Introduction

Low bandgap conducting polymers (CPs) have been considerably investigated in the last years because of their potential applications for organic optoelectronic and photovoltaic devices [1]. Modification on the bandgap of these polymers can be achieved by means of the molecular design of the bond conjugation, planarity, aromaticity, or donor/acceptor characteristics [1,2,3,4,5,6]. Among the representative low bandgap polymers, PEDOT, poly(3,4-ethylenedioxythiophene), has attracted much attention due to its many advantageous properties such as high electrical conductivity, transparency in the visible range, excellent thermal and structural stability, uniform morphology, and fast doping–dedoping redox mechanism [7]. Due to all these features, PEDOT has found valuable application as antistatic coating [7], electrochromic devices [8], hole-injection electrode for organic optoelectronic devices such as light emitting diodes (LEDs) and organic photovoltaic cells (OPVs) [9], and as material for charge storage devices such as ultracapacitors [10] and lithium-ion batteries [11].

Conducting polymers are typically synthesized electrochemically via oxidative (p-type) or reductive (n-type) coupling. Generally, the electrooxidation of the monomer results in the formation of a radical cation, which subsequently reacts with another monomer or a radical cation, leading to a dimer and then, an anion from the electrolyte solution is incorporated as a consequence of doping process. However, despite using the same terminology “doping,” the process is quite different from that occurring in inorganic semiconductors. The nature of the counterion incorporated during the polymerization plays an important role in determining the properties of the final conducting polymer. The electrochemical method has an advantage over chemical method since the polymer is obtained as a thin film on the electrode surface. In addition, some properties of these materials such as thickness, porosity, and morphology can be accurately achieved by controlling the electrochemical parameters. This strategy leads to films with higher mechanical stability and stronger adherence and better electrochemical properties than that obtained by chemical methods [1].

The dimensionality of charge carrier transport in PEDOT has shown been able to be increased through π–π stacking interactions [1]. The magnitude of inter-chain interactions, as well as the control on the polymer’s electronic properties can be achieved by varying the substituents of the conjugated backbone or the type of counterion used during the electropolymerization process. Incorporation of acceptor units (A) into the PEDOT conjugated network and other polythiophenes (PTh) has been rising as an efficient way to narrow its bandgap and consequently, being used for solar cells bulk hetero-junction [12]. Also, donor molecules (D) such as tetrathiafulvalene, TTF, derivatives have been successfully attached to PTh-based polymers aiming to reduce their bandgap. The stability observed for the oxidized TTF-electron system mainly resulted from the aromatic nature of the oxidized 1,3-dithiolium rings [12].

On the other hand, the doping process can also cause modifications on the polymer electronic structure by producing new electronic states in the bandgap (polarons and bipolarons) and might also be an accessible way of changing the gap between HOMO/LUMO. Several species have been used as a counter ion during the doping process of PEDOT [13, 14]. There are also some reports on using transition metal complexes, such as Prussian blue, as the counterion in the polymerization of PEDOT-based hybrid materials [15,16,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,16]. These studies on hybrid materials, which merge two chemistry fields (organic and inorganic), have shown significant contributions to the area of materials science due to the opportunity of developing new materials with synergic behavior.

We have been using coordination compounds containing the ligand 1,3-dithiole-2-thione-4,5-dithiolate, dmit, as the counterion in the electrochemical synthesis of conducting conjugated polymers, such as polypyrrole, PPy, aiming to develop new hybrid films [17,19,20,20]. Our previous results revealed that the use of this class of compounds has improved considerably the electronic, spectroscopic, and electrochemical properties of the polymer films. In spite of improving some properties, the electrochemical stability of the hybrid films, as well as its morphology, has not been improved. This could have been probably due to numbers of possible defects that can be produced in the PPy polymer chains during the electrochemical oxidation process [18, 19]. On the other hand, PEDOT has a linear polymeric chain growing since the monomer molecules are exclusively attached via α–α’ linkages because thiophene ring β-positions are blocked by the ethylenedioxy ring. Hence, the use of PEDOT can enhance the synergy between the sulfur-rich π–π dianion and the organic conjugated polymer chains resulting in better electronic properties and consequently decrease in its bandgap value. Furthermore, from the best of our knowledge, there are no reports on using this class of compounds in the electropolymerization of PEDOT.

