Smartphone light-driven electrocatalytic polymerization of thiophenes

Abstract

Herein, we take advantage of the photoelectric effect produced on light-emitting diodes to induce a kinetically controlled electrocatalytic polymerization of 3,4-alkoxythiophenes. Wireless polymerization was achieved by fine-tuning the irradiation time, the power density and the molecular structure of the thiophenes. A smartphone based light-driven system is introduced as a simple and low cost polymerization approach to π-conjugated films.

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Organic electroactive materials have gained considerable attention in multiple applications, ranging from energy conversion to sensing.¹ Among all these materials, π-conjugated polymers stand out due to their rather fast charging/discharging transition, good optical properties and efficient conductivity.² Commonly, these materials are synthesized via an oxidative radical polymerization mechanism,³ which is triggered chemically by using homogeneous catalysts (strong oxidizing agents)⁴ or electrochemically by controlling the applied potential or current.⁵ Due to the possible fine-tuning of the thermodynamic or kinetic parameters of the polymerization, electrochemical methods allow efficient control of the morphology and optoelectronic properties of the films.⁶ Nonetheless, this methodology requires a direct connection of the electrode to a potentiostat, limiting its use for in situ applications, thus the development of wireless strategies is highly desired.

Recently, bipolar electrochemistry (BE) has emerged as a powerful approach to achieve wireless electropolymerization.⁷ It is based on the induction of a polarization potential difference (ΔV) across a conducting object acting as a bipolar electrode (BPE), due to the presence of an electric field in solution. When ΔV overcomes the threshold polarization potential difference (ΔV_min) associated with the oxidation and reduction of the monomer and a sacrificial redox probe, respectively, electropolymerization takes place at the anodic extremity of the BPE. This approach has been extensively used for the synthesis of films and fibers of poly(3,4-ethylenedioxythiophene) (PEDOT).⁷ Although interesting, BE often requires a high-voltage power source in order to produce the appropriate electric field.

An interesting alternative is to take advantage of the voltage-generating function of light-emitting diodes (LEDs). In recent years, these microelectronic devices have gained considerable attention as optical transducers of chemical information.⁸ However, due to the n- and p-type semiconductors composing the diodes, LEDs can not only produce light, but also generate a significant difference of voltage when irradiated with photons with appropriate wavelength (ΔV_hv).⁹ This photoelectric effect has been exploited in sensing, electrochemiluminescence devices and water electrolysis.⁹ In this work, we take advantage of the light-induced ΔV_hv to power the electrocatalytic polymerization of different 3,4-alkoxy thiophenes. By irradiating a red LED with white light, a sufficient ΔV_hv is induced to trigger the oxidation and reduction of the monomer and of a sacrificial redox probe, respectively. Furthermore, by fine-tuning the chemical structure of the thiophene unit, it is possible to decrease the ΔV_min required to trigger the redox reactions. Moreover, this approach allows producing π-conjugated films with well-defined redox signals, even when irradiating with the light of a mobile phone. This straightforward method enables the wireless synthesis of π-conjugated polymers with a simple low-cost experimental setup.

As stated above, the light-driven electrocatalytic polymerization takes advantage of the ΔV_hv induced across an LED when irradiated with photons of appropriate wavelength. In a first order approximation, this ΔV_hv can trigger redox reactions at each extremity of the diode, as long as it overcomes the ΔV_min associated with the electroactive species in solution. In the present case, the oxidation of two different 3,4-alkoxythiophenes, i.e. EDOT and 3,4-propylenedioxythiophene (ProDOT) (Fig. 1a), and the reduction of benzoquinone (Q) were used as model reactions. First, the individual potentiodynamic characterization of EDOT and ProDOT (10 mM), and Q (10 mM) was performed in a 0.1 M tetrabutylammonium perchlorate (TBAP)/acetonitrile (ACN) solution. During the anodic sweep, EDOT and ProDOT present a characteristic oxidation peak with an onset potential around 1.1 V vs. Ag/AgCl and 1.2 V vs. Ag/AgCl, respectively, whereas in the cathodic direction, the reduction of Q occurs at −0.4 V vs. Ag/AgCl (Fig. S1). Thus, ΔV_min values of ≈1.5 V and ≈1.6 V are required to trigger the anodic electropolymerization of the monomers via the voltage-generating function of a diode. In order to corroborate this, the ΔV_hv induced by a red LED was evaluated as a function of power density (P_hv). The photo-induced ΔV_hv was monitored by connecting a voltammeter to the terminals of the diode. A tungsten–halogen light source coupled to a fiber-bundle was used to irradiate the diode. As shown in Fig. S2 (black line), at low P_hv values (below 2 × 10⁻³ kW cm⁻²), the induced ΔV_hv remains below 0.3 V. However, when P_hv is increased gradually, ΔV_hv increases up to 1.6 V, which is the minimum required to trigger the corresponding redox reactions.

