Diversifying peripheral aromatic units of pyrrolo[3,2-b]pyrrole-containing conjugated polymers and the resulting optoelectronic properties

Abstract

Utilizing synthetically simple monomers to create conjugated polymers with tailorable properties is important for the continued development of materials suited for organic electronics. Pyrrolo[3,2-b]pyrrole (DHPP) has been shown to reduce the synthetic complexity commonly associated with conjugated polymers but tailoring properties were limited to the choice of comonomer that is coupled with phenyl-containing DHPPs. Here, the DHPP monomer toolbox is diversified to include thienyl and benzothiadiazole aromatic units directly attached to the periphery of DHPP monomers. Dibrominated monomers are polymerized via Suzuki cross-coupling polymerizations to generate a series of DHPP “homopolymers” with tailorable optoelectronic properties. While phenyl- and thienyl-containing polymers demonstrate localized excitation properties, the benzothiadiazole-containing DHPP polymer exhibits charge-transfer character associated with donor–acceptor materials. These differences in the aromatic unit enable optical absorbances to be tuned across the visible spectrum and provide design guidelines for attaining electrochemically stable DHPP polymers. In addition to fundamental insights into the structure–property relationships of DHPP-based polymers, the synthetic simplicity and tailorability of DHPPs continue to motivate using these building blocks in conjugated materials for optical and electronic devices.

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Graham S. Collier

Graham S. Collier received his BS in Chemistry from the University of North Carolina Wilmington in 2011. Upon graduation, he joined the research group of Michael Walter and received his MS from the University of North Carolina at Charlotte in 2013. Graham received his PhD in Polymer Chemistry under the direction of Mike Kilbey at the University of Tennessee in 2017 before joining the lab of John Reynolds at Georgia Tech as a Postdoctoral Research Associate. Graham began his independent career at Kennesaw State University in August 2020 and moved to the School of Polymer Science and Engineering at the University of Southern Mississippi as an Assistant Professor in January 2024.

Introduction

The ability to systematically manipulate properties through structural alterations continues to inspire researchers to study conjugated polymers for a variety of applications including, but not limited to, organic photovoltaics (OPVs),¹ organic light-emitting diodes (OLEDs),² electrochromics,³ and bioelectronics.⁴ While the structural design motifs that may be useful for accessing conjugated materials deemed “high-performing” seem to only be limited by one's imagination, the synthetic approaches used to access such materials are often quite complex, require numerous steps, and generate large amounts of chemical waste.^5–7 As such, there is a growing desire for the field to pursue conjugated polymers with simple structures and synthetic pathways, tunable optoelectronic properties, and high-performance metrics in device-inspired measurements such as conductivity or power conversion efficiency (PCE).⁸

As the field of conjugated polymers evolved, poly(3-hexylthiophene) (P3HT) has served as a benchmark material arguably due to its availability from a reproducible, scalable, and rather simple synthesis.^9,10 Despite the popularity of P3HT, and the impact it has had on understanding the phenomenon governing polymer performance, the modest device^11,12 performance metrics with fullerene or non-fullerene acceptors dampens enthusiasm for this polymer to realize commercial success. This dwindling enthusiasm has led to the development of donor–acceptor (D–A) type copolymers that push the limits of device performance but have simultaneously increased the synthetic complexity of the polymers.⁶ For example, a donor polymer notated as D18 achieved a PCE over 18% when blended with the non-fullerene acceptor Y6¹³ but suffers low overall yields from 10 synthetic steps for one monomer.¹⁴ This challenging synthesis motivated the You group to explore new synthetic protocols that ultimately led to improved overall yields but still required 12 synthetic steps to prepare a necessary monomer.¹⁵ There are examples of D–A copolymers that have rather simple synthetic protocols without sacrificing performance metrics. For example, the polymer PTQ-10 can be synthesized in 3 synthetic steps and achieves a PCE of 19% when blended with a non-fullerene acceptor.^16–18 These attributes have motivated researchers to pursue more cost-effective synthesis¹⁹ and expand the side chain functionality that enables green solvent processing,²⁰ but synthesis still requires using Stille cross-coupling polymerizations that produce stoichiometric amounts of potentially toxic waste. Other recent efforts by the Heeney group have involved utilizing scalable nucleophilic aromatic substitution (SN_Ar) protocols to attain simple D–A polymers with tunable structures and solubility in conjunction with respectable PCEs.²¹ As it relates to redox-active polymers, propylenedioxythiophene (ProDOT)-based polymers are the benchmark for the field due to their outstanding optical contrast,^22,23 electrochemical stability,²⁴ and diverse applicability. The utility of ProDOT polymers is supported by diverse, simple, and scalable syntheses^25,26 and monomers have even been shown to be amenable to continuous flow chemistry.²⁷ Each of these systems represents respectable advancements in the synthesis science of conjugated polymers and provides inspiration to find new structures that further simplify the preparation of these materials.