Herein, we report the synthesis of new hybrid polymer films obtained by electropolymerization of poly(3,4-ethylenedioxythiophene) PEDOT using the nearly planar dianion from [Et4N]2[Pt(dmit)2] as the counterion. We also detailed their electrochemical, spectroscopic, optoelectronic, and morphological characterization. The main focus of this research was to achieve a system with tunable electronic properties obtained due to the interaction between the two sulfur-based compounds (polymer and coordination complex) envisioning the area of modified electrodes, electroanalysis, or solid-state optoelectronic devices.

Experimental

Materials and reagents

3,4-ethylenedioxythiophene, K2PtCl4, acetonitrile (ACN), LiClO4, and NaBF4 were obtained from Sigma-Aldrich. Indium-doped tin oxide, ITO glasses, (10 Ω/sqr) were purchased from Zhuhai Kaivo Optoelectronic Components-China. All other reagents and solvents were of analytical grade and were used without further purification.

Synthesis of [Et4N]2[Pt(dmit)2] complex

The starting zinc chelate, [Et4N]2[Zn(dmit)2], was synthesized according to a previously described procedure [21, 22].

$$ {\left[{\mathrm{Et}}_4\mathrm{N}\right]}_2\left[\mathrm{Zn}{\left(\mathrm{dmit}\right)}_2\right]\underset{\mathrm{reflux}\kern0.5em 5\mathrm{h}}{\overset{{\mathrm{K}}_2{\mathrm{PtCl}}_4\kern0.5em \mathrm{in}\kern0.5em {\mathrm{H}}_2\mathrm{O}}{\to }}{\left[{\mathrm{Et}}_4\mathrm{N}\right]}_2\left[\mathrm{Pt}{\left(\mathrm{dmit}\right)}_2\right] $$

The zinc chelate (1.0 mmol) was dissolved in 25 mL of methanol and to this resulting solution, was added a 10 mL of an aqueous solution of K2PtCl4 (1.0 mmol). The resulting mixture was stirred under reflux for 5 h, affording a dark green precipitate. The solid was collected by filtration and recrystallized from acetone and isopropyl alcohol solvent mixture. Yield, 88%. Mp, 197 °C. Calculated for C8H20N2S10Pt: C, 39.7%; N, 4.7%; H, 3.3%. Found: C, 40.3%; N, 3.9%; H, 3.0%.

Electropolymerization of hybrid polymer films

Hybrid films, PEDOT/[Pt(dmit)2]2−, were grown in a one-compartment electrochemical glass cell (volume, 10 mL) with a platinum wire as auxiliary electrode and an ITO glass sheet (2.5 cm2) as working electrode. Prior the film synthesis, ITO sheets were successively cleaned with solvents of a wide range of polarity, e.g., hexane, methanol, and dichloromethane in ultrasound. The applied potentials were related to that of the silver/silver chloride electrode (Ag/AgCl). Polymer films were synthesized by electropolymerization of EDOT monomer (0.1 mol L−1) in the presence of [Et4N]2[Pt(dmit)2] (0.05 mol L−1) in acetonitrile (ACN) at room temperature. Prior to polymerization, the electrochemical system was purged with ultrapure argon, and the gaseous atmosphere was kept over the solution throughout the synthesis.

The films were deposited under galvanostatic conditions using an Autolab PGSTAT 302N potentiostat/galvanostat. The constant current density (j) of 0.25 mA cm−2 was applied during 50 s, corresponding to a synthesis charge (Q s) of 12.5 mC cm−2. The thickness of the films was controlled by the total charge passed through the electrochemical cell during the electropolymerization. After the process, the obtained films were rinsed repeatedly with ACN in order to remove the electrolyte, residual monomer, and oligomers. Additionally, the films were dried and stored at room temperature under vacuum before analysis.

