Emission concentration-dependent

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

In a later work, Stokes established the relationship between the intensity of fluorescence and the concentration, pointing out that the emission intensity depended on the concentration of the sample (analyte), but that attenuation of the signal occurred at higher concentrations as well as in the presence of foreign substances. He actually was the first to propose, in 1864, the application of fluorescence as an analytical tool, based on its sensitivity, on the occasion of a conference given previously in the Chemical Society and the Royal Institution, and entitled On the Application of the Optical Properties to the Detection and Discrimination of Organic Substances [5],... 

The fluorescence decay time, calculated for the 400 nm emission of TIN in PMMA films, decreases from 1.3 0.2 ns to 0.20 0.02 ns as the concentration of TIN is increased from 0.07 mole% to 1.1 mole% respectively. This is further evidence that the TIN molecules are involved in a concentration-dependent, self-quenching... 

The dependence of the fluorescence quantum yields and lifetimes of these stabilizers on the nature of the solvent suggests that the excited-state, non-radiative processes are affected by solvation. In polar, hydroxylic solvents, values of the fluorescence quantum yield for the non proton-transferred form are significantly lower, and the fluorescence lifetimes are shorter, than those calculated for aprotic solvents. This supports the proposal of the formation, in alcoholic solvents, of an excited-state encounter complex which facilitates ESIPT. The observed concentration dependence of the fluorescence lifetime and intensity of the blue emission from TIN in polymer films provides evidence for a non-radiative, self-quenching process, possibly due to aggregation of the stabilizer molecules. 

The complex [Ru(bpy)2(2-Me0C6H4PPh2-P,0)] + exhibits a significant red shift of the absorption and emission spectra that is concentration dependent. From NMR spectroscopic data, it is concluded that this observation arises from equilibration with an aqua complex. Structural data and the solution properties of [Ru(bpy)2(2-Pr0CgH4PPh2-P,0)] have been reported." These results expand on an earlier report of [Ru(bpy)2(2-R0CgH4PPh2-P,0)] (R = Me, Et)." ... 

Fluorescence quantum yield and emission maximum determinations as a function of peptide concentration may also permit the detection of peptide self-aggregation at concentrations below 10-4 M, because the peptide fluorophore is likely to be located in a different environment in the peptide aggregate. For example, the concentration-dependent changes in the tryptophan fluorescence emission maximum of mellitin were monitored to determine the equilibrium dissociation constant and thermodynamic parameters of the monomer-tetramer self-association reaction of this peptide. 25 Similarly, measurement of the changes in the tryptophan fluorescence intensity of gramicidin A as a function of its concentration permitted the determination of an average monomer-dimer equilibrium con-stant. 26 ... 

However, simultaneously there is emission of infrared radiation from the Boltzmann distribution of molecules in excited states, which leads to a negative energy component. This emission process depends not only on the concentration of the gas but very sensitively on the temperature since this determines the population of the excited states that emit (Eq. (A)). As we shall see in some specific cases below, it is the balance between these two at any given altitude that determines the changes in fluxes and the ultimate impact of a change in a greenhouse gas concentration. 

Eximer Fluorescence. Since Forster and Kasper discovered concentration-dependent long-wavelength emission resulting from association of an electronically excited pyrene molecule with another ground state pyrene molecule,39 the phenomenon of excimer fluorescence has been studied extensively.40 The mechanism for excimer formation and emission can be represented by... 

For higher concentrations, we need all the terms in Equation 18-12. As concentration increases, a peak emission is reached. Then emission decreases because absorption increases more rapidly than the emission. We say the emission is quenched (decreased) by self-absorption, which is the absorption of excitation or emission energy by analyte molecules in the solution. At high concentration, even the shape of the emission spectrum can change, because absorption and emission both depend on wavelength. 

The photolytic flash must have enough energy to prepare, in a very short time, a detectable concentration of transient species. The lowest detectable concentration depends on the probe technique, and here the methods of UV/VIS/near IR absorption and emission spectroscopy are the best. Their drawback is that they provide very little structural information about the nature of the transient species. IR and Raman spectra are much more informative, but they present many problems in fast reaction kinetics because of the weakness of the signals. 

Since Forster s original work, a large number of aromatic compounds, including benzene, naphthalene, and anthracene, have been found to have concentration-dependent fluorescence spectra under some conditions. Most of these excimers are not as stable a.s the pyrene prototype, and require lower temperatures or higher concentrations to be observed.29 Some crystals also exhibit excimer emission. Crystalline pyrene, for example, has only a single structureless fluorescence band of the same energy as its excimer emission in solution.30... 

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