Cyclopenta[c]thiophene- and Diketopyrrolopyrrole-Based Red-Green-Blue Electrochromic Polymers

Abstract

Cyclopenta[c]thiophene (CPT)-based polymers are potential candidates in organic electronics. Here, we report the first solution-processable red homopolymer (P1) of a thiophene-capped derivative of CPT (DHTCPT), and a blue homopolymer (P2) of N-substituted thienodiketopyrrolopyrrole (DEHTDPP). Additionally, by alternatingly copolymerizing the DHTCPT and DEHTDPP units, we achieved the green copolymer P3, thus completing the red-green-blue color wheels. We have shown experimentally and computationally (time-dependent density functional theory and natural bond orbital calculations) that P1 and P2 have very different optoelectronic features. However, in a donor–acceptor (D–A) copolymer P3, the optoelectronic properties have been tuned significantly to keep it in an intermediate range of P1 and P2. P2 and P3 absorb throughout the whole UV-vis range of the solar spectrum. Furthermore, all polymers showed electrochromism to switch colors between neutral and polaronic states in solution. For P1, the maximum optical contrast (%ΔT) was observed for the SOMO→LUMO transition, whereas P3 displayed the maximum %ΔT at the HOMO→LUMO transition.

Key words

conducting polymers - solution-processable - donor–acceptor polymer - electrochromicity - optical contrast

Introduction

π-Conjugated polymers (CPs) for organic electronic devices are important materials because of their potential advantages over inorganic and small organic molecules.[4] Several applications of CPs as active materials in polymer field-effect transistors (PFETs), polymer solar cells, polymer light-emitting diodes, sensors, and electrochromic devices have been the center of attention in the past few decades.[5],[6] Their solution-processability, cost-effective synthesis, stability against aerial oxidation, amenable structural modifications and interesting structure–property relationships render them a major focus in organic chemistry.[7] Among these, a study on electrochromic CPs (ECPs) to complete the entire span of color palette received significant attention.[8] The general strategy in ECPs has been to synthesize polymers with the three primary colors of the red-green-blue (RGB) color wheel.[9] ECPs with these three primary colors showing high optical contrasts at different redox potentials are ideal candidates for electrochromicity-based applications.[10]

Covalently linked alternating donor (D) and acceptor (A)-based copolymers have been an efficient strategy to tune the optoelectronic properties in CPs.[11],[12] For example, atomistic modification in the acceptor part efficiently tuned the optical bandgap (E _g ^opt) to result in donor–acceptor (D–A) RGB polymers based on cyclopenta[c]thiophene (CPT) and benzazoles.[13],[14] It is noteworthy to mention that even cyclopenta[c]chalcogenophene-based small molecules or polymers showed tunability of optoelectronic properties by atomistic modification.[15]–[18] However, in D–A alternating copolymers, CPT/thiophene-capped CPT (TCPT)-based oligomers ([Figure 1]) and benzazole are found to be an electron donor and electron acceptor, respectively, to result in D–A intramolecular charge transfer (ICT).[19] When the benzazole was replaced with dipyrromethene difluoroborane (BODIPY), a black polymer with E _g ^opt of 1.28 eV was produced.[20] Alternate copolymers based on DCPT and thiophene showed descent hole mobility (1.4 × 10⁻² cm² · V⁻¹ · s⁻¹) in PFETs.[21] While most of the CPT-based polymers were found to be amorphous, copolymers based on CPT and bithiazole building blocks showed superior p-type hole mobility (~0.05 cm² · V⁻¹ · s⁻¹) arising from the semicrystalline nature of the polymers.[22] The semicrystalline nature of the polythiophenes is known to result in efficient inter-lamellar charge transport via hopping. This could also have a significant impact on ECPs, where a narrow switching time between the doped and dedoped states is required.[23] The coloration efficiency of the CPs is altered drastically by varying the π-backbones. One way to improve the semicrystalline nature of the polymers has been to synthesize copolymers, where the other building block carries multiple types of heteroatoms, which could lead to better inter-lamellar interactions in the solid state.[22]

Figure 1 Structures of CPT, TCPT, TDPP, and DEHTDPP.