Characterization

FTIR measurements with a resolution of 1.0 cm−1 were carried out on a Bruker Vertex-70 IR spectrometer in attenuated total reflectance (ATR) mode. Absorption spectra in the UV-Vis-NIR region were collected in absorbance mode with a Shimadzu UV-1800 spectrophotometer.

The surface morphology of electropolymerized hybrid film was analyzed by scanning electron microscopy in a Carl Zeiss LEO 1450 VP scanning electron microscope operating at an acceleration voltage of 15.0 kV. The analyzed hybrid films were grown on ITO glass electrodes (2.5 cm2). The samples were mounted in aluminum plates and coated with a gold dispersion.

The surface morphology of hybrid films was also analyzed by atomic force microscopy (AFM). The images in tapping mode (non-contact) were obtained using an atomic force microscopy Witec-Alpha300 RA.

All the electrochemical experiments were conducted on an Autolab PGSTAT 302N potentiostat/galvanostat in a one-compartment electrochemical glass cell (volume, 10 mL) under ultrapure argon atmosphere using a ITO glass electrode covered with hybrid films as working electrode, a platinum wire as auxiliary electrode, and a silver/silver chloride (Ag/AgCl) as reference electrode. All the experiments were performed using ACN as solvent and 0.1 mol L−1 LiClO4 as supporting electrolyte. Cyclic voltammetry was carried out in the range of − 0.2 to 1.0 V at a scan rate of 50 mV s−1.

The spectroelectrochemical experiments, which allowed us to monitor the response change of current and absorption as a function of applied potential, were performed using an assembly consisting of a quartz electrochemical cell with a volume of 3.0 mL and the optical path of 1.0 cm. The instrumentation setup for the spectroelectrochemical measurements comprises ITO glasses covered with the hybrid film as working electrode, a Pt wire as the auxiliary electrode, and an Ag wire as the reference electrode. All the spectroelectrochemical measurements were carried out in ACN/0.1 mol L−1 LiClO4 solutions using the Shimadzu UV-1800 spectrophotometer coupled to an Autolab PGSTAT 302N potentiostat/galvanostat.

Results and discussion

Electropolymerization of hybrid films

Several different EDOT:[Pt(dmit)2]2− ratios were evaluated in order to obtain the hybrid PEDOT/[Pt(dmit)2]2−. Nevertheless, the polymerization rate is limited by the diffusion of monomer’s molecules toward the anode surface during the chain propagation. Thus, the film formation was only observed at the ratios where the monomer was found in excess. For synthesis time lower than 200 s, the obtained PEDOT/[Pt(dmit)2]2− hybrid films were obtained. However, at synthesis time higher than 200 s, homogeneous films were unstable, poorly adherents, and easily detaching from the surface of the electrode toward the solution during the electropolymerization process. From the galvanostatic results, we could conclude that the electropolymerization follows an instantaneous nucleation mechanism, in which the formed nuclei increase in size and coalesce covering the entire surface of the electrode. The obtained films also showed excellent adherence on the electrode, which could be verified using an adhesive tape.

As previously reported by us for PPy/[M(dmit)2]2− system, the obtained films in this work displayed the same color of the coordination compound: olive green (Fig. 1).

Fig. 1
figure 1

a Proposed structure for the hybrid film and b hybrid film PEDOT/[Pt(dmit)2]2− deposited on ITO glass

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Morphology of the hybrid films

The scanning electron micrographs of the ITO glass coated with the hybrid film PEDOT/[Pt(dmit)2]−2 and pristine PEDOT (BF4 − as the counterion) obtained under the same synthesis conditions are shown in Fig. 2a, b.

