Cross-coupling Polymerization Research Articles

Homocoupling defects in diketopyrrolopyrrole-based copolymers and their effect on photovoltaic performance.

We study the occurrence and effect of intrachain homocoupling defects in alternating push-pull semiconducting PDPPTPT polymers based on dithienyl-diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT-TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5-5.6% for polymers containing 20% to 5% defects.

Fluorinated benzothiadiazole-based low band gap copolymers to enhance open-circuit voltage and efficiency of polymer solar cells

Two donor–acceptor (D–A) copolymers containing dithienosilole (DTS) donor unit and unsubstituted benzothiadiazole (BT) or fluorinated benzothiadiazole (ffBT) acceptor unit, PDTSBT and PDTSffBT, were synthesized by Stille cross coupling polymerization and tested for application in polymer solar cells (PSCs). The new alternating copolymer (PDTSffBT) possesses both a low optical bandgap (Eg=1.54eV) and a deep highest occupied molecular orbital energy level (HOMO) (−5.46eV). It was found that the introduction of the two electron-withdrawing fluorine atoms on the benzothiadiazole unit results in a decrease of the HOMO energy level with slight effect on the lowest unoccupied molecular orbital (LUMO) as observed through cyclic voltammetry (CV) analysis. Inclusion of fluorine atoms also leads to an increase in the interchain interaction in the PDTSffBT copolymer with respect to its analogue PDTSBT copolymer according to the X-ray diffraction measurements (XRD). When PDTSffBT:PC71BM blends are tested in bulk heterojunction (BHJ)-polymer solar cells, the cells display a short-circuit current (JSC) of 8.82mA/cm2, a high open circuit voltage (VOC) of 0.74V, and a Fill Factor (FF) of 45%, giving an overall power conversion efficiency (PCE) of 2.93%, compared to 1.64% for the BT-containing cells prepared in parallel under identical conditions. The significant performance enhancement results from the higher VOC and JSC in the ffBT-containing cells. These results unambiguously indicate that the fluorination is an efficient method to modulate the energetic levels and improve the photovoltaic performances of the widely used benzothiadiazole-based low bandgap copolymers.

Low-bandgap copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges

Two donor–acceptor-type alternating copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges were synthesized by Suzuki cross-coupling polymerization, and their optical, electrochemical, and photovoltaic properties were compared. The selenophene π-bridged copolymer (PCz-DSeBSe) exhibited a smaller band-gap (1.82 eV) than the thiophene-bridged polymer (PCz-DTBSe; 1.89 eV). PCz-DSeBSe also showed a deeper highest occupied molecular orbital energy level (−5.36 eV) than PCz-DTBSe (−5.20 eV). Moreover, the PCz-DSeBSe thin film showed higher crystallinity and hole mobility than the PCz-DTBSe thin film. Organic photovoltaic devices were fabricated using the polymers as the donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. The device using PCz-DSeBSe showed a higher open circuit voltage (Voc), short circuit current density (Jsc), and power conversion efficiency (PCE) than that using PCz-DTBSe. The fabricated indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/PCz-DSeBSe:PC71BM/LiF/Al device showed the maximum PCE of 2.88% with a Jsc of 7.87 mA/cm2, an Voc of 0.80 V, and a fill factor of 0.50 under AM 1.5G irradiation (100 mW/cm2).

Synthesis, characterization, optical and electrochemical properties of cyclopentadithiophene and fluorene based conjugated polymers containing naphthalene bisimide

Cyclopentadithiophene and fluorene based conjugated polymers containing naphthalene bisimide were synthesized through Suzuki cross-coupling polymerization. The resulting polymers exhibit good thermal stabilities and low crystallinities measured by thermogravimetric analysis and differential scanning calorimetry. The optical and electronic properties of polymers can be examined by UV–Vis spectroscopy and cyclic voltammetry. The UV–Vis absorption maxima of cyclopentadithiophene and naphthalene bisimide containing polymers showed longer wavelength than those of fluorene and naphthalene bisimide containing polymers. This is due to the stronger electron donating property of cyclopentadithiophene in the polymer backbone. The absorption maximum of cyclopentadithiophene and naphthalene bisimide containing polymers was red-shifted in comparison with those of low molecular weight analogous. Alternating polymers containing cyclopentadithiophene and naphthalene bisimide moieties showed a low band gap relative to that of the fluorene and naphthalene bisimide containing polymers presumably there is a strong charge transfer interaction between the electron-donating cyclopentadithiophene unit and the electron-accepting naphthalene bisimide.

