Luminescence Phosphorescence

LINEAR STRAIN Long-lived luminescence, PHOSPHORESCENCE JABLONSKI DIAGRAM LONG-RANGE LOOPS LOOSE ION PAIR ION PAIR... 

The requirement for a s-ms spectroscopic "clock implies that suitable photophysical processes must be used. Furthermore, in the case of inherently dilute systems such as suspensions of living cells, sensitivity of detection is a major consideration. In this respect, light emission processes offer significant advantages compared to measurements based upon light absorption. They include delayed luminescence (phosphorescence, fluorescence) involving the long-lived triplet state (7-12) and other two-laser techniques to be described below. 

Selectivity The selectivity of molecular fluorescence and phosphorescence is superior to that of absorption spectrophotometry for two reasons first, not every compound that absorbs radiation is fluorescent or phosphorescent, and, second, selectivity between an analyte and an interferant is possible if there is a difference in either their excitation or emission spectra. In molecular luminescence the total emission intensity is a linear sum of that from each fluorescent or phosphorescent species. The analysis of a sample containing n components, therefore, can be accomplished by measuring the total emission intensity at n wavelengths. 

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. 

Fluorescence and phosphorescence are types of luminescence, ie, emission attributed to selective excitation by previously absorbed radiation, chemical reaction, etc, rather than to the temperature of the emitter. Laser-iaduced and x-ray fluorescence are important analytical techniques (see... 

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. 

Fluorescence and phosphorescence are both forms of luminescence [3]. If the emission of radiation has decayed within 10 s after the exciting radiation is cut off it is known as fluorescence [4], if the decay phase lasts longer (because the electrons return to the ground state from a forbidden triplet state (Fig. 5), then the phenomenon is known as phosphorescence. A distinction is also made between... 

Fig. 5 Schematic representation of the electronic transitions during luminescence phenomena [5]. — A absorbed energy, F fluorescence emission, P phosphorescence, S ground state. S excited singlet state, T forbidden triplet transition.

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]. 

Phosphorescence. See under Fluorescence, Luminescence and Phosphorescence in Vol 6, F124-R... 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

The Early Period. Luminescent phenomena such as the aurora borealis, phosphorescence of the sea, luminous animals and insects, and phosphorescent wood were the earliest of spectral observations because th require nothing more than the... 

Luminescence measurements on proteins occupy a large part of the biochemical literature. In what surely was one of the earliest scientific reports of protein photoluminescence uncomplicated by concurrent insect or microorganism luminescence, Beccari (64), in 1746, detected a visible blue phosphorescence from chilled hands when they were brought into a dark room after exposure to sunlight. Stokes (10) remarked that the dark (ultraviolet) portion of the solar spectrum was most efficient in generating fluorescent emission and identified fluorescence from animal matter in 1852. In general, intrinsic protein fluorescence predominantly occurs between 300 nm and 400 nm and is very difficult to detect visually. The first... 

The active state of luminescence spectrometry today may be judged ly an examination of the 1988 issue of Fundamental Reviews of Analytical Chemistry (78), which divides its report titled Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectrometry into about 27 specialized topical areas, depending on how you choose to count all the subdivisions. This profusion of luminescence topics in Fundamental Reviews is just the tip of the iceberg, because it omits all publications not primarily concerned with analytical applications. Fundamental Reviews does, however, represent a good cross-section of the available techniques because nearly every method for using luminescence in scientific studies eventually finds a use in some form of chemical analysis. Since it would be impossible to mention here all of the current important applications and developments in the entire universe of luminescence, this report continues with a look at progress in a few current areas that seem significant to the author for their potential impact on future work. 

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. 

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