Phosphorescence quantum efficiency

Y. Kawamura, K. Goushi, J. Brooks, J.J. Brown, FI. Sasabe, and C. Adachi, 100% phosphorescence quantum efficiency of Ir (III) complexes in organic semiconductor films, Appl. Phys. Lett., 86 71104-71106 (2005). 

The fraction of triplet states that phosphoresce is given by the phosphorescence quantum efficiency (0P) ... 

The concept of quantum yield is also extensively used to characterize the various photophysical processes. A complete account of this matter may be found in the recent book by Birks (13). The "phosphorescence quantum yield" is defined there as "the ratio of the number of phosphorescence photons emitted to the number of absorbed photons." In the same book, the name "phosphorescence quantum efficiency" is given to the fraction of the molecules in the triplet state which phosphoresce. This last quantity, however, does not depend on the number of quanta absorbed, and thus the word "quantum" is inappropriate. On the other hand, the term "triplet formation efficiency" used by Parker (14) to indicate the "number of triplet molecules formed per quantum of exciting... 

Extensive research has been carried out on the luminescence of the Alqs complex which is used as an electron transport emitting layer in OLEDs, with strong green emission at 532 nm and phosphorescence quantum efficiency around 32% . ... 

Other benzenoid structures incorporating fused thiophenes are represented by syn (42a) [99] and anti (42b) [100] isomers of a benzene linked trithiophene system. The compounds were obtained via condensation reactions of thiophene carboxaldehydes 41a and 41b in 88 and 24 % yield, respectively. Absorption and emission maxima are given in Chart 3.5, but noteworthy is the reduction in phosphorescence quantum efficiency experienced by the anti conformer (0.04 compared with 0.56 for the syn conformer) [101]. This is not surprising considering the tight crystal packing structure of 42b, which has an effective volume occupation or packing index of 0.75 [100]. 

One-dimensional velocity distribution Specific conductivity, hard-sphere diameter for a collision, length parameter in Lennard-Jones potential, symmetry number of a molecule Intrinsic lifetime of a photoexcited state Azimuthal angular velocity in spherical polar coordinates, azimuthal angle in spherical polar coordinates, angle of deflection Quantum yield at wavelength A Fluorescence (phosphorescence) quantum efficiency... 

Well-known phosphorescent emitter devices are iridium(III) and platimnn(ll) complexes. When placed in a suitable host material (small molecules of polymeric materials), such emitters find applications in full-color displays. One of the main requirements for OLEDs doped with phosphorescent emitters is that the phosphorescent emitter should exhibit very high phosphorescence quantum efficiencies. Similar interesting properties are also displayed in many other metal complexes, such as osmium(II), rhemium(l), and ruthenium(II). Platimmi(ll) porphyrinas [141] or iridium(lll) complexes containing 2-phenylpylidine,... 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Thus we would expect the phosphorescence efficiency to be greater for the first case than the second. In agreement with this conclusion, Similar effects are observed for heterocycles for example, the phosphorescence quantum yield for pyrazine (lowest n, it triplet) is 0.30(119) while that for quinoline in a hydroxylic solvent (lowest 77,77 triplet) is 0.19/305... 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

In 1962, Parker and Hatchard described a photoelectric spectrometer for phosphorescence measurements with which they were capable of obtaining phosphorescence spectra, and of determining lifetimes and quantum efficiencies of a large number of organic compounds. This work stimulated intensely the interest in the phosphorimetry of diverse chemical analytes [5], and one year later, Wine-... 

Griseofulvin exhibits both fluorescence and luminescence. A report by Neely et al., (7) gives corrected fluorescence excitation (max. 295 nm) and emission (max. 420 nm) spectra, values for quantum efficiency of fluorescence (0.108) calculated fluorescence lifetime (0.663 nsec) and phosphorescence decay time (0.11 sec.). The fluorescence excitation and emission spectra are given in Figure 7. 

There is no reason why the same principle cannot be applied for light-emitting polymers as host materials to pave a way to high-efficiency solution-processible LEDs. In fact, polymer-based electrophosphorescent LEDs (PPLEDs) based on polymer fluorescent hosts and lanthanide organic complexes have been reported only a year after the phosphorescent OLED was reported [8]. In spite of a relatively limited research activity in PPLEDs, as compared with phosphorescent OLEDs, it is hoped that 100% internal quantum efficiency can also be achieved for polymer LEDs. In this chapter, we will give a brief description of the photophysics beyond the operation of electrophosphorescent devices, followed by the examples of the materials, devices, and processes, experimentally studied in the field till the beginning of 2005. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

As mentioned above, phosphorescence is observed only under certain conditions because the triplet states are very efficiently deactivated by collisions with solvent molecules (or oxygen and impurities) because their lifetime is long. These effects can be reduced and may even disappear when the molecules are in a frozen solvent, or in a rigid matrix (e.g. polymer) at room temperature. The increase in phosphorescence quantum yield by cooling can reach a factor of 103, whereas this factor is generally no larger than 10 or so for fluorescence quantum yield. 

In dianions of xanthene dyes although halosubstituents decrease the quantum efficiency of fluorescence, phosphorescence efficiency is not increased proportionately. The phosphorescence lifetime decreases with fa. It is suggested that in these dyes, besides enhancement of Sx intersystem crossing rates, S nonradiative transition is promoted by heavy atom... 

For aromatic hydrocarbons, the quantum efficiencies of fluorescence and phosphorescence in low temperature glasses (Appendix H) such as EPA (ether isopentane ethyl alcohol in the ratio 2 2 5) add up to unity. This suggests that direct nonradiative decay from Si —> S0 is of very low probability. All the nonradiative paths to the ground state are coupled via the triplet state. The sequence of transfers is ... 

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