Influence of functionality on direct arylation of model systems as a route toward fluorinated copolymers via direct arylation polymerization (DArP)

A screening of direct arylation conditions on a model small molecule system is carried out to develop suitable conditions for the direct arylation polymerization (DArP) of fluorinated copolymers, which are incompatible with conditions previously utilized successfully for nonfluorinated systems. The model system features a coupling between a 2‐substituted thiophene and a pentafluorobenzene, where one of the partners was brominated. A substantial difference in reactivity is observed, demonstrating that the optimal functionalization for direct arylation between a thiophene‐based donor and a highly fluorinated acceptor is a halogenated thiophene and an unfunctionalized fluorinated unit, which is opposite of typical cross coupling reactions, where the acceptor is typically halogenated. The best conditions are applied to the copolymerization of 1,2,4,5‐tetrafluorobenzene and 2,2'‐(2,5‐bis((2‐hexyldecyl)oxy)?1,4‐phenylene)dithiophene. Polymers are free of β‐defects and significant homocoupling. This work further underscores the attractive simplicity, relevance, and ease of DArP while reconfirming its broad compatibility with increasingly popular fluorinated copolymers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2598–2605     

Optimization of direct arylation polymerization conditions for the synthesis of poly(3‐hexylthiophene)

Direct arylation polymerization (DArP) is an emerging alternative to Stille and Suzuki polymerizations. This method is attractive as it allows preparation of high‐molecular‐weight conjugated polymers in good yield without the need to metallate monomers. Despite this promise, for poly(3‐hexylthiophene) (P3HT) and related polymers that have β‐protons on the thiophene ring, DArP is known to produce β‐defects, which make the polymer properties different from polymers produced by traditional methods. Here, we demonstrate that DArP conditions based on simple, inexpensive, and bench‐stable reagents can be tuned to limit the amount of defects and produce P3HT with properties remarkably similar to Stille P3HT. Specifically, lowering the reaction temperature, lowering the amount of catalyst, and using a bulkier carboxylate ligand is critical. Optimized conditions include reacting 2‐bromo‐3‐hexylthiophene with 0.25 mol % of Pd(OAc)₂, 1.5 equivalents of K₂CO₃, and 0.3 equivalents of neodecanoic acid in N,N‐dimethylacetamide at 70 °C and give DArP P3HT with ～60% yield, regioregularity of 93.5%, molecular weight of 20 kDa, polydispersity of 2.8, and melting point of 217 °C, providing a very close match to Stille P3HT, which is obtained with 70–80% yield, 91–94% regioregularity, molecular weight of 15–25 kDa, polydispersity of 2.5–2.8, and melting point of 214–221 °C. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2660–2668     

Optimization of direct arylation polymerization (DArP) through the identification and control of defects in polymer structure

As a newly emerged protocol for the synthesis of conjugated polymers, direct arylation polymerization (DArP) is an environmentally friendly and cost-effective alternative to traditional methods of polymerization. DArP efficiently yields conjugated polymers with high yield and high molecular weight. However, DArP is also known to produce defects in polymer chemical structure. Together with molecular weight and polydispersity, these defects are considered to be important parameters of polymer structure and they have a strong impact on optical, electronic and thermal properties of conjugated polymers. The four major classes of conjugated polymer defects inherent for DArP have been identified: homocoupling regiodefects, branching defects, end group defects, and residual metal defects. To have a precise control over the polymer properties, it is important to understand what causes the defects to form during the polymerization process and be able to control their content. Here within the scope of current literature, we discuss in detail the definition and origin of all these defects, their influence on polymer properties and effective means to control the defects through fine tuning of the DArP reaction parameters. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 135–147     

Improving the efficiency and sustainability of catalysts for direct arylation polymerization (DArP)

In the past decade, direct arylation polymerization (DArP) has rapidly developed as a sustainable synthetic protocol for cost-effective, atom-economical preparation of conjugated polymers. By circumventing monomer functionalization with toxic transmetallating reagents such as organostannane and organoboron required for Stille-Migita and Suzuki-Miyaura polymerization methods, DArP proceeds through a metal-catalyzed C H activation pathway for the preparation of high-performance conjugated polymer materials. This review evaluates the development of several classes of efficient catalysts/catalytic systems from small-molecule studies to polymerizations, including the mechanisms involved in these transformations and how they inspire catalyst and monomer design for defect-free conjugated polymer synthesis. Recent advances in developing more sustainable first-row transition metal catalysts for DArP are also highlighted, and the fundamental understanding of these efficient and sustainable catalysts should motivate the pursuit for the next generation of catalytic design to enable more effective and environmentally friendly conjugated polymer synthesis.     