Novel Thiophene–Phenylene–Thiophene Fused Bislactam-Based Donor–Acceptor Type Conjugate Polymers: Synthesis by Direct Arylation and Properties

Three new donor–acceptor copolymers based on thiophene–phenylene–thiophene fused bislactam and various donors (3,4-dodecylthiophene, 4,4′-didodecyl-2,2′-bithiophene, and ethylenedioxythiophene) were synthesized, characterized, and used in field-effect transistors. Polycondensation was performed using nonactivated thiophene derivatives by employing palladium-catalyzed direct arylation under phosphine-free conditions. This method is superior to traditional cross-coupling polymerization because it requires fewer synthetic operations and does not employ toxic organometallic intermediates. Regioselective polymers can also be generated by using β-substituted thiophene derivatives. The studied polymers were tested in a bottom gate top contact thin film transistor (OTFT) architecture. The best electronic performance was shown by polymer P3, with enhanced π-conjugation due to the appearance of intramolecular attractive interactions.

Synthesis of water-soluble, fluorescent, conjugated polybenzodiazaborole for detection of cyanide anion in water

A polymer sensor (P1) containing benzodiazaborole moieties for recognition of cyanide anion and sulfonate groups for water-solubility was synthesized by Suzuki cross-coupling polymerization, which enables cyanide detection in aqueous solution. The polymer showed visible fluorescence quenching upon exposure to cyanide anion concentrations as low as ∼2 μM. Selective detection of cyanide anions can be accomplished via fluorescence quenching, while other anions exhibited fluorescence enhancement upon exposure, mainly due to the difference in the nucleophilicity of anions, affecting benzodiazaborole group.

Selective detection of cyanide by a polyfluorene-based organoboron fluorescent chemodosimeter

Organoboron-based polymeric fluorescent probes have been designed and synthesized for the sensitive and selective detection of toxic cyanide ions by a fluorescence “turn off” mechanism. Bis-(bromohexyl)-polyfluorenes have been synthesized by Pd catalyzed Suzuki cross-coupling polymerization. Its treatment with (Mes)2BF achieved organoboron-appended polyfluorene (PFBP). PFBP that is soluble in common organic solvents shows bright blue luminescence under UV-irradiation. The substantial quenching of the luminescence of a PFBP solution in the presence of toxic cyanide ions (CN−) has been observed. This is due to the formation of strong covalent bonds between the cyanide ions and trivalent boron centres of the side chains of the PFBP, which is further supported by NMR studies. The polyfluorene backbone of the PFBP acts as the fluorophore, and the boron centre as the cyanide acceptor. This novel type of polymeric material will find a potential application in sensor technology, as the detection limit is very low at ∼0.5 μM.

T-Bu3P-Coordinated 2-Phenylaniline-Based Palladacycle Complex as a Precatalyst for the Suzuki Cross-Coupling Polymerization of Aryl Dibromides with Aryldiboronic Acids

t -Bu3P-Coordinated 2-phenylaniline-based palladacycle complex, [2'-(amino-kN)[1,1'-biphenyl]-2-yl-kC]chloro(tri-t-butylphosphine)palladium, as a general, highly efficient precatalyst for the Suzuki cross-coupling polymerization of aryl dibromides with aryldiboronic acids is described. Such t-Bu3P-coordinated 2-aminobiphenyl-based palladacycle complex-catalyzed Suzuki cross-coupling polymerization afforded polymers within an hour, with the yields and the molecular weights comparable to or higher than that of polymers obtained by using other palladium catalysts with much longer polymerization time. Our study provided a highly efficient catalyst system for the Suzuki cross-coupling polymerization of aryl dihalides with aryldiboronic acids. Our study also paved the road for us to investigate other monodentate ligand-coordinated palladacycle complexes including N-heterocyclic carbine-coordinated ones for cross-coupling polymerizations.

Electron Acceptors Based on an All‐Carbon Donor–Acceptor Copolymer

The Sonogashira cross-coupling polymerization of a dibrominated cyclopenta[hi]aceanthrylene and a diethynylfluorene derivative produced a donor–acceptor copolymer composed solely of cyclopenta-fused polycyclic aromatic hydrocarbons. The resulting polymer displays low band gaps (<1.5 eV), dual absorption bands, and electron-accepting behavior as demonstrated through fluorescence quenching of poly(3-hexylthiophene).