Contextually, 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TDPP) has been a potent building block in organic electronics.[24] Minor synthetic modifications on TDPP derivatives or polymers provide access to improved ordering in solid-state packing and excellent charge transport properties.[25] Even a small chiral side chain on the TDPP unit can induce strong optical activity via hierarchical self-assembly in TDPP-based polymers.[26] The photophysical properties can be fine-tuned by structural modifications that lead to alterations in frontier molecular orbitals (FMOs). The ICT and π–π stacking facilitate the intra- and inter-chain hopping of charges in TDPP-based small molecules or polymers, resulting in superior ambipolar charge transport with a high on/off ratio (I _on/off).[25],[27] Furthermore, the near-IR absorption of the TDPP-based polymers promises applications in electrochromic displays. However, there is only one report on the electrochromic behavior of the homopolymer of TDPP.[28]

Inspired by these, here we have synthesized a homopolymer of 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DEHTDPP) and the first solution-processable homopolymer of thiophene-capped 5,5-bis(hexyloxy)-5,6-dihydro-4H-cyclopenta[c]thiophene (DHTCPT) ([Scheme 1]). Additionally, we made the alternating copolymer based on DHTCPT and DEHTDPP. On the premise of the variations in the polymer backbone, this enabled us to investigate the optoelectronic and electrochromic properties of the resultant polymers.

Scheme 1 Synthetic pathway to the polymers P1 – P3. Conditions: (a) Pd₂(dba)₃, P(o-tolyl)₃, toluene, 110 °C, 24 h; (b) Pd(dppf)Cl₂, Na₂CO₃, toluene/ethanol/water (2.5/1/1), 85 °C, 22 h; (c) Pd₂(dba)₃, P(o-tolyl)₃, toluene, 110 °C, 28 h.

Results and Discussion

Synthesis and Characterization

The monomers 1–4 were synthesized according to the previously reported methods.[19],[29] Generally, copolymerization of the CPT-based monomers was performed by Stille coupling, which could keep the polydispersity indices (PDIs) below 2. Additionally, the Stille coupling method afforded CPT-based polymers with reproducible device properties in terms of hole mobility and on/off ratio.[21] So, P1 was synthesized by the Stille polymerization method[30] using 1 and 2. Although TCPT was used to make many copolymers by us earlier, this is the first synthesis of its solution-processable homopolymer. P2 (homopolymer of DEHTDPP) was synthesized by Suzuki polymerization of 3 and 4 as this method is known to produce highly efficient materials for electronic devices.[25],[29] Here, the alternate copolymer P3 was afforded by a Stille cross-coupling polymerization between 2 and 3. Finally, the polymers were purified by Soxhlet extraction and precipitation from the chloroform extract using methanol as a non-solvent. The presented polymerization methods furnished the three polymers with yields ranging between 62% and 74% and PDIs in the range of 1.7 – 1.8 ([Table 1]). It can be noted that the homopolymers were obtained with slightly lower yield and molecular weight. M _n values of 11.3, 8.2 and 6.2 affirm the presence of at least 22, 15 and 6 repeating units of the corresponding monomeric or comonomeric units in P1, P2, and P3, respectively ([Table 1]). The polymers exhibited ample solubility for further solution-based studies. Decomposition temperature (T _d) was calculated at 5% thermal decomposition for the polymers. P2 showed maximum stability with a T _d of 251 °C (see Figure S4 in the Supporting Information).

Table 1 Physical properties of the polymers

Polymer	M w (kD)	M n (kD)	PDI	T d (°C)
P1	19.2	11.3	1.67	251
P2	14.9	8.2	1.84	268
P3	11.0	6.2	1.7	224

