Description and importance of interchain excited states in conjugated polymer photophysics

We compare the photoluminescence (PL) of poly-p-phenyl-enevinylene (PPV) to that of one of its soluble derivatives, poly (2-methoxy, 5-(2¹-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV). We show that excimers due to π-orbital overlap of adjacent polymer backbones are formed in MEH-PPV in high quantum yield. They do not emit light at room temperature, potentially explaining the reduction of PL yield in MEH-PPV films relative to solution. The herringbone packing of polymer chains in PPV prevents π-π stacking, but low PL yields are observed anyway. We conclude that polymer design to inhibit interchain registration is a questionable strategy to increase luminescence yields. The low PL yield in PPV is ascribed to bound polaron pair formation and preliminary data suggest that these may also form upon photoexcitation of MEH-PPV films.     

Improved environmental stability in poly(p-phenylene vinylene)/layered silicate nanocomposite

Polymer-rich/clay-rich phase-separated, polymer-layered silicate nanocomposites are prepared through a solution blending of poly(xylylidene tetrahydrothiophenium chloride) with two kinds of layered silicates. By spin-coating, thin films of each phase were obtained, followed by thermal elimination of tetrahydrothiophene and HCl to final poly(p-phenylenevinylene) (PPV)/layered silicate nanocomposite. At ambient conditions, the PPV films are easily photodegraded due to the oxygen diffused in. Poly(p-phenylenevinylene)/layered silicate nanocomposite showed improved environmental stability against photodegradation under an air ambient condition. Additionally, optical absorption and photoluminescence measurements demonstrate that the thin films of clay-rich phase are much less photodegraded than those of polymer-rich and PPV reference.     

Non-isothermal elimination process in the solid state of n-alkyl-sulphinyl precursor polymers towards conjugated poly[2-(3′,7′-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene] studied with MTDSC and TGA

The elimination process of n-alkyl-sulphinyl precursor polymers towards conjugated poly[2-(3′,7′-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene], or OC₁C₁₀-PPV, was studied with modulated temperature differential scanning calorimetry (MTDSC) and thermogravimetric analysis (TGA), with a focus on the subsequent reactions of the elimination products. The latter reactions were monitored using the non-reversing heat flow and the heat capacity (C_p) measured in non-isothermal MTDSC experiments. The disproportionation reaction occurs in a temperature range between 85 and 135 °C and is seen as an increase in C_p. Water and elimination products released during the elimination reaction act as plasticizers and lower the T_g. TGA experiments show that the temperature, film thickness, and the eliminated group play an important role on the diffusion and evaporation of the elimination products. The elimination products can further decompose and interact with the conjugated system to form undesirable crosslinks (network formation) in a temperature range of 140-160 °C.     

The thermal conversion reaction of sulphonyl substituted poly(para-xylylene): evidence for the formation of PPV structures

The thermal elimination of the n-octyl-sulphonyl group from poly[1,4 phenylene (1-n-octyl-sulphonyl)-1,2-ethylene] and the phenyl-sulphonyl group from poly[1,4 phenylene (1-phenyl-sulphonyl)-1,2-ethylene] is evaluated by different analytical techniques: thermal gravimetric analysis, direct insert probe mass spectrometry, in situ ultraviolet-visible spectroscopy and in situ Fourier transform infra red spectroscopy. This thermal treatment yields PPV with improved optical properties compared to the conventional PPV concerning UV-absorption and photoluminescence efficiencies. The results obtained are consistent with the formation of a PPV with a more or less restricted conjugation length compared to the PPV material obtained from the conversion of a sulphinyl precursor polymer.     

Cationic phenyl-substituted poly(p-phenylenevinylene) related copolymers with efficient photoluminescence and synthetically tunable emissive colors

A series of amino-functionalized phenyl-substituted poly(p-phenylenevinylene) (PPV) related copolymers were synthesized by Wittig reaction. Their corresponding cationic conjugated polymers were successfully obtained via a post-polymerization approach. On the basis of FT-IR and ¹H NMR spectra, it was found that phenyl-substituted PPV related copolymers containing alkoxylated benzene (neutral polymer P1 and quaternized polymer P1′), phenylated benzene (neutral polymer P2 and quaternized polymer P2′) and fluorene (neutral polymer P3 and quaternized polymer P3′) moieties are of 55, 80, and 45% cis-vinylic linkage respectively while the polymer containing thiophene moiety (neutral polymer P4 and quaternized polymer P4′) is primarily of trans-vinylic linkage. Their photoluminescence (PL) were conveniently tuned from blue color to yellow color by introducing units with different optoelectronic properties into the PPV backbones. The polymer with fluorene unit and bulky phenylene-substituted benzene unit in the backbone exhibited the highest PL efficiency among these neutral and quaternized PPVs. P4′ containing little cis-vinylic linkage showed complete quenching while P1′-P3′ containing much more cis-vinylic linkage showed incomplete quenching, indicating that the quenching behavior of these cationic PPVs may be highly influenced by the content of cis-vinylic linkage in the PPV backbones.     