Fig. 2
figure 2

Scanning electron micrographs for a hybrid film PEDOT/[Pt(dmit)2]2−and b pristine PEDOT film deposited on ITO glasses at magnification of 15 k

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From the Fig. 2a, we observed that the surface of the hybrid films is compact, uniform, and regular revealing only few defects. It was also possible to us to identify the presence of small globular and fiber-like structures. The fibers were formed from clusters of few μm in diameter, suggesting a three-dimensional growth. At higher magnifications, it was also possible to identify globular and spheroidal-like structures, which are typical of the topology of thiophenes and PEDOT films [8, 10, 23, 24]. On the other hand, PEDOT/BF4 − films (Fig. 2b) exhibited a homogeneous and compact structure consisting of globular and popcorn-like particles.

As can be seen from SEM micrographs, the surface of the hybrid film differs from that of pristine PEDOT suggesting that the counterion plays a key role in the morphology of the hybrid films.

Atomic force microscopy (AFM) was also applied to investigate the influence of dmit-based coordination compounds on the PEDOT morphology. Figure 3 depicts the phase and topographical (2D and 3D) AFM images obtained in tapping mode of ITO glass sheets covered with PEDOT/[Pt(dmit)2]2− hybrid films, as well as a film of pristine PEDOT obtained under the same synthesis conditions.

Fig. 3
figure 3

AFM images (10 μm × 10 μm) for the hybrid film PEDOT/[Pt(dmit)2]2−deposited on ITO glass. a phase, b topography, and d 3D, and c and e for pristine PEDOT electropolymerized at the same conditions

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AFM analysis of the films has shown itself more elucidating than did SEM analysis since it also allowed us to evaluate the homogeneity in the composition of films. Figure 3a depicts the AFM phase image of hybrid films. The image shows a relatively low contrast between the phases shifts, bright (positive) and dark (negative), suggesting that the composition of hybrid films is constant and quite regular and uniform throughout the entire scanned surface.

The topographical AFM image of the hybrid film, (Fig. 3b, d), depicts a globular-like structure consisting of nano-protrusions with a regular distribution of the doped PEDOT. The surface of the film showed differences in the size of the particles with diameters ranging from 150 to 50 nm, distributed over the entire scanned area. The surface also appeared rough with RMS (roughness means square) value of 141.8 nm (higher than for pristine PEDOT films, which the RMS value was of 66.5 nm) and with small and large nuclei in granular form, due to the packing of the macromolecules. Figure 3c, e depict topographical AFM images of pristine PEDOT film. The topography of films depicts a structure formed by small clusters of molecules homogeneously distributed, due to the linear growing of polymer chains (the chains are formed exclusively via α-α’ coupling). However, the topographical AFM images of hybrids films showed that the presence of the bulky [Pt(dmit)2]2− dianion appears to promote the formation of larger and higher aggregates observed in the AFM images. These observations confirmed the influence of the counterion on the morphology of the hybrid films.

Electrochemical characterization of hybrid films

Cyclic voltammetry (CV) has been used as an easy and efficient method for studying the electroactivity of conducting polymer films as well as determining the oxidation and reduction potential values. The investigation of the redox behavior of the hybrid films is shown in Fig. 4 which reveals consecutive cyclic voltammograms obtained from the ITO glass electrode covered with an ~ 1.4 μm film of PEDOT/[Pt(dmit)2]2− in 0.1 mol L−1 LiClO4 in ACN solution at scan rate of 50 mV s−1 vs Ag/AgCl. The thickness of the film was controlled by the synthesis charge [17, 25]. The investigation was performed by sweeping the potentials in range of − 0.2 to 1.0 V in cathodic direction under argon atmosphere in a monomer-free solution.

Fig. 4
figure 4

a Cyclic voltammogram and consecutive cyclic voltammograms (inserted) for the hybrid PEDOT/[Pt(dmit)2]2− deposited on ITO glass. Supporting electrolyte 0.1 mol L−1 LiClO4 in ACN at 50 mV s−1 vs Ag/AgCl. b Electrochromic behavior of films upon potential sweep from 1.0 to − 0.2 V vs Ag/AgCl

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For sake of comparison, Fig. 5 shows the cyclic voltammogram of an ACN solution containing 10−4 mol L−1 of [NEt4]2[Pt(dmit)2] using NaBF4 0.1 mol L−1 as supporting electrolyte. From the voltammogram (Fig. 5), it was possible to identify two quasi-reversible processes E½ = − 0.164 and 0.088 V associated to the redox couples [Pt(dmit)2]2−/[Pt(dmit)2]− and [Pt(dmit)2]−/[Pt(dmit)2]0.

