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Thermally Promoted Elimination and Substitution Reactions in Poly(p-arylene vinylene) Precursor Polyelectrolytes
Abstract
The thermal conversion of dialkylsulfonium polyelectrolyte precursor polymers 1b−e to poly(p-arylene vinylene)s was studied by direct pyrolysis mass spectrometry in order to elucidate the mechanism of the sulfonium elimination reaction. The temperature dependence of the single ion currents of the four more abundant volatile products was correlated with the structural changes introduced in the arylene moiety of the polyelectrolytes and with the counterion nature. Experimental results suggest that the elimination reactions in the solid state proceed through an E2 mechanism.
In this work, we investigated the correlation between the inner structure of aqueous solution processed polymer:nanocrystals (NCs) hybrid solar cells and its performance. By altering the compositional concentration, annealing temperature, annealing time, and the thickness of the active layer, the diameter of CdTe NCs increased from 2.8 nm to tens of nanometers. Also, the performance of poly (1,4-phenylene vinylene) (PPV):CdTe hybrid solar cells changed accordingly. Systematical analysis revealed that the best performance of hybrid solar cells was achieved when CdTe was 30 mg/mL and the annealing temperature was above 250 °C with the active layer thickness of about 100 nm. The inner structure showed the size of CdTe NCs was around 26 nm, with the spacing of 6–12 nm. The data of transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed that the CdTe NCs grew mainly via dynamic coalescence.
Coordinated experimental and orthogonal space random walk (OSRW) molecular dynamics simulation studies were performed to understand the exceptionally low temperature required for thermal conversion of a poly(phenylenevinylene) (PPV) precursor containing the macromolecular counterion poly(ethylene glycol)-4-nonylphenyl-3-sulfopropyl ether (PEGNOPS). Simulations of this solvent-free system converged for starting points where the backbone was artificially expanded or compressed. In this example of a polymer organized by a polymer, the simulations revealed the ability of PEGNOPS to partially preorder the precursor chain in a conformation that favors the E2 elimination pathway.
Efforts are made to identify the irregular structures formed in poly(phenylene-vinylene) (PPV), obtained via the conversion of its sulfonium precursor by different physical and chemical methods. Analysis by X-ray photoelectron spectroscopy and Fourier transform infra-red spectroscopy reveals that the ketone structure is a common defect appearing in PPV whenever the conversion is carried out in the presence of oxygen. In contrast to the PPV obtained by pyrolysis of the precursor, the PPV obtained by treatment of the precursor with H2SO4 contains no chlorine residue but much more sulfur residue. Ion bombardment of the sulfonium precursor leads to a PPV containing no trace of sulfur or chlorine in the structure. Finally, the nature of bonding between the carbon atom and oxygen, chlorine or sulfur atoms in PPV is discussed and irregular structures are proposed.
Poly(p-phenylenevinylene), PPV, is one of the most promising candidates among the conjugated polymers for use in all-optical signal processing or as the active layer in light emitting diodes. Here, a low-temperature route to PPV is reported which makes the synthesis compatible with many of the thermoplastic polymers used in microelectronics and optics. The applicability of this route to device fabrication is also addressed.
Conjugated polymers such as poly(p-phenylenevinylene), (PPV, see Figure), show real promise in optoelectronic applications. A new synthetic route to improved PPV is presented, involving the thermal conversion of a precursor polymer containing rigid rod conjugated segments joined by flexible spacer groups. A high degree of interchain ordering results which influences the optical response of the material.
Several precursor polymer routes to poly(p-phenylenevinylene)4, poly[(2,5-dimethyl-p-phenylene)vinylene]10 and poly[(2,5-dimethoxy-p-phenylene)vinylene]15 are described. Base induced polymerisation of bis-sulfonium salt monomers afforded the polyelectrolytes 3 and 9 which were converted respectively directly into either polymer 4 or 10. Optimised conditions for the complete displacement of sulfonium groups by methanol to produce the corresponding methoxy precursor polymers 5, 11 and 17 are reported. The fully conjugated polymers obtained after heat and acid treatment of these precursors have been characterised and compared with those obtained via different precursor routes. The 1H NMR spectra of the precursor polymers, as well as IR and UV–VIS data of both the precursor polymers and the materials obtained after conversion into the poly(arylenevinylene)s are discussed.