Preparation of semi‐alternating conjugated polymers using direct arylation polymerization (DArP) and improvement of photovoltaic device performance through structural variation

The results herein expand on optimized direct arylation polymerization (DArP) conditions for defect‐free poly[(2,5‐bis (2‐hexyldecyloxy)phenylene)‐alt‐(4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5] thiadiazole)] (PPDTBT). Semi‐alternating and alternating donor–acceptor polymers containing alkoxy phenylene, dithienyl‐substituted thieno[3,4‐c]pyrrole‐4,6‐dione (DTTPD), and dithienyl‐substituted diketopyrrolopyrrole (DTDPP) were prepared via DArP, including a four‐component semi‐alternating copolymer PPDTDPPTPD. Variation of the alkoxy substituents on the phenylene donor including n‐hexyl, 2‐ethylhexyl, or 2‐hexyldecyl allowed for the tuning of thephysical and electronic properties. Molecular weights (M _n) ranged from 3.07 to 28.3 kDa for the PPDTTPD polymers and 2.63‐44.0 kDa for the PPDTDPP polymers, depending on the alkoxy substituents. Absorbance maxima and HOMO energies were varied from 550 to 602 nm and ?5.31 to ?5.69 eV for the PPDTTPD polymers and from 671 to 794 nm and ?5.41 to ?5.55 eV for the PPDTDPP polymers, respectively. Additive‐free, bulk heterojunction (BHJ) solar cells were fabricated, and the fill‐factors obtained (0.57–0.63) are some of the highest reported for polymers prepared using DArP. Higher molecular weight polymers for both PPDTTPD (28 kDa) and PPDTDPP (44 kDa) series performed poorly in solar cells. In contrast, the semi‐alternating polymers of lower M _n for the PPDTTPD (12.4 kDa) and PPDTDPP (9.05 kDa) series, incorporating both n‐hexyl and 2‐hexyldecyl alkoxy phenylene donors, provided power conversion efficiencies (PCE) of 3.26% and 3.49%, respectively. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3370–3380     

Minimization of the auxiliary reagent loading for direct arylation polymerization (DArP) of 2‐bromo‐3‐hexylthiophene

In this work, we present a powerful set of synthetic strategies aimed at minimization of auxiliary reagent loading for direct arylation polymerization (DArP) of 2‐bromo‐3‐hexylthiophene. As such, we report efficient lowering of Pd(OAc)₂ catalyst loading as well as loading of other auxiliary reagents, such as neodecanoic acid and N,N‐dimethylacetamide. Unprecedented low loadings of catalyst down to 0.0313% (313 ppm) were achieved, while producing polymer in high yield (91% after Soxhlet extraction), with a high molecular weight (24.2 kDa) and carefully controlled chemical structure thus making the optimized DArP protocol significantly more cost‐effective, convenient, sustainable, and environmentally friendly. The resulting polymer samples were thoroughly investigated in terms of their chemical structure as well as optical, thermal, chain ordering and electronic properties using GPC analysis, ¹H NMR, MALDI, UV–vis, GIXRD spectroscopy, DSC, and SCLC hole mobility measurements. The results demonstrate that the reagent lowering strategies increase the polymer regioregularity from 94.6 to 96.5% as evidenced by ¹H NMR spectra and corroborated by GIXRD, DSC, and UV–vis measurements. Additionally, polymer samples obtained at low reagent loading are more uniformly proton‐terminated as evidenced by ¹H NMR and MALDI end‐group analysis. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1492–1499     

The effect of amide solvent structure on the direct arylation polymerization (DArP) of 2‐Bromo‐3‐hexylthiophene

In this work, we investigate the influence of the amide solvent chemical structure on the properties of poly(3‐hexylthiophene) (P3HT) prepared via direct arylation polymerization (DArP). Our findings indicate that for successful polymerization the amide must possess an acyclic aliphatic structure since cyclization of an amide results in a complete shutdown of DArP reactivity as evidenced by failed polymerization in N‐methylpyrrolidone, whereas the presence of an aromatic motif renders the amide solvent susceptible to C? H activation and leads to incorporation of the solvent structure into the P3HT backbone, as demonstrated on the example of N,N‐diethylbenzamide. Additionally, we observed that the steric bulk of alkyl substituents on both the nitrogen atom and the carbonyl group within the amide structure has to be delicately balanced for optimal DArP reactivity. In the optimal cases, P3HT is obtained in high yield, with high molecular weight and contains a minimal amount of structural defects. The obtained polymer samples were comprehensively studied in terms of their chemical structure, optical, thermal and solid‐state properties in thin films using GPC analysis, ¹H NMR, MALDI, UV–vis, GIXRD spectroscopy, and DSC. We additionally note a drastic difference of the amide solvent effect between DArP and Stille polymerization. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2494–2500     