When envisioning other building blocks that may be useful for attaining structurally diverse conjugated materials via simple syntheses, 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs) garner attention due to their ease of synthesis and expansive design space facilitated by the variety of readily available aromatic aldehydes and anilines.²⁸ The synthesis involves an Fe-catalyzed multicomponent reaction (MCR) that is performed in a single aerobic step and purification is typically achieved via vacuum filtration.^29,30 Ultimately, these chromophores have been used as active layer materials in OPVs or dye-sensitized solar cells (DSSCs),^31,32 emissive materials in OLEDs,³³ as organic linkers in responsive metal–organic-frameworks (MOFs),³⁴ and more recently as color-controlled high-contrast electrochromes.³⁵ Their ease of synthesis and structural tailorability ultimately motivated our group to begin incorporating DHPPs into the main chain repeat unit of conjugated polymers via Pd-catalyzed cross-coupling^36,37 or acid-catalyzed condensation polymerizations.³⁸ Overall, our efforts led to a quantitative reduction in the synthetic complexity commonly associated with conjugated polymers while demonstrating high-contrast or multi-colored electrochromism,³⁹ PCEs ∼ 2.5% with hole mobilities similar to high-performing copolymers, or on-demand acid-catalyzed degradation. While our polymer systems demonstrate the ability to enchain DHPP monomers into a polymer repeat unit, the aromatic units directly adjacent to the DHPP core were relegated to benzene rings. The increased aromaticity in the benzene ring inhibits efficient formation of the quinoidal structure of the conjugated polymer and introduces large dihedral angles, both of which are detrimental to properties such as mobility and optical contrast upon doping. Furthermore, benzene-based electrochromes are hypothesized to be susceptible to undesired side reactions when doped that lead to decreased stability with extended switching protocols.⁴⁰ As such, there is motivation to broaden the structural-diversity of the aromatic functionalities at the periphery of DHPP monomers that will enable further tunability of polymer properties while also enhancing performance metrics.

Motivation for diversifying the aromatic functionality at the DHPP periphery is gained from the molecular DHPP literature. First, in their seminal reports of the Fe-catalyzed MCR to synthesize DHPPs, the Gryko group reported being able to incorporate electron-rich aromatics into the DHPP scaffold resulting in large fluorescence quantum yields and red-shifted absorbance/emission spectra compared to tetraphenyl-DHPPs.²⁹ This same group also showed that the aromatic functionality and positioning of a nitro group influence intersystem crossing of quadrapolar DHPPs.⁴¹ Beyond fundamental studies into structure–property relationships, heteroaromatic moieties offer useful attributes across many applications. For example, Ippolito and coworkers synthesized a thienyl-functionalized DHPP that was used in a single component, wide broadband photodetector.⁴² Pyridyl-functionalized DHPPs have been used as probes for nuclei cell imaging^43,44 or as coordinating N → B bonds that lock DHPP chromophores in a planar conformation that ultimately yields deep red emission upon photoexcitation.⁴⁵ DHPPs functionalized with coumarins⁴⁶ or tetrazolo[1,5-a]quinolines⁴⁷ generate strongly polarized chromophores that display strong two photon absorbance. Finally, thiazole-functionalized DHPPs facilitate further modifications that enable eliminating the large dihedral angles associated with phenyl-functionalized DHPPs and lead to a new approach for manipulating the optoelectronic properties of DHPP chromophores.⁴⁸

Motivated by the successes of the reported molecular systems, and as alluded to in Fig. 1, we were interested to explore how properties change when DHPP monomers, and subsequently polymers, are designed with differing peripheral aromatic units. Herein, we report the synthesis of phenyl-, thienyl-, and benzothiadiazole-functionalized DHPP monomers via an Fe-catalyzed multicomponent reaction that readily participate in Pd-catalyzed Suzuki cross-coupling polymerizations. The resulting polymers expand the structural diversity of a class of synthetically-simple conjugated polymers and lead to straightforward manipulation of optoelectronic properties. Specifically, the thienyl- and benzothiadiazole-containing polymers display distinct red shifts in their absorbance spectra compared to a phenyl-containing polymer in solution and as thin films that are attributed to the increased planarization along the polymer backbone and enhanced intramolecular charge transfer processes, respectively. Thermal characterization via thermal gravimetric analysis reveals the polymers to have excellent thermal stability while differential scanning calorimetry results indicate that the polymers are amorphous evident by the absence of distinct thermal transitions. Electrochemical properties, measured via cyclic voltammetry (CV) and spectroelectrochemistry, reveal redox properties and stability to be strongly influenced by repeat unit composition. This study represents an expansion to monomer design considerations for the synthetically simple DHPP system that enables the systematic manipulation of polymer properties and offers enhanced performance metrics, such as improved redox stability. Overall, the results presented here represent a continued demonstration of DHPP being a useful building block for attaining conjugated systems by simplified syntheses while possessing properties that will be useful as next-generation organic electronic materials.