Growth of nanostructured thin films of poly(p-xylylene) derivatives by vapor deposition

Nanostructured thin films of poly(p-xylylene) derivatives are deposited by oblique angle vapor deposition method under low-vacuum conditions. We showed deposition of columnar nanostructured poly(chloro-p-xylylene) and poly(bromo-p-xylylene) thin films, and co-deposition of nanostructured poly(o-trifluoroacetyl-p-xylylene-co-p-xylylene) thin film. Characterization of both the nanostructured and planar thin films of poly(p-xylylene) are performed with different experimental methods. We developed a generalized strategy towards depositing nanostructured poly(p-xylylene) derivatives and thus promise a new generation of thin films suitable for biomedical and antifouling applications.     

Polymeric coatings for photostability enhancement of poly(p‐phenylene vinylene) derivative films

Poly(p‐phenylene vinylene) (PPV) derivatives are an important class of conjugated polymers, known for their applications as electroluminescent materials for light‐emitting devices and sensors. These derivatives are highly susceptible to photodegradation by the combined action of oxygen and light. Here, the use of various commercial polymers as protective coatings against the photodegradation of PPV derivatives was explored. Cast films of two similar PPV derivatives, poly[(2‐methoxy‐5‐n‐hexyloxy)‐p‐phenylene vinylene] and poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐p‐phenylene vinylene], were submitted to photodegradation by exposure to white light under atmospheric conditions in order to verify if the type of side chain (linear or branched) had an effect on the photodegradation. No significant differences in the photodegradation behaviour between the two polymers were noticed. The following commercial polymers were tested as protective coatings for the PPV derivative cast films: 99 and 80% hydrolysed poly(vinyl alcohol) (PVA) and starch. The best results were achieved using coatings of 99% hydrolysed PVA, which increased about 700 times the time necessary for complete degradation of the PPV derivative films. The results show the effectiveness of this coating in minimizing and, possibly, controlling the effects of the photodegradation of PPV derivative films, which can be useful in many applications, e.g. oxygen sensors. Copyright © 2009 Society of Chemical Industry     

Investigation of orientation phenomena in drawn films of poly[(±)-β-aminobutyric acid]

Orientation data of drawn films of poly[(±)-β-aminobutyric acid] (structural unit [NHCHCH₃CH₂CO]_n), are derived from the infra-red dichroism of the NH stretching vibration and wide angle X-ray diffraction measurements. In combination with accessibility data from deuteration experiments, these parameters are used to examine the applicability of two-phase polymer models for uniaxial orientation to the investigated polymer.     

Synthesis and electroluminescent property of poly(p-phenylenevinylene)s bearing triarylamine pendants

Two poly(p-phenylenevinylene) derivatives, TPA-PPV and TPA-CN-PPV, bearing a triarylamine pendant were synthesized by the Wittig-Horner and Knoevenagel reactions, respectively. The optical properties of the polymers were improved by energy transfer from the triarylamine pendant to the poly(p-phenylenevinylene) main chain. Cyclic voltammetry revealed that the HOMO energy level of the pendant TPA moiety is higher than that of the PPV main chain, suggesting that the hole injection from ITO into the TPA moiety, then from the TPA moiety into PPV is more favorable than directly from ITO into the PPV main chain. A single-layer light-emitting diode using TPA-PPV as the emitting layer exhibited a maximum brightness of 790 cd/m² at a bias of 8 V, while the control device employing commercially available MEH-PPV as the emitter showed a maximum brightness of 250 cd/m² under 11 V. The higher brightness combined with the higher current density was attributed to the effective hole injection and/or hole transport in the TPA-PPV-based device.     

Green-blue luminescence dichroism of cyano-containing poly[(m-phenylene ethynylene)-alt-(p-phenylene ethynylene)] aggregates dispersed in oriented polyethylene

A new highly luminescent cyano-containing poly[(m-phenylene ethynylene)-alt-(p-phenylene ethynylene)] derivative (CN-PPE) has been prepared and blended in micro/nano-sized dispersions into linear low density polyethylene (LLDPE) films by melt-processing. CN-PPE macromolecules show pronounced tendency to form molecular aggregates in the excited state when dispersed in the polymer matrix with emission characteristics depending on the structural order of the luminescent chromophores. In CN-PPE/LLDPE films, the polymer matrix orientation induces effective anisotropic arrangements of the π-π staking interactions between luminescent guest aggregates, thereby generating a pronounced green-blue dichroic emission. The capability to easily modulate the luminescent properties of polyethylene films (from green to blue reversibly) simply by varying CN-PPE concentration or by stretching the polymer matrix, suggests various technological applications in the field of smart and intelligent films from thermoplastic materials.     