Application of direct (hetero)arylation in constructing conjugated small molecules and polymers for organic optoelectronic devices

Direct (hetero)arylation, as a sustainable, atom-economic and environmentally benign synthetic protocol compared to conventional coupling techniques, has been extensively applied to the sustainable preparation of π-conjugated materials for organic optoelectronic devices. In this review, we will highlight recent advances made in direct arylation for conjugated small molecules and polymers toward high performance organic optoelectronic devices. Some important insights in direct arylation for synthesizing organic optoelectronic materials are given, together with the challenges and outlook in this significant and hot research field.     

Incremental optimization in donor polymers for bulk heterojunction organic solar cells exhibiting high performance

The power conversion efficiency of an organic solar cell has now exceeded the 10% mark, which is a significant improvement in the last decade. This has been made possible due to the development of low-band-gap polymers with tunable electron affinity, ionization potential, solubility, and miscibility with the fullerene acceptor, and the improved understanding of the factors affecting the critical device parameters such as the V_OC and the J_SC. This review examines the latest strategies, results, and trends that have evolved in the design of solar cells with better efficiency and durability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Direct arylation polymerization toward ultra‐low bandgap poly(thienoisoindigo‐alt‐diketopyrrolepyrrole) conjugated polymers: The effect of β‐protection on the polymerization and properties of the polymers

Direct arylation polymerization between derivatives of dibromodiketopyrrolopyrrole (DPP) and thienoisoindigo (TIIG) resulted in two π‐conjugated copolymers with average molecular weights up to 24.0 kDa and bandgaps as low as 0.8 eV. The structural analysis of the obtained two polymers revealed well‐defined alternating conjugation backbones without obvious structural defects. The introduction of hexyl‐group in the β‐position of thiophene rings in the DPP units not only reduces the bandgap of conjugated polymer compared to a similar polymer containing bare‐thiophene flanked DPP but also affects polymer morphology in thin films. P‐type charge‐transport characteristics were observed for two polymers in organic field‐effect transistors with comparable hole mobilities. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3205–3213     

Multifunctional triphenylamine polymers synthesized via direct (hetero) arylation polymerization

The design rules for creating multifunctional organic electronic materials are currently limited. By copolymerizing twisted triphenylamine (TPA) and electron rich dioxythiophene (XDOT) monomers via Direct (Hetero) Arylation Polymerization (DHAP), a set of polymers are obtained that perform as yellow to transmissive electrochromic (EC) films with up to 45% contrast, as well as in electroluminescent (EL) applications, achieving a luminance of ∼450 cd/m² in yellow‐green polymer light‐emitting diodes (PLEDs). In addition, polymerizing TPA with a donor‐acceptor‐donor monomer results in a low‐bandgap polymer that achieves power conversion efficiencies up to 2.5% when blended with PC₇₁BM in conventional organic photovoltaic (OPV) devices. Incorporation of TPA units into the polymer backbone largely breaks any aggregation and ordering in the solid‐state, leading to highly soluble materials that form smooth, reproducible thin films. The TPA unit also serves to break conjugation throughout the polymer backbone, providing precise control over optical and electronic properties through choice of comonomer. These results suggest that TPA copolymers can be useful for achieving multi‐functionality without sacrificing facile solution processability, making them promising candidates for multifunctional devices like dual EC/EL displays. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 147–153     

Toward low‐band gap dithienophosphole copolymers for an application in organic solar cells

Two functionalized dithieno[3,2‐b:2′,3′‐d]phospholes with solubilizing groups have been synthesized that allow for the generation of a series of π‐conjugated AB‐ and ABC‐copolymers. The polymers obtained show notable optoelectronic properties with red‐shifted absorption and emission in the orange to red section of the optical solar spectrum. Although combination of dithienophosphole units with fluorene building blocks gives access to processable polymers with band gaps between 2.2 and 2.3 eV in solution and 2.0 eV in the solid state, an ABC copolymer based on dithienophosphole, fluorene, and bis(thienyl)benzothiadiazole units was found to not only exhibit a suitable band gap for solar cell applications (solution: 2.0 eV; solid state: 1.7 eV) but also showed good solubility as well as good electron transfer properties in the presence of fullerene (C₆₀). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8179–8190, 2008     