Fig. 5
figure 5

Cyclic voltammogram obtained for the coordination complex [Et4N]2[Pt(dmit)2] (10−4 mol L−1) and NaBF4 (0.1 mol L−1) solution in ACN. Pt as working electrode at 50 mV s−1and Ag/AgCl as reference

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The CV for the hybrid films (Fig. 4) showed a color change from olive green to greenish yellow during the consecutive potential sweep. Electrochromism phenomenon in conducting polymers is inherently associated with doping–dedoping process which causes modifications on the electronic structure of the polymer. These modifications result in new electronic states within the bandgap affording observable changes in color. Hence, absorption bands shift to longer wavelengths on doping, the change in color contrast amid the undoped and doped states can be associated with the polymer bandgap.

As reported in our previous works with hybrid films, PPy/[M(dmit)2]2−, a purple gradient was also observed around the working electrode, during the reduction process of this film, at potentials lower than − 0.2 V. However, for potential values higher than − 0.2 V, this gradient is suppressed. The presence of this gradient was previously attributed to an electro-dissolution process involving the counterions and polymer oligomers [17, 19].

From Fig. 4, we can observe a sharp cathodic peak at 0.05 V and two well-defined anodic peaks, a larger one at about 0.012 V and a smaller one at 0.734 V related to the redox processes undergone by the hybrid films. The half wave potential E½ of the PEDOT/[Pt(dmit)2]2− hybrid film, calculated as the average of the anodic and cathodic peak potentials (E p a and E p c, respectively), was equal to − 0.037 V. The ΔE p value, i.e., E p a–E p c, was 0.038 V characterizing a reversible electron transfer not considering the correction of solution’s ohmic drop.

During the successive potential sweeps, the CVs of the films demonstrated an obvious hysteresis altering its profile (Fig. 4 inserted). The current response for the redox peaks decreases when increasing the number of scans and the cathodic peak shifts positively. The resulted CV profile makes clear the presence of two broad peaks at 0.44 V (cathodic) and 0.30 V (anodic) driving the CV curve to a more rectangular shape (capacitive current). In the subsequent cycles, the two pairs of peaks turn ill-defined and the current response decreases continuously. This occurrence can be tentatively explained as follows: (1) The two redox peaks observed in the initial cycles may be related to the PEDOT oxidation and reduction along the insertion or expulsion of the electrolyte ions (Li+ and ClO4 −). In this way, the electroneutrality of the film is guaranteed in its oxidized state. During the reduction process (PEDOT0), the decrease in positive charge should be followed by the insertion of cations Na+ into the film in order to compensate the negatively charged [Pt(dmit)2]2− dianion. On the other hand, the dianion is slowly released from the film during the potential sweeping. Henceforward, as the positive charge in the PEDOT chains cannot be fully compensated by the remaining [Pt(dmit)2]2−, ClO4 − anions are inserted within the polymer framework to maintain its electroneutrality. Smaller the quantity of [Pt(dmit)2]2− retained, the larger the amount of ClO4 − ions needed in this process. Thus, cycle after cycle, the main counterions involved in the process are progressively exchanged from Li+ to ClO4 − ions. Consequently, insertion of the large radius ClO4 − ions into the film results in a compressional effect causing distortion in the film structure. This phenomenon leads to a broadening of the two peaks and a consequent decrease in the current [19]. The subsequent continuous changes, including the presence of the color gradient, can be attributed to a progressive interchange of the [Pt(dmit)2]2− anions from the films with ClO4 − anions from solution suggesting a partial expulsion of dmit-based dianion as follows:

