Efficient, saturated red electroluminescent devices with modified pyran-containing emitters

A series of red dopants, i.e., 4-(dicyanomethylene)-2-methyl-6-(o-methoxy(p-diethylamino))-4H-pyran (A), 4-(dicyanomethylene)-2-methyl-6-(o-ethoxy(p-diethylamino))-4H-pyran (B), 4-(dicyanomethylene)-2-methyl-6-(o-(n-proplyoxy) (p-diethylamino))-4H-pyran (C) and 4-(dicyanomethylene)-2-methyl-6-(o-(n-butoxy) (p-diethylamino))-4H-pyran (D) for application in organic light-emitting diodes (OLEDs) have been synthesized and characterized. It was found that by introducing a (diethylamino)-2-alkoxy segment as electron donor and 4-(dicyanomethylene)-2-methyl-4H-pyran moiety as electron acceptor to form the molecular skeleton, the resultant emitters have high photoluminescent quantum yields and saturated red emissions. OLEDs devices based on the present emitters at varying doping levels have been fabricated. Devices using the new emitters as the dopant at about 1 wt% doping levels show excellent performances. The luminance of them can achieve 8785–12540 cd/m² with the highest luminous efficiency around 2.0 lm/W.     

Oxide phosphor and dielectric thin films for electroluminescent devices

Since its inception in the early 1990s, the field of high brightness, high stability oxide phosphor electroluminescence (OPEL) continues to develop with efforts now underway in several groups around the world. Green-emitting phosphors Zn₂SiO₄:Mn, Zn₂Si_1−xGe_xO₄:Mn and ZnGa₂O₄:Mn achieve efficiencies of 0.5-2.5 lm/W. Lifetimes of 50 000 h in air are achieved, even without control of humidity. Red emission from Ga₂O₃:Eu and MgGa₂O₄:Eu has also been realized, with efficiencies approaching 1 lm/W. Recently, higher efficiencies of up to 10 lm/W have been reported in yellow emitting Y₂O₃:Mn and (Y₂O₃)_x(GeO₂)_y:Mn. Commercialization of OPEL for flat panel displays and lamps on glass substrates requires the development of phosphors that may be deposited at temperatures of 700 °C or lower. Several approaches are described here. A vacancy mechanism is shown to control the enhancement of crystallization in ZnGa₂O₄ by the introduction of CdO to form compounds of Zn_1−xCd_xGa₂O₄:Mn. Here, sputtering targets and as-deposited thin films contain Cd atoms, but the annealed thin films undergo complete loss of Cd as they crystallize. It is also shown that a flux such as LiF maybe added to Zn₂SiO₄:Mn to lower crystallization temperature. Here it is likely that point defects Li_Zn and F_o permit charge compensation while lowering the crystallization temperature. High breakdown strength, high dielectric constant and transparent dielectric layers have been developed to permit long life OPEL on Corning 1737 glass substrates. Whereas it is commonly reported that SrTiO₃ and other perovskite titanate and zirconate dielectrics are not self-healing, it has been shown that a SrTiO₃ dielectric of thickness 2 μm prepared using three sub-layers does achieve self-healing. In conjunction with a Zn₂Si_0.5Ge_0.5O₄:Mn phosphor, excellent steep-threshold electroluminescence (EL) behaviour is observed with L50, the brightness 50 V over threshold at 60 Hz of over 300 cd/m². Finally, results for a new phosphor system Ga_2−xIn_xO₃:Eu on glass substrates are reported, in which the electric field for EL is reduced.     

Binuclear aluminum complex as an efficient orange emitter in single-layer electroluminescent devices

A binuclear aluminum complex with a chelating anilido-imine conjugated Schiff base ligand, 1,4-{o-C₆H₄[N(2,6-ⁱPr₂C₆H₃)][CH  N]Al(CH₃)₂}₂C₆H₄ (3), was synthesized from the alkane elimination reaction of trimethylaluminum with the free ligand, 1,4-{o-C₆H₄[NH(2,6-ⁱPr₂C₆H₃)][CH  N]}₂C₆H₄ (2), at room temperature. The photophysical properties of complex 3 were briefly examined and it was found that complex 3 displays bright orange luminescence in solution and in solid state. Electroluminescent devices were fabricated by doping complex 3 in polymer blend host of poly(vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol using simple solution spin-coating technique. Efficient orange light emission was obtained from the fabricated single-layer organic light-emitting devices with a maximum current efficiency of 3.2 cd/A.     