Thieno-, Furo-, and Selenopheno[3,4-c]pyrrole-4,6-dione Copolymers: Effect of the Heteroatom on the Electrooptical Properties

New push–pull conjugated polymers based on furo[3,4-c]pyrrole-4,6-dione (FPD) and selenopheno[3,4-c]pyrrole-4,6-dione (SePD) have been synthesized and compared with their thieno[3,4-c]pyrrole-4,6-dione (TPD) analogues to investigate the effects of the heteroatom on the electrooptical properties. The copolymers were synthesized using either Stille cross-coupling or direct heteroarylation polymerization (DHAP), the latter method giving high molecular weights. Hypsochromic shifts of the band gaps were observed for FPD-based copolymers (sulfur substituted by oxygen) while small bathochromic shift was observed for SePD (sulfur substituted by selenium) when compared to its TPD analogue. These two new classes of conjugated polymers exhibit electrooptical properties that could lead to interesting bulk heterojunction plastic solar cells.

Controlled Pd(0)/t-Bu3P-Catalyzed Suzuki Cross-Coupling Polymerization of AB-Type Monomers with PhPd(t-Bu3P)I or Pd2(dba)3/t-Bu3P/ArI as the Initiator

Controlled Pd(0)/t-Bu(3)P-catalyzed Suzuki cross-coupling polymerizations of AB-type monomers via the chain-growth mechanism with an ArPd(t-Bu(3)P)I complex as the initiator are described. ArPd(t-Bu(3)P)I complexes, either prepurified or generated in situ from Pd(2)(dba)(3)/t-Bu(3)P/ArI (dba = dibenzylideneacetone) without separation/purification, were found to be efficient initiators in general for the controlled Suzuki cross-coupling polymerization, with narrow polydispersity indexes (PDIs) of 1.13-1.35 being observed. The Pd(2)(dba)(3)/t-Bu(3)P/p-BrC(6)H(4)I combination was identified as a highly robust initiator system, with PDIs of ≤1.20 in general and as low as 1.13 being obtained. Higher number-average molecular weights (M(n)) were achieved without a significant increase in the PDI (from 1.14 for a polymer with a M(n) = 9500 to 1.20 for a polymer with M(n) = 31,400) by using a smaller amount of the Pd(2)(dba)(3)/t-Bu(3)P/p-BrC(6)H(4)I initiator in the polymerization.

An Organozinc Route for the Preparation of Functionalized Poly-3-alkylthiophenes

In 1980, two groups reported the synthesis of unsubstituted polythiophene utilizing 2,5-dibromothiophene via a Ni-catalyzed method. Since then, several different types of catalysts were employed for the polymerization of 2,5dibromothiophene. Unfortunately, low molecular weight polymers generally resulted from these attempts. Even though these methods provided interesting materials, there was a big barrier to utilize this polymer due to the lack of processability. In order to overcome this difficulty, an alkyl chain was introduced on 3-position of the thiophene ring. Of the many methods, metal-catalyzed cross-coupling polymerization was most frequently used, and the synthesized polythiophenes containing an alkyl group equal or greater than C4 are soluble in common organic solvents. This wellestablished method, however, has significant drawbacks generating inconsistent results and regiorandom polymers. As is known, the regioregularity of the 3-substituted polythiophene plays a critical role in electronic and optical devices. Consequently, developing the efficient methods of preparing highly regioregular head-to-tail poly-3-alkylthiophenes is of high value. This goal was achieved by two research groups, seperately. Rieke and coworkers reported a synthetic approach to the preparation of regioregular poly-3alkylthiophene utilizing thienylzinc bromide. The other synthetic method, the McCullough method, was performed by using the Kumada cross-coupling method. Even though extensive studies have focused on the preparation and application of the linear alkyl-substituted polythiophenes, there is an increasing need to develop various types of polythiophenes because these polymers have a wide range of potential possibilities of being used in material chemistry. Recent reports also showed that the chemical composition of the P3HT significantly affect the device performance. Therefore, many efforts were devoted to finding new strategies for the preparation of functionalized P3HTs. One of the first attempts was to introduce a heteroatom on the thiophene ring. This has been accomplished wherein the thiophene has an oxygen atom directly attached to the ring. Furthermore, polythiophenes containing other functionalities such as an alcohol, ester, and urethane were prepared. In addition, thiophenes bearing a carboxylate or sulfonate functionality were introduced for the synthesis of water-soluble polythiophenes. More importantly, a recent report showed that P3HT-phosphonic ester was employed to improve the power conversion efficiencies in hybrid solar cells. We have developed a versatile synthetic method for polythiophenes bearing a phosphonic ester functionality on the 3-position. In addition, we wish to report more results obtained from the study on the preparation of polythiophenes bearing several different functional groups. In 2003, McCullough et al. reported the synthesis of poly[3-(6-diethylphosphorylhexyl)thiophene] utilizing the Stille method. This synthesis required a difficult monobromination as well as cryogenic condition (−70 C) for the preparation of the reactive organometallic intermediate. This study also pointed out that the GRIM and Kumada methods are not reliable for the preparation of polythiophenes containing the phosphonic ester moiety. Although poly[3(11-diethylphosphorylundecyl)thiophene] was also synthesized by Kowalik and Tolbert using an organozinc route, we have focused on a more practical synthetic method. It was accomplished starting with readily available 2,5-dibromo-3-(6-bromohexyl)thiophene (4), which is easily prepared by known literature methods. The use of this route provided a huge advantage over the previously reported studies requiring a regioselective monobromination of thiophene ring. As shown in Scheme 1, in our study, the phosphonate ester group was easily provided by the MichaelisArbuzov reaction with 4 which was readily obtained by the simple bromination of 3 with NBS. The oxidative addition of active zinc to 5 was completed in 20 min at 0 C showing 75:25 ratio regioselectivity. The resulting mixture of thienylzincs (6 and 7, Scheme 1) were polymerized in the presence of 0.3 mol % of Ni(dppe)Cl2 at room temperature yielding the title polymer (P1) in 93%. The UV-vis analysis exhibited a λmax of 440 nm in chloroform solution. This is compatible with the literature value (λmax 442 nm). 11a