$$ {\mathrm{PEDOT}}^{\mathrm{m}+}/{\left(\mathrm{Pt}{\left(\mathrm{dmit}\right)}_2\right)}^{\mathrm{m}-}+{\mathrm{n}\mathrm{e}}^{-}+{\mathrm{n}\mathrm{Li}}^{+}+{{\mathrm{n}\mathrm{ClO}}_4}^{-}\rightleftarrows {\mathrm{PEDOT}}^{\left(\mathrm{m}-\mathrm{n}\right)}+{\left(\mathrm{Pt}{\left(\mathrm{dmit}\right)}_2\right)}^{2-}{{\mathrm{Li}}_{\mathrm{n}}}^{+}+{{\mathrm{n}\mathrm{ClO}}_4}^{-}\rightleftarrows {\mathrm{PEDOT}}^{\mathrm{m}+}/{\left({{\mathrm{n}\mathrm{ClO}}_4}^{-}\right)}_{\mathrm{n}}+{\left(\mathrm{Pt}{\left(\mathrm{dmit}\right)}_2\right)}^{2-}+{\mathrm{Li}}^{+}-{\mathrm{n}\mathrm{e}}^{-} $$

As was said before, this is just a hypothesis, and a more solid confirmation of the process could be achieved by using electrochemical quartz crystal microbalance (EQCM) measurements, but this is not the focus of this work.

FTIR spectroscopic characterization of hybrid films

Figure 6 depicts the FTIR spectra at transmission mode of (a) PEDOT/[Pt(dmit)2]2−- (b) pristine PEDOT, and (c) [NEt4]2[Pt(dmit)2].

Fig. 6
figure 6

Comparison between the FTIR-ATR spectra of a PEDOT/[Pt(dmit)2]2−, b pristine PEDOT, and c [Et4N]2[Pt(dmit)2]. Spectra (a) and (b) obtained from ITO glass covered with the polymer films and spectrum (c) obtained from microcrystalline powder

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IR spectrum of the PEDOT/[Pt(dmit)2]2 hybrid films, (Fig. 6a), showed a complex absorption pattern when compared to pristine PEDOT (b) and [Et4N]2[Pt(dmit)2] (c). Since either dmit ligand and ethylenedioxythiophene are sulfur heterocycles, we would expect that most of the bands assigned to both compounds are overlapped. Consequently, the attribution to some vibrational modes which are common to both compounds became a hard task. We noticed that the aromatic C–C stretching, which was observed as a mid-intensity and broadband in the range of 1600 cm−1 in pristine PEDOT (Fig. 6b), was not observed in the spectrum of the hybrid film suggesting a highly-delocalized structure. Besides, IR spectrum of the hybrid film showed a large tail extending into the NIR beyond 1800 cm−1 as an indicative of polymerization. This band has been commonly associated with the presence of free charge carriers arising from interchain excitations in the PEDOT structure [17]. Bands in the region of 1430–1480 cm−1 have been assigned as belonging to the symmetric and asymmetric Cα = Cβ stretching indicating that PEDOT presents a quinoidal structure [26,28,29,29]. The symmetric C = C stretching mode associated to dmit ring should also appear in this region. However, it was probably overlapped by those of PEDOT [17,19,20,20, 30,32,33,33]. The band observed at 1242 cm−1 has been associated with the doping process and the quinoid structure of the PEDOT ring. The bands observed at 1076 and 1084 cm−1 have been assigned to the stretching modes of the ethylenedioxy group (–COROC–) [26]. The bands assigned to the C = S symmetric and asymmetric stretching of the dmit ring thione group were observed at 1016 and 1058 cm−1, respectively [17,19,20,20, 34]. These vibrational modes clearly confirmed that the ion [Pt(dmit)2]2− was successfully incorporated within PEDOT framework. Absorptions at 780 and at 930 cm−1 were assigned to the C–S stretching mode and to the deformation mode of the ethylenedioxy ring, respectively. In this same region, the vibrational modes assigned to the PEDOT thiophene ring, symmetric and asymmetric C–S stretching and deformation mode C–S–C were also observed [23]. The vibrational modes seen at 482 and 500 cm−1 were assigned to the symmetric stretching mode (breathing) and the deformation C–S–C of the dmit ring, respectively [30]. A strong band at 890 cm−1 has been frequently reported in the literature as being ascribed to the C–H bending of the monomer [26]. This band vanishes in the spectrum of hybrid film confirming that the formation of PEDOT/[Pt(dmit)2]2− hybrid film occurred via 2,5-positions (α-α’ coupling) [35, 36]. The assignments of the peaks observed in Fig. 6 are also listed in Table 1 and have shown good agreement with those reported for PEDOT electropolymerized in organic solvents [12, 26,28,29,30,31,32,33,33].