Fig. 1 Representative evolution of DHPP-containing chromophores and polymers.

Results and discussion

Monomer and polymer synthesis and characterization

DHPP-containing polymers are known to have limited solubility in common organic solvents which has required synthesizing customized anilines with branched side chains. To ensure the solubility of the resulting DHPP polymers, DHPP monomers were synthesized by reacting anilines functionalized with branched hexyldecyl side chains with 4-bromobenzaldehyde, 5-bromo-2-thiophenecarbaldehyde, or 7-bromobenzo[c][1,2,5]thiadiazole-4-carbaldehyde followed by the addition of 2,3-butanedione and Fe(ClO₄)₃ to attain m-Ph₂DHPP, m-Th₂DHPP, and m-BTD₂DHPP, respectively (Scheme 1(A)). The monomer m-Ph₂DHPP readily precipitated from the crude reaction mixture that enabled collection via vacuum filtration and purification by washing with cold MeOH and acetone to attain the desired monomer in 47% yield. The monomers m-Th₂DHPP and m-BTD₂DHPP required modifications to the purification protocols. Purifying m-Th₂DHPP required precipitating the crude reaction mixture into cold MeOH to yield a brown sludge. The brown sludge was collected via vacuum filtration and subsequently washed with cold MeOH and acetone until a pale-yellow solid remained. The pale-yellow solid was collected in 15% yield and was confirmed to be the desired thienyl-functionalized monomer. The lower yield of m-Th₂DHPP compared to that of m-Ph₂DHPP is common for electron-rich DHPPs and likely stems from the stabilization of the imine that is formed during the beginning stages of the multicomponent reaction. The monomer m-BTD₂DHPP precipitated into cold MeOH as an amorphous solid that clogged the filter during attempts to isolate the product. The pure monomer could ultimately be obtained in 12% yield after recrystallizing the crude product from ethyl acetate.

Scheme 1 (A) Synthesis of dihalogenated monomers via the Fe(III)-catalyzed multicomponent reaction. (B) Suzuki polymerizations of DHPP monomers with bis(pinacolato)boron for the synthesis of DHPP-containing polymers.

Characterization of the DHPP monomers via¹H NMR revealed differences in the aromatic chemical shifts based on the choice of the peripheral aromatic unit, as represented by the spectra presented in Fig. 2. The chemical shift associated with the 3,6-protons of the DHPP core serves as the diagnostic peak for the DHPP molecules with m-Ph₂DHPP and appears at 6.34 ppm while the same peak for m-Th₂DHPP appears at 6.23 ppm. The difference is attributed to thiophene being more electron rich than the phenyl group that leads to increased proton shielding. Alternatively, the diagnostic peak for m-BTD₂DHPP shows a drastic downfield shift to 7.01 ppm due to the electron-deficient BTD leading to increased deshielding of the DHPP core protons. This same phenomenon has been observed for donor–acceptor purine-based chromophores⁴⁹ and supported the intuition that electronic properties were susceptible changes based on the choice of the aromatic linker. The different electron distributions in this study support the notion that the optoelectronic properties of the resulting DHPP-containing polymers can be tuned based on the identity of the peripheral aromatic group similar to substituent effects previously reported by our group for molecular DHPPs.^35,39

Fig. 2 Aromatic region of ¹H NMR showing the changes in shielding effects for Ph₂DHPP (black), Th₂DHPP (red), and BTD₂DHPP (blue).

