Fluorescence excited states

In such reactions the substantial heat of the simultaneous (concerted) formation of the carbonyl groups produced meets the energy requirement (8,16). In the reaction shown (8), the product is the highly fluorescent excited state of 9,10-diphenylanthracene [1499-10-1] (2). It is not necessary for the new carbonyl groups to be a part of the stmcture of the excited product, only that the excited state be formed synchronously with two carbonyl groups. 

To confirm that the emission under each of the two maxima detected in our study was in fact that of DPA, the wavelength dependence was monitored as a function of the intensity with each one. As shown in Figure 9, the emission spectra are those of the DPA fluorescent excited state. 

Glow discharge is essentially a simple and efficient way to generate atoms. Long known for its ability to convert solid samples into gas-phase atoms, GD techniques provide ground-state atoms for atomic absorption or atomic fluorescence, excited-state atoms for atomic emission, and ionised atoms for MS [158], Commercial instrumentation has been developed for all these methods, except for GD-AFS and pulsed mode GD. 

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... 

Figure 26. Schematic representation of a sensory signal amplification upon formation of a poly-pseudorotaxane from a conjugated polymer with appended many fluorescent electron-donor macrocycles. Threading only one macrocycle by an electron acceptor can cause the quenching, by energy or electron transfer, of the fluorescent excited state of remote macrocycles [68b].

Poly(propyleneamine) dendrimers of generations 1 and 4 (89) functionalized with azobenzene groups were investigated as hosts for eosin Y (eosin = 2, 4, 5, 7 -tetrabromofluorescein dianion) in DMF solution [159]. The peripheral azobenzene groups can be switched by light excitation from the E to the Z form. The fluorescent excited state of eosin is reductively quenehed by the tertiary amine units present in the dendrimer structure. This electron transfer quenching takes place with a static... 

Photoinitiated SET has been used to drive a molecular machine and absorption and fluorescence spectroscopy have been used to monitor it. A 1 1 pseudoro-taxane forms spontaneously in solution as a consequence of the donor-acceptor interactions between the electron-rich naphthalene moiety of the thread (380) and the electron-deficient bipyridinium units of the cyclophane (381). The threading process is monitored by the appearance of a charge transfer absorption band and disappearance of the naphthalene fluorescence. Excited state SET from 9-anthracenecarboxylic acid (9-ACA) reduces a bipyridinium moiety of the cyclophane, lessening the extent of interaction between the thread and the cyclophane and dethreading occurs. On addition of oxygen the reduced cyclophane is reoxidised and threading reoccurs. ... 

At pH 3.2, on the other hand, the decay becomes markedly nonexponential (see Fig. 2.3a). Modeling by a function of the form of Equation 2.6 was not statistically adequate [90] the value is 5.75 and a marked deviation in the residuals is apparent. This infers that additional fluorescent excited states are present under these conditions. Analysis of the data in terms of a double-exponential function as described by Equation 2.7... 

The fluorescent excited state (F ) can transfer its energy to Q (which is usually nonfluorescent) over very short (i.e., contact) distances and is reflected by a decrease in the fluorescence properties such as the quantum yield (), intensity of fluorescence (/), and lifetime (t). 

Non-fluorescing excited states of alkanes. Several relatively long-lived non-fluorescing ("dark") excited states of hydrocarbons capable of sensitizing singlet solute luminescence have been proposed in recent years. 

Several concepts (categories of action) that could result in cation enhanced fluorescence can be envisioned. Five examples are summarized here (a) the ion could cause subtle change(s) in energy levels or electron densities that lead to enhanced fluorescence, (b) the cation of interest could displace a quencher complexed by the crown, (c) the complexation of a cation could interrupt a quenching mechanism operable in the free crown, (d) complexation could adjust the conformation so that a new fluorescent excited state might form, (e) a crown ether used in an extraction method could promote the solubility of a fluorescent ion in a phase that is monitored for fluorescence. The literature of crown ethers contains examples employing each of these concepts (i-2i) with the possible exception of case (b). Seve concq)ts we have attempted to employ follow. 

TPA can be measured by the transmission method or by the Z-scan technique. Moreover, two-photon fluorescence can serve to measure TPA absorption cross-sections, provided that a fluorescent excited state is reached by TPA. In nonlinear... 

The lifetime of the fluorescent excited state may be long enough (10" - lO s) to allow in addition to the intramolecular nuclear relaxation (10 s), also the solvent orientational relaxation. The latter, which is characterized by the relaxation times ranging from 1 O s up to infinity (in the case of Figure 11.1.4.TheFranck-Condontamsitionsduringthe golids) may bring up fllC additional, sol-excitation and the de-excitation of the molecule. 

Another such example is given by fluorescence quenching. If a molecule A yields a fluorescent excited state A that is partially quenched by an added material Q, then a simple kinetic analysis of the quenching data will reveal the second-order quenching constant as follows. Consider the reaction scheme ... 

If only the fluorescent excited states are harvested for light emission in an OLED, the maximum efficiency of the device is 25%. Explain why this is. What can be done to overcome the problem ... 

Closing the confocal aperture further reduces the fluorescence contribution originating outside of the primary focal volume. This works because, it is believed that some of the fluorescence is emitted away from the point of illumination following exciton migration of the fluorescent excited state. 

Principles and Characteristics The analytical capabilities of the conventional fluorescence (CF) technique (c/r. Chp. 1.4.2) are enhanced by the use of lasers as excitation sources. These allow precise activation of fluorophores with finely tuned laser-induced emission. The laser provides a very selective means of populating excited states and the study of the spectra of radiation emitted as these states decay is generally known as laser-induced fluorescence (LIF, either atomic or molecular fluorescence) [105] or laser-excited atomic fluorescence spectrometry (LEAFS). In LIF an absorption spectrum is obtained by measuring the excitation spectrum for creating fluorescing excited state... 

Figure 3 Energy transfer and charge separation scheme for the reaction centers from C aurantiacus. The corresponding rate constants determined by the global target analysis procedure are listed in Tab. 2. X and P IQ indicate the two fluorescing excited states (see text) with the species-associated spectra shown in Fig. 4...

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