Investigation of carrier injection mechanism in small molecular organic light emitting device with a mixed single organic layer

Injection properties of electrons and holes in a mixed single layer organic light emitting device with mixed small molecules tris-(8-hydroxy-quinoline) aluminum (Alq₃), 2,5-bis(6′-(2′,2″-bipyridyl))-1,1-dimethyl-3,4-diphenylsilole (PyPySPyPy), 4′-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (α-NPD), and 5,6,11,12-tetraphenylnaphthacene (rubrene) were investigated using Au/MoO₃ as hole and Al alloy as electron injection electrodes. On the basis of measuring the temperature dependence of currents through the interface between the electrodes and the mixed single organic layer, the carrier injection mechanism was primarily ascribed to the Schottky thermionic emission with the barrier height of 0.25 eV for holes and 0.67 eV for electrons. By adding the dopant material rubrene and the electron transport material PyPySPyPy into the mixed single layer, the barrier height of electrons could be reduced. The interfacial state analysis demonstrated that the electron barrier height was also dependent on the interfacial conditions of the device.     

The characterization of electroplex generated from the interface between 2-(4-trifluoromethyl-2-hydroxyphenyl)benzothiazole] zinc and N,N′-diphenyl-N,N′- bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine

The electroplex between (2-(4-trifluoromethyl-2-hydroxyphenyl)benzothiazole) zinc [Zn(4-TfmBTZ)₂] as an electron-acceptor and N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) as an electron-donor was characterized by bilayer, blend, and multilayer quantum-well (MQW) device, respectively. The blend composition and quantum-well number are effective parameters for tuning electroluminescence color. White light with high color purity and color rendering index (CRI) was observed from these devices based on Zn(4-TfmBTZ)₂/NPB. Moreover, the blend and MQW devices all exhibit high operation stability, hence excellent color stability. For the device with 5 mol% NPB in blend layer, its Commission International Del’Eclairage (CIE) coordinate region is x=0.28–0.31, y=0.33–0.35 and CRI is 83.3–91.2 at 5–9 V. For MQW structure device with NPB of 60 nm thickness, its CIE coordinate region is x=0.29–0.32, y=0.31–0.34 and CRI=87.9–92.5 at 10–15 V. Such high color stability and purity and CRI, being close to ideal white light, are of current important for white OLED.     

Magnetic Field Effects on the Photoinduced Electron-Transfer Reactions in a Ruthenium Porphyrin–C60 Ligand Complex

Magnetic field effects (MFEs) on a photoinduced electron-transfer reaction in a semi-rigid linked system (RuP–C₆₀) were examined in toluene and o-dichlorobenzene. The transient absorption spectra indicated that the photogenerated biradical produced by an intramolecular electron-transfer reaction was observed only in o-dichlorobenzene. The decay rate constants (k _d) for the biradical decreased steeply at lower magnetic fields (≤0.06 T) and rapidly recovered at 0.08 T. The k _d value was observed to decrease gradually above 0.2 T. The MFEs strongly indicated the formation of the singlet-born biradical for RuP–C₆₀. The dip in the MFE at 0.06 T was ascribed to the S–T level crossing mechanism. The MFEs above 0.2 T can be explained by a spin–spin relaxation mechanism due to the anisotropic Zeeman interaction being related to the exchange interaction.     

The effect of parametrically excited spin waves on the dispersion and damping of magnetostatic surface waves in ferrite films

This paper describes an experimental study of variations of the dispersion and damping of magnetostatic surface waves in ferrite films, caused by three-and four-magnon interactions with parametric spin waves excited by an auxiliary surface magnetostatic pump wave with frequency f _p. The variations in the dispersion and damping were identified, respectively, with variations Δk″ in the real part and Δk′ in the imaginary part of the wave number of the surface magnetostatic wave. The Δk″ and Δk′ values were determined from the ratio of the changes of the phase increment Δφ and the amplitude increment ΔA of the surface magnetostatic wave to the length L of the nonequilibrium section of the film, where the parametric spin waves exist. It is found that, when three-magnon decay processes are allowed for the pump wave and the surface magnetostatic probe wave, the probe wave can substantially alter the distribution of the parametric spin waves in the film. Zh. éksp. Teor. Fiz. 115, 318–332 (January 1999)     

