Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes.

Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable toward RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe2 complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe2 complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10 °C to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand.

Carving Out Pores in Redox-Active One-Dimensional Coordination Polymers.

Reduction of the insulating one-dimensional coordination polymer [Cu(abpy)PF6 ]n , 1 a(PF6 ), (abpy=2,2'-azobispyridine) yields the conductive, porous polymer [Cu(abpy)]n , 2 a. Pressed pellets of neutral 2 a exhibit a conductivity of 0.093 S cm-1 at room temperature and a Brunauer-Emmett-Teller (BET) surface area of 56 m2 g-1 . Fine powders of 2 a have a BET surface area of 90 m2 g-1 . Cyclic voltammetry shows that the reduction of 1 a(PF6 ) to 2 a is quasi-reversible, indicative of facile charge transfer through the bulk material. The BET surface area of the reduced polymer 2 can be controlled by changing the size of the counteranion X in the cationic [Cu(abpy)X]n . Reduction of [Cu(abpy)X]n with X=Br (2 b) or BArF (2 c; BArF =tetrakis(3,5-bis(trifluoromethyl)phenyl)), affords [Cu(abpy)]n polymers with surface areas of 60 and 200 m2 g-1 , respectively.

Influence of ß-linkages on the morphology and performance of DArP P3HT-PC61BM solar cells.

Direct arylation polymerization (DArP) has emerged as a greener and more atom-efficient alternative to Stille polymerization. Despite the attractiveness of this method, DArP is known to produce ß-linkages in polymers, which have ß-protons available for activation. Here, we report the influence of the ß-defect content in DArP poly(3-hexylthiophene) (P3HT) on the performance of bulk-heterojunction solar cells and the morphology of pristine polymers and their blends with PC61BM in thin films and compare with Stille P3HT containing 0% ß-defects as a reference point. The optical and electrochemical properties as well as the hole mobilities of pristine polymers remain virtually the same when the amount of ß-defects is limited to 0.75% or lower, as evidenced by UV-visible absorption spectra, cyclic voltammetry and space-charge-limited current (SCLC) mobility measurements. However, an increase of ß-defect concentration to 1.41% significantly affects the oxidation onset, UV-visible absorption profile and hole mobility of P3HT. The key result of this study is that the photovoltaic performance of DArP P3HT with 0% ß-defects is remarkably close to that of Stille P3HT, whereas the performance of DArP P3HT with 0-0.75% ß-defects does not differ dramatically from that of Stille P3HT and could potentially be improved upon by individual optimization of the processing conditions.

Influence of selenophene on the properties of semi-random polymers and their blends with PC61BM.

In an effort to broaden the absorption of conjugated polymers, atomistic bandgap control was applied to the semi-random polymer architecture. Here, we report the physical properties of semi-random polyselenophenes as compared to analogous polythiophenes. In order to examine the effect of the selenium heteroatom on the optical properties of the polymers, UV-vis spectra were studied and it was found that all polyselenophenes exhibit lower bandgaps and higher absorption coefficients in thin films. Further, differential scanning calorimetry and grazing incidence x-ray diffraction results indicate that semi-random polyselenophenes are semicrystalline polymers and their (100) interchain distances are shorter than in the case of semi-random polythiophenes, which may be responsible for higher absorption coefficients. To probe the effect of the selenium heteroatom on the nano-organization of these polymers and their blends with PC61BM, thin films were studied by transmission electron microscopy (TEM). The TEM images show a segregation between more densely packed areas from less densely packed areas in the pristine polymer films, which is more pronounced for polyselenophenes than for polythiophenes. The blends of polyselenophenes with PC61BM do not show the well-defined segregation observed for the polythiophene analogues. However, the broadened and extended absorption of semi-random polyselenophenes translates into an extended photocurrent response in the photovoltaic devices, as evidenced by external quantum efficiency measurements.

Zwitterionic Ring-Opening Polymerization of N-Substituted Eight-Membered Cyclic Carbonates to Generate Cyclic Poly(carbonate)s.

The zwitterionic ring-opening polymerization of N-functionalized eight-membered cyclic carbonates with N-heterocyclic carbenes (NHC) in the absence of alcohol initiators generates cyclic polycarbonates of Mn ∼ 30-100 kDa. The polymerization behavior of these eight-membered cyclic azacarbonates depends sensitively on the nature of the nitrogen substituent. The N-benzyl-substituted eight-membered cyclic carbonate (8CCBn) polymerizes readily with 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene to generate cyclic polycarbonates with molecular weights of Mn = 14 000 to 96 000 Da. In contrast, the N-phenyl-substituted cyclic carbonate (8CCPh) catalytically dimerizes in the presence of the NHC to afford the crystalline cyclic dimer. The zwitterionic ring-opening copolymerization of δ-valerolactone (VL) and the cyclic carbonates afford gradient cyclic copolymers. The cyclic topology of both the homopolymers and copolymers was supported by MALDI-TOF MS and intrinsic viscosity measurements. 13C NMR and differential scanning calorimetry of the cyclic copolymers are indicative of a gradient sequence distribution as a consequence of the more rapid enchainment of the cyclic carbonates relative to valerolactone.