Synthesis and photovoltaic properties of low band gap quarterthiophenes-alt-diketopyrrolopyrroles polymers having high hole mobility

Two low band gap copolymers (PQTDPP and PQTVTDPP) having quaterthiophene (QT) or bithiophene-vinylene-bithiophene (QTVT) as donor unit and diketopyrrolopyrrole (DPP) as acceptor unit have been synthesized via Stille cross-coupling polymerization. The polymers showed high thermal stability and broad light absorption with optical band gaps around 1.3eV. Cyclic voltammetry measurements revealed that the HOMO and LUMO energy levels of QT and DPP copolymer (PQTDPP) were found to be −5.19 and −3.64eV, respectively and those of PQTVTDPP −5.05 and −3.66eV, respectively. X-ray diffraction patterns showed highly crystalline phase in the PQTDPP film and nearly amorphous in the PQTVTDPP film. This dramatic difference in crystallinity was attributed to the structural isomerization around vinyl groups in the PQTVTDPP polymer. Field-effect transistor measurement revealed that the PQTDPP showed high hole mobility of 0.06cm2/Vs and on/off ratio of 106 while the PQTVTDPP showed hole mobility of 3.57×10−4cm2/Vs and on/off ratio of 105, which is consistent with X-ray diffraction results. Photovoltaic devices were also fabricated using polymer:PCBM blends in the structure of ITO/PEDOT:PSS/blend/Al. The PQTDPP devices exhibited an open circuit voltage (Voc) of 0.58V, a short circuit current (Jsc) of 7.16mA/cm2, a fill factor (FF) of 0.55 and a power conversion efficiency (PCE) of 2.28%, and the PQTVTDPP devices exhibited an Voc of 0.50V, a Jsc of 7.60mA/cm2, a FF of 0.51 and a PCE of 1.97% under AM 1.5 illumination.

Synthesis and characterizations of benzotriazole based donor–acceptor copolymers for organic photovoltaic applications

Donor–acceptor alternated conjugated copolymers based on benzotriazole acceptor units and electron-donating units such as fluorene, carbazole and silyl fluorene, were synthesized through palladium catalyzed Suzuki cross-coupling polymerization. NMR and elemental analyses were achieved to confirm the structure of each copolymer. Their thermal, photophysical and electrochemical properties are also reported. The photovoltaic characteristics of these polymers determined from bulk-heterojunction photovoltaic cells are discussed.

Norbornadiene End-Capping of Cross-Coupling Polymerizations: A Facile Route to Triblock Polymers.

The potential use of conjugated polymers in device applications is often limited by their less than optimal physicochemical properties. This work describes an efficient protocol to end-cap conjugated polymers synthesized via palladium-catalyzed cross-coupling polymerizations with norbornene groups. Specifically, the hydroarylation of norbornadiene is shown to be a high-yielding end-capping method. These strained bicyclic alkenyl end groups can be transformed into macroinitiators via ring-opening metathesis polymerization and can polymerize other strained monomers, such as norbornene, yielding elastomeric triblock copolymers.