Optical and Electrochemical Properties

The polymer showed three basic colors of the RGB color wheel. P1, P2 and P3 displayed red, blue and green colors, respectively, in their chloroform solutions. This shows the importance of the alternating copolymer to complete the RGB spectrum of colors. P1 showed one major absorption band around 526 nm, due to the π–π* transition ([Figure 2]). The HOMO–LUMO gap (HLG) calculated from the absorption onset of absorption (λ _onset ^abs) was found to be 2.02 eV. This is in line with the previously synthesized thin films of the homopolymers by the electrochemical method.[17] Both P2 and P3 were found to show absorption spectra covering the UV-vis range with two significant bands. The TDPP-based oligomers are known to have a strong transition in the near-IR region due to ICT between the donor-type thiophene and the acceptor-type lactam unit. The TDPP-containing polymers are known to show similar behavior as well. As a result, the longer wavelength band in TDPP-based polymers originates from the ICT, and the π→π* transition appears at a lower wavelength. The wavelength of absorption maxima (λ _max) for P2 appeared at 692 nm with two weaker peaks around 313 and 396 nm. Compared to other reported homopolymers of TDPP carrying different substitutions on N-atom, P2 exhibited a lower λ _max, which may be due to a substitution effect or an effect of a shorter chain length of the polymer backbone.[25] But, ~140 nm red-shifted λ _max of P2 compared to the DEHTDPP monomer[24] is evident of a significantly long polymeric chromophore in P2. The HLG calculated from the λ _onset ^abs of the solution-state UV-vis spectra is 1.48 eV, the lowest among the current series. The alternate copolymer P3 showed a strong transition at 678 nm and a weaker peak at 416 nm.

Figure 2 (a) Experimental UV-vis absorption spectra of the polymers in chloroform; (b) computational (TDDFT/B3LYP/6 – 31 G(d,p)) UV-vis absorption spectra of DM1, DM2, and CM1.

The electrochemical properties of the polymers were characterized by CV ([Figure 3]a). The polymers displayed peaks for both anodic oxidation and cathodic reductions. The oxidation of the polymers was irreversible. The onset of the oxidation (E _onset ^ox) for P2 was calculated to be 0.13 V higher than that of P1. The HOMO level was calculated to be located at −5.04 for P1 and −5.17 eV for P2. The presence of the lactam unit is known to stabilize the HOMO in DEHTDPP homopolymers. Compared to this, incorporating more electron-rich thiophene in the alternating copolymer backbone is expected to destabilize HOMO. Evident to this conclusion is the E _onset ^ox value of P3 (0.64 V, HOMO = −5.08 eV), which is between the two extreme values of P1 and P2. Under the cathodic voltage sweep, P1 and P2 showed irreversible behavior, whereas P2 displayed a quasi-reversible reduction due to the presence of the lactam units. The trend in electrochemical reduction was the opposite in terms of the onset of the reduction (E _onset ^red). E _onset ^red for P1 was 0.41 V higher than that of P2. However, alternating copolymer P3 showed an intermediate E _onset ^red value (−0.94 V) compared to P1 and P2. The energy of the LUMO was calculated to be −3.32 and −3.67 eV for P1 and P2, respectively. Thus, the electrochemical bandgap (E _g ^elec) values of 1.72 and 1.5 eV were achieved for P1 and P2, respectively. For completion of the RGB color wheel, intermediate FMO levels of the P1 and P2 were essential. By combining the TCPT and DEHTDPP units, we were able to achieve P3, which has intermediate properties of the FMOs (HOMO = −5.08 eV; LUMO = −1.58 eV) compared with individual homopolymers P1 and P2.

Figure 3 (a) CV of the polymers in acetonitrile as a solvent and tetrabutylammonium hexafluorophosphate (TBAPF₆) as an electrolyte; (b) experimental (electrochemical) and computational (DFT/6 – 31 G(d,p)/B3LYP) bandgap in the polymers, model dimers, and comonomers.

