Introduction

Organic polymeric and metallic characteristics are combined into one structure in conjugated organic polymers. One benefit of these materials is that the final product’s electrical and optical characteristics can be modified by modifying the molecular structure of the initial monomer. While, many studies have been conducted on conjugated polymers, one of the key objectives of scientists is still the development of low band gap polymers or zero bandgap polymers [1], and various viable avenues have been developed in order to achieve this. The bandgap of the polymer can be tuned by carefully selecting the donor (D) and acceptor (A) units, since doing so enables us to select and modify the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Synthesizing soluble conjugated polymers is another essential goal, since solubility makes their use in applications simple and inexpensive.

According to the literature, quinoline [2,3,4] and quinoxaline-fused thiadiazoles [5,6,7,8,9,10,11] are utilized as acceptors in polymers with low band gap values. One of the studies among them presented an extremely low bandgap polymer (ranging from 0.21 to 0.60 eV depending on bandgap determination method), namely poly(6,7-diphenyl-4,9-di(selenophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (P1). This polymer film was found to be responsive to both n- and p-type doping in electrochemical and optical investigations. Its neutral state is mustard in color, and it changes to brown upon oxidation and light purple with reduction [7]. Although this polymer has a relatively small band gap, neither water nor organic solvents can dissolve it.

3,4-propylenedioxythiophenes (ProDOTs) with alkyl or alkoxy substitutions are frequently utilized to synthesize soluble conjugated polymers [11,12,13,14]. The ProDOT (2) unit can be used alone or as a D in D-A-D systems and offers solubility, regioregularity, low oxidation potential (due to the effect of oxygen that donates electrons in propylenedioxy units), quick switching, robustness, and electrochemical stability [12,13,14,15,16].

As is well known, copolymerization is a method that allows some superior properties to be assembled in a single structure by using the monomers of the polymers showing superior characteristics. This research aims to combine the exceptional properties of poly(6,7-diphenyl-4,9-di(selenophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (P1) and poly(3,3’-didecyl-3,4-propylenedioxythiophene) (P2) in a single material. In order to achieve this, the monomers 1 and 2 were electro-copolymerized, and poly (1-co-2) was created (Scheme 1). Two alternative ratios of the comonomers were utilized during this polymerization, and the copolymers that resulted were called PC1 and PC2 in accordance with these various ratios. The polymers’ electrochemical, optical, and colorimetric measurements were examined, and they were contrasted with those of P1 and P2.

Scheme 1
scheme 1

Chemical structures of 1, 2, PC1, and PC2 and electrochemical synthesis of PC1 or PC2 from 1 and 2

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Experimental

Materials and instrumentation

Tetrabutylammonium hexafluorophosphate (TBAH) was used as a supporting electrolyte for the electrochemical synthesis and analysis. Commercially available acetonitrile (ACN) and dichloromethane (DCM) were purchased in extremely high purity (≥ 99.9%) and used as obtained.

Polished platinum button electrode (electrode area: 0.02 cm2) and platinum wire were used as working and counter electrode, respectively. As well as a silver wire was used as a pseudo-reference electrode (calibrated externally using a 10 mM solution of ferrocene/ferrocenium couple) or Ag/AgCl in 1.0 M NaCl reference electrode. The working electrode for SPEL analysis was an indium tin oxide coated quartz glass slide (ITO, Delta Tech. 8–12 W, 0.7–5 cm), together with counter electrodes made of platinum wire and a pseudo-reference electrode made of Ag wire.

Gamry PCI4/300 potentiostat–galvanostat was used to conduct electroanalytical measurements. On the Specord S 600 (standard illuminator D65, field of 100 observers), the electro-optical spectra and colorimetric measurements were recorded. The color space was defined by the CIE 1976 parameters luminescence (L), hue (a), and intensity (b). Iodine DIN EN 1557, platinum-cobalt DIN ISO 621, and Gardner DIN ISO 6430 are references for colorimetric measurements.

Method

Our group had previously carried out a synthesis of monomer 1 using the Stille coupling method [7].

The monomer 1:monomer 2 feed ratio was set up in two separate configurations for this purpose: 1:9.5 and 1:2.8. The final polymers were given the designations PC1 and PC2, respectively. These particular ratios were selected because, when the concentration of 2 was below these values, the outcome was nearly equal to P1, and, when it was above these values, an oily monomer 2 polluted polymer was produced on the electrode surface. Another factor in the selection of this particular range of ratios was the fact that the resulting polymer is insoluble up to a monomer 1 to monomer 2 ratio of 1:9.5.

The copolymer synthesis and the determination of the oxidation potentials of the monomers were both done using cyclic voltammetry. The spectroelectrochemical analysis was conducted using UV–Vis spectroscopy and chronoamperometry, and the stability test of the copolymers was conducted using cyclic voltammetry.

Results and discussion

The oxidation potentials of compounds 1 and 2 were determined using cyclic voltammetry for copolymerization as a preliminary step. The oxidation potential of 1 was found to be approximately 1.1 V, while that of 2 was found to be approximately 1.5 V (Fig. 1).

Fig. 1
figure 1

Cyclic voltammogram of the monomer 1 and 2 in 0.1 M TBAH/ DCM with 100 mVs−1 scan rate versus Ag/AgCl reference electrode

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For copolymerization, a potential value of 1.5 V was employed. New redox couples were seen to form throughout repetitive cycles below the oxidation potential of comonomers, and the intensity of these couples grew stronger with each cycle. This outcome was accepted as evidence that a conducting (co)polymer had formed on the surface of the electrode (Fig. 2).

