CH Activation as a Shortcut to Conjugated Polymer Synthesis

Rational Monomer Design for the Synthesis of Conjugated Polymers by Direct Heteroarylation Polymerization

This study focuses on the design concepts that contribute to the C-H activation in bithiophene-flanked monomers incorporating naphthalene diimide (NDI), perylene diimide (PDI), and fluorene (FLU) and their polymerization by direct heteroarylation. Density functional theory (DFT) calculations reveal distinct energy requirements for C-H bond abstraction, which is dictated by the electron-withdrawing strength of the central aromatic core flanked by bithiophene. These provide insights into the reactivity of each monomer for C-H bond activation. Proton NMR spectroscopic experimental results confirm the favorable energetic profiles predicted by DFT, with NDI- and PDI-flanked monomers exhibiting lower energy requirements than fluorene-flanked monomers. Successful polymer synthesis is demonstrated for NDI and PDI, while the fluorene-flanked monomer shows challenges due to its higher energy demands.

Room Temperature Synthesis of a Well-Defined Conjugated Polymer Using Direct Arylation Polymerization (DArP)

While a major improvement to the sustainability of conjugated polymer synthesis, traditional direct arylation polymerization (DArP) still requires high temperatures (typically >100 °C), necessitating a significant energy input requirement. Performing DArP at reduced or ambient temperatures would represent an improvement to the sustainability of the reaction. Here we describe the first report of a well-defined conjugated polymer synthesized by DArP at room temperature. Previous efforts toward room temperature DArP relied on the use of a near-stoichiometric silver reagent, an expensive coinage metal, which makes the reaction less cost-effective and sustainable. Here, room temperature polymerizations of 3,4-ethylenedioxythiophene (EDOT) and 9,9-dioctyl-2,7-diiodofluorene were optimized and provided molar mass (Mn) up to 11 kg/mol PEDOTF, and performing the reaction at the standard ambient temperature of 25 °C provided Mn up to 15 kg/mol. Model studies using other C-H monomers of varying electron density copolymerized with 9,9-dioctyl-2,7-diiodofluorene provided insight into the scope of the room temperature polymerization, suggesting that performing room temperature DArP is highly dependent on the electron richness of the C-H monomer.

Survey of Recent Advances in Molecular Fluorophores, Unconjugated Polymers, and Emerging Functional Materials Designed for Electrofluorochromic Use

In this review, recent advances that exploit the intrinsic emission of organic materials for reversibly modulating their intensity with applied potential are surveyed. Key design strategies that have been adopted during the past five years for developing such electrofluorochromic materials are presented, focusing on molecular fluorophores that are coupled with redox-active moieties, intrinsically electroactive molecular fluorophores, and unconjugated emissive organic polymers. The structural effects, main challenges, and strides toward addressing the limitations of emerging fluorescent materials that are electrochemically responsive are surveyed, along with how these can be adapted for their use in electrofluorochromic devices.

A novel donor-acceptor structured diketopyrrolopyrrole-based conjugated polymer synthesized by direct arylation polycondensation (DArP) for highly efficient antimicrobial photothermal therapy

A novel donor (D)-acceptor (A) structured conjugated polymer (PDPP-TP), which contains two alternating D-A pairs, namely thiophene (T)-diketopyrrolopyrrole (DPP) and thiophenen (T)-thieno[3,4-b]pyrazine (TP) along the main chain of the polymer, was synthesized by direct arylation polycondensation (DArP) for a highly efficient photothermal antibacterial treatment. The hydrophilic PDPP-TP-based nanoparticles (PTNPs) with a hydration diameter of about 120 nm were obtained by self-assembly using DSPE-mPEG₂₀₀₀ as the polymer matrix. PTNPs show strong near-infrared (NIR) absorbance with a λ_max at 910 nm (ε = 2.25 × 10⁴ L mol^-1 cm^-1) and NIR light-triggered photoactivity with a high photothermal conversion efficiency (PTCE) of 52.8% under 880 nm laser irradiation. Keeping the merits of excellent biocompatibility and photostability, PTNPs exhibited remarkable bacterial inhibition efficiency of almost 100% against Gram-negative E. coli and Gram-positive S. aureus with the help of an 880 nm laser (0.7 W cm^-2, 6 min), demonstrating its great potential as photothermal materials with a broad spectrum of activity for the effective treatment of microbial infections.

