Influence of the Ester Directing Group on the Inhibition of Defect Formation in Polythiophenes with Direct Arylation Polymerization (DArP)

Abstract

Direct arylation polymerization (DArP) has allowed for the facile preparation of a variety of conjugated polymer architectures. Because DArP proceeds through a C−H activation pathway, the selectivity of only the desired C− H bond for certain monomers has remained a challenge. Low selectivity for the desired C−H bond can lead to the introduction of various structural defects. The development of conditions through the use of additives, screening and design of phosphine ligands, and precatalysts has provided condition sets that work for some monomers to afford the desired polymers with a minimization of defects. In addition to modifying the conditions, another handle for tuning the site selectivity for C−H activation is through the introduction of a directing group, such as an ester. Ester-functionalized conjugated polymers have gone through a renaissance of sorts where the inclusion of this functionality on thiophene-based monomers has garnered increased attention. To understand how this functionality can be exploited as a directing group for DArP, we study the formation of defects for poly(3-hexyl ester thiophene-2,5-diyl) (P3HET) and select structural analogues. We develop optimized conditions that allow for the synthesis of this polymer with improved molecular weight (M n ) (up to 15.9 kDa vs 11.7 kDa) and regioregularity (rr) (up to >99% vs 96%) in a shorter polymerization time (16 h vs 48 h). On the basis of extensive defect analysis using 1 H NMR spectroscopy, we rationally design a monomer for the synthesis of the donor−acceptor polymer poly(2hexyldecyl-[2,2′-bithiophene]-4-carboxylate-5,5′-diyl) (P3HDET-T) with good M n (26.2 kDa) and regioregularity (>99%). This study shows how the inclusion of ester directing groups and the formulation of pathways for defect formation can enable the rational design of monomers yielding conjugated polymers with improved M n and rr.

Molecular Engineering and Morphology Control of Polythiophene:Nonfullerene Acceptor Blends for High‐Performance Solar Cells

“…For example, Noonan and co-workers developed Suzuki polycondensation using commercially available nickel precatalysts, and obtained carboxylate substituted PTs with controlled molecular weights; [72] Thompson and co-workers synthesized carboxylate substituted PTs by direct arylation polymerization, which is thought as a safe, atom economical, and scalable candidate to replace traditional cross-coupling methods. [73][74][75] It is expected that more efforts will be denoted to the studies on the polymerization methods of the PTs with functional side chains.…”

Advances in applying C–H functionalization and naturally sourced building blocks in organic semiconductor synthesis

“…Thompson et al have recently uncovered that using an ester as a directing group installed directly on the thiophene monomer improves the regioregularity (head-to-tail coupling) of the resulted polymer. 55 They installed a hexyl ester directing group adjacent to the C-H bond that was designated for activation on the 2-bromothiophene monomer 2 (Scheme 5c, bottom). Compared to the similar monomer 1 (Scheme 5c, top), where the ester group was installed adjacent to the bromine, the regioregularity of produced poly(3-hexyl ester thiophene-2,5diyl) (P3HET) improved from 96% to 499%.…”

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

“…Further studies by the same group demonstrated that both the steric bulk of the ester and the position of the ester relative to the bromide affected both the reaction efficacy and defect formation. [35] DArP has also been used to prepare functionalised thiophene polymers. Thompson's group reported the preparation of an amide functionalised thiophene polymer (Scheme 5a).…”

“…[34] (b) Plausible origin of branching β-defects and crosslinks when reaction time is increased. [35] Scheme 5. (a) DArP preparation of a novel amide functionalised thiophene polymer and its hole-transport properties.…”

“…However, when thieno[ 3,2‐b ]thiophene was used in place of bi‐thiophene as the coupling partner, no gelation was observed, even when the reaction was left overnight, which was attributed to the lower reactivity of the β‐hydrogen on thieno[ 3,2‐b ]thiophene. Further studies by the same group demonstrated that both the steric bulk of the ester and the position of the ester relative to the bromide affected both the reaction efficacy and defect formation [35] …”