Visible-Light Driven Synthesis of Semiconducting Polymers – PhotoSynth

Dynamic covalent imine-based conjugated polymers recently emerged as a new path toward easy-to-prepare semiconducting structures. Imines are efficiently synthesized by condensation of amine and aldehyde functionalized building blocks, generating water as a side product. Such functional groups are reasonably easy to instal on a myriad of conjugated scaffolds which could make imine-condensation a strategy of choice, if it was not for their intrinsic reversible nature. Indeed, imines are dynamic covalent bonds (DCBs), labile and sensitive to their environment, which is why this strategy has been mostly limited to the successful preparation of recyclable and degradable semiconductors to be used in transient electronic devices crucial for bio-applications such as sensing. To bridge the gap between kinetically locked and stable carbon-carbon bound conjugated polymers and dynamic imine-based materials with the aim to develop a new synthetic methodology for the preparation of organic semiconductors, we investigated a so-called “light-frozen” dynamic covalent synthesis of conjugated polymers. It aimed to combine the best aspects of conventional conjugated polymer synthesis (irreversibly bound building blocks, high molecular weights, high diversity) with the advantages of dynamic covalent chemistry (mitigation of reproducibility issues by avoiding Pd-catalysed reactions, water as waste, synthesis at thermodynamic equilibrium) into a single synthetic methodology.

We exploited the photodriven azabenzannulation of perylenediimides (PDIs) recently developed in our group to build conjugated polymers in two steps: the synthesis of reversible poly-imines at thermodynamic equilibrium, followed by and irreversible oxidative photocyclization performed by exposure to white light. (NH2)2PDIs can be efficiently combined with any optoelectronicaly active bis-aldehyde to produce a new family of BisAzaCoroneneDiimides (BACDs) rylene-based conjugated polymers. We hope that the formation of polyimines under thermodynamic control should lead to limited batch-to-batch variations and allow a certain degree of predictability or control of the molecular weight of the polymers. Optimal polymerization conditions for this polymer were determined to be 150°C in toluene at 50 mM under microwave irradiation, with TFA as a catalyst and molecular sieves (MS) to trap the water produced by imine condensation and push the equilibrium toward the elongation of polymer chains. 2,5-thiophenedicarboxaldehyde was reacted with a small excess of 1,6/7(NH2)2PDIs to afford amine-terminated dynamic covalent imines-linked PDIs polymers. The solution was then exposed to visible light in a flux photoreactor and subjected to oxidative aromatization by addition of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Then, the amine-terminated material was reacted with an excess of 2-thiophenecarboxaldehyde as end-capper, exposed to visible light and oxidized again.

After polymerization, the materials were isolated and analysed by size-exclusion chromatography (SEC) to determine their M ̅_n, M ̅_w, (DP) ̅_n and dispersity index Đ. All polymers displayed good solubility in common organic solvents and were obtained in high yields. For Method A, the chromatograms revealed several populations for reaction concentration performed at 30 mM and below, but much cleaner elution peaks were obtained above this threshold. The P(BACD-T) polymers prepared by the Method A revealed increasing M ̅_n, M ̅_w, (DP) ̅_n and Đ when the reactants concentration increased, affording polymers of Mn ranging from 12.9 up to 40.0 kDa corresponding to (DP) ̅_n of 12 to 37 repeating units. A quasi-linear relationship can be observed between the initial concentration in monomers and M ̅_n. The dispersity Đ increased along with M ̅_n although remaining in a reasonable range at 1.8 at 50 mM. This method demonstrated excellent reproducibility. By performing the polymerization of P(BACD-T) at 50 mM three times, little deviation was observed: M ̅_n = 23.8 ± 2.5 kDa, M ̅_w = 42.4 ± 6.1 kDa, (DP) ̅_n = 22 ± 2 and Đ = 1.8 ± 0.1.

Their optoelectronic properties were investigated by absorption spectroscopy in solution (CHCl3) and thin film, and Cyclic Voltammetry. P(BACD-T), P(BACD-F) and P(BACD-C) strongly absorbs light in the UV range with maximums at 408, 388 and 387 nm respectively with high absorption coefficients. They absorb visible light up to 600 nm in solution. In films, red shifts are observed resulting from aggregation. No significant changes were observed after thermal annealing at 150 °C for 10 min. P(BACD-DTII) shows a broad continuous absorption of UV and visible light up to 700 nm, with a small red shift in films. TP(BACD-T-DTII) revealed a similar spectrum with a small red-shift of its maximum. P(BACD-DTDPP) also strongly absorbs light across the visible range with large band showing a maximum at 651 nm and tailing up to 800 nm. In thin films, a broadening of the low energy band was observed, extending the absorption in the near-IR region beyond 900 nm. P(BACD-IDTTB6) also strongly absorbs light across the visible range, with a maximum around 500 nm and tailing up to 700 nm.

BACD polymers are an intriguing class of n-type materials that deserve more investigations. They are now being tested as components of organic electronic devices such as OSCs and photodetectors in the group of Prof. Thuc-Quyen Nguyen in Santa Barbara, US. These results and the investigations of their properties in thin films will guide the design of new generation of materials for specific applications in the future.

The light-driven azabenzannulation of PDIs was adapted to develop the light-frozen dynamic covalent polymerization of BACDs, enabling the dynamic growth of polyimines 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 regiopure 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.

This work resulted in the publication of 4 articles and 1 review. The main papers on the topic of polymers is soon to be submitted, and will be followed by others.

There is a growing interest in conjugated polymers, now found in most organic electronic devices. However, their preparation most of the time relies on precious metal catalysed reactions with poor atom economy and prone to introduce defects. The PhotoSynth project offers to prepare conjugated polymers with an excellent atom economy, condensing a diaminoPDI and an aldehyde together to form a poly-imine further irreversibly photo cyclized by visible light (for example in a continuous flow photo reactor) into a fully conjugated system. This approach limits waste, the use of additive, removes the need for expensive catalyst or ligands and is also versatile as the diaminoPDI building block can be combined with a wide variety of opto/electroactive bis-adehydes to tune the properties of the polymers, which will be later tested in devices. PhotoSynth paves the way toward a sustainable, modular and efficient synthesis of a new generation of organic semiconductors of interest.