Contrasting performance of donor-acceptor copolymer pairs in ternary blend solar cells and two-acceptor copolymers in binary blend solar cells.

Here two contrasting approaches to polymer-fullerene solar cells are compared. In the first approach, two distinct semi-random donor-acceptor copolymers are blended with phenyl-C61-butyric acid methyl ester (PC61BM) to form ternary blend solar cells. The two poly(3-hexylthiophene)-based polymers contain either the acceptor thienopyrroledione (TPD) or diketopyrrolopyrrole (DPP). In the second approach, semi-random donor-acceptor copolymers containing both TPD and DPP acceptors in the same polymer backbone, termed two-acceptor polymers, are blended with PC61BM to give binary blend solar cells. The two approaches result in bulk heterojunction solar cells that have the same molecular active-layer components but differ in the manner in which these molecular components are mixed, either by physical mixing (ternary blend) or chemical "mixing" in the two-acceptor (binary blend) case. Optical properties and photon-to-electron conversion efficiencies of the binary and ternary blends were found to have similar features and were described as a linear combination of the individual components. At the same time, significant differences were observed in the open-circuit voltage (Voc) behaviors of binary and ternary blend solar cells. While in case of two-acceptor polymers, the Voc was found to be in the range of 0.495-0.552 V, ternary blend solar cells showed behavior inherent to organic alloy formation, displaying an intermediate, composition-dependent and tunable Voc in the range from 0.582 to 0.684 V, significantly exceeding the values achieved in the two-acceptor containing binary blend solar cells. Despite the differences between the physical and chemical mixing approaches, both pathways provided solar cells with similar power conversion efficiencies, highlighting the advantages of both pathways toward highly efficient organic solar cells.

Random Poly(3-hexylthiophene-co-3-cyanothiophene) Copolymers via Direct Arylation Polymerization (DArP) for Organic Solar Cells with High Open-Circuit Voltage.

A family of four poly(3-hexylthiophene) (P3HT) based copolymers containing 5, 10, 15, and 20% of 3-cyanothiophene (CNT) incorporated in a random fashion with a regioregular linkage pattern (P3HT-CNT) were successfully synthesized via direct arylation polymerization (DArP). Unique reaction conditions, previously reported for P3HT, were used, which employ very low loadings of Pd(OAc)2 as a catalyst and an inexpensive bulky carboxylic acid (neodecanoic acid) as an essential part of the palladium catalytic center. The chemical structures and optoelectronic properties of DArP P3HT-CNT polymers were found to be similar to those of previously investigated P3HT-CNT polymers synthesized via Stille polycondensation. All polymers are semicrystalline with high hole mobilities and UV-vis absorption profiles that resemble P3HT, while the polymer highest occupied molecular orbital (HOMO) level decreases with increasing content of cyanothiophene in both DArP and Stille P3HT-CNT polymers. In photovoltaic devices with a PC61BM acceptor, DArP P3HT-CNT copolymers showed slightly lower open-circuit voltages (Voc) than their Stille P3HT-CNT analogues but similar fill factors (FF) and significantly enhanced short-circuit current densities (Jsc), leading to overall power conversion efficiencies for the DArP polymers that rivaled or exceeded those of the Stille polymers. This work further emphasizes the generality and relevance of DArP for the synthesis of conjugated polymers for use in organic solar cells and the attractive simplicity and ease of synthesis of random conjugated polymers.

Influence of polymer compatibility on the open-circuit voltage in ternary blend bulk heterojunction solar cells.

The evolution of the open-circuit voltage (Voc) with composition in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of the polymers. Ternary blends based on poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) with phenyl-C61-butyric acid methyl ester (PC61BM) acceptor were investigated. The Voc is pinned to the lower value of the P3HTT-DPP-10%:PC61BM binary blend even up to 95% PCDTBT in the polymer fraction. This is in stark contrast to the previously investigated system based on P3HTT-DPP-10%, poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25), and PC61BM, where the Voc varied regularly across the full composition range, as explained by an organic alloy model, implying strong physical and electronic interaction between the polymers. Photocurrent spectral response (PSR) and external quantum efficiency (EQE) measurements indicate that the present system does not exhibit the hallmarks of alloy formation. Measured values of the surface energies of the polymers support miscibility of P3HTT-DPP-10% with P3HT75-co-EHT25 but not with PCDTBT. Surface energy is proposed as a figure of merit for predicting alloy formation and compositional dependence of the Voc in ternary blend solar cells and miscibility between polymers is proposed as a necessary attribute for polymer pairs that will display alloy behavior.