Fluorescence and phosphorescence

The molecule absorbs energy and an electron is promoted to one of the higher vibrational levels in the singlet state this is a vibrationally excited electronic state. The vibrationaUy excited molecule will rapidly relax to the lowest vibrational level of the 

Fluorescence and phosphorescence are both types of luminescence. They are specifically types of photoluminescence, meaning that the excitation is achieved by absorption of light. There are other types of luminescence. If the excitation of a molecule and emission of light occurs as a result of chemical energy from a chemical reaction, the luminescence is called chemiluminescence. The light emitted by a firefly is an example of bioluminescence. 

As shown in Fig. 5.37, there are other ways for molecules to return to the ground state. Excited molecules may collide with other molecules it is possible for energy to 

The intensity of fluorescence F is proportional to the amount of light absorbed by the analyte molecule. We know from Beer s Law that 

From Eq. (5.8), it can be seen that fluorescence intensity is related to the concentration of the analyte, the quantum efficiency, the intensity of the incident (source) radiation, and the absorptivity of the analyte. is a property of the molecule, as is the absorptivity, a. A table of typical values of for fluorescent molecules is given in Table 5.12. The absorptivity of the compound is related to the fluorescence intensity [Eq. (5.8)]. Molecules like saturated hydrocarbons that do not absorb in the UV /VIS region do not fluoresce. 

Chemical Analysis Second Edition Francis and Annick Rouessac 

This emission can be classified as fluorescence, which has a very rapid decrease in intensity or phosphorescence, where emission decay is much slower. The difference between the two is characterized by the value of the constant k, which for fluorescence is much greater than for phosphorescence. The lifetime of fluorescence Tq is defined, using the rate constant k, by Tq = l/k. At the instant Tq, the intensity f will become, according to expression 11.1, 36.8 per cent of the initial intensity 7q. In other words a fluorescent compound corresponds, on the microscopic scale, to a population of individual species of which 63.2 per cent have relaxed to an non-emissive state after this brief period of time. 

In a perfect molecule, electronic transitions would go like this Absorption of a photon excites a molecule from initial (usually ground) state to excited state excited state emits a photon having the same energy/frequency/wavelength and molecule goes from excited state to previous initial ground state. The first process, excitation, would be followed by the exact opposite process, called deexcitation or decay. Such processes would follow quantum-mechanical selection rules strictly. 

In reality, electronic transitions stray somewhat from the ideal selection rules. In particular, when an excited electronic state decays to a lower electronic state, a photon having the same energy as the excitation photon might not be emitted. Instead, the molecule may de-excite by transferring the extra energy into various vibrational, rotational, or solid-state vibrational (called phonon ) modes of the sample. Ultimately, this excess energy is converted into heat energy. Such processes are called radiationless transitions. 

FIGURE 15.23 Many molecules have overlapping singlet (that is, S = 0, so 2S + 1 = 1) and triplet (S = 1, so 2S + 1 = 3) electronic states. Each of these electronic states has its own vibrational state manifold. In some cases, absorbed electronic eneigy is simply dissipated by being redistributed to the vibrations of the molecules, as shown. Normally, the singlet manifold of electronic states does not interact with the triplet manifold of electronic states via allowed electronic transitions. The numerical labels in the S and T states are used to differentiate one singlet (or triplet) state from another 

FIGURE 15.24 Fluorescence occurs when an atom or molecule absorbs a photon, vibrationally relaxes, and then emits a photon to go back to the ground state. The emitted photon is always lower in energy than the absorbed photon. Fluorescence is a relatively fast process. 

Unless otherwise noted, all art on this page is Cengage Learning 2014 

More precisely, fluorescence is the spontaneous emission of light from a higher energy excited state resulting in the formation of a lower energy excited state with the same multiplicity. Thus, a T2 — Ti transition is also termed fluorescence. Despres, A. Lejeune, V. Migirdicyan, E. Siebrand, W. /. Phys. Chetn. 1988,92,6914, reported the fluorescence of photoexcited m-xylylene biradical. 