Fig. 2
figure 2

Repetitive cyclic voltammogram of monomer 1: monomer 2 ratios as (a) 1:9.5 and (b) 1:2.8 in 0.1 M TBAH/ DCM: ACN (2:1) with 100 mVs−1 scan rate vs Ag/AgCl reference electrode to obtain PC1 and PC2, respectively (colour figure online)

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The electrochemical behaviors of copolymers were then studied at various scan rates in a monomer-free electrolyte solution (Fig. 3a and (c)). Peak current levels increased linearly with the increase in scan rate in PC1 as well as PC2 (Fig. 3b and (d)). This was seen as proof that the copolymers were firmly attached to the electrode’s surface and that the electrochemical reactions are nondiffusion controlled.

Fig. 3
figure 3

Electrochemical behavior of (a) PC1 and (c) PC2 at different scan rates; current verus scan rate graphs of (b) PC1 and (d) PC2 on Pt disk electrode in 0.1 M TBAH/ACN versus Ag/AgCl from 20 to 120 mVs−1 scan rates with 20 mVs−1 increments (colour figure online)

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In order to investigate p-doping, spectroelectrochemical (SPEL) experiments were also completed for the copolymer using tandem potentiostat/galvanostat and UV–Visible spectroscopy studies. PC2 showed three absorption bands attributed to π–π* transition at 550 nm, 593 nm (as a shoulder), and 1000 nm (Fig. 4b), compared to PC1, which had two absorption bands attributed to π–π* at 560 nm and 595 nm (nearly a shoulder) (Fig. 4a). The band at the near IR (about 1000 nm), which is a hallmark band of the P2 homopolymer, could not be seen as expected because monomer 2 composition was considerably higher in PC1. However, because the composition of monomer 1 was larger in ratio, this band was seen for PC2. During the oxidation process, the value of the π–π* transition bands declined for both copolymers, which was accompanied by the formation of new absorption bands for PC1 and PC2 above 1000 nm and around 950 nm due to polaron formation, respectively. Upon further oxidation, the absorption bands between 400 and 650 nm almost disappeared for both copolymers, and it was obviously observed for PC1 that there was a new absorption band above 1000 nm due to bipolaron formation.

Fig. 4
figure 4

SPEL behaviors of (a) PC1 and (b) PC2 at different potentials from 0 to 1.4 V in 0.1 M TBAH/ACN on ITO electrode versus Ag wire (colour figure online)

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The optical band gap of copolymer PC1 and PC2 were calculated as 1.65 eV and ≤ 1 eV, respectively (Fig. 4 and Table 1).

Table 1 Electrochemical and optical data of P1, P2, PC1 and PC2
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The switching characteristics of copolymers were also studied. In contrast to PC1, which had 29% transmittance at 550 nm with a 1.37 s switching time, PC2 had 50% transmittance at 560 nm with a 1.36 s switching time. Again, as anticipated, PC2 had a lower transmittance value percentage than PC1 because PC2 had a higher proportion of 2 than PC1. The copolymers’ switching times and coloration efficiencies were nearly identical, but as was to be predicted, PC1 had a somewhat better coloration efficiency than PC2 (171 cm2/C), at 180 cm2/C (Fig. 5 and Table 1).

Fig. 5
figure 5

Switching behaviors of (a) PC1 at 560 nm (b) PC2 at 550 nm between 0 to 1.4 V in 0.1 M TBAH/ ACN on ITO electrode versus Ag wire

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Colorimetric measurements of both copolymers were carried out because they both exhibited electrochromic characteristics. While, the color of PC1 was purple in its neutral state, it was a transparent light purple in its oxidized state (Table 2). For PC2, a color change was observed from dark purple to green as the neutral state was changed to an oxidized state (Table 3). Since, the color of homopolymer P1 changed from mustard to brown (neutral to oxidized) and that of homopolymer P2 changed from blue to transparent (neutral to oxidized), the obtained dark color and some losses of transparency were also expected (Table 1).

Table 2 Colorimetric measurements of PC1 during p-doping process and the hues that the human eye perceives (colour figure online)
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Table 3 Colorimetric measurements of PC2 during p-doping process and the hues that the human eye perceives (colour figure online)
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Stability tests for PC1 and PC2 were performed as the last step. After 2000 cycles, PC2 has retained 60% of its electroactivity compared to 66% for PC1. Since, PC1 had a larger quantity of P1, as expected, PC1 had slightly more electrochemical stability than PC2 (Fig. 6 and Table 1).

Fig. 6
figure 6

Stability test of (a) PC1 and (b) PC2 film in 0.1 M TBAH/ACN at a scan rate of 100 mV s−1 by cyclic voltammetry under ambient conditions. Qa Anodic charge stored; ipa Anodic peak current; ipc Cathodic peak current

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Conclusions

Two new copolymers (PC1 and PC2) were electrochemically produced in this study and then examined electrochemically and electro-optically. P1 was an insoluble polymer with a low bandgap (0.6 eV), but it also had a poor electrochemical stability. P2 was an extremely electrochemically stable, soluble polymer with a high band gap (1.82 eV). In order to merge these two polymers’ desirable qualities into a single material, this study attempted to do so. To achieve this, the electrochemical copolymerization of 1 and 2 was done. A more soluble and electrochemically stable polymer with a high band gap (1.65 eV) was produced because PC1 had a relatively higher concentration of 2. While, a low band gap (≤ 1 eV) polymer was achieved with PC2, where 1 composition was considerably higher. The resulting PC2, though, was slightly soluble.