix until a polarization potential of 0.9 V is achieved. On the other hand, the band at 516 cm−1 attributed to F-B bond stretching stays consistently across all investigated film polarization potentials. This implies the possible formation of a zwitterionic moiety, which remains stable throughout the p2 reduction.

IR studies also revealed interactions between C=O (carbonyl) bonds and hydrogen moieties from protonated bis-perimidine segments. These interactions are visible for all measured potentials, as corroborated by the absorption band visible at the 3700–3000 cm−1 range. Moreover, the reduction of the p2 polymer leads to the strengthening of these interactions. This behavior is visible with the band at 1340 cm−1, which comes from arised stabilization of complex formed between the hydrogens of bis-perimidine moieties and the oxygen atoms from negatively charged C=O bond. Throughout the course of the polymer reduction process, the bands spanning 1800–1640 cm-1 are decreasing in intensity. These feature, associated with the vibrational modes of the C=O bonds, proves engagement of the [3, 8]phenanthrolinedione segments within the reduction process of the polymer matrix.

The absorption band observed at the wavenumber of 1590 cm−1 is attributed to the oscillation of C=C bonds within the pyrimidine segment. Notably, during the process of reduction, the absorption band at 1590 cm−1 exhibits its highest intensity. This enhanced intensity suggests that the pyrimidine segment remains relatively uninvolved in the reduction process. However, when the polymer undergoes oxidation, the absorption band at 1590 cm−1 decreases in intensity, ultimately disappearing from the IR spectrum. This observation indicates the polarization of the pyrimidine segments during the oxidation process.

The observed range of 1140–930 cm−1 can be attributed to vibrational modes arising from C–H bonds. Upon oxidizing the polymer, we noticed an observable trend of broadening this band and it is linked to the weakening of interactions between the carbonyl functional group (C=O) and the bis-perimidine hydrogen moieties. This weakening of interactions occurs during a specific stage in which the oxygen atoms of the carbonyl groups are in their neutral state.

ESR spectroelectrochemistry

In Fig. 6, we present the ESR spectra corresponding to various polarization potentials applied to the p2 polymer. We noticed that the hyperfine coupling effect is not evident in the conducted measurements, and this is characteristic of conductive polymers34. During the ESR measurements, we identified three primary states, which we have colored as red, blue, and green. All of these states exhibit a g-factor close to 2.002335, which aligns with the value expected for free electrons. This observation suggests the presence of delocalized radicals within the polymer matrix of p2.Fig. 6 Voltage dependent in situ ESR spectroscopy of p2. The p2 polymer was deposited on a platinum wire electrode in an anodic–cathodic mode. The ESR spectra were compiled for the key p2 polarization potentials, recorded in pure 0.1 M Bu4NBF4/DCM electrolyte.

Initiating our analysis with Stage I (as depicted in Fig. 6) at − 0.75 V, we recorded the existence of intermediate species between the reduction and oxidation states of p2. The g-factor for this stage was measured at 2.0027, falling between the values of 2.0031 and 2.0024 associated with reduced and oxidized p2, respectively. Next, during oxidation process at − 0.25 V (onset of 1 ox, Stage II), the ESR signal decreases and g-factor shifts from 2.0027 to 2.0024, what indicate the presence of a stable neutral diradical state with free electrons localized on the carbon atoms between reduction and oxidation of p2.

In the range of 1ox, the concentration of spins decreases as a results of transition between diradical (I) and radical-cationic state (II) on the same segment. Here the ratio of spins can be compared, because it concerns the same type of redox center. In the range of the 2ox, the number of spins decreases almost to 0, which indicates the formation of diamagnetic states as in III and IV. In the range of 3ox, the signal reappears because at the highest potential, free perimidine groups (as in the monomer) and bis-perimidine linkages through one bond (V) can be oxidized (Fig. 7).Coming back through the oxidized states and reaching the potential just before the − 0.75 V, the ESR signal also does not disappear, despite total reversibility of each oxidation peak. Signal with g-factor equal to 2.0031 increases twice between transition from the 1red to 2red, what confirms the reduction of carbonyl moieties within the imide ring in p2 (structures VI and VII, Fig. 7).Fig. 7 Proposed mechanism of electrochemical reactions in the p2 polymer.

