Introduction

Nowadays conducting polymers such as PANI and its derivatives have attracted many attentions due to their well-defined nature and special optical and electrical properties (Bazito et al. 2008; Ćirić-Marjanović 2013; Nateghi and Savabieh 2014; Sapurina and Stejskal 2008; Stejskal et al. 2010). In recent years, aromatic diamines have been employed in the synthesis of new conjugated polymers to acquire materials which can exhibit better chemical functionality, mechanical and thermal properties than PANI (Li et al. 2002). It has been shown that a hybrid (polymer/inorganic) heterojunction device built on PANA-ES shows a photovoltaic performance which can be employed as a promised heterojunction solar cell (Al-Hossainy et al. 2018). Moreover, doped-poly (aniline-co-4-nitroaniline) thin films have shown reliable photovoltaic features (Al-Hossainy and Zoromba 2019). The most frequently used derivatives of aniline are phenylenediamines (PDA) that form polymers with lower conductivity and redox activity (Guimard et al. 2007; Li et al. 2002; Sestrem et al. 2009, 2010; Stejskal 2015; Yang and Liao 2012; Yang et al. 2014; Zoromba et al. 2018). These polymers have been studied more especially in biomedical applications with lower potential toxicity compared to aniline (Guimard et al. 2007). PpPDA nanofibers have shown high-efficiency effects on enhancing the fluorescence intensity of l-cysteine (l-Cys) (Yang and Liao 2012). Moreover, the combination of Fe3O4 with PpPDA leads to high photo catalytic activity in the degradation of the dyes under both ultraviolet and visible radiation (Yang et al. 2014).

The structures of these polymers and consequently their electronic properties are very susceptible to their polymerization procedure. Poly aniline has different structures, which depends on their synthesis method. Oxidation of aniline under acidic conditions produces the emeraldine salt with equal number of secondary amine groups and imine nitrogen atoms (Stejskal and Gilbert 2002; Stejskal et al. 1996). The emeraldine form of poly aniline can be chemically or electrochemically oxidized to pernigraniline (Stejskal et al. 1996). Some studies suggest that the pernigraniline salts are conducting (Bazito et al. 2008). The reduction of emeraldine yields leucoemeraldine form with non-conducting behavior. Various polymer structures can be considered for poly para-PDA. The poly aniline-like chain is probably the most obvious structure for this polymer (Li et al. 2013; Min et al. 2011; Wu et al. 1996). At higher oxidation concentration, a phenanzine like ladder structure is produced (Amer and Young 2013; Baibarac et al. 2011; Cheng et al. 2006; Muthirulan and Rajendran 2012; Sayyah et al. 2009). The ladder-like structure is widely accepted in the literature for pol para-PDA produced via chemical oxidation of monomer by sodium, potassium, or ammonium persulphate. The synthesized polymer shows high thermo stability and conductivity, high gas separation ability, and lyotropic liquid crystallinity (Chan et al. 1991; Cheng et al. 2006; Manivel et al. 2008; Prokeš et al. 1999; Sulimenko et al. 2001; Ullah et al. 2013). Another derivative of poly aniline is PBz which has been synthesized through chemical and electrochemical manner (D'Eramo et al. 2000; do Nascimento et al. 2004; Naveen Kumar et al. 2001). The presence of azo and phenanzine-like segments of PBz were detected by resonance Raman analysis (do Nascimento et al. 2004). However, some doubts concerning the structure of the polymer still remains. While FTIR, UV–Vis and 1HNMR spectroscopies are commonly used in the study of aromatic diamine polymers, but there is not a real conclusion about the structure of these polymers in the literatures.

In the present work aniline, para phenylenediamine and benzidine polymers were synthesized via chemical and electrochemical oxidation. Then electrical and optical properties of the synthesized polymers were studied by cyclic potential sweep, IR and UV–Vis spectroscopic techniques. Some fundamental physical parameters such as energy levels and bandgaps were assessed for the polymer both theoretically and experimentally. Finally, by comparing the experimental and theoretical results the most probable structure for each polymer is predicted. This study reveals that the polymer structure strongly depends on the synthesis procedure.