The characteristics of inorganic electroluminescent devices with an amorphous diamond film as cathode material

Diamond like carbon (DLC) thin films were used as the cathode layers of inorganic alternating current driven thick dielectric electroluminescent devices. The results indicated that electroluminescent (EL) devices with DLC cathode has superior brightness over the EL with Al or Cr-doped DLC cathodes. Cr-doping in DLC thin film can increase the electrical conductivity, but degrades the EL properties. Also, the EL device with DLC cathode possesses the lowest decay rate among various cathodes, because of the high thermal conductivity and the inert nature of DLC film.     

Efficient fluorescent red, green, and blue organic light-emitting devices with a blue host of spirobifluorene derivative

Efficient fluorescent blue, green, and red (RGB) organic light-emitting devices (OLEDs) were fabricated using a blue host material of pyrimidine-containing spirobifluorene derivative 2,7-bis[2-(4-tert-butylphenyl)pyrimidine-5-yl]-9,9′-spirobifluorene (TBPSF) doped with blue dye perylene, green dye 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l] pyrano[6,7,8-ij] quinolizin-11-one (C545T), and red dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), respectively. The brightness and current efficiency of the perylene doped blue device reached 10117 cd/m² and 2.97 cd/A. Green emission of the C545T doped device reached 8500 cd/m² and 13.0 cd/A. Red emission of the DCJTB doped device can be as high as 9000 cd/m² and 2.0 cd/A, respectively. High color purity of the blue (Commission Internationale de L′Eclairage (CIE_x,y) coordinates (CIE, x = 0.27, y = 0.24)), green (CIE, x = 0.19, y = 0.63) and red (CIE, x = 0.62, y = 0.37) emissions were achieved for RGB dyes doped TBPSF OLEDs. High brightness, large current efficiency, and good color purity of TBPSF-based RGB OLEDs were obtained by the configuration optimization device, such as inserting the hole and electron-injection materials, and suitable dopant content and light emitting layer thickness.     

Synthesis and optoelectronic properties of thieno[2,3-b]thiophene based bis 1,3,4-oxadiazole derivatives as blue fluorescent material for use in organic light emitting diodes

We report the synthesis and optoelectronic properties of a series of symmetrical disubstituted derivatives containing bis 1,3,4-oxadiazole with thieno[2,3-b]thiophene backbone (3a–f). These novel compounds were synthesized by employing simple and convenient synthetic protocol using thieno[2,3-b]thiphene-2,5,dicarboxylic acid as a starting material. The structures of these target molecules were established by their analytical and spectral data. The photo physical and electrochemical studies revealed that these compounds exhibit good blue fluorescent properties with better quantum yield. These compounds are expected to be of use in various optoelectronic applications.     

Thermally and electrochemically stable amorphous hole-transporting materials based on carbazole dendrimers for electroluminescent devices

Amorphous hole-transporting carbazole dendrimers, 1,4-bis[3,6-di(carbazol-9-yl)carbazol-9-yl]-2,6-di(2-ethylhexyloxy)benzene (G2CB) and 1,4-bis[3,6-di(carbazol-9-yl)carbazol-9-yl]-9-(2-ethylhexyl)carbazole (G2CC), were synthesized by a divergent approach involving bromination and Ullmann coupling reactions. Compounds G2CB and G2CC showed high thermal stability (T_g = 206 to 245 °C) and excellent electrochemical reversibility. Double-layer organic light-emitting diodes were fabricated by using G2CB and G2CC as hole-transporting layers (HTLs) and tris(8-quinolinato)aluminum (Alq₃) as light-emissive layer with the device configuration of indium tin oxide/HTL/Alq₃/LiF:Al. Both devices exhibited bright green emission from Alq₃. The device using G2CC as HTL has the best performance with a maximum brightness of 8900 cd/m² at 14 V and a low turn-on voltage of 3.5 V.     

Conductive polymer composites based on metallic nanofiller as smart materials for current limiting devices

The influence of the structure and the temperature on the Conductive Polymer Composites (CPC) properties has been studied. The investigated CPC are based on homo- and heterogeneous polymer blends of high density polyethylene, polybutylene terephthalate and poly(m-xylene adipamide), filled with dispersed silver nanoparticles (Ag). It is shown that by the appropriate use of the immiscible polymers blends the percolation threshold can be decreased twice, what significantly reduces costs and keeps better mechanical properties. Additionally, it was found that depending on the choice of CPC structure the commutation temperature from a conducting state to an insulating state can be observed between 45 and 180 °C. The observed high intensity of Positive Temperature Coefficient (PTC) effect, i.e. a sharp (narrow temperature range) and strong (10 orders of magnitude) resistivity increase, makes such composites promising for current limiting devices and temperature sensors.     

