Synthesis and electroluminescent properties of heterocycle-containing poly(p-phenylene vinylene) derivatives

Two novel conjugated polymers were synthesized by the Wittig reaction. The multilayer light emitting diodes were fabricated and showed green to blue electro-luminescence.     

Photoluminescent and electroluminescent properties of DCM-1 dispersed poly(p-phenylene vinylene) derivatives

The single layer devices utilizing poly[2-(carbazol-9-yl)-5-(2-ethylhcxyloxy)-1,4-phenylene vinylene] (CzEh-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-l,3,4-oxadiazolyl]-phenyl}-5-(2-ethythexyloxy)-l,4-phenylene vinylene] (OxdEh-PPV) doped with varying weight percent of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM-1) were fabricated and their photo luminescence (PL) and electroluminescence (EL) properties were discussed in this investigation. The PL spectra of DCM-1 doped polymers show that the emission is mostly from DCM-1 and Förster energy transfer may occur between DCM-1 and the two polymers. On the other hand, field-dependence of the emission spectra of EL devices was observed in detail. For the CzEh-PPV/DCM-1, the emission at the wavelength of 534 nm remains unchanged as the level of DCM-1 increases, whereas the peak at 572 nm is intensified with increasing both the additive level and applied electric field. For the OxdEh-PPV/DCM-1, the main peak is red-shift as the level of DCM-1 increases and blue-shift as the applied electric field does.     

Subgap absorption in poly(p-phenylene vinylene)

The subgap absorption of sulfonium chlorine precursor route PPV films has been studied by photothermal deflection spectroscopy. Two distinct features have been observed — a shoulder in the exponential region at 2.3 eV and a subgap absorption band centred at 1.6 eV. The latter is superimposed on an absorption smoothly decreasing to lower energy which magnitude correlates with the photoluminescence quantum efficiency. Similar features have been observed also for sulfonium precursor route dimetoxy substituted PPV and for soluble dialkoxy PPV.     

Heterocycle-substituted poly(p-phenylene vinylene) for light-emitting devices

Results are presented which explore the synthesis, characterization and electroluminescence performance of a new type of conjugated polymer, poly(2,5-diphenyl-1,3,4-oxadiazolyl)-4,4′-vinylene (O-PPV). A light-emitting diode consisting of ITO/O-PPV/Al is driven at about 6 V and has a peak emission wavelength of 483 nm, and this is in agreement with the result determined by the X-ray photoelectron spectroscopy (XPS) technique.     

Energy transfer from poly(2-carboxyphenylene-1,4-diyl) to poly(p-phenylene vinylene) in bilayer films and its effects on luminescent properties

Luminescent properties of poly(p-phenylenevinylene) (PPV) in bilayer films were found significantly enhanced by energy transfer from poly(2-carboxyphenylene-1,4-diyl) (PCPD) to PPV prepared by alternatively spin-coating the poly(xylylene tetrahydrothiophenium chloride) (PXT, dubbed as PPV-precursor) aqueous solution and PCPD pyridine solution followed by heat treatments. The energy transfer process, as verified by the photoluminescent excitation (PLE) spectroscopy and the analysis of time-resolved photoluminescence (PL) decays, was attributed to the chemical interlocking between two polymers in the interfacial region. Accordingly, the efficiency of energy transfer from PCPD to PPV was calculated. Consequently, the (glass)PPV/PCPD configuration exhibited higher energy transfer efficiency than the (glass)PCPD/PPV, resulting in higher PL and electroluminescent (EL) quantum efficiencies. Besides, the improved EL emission of indium-tin-oxide (ITO)/PPV/PCPD/Al device was attributed to the energy barrier in the PPV/PCPD interfacial region, which trapped the majority carriers, holes, to form the excitons in situ, that decayed radiatively.     

Effect of Ag nanoparticles on the electron energy structure and electrical properties of poly(p-phenylene vinylene) (PPV)

The effects of inorganic nanoparticles on a conjugated polymer were investigated by measuring the electronic properties of poly(p-phenylene vinylene) (PPV) and PPV/Ag nanocomposites. Through hybridization, an enhancement in current density was achieved with PPV/Ag nanocomposites due to an increase in the electron affinity with Ag nanoparticle content. Furthermore, roughening of the surface morphology was observed with incorporation of Ag nanoparticles. This roughness induces an enhanced applied field at the thinner region of the film and an increase in the surface area with a resulting increase of electron injection, leading to current enhancement.     

