Fluorescence, types resonance

The effect of pH on the fluorescence of some DNS- (5-dimethylaminonaphthalene-l-sulfonyl-) derivatives has been examined [32]. Fig. 2.7 shows the change in fluorescence intensity of DNS-4-methylthio-3,5-xylenol with variation in the pH of the solution. This may be explained in terms of Fig. 2.8 which shows possible resonance structures of DNS-derivatives. When protonated, as in D, the naphthoquinone-type resonance is prohibited, resulting in a loss of fluorescence. 

Figure 141 Types of atomic fluorescence (A) resonance fluorescence, (B) direct fluorescence, and (C) stepunse fluorescence (0 = ground state, 1= first excited state, 2 = second excited state)...

Time resolved fluorescence measurements have become an important tool in applied fluorescence spectroscopy. Recently, it has been pointed out that the controlled manipulation of fluorescence decay rates opens a new dimension in applied fluorescence spectroscopy. The fluorescence decay rate depends on two independent contributions, the pure rachative rate and the nonradiative rate. The latter one can be influenced by the well known Forster-type resonant energy transfer processes, while the radiative rate can be changed if the molecules are embedded or close to media comprising a dielectric constant markedly different from vacuum. Especially metal nanostructures have been used to alter both decay paths of fluorescent molecules. Apart from a change of those two rates, the absorption cross-section might also be altered. 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

The presence of metallic surfaces or particles in the vicinity of a fluorophore can dramatically alter the fluorescence emission and absorption properties of the fluorophore. The effect, which is associated with the surface plasmon resonance of the metallic surface, depends on parameters such as metal type, particle size, fluorophore type and fluorophore-particle separation. 

The dynamics of intercalation of small molecules with DNA, groove binding and binding to specific sites, such as base pair mismatches have been studied by stopped-flow,23,80 108 temperature jump experiments,26,27,94 109 120 surface plasmon resonance,121 129 NMR,86,130 135 flash photolysis,136 138 and fluorescence correlation spectroscopy.64 The application of the various techniques to study the binding dynamics of small molecules will be analyzed for specific examples of each type of binding. 

It is worth recalling that other types of probes are used in practice for example, radioactive tracers, with their well-known drawback of their radioactivity, and EPR (electronic paramagnetic resonance) probes that provide information mainly on molecular mobility. In contrast to these probes, which are used in rather limited fields of applications, fluorescent probes can offer a wealth of information in various fields, as shown in Table 1.4. The various examples described in this book will demonstrate their outstanding versatility. 

The integrated fluorescence signal //was collected with a g-in. glass light pipe and detected through a combination of dielectric and colored glass filters with a photomultiplier tube. Fluorescence excitation and elastic scattering spectra were recorded simultaneously, in order to identify the type (TM or TE) of resonance responsible for the peaks seen in the excitation spectrum. 

Nuclear magnetic resonance spectroscopy has been developed as a standard method for the determination of hydrogen types in aviation turbine fuels (ASTM D3701). X-ray fluorescence spectrometry has been applied to the determination of lead in gasoline (ASTM D2599) as well as to the determination of sulfur in various petroleum products (ASTM D2622, D4294). 

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