Fig. 1 (a) Chemical structures of EDOT and ProDOT. (b) Schematic illustration of the experimental setup and the principle of the light-driven electrocatalytic polymerization. (c) Potentiodynamic characterization of PEDOT films deposited on Pt electrodes under constant P_hv (3.7 kW cm⁻²) as a function of t_hv (indicated in the figure), obtained in a monomer-free 0.1 M tetrabutylammonium perchlorate/acetonitrile (TBAP/ACN) solution, v = 50 mV s⁻¹. The grey curve represents the potentiodynamic response in the absence of quinone. (d) Optical pictures of the electrode surfaces modified for different t_hv (indicated in the figure) by the electropolymerization of EDOT. (e) Q_a and Q_c variation as a function of t_hv obtained during the light-assisted electrocatalytic polymerization of EDOT. The green dots represent Q_a and Q_c obtained in the absence of quinone.

After this set of experiments, we evaluated the possible light-driven electrocatalytic polymerization of EDOT and ProDOT. A Pt wire and a Pt disk electrode were dipped into two chambers separated by a glass frit, filled with a 0.1 M TBAP/ACN solution containing either Q (10 mM) or a monomer (10 mM), and then connected to the cathode and anode of a red-LED, respectively (Fig. 1b and Fig. S3). It is important to highlight that under these experimental conditions, additional interfacial resistances will impact the polarization potential, leading to a 100 mV decrease in driving force (Fig. S2 red line). Nonetheless, by irradiating with a P_hv of 3.7 kW cm⁻², it is possible to produce a sufficient ΔV_hv to trigger the redox reactions, and the amount of deposited PEDOT was evaluated as a function of irradiation time (t_hv). For this, the potentiodynamic response of the films after polymerization was recorded in a monomer-free 0.1 M TBAP/ACN solution. As expected, the irreversible charge/discharge signals (ΔE_p ≈ 560 mV), characteristic of PEDOT, appeared gradually when increasing t_hv (Fig. 1c). Furthermore, the discharged polymer presents a slightly blue color that progressively changes to dark purple as t_hv increases (Fig. 1d). Moreover, the plot of the anodic (Q_a) and cathodic (Q_c) charge as a function of t_hv presents a rather linear correlation (Fig. 1e blue dots). Such a linearity is characteristic of kinetically controlled electropolymerization, thus, under these conditions, the oxidative dimerization pathway occurs at a rather slow rate.^3a This leads to an ordered monomer–oligomer coupling, which in turn favours the formation of long-chain oligomers, as evidenced by the well-defined redox peaks between 0.2 V vs. Ag/AgCl and −0.75 V vs. Ag/AgCl (Fig. 1c). The general validity of the approach was verified by performing the light-driven electropolymerization of ProDOT. For a constant P_hv value (3.7 kW cm⁻²), the characteristic irreversible redox signals of poly-ProDOT (PProDOT, ΔE_p ≈ 600 mV) appeared gradually as a function of t_hv (Fig. S4a), producing a brownish coloured film (inset of Fig. S3b). Once again, a linear correlation between Q_a and Q_c and t_hv was obtained, corroborating the kinetically controlled electropolymerization (Fig. S4b blue dots). The electrocatalyzed polymerization of ProDOT is rather surprising, since a ΔV above 1.6 V is required to trigger the redox reactions. However, it is possible to assume that under these conditions, the diode generates a sufficient ΔV_hv, triggering the oxidation of the monomer. Thus, once a few charged short-chain oligomers of PProDOT are formed, they self-catalyse the oligomerization via solid-state redox processes.¹⁰ To verify the contribution of the sacrificial redox probe on the electropolymerization kinetics, a control experiment was carried out in the absence of quinone under constant P_hv and t_hv (3.7 kW cm⁻² and 10 min, respectively). As expected, under these conditions, only the characteristic response associated with the capacitive current of the electrode/solution interface was obtained (Fig. 1c and Fig. S4a, grey line). Furthermore, the recorded Q_a and Q_c values are found to be comparable to the ones obtained in the absence of light (Fig. 1e and Fig. S4b, green dots).

