# Synthesis of Electron-Deficient BisAzaCoroneneDiimide-Conjugated Polymers by Light-Locking Dynamic Covalent Bonds

Adèle Gapin, Elarbi Chatir, Clara Pasgrimaud, Arthur David, Anaïs de Maria, Mélanie Legros, Laura Le Bras, Olivier Alévêque, Eric Levillain, Antoine Goujon

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Adèle Gapin, Elarbi Chatir, Clara Pasgrimaud, Arthur David, Anaïs de Maria, et al.. Synthesis of Electron-Deficient BisAzaCoroneneDiimide-Conjugated Polymers by Light-Locking Dynamic Covalent Bonds. Journal of the American Chemical Society, 2025, 10.1021/jacs.5c01351. [PDF] [ DOI ]

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# Synthesis of Electron-Deficient BisAzaCoroneneDiimide Conjugated Polymers by Light-Locking Dynamic Covalent Bonds

Adèle Gapin,<sup>a†</sup> Elarbi Chatir,<sup>a†</sup> Olivier Alévêque,<sup>a</sup> Clara Pasgrimaud,<sup>a</sup> Arthur H.G. David,<sup>a</sup> Anaïs De Maria,<sup>b</sup> Mélanie Legros,<sup>b</sup> Laura Le Bras,<sup>*</c</sup> Eric Levillain,<sup>*</a</sup> and Antoine Goujon<sup>*</a</sup>

<sup>a</sup>Univ Angers, CNRS, MOLTECH-Anjou, SFR MATRIX, F-49000 Angers, France. <sup>b</sup>Université de Strasbourg, CNRS, Institut Charles Sadron, F 67000, Strasbourg, France. <sup>c</sup>Université Marie et Louis Pasteur, CNRS, Chrono-environnement (UMR 6249), F-25000.

<sup>†</sup>These authors contributed equally to this work.

KEYWORDS: Conjugated Polymers, rylene diimides, electrochemistry, organic materials, photochemistry.

**ABSTRACT:** We present a novel light-locked dynamic covalent polymerization methodology to synthesize conjugated polymers based on BisAzaCoroneneDiimides (BACDs). This metal-free process converts reversible poly-imines into kinetically locked conjugated polymers using visible light, generating minimal side-products. By incorporating aldehyde-functionalized comonomers, the approach enables the creation of diverse n-type semiconducting polymers with tunable optical band gaps and low LUMO levels. The polymers exhibit exceptional thermal, electrochemical, and photostability, with strong interchain interactions upon electrochemical reduction observed in solution, attributed to the BACD core. Broad absorption from the visible to near-infrared range underscores their potential in charge and energy transport applications for organic electronics. This scalable, sustainable strategy unlocks access to a versatile class of n-type diimide polymers.

## INTRODUCTION

Organic semiconducting conjugated polymers are ubiquitous in organic electronics.¹ Present in cutting-edge devices like organic photovoltaics,²,³ organic light-emitting diodes,⁴,⁵ and organic field-effect transistors,⁶,⁷ they also hold great promise for high performance organic batteries,⁸,⁹ thermoelectric generators,¹⁰,¹¹ photodetectors,¹²,¹³ and organic electrochemical transistors.¹⁴ Their development is essential in driving the next generation of low-energy, low-carbon-emission technologies. What sets these polymers apart from their small organic semiconducting molecules counterparts is their advantageous solution-processability for large-scale device fabrication, superior chemical and photostability, and enhanced film morphology well-suited for flexible electronics.¹⁵⁻¹⁷ The design and synthesis of conjugated polymers have rapidly evolved, and they increased in complexity, diversity and scope of applications.¹⁸ Currently, most high-performance conjugated polymers are synthesized through palladium-catalyzed cross-coupling reactions, particularly using the Stille polycondensation, which involve halogenated derivatives with stannylated reagents.¹⁹,²⁰ This methodology has afforded numerous state-of-the-art materials now routinely used in research laboratories to push the boundaries of organic electronic devices performances. Despite its efficacy and versatility, this reaction presents significant challenges for large-scale production.²¹ Issues like batch-to-batch variability, problematic

by-products, and the costly and environmentally taxing removal of residual palladium and toxic²² stannylated compounds hinder its practicality for industrial applications and the mass-production of reproducible and stable devices. Moreover, hard-to-remove residual ligand or metallic impurities lower the material performances and stability over time.²³⁻²⁵ Many precautions and parameters need to be taken into account to obtain a high quality material.²⁶ Recent advances in C-H arylation reactions have offered elegant and promising alternatives, eliminating the need for toxic stannylated reagents.¹⁸,²⁷,²⁸ That being said, these reactions still suffers from similar pitfalls regarding side-reactions. Other promising methods, such as Aldol condensation, have successfully produced low-bandgap, high-performance semiconducting polymers presenting low LUMO levels with minimal waste.²⁹ However, this reaction is limited by the functional groups required for polymerization, which are less versatile compared to halogens or organotins and harder to install on every aromatic building block of interest. To overcome these limitations, innovative approaches are needed, which consist of: strategies that enable precise polymer structures with minimal waste and purification are particularly attracting.¹,³⁰⁻³² Such novel polymerization methodologies have been pursued with success. A family of material answering some of these requirements is dynamic covalent imine-based conjugated polymers.³³,³⁴ These
Light-Locked Dynamic Covalent Synthesis of n-type Conjugated Polymers