Mechanism of electrochemical reactions

P1 and p2 were produced via electropolymerization of perimidine units. These materials arose due to the recombination reactions of perimidine radical cations, generated by the applying an oxidizing potential. As shown above, the spin density of radical cations was concentrated at positions 1, 3, 4, and 6 in the perimidine core. Therefore binding at these positions most probably occur. The bond at position 1 was in a non-protonated form due to the stabilization of the proton with the nearby amide group. Binding through the remaining positions stemmed from the formation of deprotonated bis-perimidine segment first (semi-ladder segments). In the subsequent oxidation cycles, a certain amount of semi-ladder segments could condense into a ladder, which is especially favored by the planarity of the monomer and product structure (Fig. 7, Figs. S10, and S11). In the polymer, protonated bis-perimidine segments are present due to hydrogen bonding interactions with the C=O group. These interactions can manifest in different forms, including the possibility of proton transfer to generate an hydroxy group, as demonstrated in this study36.

Based on the spetroelectrochemical measurements, we have proposed a comprehensive mechanistic understanding of the electrochemical reactions that occur during the p2 polymer polarization process as illustrated in Fig. 7. Starting from the initiation of Stage II at a potential of 1ox, we observe the formation of radical states arising from bis-perimidines species. A slight decrease in potential to − 0.7 V leads to a notable increase in the intensity of the peak at 734 nm (see Fig. S8). Further computational investigations were carried out to generate UV–Vis–NIR spectra for both the dicationic and radical cationic states of bis-perimidines (refer to Fig. S12). These calculations yielded an absorption peak at 728 nm, which corresponds to the radical cationic state of bis-perimidines. A subsequent decrease in potential to − 0.8 V results in the disappearance of this absorption band. At this juncture, the gradual generation of zwitterions occurs (Stage I), taking into account IR measurements, which confirm the presence of both dopant ions within the polymeric matrix. Subsequently, we obtain radical anions at a potential corresponding to Stage VI, followed by the formation of diradical dianions at the potential corresponding to Stage VII, involving all [3, 8]phenanthrolinedione cores. This is proved by IR measurements, where the disappearance of bands corresponding to C=O vibrations is observed.

The oxidation process of the p2 polymer progresses through several stages. Initially, at the 1ox potential (Stage II), it undergoes the formation of radical cations. Subsequently, at the 2ox potential (Stages III and IV), the gradual formation of dicationic species occurs, although these remain undetectable in ESR measurements. Radicals are generated at the 3ox potential (Stage V), and these are primarily localized on the perimidine segments, as confirmed by UV–vis–NIR and IR measurements. These results collectively demonstrate that during these electrochemical reactions, the perimidine segments within the polymer are polarized.

Electrochemical properties of p2 polymer

Notably, p1 polymer was not stable during prolonged polarization at both reducing and oxidizing potentials. Its low stability, compared to p2, was caused by the considerably smaller number of interactions between planar pyrrolo[3,4-m]phthaloperinedione central units. Therefore, we will present in this work further research only for p2 material.

The energy value between reduction and oxidation for p2 material was impossible to determine using CV Cardona’s method37, because, according to calculations, it had an energy gap below zero (Fig. S13). By measuring the admittance of p2 material, the electrochemical energy gap of the material deposited during 10, 20, and 30 cycles were determined (Fig. 8). The admittance measurement revealed that with an increasing number of deposition cycles, the value of LUMO orbital energy for the material decreased, while HOMO energy remained unchanged. The decreasing LUMO value with each deposition cycle tightened the energy gap to the value of 0.09 eV for the 30th cycle layer.Fig. 8 Admittance for p2 layers. Admittance of p2 layers deposited with 10, 20, and 30 CV cycles, calculated for points at 1 Hz frequency. HOMO and LUMO were determined graphically—the intersection of the background line and the oxidation or reduction trend line. The background line represents the admittance of electropolymerized PEDOT (poly(3,4-ethylenedioxythiophene) in a non-conductive region (1.0 μS).

