Ligand effect on the performance of organic light-emitting diodes based on europium complexes

A series of europium complexes were synthesized and their electroluminescent (EL) characteristics were studied. It was found by comparison that the different substituted groups, such as methyl, chlorine, and nitryl, on ligand 1,10-phenanthroline affect significantly the EL performance of devices based on these complexes. The more methyl-substituted groups on ligand 1,10-phenanthroline led to higher device efficiency. A chlorine-substituted group showed the approximate EL performance as two methyl-substituted groups, whereas a nitryl substituent reduced significantly the EL luminous efficiency. However, beta-diketonate ligand TTA and DBM exhibited similar EL performance. The improved EL luminous efficiency by proper substituted groups on the 1, 10-phenanthroline was attributed to the reduction of the energy loss caused by light hydrogen atom vibration, as well as concentration quenching caused by intermolecular interaction, and the match of energy level between the ligand and Eu3+.

Improvement of amplified spontaneous emission performance by doping tris(8-hydroxyquinoline) aluminum (Alq3) in dye-doped polymer thin films

A well-known red fluorescent dye 4-(dicy-anomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)4H-pyran (DCJTB) was codoped with an electron transport organic molecule tris(8-hydroxyquinohne) aluminum (Alq3) in a host matrix of polystyrene (PS), and the amplified spontaneous emission (ASE) was studied by optically pumping. It was found that the ASE performance was significantly improved by the introduction of Alq3. The Alq3:DCJTB:PS blending thin films showed a low threshold (2.4 mu J/pulse) and a high net gain coefficient (109.95 cm(-1)) compared with the pure DCJTB:PS system (threshold of 15.2 mu J/pulse and gain of 35.94 cm(-1)). The improvement of the ASE performance was considered to be attributable to the effective Foster energy transfer from Alq(3) to DCJTB. Our results demonstrate that the Alq(3):DCJTB could be a promising candidate as gain medium for red organic diode lasers.

Improved amplified spontaneous emission by doping of green fluorescent dye C545T in red fluorescent dye DCJTB : PS polymer films

Amplified spontaneous emission (ASE) characteristics of a red fluorescent dye, 4-(dicy-anomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), and a green fluorescent dye, (10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1] benzopyrano [6,7,8-ij]quinohzin-11-one) (C545T) codoped polystyrene (PS) as the active medium were studied. It was found that the performance of ASE is greatly improved due to the introduction of C545T. By optimizing the concentrations of C545T and DCJTB in PS, an ASE threshold of 0.016 mJ pulse(-1), net gain of 52.71 cm(-1), and loss of 11.7 cm(-1) were obtained. The efficient Forster energy transfer from C545T to DCJTB was used to explain the improvement of the ASE performance in the coguest system.

Single-walled carbon nanotubes binding to human telomeric i-motif DNA: significant acceleration of S1 nuclease cleavage rate

Single-walled carbon nanotubes (SWNTs) binding to human telomeric i-motif DNA can significantly accelerate S1 nuclease cleavage rate by increasing the enzyme turnover number.

Phase conversion and spectral properties of long lasting phosphor Zn-3 (PO4)(2): Mn2+, Ga3+

A red long lasting phosphor Zn-3(PO4)(2): Mn2+ Ga3+ (ZPMG) was prepared by ceramic method, and phase conversion and spectral properties were investigated. Results indicated that the phase conversion from alpha-Zn-3(PO4), beta-Zn-3(PO4)(2) to gamma-Zn-3(PO4)(2) occurs with different manganese concentration incorporated and sinter process. The structural change induced by the phase transformation results in a remarkable difference in the spectral properties. The possible luminescence mechanism for this red LLP with different forms has been illustrated.

Oriented growth and luminescence of ZnO : Eu films prepared by sol-gel process

A series of Eu3+-doped ZnO films have been prepared by a sol-gel method. These films were characterized by X-ray diffraction (XRD) and photoluminecent spectra (PL). Effects of synthetic parameters, such as annealing atmosphere, temperature and concentration of doped ions, on the highly oriented crystal growth were studied in detail. The crystalline structures of films annealed in vacuum have a wurtzite symmetry with highly c-axis orientation. A characteristic D-5(0) -> F-7(J)(J = 1, 2, 3 and 4) red emission is observed due to energy transfer from the ZnO host to the doped Eu3+ in the c-oriented ZnO films.