Preparation and characterization of organic–inorganic hybrid compound [N(C<Subscript>4</Subscript>H<Subscript>9</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>Cu<Subscript>2</Subscript>Cl<Subscript>6</Subscript>

Organic–inorganic hybrid sample [N(C₄H₉)₄]₂Cu₂Cl₆ was prepared via the reaction between copper chloride and tetrabutylammonium chloride. The compound was characterized by X-ray powder diffraction, IR, Raman, differential scanning calorimetry (DSC), DTA-TGA analysis and electrical impedance spectroscopy. DSC studies indicate a presence of one-phase transition at 343 K. The complex impedance of compound [N(C₄H₉)₄]₂Cu₂Cl₆ have been investigated in temperature and frequency ranges 300–380 K and 200 Hz–5 MHz, respectively. The Z′ and Z″ versus frequency plots are well fitted to an equivalent circuit model. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements CPE. The frequency dependence of the conductivity is interpreted in term of Jonscher's law: s(w) = s_dc + Awⁿ \sigma (\omega ){ } = {\sigma_{\rm{dc}}} + { }A{\omega^n} . The conductivity follows the Arrhenius relation. The variation of the value of these elements with temperatures confirmed the availability of the phase transition at 343 K detected by DSC and electrical measurements.     

Relaxation phenomena of polar non-polar liquid mixtures under low and high frequency electric field

Simultaneous calculation of the dipole moment μ_j and the relaxation time τ_j of a certain number of non-spherical rigid aliphatic polar liquid molecules (j) in non-polar solvents (i) under 9.8 GHz electric field is possible from real ε′_ij and imaginary ε″_ij parts of the complex relative permittivity ε*_ij. The low frequency and infinite frequency permittivities ε_0ij and ε_∞ij measured by Purohitet al [1,2] and Srivastava and Srivastava [3] at 25, 35 and 30°C respectively are used to obtain static μ_s. The ratio of the individual slopes of imaginary σ″_ij and real σ′_ij parts of high frequency (hf) complex conductivity σ*_ij with weight fractionsw _jatw _j → 0 and the slopes of σ″_ij— σ′_ij curves for differentw _js [4] are employed to obtain τ_js. The former method is better in comparison to the existing one as it eliminates polar-polar interaction. The hf μ_js in Coulomb metre (C m) when compared with static and reported μs indicate that μ_s s favour the monomer formations which combine to form dimers in the hf electric field. The comparison among μs shows that a part of the molecule is rotating under X-band electric field [5]. The theoretical μ_theos from available bond angles and bond moments of the substituent polar groups attached to the parent molecules differ from the measured μ_js and μs to establish the possible existence of mesomeric, inductive and electromeric effects in polar liquid molecules.     

CORAL: QSPR models for solubility of [C60] and [C70] fullerene derivatives

Quantitative structure–property relationships (QSPRs) between the molecular structure of [C₆₀] and [C₇₀] fullerene derivatives and their solubility in chlorobenzene (mg/mL) have been established by means of CORAL (CORrelations And Logic) freeware. The CORAL models are based on representation of the molecular structure by simplified molecular input line entry system (SMILES). Three random splits into the training and the external validation sets have been examined. The ranges of statistical characteristics of these models are as follows: n = 18, r ² = 0.748–0.815, s = 15.1 –17.5 (mg/mL), F = 47–71 (training set); n = 9, r ² = 0.806–0.936, s = 12.5–17.5 (mg/mL), F = 29–103 (validation set).     

Interaction with DNA of the novel cationic complex (2, 2′: 6′, 2″-terpyridine)methyl platinum(II)

Summary The planarity and electron delocalization of the 2, 2′:6′, 2″-terpyridine (terpy) moiety in salts of the type [Pt(terpy)Me]X leads to extensive stacking interactions in aqueous solution. UV/VIS,¹H NMR and resonance light scattering spectra provide evidence for the tendency of the complex to form large aggregates even under low ionic strength conditions. The interaction of the cationic complex with calf thymus DNA was investigated by spectroscopic techniques and mobility assays. At highr _f ratios the complex seems to form extended aggregates on the surface of the nucleic acid but at lowerr _f evidence was obtained for intercalation. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

New Luminescent Bioprobes Eu(lll)-phloroglucinol Derivatives and Their Spectrofluorimetric, Electrochemical Interactions with Nucleotides and DNA

