Evolution of emission manners of organic light-emitting diodes: From emission of singlet exciton to emission of doublet exciton

The emission manners of organic light-emitting diodes(OLEDs) have experienced almost three-decade evolution.In this review,we briefly summarized the emission manners of OLEDs including:(ⅰ) emission from singlet exciton;(ⅱ) emission from triplet exciton;(ⅲ) emission from singlet exciton converted from triplet exciton.Then we introduced a new type of OLEDs with the emission from doublet exciton,wherein organic neutral radicals are used as emitters.Due to the spin-allowed transition of doublet excitons,using neutral radicals as emitters is believed to be a new way to break the 25%upper limit of internal quantum efficiency of OLEDs.The progress of emissive stable neutral radicals is also shortly reviewed.     

压致变色聚集诱导发光材料

聚集诱导发光化合物分子具有特殊的螺旋桨形扭曲构象结构, 导致其很难在结晶状态下进行紧密堆砌, 使得其结晶结构容易在外力的作用发生改变, 致使其分子能级水平和发光光谱发生变化, 产生压致发光变色现象. 因此, 聚集诱导发光化合物是压致发光变色材料的一个重要来源. 压致变色聚集诱导发光材料是一类重要的压致发光变色材料, 其既具有压致发光变色的性能, 又具有聚集诱导发光的性能. 它是一类智能材料, 在应力传感、商标防伪和发光器件等领域具有重要的潜在应用, 近年来受到人们极大的关注. 本文分类介绍了近年来压致变色聚集诱导发光材料的研究进展.     

Excimer emission in 9-aminoacridine hydrochloride

Excimer emission in concentrated aqueous solutions of 9-aminoacridine hydrochloride has been observed at 555 nm. A study of monomer, dimer and excimer emission, absorption and excitation spectra shows that the excited dimer emission is transformed into excimer emission at higher temperatures. Unusually large hypochromism is observed on dimer formation.     

Acoustic emission in solid-liquid decomposition reactions

The chemical acoustic emission method was used to study solid-liquid decomposition reactions for the example of the interaction of potassium carbonate pellets with solutions of hydrochloric acid (HCl). The reaction was shown to follow different mechanisms at different hydrochloric acid concentrations (the concentration was varied from 0.12 to 12 M). An increase in the concentration of HCl not always caused an increase in the rate of generation of acoustic emission signals and, therefore, the reaction rate. The generation of acoustic emission was studied at various reaction stages. The advantages of the chemical acoustic emission method include measurements in real time (this is of importance for fast reactions, the reaction studied took no more than 5 min) and the possibility of determining rates at separate reaction stages. An automated acoustic emission unit was tested. The unit showed high reproducibility of measurement results, reliability, and rich possibilities for studying chemical acoustic emission in solids.     

Wavelength table of chromium emission lines in argon glow discharge optical emission spectrometry

A wavelength table of chromium lines emitted from an argon glow discharge plasma, which comprises 2049 atomic and ionic emission lines in the wavelength range of 200–440 nm, is presented. The relative intensities are rather different from the data of published wavelength tables based on arc-excited and spark-excited spectra. Emission lines of Ar, Ti, V, Fe, Ni, and Cu in the neighborhood of the prominent Cr emission lines are also compiled as a table. These tables could be employed for the analytical applications in glow discharge optical emission spectrometry. All of the data are presented as Supplementary Electronic Material.     

Quantification of acetone emission from pine plants

Acetone emission from pine plants (pinus sylvestris) is measured by continuously stirred tank reactor. Under a constant light intensity, acetone emission rates increase exponentially with leaf temperature. When leaf temperature is kept constant, acetone emission increases with light intensity. And acetone emission in darkness is also detected. Acetone emitted from pine is quickly labeled by 13C when the plants are exposed to air with 630 mg/m313CO2. However, no more than 20% of acetone is 13C labeled. Acetone emission from pine may be due to both leaf temperature-controlled process and light intensity-controlled process. Based on these understandings, an algorithm is used to describe the short term acetone emission rates from pine.     

Wavelength table of chromium emission lines in argon glow discharge optical emission spectrometry

A wavelength table of chromium lines emitted from an argon glow discharge plasma, which comprises 2049 atomic and ionic emission lines in the wavelength range of 200–440 nm, is presented. The relative intensities are rather different from the data of published wavelength tables based on arc-excited and spark-excited spectra. Emission lines of Ar, Ti, V, Fe, Ni, and Cu in the neighborhood of the prominent Cr emission lines are also compiled as a table. These tables could be employed for the analytical applications in glow discharge optical emission spectrometry. All of the data are presented as Supplementary Electronic Material. Recieved: 22 December 1999 / Revised: 25 February 2000 / Accepted: 25 February 2000     