Facile sonochemical synthesis of single-crystalline europium fluorine with novel nanostructure

A single-crystalline EuF3 nanoflower with a novel three-dimensional (3D) nanostructure has been successfully synthesized via a facile, fast, efficient, and mild ultrasonic irradiation solution route employing the reaction of Eu(NO3)(3) and KBF4 under ambient conditions without any template or surfactant. The ultrasonic irradiation plays an important role and is necessary for the synthesis of EuF3 with the complex structure. The formation mechanism of this complex nanostructure is proposed in this paper. No template or surfactant is used in this method, which avoids the subsequent complicated workup for the removal of the template or surfactant. Furthermore, a substantial reduction in the reaction time as well as the reaction temperature is observed compared with the hydrothermal process.

Molecular design on highly efficient white electroluminescence from a single-polymer system with simultaneous blue, green, and red emission

A highly efficient white electroluminescent polymer with simultaneous blue, green, and red emission is reported, developed using a dopant/host strategy by covalently attaching both a green- and a red-light-emitting dopant to the side chain of a blue-light-emitting polymer host (see figure). In a single-layer device a maximum luminance efficiency of 7.3 cd A(-1) with CIE coordinates of (0.31,0.32) is achieved.

Synthesis and characterization of white-light-emitting polyfluorenes containing orange phosphorescent moieties in the side chain

Two orange phosphorescent iridium complex monomers, 9-hexyl-9-(iridium (III)bis(2-(4'-fluorophenyl)-4-phenylquinoline-N, C-2')(tetradecanedionate-11,13))-2,7-dibromofluorene (Br-PIr) and 9-hexyl-9-(iridium(III)bis(2-(4'-fluorophenyl)-4-methylquinoline-N, C-2')(tetradecanedionate-11,13))-2,7-dibromofluorene (Br-MIr), were successfully synthesized. The Suzuki polycondensation of 2,7-bis(trimethylene boronate)-9,9-dioctylfluorene with 2,7-dibromo-9,9-dioetylfluorene and Br-Plr or Br-MIr afforded two series of copolymers, PIrPFs and MIrPFs, in good yields, in which the concentrations of the phosphorescent moieties were kept small (0.5-3 mol % feed ratio) to realize incomplete energy transfer. The photoluminescence (PL) of the copolymers showed blue- and orange-emission peaks. A white-light-emitting diode with a configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/PIr05PF (0.5 mol % feed ratio of Br-PIr)/Ca/Al exhibited a luminous efficiency of 4.49 cd/A and a power efficiency of 2.35 lm/W at 6.0 V with Commission Internationale de L'Eclairage (CIE) coordinates of (0.46, 0.33). The CIE coordinates were improved to (0.34, 0.33) when copolymer MIr10PF (1.0 mol % feed ratio of Br-MIr) was employed as the white-emissive layer. The strong orange emission in the electroluminescence spectra in comparison with PL for these kinds of polymers was attributed to the additional contribution of charge trapping in the phosphorescent dopants.

Fabrication and luminescent properties of rare earths-doped Gd-2(WO4)(3) thin film phosphors by Pechini sol-gel process

Rare earth ions (Eu3+ and Dy3+)-doped Gd-2(WO4)(3) phosphor films were prepared by a Pechini sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting powders and films. The results of XRD indicate that the films begin to crystallize at 600degreesC and the crystallinity increases with the elevation of annealing temperatures. The film is uniform and crack-free, WO(4)(2-)mainly consists of closely packed fine particles with an average grain size of 80 nm. Owing to an energy transfer from 4 groups, the rare earth ions show their characteristic emissions in crystalline Gd-2(WO4)(3) phosphor films, i.e., D-5(J) -F-7(J), (J = 0, 1, 2, 3; J' = 0 1, 2, 3, 4, not in all cases) transitions for Eu3+ and F-4(9/2)-H-6(J) (J = 13/2, 15/2) transitions for D Y3+, with the hypersensitive transitions D-5(0)-F-7(2) (Eu3+) and F-4(9/2) - H-6(13/2) (Dy3+) being the most prominent groups, respectively.