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Two-steps synthesis of BisAzaCoroneneDiimides (BACDs) conjugated polymers. The first step is the reversible formation of polyimines. The second step is an irreversible visible light triggered oxidative photocyclization, which locks the imines into a fused system and “freezes” or “lock” the system into a kinetically stable conjugated polymer.</figcaption></figure>

structures are formed via the condensation of amine and aldehyde functionalized building blocks, yielding water as the only by-product. While imines are attractive due to their ease of synthesis, their reversible nature and the lower conjugation in the resulting polymer have so far limited their application in organic electronics.35 However, imines now shine as key functional groups in the preparation of recyclable or transient electronics, especially in bio-sensing devices, where their progressive degradation by hydrolysis is a desired feature.36–40 In this work, we report a novel approach to bridge the gap between traditional stable conjugated polymers and dynamic imine-based materials. We exploit the light-driven azabenzannulation41 of perylene-diimides (PDIs) into BisAzaCoroneneDiimides42 (BACDs) discovered in our group to develop a so-called “light-frozen” dynamic covalent synthesis (Figure 1), combining the use of dynamic building blocks with the stability of kinetically inert materials. We produce conjugated polymers in two steps: a dynamic poly-imine formation followed by irreversible oxidative photocyclizations under white light irradiation. This method enables the creation of a new class of electron-accepting polycyclic aromatic hydrocarbon (PAH) diimide-based polymers.43,44 If other strategies exist to freeze or lock the imine bond, most of them lead to a decrease or disappearance of electronic communications.45,46 Here, a new conjugated C-C bond is formed. Not only diimide-based materials are promising electron-acceptors,47–49 but their development is also crucial in addressing the current limitations of n-type semiconducting polymers compared to their more advanced p-type counterparts.50–53 BACD-based polymers are prepared without the use of toxic or precious metals and homo-coupling reactions are not

achievable in these conditions, greatly limiting the introduction of defects. $(NH_2)_2PDIs$ can be reacted with any optoelectronically active ditopic aldehyde of choice, which ensure a wide diversity of accessible materials knowing that formylated conjugated building blocks are easily accessible synthetically and commercially. We explored and optimized their synthesis and investigated their promising optoelectronic properties. BACD-based polymers can be tuned to strongly absorb light across the visible spectrum and display low LUMO energy. Remarkably, they demonstrate strong inter-chains interactions upon electrochemical reduction in solutions, which is highly promising for their future applications as active thin films in organic electronic devices.

RESULTS AND DISCUSSION

First, the thermodynamic reversibility of imine-linked PDI polymers was tested by reacting $NH_2-PDI$ with 2-thiophenecarboxaldehyde in the presence of trifluoroacetic acid (TFA) in the dark to yield the corresponding imine (See Supplementary Information). After isolation, the product was subjected to the same reaction conditions with *p*-anisaldehyde, resulting in imine exchange and the coexistence of both imines (Scheme S1, Figure S1 and S2). This demonstrates that imine-linked PDIs behave as dynamic covalent materials. Subsequently, the two-steps/one-pot polymerization of 1,6/7$(NH_2)_2PDIs$ 1 and 2,5-thiophenedicarboxaldehyde 2 was developed to form a BACD-thiophene polymer P(BACD-T) (Figure 2a, Method A). $(NH_2)_2PDIs$ were used as a 1:1 mixture or regioisomers, which is represented as drawn on Figure 1. For the sake of clarity, only the 1,7 linkages will be represented in polymeric materials. See the

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Synthesis of BACD polymers via Method A. Only the 1,7 linkage is represented in polymers for the sake of clarity</figcaption></figure>
Synthetic Note in the SI for more details. The optimal polymerization conditions were determined to be 150°C in toluene at 50 mM under microwave irradiation (MW), using TFA as a catalyst. A slight excess of **1** was used to form amine-terminated imine-linked PDI polymers. This solution was then exposed to visible light and subjected to oxidative rearomatization by the addition of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The amine-terminated material was further reacted with an excess of 2-thiophenecarboxaldehyde **3** as an end-capper, exposed to visible light, and oxidized again. The photocyclization steps of this sequence were conducted in a continuous flow photoreactor. Prior to its applications to polymer, this reaction demonstrated to be a highly selective, clean and efficient method for the preparation of AzaBenzannulated PDIs.<sup>41,54</sup> We performed an optimization of the photocyclization on model dimer **4** obtained from the condensation of 2 equivalent of the diamine **1** on 1 equivalent of the ditopic aldehyde **3**, to determine the ideal flow-rate and concentration for this reaction (**Scheme S2**, **Figure S3**). Remarkably, the monitoring of the photocyclization by MALDI-TOF mass spectrometry revealed here again an efficient and clean process, with no decomposition or side reactions observed after exposure to strong white light. This importantly highlights the high photostability of the studied materials. Several polymerization attempts were performed at initial monomer concentrations of 6, 10, 30, 50 and 86 mM. The polymers were purified by precipitation and Soxhlet extraction and analyzed by size-exclusion chromatography (SEC) to determine their $\overline{M}_n$, $\overline{M}_w$, $\overline{D}\overline{P}_n$, and dispersity index $\mathcal{D}$ (see Supplementary Information). Chromatograms revealed multiple populations for polymerization concentrations of 30 mM and below, but cleaner elution peaks were obtained above this threshold, leading to the selection of 50 mM as the optimal concentration for further studies. **P(BACD-T)** polymers produced by Method A were obtained in high yields and displayed increasing $\overline{M}_n$, $\overline{M}_w$, $\overline{D}\overline{P}_n$ with higher monomer concentrations, reaching $\overline{M}_n$ values from 12.9 to 40.0 kDa, corresponding to $\overline{D}\overline{P}_n$ of 12 to 37 repeating units (**Table S1**). A near-linear relationship was observed between initial monomer