Electron emission during deformation of polymers

Electron emission was detected during deformation of both carbochain and heterochain polymers in vacuum. It was found that the features of emission are similar to those observed in molecular scissions under drawing of unoriented and oriented polymers. This fact indicates that there is a relationship between the fracture process and electron emission under deformation of polymers. This relationship is also obvious from the experiments with interruption of loading when electron emission during the repeated loading does not begin immediately at the moment of load application, but can be recorded only at the degree of deformation which is higher than that reached during the first loading. The interconnection between deformational electron emission and molecular scissions allowed visualization of the fracture process in the subsurface layers of polymers using an electron-optical convertor which gives a mechanoemission image of a stretched sample. It is supposed that the deformation-induced electron emission of polymers is caused by ionization of stressed macromolecules resulting from tunnel transitions of electrons into deep traps. During deformation, the traps are destroyed and a part of electrons escapes in vacuum.     

Optical emission of a molecular nanoantenna pair

The optical emission from a pair of nanoantennas is investigated within the theoretical framework of quantum electrodynamics. The analysis of fluorescent emission from a pair of molecular antenna species in close proximity is prompted by experimental work on oriented semiconductor polymer nanostructures. Each physically different possibility for separation-dependent features in photon emission by any such pair is explored in detail, leading to the identification of three distinct mechanisms: emission from a pair-delocalized exciton state, emission that engages electrodynamic coupling through quantum interference, and correlated photon emission from the two components of the pair. Although each mechanism produces a damped oscillatory dependence on the pair separation, each of the corresponding results exhibits an analytically different form. Significant differences in the associated spatial frequencies enable an apparent ambiguity in the interpretation of experiments to be resolved. Other major differences are found in the requisite conditions, the associated selection rules, and the variation with angular disposition of the emitters, together offering grounds for experimental discrimination between the coupling mechanisms. The analysis paves the way for investigations of pair-wise coupling effects in the emission from nanoantenna arrays.     

Radiation-induced emission from golden hamster embryo cells

Emission from high-energy-electron-irradiated golden hamster embryo (GHE) cells has been studied over the temperature range 12–300 K both by a one-shot-single-photon-counting method and by photocurrent measurements with an oscilloscope. Emission from the irradiated phosphate buffered saline (PBS) also has been studied. The emission spectra from PBS at 12 and 77 K show a maximum around 330 and 380 nm, respectively, which are the same spectra as those from irradiated pure H₂O. The emission from irradiated GHE consists of the new band at 480 nm in addition to the emission from H₂O. The 480 nm emission is observed at the temperature range of 12–300 K, though the emission at 300 K is much lower than that at low temperature. The 480 nm emission is ascribed to the transition from excited organic substances in GHE cells. The intensity of 480 nm emission at 300 K increases linearly with increasing irradiation-dose in the range of 11–600 Gy.     

Time-resolved measurement of emission profiles in pulsed radiofrequency glow discharge optical emission spectroscopy: Investigation of the pre-peak

Radiofrequency glow discharge coupled to optical emission spectroscopy has been used in pulsed mode in order to perform a detailed study of the measured temporal emission profiles for a wide range of copper transitions. Special attention has been paid to the early emission peak (or so-called pre-peak), observed at the beginning of the emission pulse profile. The effects of the important pulse parameters such as frequency, duty cycle, pulse width and power-off time, have been studied upon the Cu pulse emission profiles. The influence of discharge parameters, such as pressure and power, was studied as well.     

Circular emission from molecular iodine

The resonance emission from molecular iodine excited by circularly polarised light is circularly polarised. We report the circular emission spectrum of iodine excited by the 5145 A line of Ar⁺. In zero field the prominent rotational triplet and the high energy P,R doublet are strongly circularly polarisation ratios close to theoretical.     

Photon emission from massive projectile impacts on solids

First evidence of photon emission from individual impacts of massive gold projectiles on solids for a number of projectile-target combinations is reported. Photon emission from individual impacts of massive Au(n) (+q) (1 ≤ n ≤ 400; q = 1-4) projectiles with impact energies in the range of 28-136 keV occurs in less than 10 ns after the projectile impact. Experimental observations show an increase in the photon yield from individual impacts with the projectile size and velocity. Concurrently with the photon emission, electron emission from the impact area has been observed below the kinetic emission threshold and under unlikely conditions for potential electron emission. We interpret the puzzling electron emission and correlated luminescence observation as evidence of the electronic excitation resulting from the high-energy density deposited by massive cluster projectiles during the impact.     