Table 1 Assignments for the vibrational frequencies in the [NEt4]2[Pt(dmit)2], pristine PEDOT, and PEDOT/[Pt(dmit)2]2− hybrid film
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Most of the bands assigned to PEDOT and dmit ring shifted to higher frequencies. Special attention must be given to the absorption modes at 1047 and 1025 cm−1 assigned to the C = S stretches which appear as quite intense bands in the dmit complex, but decrease in intensity suffering some shift in the hybrid film. The band which has been assigned to the symmetric C = S stretching blueshifts to 1058 cm−1, and the band assigned to the asymmetric C = S stretching redshifts to 1019 cm−1 suggesting that the interaction between polymer framework and dmit-based dianion might be occurring through the thione groups. Furthermore, an absence of the broadband at 1600 cm−1 and bands associated with the defects in the polymer chain rise as evidence that the bulk dianion tends to stabilize the quinoid structure of PEDOT [37].

From the presented data and the resemblance between the spectra of the hybrid polymer film [ET4N]2[Pt(dmit)2] and pristine PEDOT, we can confirm that the incorporation of counterion into polymer chains of PEDOT was successfully achieved during the electropolymerization process, and neither the structure of PEDOT or dmit-based dianion was destroyed during the synthesis process. The results also indicated the presence of strong intermolecular interactions between the polymer framework and its counterion.

Spectroelectrochemical characterization of the hybrid films

Spectroelectrochemistry is the most efficient technique for studying in-situ modifications in optical properties of a hybrid composite film formed on the surface of ITO glass electrode upon applied potentials. The intrinsic optical properties can be determined from the polymer’s π–π* transition, and the value of the optical bandgap, E g, is commonly estimated from the onset of this transition for the polymer in its undoped state. Besides, it also delivers information about the inter-gap states which appear upon doping process of PEDOT.

In order to evaluate the optical properties of the hybrid films, ITO glass was coated with a film of PEDOT/[Pt(dmit)2]2− and was investigated by applying potential step from + 1.0 to − 0.1 V in a monomer free ACN/LiClO4 (0.1 mol L−1) solution.

As can be seen in Fig. 7, an intense band arises at λmax equal to 424 nm being assigned to π–π* intraligand transition in the dmit ring of [Pt(dmit)2]2− anion. The presence of this absorption is a characteristic for this class of ligand and confirms its presence in the polymer film [17,19,20,20, 33]. The band with λmax in the visible region, which has been assigned to the high energy π–π* transition of neutral state PEDOT backbone, was observed as a wide absorption at 786 nm.

Fig. 7
figure 7

In-situ spectroelectrochemical spectra of hybrid film PEDOT/[Pt(dmit)2]2− on ITO glass at various applied potentials. 0.1 mol L−1 LiClO4 in ACN

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Upon reduction of the hybrid films, new absorption bands emerged as a shoulder at 626 nm and into NIR region. As the applied potential was decreased, the intensity of these new absorption bands was increased, meanwhile, the intensity of maximum absorption band at 786 nm gradually decreases and has its maximum wavelength blueshifted to 762 nm. The lower energy transition is due to the formation of charge carrier species (polarons and bipolarons) on the polymer chains. These charge carriers lead to different coloration for the hybrid composite films on generating new energy transitions between HOMO and LUMO.