concentration and $\overline{M}_n$. The dispersity $\mathcal{D}$ increased with $\overline{M}_n$ but remained at a reasonable 1.8 for the 50 mM concentration. This method showed excellent reproducibility, with repeated polymerizations of **P(BACD-T)** at 50 mM yielding minimal deviation: $\overline{M}_n = 22.6 \pm 2.5$ kDa, $\overline{M}_w = 39.7 \pm 6.1$ kDa, $\overline{D}\overline{P}_n = 21 \pm 2$, and $\mathcal{D} = 1.8 \pm 0.1$. We attribute this consistent result to the first step involving dynamic covalent bonds, ensuring that, for a given set of reaction parameters, similar poly-imine populations are formed at thermodynamic equilibrium before being "frozen" by visible light. Additional insights into the polymer structure were revealed through infrared (IR) spectroscopy (See Supplementary Information, **Figure S13 to S15**). Vibrational spectra of amine **1**, aldehyde **2**, and **P(BACD-T)** polymer synthesized at 50 mM and a model monomeric compound, **1,6/7-BACD-T₂** (synthesis in **Scheme S18**), were compared. In the high-wavenumber region, **1** exhibited broad peaks between 3500 and 3200 cm⁻¹, attributed to N-H bond vibrations. These IR absorptions were absent in the model compound **1,6/7-BACD-T₂** and in **P(BACD-T)**, indicating the successful transformation of NH₂ groups. In the carbonyl region, the spectra of **1,6/7-BACD-T₂** and the **P(BACD-T)** polymer displayed similar vibrational profiles, without the aldehyde signature seen in the aldehyde monomer **3**. The spectra of both materials are almost identical between 1750 and 1650 cm⁻¹. By comparison, we can attribute the band around 1710 cm⁻¹ to C=O bonds and the one around 1665 cm⁻¹ to C=N bonds (**Figure S15**). It supported the successful polymerization with no detectable amount of amine- or aldehyde-terminated chains. Based on our previous work on the photoazabenzannulation reaction of PDIs<sup>41,42</sup> and the results presented herein, we can state that this process produces well-defined BACD-based conjugated polymers, with no evidence of homocoupling, undesired terminations, or any residual reactive chain ends, an attractive combination of advantages for the synthesis of new n-type conjugated polymers. With this new methodology in hand, we prepared a range of five additional polymers from more complex aldehyde monomers: **P(BACD-C)** from 9-(2-Ethylhexyl)carbazole-3,6-dicarboxaldehyde, **P(BACD-F)** from 9,9-Dioctyl-

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Synthesis of BACD polymers P(BACD-CHO) and terpolymers TP(BACD-CHO). Method A: (i) 0.9 equiv. bis-aldehyde, TFA cat., toluene, 150 °C, MW; (ii) 10 equiv. **2**, TFA cat. CHCl₃, 80°C, MW. Method B: (i) 0.94 equiv. bis-aldehyde, TFA cat., chlorobenzene, 170 °C, MW; (ii) 10 equiv. **3**, TFA cat. CHCl₃, 80°C, MW. Method C, the "terpolymer approach", is identical to Method A but starts from a dimer **4** as co-monomer. Dimer **4** exists as a mixture of 3 regioisomers. Only one is represented for the sake of clarity.</figcaption></figure>
9H-fluorene-2,7-dicarbaldehyde, P(BACD-IDTTB6) by reacting with IDTTB6-dicarboxaldehyde, P(BACD-TII) by reacting with a dithienoisoindigodicarboxaldehyde monomer, and P(BACD-DPP) through polymerization with a dithienodiketopyrrolopyrroledicarboxaldehyde monomer. (Figure 3a, Scheme S2 to S10). All polymers were first synthesized using Method A, with results summarized in Table S2/Figure 4a. The polymerization of P(BACD-C) resulted in moderate yields, and P(BACD-IDTTB6) and P(BACD-DPP) were obtained as a mixture of oligomers. P(BACD-TII) displayed a single peak in SEC with a moderate M~n~ of 10.9 kDa. In contrast, P(BACD-F) was obtained in good yield with a M~n~ of 38.6 kDa. To improve these results, we developed Method B, using chlorobenzene as a stronger solubilizing solvent for polycyclic aromatic derivatives, to overcome the eventual solubility limitations of poly-imes intermediates. We also adjusted the monomer ratio slightly to 1/0.94 to aim for longer polymer chains. Method B resulted in improvements across all materials (Figure 4b, Figure S5 to S7), with some results showing significant enhancements. P(BACD-F) afforded an insoluble material probably because of a high M~n~, P(BACD-C) improved its molar mass and the M~n~ of P(BACD-TII) improved from 10.9 to a good value of 29.6 kDa. However, both P(BACD-IDTTB6) and P(BACD-DPP) remained highly polydisperse materials showing multiple peaks when analyzed by SEC and showed small M~n~ < 10 kDa. We envisioned a new approach: Method C, a terpolymerization strategy, designed to address the two primary hypothetical limitations: a possible poor solubility of growing polymer chains and the possible low nucleophilicity of the diamine monomer 1. First, 2,5-thiophenedicarboxaldehyde was reacted with a small excess of 1 to form the dimer 4 (see Supplementary Information and Scheme S11). This highly soluble monomer, slightly electronically enriched by the thiophene core, was then polymerized with the previous aldehyde monomers using the reaction conditions of Method A (Scheme S12 to S19, Figure 3b and Figure 4c).

