Fluorescence spectra characterization

A fluorescence spectrum may result from overlapping spectra of several fluorescent species (or several forms of a fluorescent species). If each of them is characterized by a single lifetime, it is possible to decompose the overall spectrum into its components. 

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

Observation of reorientational dynamics of dipolar groups surrounding the fluorophore in response to changes in the dipole moment of the fluorophore occurring upon electronic excitation. Such dynamics result in the appearance of spectral shifts with time,(1 ) in changes of fluorescence lifetime across the fluorescence spectrum,(7,32) and in a decrease in the observable effects of selective red-edge excitation.(1,24 33 34) The studies of these processes yield a very important parameter which characterizes dynamics in proteins— the reorientational dipolar relaxation time, xR. 

Readily measurable fluorescence intensities are found for molecules having aromatic and heteroaromatic rings, in particular when annulated rings are present, and in the case of conjugated 7x-electron systems. If the polymer molecules contain such fluorescence-active subunits they can be characterized by this technique, either directly via their fluorescence spectrum or via fluorescence quenching experiments (for polymers with appropriate quencher groups). It is... 

Imre, D.G., Kinsey, J.L., Field, R.W., and Katayama, D.H. (1982). Spectroscopic characterization of repulsive potential energy surfaces Fluorescence spectrum of ozone, J. Phys. Chem. 86, 2564-2566. 

The intensity, position of the emission wavelength, and lifetime are some of the observables that will characterize a fluorophore. Each fluorophore has its own fluorescence properties and observables. These properties are intrinsic to the fluorophore and are modified with the environment. A fluorescence spectrum is the plot of the fluorescence intensity as a function of wavelength (Figure 7.3). 

The fluorescence excitation spectrum characterizes the electron distribution of the molecule in the ground state. Excitation is, in principle and for a pure molecule, equivalent to absorption. The fluorescence excitation spectrum is obtained by fixing the emission wavelength and by running the excitation monochromator (Figure 7.3). 

While luminescence in vapor-deposited matrices accordingly should be a powerful technique for detection and quantitation of subnanogram quantities of PAH in complex samples, it suffers from two major limitations. First, it is obviously limited to the detection of molecules which fluoresce or phosphoresce, and a number of important constituents of liquid fuels (especially nitrogen heterocyclics) luminesce weakly, if at all. Second, the identification of a specific sample constituent by fluorescence (or phosphorescence) spectrometry is strictly an exercise in empirical peak matching of the unknown spectrum against standard fluorescence spectra of pure compounds in a hbrary. It is virtually impossible to assign a structure to an unknown species a priori from its fluorescence spectrum qualitative analysis by fluorometry depends upon the availabihty of a standard spectrum of every possible sample constituent of interest. Inasmuch as this latter condition cannot be satisfied (particularly in view of the paucity of standard samples of many important PAH), it is apparent that fluorescence spectrometry can seldom, if ever, provide a complete characterization of the polycyclic aromatic content of a complex sample. 

Snowden used a (1 1) mixture of KBF and BgOg placed in a 13 mm Vycor tube. The mixture was heated to produce a vapor which passed through an external electrodeless discharge. The Intense emission spectrum of the BOg molecule was observed. The fluorescence spectrum obtained by Johns was too weak for a full characterization of the electronic ground state. 

Conformations of oligo(pyridine-aA-pyrimidine)s 119 have been studied. On the basis of NMR analysis and the fluorescence spectrum in solution, the oligomers were found to take a helical conformation.203 The conformation was characterized by distinct chemical shifts (upheld shift), NOE effects, and excimer emission arising from the overlap of aromatic groups. The helical structure was confirmed for 120 in the... 

The fluorescence excitation spectrum characterizes the state of the molecule at the fundamental state. Figure 10.9 shows the fluorescence excitation spectra of the standard (spectrum 3), the AE (spectrum 2) and a hybrid sweet (1) corn, obtained with emission wavelength of 435 nm (spectra a ) and of 540 nm (spectra b). 

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