Photochcm. 1963,1,23. In order to facilitate the comparison, the absorption and emission intensities are normalized so that the 0,0 transition has the same intensity in each. 

The energy difference between the values for absorption and emission is called the Stokes shift. 

Mirror image relationship in absorption and fluorescence spectra of anthracene. (Note that the horizontal scale is linear in wave numbers, so higher energy is to the right. Reproduced from reference 41.) 

Effect of change in excited state geometry on fluorescence spectrum of 1. (The horizontal scale is linear in nm, so higher energy is to the left. Adapted from reference 43.) 

The efflciency of these radiative processes often increase at low temperatures or in solvents of high viscosity. Consequently emission spectra are generally run in a low-temperature matrix (glass) or in a rigid polymer at room temperature. The variation in eiflciency of these processes as a function of temperature and viscosity of the medium indicates that collisional processes compete with radiative and unimolecular nonradiative processes for deactivation of the lowest singlet and triplet states. 

Kasha s rule and the rate constants discussed are applicable only for molecules in solution. In the gas phase, where collisions are few, transitions from vibrationally hot states and from electronic states other than the lowest state in each manifold are common. 

Most dyes and pigments owe their colour to the selective absorption of incident light. In some compounds, colour can also be observed as a result of the emission of visible light of specific wavelengths. These compounds are referred to as luminescent. The most commonly encountered luminescent effects are fluorescence and phosphorescence. The transitions which can occur in a molecule exhibiting either fluorescence 

The traditional use of dyes is in the coloration of textiles, a topic covered in considerable depth in Chapters 7 and 8. Dyes are almost invariably applied to the textile materials from an aqueous medium, so that they are generally required to dissolve in water. Frequently, as is the case for example with acid dyes, direct dyes, cationic dyes and reactive dyes, they dissolve completely and very readily in water. This is not true, however, of every application class of textile dye. Disperse dyes for polyester fibres, for example, are only sparingly soluble in water and are applied as a fine aqueous dispersion. Vat dyes, an important application class of dyes for cellulosic fibres, are completely insoluble materials but they are converted by a chemical reduction process into a water-soluble form that may then be applied to the fibre. There is also a wide range of non-textile applications of dyes, many of which have emerged in recent years as a result of developments in the electronic and reprographic 

The transition from the singlet ground state to a triplet state is a forbidden transition. However, an excited singlet state can undergo a radiationless transition to the triplet state by reversing the spin of the excited electron. This is an energetically favorable process since the triplet state is at 

Practical Fluorescence, 2nd edn Marcel Dekker, Inc., New York, 1990. With 

---

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. 

Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section. 

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. 

Time, Cost, and Equipment As with other optical spectroscopic methods, fluorescent and phosphorescent methods provide a rapid means of analysis and are capable of automation. Fluorometers are relatively inexpensive, ranging from several... 

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... 

Fluorometry and Phosphorimetry. Modem spectrofluorometers can record both fluorescence and excitation spectra. Excitation is furnished by a broad-band xenon arc lamp foUowed by a grating monochromator. The selected excitation frequency, is focused on the sample the emission is coUected at usuaUy 90° from the probe beam and passed through a second monochromator to a photomultiplier detector. Scan control of both monochromators yields either the fluorescence spectmm, ie, emission intensity as a function of wavelength X for a fixed X, or the excitation spectmm, ie, emission intensity at a fixed X as a function of X. Fluorescence and phosphorescence can be distinguished from the temporal decay of the emission. 

Cadmium Silicates. Cadmium orthosihcate [15857-59-2] Cd2SiO, (mp 1246°C d = 5.83 g/ inL) and cadmium metasihcate [13477-19-5] CdSiO, are both prepared by direct reaction of CdO and Si02 at 390°C under 30.4 MPa (300 atm) or at 900°C and atmospheric pressure ia steam. The materials are phosphors whea activated with Mn (IT) ioa and are both fluorescent and phosphorescent. 