Spectroelectrochemistry experiments showed that, unlike PEDOT films, hybrid films PEDOT/[Pt(dmit)2]2− showed color changes from a dark green absorbing state (neutral form) at 0.8 V to a highly transmissive light green state (fully reduced form) at − 0.1 V. They also showed that the peak in visible region of spectrum (786 nm) decreases upon the reduction potential applied. This fact suggests that the obtained hybrid film is an n-doped conducting polymer rather than p-type one. The experimental data also showed that the introduction of [Pt(dmit)2]2− ion in the π-conjugated polymer chains resulting in changes in opto-electrochemical characteristics of these novel hybrid films.

The optical bandgap (E g) of the PEDOT/[Pt(dmit)2]2− films deposited on ITO glass was calculated starting from higher to lower wavelength values in the UV-vis spectrum using the Tauc relation: [38, 39]

$$ \mathrm{Ah}\upnu ={\left(\mathrm{h}\upnu -{E}_{\mathrm{g}}\right)}^n $$

where A is the absorbance, hν is the energy of the photon. The exponential factor n characterizes the electronic transition as direct or indirect during the absorption process. It can assume values 1/2, 3/2, 2, and 3 for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively. E g is the value of the optical band-gap between the valence band and the conduction band. The hybrid film’s indirect bandgap was estimated from the extrapolation of the straight line from the linear portion of the graph of (Ahν)2 as a function of hν when A = 0 (Fig. 8).

Fig. 8
figure 8

Tauc plots of (Ahν)n vs photon energy (hν) for PEDOT/[Pt(dmit)2]2− hybrid films deposited on ITO. A linear fit (dashed line) was used to estimate the bandgap by extrapolating to zero absorption, the direct-bandgap energy (n = 2) was around 1.42 eV and the indirect-bandgap energy (n = 3/2) was around 1.45 eV

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For this film, the direct optical bandgap was calculated as 1.42 eV, which was slightly lower than those optical bandgaps reported for other PEDOT systems [8, 29, 40]. The obtained bandgap value suggests that the insertion of the [Pt(dmit)2]2− strongly influences the optoelectronic properties of the film via the interaction of the polymer backbone with the counterion. This interaction lowers the energy difference between the HOMO and LUMO of the PEDOT when compared to the reported values [41] leading to a new low-band-gap conducting polymer film.

It is known that the photovoltaic performance of organic photovoltaic cells (OPV) is greatly influenced by the magnitude of the bandgap. Hence, this low bandgap film can be a serious candidate to be used as electrode material in OPV.

Conclusions

The electrochemical polymerization of 3,4-ethylenedioxythiophene, EDOT, using the complex anion bis(1,3-dithiole-2-thione-4,5-dithiolate)platinate(II) as counterion ion, was successfully accomplished resulting in the hybrid polymer films PEDOT/[Pt(dmit)2]2− deposited on ITO glass electrode. From the obtained results, it was possible to conclude that the morphology of the PEDOT suffers modifications due the presence of the bulk species into the polymer chain resulting in a rough surface (RMS = 141.8 nm) but with quite homogeny composition. The results also allowed us to conclude that the hybrid film absorbs strongly in the visible region with a maximum at 786 nm. Electrochemical results led us to conclude that the hybrid film is electroactive and undergoes redox processes which resemble that of confined species with well-defined peaks and potential values. It was also possible to conclude that the presence of the dmit-based anion induces conformational effects in the polymer chain leading to multicolor electrochromism behavior (greenish yellow-deep green). It was also observed that the neither dmit-based dianion structure or PEDOT structure was destroyed during the polymerization process since the IR bands assigned to both species are present in the spectra. However, most of these bands were shifted to higher wavenumbers suggesting a strong interaction between both systems. The optical and electronic analysis also corroborates the observations of IR measurements indicating that the presence of dmit dianion alters the electronic properties of the material by reducing the difference between the HOMO/LUMO, bandgap (E g = 1.42 eV). In summary, we could conclude that hybrid films showed interesting properties which point toward a possible application as an electrode in organic optoelectronic devices.