Methodology

Experimental

Materials

Aniline, para phenylenediamine and benzidine (reagent grad) were purchased from Merck. Aniline was distilled under reduced pressure and kept far from the light in the refrigerator for subsequent uses. Other chemicals (reagent grade) were purchased from Merck and used without further purification. Conductive glass (FTO glass, sheet resistance 8 Ωsq−1 purchased from South Korea) was used as working electrode.

Methods

PANI, PpPDA, PBz were synthesized by a chemical and electrochemical oxidative polymerization method at ambient temperature. Ammonium peroxy disulphate (APS) and HCl were used as oxidizing agent and supporting electrolyte, respectively. In a typical procedure, 10 mL (0.1 M) of HCl were mixed with 1.14 g sodium dodecyl sulfate (SDS) and stirred. 10 mL (0.15 M) of each monomer (aniline, para phenylenediamine and benzidine) were added and stirred for one hour. Then 10 mL of APS (0.04 M) were also mixed. The reaction was continued for 5 h in stirred solution. The mixture was filtered and washed with distilled water, methanol and then with HCl (0.1 M).

The electrochemical method such as cyclic voltammetry (CV), is commonly used in studies of small molecules and polymers (Heinze 1984). Oxidation (πox) and reduction(πred) potentials can be used for estimating the values of solid state ionization potentials(I), electron affinities and experimental band gap (Sworakowski 2018; Sworakowski and Janus 2017; Sworakowski et al. 2016). The electrochemical polymerizations were also performed in a single compartment cell housing three-electrodes at room temperature using cylic potential sweep ethodd. FTO was used as working and a Pt rod as counter electrodes, respectively. All reported potentials were measured relative to Ag/AgCl/Cl−(sat.) reference electrode. The electrochemical polymerization was carried out in NaClO4 solution containing 0.04 M monomer on FTO working electrodes. Polymers were deposited by potential cycling between − 0.3 and 1.4 V at scan rate of 50 mVs−1. Experimental band gaps were calculated by the difference between HOMO and LUMO states which are the negative values of ionization potentials (I) and the electron affinities (A). Commonly used procedure (Cardona et al. 2011) (Eq. 1) were for calculating I and A values.

$$I = e\pi_{{{\text{ox}}}} + 4.8{\text{ eV}};\quad A = e\pi_{{{\text{red}}}} + 4.8{\text{ eV}}$$
(1)

In the above equation, the parameter e stands for the unite charge. The energies are expressed in eV.

Electrochemical studies were carried out using an Autolab general purpose electrochemical potentiostat/galvanostat system PG302N.

Electronic absorption spectra were obtained on an Agilent UV–visible spectrometer (model 8453, USA). IR spectra were recorded using a Shimadzu model 470 grating IR measurement instrument by the KBr pellet technique.

Computational methods

DFT and time dependent DFT calculations (TDDFT) were performed to determine the approximate structure of poly aniline, poly para phenylenediamine and polybenzidine. It was shown that oligomers up to five or six repeating units can accurately represent the polymeric properties (Kamran et al. 2015; Ullah et al. 2014). Different types of geometries were considered for these polymers that are presented in Figs. 1, 8 and 18. All structures were optimized using B3LYP/6-31G(d) method which was proposed to give fruitful results especially in the field of conducting polymers (Kamran et al. 2015; Salzner 2008; Ullah et al. 2013; Ullah et al. 2014; Yang et al. 2006). HOMO-LUMO energy gaps and IR spectra of all oligomers were simulated using the same method and scaled with a scaling factor of 0.9631 (Casado et al. 1999; Kamran et al. 2015). The UV–Vis spectra and optical energy gaps were simulated at TD-DFT/6-31G(d) level of theory using polarized continuum solvation model (PCM) in dimethylsulfoxide medium (Tomasi et al. 2005). All calculations were performed with GAUSSIAN 09 (Frisch et al. 2009) program and visualization of results were done with Gauss View 5 visualization software (Dennington et al. 2009).