<figure><img src="image_5.png" /><figcaption>**Figure 4.** M~n~ and M~w~ of BACD polymers prepared via a) Method A, b) Method B and c) Method C (terpolymers). d) Visualization of the evolution of M~w~ for Method A/B/C, for a selection of monomers.</figcaption></figure>

Remarkably, the SEC analysis (Figure S8 to S12) of TP(BACD-T-TII) revealed a significantly improved M~n~ of 45.3 kDa. Similarly, SEC analysis of TP(BACD-T-DPP) showed clean elution peak and a notable increase in both M~n~/M~w~, confirming the success of this approach (Figure 4d). The SEC data of TP(BACD-T-IDTTB6) was much more satisfying even if a shouldering on the main elution peak remained. For all terpolymers, vibrational spectra were recorded and demonstrated no significant traces of remaining amine, and similar signatures in the C=O and C=N bonds region compared to the starting dimer 4 (Figure S16) and model monomer 1,6/7-BACD-T~2~, supporting an efficient end-caping and photocyclization/aromatization process (Figure S17 to S21). This demonstrates that depending on the monomer's nature and reactivity, the polymerization strategy can be adapted to reach the desired materials. The more difficult polymerization of the IDTTB6 fragment is attributed to its strong electron donor nature rendering the aldehydes poorly electrophilic. Although polymers produced by Method A/B and Method C are not strictly identical, the flexibility of both methods potentially enables the synthesis of a wide and diverse collection of new electron-deficient conjugated polymers by simply combining dianines 1 or 4 with any optoelectronically active bis-aldehyde monomers. The success of Method C, the terpolymer approach, opens the door to the easy synthesis of more complex terpolymers combining complementary redox-active chromophores in structure exhibiting a well-defined sequence. The ¹H NMR of the polymers performed in C~2~D~2~Cl~4~ showed only barely visible broad peaks, probably owing to strong intermolecular interactions in solution (Figure S64 to S76).⁵⁵ Thermogravimetric analyses (TGA) were performed to probe the thermal stability of these BACD polymers. High degradation temperatures are crucial to ensure

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Normalized absorption and spin-coated thin-films (full lines) of a) P(BACD-T), b) P(BACD-TII), c) TP(BACD-T-TII), d) TP(BACD-T-IDTTB6) and e) TP(BACD-T-DPP). f) Absorption of a 1 × 10⁻⁵ M solution of TP(BACD-T-DPP) in toluene before and after 120 minutes of irradiation with a 18W white lamp, showing no variation.</figcaption></figure>
a good resilience to various device processing and fabrication methods. All polymers displayed excellent thermal stability with no degradation observed before 400 °C (Figure S57 to S65). Differential scanning calorimetry (DSC) revealed a similar exothermic peak around 280-290°C for all polymers except P(BACD-T-F) which showed an endothermic signal at 81 °C (Figure S90). According to the TGA data, the high-temperature signals are not associated with the decomposition of the polymer and could be interpreted as phase transitions of crystallizations processes. For P(BACD-T-F), the endothermic peak could be linked to the melting temperature of the polymer. The optoelectronic properties of the new polymers were investigated by absorption spectroscopy in solution (CHCl₃) and thin film, and Cyclic Voltammetry (CV). For the sake of clarity and space, we will limit the discussion here to the polymers that were obtained with a satisfying M̄n, but the complete optoelectronic characterizations of all polymers can be found in the Supplementary Information. P(BACD-T), P(BACD-F) and P(BACD-C) strongly absorbs light in the UV range (Figure 22 to S39), with maximums at 412, 511 and 403 nm respectively with high extinction coefficient of ε = 3.0, 1.4 and 3.8 × 10⁻⁴ M⁻¹. cm⁻¹ (Figure 5). They absorb visible light up to 550 nm in solution. P(BACD-T) can be directly compared to its monomeric analogue 1,6/7-BACD-T₂. The monomeric material showed a maximum around 373 nm spectra in CH₂Cl₂, and an overall absorption reaching only 530 nm, thus the polymerization of the motif leads to a significant red-shift. In films, red-shifts are observed, resulting from aggregation (Figure 23, 25, 27, 29, 31, 33, 35, 37 and 39). In the light of the DSC data, the films were thermally annealed at 300 °C for 5 min. No significant changes were observed in absorption after this process (Figure 40 and 41). This could be explained by the fact that we used 1 as its regioisomeric mixture of 1,6 and 1,7 isomers, which results in regio-irregular polymers thus probably poorly crystalline in thin films. P(BACD-TII) shows a broad continuous absorption of UV and visible light up to 700 nm, with a maximum strongly absorbing with ε = 3.8 × 10⁻⁴ M⁻¹. cm⁻¹. In films, a significant shoulder appears between 500 and 700 nm, reinforcing its absorption coverage tailing up to 800 nm. TP(BACD-T-TII) revealed a similar spectrum with a stronger shouldering around 450 nm (ε = 5.1 × 10⁻⁴ M⁻¹. cm⁻¹), reinforced and red shifted in thin-films. These two polymers display a quasi-panchromatic absorption of visible light up to the NIR. P(BACD-IDTTB6) also strongly absorbs visible light with a maximum around 475 nm and tailing up to 700 nm in thin films.

