Sensitized phosphorescence

MA Baldo, ME Thomson, and SR Forrest, High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer, Nature, 403 750-753, 2000. 

S. Liu, J. Feng, and Y. Zhao, Enhanced red emission from fluorescent organic light-emitting devices utilizing a phosphorescent sensitizer, Jpn. J. Appl. Phys., 43 2320-2322 (2004). 

G He, S Chang, F Chen, Y Li, and Y Yang, Highly efficient polymer light-emitting devices using a phosphorescent sensitizer, Appl. Phys. Lett., 81 1509-1511, 2002. 

Luhman WA, Holmes RJ (2009) Enhanced exciton diffusion in an organic photovoltaic cell by energy transfer using a phosphorescent sensitizer. Appl Phys Lett 94 153304... 

The room-temperature biacetyl phosphorescence sensitized by PNA was much more intense in the presence of P-CD [202]. This was attributed to the organization of the donor-acceptor pair in a smaller reaction volume, which facilitates the triplet-triplet energy transfer. The dimensions of the CD cavity set restrictions on the number of triplet energy donors for which the enhancement was observed. The large cavity of the y-CD is, in general, the best medium to observe this phenomenon [203],... 

Figure 2.14 Spectral characteristics of luminescence observed for Nonox Cl. (1) Excitation and emission spectra at room temperature, sensitivity scale 0.01, (2) excitation and emission spectra at -196 °C for total luminescence, sensitivity scale 0.01, (3) excitation and emission spectra at -196 °C for phosphorescence, sensitivity scale 0.01 Reproduced from Kirkbright and co-workers, Elsevier [86]...

In a first part, aU the concepts needed to understand the luminescence of lanthanides are clarified. It encloses a brief overview or summary of the electronic structure of 4f-elements that then extend toward the absorption of light, the formation of excited states, and the emission of light, in other words, the luminescence phenomena. Notions such as fluorescence, phosphorescence, sensitization, and charge transfer (CT) are detailed. Again, the intent is to provide the reader with the essential tools to understand and develop... 

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. 

Sensitivity From equations 10.32 and 10.33 we can see that the sensitivity of a fluorescent or phosphorescent method is influenced by a number of parameters. The importance of quantum yield and the effect of temperature and solution composition on f and p already have been considered. Besides quantum yield, the sensitivity of an analysis can be improved by using an excitation source that has a greater... 

The relatively simple study of fluorescence and phosphorescence (based on the action of colour centres) has nowadays extended to nonlinear optical crystals, in which the refractive index is sensitive to the light intensity or (in the photorefractive variety (Agullo-Lopez 1994) also to its spatial variation) a range of crystals, the stereotype of which is lithium niobate, is now used. 

Colorless substances absorb at wavelengths shorter than those of the visible range (the UV range normally amenable to analysis X = 400...200 nm). Such compounds can be detected by the use of UV-sensitive detectors (photomultipliers. Sec. 2.2.3.1). Substances that absorb in the UV range and are stimulated to fluorescence or phosphorescence (luminescence) can be detected visually if they are irradiated with UV light. 

However, the direct determination of absorption at the wavelength of maximum absorption is more sensitive (or in the worst case at least as sensitive) as the indirect measurement of absorption by fluorescence or phosphorescence quenching. 

Differences in the materials employed for the layers can also become evident when chemical reactions are performed on them. Thus, Macherey-Nagel report that the detection of amino acids and peptides by reaction with ninhydrin is less sensitive on layers containing luminescent or phosphorescent indicators compared to adsorbents which do not contain any indicator [7]. 

Both types of processes, 7r -assisted y, -bond cleavage and P -bonding, have been invoked to operate in the phototransformations of the aldehyde-ketone (153) to products (155), (156) and (158). The conversions have been observed at room temperature in dioxane, t-butanol, ethanol and benzene using light of wavelengths 2537 A or above 3100 A or sensitization by acetophenone. The phosphorescing excited triple state of (153) is very similar to that of testosterone acetate (114), but its reactions are too rapid... 

In 1944, Lewis and Kasha (52) identified phosphorescence as a forbidden" transition from an excited triplet state to the ground singlet state and suggested the use of phosphorescence spectra to identify molecules. Since then, phosphorimetry has developed into a popular method of analysis that, when compared with fluorometry, is more sensitive for some organic molecules and often provides complimentary information about structure, reactivity, and environmental conditions (53). 

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. 

Interactions in Solid-Surface Luminescence Temperature Variation. Solid-surface luminescence analysis, especially solid-surface RTF, is being used more extensively in organic trace analysis than in the past because of its simplicity, selectivity, and sensitivity (,1,2). However, the interactions needed for strong luminescence signals are not well understood. In order to understand some of the interactions in solid-surface luminescence we recently developed a method for the determination of room-temperature fluorescence and phosphorescence quantum yields for compounds adsorbed on solid surfaces (27). In addition, we have been investigating the RTF and RTF properties of the anion of p-aminobenzoic acid adsorbed on sodium acetate as a model system. Sodium acetate and the anion of p-aminobenzoic acid have essentially no luminescence impurities. Also, the overall system is somewhat easier to study than compounds adsorbed on other surfaces, such as filter paper, because sodium acetate is more simple chemically. 

The third process sensitive to heavy-atom perturbation is the radiative decay from the triplet to the ground state (phosphorescence). Since phosphorescence is commonly not observed in fluid solution at room temperature, the rate of phosphorescence in the presence of heavy-atom perturbation relative to the rate of intersystem crossing and nonradiative decay need not be considered. At low temperatures in a rigid glass, however, phosphorescence... 

For compounds that are very weakly phosphorescent or that phosphoresce at wavelengths out of the normal range of sensitivity of the spectrometer this method of triplet energy determination cannot be applied. For these compounds triplet energies can sometimes be determined by measuring their E-type or P-type delayed fluorescence. 

The original stabilizer (HBC) was modified as the rapid radiationless deactivation of the stabilizer is (at least partly) due to the intramolecular hydrogen bond, the H-atom was substituted by a methyl group (MBC). This "probe molecule" showed fluorescence and phosphorescence and enabled us to demonstrate the energy transfer to the stabilizer, simply by studying its sensitized luminescence. 

Fluorescence and phosphorescence spectra corrected for the instrumental sensitivity were measured with a spectrometer described previously (()). Corrected excitation spectra were obtained with constant excitation intensity controlled by a rhodamine B quantum counter. For phosphorescence polarization measurements the apparatus was set up in an "In Line" arrangement (j ) and equipped with a Glan-Thomson polarizer and a sheet polarizer (analyser) (10). 

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