Organic colors caused by this mechanism are present in most biological colorations and in the triumphs of the dye industry (see Azinedyes Azo dyes Eluorescent whitening agents Cyanine dyes Dye carriers Dyes and dye intert diates Dyes, anthraquinone Dyes, application and evaluation Dyes, natural Dyes, reactive Polymethine dyes Stilbene dyes and Xanthenedyes). Both fluorescence and phosphorescence occur widely and many organic compounds are used in tunable dye lasers such as thodamine B [81-88-9], which operates from 580 to 655 nm. 

Of interest, and occasional importance (in whiteness enhancers, for instance), are the fluorescent properties of heterocyclic compounds. Fluorescence is quite frequently found in the compounds relevant to this volume the acridines and acridones show it particularly often, but it appears in a number of very diverse systems. The fluorescence and phosphorescence of heterocyclic molecules have been reviewed by Schulman <74PMH(6)147). 

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. 

Physical methods Physical methods include photometric absorption and fluorescence and phosphorescence inhibition, which is wrongly referred to as fluorescence quenching [1], and the detection of radioactively labelled substances by means of autoradiographic techniques, scintillation procedures or other radiometric methods. These methods are nondestructive (Chapt. 2). 

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... 

R. S. Becker, Theory and Interpretation of Fluorescence and Phosphorescence , J. Wiley, New York, 1969. 

H H Willard, L L Merritt, J R Dean and F A Settle, Instrumental methods of analysis, Molecular Fluorescence and Phosphorescence Methods, 6th edn, Van Nostrand Reinhold, New York, 1981, Chapter 5... 

Fluorescence and phosphorescence spectra of poly(propynoic acid)(FPA), polyphenylene (PP), and DPAcN show that the difference of energies between the lower excited singlet and triplet states, as observed in the case of PP (583 nm) and DPAcN (528 nm), is considerably greater than that of poly(propynoic acid) (270—300 nm) which besides transitions may undergo rr - transitions. PCSs showing only... 

In order to clear up the mechanism of inactivation of excited states, we examined the processes of quenching of fluorescence and phosphorescence in PCSs by the additives of the donor and acceptor type253,2S5,2S6 Within the concentration range of 1 x 1CT4 — 1 x 10"3 mol/1, a linear relationship between the efficiency of fluorescence quenching [(/0//) — 1] and the quencher concentration was found. For the determination of quenching constants, the Stem-Volmer equation was used, viz. 

UV spectra are, however, very useful for the determination of acid-base and ion pair formation equilibria, and for photochemical investigations (e. g., determination of quantum yield in photolytic dediazoniation, Tsunoda and Yamaoka, 1966 fluorescence and phosphorescence at low temperature, Sukigahara and Kikuchi, 1967a). 

The use of emission (fluorescence and phosphorescence) as welt as absorption spectroscopy. From these spectra the presence of as well as the energy and lifetime of singlet and triplet excited states can often be calculated. 

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. 

Solid-surface luminescence analysis involves the measurement of fluorescence and phosphorescence of organic compounds adsorbed on solid materials. Several solid matrices such as filter paper, silica with a polyacrylate binder, sodium acetate, and cyclodextrins have been used in trace organic analysis. Recent monographs have considered the details of solid-surface luminescence analysis (1,2). Solid-surface room-temperature fluorescence (RTF) has been used for several years in organic trace analysis. However, solid-surface room-temperature phosphorescence (RTF) is a relatively new technique, and the experimental conditions for RTF are more critical than for RTF. 

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. 

Solid-surface fluorescence and phosphorescence quantum yield values were obtained from +23° to -180°C for the anion of p-aminobenzoic acid adsorbed on sodium acetate (11). Fhosphorescence lifetime values were also obtained for the adsorbed anion from +23° to -196°C. Table 1 gives the fluorescence and phosphorescence quantum yield values acquired. The fluorescence quantum yield values remained practically constant as a function of temperature. However, the phosphorescence quantum yield values changed substantially with temperature. The phosphorescence lifetime experiments indicated two decaying components. Each component showed a gradual increase in phosphorescence lifetime with cooler temperatures, but then the increase appeared to level off at the coldest temperatures. 

---