Fig. 1
figure 1

Proposed structures of aniline a Leucoemeraldine (PAni-a), b Emeraldine (PAni-b)

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Results and discussion

Poly aniline

Structural and electronic properties

Figure 1 shows two oxidation states of poly aniline that may be obtained through our synthesized procedure which are called PAni-a and PAni-b for leucoemeraldine and emeraldine structures, respectively. The optimized geometry structures are presented in Fig. 2 showing that both structures are twisted to minimize steric hindrance of hydrogen atoms. Table 1 shows the inter ring bond lengths and torsional angles. This non planar structure reduces the delocalization of π- electrons along the polymer backbone leading to reduction of polymer conductivity. The C-N bond lengths and torsional angles in PAni-a structure were calculated to be about 1.40 Å and 35-40º, respectively.

Fig. 2
figure 2

Optimized structures and frontier orbitals at isovalue = 0.02 of a Leucoemeraldine (PAni-a), b Emeraldin (PAni-b)

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Table 1 Torsional angles (°) and bond lengths (Å) of Ani-a and Ani-b. The parameters are shown in studied structure figures
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In PAni-b structure, torsional angles around C=N bonds are lower than those of calculated for PAni-a. The lower torsion increases the electron delocalization and reduces the bond length in PAni-b chain. The HOMO–LUMO delocalization of both structures are presented in Fig. 2. It can be seen that HOMO is mostly distributed through C=C bonds of benzene and N atoms of polymers. However, in PAni-b it is distributed over C=N inter ring bonds and helps the electron delocalization. LUMO is located on C-N and C-H bonds of PAni-a with antibonding character. The LUMO state of PAni-b is mostly located on C atoms of benzene rings. However the band gap which was calculated by the difference between HOMO and LUMO levels is about 3.93 eV and 2.16 eV in PAni-a and PAni-b chains, respectively.

Figure 3 shows the CV for electrochemically synthesized PANI obtained in CH3CN solution containing 0.1 M NaClO4 by scan rate of 50 mV/s. Oxidation and reduction potentials are presented in Table 2. The anodic peak relevant to the oxidation of poly aniline is observed at 1.48 V and its cathodic peak is appeared at -1.41 V. Using Eq. 1 gives the band gap of 2.89 eV which is close to the band gap of PAni-b structure. In order to clear the ambiguity in the determination of the band gap and the structure of the polymers spectroscopic techniques were considered.

Fig. 3
figure 3

Cyclic votammogram of PANI coated FTO electrode immersed in 0.1 M NaClO4 in CH3CN. sweep rate; 50 mVs−1

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Table 2 The experimental and calculated HOMO–LUMO energies with electronic (ΔEgH−L) and optical (ΔEgopt) band gaps of all proposed structures of PANI
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UV–Vis characterization of polymers

The experimental UV–Vis spectrum of aniline dissolved in DMSO is shown in Fig. 4a and the simulated one is depicted in Fig. 5. The experimental UV–Vis spectrum shows distinctive absorption peaks at 330, 390 and 540 nm. The peak appeared at 330 nm corresponds to π-π* excitation of para substituted benzoid segment in PANI which is in close agreement with the peaks appeared at 320 and 354 nm for PAni-a and PAni-b structures, respectively (Nateghi and Savabieh 2014; Shahhosseini et al. 2016). The peak observed at 390–460 nm region in experimental spectrum is attributed to polaronic transitions of quinoidal structure which can be correlated to the peak presents at 480–500 nm in simulated spectrum of PAni-b structure.

Fig. 4
figure 4

a UV–Vis spectrum, b Tauc's plot from UV–visible analysis of poly aniline dissolved in DMSO

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Fig. 5
figure 5

Simulated UV–Vis spectra of aniline a Leucoemeraldine (PAni-a), b Emeraldin (PAni-b)

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The broad peak observed at 500–800 nm in experimental spectrum shows nice correlation with the low intensity peak appeared in PAni-b spectrum. On the basis of this similar assignments and band peak position it can be concluded that the PANI structure of synthesized sample is PAni-b. Simulated optical band gaps (ΔEgopt) of two structures are presented in Table 2.

