Quenching, of excited molecules

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

Quenching of Excited Molecules Through Proton Transfer. An aromatic molecule such as benzene cannot be protonated in the ground state even in strong acids... 

Reactions that occur between components in the bulk solution and vesicle-bound components, i.e., reactions occurring across the membrane interface, can be treated mathematically as if they were bimolecular reactions in homogeneous solution. However, kinetic analyses of reactions on the surface of mesoscopic structures are complicated by the finiteness of the reaction space, which may obviate the use of ordinary equations of chemical kinetics that treat the reaction environment as an infinite surface populated with constant average densities of reactant molecules. As was noted above, the kinetics of electron-transfer reactions on the surface of spherical micelles and vesicles is expressed by a sum of exponentials that can be approximated by a single exponential function only at relatively long times [79a, 81], At short times, the kinetics of the oxidative quenching of excited molecules on these surfaces are approximated by the equation [102]... 

Another useful technique for measuring the rates of certain reactions involves measuring the quantum yield as a function of quencher concentration. A plot of the inverse of the quantum yield versus quencher concentration is then made Stern-Volmer plot). Because the quantum yield indicates the fraction of excited molecules that go on to product, it is a function of the rates of the processes that result in other fates for the excited molecule. These processes are described by the rate constants (quenching) and k (other nonproductive decay to ground state). 

Overview of the intermolecular de-excitation processes of excited molecules leading to fluorescence quenching... 

Summarizing the results obtained up to now with semiconductor electrodes and excited dye molecules one can say that it is possible by using different semiconductors to get an approximate information on the energy position of the electron exchange orbitals of excited molecules in solution. In future it should become possible to study photochemical reactions with other reactants by competition with the electrode reaction (quenching for instance) and to analyse in this way... 

Rate constants for Oj-quenching of excited singlet and triplet state molecules. 

Figure 3.39 The Perrin action volume model of static quenching. Each excited molecule is surrounded by a sphere which can contain one (or several) quenchers (a), or no quencher (b). In (a) quenching is instantaneous, in (b) there is no quenching at all...

Quenching the luminescence of TMPD by phthalic anhydride (PA) and of pyromellitic dianhydride (PMA) by hexamethyl triindan (HMTI) in vitreous MTHF at 77 K has been discussed by Miller et al. [34]. The donors were the excited molecules of TMPD or molecules of HMTI in the ground state while PA molecules in the ground state or the excited molecules of PMA served as the electron acceptor. Quenching of both the fluorescence and the phosphorescence of excited molecules was observed. The authors deny the possibility of quenching the luminescence of TMPD and PMA via the mechanism of energy transfer since the luminescence has been quenched by mol-... 

The exponential dependence of the efficiency of fluorescence quenching on the distance between a donor and an acceptor may be explained by the tunneling mechanism of electron transfer from a singlet-excited molecule of the donor to the acceptor. Indeed, in case of stationary excitation of donor particles, the value of J is determined by the stationary concentration n of the excited donor particles J = An where A is a constant. The value of n is, in its turn, inversely proportional to the rate constant, k, of deactivation of excited particles nft = nJexcexciting light, quantum yield of excited molecules, and n is the concentration of non-excited donor molecules. Thus, J = AnJexc4>lk. Hence, one can easily obtain... 

Quenching of triplet states of aromatic hydrocarbons and carbonyl compounds by inorganic anions (I-, Br , NOj, Cl-) Quenching of excited aromatic molecules by aromatic hydrocarbons, nitriles, methoxy- and amino-aromatics Quenching of excited aromatic molecules by methoxy and amino-aromatics Quenching of excited cyanoanthracenes, by aromatic hydrocarbons, methoxy-aromatics and sulfides... 

Apart from iodide ion, radicals are efficient quenchers of excited states of molecules [16] the processes of quenching of excited states of various molecules by radicals were studied earlier in detail [17 - 19]. It was shown that the triplet states of usual cyanine dyes are mainly quenched by the mechanism of acceleration of the intersystem crossing to the ground state (T-So). In this case, the quenching process is described by the following scheme ... 

No, we have not although I should emphasize that, using the newly developed laser spectroscopic tools described in my paper, studies of the chemistry of vibrationally excited states now become feasible. Destruction of ozone following collisions with vibrationally excited molecules is known, but the importance of vibrationally excited species in the atmosphere is a matter of some debate. Collisional quenching of these molecules must always compete with chemistry and so the question becomes one of understanding the relative efficiencies of such processes. 

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