Motivated by the propensity of DHPP scaffolds to participate in Pd-catalyzed cross-coupling reactions and polymerizations, in addition to our successes synthesizing biphenyl DHPP chromophores, the brominated monomers were subjected to Pd-catalyzed Suzuki polymerization protocols using bis(pinacolato)diboron as a comonomer and Pd(PPh₃)₂Cl₂ as the palladium source (Scheme 1(B)). The monomers and catalyst were heated at 110 °C in a 4 : 1 toluene/K₂CO₃(aq) solvent mixture under an argon (Ar) atmosphere. After purification via Soxhlet washes with methanol (MeOH) and acetone, the polymers were extracted with chloroform. Both Ph₂DHPP and Th₂DHPP were collected with respectable yields of 70% and 77%, respectively, while yields for BTD₂DHPP were ∼53% due to a large fraction of the insoluble material remaining in the Soxhlet thimble. Each polymer retained its respective diagnostic peak in its resulting NMR spectrum, but the line broadening and new chemical shifts corresponding to protons on the peripheral aromatic units in Fig. S9–S11 (ESI†) support successful coupling. The number-average molecular weights (M_n) for Ph₂DHPP and Th₂DHPP were estimated to be 12.0 and 14.0 kg mol⁻¹, respectively, via size-exclusion chromatography (SEC) versus polystyrene standards using CHCl₃ as the eluent. Both polymers have monomodal SEC traces and dispersity values (Đ = M_w/M_n) of 1.6 for Ph₂DHPP and 2.1 for Th₂DHPP (Fig. S12 and S13, ESI†). The solubility constraints for BTD₂DHPP inhibited molecular weight estimations due to the inability of BTD₂DHPP to pass through a GPC prefilter, even when using trichlorobenzene at elevated temperatures. However, as shown in Fig. S14 (ESI†), there is a significant red shift in the absorbance of the BTD polymer compared to that of the BTD monomer which supports successful coupling and extension of the π-conjugation. Due to the appreciable molecular weights and Gaussian molecular weight distributions for Ph₂DHPP and Th₂DHPP, we are confident that an adequate number of aromatic rings have been enchained to reach the effective conjugation length (ECL) for BTD₂DHPP and enable accurate elucidation of structure–property relationships.

The synthetic complexity for each polymer was calculated using eqn (S1) (ESI†) and the results are tabulated in Table S1 (ESI†).⁶ Ph₂DHPP has the lowest synthetic complexity of 14.5 followed by Th₂DHPP (20.9) and then BTD₂DHPP (22.5). The higher synthetic complexity for Th₂DHPP and BTD₂DHPP compared to that for Ph₂DHPP is attributed to the lower yields during monomer synthesis, but each polymer is still calculated to be more synthetically simple than many conjugated polymers.^36,37

Optical properties and structural analysis

To begin understanding the influence of repeat unit composition on optical properties, the absorbance spectra for each polymer dissolved in toluene were measured by UV-vis absorbance spectroscopy. As shown by the solid traces in Fig. 3 and reported in Table 1, there are distinct differences in the absorbance properties dependent on the choice of the peripheral aromatic unit. First, Ph₂DHPP absorbs in the high energy portion of the electromagnetic spectrum with an absorbance maximum (λ_max) at ∼430 nm. This absorbance of Ph₂DHPP is slightly blue-shifted compared to that of the ProDOT-containing DHPP copolymer we reported in 2022 that had a λ_max of 450 nm. Th₂DHPP and BTD₂DHPP show the expected red-shifted absorbance profiles with Th₂DHPP having a λ_max ≈ 515 nm and BTD₂DHPP having a λ_max measured to be ∼630 nm. The extended red shift measured for BTD₂DHPP is attributed to the electron-deficient nature of the BTD units coupled to the electron-rich DHPP core where a donor–acceptor (D–A) motif is attained that facilitates intramolecular charge transfer (ICT) transitions. While not entirely clear in the experimental measurement range (325–800 nm), there appears to be a high energy transition at ∼325 nm that would arise from the dual-band absorbance characteristics commonly associated with D–A polymers. All three polymers display featureless absorbance profiles in solution, which is indicative of the polymers being well-solvated without any noticeable vibronic features that would arise if there were increased levels of aggregation between the polymer chains.

Fig. 3 UV-vis absorbance spectra of Ph₂DHPP (black), Th₂DHPP (red), and BTD₂DHPP (blue) dissolved in toluene (solid lines) and as thin films (dashed lines).