Callindra haematocephata and Peltophorum pterocarpum flowers as natural sensitizers for TiO2 thin film based dye-sensitized solar cells

We have studied the performance of dye-sensitized solar cells employing natural dye extracted from the flowers Callindra haematocephata and Peltophorum pterocarpum as sensitizers for TiO₂ photoanode. The extracts have shown appreciable absorption in the visible region. FTIR studies indicated the presence of anthocyanins and β-carotene in the flowers of C. haematocephata and P. pterocarpum respectively. The extracts were anchored on TiO₂ film deposited on transparent conductive glass (FTO) which were used as photoanode. The dye coated TiO₂ film electrode, Pt counter electrode and electrolyte (I⁻₃) assembled into a cell module was illuminated by a light source with intensity 100 mW/cm² to measure the photoelectric conversion efficiency of the DSSCs. From the J-V characteristic curves of cells, the parameters related to the solar cell performance were determined. The conversion efficiency of the DSSC employing natural dye extract from the flower C. haematocephata and P. pterocarpumwere was found as 0.06% and 0.04%, with open-circuit voltage (V_OC) of 370 mV & 400 mV, short-circuit current density (J_SC) of 0.25 mA/cm² & 0.15 mA/cm², fill factor (FF) of 0.70 & 0.71 and P_max of 65 & 45 μW cm⁻² respectively. The extract of the flower C. haematocephata exhibited better photosensitization action compared to the flower of P. pterocarpum.     

An in vitro screening system for protein splicing inhibitors based on green fluorescent protein as an indicator

This paper describes an in vitro fluorometric assay system for protein splicing based on the RecA intein of Mycobacterium tuberculosis and a modified green fluorescent protein (GFP). The assay takes advantage of the fact that polypeptides inserted adjacent to residue 129 of GFP cause the protein to form inclusion bodies when expressed in Escherichia coli and to be incapable of fluorophore formation. However, when the inserted polypeptide is an intein, the renatured fusion protein can undergo protein splicing and chromophore formation. Comparison of chromophore formation by renatured GFP-intein fusion and renatured GFP showed that under optimal conditions (pH 6.5 and 20 degrees C) protein splicing is significantly slower than GFP chromophore formation. Taking advantage of the reversible inhibition of protein splicing by zinc ion, a fluorometric protein splicing assay was developed in which the denatured fusion protein of GFP and the RecA intein was purified on a metal ion affinity column and renatured in the presence of 2 mM ZnCl2. When diluted into appropriate buffers, protein splicing could be initiated by the addition of a molar excess of EDTA and followed fluorometrically. This assay should be valuable as a high-throughput screening system for protein splicing inhibitors as potential antimycobacterial agents and as tools for studying the mechanism of protein splicing.     

Tunable color emitting of CaAl2Si2O8: Eu,Tb phosphors for light emitting diodes based on energy transfer

A series of single-phased CaAl₂Si₂O₈: Eu, Tb phosphors have been synthesized at 1400 °C via a solid state reaction. The emission bands of Eu²⁺ and Eu³⁺ were observed in the air-sintered CaAl₂Si₂O₈: Eu phosphor due to the self-reduction effect. Tb³⁺ ions that typically generated green emission were added in CaAl₂Si₂O₈: Eu phosphor for contributing for a wider-range tunable emission. Energy transfer from Eu²⁺ to Tb³⁺ and the modulation of valence distribution of Eu²⁺/Eu³⁺ that contributes to the tunable color emitting were elucidated. More importantly, a white emission can be obtained by controlling the codoped contents of Li⁺ as well as suppressing the self-reduction degree of Eu. The white light emitting with the color coordinate (0.326, 0.261) was obtained, which indicates that CaAl₂Si₂O₈: Eu, Tb is a promising tunable color phosphor for application in ultraviolet light emitting diodes (UV-LEDs).     