<table><caption>**Table 1.** Optoelectronic properties of a selection of BACD polymers.</caption><thead><tr><th>Polymer</th><th>ε<sup>a</sup></th><th>E<sub>g</sub><sup>b</sup></th><th>HOMO<sup>c</sup></th><th>LUMO<sup>d</sup></th></tr></thead><tbody><tr><td>P(BACD-T)</td><td>3.0</td><td>1.87</td><td>-5.69</td><td>-3.82</td></tr><tr><td>P(BACD-TII)</td><td>3.8</td><td>1.69</td><td>-5.44</td><td>-3.75</td></tr><tr><td>TP(BACD-T-TII)</td><td>5.1</td><td>1.79</td><td>-5.56</td><td>-3.77</td></tr><tr><td>TP(BACD-T-IDTTB6)</td><td>10.4</td><td>1.76</td><td>-5.59</td><td>-3.83</td></tr><tr><td>TP(BACD-T-DPP)</td><td>3.1</td><td>1.57</td><td>-5.19</td><td>-3.72</td></tr></tbody></table>

TP(BACD-T-DPP) shows a maximum at 410 nm and a broad peak at larger wavelength with two maxima at 660 and 706 nm. Its spectrum is red-shifted in film with an increase of absorption of the low-energy bands, allowing it to efficiently absorb all visible light up to 800 nm. A crucial photophysical properties of such materials is their photostability, which ensure that they will deliver long lasting and reproducible performances in devices. Toluene solutions of polymers P(BACD-T), P(BACD-C), P(BACD-F), P(BACD-TII), TP(BACD-T-TII), TP(BACD-T-DPP) and TP(BACD-T-IDTTB6) at 10⁻⁵ M were exposed to continuous irradiation from a 18W white LEDs lamp in presence of oxygen. After two hours, the absorption spectra of the solution remained unchanged, demonstrating an outstanding photostability (Figure 5d and Figure S52 to S56). Moreover, this experiment provides additional proof that all imine linkages were properly photocyclized. All polymers were studied by CV (Figure S42 to S51) to estimate their frontier orbital energy. All the polymers displayed excellent electrochemical stability. Combined with the spectroscopy measurements previously detailed, HOMO and LUMO level energies could be estimated. All BACD-based conjugated polymers are good electron acceptors with LUMO energies comprised between -3.72 and -3.96 eV (Table S3). With their low LUMO energies combined with broad and strong visible-light absorption, P(BACD-TII), TP(BACD-T-TII), TP(BACD-T-DPP) and TP(BACD-T-IDTTB6) have the potential to be potent polymeric non-fullerene electron acceptors for organic electronics devices such as organic solar cells or photodetectors for examples. The combination of BACDs with strong electron donors such as IDTTB6 does not seem to lead to a strong intermolecular charge-transfer significantly improving the visible-light absorption coverage of the final material. The most compelling results were obtained when combining the BACD core with electron-deficient fragments such as DPP and TII, leading to a wide absorption clearly different from the simple addition of the absorption spectrum of both subcomponents. Combining the BACD scaffold with electron acceptors along the polymer chains thus seems to be a promising strategy for the synthesis of future materials. To understand better their electronic structure and establish structure/properties relationships for this new family of materials, TD-DFT calculations were performed on dimeric fragments of polymers (P(BACD-T), P(BACD-TII) and terpolymers (TP(BACD-T-TII), TP(BACD-T-IDTTB6), TP(BACD-T-DPP)). Computational details can be found in Supplementary Information. The calculated absorption properties of the polymers and the terpolymers are gathered in Table S4 and S5. All the 5 molecules display strong absorption peaks in the visible spectrum. For the terpolymers, λ<sub>calc</sub> lies between 662 and 818 nm for TP(BACD-T-TII) and TP(BACD-T-DPP) respectively. The first main absorption peak always arises from a S₀→S₁ transition and is described by HOMO-LUMO orbitals, except for TP(BACD-T-IDTTB6) with a HOMO-LUMO+1 transition.

