Fluorescence theory example

In certain situations involving coherently interacting pairs of transition dipoles, the initial fluorescence anisotropy value is expected to be larger tlian 0.4. As mdicated by the theory described by Wyime and Hochstrasser [, and by Knox and Gtilen [, ], the initial anisotropy expected for a pair of coupled dipoles oriented 90° apart, as an example. 

The aim of this review was to summarize those aspects of fluorescence spectroscopy that may have value for solving problems in food science and technology. The techniques described, which are mainly based on front-face fluorescence spectroscopy coupled with multidimensional statistical methods, have been illustrated by examples taken from the literature and the work done in our laboratory. Although fluorescence spectroscopy is a technique whose theory and methodology have been extensively exploited for studies of both chemistry and biochemistry, the utility of fluorescence spectroscopy for molecular studies has not yet been fully recognized in food science. Fluorescence spectroscopy has the same potential to address molecular problems in food science as in the biochemical science field, because the scientific questions that need to be answered are closely related. We hope that this coverage will introduce a novel class of techniques in the emulsion and gel fields. 

The examples in the previous section give a comprehensive overview of application areas where molecular rotors have become important fluorescent reporters. Current work on the further development of molecular rotors can broadly be divided into three areas photophysical description, structural modification, and application development. Although a number of theories exist that describe the interaction between a TICT fluorophore and its environment, the detailed mechanism of interaction that includes effects such as polarity, hydrogen bonding, or size and geometry of a hydrophobic pocket are not fully understood. Molecular simulations have recently added considerable knowledge, particularly with... 

When the emissive state is a charge transfer state that is not attainable by direct excitation (e.g. which results from electron transfer in a donor-bridge-acceptor molecule see example at the end of the next section), the theories described above cannot be applied because the absorption spectrum of the charge transfer state is not known. Weller s theory for exciplexes is then more appropriate and only deals with the shift of the fluorescence spectrum, which is given by... 

The polarization properties of the evanescent wave(93) can be used to excite selected orientations of fluorophores, for example, fluorescent-labeled phosphatidylethanolamine embedded in lecithin monolayers on hydrophobic glass. When interpreted according to an approximate theory, the total fluorescence gathered by a high-aperture objective for different evanescent polarizations gives a measure of the probe s orientational order. The polarization properties of the emission field itself, expressed in a properly normalized theory,(94) can also be used to determine features of the orientational distribution of fluorophores near a surface. 

Steady-state fluorescence spectroscopy has also been used to study solvation processes in supercritical fluids. For example, Okada et al. (29) and Kajimoto and co-workers (30) studied intramolecular excited-state complexation (exciplex) and charge-transfer formation, respectively, in supercritical CHF3. In the latter studies, the observed spectral shift was more than expected based on the McRae theory (56,57), this was attributed to cluster formation. In other studies, Brennecke and Eckert (5,31,44,45) examined the fluorescence of pyrene in supercritical CO2, C2HSteady-state emission spectra were used to show density augmentation near the critical point. Additional studies investigated the formation of the pyrene excimer (i.e., the reaction of excited- and ground-state pyrene monomers to form the excited-state dimer). These authors concluded that the observance of the pyrene excimer in the supercritical fluid medium was a consequence of increased solute-solute interactions. 

In recent years numerous experiments have been reported on the fluorescence and energy transfer processes of electronically excited atoms. However, for flame studies the rates of many possible collision processes are not well known, and so the fate of these excited atoms is unclear. An interesting example concerns the ionization of alkali metals in flames. When the measured ionization rates are interpreted using simple kinetic theory, the derived ionization cross sections are orders of magnitude larger than gas kinetic (1,2,3). More detailed analyses (4,5) have yielded much lower ionization cross sections by invoking participation of highly excited electronic states. Evaluation of these models has been hampered by the lack of data on the ionization rate as a function of initial state for the alkali metals. 

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