Cross-Dehydrogenative Coupling Polymerization via C-H Activation for the Synthesis of Conjugated Polymers

Owing to their versatile (opto)electronic properties, conjugated polymers have found application in several organic electronic devices. Cross-coupling reactions such as Stille, Suzuki, Kumada couplings, and direct arylation reactions have proved to be effective for their synthesis. More atom-efficient oxidative direct arylation polymerization has also been reported for making homopolymers. However, growing interest toward donor-acceptor polymers has led to the recent emergence of cross-dehydrogenative coupling (CDC) polymerization to synthesize alternating copolymers without any prefunctionalization of monomers. Metal-catalyzed cross-coupling of two simple arenes via double C-H activation, or of an arene with an alkene via oxidative Heck-type reaction have been used so far for CDC polymerization. In this article, we discuss the development of CDC polymerization protocols along with the relevant small molecule CDC reactions for an improved understanding of these reactions.

A Route to Conjugated Monomers and Polymers Incorporating 2,5-Connected Oxazole in the Backbone

Joining of imidazole, pyrimidine, and oxazole to other conjugated core units was explored in pursuit of yielding monomers to synthesize organic semiconducting polymers. Regioregular oxazole-flanked thiophene, benzothiadiazole, naphthalene diimide (NDI), and thienopyrroledione (TPD) were successfully isolated via stannylation of oxazole and the Stille coupling of brominated core units (overall yields ranging from ca. 40 to 60%). From subsequent direct arylation polymerization, NDI/oxazole/TPD-containing regioisomeric polymers were obtained with optical and electrochemical orbital energetics in the semiconducting regime.

Circular Discovery in Small Molecule and Conjugated Polymer Synthetic Methodology

Simple and efficient methods are a key consideration for small molecule and polymer syntheses. Direct arylation polymerization (DArP) is of increasing interest for preparing conjugated polymers as an effective approach compared to conventional cross-coupling polymerizations. As DArP sees broader utilization, advancements are needed to access materials with improved properties and different monomer structures and to improve the scalability of conjugated polymer synthesis. Presented herein are considerations for developing new methods of conjugated polymer synthesis from small molecule transformations, exploring how DArP has successfully used this approach, and presenting how emerging polymerization methodologies are developing similarly. While it is common to adapt small molecule methods to polymerizations, we demonstrate the ways in which information gained from studying polymerizations can inform and inspire greater advancements in small molecule transformations. This circular approach to organic synthetic method development underlines the value of collaboration between small molecule and polymer-based synthetic research groups.

Synthesis of Conjugated Polymers via Transition Metal Catalysed C-H Bond Activation

Transition metal catalysed C-H bond activation chemistry has emerged as an exciting and promising approach in organic synthesis. This allows us to synthesize a wider range of functional molecules and conjugated polymers in a more convenient and more atom economical way. The formation of C-C bonds in the construction of pi-conjugated systems, particularly for conjugated polymers, has benefited much from the advances in C-H bond activation chemistry. Compared to conventional transition-metal catalysed cross-coupling polymerization such as Suzuki and Stille cross-coupling, pre-functionalization of aromatic monomers, such as halogenation, borylation and stannylation, is no longer required for direct arylation polymerization (DArP), which involve C-H/C-X cross-coupling, and oxidative direct arylation polymerization (Ox-DArP), which involves C-H/C-H cross-coupling protocols driven by the activation of monomers' C(sp² )-H bonds. Furthermore, poly(annulation) via C-H bond activation chemistry leads to the formation of unique pi-conjugated moieties as part of the polymeric backbone. This review thus summarises advances to date in the synthesis of conjugated polymers utilizing transition metal catalysed C-H bond activation chemistry. A variety of conjugated polymers via DArP including poly(thiophene), thieno[3,4-c]pyrrole-4,6-dione)-containing, fluorenyl-containing, benzothiadiazole-containing and diketopyrrolopyrrole-containing copolymers, were summarized. Conjugated polymers obtained through Ox-DArP were outlined and compared. Furthermore, poly(annulation) using transition metal catalysed C-H bond activation chemistry was also reviewed. In the last part of this review, difficulties and perspective to make use of transition metal catalysed C-H activation polymerization to prepare conjugated polymers were discussed and commented.