<footnote>aMolar extinction coefficient of the most absorbing band (10⁻⁴ M⁻¹. cm⁻¹).</footnote>

<footnote>bOptical band gap in eV determined from the onset of the absorption spectrum recorded in thin films. cHOMO and dLUMO energies in eV.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Representation of the HOMO, LUMO and LUMO+3 of P(BACD-T).</figcaption></figure>

All these transitions possess a π-π* character and are delocalized through the polymer chain (see **Figures S68** to **S72**). All HOMO orbitals are centred on the aldehyde co-monomer except for **P(BACD-T)** where the electronic density is delocalized throughout the entire molecule (**Figure 6**). Even if all the polymers are conjugated systems, **P(BACD-T)** is the most planar one and it can explain the more pronounced delocalization of the electronic density over both the BACD unit and the co-monomers. For LUMO orbitals, the electronic density is mainly centred on the BACD units for the two polymers and the three terpolymers. The main absorption peak always corresponds to a HOMO/LUMO+3 transition with LUMO+3 orbitals being localized either on the entire polymer **P(BACD-T)**, on the co-monomer **P(BACD-TII)** or on some BACD and co-monomer units in the terpolymers). A charge transfer (CT) character can be attributed to each transition, as illustrated through the calculation of the amount of transferred charge (**Table S3**). Due to the symmetry of the polymers, values for the charge transfer distance, *d*<sub>CT</sub>, and charge transfer dipole moment, ||**μ**<sub>CT</sub>|| are very small. For terpolymers, the loss of the symmetry leads to an increase of all the CT parameters, with a transfer from either TII, IDTTB6 or DPP towards BACD unit or even BACD-T unit (for **TP(BACD-T-TII)**). By introducing either electron donor or electron acceptor co-monomers within the BACD-based polymer chain, the absorption behaviour of the polymer can be effectively modified. **P(BACD-TII)** displays the highest global charge transfer character among the polymers and has the highest *f*(*λ*<sub>calc</sub>)/*f*(*λ*<sub>max</sub>) ratio with a value equal to 0.95. It highlights its ability to absorb effectively across a large area of the visible spectrum. Terpolymers show some attenuation of the absorption process at higher wavelength. Indeed, the *f*(*λ*<sub>calc</sub>)/*f*(*λ*<sub>max</sub>) ratio is equal to 0.69, 0.24, and 0.20 for **TP(BACD-T-TII)**, **TP(BACD-T-DPP)**, and **TP(BACD-T-IDTTB6)**, respectively. Introducing various aldehyde monomers and combining them is thus an effective way to modulate the absorption properties of the resulting polymer or terpolymer, and the combination of BACDs and electron-deficient co-monomers lead to low-LUMO materials absorbing light across the whole visible range up to the NIR. Most

rylene-based polymers and dyes typically undergo the formation of mono-anionic and bis-anionic species centered on BACD motifs during CV experiments when scanned toward negative potentials. Recently, we reported that *ortho-pentannulated BACDs*, large π-extended analogues of BACDs, exhibit an unusual electrochemical behaviour. In CV experiments, the first reduction event is split into two distinct waves. We demonstrated that these derivatives form strong intermolecular mixed-valence (MV) species through supramolecular dimerization of a radical anion and a neutral species, followed by π-dimers (π-DIM) formed *via* dimerization of two radical anions.⁵⁵ These two electrochemical processes explain the splitting of the first reduction wave. Motivated by these findings, we investigated whether similar phenomena occur in BACD-based polymers. Detailed electrochemical studies were conducted on the polymer **P(BACD-T)**. CV experiments (0.5 mM, 0.1 NBu₄PF₆ in CH₂Cl₂) revealed two distinct reduction events: a first reduction split into two peaks and a second, regular reduction (**Figure 7a**). Additionally, a scan-rate-dependent event emerged around -1.2 mV, which will be addressed later in the full interpretation of the CVs. The splitting of the first reduction wave, which was concentration-dependent, indicates intermolecular interactions. This can be attributed to the rapid formation of strong inter-chain MV complexes through interactions between reduced and neutral BACD fragments during partial reduction. As the potential decreases further, all BACD units along the polymer chain are reduced to their anionic forms, enabling the formation of π-DIM complexes between anionic diimides along the polymer chains. These processes were further elucidated using real-time spectroelectrochemistry (SEChem) under thin-layer cyclic voltammetry (TLCV) conditions (10 mV/s, **Figure 7f**). Under blocked-diffusion conditions, the absorption spectra of the solution were recorded in real time. Three distinct species were observed, as expected (**Figures 7b, 7c**), and isolated via principal component (PC) analysis, which extracted three unique optical signatures (**Figures 7d, 7e**). PC1 displayed a depletion of absorption around 400 nm, strong absorption centered at 620 nm with a shoulder around 580 nm, and a new band near 960 nm. A weak increase beyond 1350 nm suggests an absorption band extending beyond the detection limit (**Figure 7d**, indicated by the *). PC2 showed similar depletion at 400 nm with absorption increases at 580 and 850 nm, alongside a broad NIR band peaking at 1400 nm. PC3 exhibited strong, broad absorption between 600 and 800 nm. Based on our prior studies, these signatures correspond to distinct stages of reduction: PC1 represents inter-chain MV complexes, PC2 corresponds to π-DIM species, and PC3 reflects bis-anionic diimides or fully reduced polymer chains (**Figure 7g**). PC1's broad absorption beyond 1400 nm is characteristic of the intervalence charge-transfer (IVCT) band in MV complexes. PC2's π-DIM complexes feature broad NIR absorption near 1400 nm and a new band at 850 nm. The absorption at 580 and 960 nm, shared by PC1 and PC2, arises from free monoreduced BACD fragments in equilibrium with MV and π-DIM species. The first reduction wave showed a splitting of ΔE =
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Cyclic voltammetry and spectroelectrochemical studies of P(BACD-T) polymer. a) CVs recorded at low (50 mV/s) and high (2000 mV/s) scan-rates and their respective deconvoluted versions. The * show the waves attributed to adsorption. b) Schematic representations of BACD fragments of P(BACD-T) in their neutral state (P), as mixed-valence complexes (MV•••), π-dimers complexes (πD²•••), free reduced fragment (P•••) and bisanionic fragment (P²•••). c) Absorption map in function of time from spectroelectrochemical experiments. d) Extracted PCs of the three electrochemical events. e) Optical density variation associated to each PCs. f) TLCV performed at 10 mV/s. g) Schematic representation of the full electrochemical mechanism. h) DigiElch simulation of CVs performed at 50 and 2000 mV/s.</figcaption></figure>