Electroabsorption spectroscopic study of cyano-substituted phenylene vinylene polymer (CN-PPV)

We report electroabsorption spectroscopy measurements on a single layer cyano-substituted phenylene vinylene (CN-PPV) polymer device and also contact potential difference (CPD) in the device structure ITO/PEDOT: PSS/CN-PPV/Al. The electroabsorption (EA) spectrum shows well developed structure of a peak and valley at 2.2 and 2.56 eV, respectively. It is demonstrated that the line shape more closely reflects second derivative of absorption spectra than corresponding first derivative. The origin of dominant features in EA spectra in the range of 1.5–3.2 eV are discussed qualitatively without taking recourse to fitting a linear combination of derivatives of linear absorption. The contact potential difference (CPD) measured using this technique shows a value 0.6 V that is exactly the work function difference between ITO and Al electrode used in this structure. These features in EA measurements of CN-PPV are in contrast to the ones observed in the case of PPV thin films.     

Alteration of the photo and electroluminescent properties of poly(p-phenylene vinylene) upon addition of indium chloride

We investigate the modification of the optical properties of poly(p-phenylene vinylene), PPV, upon addition of indium chloride, InCl₃, to its precursor solution. We find that the most significant decrease of the photoluminescence efficiency occurs after addition of ≈0.5% molar of InCl₃ to the PPV precursor. The PL efficiency either remains constant or increases, depending on the conversion conditions, for further increase of the salt content. Interestingly, we find instead that the electroluminescence efficiency increases upon addition of InCl₃. Absorption and photo-deflection spectroscopy yield no evidence of charge-transfer doping of PPV+InCl₃ blends converted on spectrosil. We discuss these results in the context of previous reports on the indium–tin oxide influence on the transport and luminescent properties of PPV.     

Synthesis and characterization of a partial-conjugated hyperbranched poly(p-phenylene vinylene) (HPPV)

A novel partial-conjugated HPPV has been designed and easily synthesized from A₂ + B₃ monomers, employing distrylbenzene, an oligomer of poly(p-phenylene vinylene) with high fluorescence quantum yield, as one of starting materials. HPPV was characterized by FT-IR, H-NMR, GPC and elemental analysis. Its properties, such as thermal, electrochemical and photoluminescent, etc., have been investigated systematically. HPPV exhibits good solubility and processability, a high glass transition temperature (T_g) and high fluorescence quantum yield in chloroform solution (much higher than fully conjugated hyperbranched and linear PPV).     

Change of photoluminescence properties of cyano-substituted poly p-phenylene vinylene controlled by photoirradiation

Photoluminescence (PL) changes in cyano-substituted poly p-phenylene vinylene (PPV) derivatives caused by photooxidation are discussed.     

The surface species of poly (p-phenylene vinylene) and their effects on metal interface formation

We used X-ray photoemission spectroscopy (XPS) to investigate the surface species of poly (p-phenylene vinylene) (PPV) and its interface formation with Ca and Al. PPV surface compositions varied with sample preparation. For relatively ‘clean’ surfaces with 4–5% O, analysis of the O( Is) peak revealed four types of oxygen species, namely carbonyl (C=0), hydroxyl (C-OH), ether (C-O-C) and the carboxylic groups (HO-C=O). The oxygen groups, excluding ether, reacted with Al or Ca to form the corresponding metal oxides. Chemical interactions between the metals and the phenylene and vinylene units to yield new species were not detected. For sulfur-free surfaces, a C(Is) peak shift of +0.5 eV followed the deposition of 15-30 Å of Ca on PPV. For sulfur-containing surfaces, the C(Is) peak shift was -0.5 eV. We attribute this difference to the interaction of metal atoms with the sulfur impurities. For Al/PPV, a C(Is) peak shift occurred at < 2 Å of Al deposition and reached a constant value of about + 0.4 eV after ∼ 8 Å of Al. Again, the direction of the peak shift depended on the presence of sulfur impurities. We attribute the C(Is) peak shifts to surface band bending and to Schottky-barrier formation. Since surface oxidation of PPV can inhibit band bending, our overall results suggest that the barrier height at the metal/PPV interface is highly sensitive to the surface preparation and relatively insensitive to the work function of the metals. A qualitative band picture is presented to account for the different directions of band bending.     

Light-emitting electrochemical cells based on poly(p-phenylene vinylene) copolymers with ion-transporting side groups

We present the electro-optical characteristics of LECs based on two copolymers derived from poly(p-phenylene vinylene), DB-alt-BTEM-PPV and BDMOS-co-BTEM-PPV, having ion-transporting side groups. We find typical LEC behaviour upon addition of lithium triflate. Addition of poly(ethylene oxide), PEO, improves the EL efficiency, but the response time increases. We discuss these observations on the basis of PEO-induced phase separation.     

Conversion of precursor polymer into poly(p-phenylene vinylene) by XeCl excimer laser

A pulsed-laser beam was used to induce the conversion of a sulfonium precursor polymer into the light-emitting conjugated polymer, poly(p-phenylene vinylene) (PPV). The light-emitting PPV could be prepared by way of a conversion process using a XeCl excimer laser and simultaneously patterning a thin polymer line on a substrate. The main advantage of the method used in this study over the conventional method, thermal annealing, is the short tack time. It also has the possibility of easily patterning the conjugated polymer via a wet-processing route through the selective irradiation of a pulsed-laser beam for pixel formation. The surface morphology of the polymer film converted by laser annealing was superior to that of the film converted by thermal annealing.     