The [3+2] Annulation of CF3-Ketimines by Re Catalysis: Access to CF3-Containing Amino Heterocycles and Polyamides

Transition metal catalyzed [3 + 2] annulation of imines with double bonds via directed C-H activation offers a direct access to amino cyclic motifs. However, owing to weak coordination and steric hindrance, trifluoromethylated ketimines have been an unaddressed challenge for TM-catalyzed annulations. Here, a rhenium-catalyzed [3 + 2] annulation of trifluoromethylated ketimines with isocyanates via C(sp2)-H activation has been disclosed. This approach provides an efficient platform for rapid access to a privileged library of CF3-containing iminoisoindolinones and polyamides by utilizing challenging CF3-ketimines as the annulation component. The capability of gram scale synthesis, the post-functionalization of the cyclization adduct, the derivation of complex natural molecules and the facile synthesis of polyamides highlight a diversity of synthetic potential of the current methodology.

An Efficient Precatalyst Approach for the Synthesis of Thiazole-Containing Conjugated Polymers via Cu-Catalyzed Direct Arylation Polymerization (Cu-DArP)

Over the past decade, direct arylation polymerization (DArP) has emerged as a facile and sustainable methodology for the synthesis of conjugated polymers. Recently, we developed Cu-catalyzed DArP (Cu-DArP) as a low-cost, Pd-free synthetic pathway, which enables conjugated polymers to be synthesized with high molecular weights and minimization of defects. However, the lack of study on the use of Cu-precatalysts in small-molecule direct arylation poses significant limitations for Cu-DArP to potentially overtake conventional Pd-catalyzed methodology, such as the low solubility and stability of the previously employed CuI. Therefore, in this report, we decide to explore the utility of a well-defined, easy-to-prepare, highly soluble, and stable precatalyst, Cu(phen)(PPh₃)Br, as an alternative to the CuI, 1,10-phenanthroline catalytic system previously used for Cu-DArP. Herein, we report a drastic improvement of Cu-DArP methodology for the synthesis of 5,5'-bithiazole (5-BTz)-based conjugated polymers enabled by an efficient precatalyst approach, affording polymers with good M_n (up to 16.5 kDa) and excellent yields (up to 79%). ¹H NMR studies reveal the exclusion of homocoupling defects, which further verifies the excellent stability of Cu(phen)(PPh₃)Br compared to CuI. Furthermore, we were able to decrease the catalyst loading from 15 mol % to only 5 mol % (M_n of 11.8 kDa, 64% yield), which is unprecedented when aryl bromides are employed for Cu-DArP. Significantly, 5-BTz was shown to be inactive under various of Pd-DArP conditions, which demonstrates the high compatibility of Cu-DArP as the only pathway for the C-H activation of the 5-BTz unit and a clear case demonstrating an advantage of Cu-DArP relative to Pd-DArP.

