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synthetic metals, 9 (1984) 77 - 86 77 synthesis and properties of chemically coupled poly(thiophene) m
ForsythEnhanced properties in chemically polymerized poly ..
Traditional DArP utilizes the successive coupling of conjugated monomers, whereby the activation of a C–H bond of one monomer reacts with a typically electrophilic functionality, such as an aryl halide, on another monomer (). For a homopolymer, a DArP-type monomer adopts an AB-type configuration where A and B represent ideally orthogonal reactive groups. For the purpose of DArP, at least one is a hydrogen, while the other is often a halide. In the case of copolymers, monomers may also adopt an AA- and BB-type configuration where C–H activation only occurs on one of the monomers. Many studies for DArP have been performed on homopolymers, for example, poly-3-alkylthiophenes (P3AT). Owing to their ubiquitous appearance within the semiconducting polymer field, well-studied characterization, and well-understood reactivities, thiophene monomers are an understandably popular choice for exploring fundamental aspects of DArP. An early report from Lemair et al. was published in 1999, where 2-iodo-3-alkyl-thiophenes were reacted in accordance to Heck’s method using Jeffery conditions with Pd(OAc)2 in the presence of K2CO3 as base and tetrabutylammonium bromide (TBAB) as a phase transfer catalyst. Optimized conditions gave poly(3-hexylthiophene) (P3HT) in good yields (up to 93%) though the reported molecular weights (n = 2.9–3.1 kg/mol) and regioregularities (%RR 2CO3 as base, in superheated THF, were the keys to synthesize P3HT with high molecular weights, 31 kg/mol, and high regioregularity (%RR More recent studies have taken advantage of carboxylic acids acting as a proton shuttle, facilitating reactivity and improving the properties of the polymers. These early DArP studies continue to influence general reaction conditions applied to newer systems, which have sought to incorporate a variety of solvents and ligand types at milder heating conditions. As the substrate scope increases, it is important to still maintain control over important properties like molecular weight and regioregularity, both important in creating functional materials. Undesirable deviation from either can be attributed to some degree to the presence of in situ polymer defects, which is a key consideration to the study and development of any DArP system.
Four thiophene-based monomers have been synthesized by Stille- or Suzuki-type couplings followed by chemical or electrochemical polymerization into microporous polymer networks (MPNs) with high BET surface areas (BET). Similar BET values of up to 2020 and 2135 m2 g–1 have been determined for tetraphenylmethane-cored bulk MPN powders and thin films, respectively. Electrochemical polymerization in boron trifluoride diethyl etherate (BFEE)/dichloromethane (DCM) mixtures allows for the generation of MPN films with optimized porosity. Moreover, an interesting effect of boron trifluoride on the connectivity of the monomeric units during electropolymerization is observed for 3-thienyl-based monomers. Finally, the electrochemical reduction of 1,3,5-trinitrobenzene at MPN-modified glassy carbon (GC) electrodes shows increased cathodic responses compared to nonmodified GC electrodes due to interaction between electron-deficient nitroaromatic analyte and electron-rich MPN film. The influence of the specific surface area of MPNs on the electrochemical response is also studied for this class of materials.
thiophene): optical properties and electrical ..
Homocoupling, which is well-known from traditional cross-coupling chemistry, has a major impact on homopolymers with asymmetric repeat units as well as polymer yield and dispersity. Thiophenes, which are not just used in homopolymers but also as linkers in copolymers, have a tendency to form homocoupling byproducts as a result of their relative ease of reactivity. In P3HT synthesis, this side reaction notoriously leads to low regioregularity and thus distortion of the polymer backbone. This intrachain defect has a detrimental effect on the device performance of such materials due to consequences imparted onto the interchain packing. Thompson et al. have optimized reaction conditions for the synthesis of P3HT by carefully varying catalyst loading, temperature, and the type of carboxylic acid proton shuttle. They succeeded in preparing P3HT with properties comparable to those synthesized by Stille cross coupling, exemplifying the feasibility of DArP as a potential replacement technique. For copolymers, homocoupling can often manifest itself in reduced yield or molecular weight, which leads to necessarily using heavy excess equivalence of one comonomer, an undesirable effect on atom economy.