Density functional theory (DFT/B3LYP-6 – 31 G(d,p)) was used to optimize the polymers in order to thoroughly examine the nature of the electronic transition, FMOs, and electrochemical characteristics (see Figure S6 in the Supporting Information). Generally, time-dependent DFT (TDDFT) studies are conveniently performed upon the gas-phase optimized geometry. So, the model comonomer for P3 (CM1) and the dimers for DHTCPT (DM1) and DEHTDPP (DM2) were optimized (see Figure S5 in the Supporting Information) using DFT (DFT/B3LYP-6 – 31 G(d,p)) and TDDFT calculations were performed at the same level of computations. The optimized geometry of the polymers was found to be nearly planar. Computationally obtained results ([Figure 3]b) complemented multiple experimental observations. DM1, DM2 and CM1 displayed planarity in the gas-phase optimized geometry. The electron density in the HOMO and LUMO for DM1 was delocalized all over the polymer backbone ([Figure 4]). However, significant localization of electron density in DM2's HOMO and LUMO was seen over lactam rings, indicating a key role for those in regulating the energies of the FMOs. In DM1, the transition at 516 nm could be related to the electronic transition of P1 at 526 nm. This peak is clearly due to π→π* transition within the HOMO→LUMO (f = 1.9341) levels. In DM2, the HOMO→LUMO (f = 1.7182) transition was calculated at the longest wavelength (701 nm) among the three. This can be compared to the peak at 692 nm for the P2. Interestingly, the D–A copolymer P3 showed this transition at 678 nm in solution, which can be correlated well to the peak at 618 nm (f = 1.6278) for CM1. The peaks at a shorter wavelength for P2 and P3 in a solution can be compared to those of DM2 and CM1 studied at gas-phase calculation. For DM2, the band at 405 nm (f = 0.2635) consisted of two major transitions from HOMO−2→LUMO (46%) and HOMO→LUMO+2 (37%), which can be correlated to the band at 397 nm for P2. Similarly, for CM1, the band at 399 nm (f = 0.2269) arising from a combination HOMO−1→LUMO+1 (66%) and HOMO→LUMO+2 (22%) of two transitions can be correlated to the band at 405 nm for P3. The computed gap between the highest occupied crystal orbital and lowest unoccupied crystal orbital of 2.07, 1.78 and 1.45 eV for P1, P2, and P3, respectively, reflects the trend in both E _g ^opt and E _g ^elec.

Figure 4 Contour maps of the HOMO and LUMO of the optimized (DFT/B3LYP/6 – 31 G(d,p) structures of (a) DM1, (b) DM2, and (c) CM1 (isovalue = 0.025).

For further insight into the most critical electron delocalization pathways, which are responsible for the stabilization of the molecules, natural bonding orbital (NBO) analyses were performed on DM1, DM2, and CM1 at the 6 – 31 G(d,p)/M06 – 2X level of theory (see Figure S8 and Table S2 in the Supporting Information). In DM1, the π-electron delocalization throughout the molecule provided stability. The most significant charge transfer which stabilizes the system is from the adjacent thiophene rings to the CPT moiety. The optimized structures of DM2 and CM1 displayed some distinct variations in the accumulation of charges over the lactam ring compared to other parts of the molecules. The primary ICT in DM2 is from the lone pairs of electrons on N-atoms to the antibonding orbitals of the C=O bonds. This was also found with similar stabilization energies in the CM1. There were two crucial observations from the NBO calculations of DM1 and DM2 when compared independently to the CM1. Firstly, stabilization energy is associated with the charge transfer from the adjacent thiophene ring to the lactam ring. In DM2, the charge transfer was found between the antibonding orbital of C14 – C15 (ED_i = 0.35 702) to C9–C10 (ED_j = 0.39 688) with stabilization energy (E ²) of 324.41 kcal/mol). However, E ² within identical NBOs in CM1 was remarkably decreased by 127.49 kcal/mol BD* of C14 – C15 (ED_i = 0.37 067) to BD* of C9–C10 (EDj = 0.38 225), E ² = 196.92 kcal/mol). Secondly, E ², arising from the charge transfer from the thiophene ring to the CPT ring in CM1, is reduced by 43.18 kcal/mol ((BD* of C43 – C48 (ED_i = 0.02 064) to BD* of C37 – C41 (ED_j = 0.34 568), E ² = 147.87 kcal/mol)), compared to E ² for charge transfer within identical NBOs in DM1 ((BD* of C52 – C57 (ED_i = 0.34 950) to BD* C46 – C49 (ED_j = 0.3530), E ² = 191.05 kcal/mol)). So, the combination of TCPT and DEHTDPP in an alternate fashion resulted in a delicate balance to keep the intermediate stabilization energies of the NBOs compared to their individual components. Also, in contour maps of the LUMO of CM1, a significant localization of the electron density over the TDPP unit could be attributed to significant ICT. Similar phenomena could be expected for the polymers comprising the DM1, DM2, and CM1 as repeating units. It was very important to achieve the energies of the FMOs for the P3 as an intermediate of P1 and P2 to achieve a variable band gap and to achieve the RGB color wheel.