Synthesis and characterization of beta-diketiminate terbium complex

The terbium complex supported by beta-diketiminate was synthesized and structurally characterized. Due to an efficient energy transfer from the ligand to the central Tb3+, this complex shows a strong emission corresponding to Tb3+5D4-F-7(J) (J = 6,5,4,3) transitions, with D-5(4)-F-7(5) (550 nm) green emission as the most prominent group. The decay behavior of Tb3+ luminescence depends strongly on the excitation wavelengths.

Luminescent properties of Gd2Ti2O7 : Eu3+ phosphor films prepared by sol-gel process

Gd2Ti2O7: Eu3+ thin film phosphors were fabricated by a sol-gel process. X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 800 degreesC and the crystallinity increased with the elevation of annealing temperatures. Uniform and crack free phosphor films were obtained, which mainly consisted of grains with an average size of 70 nm. The doped Eu3+ showed orange-red emission in crystalline Gd2Ti2O7 phosphor films due to an energy transfer from Gd2Ti2O7 host to them. Both the lifetimes and PL intensity of the Eu3+ increased with increasing the annealing temperature from 800 to 1000 degreesC, and the optimum concentrations for Eu3+ were determined to be 9 at.%. of Gd3+ in Gd2Ti2O7 film host.

Amplified spontaneous emission and gain from optically pumped films of dye-doped polymers

The amplified spontaneous emission and gain characteristics of various fluorescent dyes, 2-(1,1-dimethylethyl)-6(2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij] quinolizin-9-1)ethenyl)-4H-pyran-4-ylidene) propanedinitrile (DCJTB) and 4-dicyanomethylene-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM), doped in polystyrene (PS) matrices were studied and compared. It was found that DCJTB has a larger net gain, 40.72 cm(-1), a lower loss, 2.49 cm(-1), and a lower threshold, 0.16 (mJ/pulse)/cm(2), than DCM, which has a net gain of 11.95 cm(-1), a loss of 9.25 cm(-1), and a threshold of 4(mJ/pulse)/cm(2). The improvement of performance in DCJTB PS films is attributed to the larger free volume of DCJTB caused by the introduction of steric spacer groups into the DCJTB molecule.

Self-assembled multilayer films of europium-substituted polyoxometalate and their luminescence properties

A new kind of hybrid self-assembled film was obtained by means of alternating deposition of the polyoxometalate (POM), K-13[Eu(SiW11-O-39)(2)], and polyacrylamide (PAA) on the 3-aminopropylsilanized precursor film. The experimental results showed that the polyanions were successfully incorporated into the self-assembled multilayers of the polyacrylamide. The scanning electron microscopy (SEM) was taken to study the surface morphology of the film. The X-ray photoelectron spectra (XPS) verified that the polyoxometalates were incorporated into the multilayer films with a certain adsorption interaction. The effects of the polyacrylamide on the luminescence of the polyoxometalate were discussed in detail. The luminescence spectra showed that the energy was transferred from the ligands to the Eu3+ ions in the self-assembled films.

Electrolummescence from (5)Do -> F-7(J) and D-5(1) -> F-7(J) (J=0-4) transitions with a europium complex as emitter

Electroluminescence (EL) devices with Eu(HTH)(3)phen [HTH: 4,4,5,5,6,6,6-heptafluoro-1-(2-thienyl)-1,3-hexanedione, phen: I 10-phenanthroline] as an emissive centre were fabricated using vacuum evaporation. In addition to the Eu3+ 5D0 --> F-7(J) (J = 0-4) lines that were visible in the photoluminescence signal, the device also showed strong emission from the D-5(1) --> F-7(J) (J = 0-4) transitions. The enhanced emission from the D-5(1) F-7(J) (J = 0-4) transitions was attributed to the increased excitation intensity in the EL device. The luminescence lifetimes of the 5 D, and 5 Do levels were measured to be 0.6 mus and 866 mus, respectively.