Novel heterocyclic based blue and green emissive materials for opto-electronics

Two different novel heterocyclic compounds namely 2,5-bis(1,3-diphenyl-4,5-dihydro-1H-pyrazol-5-yl)thiophene (Material I) and 2,5-bis(3-(naphthalen-1-yl)-1-phenyl-4,5-dihydro-1H-pyrazol-5-yl)thiophene (Material II) were designed, synthesized and characterized by spectral methods. The synthesized materials were confirmed by standard techniques such as FT-IR, ¹H NMR and elemental analysis. Physical properties include thermal, surface morphology of the materials were explained from TGA, DSC and SEM analysis. Optical properties such as absorption, emission, solvent effect have been investigated by UV–Visible and fluorescence spectrophotometers. The blue and green emission of the materials was confirmed by using UV light as well as fluorescence spectrophotometers. Bandgap energies of these materials were obtained by both experimental and theoretical calculation from of cyclic voltammetry, UV–Visible spectrophotometer and DFT calculation. I–V characteristic analysis used to determine the threshold voltage (V_on) of the two materials. The obtained results of the materials have promising to be applicable for opto-electronic applications.     

Dipolar versus octupolar triphenylamine-based fluorescent organic nanoparticles as brilliant one- and two-photon emitters for (bio)imaging

Two related triphenylamine-based dipolar and octupolar fluorophores are used to prepare aqueous suspensions of fluorescent organic nanoparticles (FONs) via the reprecipitation method. The obtained spherical nanoparticles (30-40 nm in diameter) are fluorescent in aqueous solution (up to 15% fluorescence quantum yield) and exhibit extremely high one- and two-photon brightness, superior to those obtained for quantum dots. Despite the two chromophores showing similar fluorescence in solution, the fluorescence of FONs made from the octupolar derivative is significantly red-shifted compared to that generated by the dipolar FONs. In addition, the maximum two-photon absorption cross section of the FONs made from the octupolar derivative is 55% larger than that of the dipolar derivative FONs. The experimental observations provide evidence that the different molecular shape (rodlike versus three-branched) and charge distribution (dipolar versus octupolar) of the two chromophores strongly affect the packing inside the nanoparticles as well as their spectroscopic properties and colloidal stability in pure water. The use of these FONs as probes for biphotonic in-vivo imaging is investigated on Xenopus laevis tadpoles to test their utilization for angiography. When using FONs made from the octupolar dye, the formation of microagglomerates (2-5 μm scale) is observed in vivo, with subsequent lethal occlusion of the blood vessels. Conversely, the nanoparticles of the dipolar dye allow acute imaging of blood vessels thanks to their suitable size and brightness, while no toxic effect is observed. Such a goal cannot be achieved with the dissolved dye, which permeates the vessel walls.     

Synthesis and electroluminescent properties of blue emitting materials based on arylamine-substituted diphenylvinylbiphenyl derivatives for organic light-emitting diodes

This paper reports the synthesis and electroluminescent properties of a series of blue emitting materials with arylamine and diphenylvinylbiphenyl groups for applications to efficient blue organic light-emitting diodes (OLEDs). All devices exhibited blue electroluminescence with electroluminescent properties that were quite sensitive to the structural features of the dopants in the emitting layers. In particular, the device using dopant 4 exhibited sky-blue emission with a maximum luminance, luminance efficiency, power efficiency, external quantum efficiency and CIE coordinates of 39,000 cd/m², 12.3 cd/A, 7.45 lm/W, 7.71% at 20 mA/cm² and (x = 0.17, y = 0.31) at 8 V, respectively. In addition, a blue OLED using dopant 2 with CIE coordinates (x = 0.16, y = 0.18) at 8 V exhibited a luminous efficiency, power efficiency and external quantum efficiency of 4.39 cd/A, 2.46 lm/W and 2.97% at 20 mA/cm², respectively.     

Syntheses and characterization of 6,6′-diaryl-9,9′-dialkyl[3,3′]bicarbazoles as materials for electroluminescent devices

Electro-active materials containing 6,6′-diaryl-substituted [3,3′]bicarbazole core were synthesized by multistep synthetic rote and characterized. The derivatives were examined by differential scanning calorimetry and demonstrated formation of amorphous materials with rather high glass transition temperatures. The synthesized compounds have been tested as hole transporting layers in simple OLED devices with Alq₃ as the emitter/electron transporting layer. The green devices containing hole transporting films of diphenyl-9,9′-diethyl-[3,3′]bicarbazole exhibited the best overall performance (turn-on voltage: 3.5 V, maximum photometric efficiency: 4.2 cd/A, maximum brightness: ∼12,200 cd/m²).     