0.185 V, yielding a high proportionation constant⁵⁶ $K_{com}=1.5×10^3$, similar to ortho-pentannulated BACDs.⁵⁵ Comparing the SEChem data of P(BACD-T) with that of the monomeric model compound **1,6/7-BACD-T₂** (Figure S66) revealed a smaller splitting ($\Delta E$ = 0.130 V, $K_{com}=1.6×10^2$), highlighting weaker intermolecular interactions in the monomer. This demonstrates that polymerization enhances intermolecular interactions by increasing conjugation and interaction sites. We previously found that phenyl-functionalized monomeric BACDs lacked significant CV splitting, despite concentration-dependent broadening and shouldering.⁵⁵ The splitting observed in $CH_2Cl_2$ in **1,6/7-BACD-T₂** likely arises from N···S chalcogen bonds, which planarize the molecule and enhance intermolecular interactions compared to phenyl-substituted BACDs. In the polymer, enhanced planarity and multiple interaction sites stabilize radical anions and facilitate MV and π-DIM formation. The second reduction step, corresponding to the reduction of mono-anionic chains into bis-anions, is observed

around –0.95 V at a scan rate of 2000 mV/s but is significantly influenced by the limited solubility of the bis-anionic forms (Figure 7a). At slower scan rates, an adsorption peak emerges near –1.15 V at 50 mV/s, attributed to the reduction of bis-anions adsorbed on the electrode surface. Deconvoluted cyclic voltammograms confirm this interpretation, showing a pronounced asymmetry between the reduction and oxidation waves of the P²⁻/P••• redox couple (Figure 7a), indicative of non-ideal electrochemical behaviour. As expected, this adsorption process is amplified under diffusion-blocked conditions (Figure 7f) but does not lead to additional spectroscopic features beyond that characteristic of the bis-anionic forms. This electrochemical behaviour was successfully reproduced through simulations performed using DigiElch software (Figure 7h and Figure S67). To the best of our knowledge, this study provides the first characterization of inter-chain electronic interactions in reduced n-type polymers in solution. Such interactions are intrinsic to the BACD core, highlighting its potential as a building
block for high-performance semiconductors. The strong interchain electron delocalization observed even in dilute solutions suggests the possibility of designing BACD-based polymers with enhanced charge transport properties. These findings are consistent with the known high-performance n-type polymer PNDI(2OD)₂T, whose efficiency derives from interchain charge hopping in the solid state.⁵⁷ At last, one could argue that the observed splitting and the NIR signatures subsequently generated could be attributed to the formation of anionic polaronic and bipolaronic states. Such species are associated with the appearance of strongly absorbing broad transitions at low energy characterized by high absorption coefficients.⁵⁸,⁵⁹ However, the weak bands linked to a probably low absorption coefficient observed in the NIR window for P(BACD-T) do not match the characteristics of polaronic/bipolaronic species. The calculated LUMOs centred on the BACD cores of P(BACD-T) further support this interpretation (Figure 6).