To gain a better understanding of the doping behavior, p2 material was studied using electrochemical impedance spectroscopy (EIS) for 10 and 20 cycles of deposition. This technique allows for the analysis of complex electrode processes and has previously been used to describe polarization-induced ion antiport38 as well as the determination of the conductivity type of semiconducting materials39. We proposed an equivalent circuit describing the polymer film under polarization, which is shown in Fig. 9. The circuit was not processable due to an infinite number of parameters, hence a simplified approach was employed. The circuit described charge relaxation, meaning the propagation of the charge from the polymer-electrode to the polymer-solution interface. Each R–C branch stood for a charge transfer process, which was defined by its rate and sensitivity to the potential change.Fig. 9 Equivalent electrical circuit of the conducting polymer film. (a) Proposed equivalent circuit for p2; (b) and its simplified version with frequency-dependent parameters.

The introduction of a time-dependent element in the equivalent circuit for impedance measurement allowed for the characterization of p2 films for 10 and 20 deposition cycles (Fig. 10). The processes that occur in the thin and thick films were the same, but observation of the processes was much more pronounced in the case of the thicker film (20 cycles of deposition). The fastest processes characterized by a time-constant below 50 μs (Fig. 10, b fast) were not present in this material. However, processes for a medium and slow time-constant above 0.1 ms were detected. The impedance measurements revealed that two electrochemical processes occur in p2 polymer with a time-constant above 10 ms. These processes were caused by the dedoping of the polymer, i.e., the migration of counter ions from the material to the electrolyte due to the changed polarization of the working electrode. They occurred at potentials of c.a. − 0.75 and 0.3 V (Fig. 10b slow). Other processes were considered as oxidation or reduction of individual segments of the material, which was accompanied by the migration of electrolyte ions into the material (doping). Therefore, p2 was mainly a redox polymer. However, high charging currents of the material, especially in the range from − 0.6 to − 0.2 V, indicated conductivity along the polymer chain and intermolecular conductivity. Most likely, the conductivity in this range increased due to trapped counter ions in the material’s structure.Fig. 10 Intrinsic charge transfer processes in p2 material. All were characterized by inverse charge transfer resistance and relaxation time (in legend) as a function of the electrode potential. The graphical data were divided into three parts for visual clearance.

Discussion

The presented study has shown that it is possible to obtain new perinone polymers from cheap and available perimidine precursors by electrochemical polymerization. The oxidation coupling of perimidine units was performed, followed by deprotonation of new bonds at a lower potential to obtain deprotonated single (semi-ladder unit junction) and ladder (double unit junction) in the polymers chain. Importantly, extremely stable protonated bonds existed in the bonding population between these chains, i.e., with at least one carbon C1 closest to the amino group, which stabilized the protonated bonds by hydrogen interaction and stated of additional redox center in the polymer. Protonated bis-perimidine undergoes transformation through the radical cation – dication, and radical cation – diradical states, while semi-ladder and ladder bis-perimidine segments undergo reversible oxidation through the neutral – dication and neutral – di(radical cationic) states, respectively. In turn, aromatic central cores of benzene and naphthalene are reduced through radical anion to di(radical anion) species. The polymer p2 was stable over a wide potential range. The EIS results showed that the charge transfer occurs mostly via the intermolecular channels which promote the narrowing of the gap between oxidation and reduction. This parameter is crucial for material application in semiconductor electronics. The target applications should take into account the chemical environment which favors intermolecular conductivity e.g. electrochemical photovoltaic cells, electrochemical supercapacitors.

The monomers 1 and 2 demonstrate modification potential through the introduction of electron-donating and electron-accepting substituents. This strategy may lead to the creation of novel materials with new interesting propertiesThis development holds great promise, charting a path toward a family of conductive organic materials with wide-ranging aplication.

Methods

Synthesis of monomers 1 and 2

Monomers 1 and 2 were prepared via the condensation reaction of aromatic dianhydrides with 1,8-diaminonaphthalene. NMR analysis confirmed that a mixture of syn- and anti-isomers was obtained in a ratio of approximately 1:0.25 for 1, and 0.75:1 for 2. A detailed description of the reaction, purification method, and characterization of the structure are presented in Supplementary Information.