Two new ligands derived from phloroglucinol 2-{[(4-methoxy benzoyl ) oxy ] } methyl benzoic acid[L1] and 2-{[(4-methyl benzoyl )oxy] methyl} benzoic acid[L2] were synthesized. The solid complex Eu(III)-L2 has been synthesised and characterized by elemental analysis ,UV and IR spectra. The reaction of Eu(III) with the two synthesized ligands has been investigated in I = 0.1 mol dm^-3 p-toluene sulfonate by cyclic voltammetry and square wave voltammetry. The reaction of Eu (III)–L1 and Eu (III)–L2 binary complexes with nucleotide 5′-AMP , 5′-ADP ,5′-ATP , 5′- GMP , 5′-IMP , and 5′-CMP has been investigated using UV, fluorescence and electrochemical methods. The experimental conditions were selected such that self-association of the nucleotides and their complexes was negligibly small, that is, the monomeric complexes were studied. The interaction of the Eu(III)–L1 or L 2 solid complexes with calf-thymus DNA has been investigated by fluorescence and electrochemical methods including cyclic voltammetery(CV) ,differential pulse polarography (DPP) and square wave voltammetry (SWV) on a glassy carbon electrode. The fluorescence intensity of Eu(III)-L2 complex was enhanced with the addition of DNA. Under optimal conditions in phosphate buffer pH 7.0 at 25 °C the linear range is 3–20 μM for calf thymus DNA (CT–DNA) and the corresponding determination limit is 1.8 μM.     

Determination of the spectral dependence [n=f(λ)] of strontium fluoride thin films in the wavelength range of 460–1000 nm

The optical properties of strontium fluoride thin films prepared by evaporation indicated the inhomogeneity of these films. This is attributed to the difference in the measured refractive indexn′ ₁ of the film layer adjacent to air and that of the film layer adjacent to the glass substrate,n″ ₁. It was also found that the refractive indicesn′ ₁ andn″ ₁ for fresh strontium fluoride films are independent on the film thickness. The optical properties of strontium fluoride thin films showed no sign of change upon aging within 3–4 weeks.     

Pulsed KrF-laser synthesis of single-wall-carbon-nanotubes: effects of catalyst content and furnace temperature on their nanostructure and photoluminescence properties

In this article, we report on the use of a pulsed KrF-laser (248 nm, 20 ns) for the synthesis of single wall carbon nanotubes (SWCNTs) from the ablation of a graphite target loaded with Co/Ni catalyst, under various growth conditions. By varying the Co/Ni catalyst load of the graphite target, from 0 to 2.4 at.%, the laser synthesized SWCNTs, under a furnace temperature (T _f) of 1,100 °C, were found to be decorated by C₆₀ buckyballs, of which the density decreases as the catalyst content is increased. The effect of the catalyst content of the laser-ablated graphite target on the produced carbon nanostructures (C₆₀ vs. SWCNTs) was systematically investigated by means of various characterization techniques, including Raman spectroscopy, thermogravimetry, and SEM/HR-TEM microscopies. A [Co/Ni] ≥ 1.2 at.% was identified as the optimal concentration for the production of SWCNTs without any detectable presence of C₆₀ buckyballs. Thus, under the optimal growth conditions (i.e., [Co/Ni] = 1.2 at.% and T _f = 1,100 °C), the produced SWCNTs were found to be characterized by a very narrow diameter distribution (centered on 1.2 nm) with lengths in excess of 10 μm. By increasing T _f from 900 to 1,150 °C, the diameter of the SWCNTs can be varied from ~0.9 to ~1.3 nm. This nanotube diameter variation was evidenced by Raman and UV–Vis absorption measurements, and its effect on the photoluminescence of the SWCNTs is presented and discussed.     

Magnetic properties of ordered nanowires of the quasi-two-dimensional antiferromagnet SpFeMn(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript>

Ordered arrays of nanowires of the photochromic antiferromagnet SpFeMn(C₂O₄)₃ (where Sp is 1-{(1′,3′,3′-trimethyl-6-nitro-5′-chlorospiro[2H-1-benzopyran-2,2′-indolin]-8-yl)methyl}pyridinium) have been fabricated in anodized aluminum oxide pores with diameters of 20 and 200 nm. It has been revealed that the growth of the spin-glass phase with noncollinear ordering of spins in nanowires is suppressed in favor of the uniaxial antiferromagnetic phase. A decrease in the nanowire diameter leads to an increase in the anisotropy of the magnetic resonance spectra. This is associated with the magnetocrystalline anisotropy that considerably exceeds the anisotropy of the nanowire shape.     