Spectroelectrochemistry and electrochromicity

To fully utilize the potential of the spectral and electrochemical profile of the polymers, spectroelectrochemistry was performed in an oxidative manner. The spectroelectrochemical behavior of the molecules was studied in the absorption range of 300 – 1600 nm to study the electrochromism of the polymers and the singly charged polarons. P1, P2, and P3 were studied under potential windows of 0.1 – 0.8, 0.1 – 0.9 and 0.3 – 0.9 V, respectively ([Figure 5]). P1 changed color from red to transparent blue when switched reproducibly between 0.1 and 0.8 V. Similarly, P2 (under 0.1 – 0.9 V) and P3 (0.3 – 0.9 V) exhibited the reproducible color transitions, blue→transparent and green→transparent, respectively. Upon gradual oxidation, the intensity of the peak at λ _max for P1 faded, while the polaronic absorption band originating from singly charged polymer (P1^•+ ) started appearing at a higher wavelength and intensified further with an increase in potential and saturates after 0.8 V.

Figure 5 Spectroelectrochemistry of (a) P1, (b) P2, and (c) P3 in dichloromethane solution using TBAPF₆ as an electrolyte.

For P2^•+ and P3^•+ , the absorption peaks were observed at 1015 and 1057 nm, respectively. In spectroelectrochemistry, the polymers showed one clear isosbestic point between the peaks related to absorption from the neutral polymer and polaron. The appearance of this isosbestic point indicates the transformation of the polymer chain into only one other species in solution, which is polaron. There may be the formation of bipolaron in an interim applied potential, but the limitation in the measurement window[31] of the used UV-vis spectrophotometer restricted us from further discussion.

The chromaticity diagram for the polymers represents the variety in the visual appearance of the polymers in neutral states ([Figure 6]a). The polymers showed a range of optical contrasts in different wavelengths ([Figure 6]b). The %ΔT values recorded at the HOMO→LUMO transition from the undoped polymer solution and singly occupied molecular orbital (SOMO)→LUMO transition from the polaronic species are summarized in [Figure 6]b. P1, which changed color from red to transparent blue upon oxidation, demonstrated a %ΔT of 29 and 44 arising from the change in optical density at the HOMO→LUMO and HOMO→SOMO transitions, respectively. Similarly, P2 displayed %ΔT of 39 and 17 when measured at their major peaks due to neutral and polaronic states. In P3, %ΔT of 53 and 26 were found at the HOMO→LUMO and SOMO→LUMO transitions, respectively. We achieved 1.8 and 1.4 times higher %ΔT for the HOMO→LUMO transition in the UV-vis region for P3, compared to those of P1 and P2, respectively, as well as decent %ΔT in the near-IR region, by alternately polymerizing the DHTCPT unit with the DEHTDPP unit in P3. It is non-trivial to correlate this to a particular reason for structurally different polymers, unlike other polymer backbones, where a single hetero atom changes the nature of conjugation.[32] Here, the ICT transition in both P2 and P3 in the D–A alternating copolymer backbone is observed to be affected mainly through extrinsic doping.

Figure 6 (a) CIE color coordinates 1931 (2° observer) for P1-0.23 (x), 0.38 (y), P2-0.40 (x), 0.32 (y) and P3-0.43 (x), 0.37 (y) are obtained from absorption spectra of the polymers in chloroform. (b) Change in %T values of the polymers during spectroelectrochemistry. On the XY plane, the bottom of the pillars represents the initial values of %T, and the Z-axis represents the final values of %T of the polymers. The frontal and rear pillars represent %T values for SOMO→LUMO and HOMO→LUMO transitions, respectively. X-axis: polymers 1 (P1), 2 (P2), and 3 (P3).

