Quantum yields of fluorescence

Iq/I — t — KgI0 [Q], in which Kg is the bimolecular rate constant of interaction of quencher Q with the excited states of the PCS, t is the lifetime of excited molecules with no quencher, I0 is the quantum yield of fluorescence in the absence of the quencher, and I is the quantum yield of fluorescence in the presence of the quencher. 

As discussed earlier by Senthilnathan and Hurtubise (4), before a saturated ethanol solution of sodium acetate is formed, the solid-surface RTF is less than the RTF from samples prepared with solutions that are saturated with sodium acetate. It was shown by Ramasamy and Hurtubise (12) that both the RTF and RTF quantum yields of the p-aminobenzoic acid anion increased as the amount of sodium acetate increased in the solid mixtures. Figure 4 shows the quantum yield of fluorescence and the quantum yield of phosphorescence )... 

Dansyl chloride does have some drawbacks when used with reverse-phase HPLC. One of these is that the quantum yield of fluorescence varies greatly with the polarity of the solvent for dansyl derivatives (10,11). As the polarity of the solvent increases, the... 

The fluorescent signal will change with variation in quantum yield of fluorescence and with molar absorptivity. Not only do fluorescence quantum yields vary with the different dansyl derivatives formed, but so do the molar absorptivities (12). Another problem is exemplified by the 30-nm difference in the emission maxima of the dansyl derivatives of phenol and 2,4,5-trichlorophenol (13). 

This fluorescent acid chloride can be used to form derivatives of alcohols, amines, and phenols. Using these fluorescent derivatives, an analysis of a series of n-alcohols from Ci to C4 was developed. A chromatogram produced by this technique is shown in Figure 3. Derivatives were also formed from ammonia, dimethylamine, and phenol. A derivative was formed from pentachlorophenol but was not fully characterized. The quantum yields of fluorescence of the alcohol derivatives of V were lower than those of the alkyl halide derivatives of III. 

Quantum Yields of Fluorescence. Table III lists the relative quantum yields of fluorescence of 24 3-substituted 2(lH)-pyridones. Pyridone I has the highest yield measured, which is set at 1.00. An attempt was made to measure the absolute quantum yield of I relative to rhodamine B using ferrloxalate actinometry. A Vpj value of 0.98 0.02 was obtained. However, the determination of absolute ... 

Quimtua Yields of Fluorescence Measurements. All of the quantum yields of fluorescence were measured by the relative fluorescence measurement technique of Parker and Rees (24). This method compares the fluorescence of the compound of interest to the fluorescence of some known compound. All of the fluorescence quantum yields were measured using I as a reference. Compound I had previously been measured by this same method using rhodamine B as a standard. 

The quantum yield of fluorescence (ratio between the number of photons emitted by Si and the number of absorbed photons) and phosphorescence (ratio between the number of photons emitted by Tj and the number of absorbed photons) can range between 0 and 1 and are given by the following expressions ... 

There are methods available to quantify the total mass of americium in environmental samples. Mass spectrometric methods provide total mass measurements of americium isotopes (Dacheux and Aupiais 1997, 1998 Halverson 1984 Harvey et al. 1993) however, these detection methods have not gained the same popularity as is found for the radiochemical detection methods. This may relate to the higher purchase price of a MS system, the increased knowledge required to operate the equipment, and the selection by EPA of a-spectrometry for use in its standard analytical methods. Fluorimetric methods, which are commonly used to determine the total mass of uranium and curium in environmental samples, have limited utility to quantify americium, due to the low quantum yield of fluorescence for americium (Thouvenout et al. 1993). 

The photophysical properties of adenine have intrigued chemists from early on. Broo studied adenine and 2-aminopurine (2AP) in order to understand their differences in photophysical properties. Adenine like all natural nucleobases has very short excited state lifetimes and low quantum yields of fluorescence, while 2AP, which differs from adenine in the position of the amino group, has long lifetimes and strong fluorescence, making it a very useful fluorescent probe. In Broo s work it was observed that the first excited state is a nn at vertical excitation but crosses with an nn state which becomes the Si state adiabatically at the minimum. The large out-of-plane distortion on the nn state opens up a deactivation channel in adenine compared to 2AP. In 2AP, on the other hand, the Si state always has a 7T7r character. 

In practice it is much simpler to determine the relative quantum yield of fluorescence than the absolute quantum yield (see Table 2.1). This is done by comparing the fluorescence intensity of a given sample to that of a compound whose fluorescence quantum yield is known. For this one must... 