Table 1 Optoelectronic and thermal properties of the DHPP-containing polymers reported in this manuscript

Polymer	λ ^solnmax (nm)	λ ^filmmax (nm)	E ^oxonset (V)	T d (°C)
Ph2DHPP	430	420	0.85	424
Th2DHPP	515	540	0.70	412
BTD2DHPP	630	725	1.0	318

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Turning to computation for a deeper understanding of the optical properties, frequency-verified optimization followed by excited state generation at the mPW3PBE/SV level was performed and the geometries and excited states of this set of systems were analyzed.^50–52 This level of theory was chosen due to the ability to accurately relate theory to experimental data while avoiding deficiencies of other functionals, such as the localization/delocalization error associated with the B3LYP global hybrid functional.⁵³ Initial analysis of the dihedral angles for each polymer is shown in Fig. 4 where starting from the center (D₃) and then stepping to the peripheral rings shows that Ph₂DPP is the most twisted out-of-plane while Th₂DHPP and BTD₂DHPP are perfectly planar where the two subunits connect. For D₄ and D₂, the planarity increases going from Ph₂DHPP to Th₂DHPP to BTD₂PHDD. It is only when analyzing D₁ and D₅ at the outer edges of the oligomer, which would be analogous to polymer chain ends, that the contortion out-of-plane changes between Th₂DHPP (most planar) and BTD₂DHPP (least planar). Overall, the geometry is consistent with the trends measured by the experimental UV-vis where Ph₂DPP (least planar, λ_max,DFT = 414 nm) is more blue-shifted than Th₂DHPP (λ_max,DFT = 524 nm) and then BTD₂DHPP (most planar, λ_max,DFT = 838 nm).

Fig. 4 The five dihedral angles (D₁–D₅) are shown for each of the 3DHPP oligomers constructed for the modeling and elucidation of structural properties.

Examination of the frontier molecular orbitals (FMOs) enables identifying the type of excited state transition from which the absorption peak maximum arises (Fig. 5). Both Ph₂DHPP and Th₂DHPP show only a slight reorganization of the electron density from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which is indicative of a local excitation. Alternatively, BTD₂DHPP shows a more significant electronic redistribution in which the HOMO electron densities are evenly dispersed along the backbone while the electron density in the LUMO is absent from the DHPP units and localized on the BTD rings. This trend is expected due to the electron-accepting nature of BTD coupled with the electron-donating DHPP that would produce a charge transfer type of excitation through the donor–acceptor approach. This behavior is further supported by the calculated energy gaps and oscillator strengths which were similar for Ph₂DHPP and Th₂DHPP but reduced nearly by half (relative to Ph₂DHPP) for BTD₂DHPP. The much narrower band gap is another indicator of BTD₂DHPP as a donor–acceptor motif and supports the experimental absorbance results reported above.

Fig. 5 The energy band diagrams of the HOMO and LUMO levels for Ph₂DHPP, Th₂DHPP, and BTD₂DHPP are shown with the corresponding frontier molecular orbital diagrams, optical gaps (E_g), and oscillator strengths (f).

Turning to studying the optical properties of these materials in the solid state, each polymer was dissolved in toluene, solution-processed into thin films via spin-coating, and had their absorbance measured. Th₂DHPP and BTD₂DHPP display the characteristic red-shifted absorbance associated with polymers as thin films when compared to dissolved in a solvent (dashed lines in Fig. 3). This red shift is attributed to a larger degree of the π-orbital overlap as well as an increased planarization of the polymer backbone in the solid state compared to solvated in solution. BTD₂DHPP shows the lowest onset of absorbance (λ_onset) at ∼820 nm and a λ_max of ∼725 nm while the Th₂DHPP λ_onset begins at ∼630 nm and the λ_max shifts to 540 nm. Alternatively, Ph₂DHPP unexpectedly showed a blue shift in the absorbance spectrum from 430 nm to 420 nm. Polymers containing DHPP units flanked by phenyl substituents are known to have minimal changes in their optical properties going from solution to thin films, which has been attributed to the larger dihedral angles through the polymer backbone preventing efficient interchain packing, but the blue-shifted absorbance warranted further probing to understand this phenomenon.

Our initial intuition leading to the blue-shifted absorbance was the polymer was “trapped” in an unfavorable morphology, perhaps due to the processing protocols used to produce films of Ph₂DHPP. If this were the case, we suspected that the blue shift would be corrected or reversed by annealing films at elevated temperatures. However, as shown in Fig. S15 (ESI†), after annealing at 100 °C for 1 h, there were no discernable changes in the absorbance maxima. This led us to believe that the blue shift is attributed to an unexpected aggregate phenomenon. To confirm the formation of aggregates, we chose to measure the absorbance of Ph₂DHPP in solution with varying concentrations of a poor solvent (MeOH) and monitor for any changes in the absorbance. Excitingly, our intuition was confirmed that by increasing the amounts of the poor solvent would lead to changes in the absorbance spectra (Fig. 6(A)), evident by measuring a 10 nm blue shift in a 50% (v:v) toluene:MeOH solvent mixture. Qualitative fluorescence observations, shown as a photograph in the inset of Fig. 6, also support the formation of aggregates due to increased quenching of the emission with increasing MeOH caused by aggregation-induced quenching.^54,55 The absorbance spectrum for the polymer aggregated in solution was plotted with the absorbance spectrum of Ph₂DHPP as a thin film (Fig. 6(B)) and the two absorbance profiles are nearly identical. These data indicate that the blue shift measured going from solution to film is attributed to the formation of the same aggregate species formed during solution titration experiments.