The optical band gap of synthesized structure can be determined by calculating the electron transitions between the highest occupied valence band and the lowest unoccupied conduction band started at the absorption edge of the absorption spectrum. The optical band gap energy is obtained by Tauc’s equation (Bhatt et al. 2012):

$$\alpha h\nu = A\left( {h\nu - \Delta E_{{\text{g}}} } \right)^{n}$$
(2)

where hν is the photon energy, h is Planck's constant, α is the absorption coefficient, ΔEg is the optical energy gap, A is a constant, and n is an index which shows the characteristics of electron transition in a semiconductor, i.e., n = 1/2 for allowed direct transition and n = 2 for allowed indirect band transition (Davis and Mott 1970; Shahhosseini et al. 2016). The optical band gap is calculated by this equation and (αhν)2 vs. hν was plotted and the extrapolated to (αhν)2 = 0 (Fig. 4b). ΔEg was obtained to be 1.90 eV for poly aniline which is in good correlation with theoretical one calculated for PAni-b structure (1.71 eV). However, PAni-a gives the band gap of 3.37 eV. Therefore, it can be concluded that poly aniline is synthesized with the structure labeled as PAni-b (Fig. 1b).

Vibrational spectral characteristics of poly aniline

Calculated IR spectra of proposed structures of poly aniline and corresponding experimental spectra are presented in Figs. 6 and 7, respectively. Comparisons between observed and simulated bands are listed in Table 3. The experimental N–H stretching band is appeared at about 3580 cm−1 while the simulated bands are computed to be at about 3477 and 3467 cm−1 in PAni-a and PAni-b structures, respectively. The discrepancies from experimental data are due to the fact that the theoretical data are obtained for an isolated oligomer in the vacuum state whereas the experimental one is achieved in condensed phase. The band observed at about 3230 cm−1 in experimental spectrum is attributed to C–H stretching vibrations of aromatic rings. The corresponding bands in simulated spectra can be observed at about 3100 and 3106 cm−1 in PAni-a and PAni-b, respectively.

Fig. 6
figure 6

Theoretical IR spectra of aniline a Leucoemeraldine (Ani-a), b Emeraldin (Ani-b)

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Fig. 7
figure 7

Experimental IR spectrum of poly aniline in KBr pellet

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Table 3 Experimental and calculated frequencies (cm−1) of poly aniline
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The appearance of distinct band relevant to C=N vibration at 1567 cm−1 which is clearly observable in IR spectrum of PAni-b is a very good evidence of formation of PAni-b structure. In corresponding simulated spectrum, C=N vibration is appeared at 1517 cm−1. Moreover, presence of a broad peak at 853 cm−1 which is relevant to C-H out of plane vibration is another proof to adopt the PAni-b as the structure of synthesized poly aniline.

Poly para phenylenediamine

Structural and electronic properties

The proposed and optimized structures for poly para phenylendiamines are depicted in Fig. 8, 9, 10, 11, 12, respectively. Inter ring bonds and angles are presented in Table 4. It can be seen that the structures are twisted to minimize steric hindrance. The bridging angles are about 150° in PPDA-a structure with a bond length of 1.41 Å. The high conjugated PPDA-b structure is planar and the bond lengths are about 1.31–1.36 Å. PPDA-c structure is also twisted to reduce the steric hindrance of amine groups. The bridging angles are alternating between 48° and 15° and the bond lengths are about 1.41 Å in this structure.

Fig. 8
figure 8

Proposed structures of poly para phenylenediamine a PPDA-a, b PPDA-b, c PPDA-c, d PPDA-d

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Fig. 9
figure 9

Optimized structures and frontier orbitals at isovalue = 0.02 of PPDA-a structure of PpPDA

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Fig. 10
figure 10

Optimized structures and frontier orbitals at isovalue = 0.02 of PPDA-b structure of PpPDA

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Table 4 Torsional angles (°) and bond lengths (Å) of proposed PpPDA structures. The parameters are shown in studied structure figures
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The structure is twisted the most in PPDA-d. Bridging angles are about 44° around single C–N bonds and it decreases to 10° around C=N bonds. The bond lengths are alternating between 1.31 Å and 1.40 Å for double and single C–N bonds, respectively.

The twisted structure in PPDA-d leads to the separation of HOMO–LUMO states and the conductivity reduction. The HOMO–LUMO distributions of all structures are depicted in Figs. 9, 10, 11, 12. The results show that in PPDA-a, the HOMO is mostly distributed over the benzene rings and N–N bonds with bonding property whereas the LUMO is located on C atoms of benzene rings. This distribution leads to the band gap of 4.69 eV in this structure. In PPDA-b, HOMO–LUMO states that are distributing on C=C and C=N bonds overlap, leading to easier electron transfer in this structure. Therefore the band gap reduces to 0.91 eV in agreement with previous calculations (Yang et al. 2014).