Sheath and electron density dynamics in the normal and self-pulsing regime of a micro hollow cathode discharge in argon gas

A microplasma is generated in the microhole (400 μm diameter) of a molybdenum-alumina-molybdenum sandwich (MHCD type) at medium pressure (30–200 Torr) in pure argon. Imaging and emission spectroscopy have been used to study the sheath and electron density dynamics during the stationary normal regime and the self-pulsing regime. Firstly, the evolution of the microdischarge structure is studied by recording the emission intensity of the Ar (5p[3/2]₁–4s[3/2]₁)_{1}) line at 427.217 nm, and Ar⁺ (4p′ ²P_3/2–4s′ ²D_5/2)_{5/2}) line at 427.752 nm. The maximum of the Ar⁺ line is located in the vicinity of the sheath-plasma edge. In both regimes, the experimental observations are consistent with the position of the sheath edge calculated with an ionizing sheath model. Secondly, the electron density is recorded by monitoring the Stark broadening of the H_b_\beta-line. In the self-pulsing regime at 150 Torr, the electron density reaches its maximum value of 4 × 10¹⁵ cm^-3, a few tens of ns later than the discharge current maximum. The electron density then decays with a characteristic decay time of about 2 μs, while the discharge current vanishes twice faster. The electron density in the steady-state regime is two orders of magnitude lower, at about 6–8 × 10¹³ cm^-3.     

Measuring the Michel parameter ξ'

Unlike the majority of Michel parameters which are consistent with the Standard Model V-A interaction, the experimental value of ξ″(=0.65±0.36) [1] is poorly known. Our experiment will measure the longitudinal polarization, P _L , of positrons emitted from the decay of polarized muons. The value of P _L , equal to unity in the Standard Model, will decrease for high energy positrons emitted antiparallel to the muon spin if the combination of Michel parameters ξ″/ξξ′ − 1 deviates from the Standard Model value of zero. This revised version was published online in August 2006 with corrections to the Cover Date.     

Experimental study of signal trapping of OH laser induced fluorescence and chemiluminescence in flames

The absorption of OH^∗ chemiluminescence and laser-induced fluorescence (LIF) in the exhaust gas of confined premixed laminar CH₄/air flames at atmospheric pressure was investigated. One flame was used as source and a second as absorber. OH LIF was excited in the ν″=0→ν′=1 band of the A–X electronic system around ≈283 nm and spectrally resolved detected in the (0,0) and (1,1) vibrational bands around 305–320 nm. For OH^∗ chemiluminescence, spectrally resolved detection was performed in the wavelength range 280–340 nm. For an absorption path of 54 mm and at T≈2000 K, signal trapping on the order of 10–40% was observed. Signal trapping was most pronounced in the (0,0) band, as expected from the thermal population distribution of OH in the electronic ground state. The spectral distribution of the signals and the wavelength dependence of the signal trapping are addressed in this paper. Implications from the results with respect to detection strategies and chemiluminescence-based equivalence ratio measurements are discussed.     

Performance improvement of rubrene-based organic light emitting devices with a mixed single layer

We have investigated the performance of organic light-emitting devices (OLEDs) with a rubrene-doped mixed single layer by using 4,4′-bis[N-(1-napthyl)-N-phenyl- amion] biphenyl (α-NPD) as hole transport layer. Comparing to a conventional heterostructure OLED, equal luminance vs. current density characteristics were obtained. In addition, maximum power efficiency was threefold improved, and the achieved value was 5.90 lm/W by optimizing a mixing ratio of hole and electron transport materials. By evaluating the temperature dependence of the J – V characteristics for electron-injection dominated device, the electron injection from Al/LiF to mixed organic layer is attributed to Schottky thermal emission model. And the barrier height of the electron injection from Al/LiF into mixed single layer was obtained to be 0.62 eV, which is lower than Al/Alq₃ interface. Meanwhile, the mixed single-layer device exhibited superior operational durability at a half-luminance of 2,250 h under a constant current operation mode. The reliability was improved with a factor of two compared to the heterostructure device due to the improvement of stability in mixed organic molecules and removal of the heterojunction interface in the mixed single-layer device.     