Picosecond emission anisotropy decays of dimethyl aminobenzonitrile

Picosecond polarized fluorescence experiments on DMABN (4-(dimethyl amino) benzonitrile) in polar solvents and toluene are reported. From the detected polarized fluorescence components emission anisotropy histograms are constructed and compared with the synthetic data simulated for the energy levels scheme in DMABN. For the short-wavelength emission of DMABN vibronic coupling between the close-lying S₁ and S₂ energy levels occurs and leads to a vibronically mixed polarization of the S₁ (F_B) fluorescence. Low initial values of the emission anisotropy decays for the long-wavelength (F_A) emission are observed. This observation can be explained by an assumption that the internal twisting of the amino group in DMABN changes the angular orientation of DMABN and by an assumption that the emission dipole moment in the F_A emission band is not parallel to the absorption dipole moment in the S₂ (¹L_a) absorption band.     

Observation of emission from XeOH exciplexes

The first observation of emission from a noble gas hdyroxide exciplex (XeOH) is reported. Preliminary measurements indicate an emission band of wavelength λ = 234 nm and 3 nm bandwidth. The spontaneous decay time is ? 4 ns.     

Green emission from end-group-enhanced aggregation in polydioctylfluorene

Green emission in polyfluorenes (PFs) has been attributed to aggregation or excimer emission, but recently it was reassigned as an on-chain fluorenone defect. We show here that, in dialkyl-substituted PFs that is hydrogen-free at the 9'-position of the fluorene, blue emission with very weak green emission is observed from end-capped polydioctylfluorene (PFO) for both photoluminescence and electroluminescence spectra, while the low-energy green emission at 507 nm is very pronounced only in uncapped PFO (PFOun). The facts that there is no detectable infrared absorption at around 1721 cm(-1) due to >C=O stretching vibration in PFOun and no charge-trapping occurring in the light-emitting device from PFOun are in contrast with those found in the literature-reported copolymers with fluorenone units, which have detectable infrared absorption at 1721 cm(-1) and charge-trapping in devices. We found that this green emission at around 507 nm originates from the end-group-enhanced aggregation by use of UV-vis absorption, photoexcitation spectra, and steady-state photoluminescent and electroluminescent spectra. The end-group-enhanced aggregation is much weaker in other PFs with less-ordered structures.     

Acoustic emission of a crazing polymer

The acoustic emission from a crazing polyvinyltoluene in a tensile and bending experiment is described. Acoustic emission appears as a series of bursts which most likely correspond to the initiation and growth of crazes. The emission intensity is characterised by acoustic activity (pulse rate) measured by the ring-down technique. The average activity increases with strain. During repeated loading the acoustic activity shows a measurable intensity and significant rise only beyond the maximum strain of the former runs. This is equivalent to Kaiser's effect in metals. Acoustic emission during the creep experiment occurs in three characteristic periods. They are characterized as the relaxation, fatigue, and breakdown periods. Visual observations indicate that the relaxation period corresponds to the initiation, and the fatigue period to the growth of crazes. In the breakdown period a macroscopic crack develops and the sample fails.     

Spectroscopic investigation of defect-state emission in CdSe quantum dots

CdSe quantum dots are the most studied Cd-based quantum dots with their high quantum yield, high photostability, narrow emission band, and easy synthesis procedure. They are frequently used to develop light emitting diode (LED) due to their unique photophysical properties; however, their narrow emission band causes a challenge to design white LEDs because white light emission requires emission in multiple wavelengths with broad emission bands. Here in this study, we developed CdSe quantum dots with a narrow band-edge emission band and broad defect-state emission band through a modified two-phase synthesis method. Our results revealed that defect-state emission is directly linked to the surface of quantum dots and can be excited through exciting surfactant around the quantum dot. The effect of surfactant on emission properties of CdSe quantum dots diminished upon growing a shell around CdSe quantum dots; as a result, surface-dependent defect-state emission cannot be observed in gradient heterogeneous alloyed CdS_xSe_1-x quantum dots.     

Field emission spectroscopy

The first experimental evidence of the existence of electron states localised on a clean metal surface was available to us since 1967 in the form of the well known Swanson-Crouser hump in the energy distribution of electrons field-emitted from the W(100) plane, although it took some time before its interpretation as such, in 1976, was established beyond any doubt. Subsequently, angle-resolved photoemission proved to be a more competitive method for the investigation of surface states of metals and semiconductors. However, field emission remains a useful tool in surface physics research.

After a brief introduction to field emission spectroscopy, we review recent work which shows the potentiality of this technique in the investigation of the surface properties of semiconductors, ferromagnetic metals, thermal superconductors, and of the electronic structure of supported clusters.     

Photoluminescence emission spectroscopic study of molybdenum-based heteropolyacid catalysts

Summary Photoluminescence emission spectroscopy studies of heteropolyacid (HPA) catalysts have been carried out. All the HPA samples gave rise to violet emission peak (emission peak I) at the same wavelength of 418 nm. However, a shoulder peak in the visible region (emission peak II) appeared at different wavelengths depending on the identity of the HPA sample. Photoluminescence emission peak (II) energies could be correlated with the negative differential resistance (NDR) peak voltages of nanostructured HPA monolayers measured using scanning tunneling microscopy (STM).