## CONCLUSION

The light-driven azabenzannulation of PDIs was adapted to develop the light-frozen dynamic covalent polymerization of BACDs, enabling the dynamic growth of poly-imines that are subsequently converted into kinetically locked conjugated polymers using visible light. This process avoids homocoupling defects, the use of toxic or precious metals, and additives beyond an acid catalyst, generating minimal waste. Considering that any aldehyde-decorated molecule can be polymerized along the BACDs, diverse electron-deficient or electron-rich functional co-monomers of interest have been successfully introduced into the polymer chains to obtain electron-deficient conjugated polymeric materials with tuneable optical band gaps and energy levels, illustrating a potential high-adaptability to various applications. The resulting BACD-based polymers absorb across the visible to NIR spectrum, combining outstanding thermal, electrochemical, and photostability with strong interchain electronic interactions observed even in solution. These properties position them as promising materials for charge and energy transport in organic electronics. Future work focusing on regioisomerically pure 1,6- and 1,7-BACD polymers could enhance intrachain communication and interchain interactions through improved crystallinity. The combination of metal-free synthesis, tunability, and excellent stability makes BACD polymers a highly attractive new class of n-type semiconductors. Their development addresses the organic electronics community's ongoing need for high-performance electron-deficient materials to complement the capabilities of p-type semiconductors.

## ASSOCIATED CONTENT

Procedures and characterization can be found in Supporting Information.

## AUTHOR INFORMATION

Corresponding Authors

Laura Le Bras - Université Marie et Louis Pasteur, CNRS, Chrono-environnement (UMR 6249), 16 route de Gray, F-25000 Besançon, France. laura.le_bras@univ-fcomte.fr

Eric Levillain - MOLTECH-Anjou, SFR MATRIX, Univ Angers, CNRS, 2 Bd Lavoisier, 49000 Angers, France. eric.levillain@univ-angers.fr

Antoine Goujon - MOLTECH-Anjou, SFR MATRIX, Univ Angers, CNRS, 2 Bd Lavoisier, 49000 Angers, France. antoine.goujon@univ-angers.fr

## Authors

Adèle Gapin - MOLTECH-Anjou, SFR MATRIX, Univ Angers, CNRS, 2 Bd Lavoisier, 49000 Angers, France.

Elarbi Chatir - MOLTECH-Anjou, SFR MATRIX, Univ Angers, CNRS, 2 Bd Lavoisier, 49000 Angers, France.

Olivier Alévêque - MOLTECH-Anjou, SFR MATRIX, Univ Angers, CNRS, 2 Bd Lavoisier, 49000 Angers, France.

Clara Pasgrimaud - MOLTECH-Anjou, SFR MATRIX, Univ Angers, CNRS, 2 Bd Lavoisier, 49000 Angers, France.

Arthur H. G. David - MOLTECH-Anjou, SFR MATRIX, Univ Angers, CNRS, 2 Bd Lavoisier, 49000 Angers, France.

Anaïs De Maria - Université de Strasbourg, CNRS, Institut Charles Sadron, F 67000, Strasbourg, France.

Mélanie Legros - Université de Strasbourg, CNRS, Institut Charles Sadron, F 67000, Strasbourg, France.

## Author Contributions

The manuscript was written through contributions of all co-authors. All authors have given approval to the final version of the manuscript. †These authors contributed equally.

## Present Address

Adèle Gapin: Univ. Bordeaux, CNRS, Bordeaux INP, LCPO, UMR 5629, F-33600 Pessac, France.

Arthur H. G. David: Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, ScanMAT - UAR 2025, Rennes (France).

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENT

The authors acknowledge the Université d'Angers and SFR MATRIX. Benjamin Siegler is acknowledged for high temperature NMR experiments, along with Ingrid Freuze for mass spectrometry measurements. L. L. B thanks the supercomputer facilities of the Mésocentre de calcul de Franche-Comté. This work received support from the "Étoiles Montantes" funding scheme (A. Goujon, project CURVY, 2021_11614, fellowship for A.H.G. David) funded by the PULSAR 2021 program from the Région Pays de la Loire and the Université d'Angers, and the Agence Nationale de la Recherche (A. Goujon, PhotoSynth ANR JCJC 2021, ANR-21-CE06-0015-01, fellowship to A. Gapin and E. Chatir). This work is funded by the European Union (ERC Starting Grant, PhotoFreeze, project n°101116355, fellowship to E. Chatir and C. Pasgrimaud).

## ASSOCIATED CONTENT
The Supporting Information is available free of charge at .... It contains experimental procedures, characterization data of the synthesized monomers and polymers (NMR, size exclusion chromatography, infrared spectroscopy), optical and electronic properties of the polymers (absorption/emission spectra, cyclic voltammetry, spectroelectrochemistry), details about DFT calculations.

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SYNOPSIS TOC: A visible light-induced, metal-free method to synthesize BisAzaCoroneneDiimides (BACDs) n-type conjugated polymers is introduced. This process irreversibly transforms dynamic poly-imines into kinetically stable conjugated polymers with tunable band gaps, low LUMO levels, and exceptional thermal and photostability. Strong interchain interactions and broad visible-to-NIR absorption highlight their potential in organic electronics, offering a scalable, eco-friendly pathway to versatile semiconducting materials.

Light-Locked Dynamic Covalent Synthesis of n-type Conjugated Polymers

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