Fig. 11
figure 11

Optimized structures and frontier orbitals at isovalue = 0.02 of PPDA-c structure of PpPDA

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Fig. 12
figure 12

Optimized structures and frontier orbitals at isovalue = 0.02 of PPDA-d structure of PpPDA

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In PPDA-c structure, HOMO is mostly distributed over C=C of benzene rings and C–N inter ring bonds. LUMO is mostly located over C atoms of aniline rings and some C–N bonds which are overlapping with HOMO. The calculated band gap for this structure is 3.93 eV. HOMO state of PPDA-d is mostly distributed over the C=C and C=N bonds which overlaps with LUMO state and is distributed over C–N bonds and resulting delocalization of electrons through the polymer backbone. The HOMO–LUMO gap which is theoretically obtained for this structure is 2.27 eV. The experimental band gaps obtained electrochemically are presented in Table 5. The cyclic voltammogram in Fig. 13 shows that PpPDA is electroactive and the peaks occurring at the potentials πox = 1.60 and πred = − 1.35 V correspond to the oxidation and reduction of the polymer, respectively.

Table 5 The experimental and calculated HOMO–LUMO energies with electronic (ΔEgH−L) and optical (ΔEgopt) band gaps of proposed structures of PpPDA
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Fig. 13
figure 13

Cyclic votammogram of PpPDA coated FTO electrode immersed in 0.1 M NaClO4 in CH3CN. sweep rate; 50 mVs−1

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The results show that the experimental band gap obtained using Eq. 1 is close to that of PPDA-d structure. Therefore, in order to determine the most probable structure in accordance with the obtained results, it is necessary to investigate the optical properties of the proposed structures.

UV–Vis spectra characterization

Simulated UV–visible spectra of all structures are depicted in Fig. 14. It can be seen that, among proposed structures, PPDA-a has the lowest absorption coefficient. The absorption peak appeared at short wavelengths (λmax = 303 nm) indicates low conjugation compared to other structures. On the other hand the highest absorption coefficient and λmax are related to the PPDA-b structure since of its highly conjugated form. The peaks related to π–π* electron transitions are appeared at 859 and 592 nm. PPDA-c shows two peaks that are appeared at 368 and 283 nm.

Fig. 14
figure 14

simulated UV–Vis spectra of poly para phenylenediamine a PPDA-a, b PPDA-b, c PPDA-c, d PPDA-d

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Figure 15 shows the experimental spectrum of PpPDA. Two peaks that are appeared in 345 and 667 cm−1 show nice correlation with PPDA-d structure. The optical band gap calculated using DFT calculations is 1.86 eV for PPDA-d structure and that one obtained by Tauc’s plot (Eq. 2) is 2.48 eV (Fig. 15b) which is in more agreement with that obtained for the PPDA-d structure (Table 5). The discrepancy between experimental and theoretical data can be attributed to the limited length of oligomer and the approximation that was taken into account for solvation model in simulated spectrum.

Fig. 15
figure 15

a UV–Vis spectrum, b Tauc's plot from UV–visible analysis of PpPDA dissolved in DMSO

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Vibrational spectral characteristics of poly para phenylendiamines

Calculated IR spectra of all structures are shown in Fig. 16 and the experimental one is depicted in Fig. 17. Table 6 shows the experimental and calculated vibration frequencies of PpPDA. N–H stretching band is appeared at about 3303 cm−1 in PPDA-a spectrum. However it is observed at 3406 cm−1 in PPDA-c and PPDA-d spectra. Since there are two different types of N–H bonds (NH and NH2) in two last structures, the broad band at 3210–3365 cm−1 of experimental spectrum confirm that synthesized form of PpPDA corresponds to PPDA-c or PPDA-d structures. NH2 bending is observed at 1631 and 1626 cm−1 in PPDA-a and PPDA-b spectra, respectively. The corresponding band in experimental spectrum is appeared at about 1634 cm−1.