Experimental and theoretical studies of absorption and photoluminescence excitation spectra of poly(p-phenylene vinylene)

In this paper, we present the experimental absorption spectra and photoluminescence excitation spectra of poly(p-phenylene vinylene) (PPV). We also present the theoretically calculated absorption and photoluminescence spectra of PPV. The singlet-to-singlet transition energies and moments calculated from the intermediate neglect of differential overlap with the spectroscopic parametrization (INDO/S) semi-empirical method were used to analyze the absorption spectra. The symmetrical vibrational modes calculated from the abinitio method were used to obtain the vibronic features in the spectra of PPV. It is demonstrated that the inhomogeneity observed in the absorption spectra of PPV can be modeled by summing up the absorption of PPV oligomers with different chain lengths. The difference between absorption and photoluminescence excitation spectra is explained.     

Controlling the luminescence properties of poly(p-phenylene vinylene) entrapped in Langmuir and Langmuir–Blodgett films of stearic acid

Properties of hybrid films can be enhanced if their molecular architecture is controlled. In this paper, poly (p-phenylene vinylene) was mixed with stearic acid in order to form stable hybrid Langmuir monolayers. Surface properties of these films were investigated with measurements of surface pressure, and also with polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS). The films were transferred from the air–water interface to solid supports through the Langmuir–Blodgett technique, and the viability of the film as optical device was investigated with fluorescence spectroscopy. Comparing the fluorescent spectra for the polymer in solution, as a casting film, and as an LB film, the emission bands for LB films were narrower and appeared at lower wavelengths. The interactions between the film components and the design for the LB film may take advantage of the method to immobilize luminescent polymers in mixed ultrathin films adsorbed in solid matrices.     

Chlorine precursor route to poly(2-phenoxy p-phenylene vinylene): synthesis and characterization

A new PPV derivative, poly(2-phenoxy p-phenylene vinylene)(PO-PPV), has been synthesized via the chlorine precursor route (CPR). PO-PPV and its chlorine precursor polymer were fully characterized by thermal (TGA and DSC), mechanical and spectroscopic (UV–vis, PL and IR) methods. The results indicate that the precursor polmer and the yellow-green light-emitting PO-PPV have excellent stability and mechanical properties. The easily processible precursor is stable at room temperature and therefore has a better shelf-life than PPV synthesized via the sulfonium precursor route. Thermal conversion provides high quality films, relatively free of oxidative defects.     

Density matrix renormalisation group calculations of molecular exciton energies in poly(p-phenylene vinylene)

Starting from the Pariser-Parr-Pople theory of π-conjugated systems, we construct a model of the low lying excitations of poly(p-phenylene vinylene). The model is based on the bonding HOMO and LUMO states of the molecular repeat units. The model is numerically tractable in that it is solved for oligomers of up to 15 units using the density matrix renormalisation group method. The energy of the 1¹ B_u ⁻ exciton is in good agreement with experimental results for oligomers, and approaches ca. 2.7 eV for oligomers of 15 units. Likewise, we predict a 2¹Ag⁺ exciton at ca. 2.8 eV, a 1³B_u⁺ exciton at 1.6 eV and the singlet exciton binding energy as being 1.4 eV for single chains. We extend this approach to target other absorption bands. For example, we find a localised Frenkel exciton at 5.8 eV, in excellent agreement with the 6 eV absorption peak in PPV.     

Synthesis and characterization of low bandgap poly(dithienosilole vinylene) derivatives

Two low bandgap conjugated polymers containing vinylene unit on their main chains, poly(dithienosilole vinylene) derivatives P1 and P2, were synthesized by Pd-catalyzed Stille-coupling method. The two polymers show good solubility in common organic solvents, broad absorption bands from 500 nm to 700 nm and lower energy bandgap of 1.72 eV for P1 and 1.77 eV for P2. The electronic energy levels of the polymers were measured by electrochemical cyclic voltammetry. The HOMO and LUMO energy levels of P1 and P2 are at ?5.03 eV, ?5.18 eV and ?3.39 eV, ?3.43 eV, respectively. The polymers P1 and P2 containing dithienosilole units show lower HOMO and LUMO energy levels, in comparison with other poly(thienylene vinylene) derivatives.     

Syntheses and optical properties of poly(o-phenylene vinylene)s

Poly(p-phenylene vinylene) (PPV) derivatives have been extensively studied as photoluminescence and electroluminescence materials. Recently, meta-phenylene has been incorporated into PPVs for conjugation-length control and luminescence enhancement. Here, we report three poly(o-phenylene vinylenes) (oPVs). Among them, Polymer II exhibits high molecular weight, good processibility and high photoluminescence quantum efficiency in both solution and solid state, making it an attractive candidate for light-emitting diode applications.