Fig. 6 (A) UV-vis absorbance spectra of Ph₂DHPP in toluene solutions with varying fractions of MeOH as a poor solvent. The photograph in the inset shows the emission intensities of the same solutions irradiated with UV light. (B) Overlayed UV-vis absorbance spectra of Ph₂DHPP as a thin film (black) and in a 50% toluene:MeOH solution (red). The near identical spectra are indicative of similar aggregation effects in solutions with the poor solvent and as dried films.

Thermal properties

Given the history of DHPP molecules and polymers finding applicability in organic electronic applications and devices, the thermal stability of Ph₂DHPP, Th₂DHPP, and BTD₂DHPP were studied using thermal gravimetric analysis (TGA). Ph₂DHPP showed the highest thermal stability with a degradation temperature (T_d) of 424 °C, followed by Th₂DHPP (T_d = 412 °C), and finally BTD₂DHPP (T_d = 318 °C). The mass loss at the T_d for each polymer corresponds to the loss of the hexyldecyl sidechains attached to the 1,4-phenyl groups on DHPP (Fig. S16, ESI†). The thermal properties were further probed via differential scanning calorimetry (DSC) to elucidate any thermal transitions (i.e. T_g or T_m). As shown in Fig. S17 (ESI†), Th₂DHPP and BTD₂DHPP do not display any distinct thermal transitions in the experimental window (−25 °C to 250 °C), which is analogous to previous iterations of DHPP-containing copolymers. Alternatively, Ph₂DHPP seems to have a small melting transition just over 100 °C and a corresponding crystallization (T_c) transition at ∼70 °C upon cooling. The observation of this deviation from other DHPP polymer systems is ostensibly due to the aggregation phenomenon described during UV-vis absorbance experiments that lead to the formation of small, but not insignificant, crystalline regions in bulk Ph₂DHPP samples.

Redox and spectroelectrochemical properties

The redox properties of the DHPP polymers as thin films were measured via cyclic voltammetry (CV). The films were created by drop-casting 2 mg mL⁻¹ polymer solutions dissolved in toluene onto a glassy carbon electrode. The CV results reported in Fig. 7 show that Ph₂DHPP has an onset of oxidation (E^ox_onset) ∼0.85 V (vs. Ag/AgCl in a 0.5 M TBAPF₆/acetonitrile (ACN) supporting electrolyte), which is slightly higher than our ProDOT-containing DHPP copolymer due to the removal of the electron-rich dioxythiophene motif. When the Ph units are replaced with flanking thiophenes, the E^ox_onset is lowered to ∼0.7 V. This reduction in potential is attributed to the more electron rich nature, higher degree of planarization, and more quinoidal character of the thiophene unit compared to benzene. When the BTD unit is the flanking moiety, the E^ox_onset is increased to ∼1.0 V vs. Ag/AgCl, which is attributed to the strong acceptor character of the BTD deepening the highest occupied molecular orbital (HOMO) compared to Th and Ph. All three polymers show a corresponding reduction process after oxidation with varying degrees of stability (vide infra).

Fig. 7 CV traces depicting the redox response for Ph₂DHPP (black), Th₂DHPP (red), and BTD₂DHPP (blue) as films deposited onto glassy carbon electrodes. Measurements were performed using a 0.5 M TBAPF₆/ACN supporting electrolyte and the Ag/AgCl reference electrode.