Fig. 16
figure 16

Scaled IR spectra of poly para phenylenediamine a PPDA-a, b PPDA-b, c PPDA-c, d PPDA-d

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Fig. 17
figure 17

Experimental IR spectrum of PpPDA in KBr pellet

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Table 6 Experimental and calculated frequencies (in cm−1) of PpPDA
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A high intensity band that is observed at 1514 cm−1 in experimental spectrum can be attributed to C=C and C=N stretching in all proposed structures. However, C=N stretching of PPDA-b is observed at 1442 cm−1 and that of PPDA-d is appeared at 1520 cm−1 which is in very good accordance with experimental one. N–N stretching in PPDA-a structure is appeared at 1128 cm−1 with high intensity which is not observed in experimental spectrum. Therefore, PPDA-a cannot be the right structure of synthesized PpPDA. Also the calculated C–N stretching band for PPDA-d (1238 cm−1) is nearly same as experimental one (1232 cm−1) indicating that the PPDA-d structure is the most probable to consider for PpPDA.

Poly benzidine

Structural and electronic properties

Different proposed structures of poly benzidine (PBz) are shown in Fig. 18. All hexamer structures were optimized and are presented in Figs. 19, 20, 21. Figure 19 shows that PBz-a is twisted more than other structures which resulted in the lowest conjugation of the structure. The structural parameters presented in Table 7 show that the dihedral angles about NH bridge are about 143–145°. However, the torsional angles between two benzene rings are about 36°. This twisted structure with long bond lengths limits the electron conjugations and increases the band gap.

Fig. 18
figure 18

proposed structures of poly benzidine a PBz-a, b PBz-b, c PBz-c

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Fig. 19
figure 19

Optimized structures and Frontier orbitals at isovalue = 0.02 of PBz-a structure of poly benzidine

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Fig. 20
figure 20

Optimized structures and Frontier orbitals at isovalue = 0.02 of PBz-b structure of poly benzidine

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Fig. 21
figure 21

Optimized structures and Frontier orbitals at isovalue = 0.02 of PBz-c structure of poly benzidine

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Table 7 Torsional angles (°) and bond lengths (Å) of proposed PBz structures. The parameters are shown in studied structure figures
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The dihedral angles on N–N bridges of PBz-b are about 180°. The dihedral angles between two benzene rings with single and double bonds are about 33° and 2–5°, respectively. The N–N and C–N bond lengths are about 1.33 Å and the inter ring bond lengths vary between 1.48 and 1.41 Å for single and double bonds, respectively. These structures compared to, show more electron mobility through the polymer backbone and reduced band gap compared to PBz-a. In PBz-c, the C–C bonds are twisted about 33°. The C–C bond lengths are about 1.48 Å.

HOMO–LUMO distributions of proposed structures are also shown in Figs. 16, 17, 18. It can be seen that HOMO states of PBz-a are mostly distributed over the benzene rings and N–N inter ring bonds. LUMO states are located on C–C inter ring bonds and C atoms of benzene segment. This distribution leads to the energy gap of 4.53 eV. However, HOMO is delocalized over the C=C and C=N bonds of PBz-b. The LUMO states are also distributed over the next part of the chain and help the electron transfer through the polymeric chain. Consequently, the band gap reduces to 1.23 eV. The twisted structure in PBz-c decreases the HOMO delocalization over the chain. However, they are distributed over the benzene rigs and C-N bond. LUMO states are distributed over the rings and N atoms. They overlap with HOMO states to help the electron transfer through the chain. The band gap in this structure is 2.55 eV.

Cyclic voltammetry technique was used to find the experimental HOMO–LUMO gap of synthesized polybenzidine. The PBz voltammogram presented in Fig. 22 shows an anodic peak at 1.58 V and a broad reduction peak centered at about − 1.35 V. The HOMO–LUMO gaps calculated using Eq. 1 are depicted in Table 8.