Conclusions

In summary, we have synthesized three new soluble and thermally stable polymers based on TCPT and DEHTDPP by Pd-catalysed Stille and Suzuki coupling methods. TCPT and DEHTDPP afforded the red and blue polymers P1 and P2, respectively, upon homopolymerization. In contrast, their copolymerization in an alternate fashion resulted in the green polymer P3. The HOMO level is the most stable in the case of P2 and the least in the case of P1. The trend in LUMO energy is the opposite. The D−A copolymer P3 possesses HOMO and LUMO energies in between those of the P1 and P2. The computationally obtained contour maps of HOMO and LUMO of the P3 show that the electron-rich DHTCPT controls the HOMO and the electron-deficient DEHTDPP units control the LUMO energy. DFT calculations supported the trends in HLGs from UV-vis spectra and CV. Additionally, NBO calculations hinted at the presence of several energy-stabilizing phenomena in the dimer model of the polymers. The ICT from the thiophene to the lactam ring in DEHTDPP was found to be the dominating factor in the energy stabilization of the molecular orbitals. Very importantly, the polymers displayed electrochromicity, measured at the solution state. The most significant %ΔT of 53 was obtained for the copolymer P3, compared to the homopolymers at the λ _max of the HOMO→LUMO transition. However, P1 showed the most superior %ΔT of 44 in the case of the SOMO→LUMO transition. So, the judicial design of CPT- and TDPP-based polymers could furnish materials for tunable optoelectronic properties and color-switching abilities in relevant future devices.

Experimental Section

Most of the reagents and solvents obtained from commercial sources were used without any further purification. Reagent-grade toluene was dried over sodium. NMR spectra were recorded on a Bruker AVANCE 500 FT-NMR spectrometer using CDCl₃ as the solvent and chemical shifts are reported in parts per million (δ scale) relative to TMS as the internal standard. Column chromatography was performed using silica gel (100 – 200 mesh). Molecular weight and polydispersity of the polymers were obtained from gel permeation chromatography (GPC, Waters 2414) analyses against polystyrene standard using THF at a flow rate of 0.3 mL/min at r. t. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA 851 analyzer at a heating rate of 10 °C · min⁻¹ under nitrogen. UV-vis-NIR spectra of the polymers were recorded on a HITACHI U-4100 UV-vis-NIR spectrophotometer in polymer solutions having concentration of 0.15 mg/mL. Electrochemical analysis was carried out with a Princeton Applied Research 263A potentiostat using a platinum (Pt) disk electrode as the working electrode, a platinum wire as the counter electrode, and an AgCl-coated Ag wire as the reference electrode. Pt disk electrodes were gleamed with alumina, water, and acetone and were dried under nitrogen flow to remove any incipient oxygen. 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) in dichloromethane (DCM) was used as an electrolyte. Films were dropcast on a Pt-disk electrode for CV. In situ spectroelectrochemistry was performed by synchronizing the operations on the potentiostat and UV-vis spectrophotometer, simultaneously. Spectroelectrochemistry of the polymers was recorded in 0.75 mg/mL DCM solution of the polymers using 0.1 M TBAPF₆ as an electrolyte and a Pt-mesh electrode (5 mm × 8 mm) as the working electrode.

Procedures

Synthesis of P1 . To an oven-dried 50 mL two-neck round-bottom flask under nitrogen was added dry toluene (10 mL) along with 2 (100 mg, 0.1190 mmol) and 1 (80 mg, 0.1190 mmol). The resulting solution was purged with dry nitrogen for 20 min. Pd₂(dba)₃ (11 mg, 0.0119 mmol) and P(o-tolyl)₃ (18 mg, 0.0595 mmol) were added into the flask. The reaction mixture was stirred at 110 °C under a nitrogen atmosphere and was monitored with thin layer chromatography. The reaction vessel was cooled to r. t. after 24 h and cold methanol was added to the reaction mixture to find dark red precipitate. The precipitate was collected by filtration on a Whatmann^®42 filter paper and was washed sequentially with methanol and acetone in a Soxhlet extraction apparatus for 28 h until the wash solution was colorless and then extracted by chloroform. To the chloroform fraction, methanol was added and the resulting precipitate was filtered. The residue was dried under vacuum (10⁻² bar) to afford the red polymer P1 (46 mg, 74%).

