Direct (hetero)arylation: a new tool for polymer chemists

Microwave-assisted, ligand-free, direct C–H arylation of thiophenes in biomass-derived γ-valerolactone

We report a ligand-free green synthesis of arylthiophenes and poly(3-hexyl)thiophene in GVL under microwave irradiation.

Direct arylation polymerization of asymmetric push–pull aryl halides

Recently, direct arylation polymerization (DArP) has emerged as a greener polymerization method for donor–acceptor (D–A) type conjugated polymers.

Pd-Catalyzed regioselective sequential meta-, ortho-C–H functionalization of arenes: a predictable approach to the synthesis of polysubstituted β-arylethylamines

Pd-Catalyzed sequential, controllable C–H functionalization of β-arylethylamines at different positions for the construction of polysubstituted arenes is reported.

Direct Arylation Strategies in the Synthesis of π-Extended Monomers for Organic Polymeric Solar Cells

π-conjugated macromolecules for organic polymeric solar cells can be rationally engineered at the molecular level in order to tune the optical, electrochemical and solid-state morphology characteristics, and thus to address requirements for the efficient solid state device implementation. The synthetic accessibility of monomers and polymers required for the device is getting increasing attention. Direct arylation reactions for the production of the π-extended scaffolds are gaining importance, bearing clear advantages over traditional carbon-carbon forming methodologies. Although their use in the final polymerization step is already established, there is a need for improving synthetic accessibility to implement them also in the monomer synthesis. In this review, we discuss recent examples highlighting this useful strategy.

Tin-Free Direct C-H Arylation Polymerization for High Photovoltaic Efficiency Conjugated Copolymers

A new and highly regioselective direct C-H arylation polymerization (DARP) methodology enables the reproducible and sustainable synthesis of high-performance π-conjugated photovoltaic copolymers. Unlike traditional Stille polycondensation methods for producing photovoltaic copolymers, this DARP protocol eliminates the need for environmentally harmful, toxic organotin compounds. This DARP protocol employs low loadings of commercially available catalyst components, Pd₂(dba)₃·CHCl₃ (0.5 mol%) and P(2-MeOPh)₃ (2 mol%), sterically tuned carboxylic acid additives, and an environmentally friendly solvent, 2-methyltetrahydrofuran. Using this DARP protocol, several representative copolymers are synthesized in excellent yields and high molecular masses. The DARP-derived copolymers are benchmarked versus Stille-derived counterparts by close comparison of optical, NMR spectroscopic, and electrochemical properties, all of which indicate great chemical similarity and no significant detectable structural defects in the DARP copolymers. The DARP- and Stille-derived copolymer and fullerene blend microstructural properties and morphologies are characterized with AFM, TEM, and XRD and are found to be virtually indistinguishable. Likewise, the charge generation, recombination, and transport characteristics of the fullerene blend films are found to be identical. For the first time, polymer solar cells fabricated using DARP-derived copolymers exhibit solar cell performances rivalling or exceeding those achieved with Stille-derived materials. For the DARP copolymer PBDTT-FTTE, the power conversion efficiency of 8.4% is a record for a DARP copolymer.

Syntheses via a direct arylation method of push-pull molecules based on triphenylamine and 3-cyano-4-hexyloxythiophene moieties

Synthetic access to new push-pull molecules based on 3-cyano-4-hexyloxythiophene and triphenylamine moieties is presented herein using a clean methodology. The key step involves a direct heteroarylation coupling reaction in the presence of a homogeneous or heterogeneous [Pd] catalyst followed by Knoevenagel condensation performed in ethanol as a solvent. Structure-electronic property relationships of the new molecular materials are discussed and then their use as donors in bilayer planar heterojunction solar cells is investigated.

Conjugated Thiophene-Fused Isatin Dyes through Intramolecular Direct Arylation

We report on the design, synthesis, and properties of innovative, planar, π-conjugated compounds in which a thiophene ring is fused with the skeleton of the naturally occurring dye isatin. The synthesis is achieved in high yields making use of an intramolecular direct arylation reaction as the key step, making the overall process potentially scalable. The synthetic sequence has been demonstrated also for an isatin bearing fluorine substituents on the aromatic ring. NMR and X-ray studies demonstrate the crosstalk occurring between the fused, coplanar, and conjugated moieties, making these novel dyes with a donor-acceptor character. Cyclic voltammetry and UV-vis studies confirm very interesting HOMO-LUMO levels and energy gaps for the new compounds.

Oxidative Direct Arylation Polymerization Using Oxygen as the Sole Oxidant: Facile, Green Access to Bithiazole-Based Polymers

The most appealing oxidant, molecular oxygen, is employed for the first time as the sole oxidant in the transition metal-catalyzed oxidative direct arylation polymerization (C-H/C-H-type DArP), which eliminates by-product formation of stoichiometric metal salts except for water . Compared to conventional approaches, other than the avoidance of an end-capping procedure, the current protocol is remarkably advanced in the aspect of eco-friendliness, step- and cost-economy, and, of special significance, the purity of polymer products. As illustrative examples, six 5,5'-bithiazole-based polymers are synthesized using this new method, demonstrating higher number-average molecular weight (M_n up to 33 700) in better yields (up to 93 %) through only one step. The evolution of C-H/C-H-type DArP from heavy metal salts to O₂ alone as the oxidant could solve the problem of metal residues in polymers, which is considered harmful to the performance of devices.