The thermal conversion of poly(p-xylene-α-dimethylsulphonium halides) into poly(p-phenylene vinylene) (PPV) can occur through two concomitant reactions. The principal reaction is an elimination of dimethyl sulphide and halogen acid, while the second reaction, an undesirable one, is the nucleophilic attack of the halide counterion on a methyl group to form the methyl halide and a methyl sulphide. The two thermal processes were observed in the pyrolysis of both poly(p-xylene-α-dimethylsulphonium bromide) and poly(p-xylene-α-dimethylsulphonium chloride). On storage at room temperature, poly(p-xylene-α-dimethylsulphonium halides) undergo partial decomposition to PPV, as shown by thermogravimetric experiments. The flash pyrolysis-gas chromatography-mass spectrometry of PPV obtained by the thermal conversion of poly(p-xylene-α-dimethylsulphonium bromide) at 400°C was also investigated.
The rate of sodium methoxide-induced dehydroacetoxylation of the diesters p-RC6H4CH(OAc)CXHCO2Me (X = Br or Cl) has been studied as a function of the para-substituent. The results correspond to the para-substituent effects expected for proton abstraction from C-2, as estimated from the diastereoisomerisation of model compounds [p-RC6H4CH(OMe)CXHCO2Me] under similar conditions. This suggests that the elimination occurs via a carbanion mechanism, with C(2)–H bond-breaking rate-determining. The apparent insensitivity of the kinetic behaviour to configuration differences, together with the response of the reactions to the effect of the 3-substituent, is interpreted in terms of inductive stabilisation by the 2-halogen atom, which tends to localise the incipient negative charge at C-2, inhibiting distortion of its tetrahedral geometry.
The dimethylsulfonium polyelectrolyte precursor polymer 1 afforded high quality poly(p-phenylenevinylene), PPV, when the sulfonium elimination was performed under acidic conditions; it is proposed that the elimination of the sulfonium group to form a vinylene unit proceeds by an E1 mechanism in the solid state.
The first anthracene-containing conjugated polymer, poly(1,4-anthrylenevinylene) (PAV), was synthesized. Its processability was accomplished by using the precursor polymer route. Consideration of the mechanism responsible for the polymerization reaction led to selection of the adequate monomer to achieve polymerizability. UV spectra of PAV films showed a band-gap energy 0.3 eV smaller compared to that of poly(phenylenevinylene) (PPV) as a result of the minimization of the energy difference between aromatic and quinoid resonance structures by the anthracene unit. Model compound 13 was synthesized to exclude structural defects of PAV as well as to clarify the role of the steric and electronic effects on the conformation of the main chain. The optical properties of PAV thin films drastically changed upon reaction with a dienophile in the solid state.
Poly(xylylidenetetrahydrothiophenium) (PXT) precursors to poly(phenylenevinylene) (PPV) containing fluoride, chloride, bromide, and acetate counterions were prepared by ion-exchange dialysis of the chloride polyelectrolyte. Thermal gravimetric analysis (TGA) of cast films of PXT's indicates that elimination of the PXT acetate to PPV occurs at a much higher temperature than the halides. In situ conductivity measurements on sulfuric acid doped PPV's show that the identity of the anion has a marked influence on the limiting electronic conductivity. Studies on solution elimination kinetics reveal ion association of acetate with aqueous PXT. The products of elimination were followed by TGA coupled to IR analysis and were found to include tetrahydrothiophene, the conjugate acid of the anion, and water. Evidence for hydrogen bonding of water to PXT was found.
Computational studies of the stereochemistry of E2 reactions of ethyl derivatives, EtX, at MP2/6-31+G*//MP2/6-31+G* give the following results (X, base, ΔΔH(syn − anti in kcal mol-1)): F, OH-, 6.3; F, LiOH, −21.4; F, NaOH, −11.6; Cl, OH-, 8.0; Cl, LiOH, −18.7; Cl, NaOH, −12.4; NMe3+, OH-, −11.1; NMe3+, LiOH, 3.7. The activation barriers are much higher for the ion-paired bases than for hydroxide, reflecting their much weaker basicity. As deduced from results in solution, ion pairing of the base promotes syn elimination with negative leaving groups (F-, Cl-) but the reverse is true for neutral leaving groups (NMe3). The effects are strikingly larger in the gas phase than the typically modest ones found in solution. The effects of the leaving group and the base on the nature of the transition structure are discussed.