They observed a constant quantum yield of fluorescence (Or = 0.3) for all members of the series independent of whether the anthracene moiety absorbed and emitted the energy or the naphthalene moiety absorbed the energy and transferred it to the anthracene moiety. Thus at these short distances singlet energy transfer is 100% efficient. 

For the photodiagnostic use of these compounds, a high quantum yield of fluorescence, r, is desirable. The metal complexes of the common first-row transition metals are not suitable, because they show very low 4>f values. On the other hand, porphyrin complexes of d° and d10 elements show appreciable fluorescence, although generally less than that of the metal-free compounds, presumably because of the heavy-atom effect (e.g., TPP ZnTPP, Table 5). The further operation of the heavy-atom effect, which increases the rate of intersystem crossing (/cisc) by... 

To get a rough idea of the rate of radiationless transition from higher excited states, consider the quantum yield of fluorescence from that state as the ratio of nonradiative (t ) to radiative (t ) lifetimes as follows ... 

In choosing a fluorescent tag, the most important factors to consider are good adsorption (high extinction coefficient), stable excitation without photobleaching, and efficient, high quantum yield of fluorescence. Some fluorophores, such as fluorescein, exhibit rapid fluorescent quenching which lowers the quantum yield over time. Up to 50 percent of the fluorescent intensity observed on a fluorescein-stained slide can be lost within 1 month in storage. AMCA and... 

Thus, the greater the numbers or rates of processes competing with fluorescence for deactivation of the lowest excited singlet state, the lower the value of <()/. The quantum yield of fluorescence is important in determining how intense chemiluminescence can be for a particular reaction. 

Another important property of fluorescing molecules is the lifetime of the lowest excited singlet state (X/). If the mean rate of fluorescence is the number of fluorescence events per unit of time, the mean lifetime of the excited state is the reciprocal rate, or the mean time per fluorescence event. The quantum yield of fluorescence and the lifetime of the excited state are related by... 

Among other factors, the quantum yield of fluorescence determines the intensity of light emission in a CL. This, as well as the position in the spectrum occupied by the fluorescence band, is largely a function of the molecular structure. 

The value of the quantum yield of fluorescence of TIN in the PMMA film (calculated using the total film absorbance at the excitation wavelength) decreases from 1.2 x 10-3 to 5.0 x 10"4 when the concentration of TIN is increased from 0.07 mole% to 5.0 mole%. This suggests that the TIN molecules are involved in a concentration-dependent, self-quenching process. 

In fac-(bpy)Re(I) (CO)3-A (where bpy is 2,2 -bipyridine and A is an aromatic amine), the d-7t(Re)—>jr (bpy) MLCT fluorescent excited state is strongly quenched via intramolecular aniline-Re charge transfer leading to a nonfluorescent LLCT state. By incorporating the donor amino group belonging to the A moiety into a crown-macrocycle, Schanze and Mac Queen(137) have provided a new luminescent cation sensor whose quantum yield of fluorescence raises from 0.0017 (without cation) to... 

Quantum yield of fluorescence measured at 165-nm excitation wavelength. 

The type II direct photooxygenation of anthracene and its derivatives has been studied quantitatively by Bowen,2 Livingston,3 and their co-workers. Quantum yields of fluorescence, dimerizations, and... 

Dialkyl ketones can undergo radiative deactivation (fluorescence) from the singlet in solution. For example, the quantum yield of fluorescence of acetone at 25°C is 0.01.30 However, the addition of maleic anhydride quenches this fluorescence, and the photocycloaddition product is obtained. Thus, it appears that, in this case, the photocycloaddition reaction can compete with fluorescence for the n,n singlet.32 Such is not the case with ordinary olefins. For example, the fluorescence of acetone was completely unaffected by the addition of 2-pentene,30 and the photocycloaddition reaction is inefficient.32... 

In the absence of the reverse absorption the radiative transition probability fquantum yield of fluorescence qmC) and the decay constant l/r (C)= 2 [Pg.200]

Table 22. Excited State Parameters of the Helicenes. Quantum yield of fluorescence (

The spontaneous fluorescent decay time tf is connected with the radiative lifetime tfr and the quantum yield of fluorescence ijf by jjf =tf/tfr. Since the radiative lifetime is of the order of a few nanoseconds in most dyes, the spontaneous fluorescent decay time is about the same for quantum yields of fluorescence near unity (i.e., k Q ttksT 0) and decreases to a few picoseconds for quantum yields of the order of 10-3. 

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