¹H NMR (400 MHz, CDCl₃): δ 7.00 (br), 3.48 (br), 2.79 (br), 1.33 – 1.25 (br, 4 H), 0.89 (br).

GPC (THF, 298 K, polystyrene standard): M _w = 19,200 Da, M _n = 11,300 Da, and PDI = 1.67.

UV-vis (CHCl₃): 526 nm.

Synthesis of P2 . 3 (100 mg, 0.1470 mmol), 4 (114 mg, 0.1470 mmol), Pd(dppf)Cl₂ (10 mg, 0.0147 mmol), and Na₂CO₃ (155 mg, 1.4700 mmol) were taken in a 100 mL Schlenk flask, and the flask was evacuated and back-filled with argon three times. A degassed solvent mixture consisting of toluene (25 mL), ethanol (9 mL) and water (10 mL) was transferred to the Schlenk flask through a septum. The reaction mixture was purged with argon for 15 min. The reaction was carried out at 85 °C for 22 h under an argon atmosphere resulting in blue precipitates on the wall of the flask. There was added a 50 mL solvent mixture containing methanol/water (1/1) to complete the precipitation from the reaction mixture. After completion of the precipitation, the resulting reaction mixture was filtered through a Whatmann^®42 filter paper. The residue was washed with methanol and acetone to confirm removal of catalyst residues and short oligomers. Finally, the Soxhlet extraction method using chloroform was performed and the concentrated polymer solution (~20 mL) was poured slowly into 250 mL of cold methanol under vigorous stirring. After 15 min of stirring, polymer was collected by vacuum filtration and dried under vacuum (10⁻² bar) to afford the blue polymer P2 (99 mg, 69%).

¹H NMR (400 MHz, CDCl₃): δ 9.01 – 8.92 (br), 7.59 – 7.09 (br), 4.03 (br), 1.89 (br, 4 H), 1.38 – 1.25 (br), 0.89 (br).

GPC (THF, 298 K, polystyrene standard): M _n = 14,900 Da, M _w = 8,200 Da, and PDI = 1.84.

UV-vis (CHCl₃): 397, 692 nm.

Synthesis of P3 . To an oven-dried 50 mL two-neck round-bottom flask under nitrogen was added dry toluene (10 mL) along with 2 (100 mg, 0.1190 mmol) and 3 (81 mg, 0.1190 mmol). The resulting solution was purged with dry nitrogen for 20 min. Pd₂(dba)₃ (11 mg, 0.0119 mmol) and P(o-tolyl)₃ (18 mg, 0.0595 mmol) were added into the flask. The reaction mixture was stirred at 110 °C under a nitrogen atmosphere and was monitored with thin layer chromatography. The reaction vessel was cooled to r. t. after 28 h and cold methanol was added to the reaction mixture to find dark red precipitate. The precipitate was collected by filtration on a Whatmann^®42 filter paper and was washed sequentially with methanol and acetone in a Soxhlet extraction apparatus for 28 h until the wash solution was colorless, followed by extraction with chloroform. To the chloroform fraction, methanol was added and the resulting precipitate was filtered. The residue was dried under vacuum (10⁻² bar) to afford the green polymer P3 (76 mg, 62%).

¹H NMR (400 MHz, CDCl₃): δ 8.98 (br), 4.04 (br), 3.45 (br), 2.77 (br), 1.94 (br, s), 1.62 (br, s), 1.31 (br), 0.88 (br).

GPC (THF, 298 K, polystyrene standard): M _w = 11,000 Da, M _n = 6,200 Da, and PDI = 1.7.

UV-vis (CHCl₃): 416, 678 nm.

Funding Information

We gratefully acknowledge financial support from SERB, India (SERB/SRG/2021/000 648) for funding this research.

Acknowledgment

High Performance Computing Centre, SRM Institute of Science and Technology is acknowledged for providing the computational facility.

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Publication History

Received: 03 September 2022

Accepted: 09 November 2022

Article published online:
13 December 2022