Copolymers from naphtho[2,3-c]thiophene-4,9-dione derivatives and benzodithiophene: synthesis and photovoltaic applications

A series of donor–acceptor (D–A) copolymers from a benzodithiophene (BDT) donor unit and a naphtho[2,3-c]thiophene-4,9-dione (NTDO) acceptor unit with different side chains, PBDTNTDO-C1, PBDTNTDO-C2 and PBDTNTDO-C3, were synthesized by a standard Stille cross-coupling polymerization. The thermal, optical and electrochemical properties of the copolymers were well investigated. Preliminary investigations of the copolymers based on the device structure of ITO/PEDOT : PSS/polymer: PC71BM (1 : 2)/Ca/Al showed power conversion efficiencies (PCEs) of 1.96% for PBDTNTDO-C1, 1.01% for PBDTNTDO-C2 and 2.21% for PBDTNTDO-C3 under the illumination of AM1.5, 100 mW cm−2.

Effect of ligand electronic properties on precatalyst initiation and propagation in Ni-catalyzed cross-coupling polymerizations

The role of ligand-based electronic effects was investigated in the Ni-catalyzed polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. The catalyst with the most electron-donating ligand outperformed the other catalysts by providing polymers with narrower molecular weight distributions. This result is attributed to both a suppression of competing reaction pathways (e.g., chain transfer and termination) as well as a relative acceleration of precatalyst initiation compared to propagation. Further studies revealed that, for all three catalysts, precatalyst initiation is slower than propagation, despite the fact that they exhibit the same rate-determining steps (i.e., reductive elimination) and have similar catalyst resting states. These results suggest that better control over the polymer molecular weight, end-functionality and sequence can be obtained with electron-rich catalysts, such as those described herein.

Synthesis and Photovoltaic Properties of a Thienylenevinylene and Diketopyrrolopyrrole Copolymer with High Mobility

A novel donor-acceptor-type polymer with a low band-gap that alternates electron-rich thienylenevinylene groups with electron-deficient diketopyrrolopyrrole (DPP) units (PETVTDPP) has been synthesized by Pd-catalyzed Stille cross-coupling polymerization. The polymer shows a broad absorption band of wavelengths that range from 330 to 900 nm, and a low band-gap value of 1.43 eV. The field-effect mobility of an organic thin-film transistor based on this polymer is 0.05 cm(2 ) · Vs(-1) . Bulk-heterojunction solar cells using a mixture of PETVTDPP and PC[71] BM for the active layer show a power conversion efficiency (PCE) of 1.94% under simulated AM 1.5 G solar irradiation at 100 mW · cm(-2) .

Effect of thiophene donor units on the optical and photovoltaic behavior of fluorene-based copolymers

A series of novel conjugated polymers containing alternating electron-donating and electron-accepting units based on 9,9-dioctylfluorene, 4,7-dithienyl-2-yl-2,1,3-benzothiadiazole, and (oligo)thiophene were synthesized. The polymers were synthesized by the Suzuki cross-coupling polymerization of 9,9-dioctylfluorene-2,7-diboronic acid and 4,7-di(2-bromothien-5-yl)-2,1,3-benzothiadiazole with dibromo(oligo)thiophene (thiophene, bithiophene, and terthiophene). Optical properties of the copolymers were highly dependent on the length of the (oligo)thiophene unit. With the incorporation of three thiophene units in the polymer backbone, the copolymer absorption covers a broad range of the visible spectrum from 300 to 700 nm. The band gap energies derived from the absorption edge of the thin film spectra were in the range of 1.83–1.94 eV. The photovoltaic performance increases as the length of the (oligo)thiophene segments in the polymer backbone increases. The best performance of photovoltaic device was obtained by PFTBzTTT as the electron donor material with PCE of 1.25% under an AM 1.5 solar simulator.

Poly(3-hexylthiophene)-block-poly(pyridinium phenylene)s: Block Polymers of p- and n-Type Semiconductors

Conjugated crystalline−crystalline donor−acceptor−donor block copolymer semiconductors, with regioregular poly(3-hexylthiophene) as a donor (p-type) block and poly(pyridinium pheneylene) as an acceptor (n-type) block within the backbone, were produced by sequential Grignard metathesis synthesis of poly(3-hexylthiophene), a Yamamoto-type cross-coupling polymerization−cyclization sequence. These conjugated block copolymers are soluble in organic solvents and display broad optical absorption bands extending close to the near-infrared region. They show reversible ambipolar redox properties with high electron affinities of 3.8−4.0 eV as well as useful ionization potentials of 5.1 eV that are characteristic of the respective blocks. Block copolymers from p- and n-type organic semiconductors are of interest for the formation of nanostructured bulk heterojunctions in photovoltaic devices.