This paper reports that carbonyl moieties are present as defects, formed during the thermal conversion of a precursor to poly(p-phenylenevinylene) (PPV), a polymer used in light-emitting diodes. The increase in carbonyl groups can be correlated with a dramatic reduction of PPV photoluminescence. We have discovered that if the conversion is carried out in a reducing atmosphere, e.g., 15% hydrogen in nitrogen, the amount of carbonyls is substantially reduced and the photoluminescence intensity of the polymer increases as much as 5-fold. These results also have important implications for theories, advanced by other authors, of the effect of conjugation length or chain alignment on photoluminescence and electroluminescence in these systems.
The highly conjugated aromatic polymers, poly(2,5-dimethoxyphenylene vinylene) and poly(2,5-dimethylphenylene vinylene), were obtained from their water soluble, sulfonium salt precursor polymers. Films of these polymers were reacted with either AsF5 or I2 vapor. Poly(2,5-dimethoxyphenylene vinylene) showed increases in electrical conductivity of up to 14 to 15 orders of magnitude for these two dopants, while an 8 to 9 order of magnitude increase was observed for poly(2,5-dimethylphenylene vinylene) with the same dopants. The synthesis of the precursor polymers, the properties and elimination reactions of films of the precursors, the doping reactions, and the conductivities of the resulting phenylene vinylene films are discussed.
Preparation par elimination thermique du poly(naphtalenedimethylene-1,4-α, -dimethylsulfonium chlorure) prepare par reaction de NaOH avec le chlorure de bis-dimethylsulfonium du dimethyl-1,4 naphtalene
Two new types of p-xylene bis-sulfonium chloride monomers were prepared from cycloalkylene sulfides. The polymerization characteristics of these monomers to form poly(p-xylene sulfonium chlorides), and the thermal elimination reactions of their polymers to poly(p-phenylene vinylene), were compared with those of two monomers prepared from dialkyl sulfides. The cycloalkylene sulfonium chloride monomer polymerized to higher yields and to higher molecular weight polymers, which showed more efficient elimination reactions.
A very stable and accurate semi-integrated Mach-Zehnder interferometer setup, that was used to obtain the sign and magnitude of the nonlinear coefficient n2 of two different main chain organic poly(phenylene-vinylene) polymer derivatives in the wavelength region between 880–990 nm, is presented. The nonlinear refractive index coefficient n2 is of the order of 10−14 cm2/W for both polymers. Power dependent transmission measurements provided the two-photon absorption coefficient β. It ranges from almost zero to 0.16 cm/GW depending on the wavelength. The results show that the poly(phenylene-vinylene)-derivatives are promising candidates for opto-optical waveguide switching elements.
We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W−1 and peak brightnesses up to 90 000 cd m−2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.
High molecular weight poly(p-phenylene vinylene) (PPV) has been synthesized starting from the monomer p-xylylene-bis(dimethylsulphonium chloride). The latter was polymerized to yield a water-soluble sulphonium salt polyelectrolyte, which was converted to PPV by the thermal elimination of (CH3)2S and HCl from films cast from aqueous solution. The elimination reaction was studied by elemental analysis and by thermogravimetric analysis and mass spectrometry. The PPV films had good mechanical properties and could be n- and p-doped to yield material with electrical conductivities approaching those of highly doped polyacetylene. The degree of conversion of the intermediate polyelectrolyte to PPV could be controlled and the conductivities of these doped films could be related to the average conjugation length.
Poly(p-phenylene vinylene) (PPV) and its derivatives have recently received considerable attention in the field of electroactive organic materials chiefly because of their potential to replace the standard inorganic semi-conducting materials in light-emitting devices. Currently efforts are being made to reduce the carbonyl and hydroxyl defects in these materials which act as quenching sites for the injected charge. Thus, the understanding of the origin of these defects in PPV is extremely important in order to improve its electroluminescence efficiency. These defects are found to appear during the thermal conversion of PPV from its precursor. In this paper we have studied the thermal elimination reaction sequence in a PPV precursor using thermogravimetry and mass spectrometry and the observed results have been associated with standard reaction mechanisms to explain the formation of the aforementioned defects.