Fig. 22
figure 22

Cyclic votammogram of PBz coated FTO electrode immersed in 0.1 M NaClO4 in CH3CN. sweep rate; 50 mVs−1

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Table 8 The experimental and calculated HOMO–LUMO energies with electronic (ΔEgH−L) and optical (ΔEgopt) band gaps of all proposed structures of PBz
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UV–Vis spectra characterization

Simulated UV–visible spectra of all proposed structures are presented in Fig. 23. A sharp peak at 310 nm is observed for PBz-a structure. Higher conjugation in PBz-b shifts the π–π* peak to 746 nm.

Fig. 23
figure 23

Simulated UV–Vis spectra of poly benzidine a) PBz-a, b) PBz-b, c) PBz-c

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The broad peak observed at 565 nm in PBz-c spectrum has the best correlation with experimental one (λmax = 455 nm) (Fig. 24a). The experimental optical gap is obtained 2.25 eV using Tauc's equation (Fig. 24b). As can be observed from Table 8, this band gap has also the best correlation with PBz-c structure. Therefore, it can be concluded that the PBz-c is the most probable structure for synthesized PBZ.

Fig. 24
figure 24

a UV–Vis spectrum, b Tauc's plot from UV–visible analysis of poly benzidine dissolved in DMSO

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Vibrational spectral characteristics of poly benzidine

Simulated IR spectra of three proposed structures and the experimental spectrum are presented in Figs. 25 and 26, respectively. Table 9 shows the calculated vibration frequencies of all structures. N–H stretching band is appeared at 3460 cm−1 in PBz-a structure. It is observed at 3264 cm−1 with lower intensity in PBz-b spectrum. The peaks for N–H in PBz-c are due to the ends of the oligomer. However, there is no distinguished peak for N–H in experimental spectrum which confirms the PBz-c structure. C=N stretching which is expected to be appeared in PBz-b and PBz-c spectra are located at 1519 and 1506 cm−1, respectively. It is appeared at 1487 cm−1 in experimental spectrum. C-H stretching at 2845 cm−1 of experimental spectrum has the best correlation with PBz-c structure (3099 cm−1). Therefore, vibrational results also confirm that the synthesis procedure leads to the ortho-coupling of benzidine and formation of phenazine segment (PBz-c) structure.

Fig. 25
figure 25

Scaled simulated IR spectra of poly diphenylamine poly benzidine a PBz-a, b PBz-b, c PBz-c

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Fig. 26
figure 26

Experimental IR spectrum of PBz in KBr pellet

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Table 9 Experimental and calculated frequencies (in cm−1) of PBz
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Table 10 shows a comparison between three aniline based structures. It can be seen that our synthesized procedure leads to the formation of benzoid form of PANI and PpPDA (PAni-b and PPDA-d) and phenazine form of PBZ (PBz-c). Experimental HOMO–LUMO gaps are ~ 2.9 eV in these structures which show that the electrochemical oxidative polymerization can affect the conductivity of synthesized polymers. The results also show that B3LYP calculations underestimate the HOMO–LUMO gap in all cases. Calculated ΔEgH−L of PANI is slightly smaller than those for other aniline based polymers. The results show that ΔEgH−L values are all greater than ΔEgopt values since it takes additional energy to fully separate electrons and holes into free carriers (Franco Jr and Padama 2016). Obtained experimental results for optical band gaps show that PpPDA has the broadest band gap which leads to its lowest conductivity.

Table 10 The experimental and calculated electronic (ΔEgH−L) and optical (ΔEgopt) band gaps of all studied structures
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Conclusion

Various structures have been proposed for poly p-phenylene diamine in which monomers are linked via different reactive sites (ortho or para). Our combinatorial (theoretical and experimental) studies reveal that the ladder-type phenazine-like structure (PPDA-b) is the most probable among the proposed structures. Whereas, aniline monomers are linked together from para position to form polyaniline as emeraldine form. In the case of benzidine which is one of the dimers of aniline, most probable configuration is formed by binding the monomers via ortho and para positions, alternatively. Our results obtained from combinatorial studies indicated that for investigation of the complex structure of the conducting polymers it is necessary to hire various theoretical calculations and experimental techniques. Experimental band gaps were obtained through cyclic voltammetry and absorption spectroscopy and compared to HOMO–LUMO gaps from DFT calculations. Among the polymers investigated in this study the PANI shows lowest band gap (1.71 eV) which can be a good candidate for application as semiconductor in organic electronic.