Synthesis of laterally attached side-chain liquid crystalline poly(p-phenylene vinylene) and polyfluorene derivatives for the application of polarized electroluminescence

Two series of poly(p-phenylene vinylene) and polyfluorene derivatives (PPV1-PPV4 and PF1-PF5) containing laterally attached penta(p-phenylene) mesogenes were synthesized and characterized. These polymers show nematic liquid crystalline behavior. The optical properties of the polymers were investigated by UV-vis absorption and photoluminescence spectrometers and these polymers were fabricated to form the polarized electroluminescent devices using poly(ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) as an alignment layer. In the series of poly(p-phenylene vinylene) derivatives, polymer PPV4 offered the best EL device performance. It emitted yellow light at 588 nm at 4 V. The maximum brightness was about 1337 cd/m² at 9 V with a polarized ratio of 2.6. In another series of polyfluorene derivatives, PF4 offered the best EL device performance with the polarized ratio of 12.4 and a maximum luminescence of 1855 cd/m². In the case of polarized white light, as a consequence of blending small amount of PF4 and PF5 with a host polymer PF2, polarized ratio of up to 10.2 and a maximum brightness of 2454 cd/m² have been attained. The aligned films exhibited pronounced polarized ratio, implying that the polymers exhibit potential for linearly polarized LED application.     

Synthesis,doping, and electrical conductivity of high molecular weight poly(p-phenylene vinylene)

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.     

Electrical and electroluminescent properties of poly(p-phenylene vinylene) LB films based LEDs using aluminum metal as the electrode

The electrical properties of poly(p-phenylene vinylene) (PPV) Langmuir-Blodgett (LB) films were investigated, wherein transparent indium-tin oxide (ITO) and aluminum (Al) were used as the electrodes. Their I–V characteristics depended strongly on the thickness of PPV LB film, and the electric conductivity of the PPV LB film was in the range of 10⁻²–10⁻¹⁵ (S/cm). Their C–V characteristics showed that the capacitance was reversibly proportional to the thickness of PPV LB film and was kept constant when the applied voltage changed from −1.0 V to +1.0 V. These results indicated that PPV LB film was an insulator in this range of applied voltage without doping. Under forward bias, yellow-green light emission was observed in PPV LB film based light emitting diodes (LEDs), the highest light emission reached more than 100 cd/m² in the case that PPV LB film was deposited for 80 layers, i.e. a ITO/PPV(80L)/Al device.     

Poly(p-phenylene vinylene) derivatives with ester- and carboxy-functionalized substituents: a versatile platform towards polar functionalized conjugated polymers

A polar functionalized poly(p-phenylene vinylene) (PPV) derivative, ester containing poly(1,4-(2-(pentyloxy-5-carboxy methylester)-5-methoxyphenylene)vinylene), has been prepared via the sulfinyl precursor route. Subsequently, the carboxylic acid containing poly(1,4-(2-(5-carboxypentyloxy)-5-methoxyphenylene)vinylene) is readily obtained by the basic hydrolysis of the ester side groups. This carboxylic acid substituted polymer exhibits significantly improved optical properties as compared to previously reported similar polymers. To obtain polar functionalized PPV derivatives with more complex tailored substituents a versatile novel approach is presented based on the Mitsunobu reaction conditions via a post-polymerization functionalization of the carboxylic acid side groups. Using these reaction conditions substituents containing various ester groups, i.e. 4-nitrobenzyl, 2-nitrobenzyl, 5-(methyl)furfural and 3-N,N-dimethylamino-1-propane, have been covalently attached to the phenylene ring. Analytical data of the functionalized polymers are consistent with a quantitative functional group substitution. The results demonstrate that the employed functionalization method allows for the introduction of a large variety of polar substituents in a straightforward manner.     