Electropolymerization and electrochemical analysis

CV and DPV were determined using a CH Instruments Electrochemical Analyzer, model 620. All analyses were carried out on compound 1 or 2 saturated solutions (28 μM and 23 μM, respectively), which were prepared in anhydrous dichloromethane (DCM). In all cases, tetrabutylammonium tetrafluoroborate (Bu4NBF4, 0.1 M) was used as the supporting electrolyte. The solutions were prepared directly in the electrolytic cell, under an Ar atmosphere, and the weighted quantity of Bu4NBF4 was added to the filtered solution of monomers 1 or 2 in DCM. Then, Ar was bubbled through the solution for 15 min. The experiments were carried out at 25 °C, and the experimental setup included a single electrochemical cell with three electrodes: a platinum disc (area = 1 mm2) as the working electrode (WE), platinum coil as the counter electrode, and a silver electrode as the Ag/Ag+ pseudoreference electrode, which was previously calibrated with ferrocene (Fc/Fc+). Materials p1 and p2 were deposited on the platinum disc electrodes by cyclic electro-oxidation within the potential range covering the first oxidation peak. The cyclic voltammograms were obtained at the potential scanning rate of 100 mV/s. For stability measurements and DPV of films, WE with deposited material (p1 or p2) was washed with anhydrous DCM and the experiments were conducted in the solution of the supporting electrolyte.

Impedance spectroscopy measurements were carried out using a BioLogic SP-150 potentiostat with a built-in frequency response analyzer. The material was deposited onto the electrode using DCM solution containing 23 μM of monomer 2 in Bu4NBF4 0.1 M over 10 cycles (“thick film”) in the potential range of − 1.69 to + 1.01 V (vs. Fc/Fc+). Before the impedance measurement, the film was stabilized with 5 more cycles at the same potential range as for depositing, and in the supporting electrolyte solution. The impedance measurement of the resulting film was carried out in DCM solution containing Bu4NBF4 0.1 M, in the potential range of − 1.79 to + 1.11 V. Impedance spectra were obtained in 1 MHz-1 Hz frequency range with 3 points per decade in the logarithmic scale (the total number of frequencies in one spectrum was 19). Analysis of electrochemical impedance spectra, determination of equivalent circuit parameters, and other calculation procedures were processed using Microsoft Excel 2013.

Impednce calculations

The admittance of the circuit presented in Fig. 9a was1 Y=∑i=1n1Ri-j1ωCi=∑i=1n11RiRi1-j1ωRiCi

The product RiCi of the resistor and capacitor connected in series is known in physics as a relaxation time-constant, usually assigned as τ. This parameter characterized the rate of the current exponential drop (current relaxation) when the capacitor was connected to a power source via a resistor. Then2 Y=∑i=1n11RiRi1-j1ωτi=∑i=1n11RiRi1+1ωτi2+j1ωτi11RiRi1+1ωτi2

When ωi∙τi >  > 1 was assumed. Then the impact of the i-th process into the total admittance was3 Yi=11RiRi1+1ωτi2+j1ωτi11RiRi1+1ωτi2≈11RiRi

In the opposite case, when ωi∙τi <  < 1:4 Yi=0

Thus, the imaginary component of a process with time-constant τi was evident in the system impedance only at the frequency of ω ≈ 1/τi. Moving from this value to lower frequencies decreased the admittance to 0. Increasing the frequency caused the disappearance of the imaginary component and only 1/Ri remains. It was confirmed that fragmentary analysis of experimental spectra was possible using a three-element equivalent circuit (Fig. 2, right). If the frequency range covered by the considered fragment was narrow enough, then it was assumed that measured Rs and C described a single process, whereas Rp stood for all the processes where the relaxation was not observed in this range, due to the conditions ωi∙τi <  < 1 and ωi∙τi >  > 1.