Click-chemistry approaches to π-conjugated polymers for organic electronics applications

Given the wide utility of click-chemistry reactions for the preparation of simple moieties within large architecturally complex materials, this minireview article aims at surveying papers exploring their scope in the area of π-conjugated polymers for application in organic electronics to enable advanced functional properties.

Metal-Free, Multicomponent Synthesis of Pyrrole-Based π-Conjugated Polymers from Imines, Acid Chlorides, and Alkynes

Multicomponent coupling reactions (MCRs) are becoming increasingly used in the synthesis of macromolecules, as they can allow the rapid generation of libraries of materials as a method to tune properties. MCRs could prove particularly useful in the synthesis of π-conjugated polymers in which structural changes are necessary for fine-tuning of electronic properties. We describe here the first metal-free multicomponent approach to conjugated polymers. This reaction exploits the coupling of imines, acid chlorides, and (catechyl)PPh to generate phospha-münchnone-containing polymers, which can be converted to poly(pyrroles) via cycloaddition. The platform allows for the efficient synthesis of families of high molecular weight polymers in one step from readily available monomers.

Heteroatom Role in Polymeric Dioxyselenophene/Dioxythiophene Systems for Color and Redox Control

The first example of a solution processable dioxythiophene-alt-dioxyselenophene polymer, ProDOT-EDOS, prepared via direct (hetero)arylation polymerization is reported and its optical and electrochemical properties are compared to the all thiophene analog, ProDOT-EDOT, and other relevant dioxythiophene polymers. By substituting the sulfur atom for a selenium atom on one of the monomers in the repeat unit a significant red-shift of both the neutral (41 nm, 132 meV) and polaronic (106 nm, 126 meV) absorbances results, as well as a 160 mV reduction in the onset of oxidation compared to ProDOT-EDOT. Spray-cast films of the polymer electrochemically switch from a vibrant blue charge neutral state to a color neutral and highly transmissive oxidized state.

C-H Arylation in the Synthesis of π-Conjugated Polymers

C-H activation reactions have allowed us to react traditionally chemically inert bonds in molecules to develop new methods for cross-coupling reactions. This type of reactivity can be applied to conjugated polymer materials in an effort to improve existing synthetic difficulties including harsh reaction conditions, multiple monomer functionalization steps, and organometallic reagent waste. In this Viewpoint, we highlight some of the encouraging advances in direct arylation polymerization (DArP) as well as ongoing challenges for future improvement and utility.

Synthesis and characterization of acceptor–donor–acceptor-based low band gap small molecules containing benzoselenadiazole

Acceptor–donor–acceptor-type compounds 5′,5″-(benzo[c][1,2,5]selenadiazole-4,7-diyl)bis(3′-dodecyl-2,2′-bithiophene-5-carbonitrile) (9) and 4,4′-(5,5′-(benzo[c][1,2,5]selenadiazole-4,7-diyl)bis(3-dodecylthiophene-5,2-diyl))dibenzonitrile (10) were designed and synthesized. These compounds differ in terminal positions, compound 9 with a thiophene-containing nitrile group and compound 10 with a phenyl-containing nitrile group. Both compounds have shown good thermal stability and low band gap. The band gaps of compounds 9 and 10 were 1.74 and 1.83 eV, respectively. These results indicate that they are promising materials for use in optoelectronics.

Synthesis and orthogonal functionalization of oxazolo[5',4':4,5]pyrano[2,3-b]pyridine by intra- and intermolecular Pd-catalyzed direct C-H bond heteroarylation

The construction and subsequent orthogonal functionalization of a hitherto unknown oxazolo[5',4':4,5]pyrano[2,3-b]pyridine are reported. A palladium-catalyzed direct C-H bond functionalization methodology was used to build the tricyclic scaffold as well as to achieve the subsequent C-H bond functionalization at the C-2 position of the oxazole unit with various (hetero)aryl iodides. Remarkably, selective C-H construction and functionalization procedures preserve the chorine atom on the pyridine moiety offering a late-stage substitution site to progress drug design.

Hydrophobic-Sheath Segregated Macromolecular Fluorophores: Colloidal Nanoparticles of Polycaprolactone-Grafted Conjugated Polymers with Bright Far-Red/Near-Infrared Emission for Biological Imaging

This article describes molecular design, synthesis and characterization of colloidal nanoparticles containing polycaprolactone-grafted conjugated polymers that exhibit strong far red/near-infrared (FR/NIR) fluorescence for bioimaging. Specifically, we synthesized two kinds of conjugated polymer bottle brushes (PFTB(out)-g-PCL and PFTB(in)-g-PCL) with different positions of the hexyl groups on the thiophene rings. A synthetic amphiphilic block copolymer PCL-b-POEGMA was employed as surfactants to encapsulate PFTB-g-PCL polymers into colloidal nanoparticles (denoted as "nanoREDs") in aqueous media. The chain length of the PCL side chains in PFTB-g-PCL played a critical role in determining the fluorescence properties in both bulk solid states and the colloidal nanoparticles. Compared to semiconducting polymer dots (Pdots) composed of PFTB(out) without grafted PCL, nanoRED(out) showed at least four times higher fluorescence quantum yield (∼20%) and a broader emission band centered at 635 nm. We further demonstrated the application of this new class of nanoREDs for effective labeling of L929 cells and HeLa cancer cells with good biocompatibility. This strategy of hydrophobic-sheath segregated macromolecular fluorophores is expected to be applicable to a broad range of conjugated polymers with tunable optical properties for applications such as bioimaging.