Design and Synthesis of a Terbium(III) Complex-Based Luminescence Probe for Time-Gated Luminescence Detection of Mercury(II) Ions

Time-gated luminescence detection technique using lanthanide complexes as luminescent probes is a useful and highly sensitive method. However, the effective application of this technique is limited by the lack of the target-responsive luminescent lanthanide complexes that can specifically recognize various analytes in aqueous solutions. In this work, a dual-functional ligand that can form a stable complex with Tb³⁺ and specifically recognize Hg²⁺ ions in aqueous solutions, N,N,N ¹ ,N ¹ -{[2,6-bis(3′-aminomethyl-1′-pyrazolyl)-4-[N,N-bis(3″,6″-dithiaoctyl)-aminomethyl]- pyridine]} tetrakis(acetic acid) (BBAPTA), has been designed and synthesized. The luminescence of its Tb³⁺ complex is weak, but can be effectively enhanced upon reaction with Hg²⁺ ions in aqueous solutions. The luminescence response investigations of BBAPTA-Tb³⁺ to various metal ions indicate that the complex has a good luminescence sensing selectivity for Hg²⁺ ions, but not for other metal ions. Thus a highly sensitive time-gated luminescence detection method for Hg²⁺ ions was developed by using BBAPTA-Tb³⁺ as a luminescent probe. The dose-dependent luminescence enhancement of the probe shows a good linearity with a detection limit of 17 nM for Hg²⁺ ions. These results demonstrated the efficacy and advantages of the new Tb³⁺ complex-based luminescence probe for the sensitive and selective detection of Hg²⁺ ions.     

Functionalized perylenes: origin of the enhanced electrical performances

In this letter we compare the transistor performances of two solution-processed perylene derivatives: N,N′-bis (n-octyl)- dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN₂) and N,N′-1H,1H-perfluorobutyl dicyanoperylenediimide (PDIF-CN₂). Perylenediimide nitrogen functionalization with perfluoroalkyl vs. alkyl chains improves the electron mobility of solution-processed organic field effect transistors (OFETs) by one order of magnitude. Time resolved spectroscopy allows attributing this increment to a higher degree of co-facial arrangement of the fluorinated molecules. This supramolecular arrangement enhances the π–π overlap leading to more efficient electron transport.     

Influence of local excitation of octupolar Oligophenylenevinylenes on their dipole moments

Absorption and fluorescence spectra of octupolar centrosymmetric oligophenylenevinylene dyes (E,E)-bis[2-(4-hexyloxyphenyl)ethenyl]-(E,E)-3,6-bis-[2-(4-N,N-dipropylaminophenyl)ethenyl]pyrazine and (E,E,E,E)-2,3,5,6- tetra-[2-(4-hexyloxyphenyl)ethenyl]pyrazine were measured in various solvents. An electro-optical absorption method was used to determine their dipole moments as μ _g  = 6.1∙10^–30 and 3.4∙10^–30 C∙m in the equilibrium ground state and the increased values ∆ ^a μ = 11.9∙10^–30 and 8.2∙10^–30 C∙m upon excitation into a Franck–Condon state. Quantum-chemical calculations showed that the molecules had non-planar configurations. The π,π-conjugated system was localized on the most planar part of the molecule that was responsible for light absorption in the range 300–450 nm due to a change in the geometry of the molecules in the ground state. Localized excitation of the molecules caused their dipole moments ∆ ^a μ to change significantly.     

Nanoclusters in mixed crystals Hg<Subscript>2</Subscript><Emphasis Type="Italic">Hal</Emphasis><Subscript>2</Subscript>

The Raman spectra of mixed Hg₂(Br,I)₂ and Hg₂(Cl,Br)₂ crystals were investigated. The multimode behavior of optical vibrations induced by presence of three types of molecules (Hg₂ Hal′₂, Hg₂ Hal″₂ and Hg₂ Hal′Hal″) in the crystals was observed experimentally and is discussed theoretically. Phase transition effects such as soft modes, density of states, IR vibration branches, and anti- and ferroelectric nanoclusters induced by polar Hal′-Hg-Hg-Hal″ molecules and their immediate environment, were observed in the Raman spectra.