Although C–H bonds have traditionally been regarded as chemically inert, in the last three decades, chemists from various disciplines have started to investigate utilization of the C–H bond in chemical transformations, especially in the formation of new C–C bonds. To achieve this, C–H bonds are often activated by a metal catalyst and coupled with an electrophilic carbon atom such as a (hetero)aryl halide (C–H/C–X coupling) to achieve what is typically called direct arylation. Other direct arylations, which proceed in the presence of an oxidant, can include a nucleophilic carbon (C–H/C–M coupling) or another C–H “functional group” (C–H/C–H coupling). In the past two decades, this C–H activation chemistry has gained great momentum, and nowadays, C–H bonds are widely used in an effort to streamline syntheses in pharmaceutical drug synthesis and eliminate toxic wastes arising from often unstable organometallic reagents found in classical cross-coupling reactions. Extending these reactions to aryl monomers, we can begin to think about applying direct arylation as a powerful tool in the synthesis of conjugated homo- and copolymers for materials chemistry (). Such polymers with a conjugated π-system in the backbone are used as active materials in the field of organic electronics and are of particular interest as semiconductors in organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaic (OPV) devices.
Electron spin resonance determination of the magnetic ..
Kobayashi M, Chen J, Chung TC, Moraes F, Heeger AJ, Wudl F. Synthesis and properties of chemically coupled poly (thiophene). Synth met. 1984, 9:77-86
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.
of chemically coupled poly(thiophene) ..
A Simple Method to Prepare Head-to-Tail Coupled, Regioregular Poly ..
Synthesis and Photovoltaic Properties ..
Following a ..
Review on Structure, Properties and Appliance of …
Thiophene-Based Microporous Polymer Networks via …
Significant progress has been made to develop DArP methods leading to truly clean polymers and thus minimize polymer defects arising from homocoupling and branching. Nevertheless, the still remaining low degree of control over molecular weight, dispersity, and end group functionality as well as a resulting batch-to-batch variation is an issue that needs to be addressed. In traditional coupling chemistry, specifically for homopolymers such as P3HT, Kumada Catalyst-Transfer Polymerization (KTCP) is an established controlled polymerization method for the synthesis of highly regioregular P3HT of defined molecular weight and dispersities close to 1 (). The reason for this great efficiency lies in the living character of this polymerization method arising from a stable complex formed by the growing polymer chain and the catalytic nickel species preventing the catalyst from dissociation. The hereby-used Grignard-type monomer is commonly prepared from 2-bromo-5-halo-3-hexyl-thiophene requiring additional synthetic steps prior to polymerization. It is important to recognize such a utilitarian traditional polymerization method in order to propose an overall goal toward which we can aspire to develop DArP to be competitive.
Thiophene-Based Microporous Polymer Networks via Chemical or ..
Heteropolymers of the donor–acceptor type are typically synthesized from two symmetric monomers (). Homocoupling of these monomers can be particularly problematic because defects in the conjugated backbone change the sequence of donor–acceptor, which might influence the electronic properties of the materials. In general, it is advantageous to use aryl halides such as fluorenes, carbazoles, or isoindigo because they seem to be more inert to homocoupling and β-branching than thienyl halides. On the other hand, C–H activation is often performed with α-C–H bonds of sulfur containing five-ring heterocycles such as thiophenes or thiazoles. The possibility of C–H activation of electron-deficient monomers with electron-withdrawing groups would be particularly useful since transformation into a (hetero)aromatic halide is often more troublesome (). This strategy has been used for example with 1,2,4,5-tetrafluorobenzene 4 or 2,1,3-benzothiadiazole 5. Interestingly, in the case of monomer 4, there have been multiple comparative studies illustrating how, depending on the catalyst and reaction conditions (Fagnou-derived or Ozawa-derived conditions), choosing to brominate the electron-deficient species can lead to better polymer yields. These variations highlight the intricacies and sensitivities in developing reaction conditions toward new substrates.
The McCullough Group - Research
Finally, in order to discover "good" materials manycompounds must be synthesized and many systematic NLOmeasurements need to be made in order to develop useful structure-propertycorrelation. Since in 1o band gap materials electrons andnuclear motions are strongly coupled through highly polarizableelectrons we let the nuclei move with a vibrational motion andtry to locate the motions which are mostly effective in changingthe electronic delocalisation (i.e. conjugation). It turns outthat most of the conjugated systems (especially one-dimensionaloligomers andlor polymers) perform a collective motion (the socalled 5I mode or bond length alternation mode) for whichelectron-phonon coupling is the largest. The mode becomesselectively enhanced in the Raman spectra of"pristine"materials or generates the so called "doping inducedinfrared absorption" in doped systems.
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