Synthesis and Characterization of a 2,3-Dialkoxynaphthalene-Based Conjugated Copolymer via Direct Arylation Polymerization (DAP) for Organic Electronics

Poly[(5,5'-(2,3-bis(2-ethylhexyloxy)naphthalene-1,4-diyl)bis(thiophene-2,2'-diyl))-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PEHONDTBT) was synthesized for the first time and through direct arylation polymerization (DAP) for use as p-donor material in organic solar cells. Optimized reaction protocol leads to a donor-acceptor conjugated polymer in good yield, with less structural defects than its analog obtained from Suzuki polycondensation, and with similar or even higher molecular weight than other previously reported polymers based on the 2,3-dialkoxynaphthalene monomer. The batch-to-batch repeatability of the optimized DAP conditions for the synthesis of PEHONDTBT was proved, showing the robustness of the synthetic strategy. The structure of PEHONDTBT was corroborated by NMR, exhibiting good solubility in common organic solvents, good film-forming ability, and thermal stability. PEHONDTBT film presented an absorption band centered at 498 nm, a band gap of 2.15 eV, and HOMO and LUMO energy levels of -5.31 eV and -3.17 eV, respectively. Theoretical calculations were performed to understand the regioselectivity in the synthesis of PEHONDTBT and to rationalize its optoelectronic properties. Bilayer heterojunction organic photovoltaic devices with PEHONDTBT as the donor layer were fabricated to test their photovoltaic performance, affording low power-conversion efficiency in the preliminary studies.

Exploring the Utility of Buchwald Ligands for C-H Oxidative Direct Arylation Polymerizations

Oxidative C-H/C-H cross-coupling polymerizations provide an opportunity to synthesize conjugated polymers with an increased ease of monomer preparation, reduced environmental impact, and increased sustainability. Considering these attributes, it is necessary to expand the diversity of monomers that readily and efficiently participate in this coupling strategy to enable the development of conjugated polymers with a wide range of properties. Herein, the oxidative direct arylation polymerization toolbox is expanded to include 3,4-propylenedioxythiophene being synthesized via C-H/C-H cross-coupling methodologies. In conjunction with these efforts, the utilization of Buchwald ligands in C-H/C-H cross coupling polymerizations also is reported, and variations in the ligand structure provide insight into the role ligand choice has on C-H cross-coupling polymerizations. Specifically, it is determined that the phosphine functionality affects the rate-determining, concerted metalation-deprotonation step of the catalytic cycle, while bulky isopropyl substituents on the ligand's lower aryl ring promote reductive elimination. By balancing these steric effects on the ancillary ligands, polymers are synthesized to exhibit molecular weights above the effective conjugation length, with recovered yields >90%. In addition to expanding the scope of conjugated polymers accessible via oxidative direct arylation polymerization, these results provide the foundational understanding for utilizing Buchwald-type ligands in C-H-activated polymerizations.

Significant Difference in Semiconducting Properties of Isomeric All-Acceptor Polymers Synthesized via Direct Arylation Polycondensation

The direct arylation polycondensation (DArP) appeared as an efficient method for producing semiconducting polymers but often requires acceptor monomers with orienting or activating groups for the reactive carbon-hydrogen (C-H) bonds, which limits the choice of acceptor units. In this study, we describe a DArP for producing high-molecular-weight all-acceptor polymers composed of the acceptor monomers without any orienting or activating groups via a modified method using Pd/Cu co-catalysts. We thus obtained two isomeric all-acceptor polymers, P1 and P2, which have the same backbone and side-chains but different positions of the nitrogen atoms in the thiazole units. This subtle change significantly influences their optoelectronic, molecular packing, and charge-transport properties. P2 with a greater backbone torsion has favorable edge-on orientations and a high electron mobility μ_e of 2.55 cm²  V^-1  s^-1 . Moreover, P2-based transistors show an excellent shelf-storage stability in air even after the storage for 1 month.

Synthesis of conjugated polymers using aryl-bromides via Cu-catalyzed direct arylation polymerization (Cu-DArP)

The first report on direct arylation polymerization with copper catalysts and aryl-bromide monomers expands the sustainability and practicality of DArP.