DHPP-based polymers and molecules have shown the ability to serve as active-layer material for multi-colored and high-contrast electrochromics^35,36,39 which necessitates studying the redox stability of the polymers reported in this study. While BTD₂DHPP shows an accompanying reduction for the redox process during the first redox cycle, the films quickly degrade (within 5 cycles) which is not uncommon for D–A and n-type conjugated polymers studied as electrochromes. As shown in Fig. 8(A), Ph₂DHPP also shows depleting redox activity over 10 cycles, although not as drastic as BTD₂DHPP. The decreasing redox activity for Ph₂DHPP is not surprising since our ProDOT-containing polymer showed an approximate 25% decrease in electrochromic contrast with repeated electrochemical switching.³⁶ Additionally, phenylene-based polymers studied as electrochromic polymers are likely susceptible to undesired side reactions upon oxidation⁴⁰ and have shown lower switching stability. Th₂DHPP, on the other hand, shows excellent redox stability with extended electrochemical switching experiments. As shown in Fig. 8(B), Th₂DHPP shows no signs of diminishing redox activity after 400 CV cycles and the only changes in the CV profiles is a result of “electrochemical annealing”,^56,57 where the shape of CV traces will change over time due to the flux of the solvent and ions into and through the film. Overall, BTD₂DHPP and Ph₂DHPP may be suited for redox applications where a single switch or a small number of switches is needed while Th₂DHPP represents a design motif that has overcome stability issues of DHPP-containing polymers used in redox-active applications.

Fig. 8 Redox response of (A) Ph₂DHPP and (B) Th₂DHPP upon repeated electrochemical sweeping cycles. Measurements were performed using a 0.5 M TBAPF₆/ACN supporting electrolyte and an Ag/AgCl reference electrode.

Changes in the absorbance with increasing electrochemical potential was used to gain insights into the charge-carrier formation of each polymer. While donor–acceptor type polymers have exhibited spectral changes with changing electrochemical potential, BTD₂DHPP did not show a spectroelectrochemical response upon oxidation⁵⁸ or reduction^59,60 but did show some spectral features that are worth noting. First, attempts at electrochemical conditioning led to diminished absorbance (∼50%) if swept in the cathodic (oxidation first) direction. However, as shown in Fig. S18 (ESI†), the absorbance was retained when sweeping towards the negative potentials (reduction) first. Scanning in the reduction direction first also led to increased cycling stability of the polymer during CV experiments (Fig. S19, ESI†). However, without the accompanying reduction sweep, the electrochemical response once again rapidly diminished during attempts at oxidizing and measuring the spectroelectrochemical response (Fig. S20, ESI†).

Turning to the phenyl- and thienyl-containing polymers, potential-dependent absorbance spectra are presented in Fig. 9. The polymer Ph₂DHPP shows a noticeable decrease in the absorbance at ∼0.8 V that corresponds to the onset of oxidation measured via CV. Similar to our first generation DHPP-containing copolymer, Ph₂DHPP retains significant absorbance across the visible spectrum upon oxidation but the absorbance features in the near-IR are less intense. Finally, the electrochemical instability observed in CV experiments is confirmed spectroscopically by measuring a blue-shifted and decreased absorbance of a Ph₂DHPP film after attempting to reduce the polymer back to the neutral state. Alternatively, the Th₂DHPP polymer demonstrated signs of efficient electrochemical doping. As shown in Fig. 9(b), a neutral Th₂DHPP film transitions from absorbing ∼550 nm to the IR with increasing electrochemical potential. While the optical contrasts of these films do not match the contrasts of dioxythiophene-type polymers, Th₂DHPP shows a full transition from a neutral film to the doped state similar to P3HT (Fig. S21, ESI†). These results suggest that Th₂DHPP possesses semiconducting properties based on the ability to transition from an insulating neutral polymer to a conductive polymer in its doped form.

Fig. 9 Absorbance spectra as a function of electrochemical potential for (A) Ph₂DHPP and (B) Th₂DHPP films that were processed via spray-casting from 5 mg mL⁻¹ toluene solutions by applying varying potentials in a 0.5 M TBAPF₆/ACN electrolyte.

To develop a deeper understanding of the optical transitions for electrochemically doped DHPP polymers, DFT calculations at the mPW3PBE/SV level followed by time-dependent (TDDFT) treatment were performed for the radical cation forms of Ph₂DHPP and Th₂DHPP (BTD₂DHPP does not switch and as such was not included). To this end, the type of excitation may be identified through a study of electron distribution changes between the molecular orbitals involved in the excitation transition. A local excitation presents as small changes in the electron distribution with mostly delocalized molecular orbitals. On the other hand, a charge transfer excitation will display redistribution of the electron density from one region of the molecule to another. Fig. 10 shows the FMOs for the two systems as well as their corresponding transition levels and excited state oscillator strengths. In the case of Ph₂DHPP, the strongest transition resulted from the SOMO_α to the LUMO_α with an oscillator strength of 1.0600 and a peak maximum of 528 nm. There is a small redistribution of the electron density from the periphery to the center which is indicative of some charge transfer character for this SOMO to the LUMO transition. For Th₂DHPP, the SOMO_α to the LUMO_α was red-shifted by 76 nm (528 nm versus 604 nm, Fig. S25 and S29, ESI†) and possessed a nearly doubled oscillator strength (1.0600 versus 2.0932) compared to Ph₂DHPP but the FMO pattern was similar to that of Ph₂DHPP and indicative of the charge transfer character. Notably, the calculations agree with the trends observed from spectroelectrochemical measurements where Th₂DHPP absorbs more strongly (i.e. larger oscillator strength) than Ph₂DHPP and transitions to longer wavelengths upon electrochemical doping.