Synthesis and gas permeability of ester substituted poly(p-phenylene)s

Polymerizations of various ester substituted 2,5-dichlorobenzoates [substituent: linear alkyl groups (1a-f), branched alkyl groups (1g-l), cyclohexyl groups (1m-o), phenyl groups (1p-r), and oxyethylene units (1s-v)] were investigated with Ni-catalyzed/Zn-mediated system in 1-methyl-2-pyrrolidone (NMP) at 80 °C. Most of monomers bearing linear and branched alkyl groups successfully polymerized to give relatively high-molecular-weight polymers (M_n = 10,000-20,800). However, the molecular weight of the polymer having eicocyl groups was low because of steric hindrance of long alkyl chain. The polymerizations of cyclohexyl 2,5-dichlorobenzoate and phenyl 2,5-dichlorobenzoate produced low-molecular-weight polymers, while the polymerizations of monomers with alkyl cyclohexyl and alkyl phenyl groups proceeded to afford polymers with relatively high-molecular-weights. The polymers possessing oxyethylene units were obtained, but the molecular weights were low when the oxyethylene chains were long. The gas permeability of membranes of poly(p-phenylene)s with alkyl chains increased as increasing the length of alkyl chain. The membranes of poly(p-phenylene)s with phenyl groups and oxyethylene units exhibited high densities and relatively low gas permeability. However, the CO₂/N₂ separation factor of membrane of poly(p-phenylene) having oxyethylene units was as large as 73.6.     

Synthesis and electrochemical characterization of soluble poly(p-phenylene vinylene) derivatives containing olefinic bonds at the side chain

A new kind of soluble poly(p-phenylenevinylene) (PPV) derivative containing free olefinic bonds at the side chain is prepared and studied by electrochemical measurement. The electrochemical investigations reveal that the free olefinic bonds in these polymers are electroactive; a new redox reaction occurs prior to the oxidation of PPV backbone in the cyclic voltammetry. The lower oxidation potential of the olefinic bonds hints that it is possible for these olefinic bonds to react with oxygen, which is desirable to remove the harmful oxygen in the light-emitting polymer devices. The merits and possibility of such polymers containing olefinic bonds in the fabrication of the light-emitting devices are discussed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2535–2539, 1999     

Synthesis and properties of poly(p-phenylenevinylene-co-sulfonylene) for a blue light-emitting diode

Summary Novel class of p-phenylenevinylene copolymer with sulfonylene group in the main chain (PPVS) was synthesized through the Wittig reaction of bis(bromomethyl-p-phenyl)-sulfone triphenylphosphonium salt with terephthaldehyde. PPVS was highly soluble in common organic solvents and showed strong blue-shifted fluorescence because the polar sulfonylene group provided better solubility and limited π-conjugation in polymer chain. A single layer device with PPVS sandwiched between indium-tin-oxide (ITO) and aluminum electrodes showed a blue electroluminescence (EL) peaked at 470 nm under the threshold voltage of 14 V. HOMO and LUMO levels of PPVS were found to be 0.46 and 0.28 eV, respectively, lower than those of poly(p-phenylenevinylene) (PPV). Received: 20 April 1999/Revised version: 31 May 1999/Accepted: 28 June 1999     

Morphological investigations of self-assembled films from a pyridine -incorporated poly (p -phenylene)

Well-organized structures of polymers in thin films have been widely investigated. Amphiphilic polymers, star-shaped polymers and modified polystyrene-based polymers have been shown to form self-assembled honeycomb patterns on substrates. Herein, we report a poly (p-phenylene) derivative capable of producing self-assembled regular structures. Highly fluorescent honeycomb structures were observed on various substrates. The nature of the substrate and polymer influences the morphology of the film.     

Generation of polyacetylene sulfoxide radicals through spin migration from the main-chain to the sulfoxide moiety in the side chain of poly[p-(n-butylsulfoxide)phenylacetylene] prepared with a [Rh(norbornadiene)Cl]2 catalyst

A phenylacetylene bearing an n-butylsulfoxide group, i.e., p-(n-butylsulfoxide)phenylacetylene (1) was prepared in high yields using the [Rh(norbornadiene)Cl]₂-NEt₃ catalyst in the presence of various solvents under mild conditions. The resulting polymer, poly[p-(n-butylsulfoxide)phenylacetylene] (poly(1)), was characterized in detail by ¹H NMR, ESR, laser Raman, and diffuse reflective UV-vis methods. The data clearly showed that cis-to-trans isomerization of the polymer can be induced when pressure is imposed to the polymer at room temperature, rotationally breaking the cis CC bonds to generate the cis and trans radicals. Further, the spin density in the cis radical was migrated from the main-chain to the sulfoxide moiety as the side chain of the phenyl ring to magnetically interact with the first two methylene protons in the n-butyl group giving a triplet line ESR spectrum with an extremely large g value, g = 2.0081.     

Morphological investigations of self-assembled films from a pyridine -incorporated poly (p-phenylene)

Well-organized structures of polymers in thin films have been widely investigated. Amphiphilic polymers, star-shaped polymers and modified polystyrene-based polymers have been shown to form self-assembled honeycomb patterns on substrates. Herein, we report a poly (p-phenylene) derivative capable of producing self-assembled regular structures. Highly fluorescent honeycomb structures were observed on various substrates. The nature of the substrate and polymer influences the morphology of the film.