ElectrochemistryElectrochemical 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 cations15,16,17,25. 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 calculationsQuantum 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. 2Structures 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. 3Mechanism 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 Ï€-electron interactions (such as Ï€-stacking) may occur, which stabilized the structure and reduced the band gap of the material29,30,31.UV–Vis-NIR spectroelectrochemistryDuring the electro-oxidation process, when the central naphthalene unit undergoes Ï€-Ï€* 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. 4UV–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 unit14,15,16,17. 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 Ï€-Ï€ type, which can in turn decrease the energy gap (Eg) value between the HOMO and LUMO.IR spectroelectrochemistryThe 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. 5Changes 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 spectroelectrochemistryIn 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. 6Voltage 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. 7Proposed mechanism of electrochemical reactions in the p2 polymer.Mechanism of electrochemical reactionsP1 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 polymerNotably, 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. 8Admittance 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. 9Equivalent 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. 10Intrinsic 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.