Structure modification and annealing effect of polymer bulk heterojunction solar cells based on polyfluorene derivatives

Three low bandgap polyfluorene copolymers containing a donor–acceptor–donor moiety have been synthesized via Suzuki and Stille polymerization reactions. Their bandgaps and molecular energy levels (highest occupied molecular orbital and lowest unoccupied molecular orbital) varied with different polymerization methods. The molecular weight of the copolymer increased significantly through copolymerizing with a monomer having a long alkyl side chain. In order to investigate their photovoltaic properties, polymer solar cell devices based on the copolymers were fabricated with a structure of indium tin oxide/poly(styrene sulfonic acid)‐doped poly(ethylene dioxythiophene)/copolymers:[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/LiF/Al under the illumination of AM 1.5G, 100 mW/cm². We found that the annealing temperature had a profound effect on the power conversion efficiency (PCE) of the devices with a blend of poly[9,9‐didodecylfluorene‐alt‐(bis‐thienylene) benzothiadiazole] (PF12‐TBT) and PCBM. The PCE of the solar cell based on PF12‐TBT/PCBM (1:4) annealing at 70 °C for 20 min was 4.13% with an open‐circuit voltage (V_oc) of 1.02 V, fill factor of 55.9%, and a short‐circuit current (J_sc) of 7.24 mA/cm². © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and characterization of benzobisthiazole based polymers as donor materials for organic solar cells

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Phosphine‐free direct arylation synthesis and self‐assembled nanostructure analysis of poly(3‐hexylselenophene)

Poly(3‐hexylselenophene)s (P3Hs) with high regioregularity (RR = 92–96%), that is, regioregular poly(3‐hexylselenophene)s (rr‐P3HSs), have been synthesized under the phosphine‐free direct arylation conditions in the presence of PdCl₂ as a precatalyst. rr‐P3HS with the high molecular weight (M_n ～ 10,000) was obtained as a result of screening of direct arylation conditions. Subsequently, the influences of primary structure, molecular weight (M_n = 3900–10,000) and regioregularity (RR = 57–96%), on optical properties and self‐assembled nanostructure of P3HS were investigated. X‐ray diffraction demonstrated that molecular weight, regioregularity, and preparation method of films dominate the crystallization behavior of P3HS. Among these parameters, it was evident that a high degree of regioregularity was the most fundamental contributor to achieve pure crystalline nanostructure. Furthermore, nanoassembly based on pure crystalline nanostructure, such as non‐woven fibrous and bundle‐like spherulitic self‐assembled nanostructures, was successfully prepared in rr‐P3HS, respectively, by appropriate modulation of the aforementioned parameters. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2749–2755     

Synthesis of D‐π‐A‐π type benzodithiophene‐quinoxaline copolymers by direct arylation and their application in organic solar cells

Conjugated copolymers based on benzodithiophene (BDT) derivatives and thiophene‐quinoxaline‐thiophene (TQT) segments represent an efficient class of light harvesting materials for organic photovoltaic (OPV) applications. Commonly, BDT‐TQT copolymers are synthesized by Stille cross‐coupling polymerization. In this study, alkoxy and thienyl functionalized alternating BDT‐alt‐TQT copolymers are synthesized by direct arylation polymerization (DArP), using Ozawa conditions. An extensive optimization of the reaction conditions such as the catalytic system, solvent, temperature, base, and the concentration of the catalyst is accomplished. The optical and electrochemical properties of the copolymers obtained by DArP are compared to the reference polymers synthesized by Stille cross‐coupling polymerization. Finally, the optimized BDT‐alt‐TQT copolymers are incorporated into organic solar cells as electron donors. The solar cells of the DArP copolymers exhibit power conversion efficiencies up to 80% (rel.) of their Stille cross coupling analogues. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1457–1467     

Synthesis of the (X-DADAD)n-type conjugated polymers with 2,1,3-benzoxadiazole acceptor blocks and their application in organic solar cells

Two polymers with benzoxadiazole acceptor units were synthesized and investigated as electron donor materials in organic solar cells. Variation of the alkyl substituents was shown to significantly affect the optoelectronic properties of the polymers. In particular, the polymer HOMO energy level was lowered by 0.1 eV, while maintaining the same band gap, by replacement of the 2-ethylhexyl side-chains with the 2-hexyldecyl group. This modification also resulted in a higher open circuit voltage of the solar cells.