Light phosphorescence

Osmium complexes. Red light phosphorescence emitting devices have been reported using osmium complexes. Efficient red emission was achieved using an in situ pol mierized tetraphenyldiaminobiphenyl-con-taining polymer as the hole-transporting layer and a blend osmium complexes of PVK and 2-rm-butylphenyl-5-biphenyl-l,3,4-oxadiazole (BPD)... 

Following exposure to light, phosphorescent substances glow for several minutes as the excited phosphors return to the ground state very slowly. 

The excited molecule can cross over to its triplet state (intersystem crossing), and from its triplet state, lose energy in the form of heat (internal conversion) and/or light phosphorescence, as it returns to its ground state. 

Spectroscopists observed that molecules dissolved in rigid matrices gave both short-lived and long-lived emissions which were called fluorescence and phosphorescence, respectively. In 1944, Lewis and Kasha [25] proposed that molecular phosphorescence came from a triplet state and was long-lived because of the well known spin selection rule AS = 0, i.e. interactions with a light wave or with the surroundings do not readily change the spin of the electrons. 

White phosphorus is very reactive. It has an appreciable vapour pressure at room temperature and inflames in dry air at about 320 K or at even lower temperatures if finely divided. In air at room temperature it emits a faint green light called phosphorescence the reaction occurring is a complex oxidation process, but this happens only at certain partial pressures of oxygen. It is necessary, therefore, to store white phosphorus under water, unlike the less reactive red and black allotropes which do not react with air at room temperature. Both red and black phosphorus burn to form oxides when heated in air, the red form igniting at temperatures exceeding 600 K,... 

The total fluorescence (or phosphorescence) intensity is proportional to the quanta of light absorbed, To — T, and to the efficiency (f>, which is the ratio of quanta absorbed to quanta emitted ... 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

An x-ray area detector can be used to collect the intensities of many reflections at a time. The crystal must be oriented in many different settings with respect to the incident beam but the detector needs to be positioned at only a few positions to collect all of the data. A charge coupled device (CCD) is used as the area detector on the Siemens SMART single crystal diffractometer system. The SMART detector consists of a flat 6-cm circular phosphorescent screen that converts x-ray photons to visible light photons. The screen is coupled to a tapered fiber optics bundle which is then coupled to a one inch by one inch square CCD chip. The CCD chip has 1024 x 1024 pixels each of which stores an electrical charge proportional to the number of... 

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. 

Fluorescent and phosphorescent substances are excited into an unstable energy state by UV light. When they return to the ground state they release a part of the energy taken up in the form of radiation. The emitted radiation is less energetic than the light absorbed and usually lies in the visible part of the spectrum. Since absorption (excitation) and emission obey a linear relationship over a certain range a reduction in absorption leads to a reduction in the luminescence, too. 

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... 

The commonest form of phosphorus, and the one which is usually formed by condensation from the gaseous or liquid states, is the waxy, cubic, white form o -P4 (d 1.8232 gcm at 20°C). This, paradoxically, is also the most volatile and reactive solid form and thermodynamically the least stable. It is the slow phosphorescent oxidation of the vapour above these crystals that gives white phosphorus its most characteristic property. Indeed, the emission of yellow-green light from the oxidation of P4 is one of the earliest recorded examples of chemiluminescence, though the details of the reaction... 

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