After this set of experiments, demonstrating the light-driven electropolymerization, we evaluated the influence of the magnitude of P_hv on the synthesis of π-conjugated polymers. In this case, the electrolysis was carried out with a constant t_hv (10 min) in a 10 mM EDOT, 0.1 M TBAP/ACN solution. At low P_hv values (below 1.5 kW cm⁻²), the potentiodynamic analysis of the modified electrode in a monomer-free solution exhibits only the capacitive response of the electrode/solution interface (Fig. S5). Above this value, the polymerization begins, as evidenced by the low intensity redox signals of PEDOT. From this point onwards, the charging/discharging response of the film increases (Fig. S5). This provides evidence that the electropolymerization takes place only when the induced ΔV_hv exceeds the ΔV_min associated with the oxidation and reduction of EDOT and quinone, respectively (above 1.5 V). In order to overcome this thermodynamic limitation, we decided to fine-tune the chemical structure of 3,4-alkoxythiophenes. It is well-established that the oxidation potential of π-conjugated groups decreases with an increase in the conjugation length, meaning that the oxidation potential of the monomer is higher than those of the dimer and the trimer.^3a In particular, π-conjugated trimeric units have gained considerable attention due to their low oxidation potential and easy synthesis.¹¹ Thus, two different 3,4-alkoxythiophene trimers were synthesized, bis[3,4-ethylenedioxythiophene]-2-hexyl-EDOT (C₆-Tri-EDOT) and bis[3,4-ethylenedioxythiophene]-3,4-(2,2-dimethylpropylenedioxy)thiophene (C₂-EPE) (Fig. 2a). Both target trimers were synthesized following a three-step procedure; (i) formation of the EDOT/ProDOT derivatives, (ii) an α-,α-di-bromination, and finally (iii) a Suzuki coupling with EDOT boronic ester (Schemes S1 and S2). All intermediates and final products were characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, and the trimeric structures were further confirmed by high resolution mass spectroscopy (HRMS). Detailed synthetic procedures and characterization data are provided in the SI (Fig. S6–S17).

Fig. 2 (a) Chemical structures of C₆-Tri-EDOT and C₂-EPE. (b) Potentiodynamic characterization of poly-C₆-Tri-EDOT films deposited on Pt electrodes as a function of P_hv (indicated in the figure) for a constant t_hv (10 min), obtained in a monomer-free 0.1 M TBAP/ACN solution, v = 50 mV s⁻¹. (c) Optical pictures of the electrode surface obtained before (top) and after electropolymerization of C₆-Tri-EDOT (middle) and C₂-EPE (bottom) at a constant P_hv and t_hv (2.1 kW cm⁻² and 10 min, respectively). (d) Q_a and Q_c variation as a function of P_hv used for the light-driven electrocatalytic polymerization of C₆-Tri-EDOT.

The oxidation of C₆-Tri-EDOT and C₂-EPE presents a peak between 0.45 V vs. Ag/AgCl and 0.5 V vs. Ag/AgCl, with a threshold potential of 0.3 V vs. Ag/AgCl and 0.4 V vs. Ag/AgCl, respectively (Fig. S18). With these potentials, ΔV_min values of 0.7 V for C₆-Tri-EDOT and 0.8 V for C₂-EPE were calculated, which are significantly lower than those required for EDOT and ProDOT. Thus, in theory, it is possible to trigger the electropolymerization with lower P_hv values (Table S1). In order to test this, the amount of poly-C₆-Tri-EDOT was evaluated as a function of P_hv under constant t_hv (10 min). Notably, when irradiating the red LED with a P_hv of 7 × 10⁻³ kW cm⁻², the potentiodynamic response of poly-C₆-Tri-EDOT, in a monomer-free 0.1 M TBAP/ACN solution, presents two main redox peaks (≈−0.1 V vs. Ag/AgCl and −0.4 V vs. Ag/AgCl, respectively) which are associated with the charge/discharge of the film (Fig. 2b). Hence, the use of the trimer unit allows decreasing by three orders of magnitude the P_hv required to initiate the electropolymerization, compared with the monomeric units. This was further corroborated by the electropolymerization of C₂-EPE, since under the same experimental conditions, charge/discharge signals around −0.2 V vs. Ag/AgCl and −0.5 V vs. Ag/AgCl were observed during the potentiodynamic analysis (Fig. S19). Moreover, for both trimers, the magnitude of the main redox signals increases as a function of the applied P_hv (Fig. 2b and Fig. S19). These conditions produce light and dark purple films for C₆-tri-EDOT and C₂-EPE, respectively (Fig. 2c).