An efficient nanocomposite based on carbon nanotubes functionalized with a fluorescent ink for ultrafast optical limiting

Functionalization of CNTs with different kinds of molecules is known to alter the optical properties of CNTs, leading to the development of new nanocomposites of interest to optoelctronics. This paper presents the design of a new and efficient optical limiting material based on carbon nanotubes (CNTs) functionalized with a commercial fluorescent ink. High resolution transmission electron microscopy (HRTEM) and optical spectroscopy indicate that the ink molecules get noncovalently attached to the surface of the CNTs. The mechanism of optical nonlinearity of the CNTs gets modified after functionalization. Z-scan studies on functionalized CNTs performed using mode-locked Ti:Sapphire femtosecond pulses at 780 nm reveal the predominance of two photon absorption (TPA). Efficient and low threshold ultrafast optical limiting is demonstrated in the new nanocomposite.     

The application of optical fibres as witness devices for the detection of plastic strain and cracking

Low cost optical fibres have recently become readily available for telecommunications purposes. Silica fibres are characterised by high elastic strains to failure. The feasibility of using these fibres for structural integrity monitoring particularly for offshore structures is investigated. The basis of the technique is that a fibre may be bonded to a critical part of a structure and provides an optical path which will be broken if the fibre fails due to plastic strain or crack opening in the critical area.
Groups of fibres which have been given predetermined fracture strains by surface etching were encapsulated in special packs. These packs were bonded to steel and concrete tensile specimens. Strain transfer occurred successfully between the specimens and individual fibres. The distribution of strain to fibre fracture appeared to be uniform along the fibre. The use of several fibres with a range of fracture strains caused fibres to break progressively with increasing strain. For applications to offshore structures it has been found possible to use water-repellent adhesives which can be applied and cured in sea water and suffer no deterioration.
The advantages of this system include versatility, relatively low cost, adaptability to continuous monitoring and the possibility of being fitted retrospectively and refitted after repair operations.     

Optical property of poly(9,9-dioctylfluorene) gel with β phase and application to polymer light emitting diode

Poly (9,9-dioctylfluorene)(F8) gel with β phase has been investigated in terms of optical absorption, photoluminescence measurements and Fourier transform infrared spectroscopy. The optical properties of the F8 gel markedly changed in the temperature range from 70 to 80 °C owing to the vibration of polymer chain related to the glass transition temperature of F8. F8 films by thermal printing method had the characteristic particulate morphology. Current efficiencies of polymer light emitting diodes (PLEDs) with the β phase of F8 fabricated by the thermal printing method were better than that with the amorphous phase of F8 by the spin-coating method. We demonstrate the β phase effects of PLEDs characteristics by the thermal printing method.     

Fiber-optic Bragg gratings as magnetic field-insensitive strain sensors for the surveillance of cryogenic devices

While conventional electrical resistance strain gages show increasing cross-sensitivities to temperature and magnetic field with decreasing temperature down to liquid helium, it has been found that fiber-optic Bragg grating strain sensors show negligible thermo-optic and magneto-optic effects in cryogenic environments; therefore, they allow reliable strain measurements. These specific application advantages of optical fiber Bragg grating sensors at low temperatures, together with the electrical isolation and low electro-magnetic interference, low thermal conductivity and their multiplexing capability, make them attractive for structural health monitoring in cryogenic devices such as superconductive magnets. In this paper we present low temperature characteristics of fiber Bragg grating-based sensors and address application-based side effects such as induced birefringence.     

Evaluation of the performance of calix[8]arene derivatives as liquid phase extraction material for the removal of azo dyes

This paper presents a study on the removal of azo dyes (Reactive Black 5, Trapaeolin 000, Methyl Orange and Direct Violet 51) with calix[n]arene derivatives from aqueous solution into the organic phase in order to explore the potential use of calixarenes as low-cost efficient extractants for wastewater dye removal. The carboxylic acid derivative of calix[8]arene shows highest affinity towards the azo dyes. The influence of NaCl (present in the solution) on extraction process was also studied. The extent of the dye removal increased with the addition of NaCl. The proposed extraction mechanism involves several kinds of interactions: electrostatic repulsion between carboxylic acid groups of calix[8]arenes and sulfonate groups of azo dyes, hydrogen bonding and formation of an inclusion complex due to three dimensional cavity type calix[n]arene molecules through host-guest interactions.