Then, every relaxation process was characterized by two parameters: its intensity 1/Ri expressed in the units of conductivity and its rate expressed in the units of time as a time constant. Both polymer layers were analyzed using this method. The results are shown in Fig. 6.

UV–vis–NIR spectroelectrochemistry

UV–Vis–NIR spectroelectrochemical measurement was performed using Agilent/HP 8453 UV–Visible Spectrophotometer G1103A. The measuring system was a quartz cuvette with an optical length equal to 2 mm with platinum mesh as working electrode, Ag|Ag+ as pseudo-reference electrode and platinum coil as counter electrode. The concentration of the monomer 2 was equal to 23 μM. For spectroelectrochemical measurement of deposited p2 was used ITO (Indium Tin Oxide)/quartz electrode (20 ± 5 Ω/sq, Praezisions Glas & Optic GmbH, Iserlohn, Germany) as a working electrode. Sample preparation: The p2 material was deposited onto the ITO electrode using an anodic–cathodic mode, 25 cycles of anodic and cathodic potential sweeps. Following the deposition process, the sample was thoroughly washed with pure DCM. This step served to remove any residual impurities or byproducts that might have formed during the deposition process, ensuring a clean and well-prepared sample surface. After the post-electropolimerization treatment, the sample was immersed in a clean 0.1 M Bu4NBF4/DCM electrolyte solution.

IR measurements

IR measurements were conducted using a Perkin-Elmer Spectrum Two spectrometer located in Waltham, MA, USA. The spectrometer was equipped with a Universal Attenuated Total Reflectance (UATR) module featuring a Single Reflection Diamond crystal. Sample Preparation: A platinum electrode with polymer p2, deposited as 20 cycles in anodic–cathodic mode, was utilized for the IR measurements. Prior to measurement, the polymer-coated electrode was polarized in a 0.1 M Bu4NBF4/DCM electrolyte for a duration of 2 min at a specific potential. Following the polarization process, the electrode underwent a thorough rinsing with pure DCM to eliminate any residual electrolyte. Subsequently, the electrode was carefully dried.

ESR spectroelectrochemistry

Spectra were acquired using a JEOL JES FA-200 X-band spectrometer with the following parameters: modulation width of 1.4 mT, microwave power of 1 mW, and amplitude of 100. A capillary quartz spectroelectrochemical cell was used equipped with a platinum wire working electrode in 0.1 M Bu4NBF4/DCM electrolyte, Ag wire pseudo-reference electrode, and platinum coil counter electrode. Sample Preparation: Prepared with the same procedure, as for UV-Vis_NIR spectroelectrochemistry, but p2 was deposited on platinum wire.

Quantum chemical calculations

For calculations, DFT/TDDFT (Time-Dependent Density Functional Theory) was used with B3LYP40 hybrid functional combined with 6-31G(d) basis set. For all optimized structures, the frequency calculations were systematically achieved (at the same level of theory) to confirm the minimum nature of the optimized geometries. All calculations in this work were performed using the ORCA 4.1.141 package programs. Input files and molecular orbital plots were prepared with Gabedit 2.4.7 software42. The Conductor-like Polarizable Continuum Model (CPCM) was used to examine the effect of solvent on the calculated structures43.

Supplementary Information

Supplementary Information.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-71842-0.

Acknowledgements

Authors acknowledge the Polish National Science Centre (NCN) (project No. 2021/41/B/ST5/03221). This research was also supported by the Polish Budget Founds for Scientific Research in 2022 as a core funding for R&D activities in the Silesian University of Technology – funding for young researchers (Grant No. 04/040/BKM21/0161), and Excellence Initiative – Research University at the Silesian University of Technology in Gliwice (Grant Nos. 32/014/RGJ22/2007, 04/040/RGJ23/0238, and 04/040/RGJ23/0242).

Author contributions

Patryk Janasik: Methodology, Investigation, Funding acquisition, Formal analysis, Data curation. Malgorzata Czichy: Writing – original draft, Validation. Pavel Chulkin: Electrochemical impedance measurements, Mieczyslaw Lapkowski: Writing—review & editing, Supervision, Resources, Project administration, Conceptualization.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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