It is noteworthy that at P_hv values above 6 × 10⁻² kW cm⁻², the ΔV_hv is high enough to surpass the oxidation and reduction thresholds of the trimers and Q, respectively. Nonetheless, as it is well-known, when performing the electropolymerization at high potential values (above the oxidation peak), the interface is enriched with highly reactive species (radical cations), which catalyse the oligomerization.¹² This is evidenced by the plot of the Q_a and Q_c as a function of P_hv (Fig. 2d). For an energy density of less than 6 × 10⁻² kW cm⁻², the redox charge of the polymer increases gradually up to 50 µC (Fig. S20). Above this value, Q_a and Q_c increase in a non-linear way by up to one order of magnitude, indicating catalyzed oligomerization (Fig. 2d). In addition, when applying high P_hv values (above 2.1 kW cm⁻²) the potentiodynamic response of both films presents wider peaks and capacitive-like responses at potentials above the main redox process (Fig. 2b and Fig. S19). Thus, under these conditions, the formation of oligomers with a larger range of chain lengths is favored.¹²

Finally, we decided to further simplify this approach by exploring the possible use of a commercial light source from a mobile phone. First, the power density of the lamp and the ΔV_hv induced on the red diode were evaluated. Surprisingly, the lamp of the mobile phone produced a P_hv of 4.2 × 10⁻³ kW cm⁻², which, when illuminating the diode from a distance of 1 cm, induced a ΔV_hv of 0.81 V. According to the potentiodynamic analysis, this is sufficient to trigger the redox reactions. Therefore, the light-driven electropolymerization was tested in 0.1 M TBAP/ACN solution containing 10 mM of trimer with a constant t_hv (30 min). After this time, the potentiodynamic response of poly-C₆-Tri-EDOT and poly-C₂-EPE, in a monomer-free 0.1 TBAP/ACN solution, exhibits the characteristic irreversible charge/discharge signals with a ΔE_p of 570 mV and 460 mV, respectively (Fig. 3a and b). Moreover, the Q_a and Q_c values (between 70 µC and 110 µC and −50 µC and −84 µC, respectively), measured for both polymers, are comparable to the ones obtained for PEDOT and PProDOT when applying 3-orders of magnitude higher P_hv (3.7 kW cm⁻²) for 20 min. These results demonstrate the possible use of a standard phone light to trigger the electrocatalytic polymerization of π-conjugated trimers.

Fig. 3 Potentiodynamic characterization of (a) poly-C₆-Tri-EDOT and (b) poly-C₂-EPE films deposited on Pt electrodes with a constant P_hv (4.2 × 10⁻³ kW cm⁻²) and t_hv (30 min under mobile phone light), obtained in a monomer-free 0.1 M TBAP/ACN solution, v = 50 mV s⁻¹.

In conclusion, a novel wireless electropolymerization approach, based on the voltage generating function of a red LED, was developed. The linear correlation between the irradiation time, under constant power density, and the anodic and cathodic charge of PEDOT and PProDOT demonstrates a kinetically controlled polymerization. By fine-tuning the chemical structure of the thiophene, a 3-fold decrease in the power density required to trigger the redox reactions was observed. Moreover, a one order of magnitude increase of the anodic and cathodic charge of the films was obtained during the electrocatalytic polymerization of the 3,4-alkoxythiophene trimers at high power densities. Finally, the smartphone light-driven electrocatalytic polymerization allows the production of films with well-defined redox signals. This proof-of-concept study opens up new interesting alternatives for light-driven wireless synthesis of π-conjugated polymers with a simple experimental setup and low-cost ingredients (Pt electrodes, TBAP/ACN, a monomer, a smartphone and an LED).

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI): detailed description of the methodology and additional results. See DOI: https://doi.org/10.1039/d5cp03956h.

Acknowledgements

This work has been supported by the French National Research Agency (ANR) in the frame of the project CHIRA-SENSEO (ANR-23-CE42-0004-01).

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Footnote

† Dedicated to Prof. Jumras Limtrakul, VISTEC, on the occasion of his 72nd birthday, for his important contributions to the field of catalysis.

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