Fig. 10 The lowest lying and most significant excited states are shown where the identity and energy of the original level and final level are provided as well as the corresponding FMO for each. The oscillator strengths (f) and arrow are provided for each of the individual excited states for clarity.

Conclusions

Pyrrolo[3,2-b]pyrroles are useful scaffolds that can be exploited for creating π-conjugated materials with tailorable optoelectronic properties from simple synthetic protocols. Their propensity to participate in Pd-catalyzed polymerizations enables these attributes to be expanded to polymeric materials and study the resulting structure–property relationships. As a way to expand the monomer toolbox, three DHPP monomers – m-Ph₂DHPP, m-Th₂DHPP, and m-BTD₂DHPP – were synthesized via aerobic Fe-catalyzed reactions between functionalized anilines and the corresponding aldehydes in the presence of 2,3-butanedione and purified without using column chromatography. The monomers were polymerized via Suzuki cross-coupling polymerizations to yield a novel class of synthetically simple conjugated polymers with tunable properties. The absorbance spectra of Th- and BTD-containing polymers are red-shifted compared to that of the Ph-containing polymer due to the increased planarity and ICT character, respectively. The choice of the peripheral aromatic unit also has drastic effects on the electrochemical properties of the resulting polymers, where the Th-containing polymer demonstrates superior redox stability and the ability to switch between doped and dedoped states. DFT calculations and analysis of FMOs support experimental measurements and provide insight into structural influences on the optical properties of neutral and oxidized materials. Furthermore, thermal analyses show that the DHPP polymers are thermally stable and amenable to thermal treatment techniques commonly used for organic electronic applications. The large number of commercially available aldehyde building blocks and synthetic transformations used to access aromatic aldehydes suggests that DHPP-based polymers may be further functionalized for desired applications. Overall, these findings reinforce the utility of DHPPs as a highly tailorable building block for conjugated materials and suggests these materials may find applicability in high-performance organic electronic devices.

Experimental and computational methodology

Comprehensive details of the experimental approaches are assembled and reported in the ESI.† The structure and connectivity of the synthesized molecules and polymers were confirmed via¹H and ¹³C NMR. Full characterization including, NMR, elemental analysis, and size-exclusion chromatography (SEC) data can be found in the ESI.† To prepare spray-coated films, polymers were dissolved in toluene with a concentration of 5 mg mL⁻¹ before deposition onto ITO-coated glass slides with an Iwata airbrush. Electrochemical and spectroelectrochemical measurements performed on the films used an electrolyte solution of 0.5 M tetrabutylammonium hexafluorophosphate (TBAPF₆) in propylene carbonate (PC), ITO/glass (7 × 50 × 0.7 mm, sheet resistance, R_s 8–12 Ω sq⁻¹) as the working electrode, an Ag/AgCl reference electrode (calibrated versus the Fc/Fc⁺ redox couple, E_1/2 = 46 mV), and a Pt flag as the counter electrode.

All systems were geometry optimized and frequency verified at the mPW3PBE level in the gas phase. The excited states were generated through time-dependent density functional theory (TDDFT) treatment for the lowest lying 15 excited states. The Cartesian coordinates of the optimized geometries, energy levels, a table of the lowest lying excited states with energy, oscillator strength, the most significant transition and the percent this transition contributes to the excited state, geometric images and simulated UV-vis spectra are provided for each system in the ESI.†

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors have no conflicts to declare.

Acknowledgements

This material is based upon the work supported by the National Science Foundation under grant no. 2448404. The authors acknowledge funding from the Department of Chemistry and Biochemistry at Kennesaw State University and the University of Southern Mississippi for start-up funds and the Kennesaw State University Academic Affairs for support of the NMR facility, which made possible the research necessary for the completion of this project. The computations in this work used the Expanse Cluster at the San Diego Supercomputer Center through allocation DMR160146 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by the U.S. National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296.

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthetic protocols, materials and methods, ¹H and ¹³C NMR spectra, and supporting spectroscopic, electrochemical, and electronic data. See DOI: https://doi.org/10.1039/d5tc00292c

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