Transfer reactions quenching

Electron-transfer reactions producing triplet excited states can be diagnosed by a substantial increase in luminescence intensity produced by a magnetic field (170). The intensity increases because the magnetic field reduces quenching of the triplet by radical ions (157). 

The rates and chemiluminescent intensities of atom-transfer reactions are proportional to the concentrations of the reactants, but the intensity is inversely proportional to the concentration of inert gas present. The latter quenches the excited state through coUision with an efficiency dependent on the stmcture of the inert gas. Chemiluminescence Qc increases with temperature, indicating that excitation has a higher activation energy than the ground state... 

Chain polymerisation necessarily involves the three steps of initiation, propagation, and termination, but the reactivity of the free radicals is such that other processes can also occur during polymerisation. The major one is known as chain transfer and occurs when the reactivity of the free radical is transferred to another species which in principle is capable of continuing the chain reaction. This chain transfer reaction thus stops the polymer molecule from growing further without at the same time quenching the radical centre. 

Taylor and Jarman [1] observed SL spectra in the range of 280-740 nm from 2 M NaCl solutions saturated with argon, krypton and xenon sonicated at frequencies of 16 and 500 kHz. The spectra showed a continuum background with bands at about 310 nm and a peak of sodium D line, which exhibited appreciable asymmetric broadening, as shown in Fig. 13.2. The bands around 310 nm result from the A2L+ — X2n transition of OH radicals. The OH bands are quenched in salt solutions compared with those in water, which suggests the energy transfer reaction... 

Oncul S, Demchenko AP (2006) The effects of thermal quenching on the excited-state intramolecular proton transfer reaction in 3-hydroxyflavones. Spectrochim Acta A Mol Biomol Spectrosc 65 179-183... 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

Quenching is the reduction in fluorescence intensity and can be caused by various processes. It occurs either during the lifetime of the excited state or in the ground state. Quenching processes that happen in the excited state are collisional quenching, charge transfer reactions, or energy transfer. The latter is the basis for FRET probes but the other events happen as well under certain conditions and it is important to consider them. 

A.B.P. Lever, York University Most model photocatalysts undergo electron transfer reactions via their spin triplet states. You point out that there are advantages to using the spin singlet state for electron transfer as exemplified by chlorophyll. What kind of structural or electronic features should be built into model photocatalysts to favour use of their spin singlet states for electron transfer quenching ... 

In the scheme, the assumption is made that the only important quenching event is electron transfer and that energy transfer quenching is negligible. The series of electron transfer events in the scheme are initiated by optical excitation to give the excited state and the electron transfer reactions which occur fol-... 

A.J. Bard, University of Texas The fact that one can generate chemiluminescence in polymer films containing Ru-(bpy)3 2 implies that the excited state may not be quenched completely by electron transfer reactions. Are the photoreactions you describe thermodynamically uphill (i.e., with chemical storage or radiant energy) or are they photocatalytic ... 

In a number of cases ITIES can be used to separate the products of a photoinduced electron-transfer reaction. An early example is the work by Willner et al. [7] at the water/toluene interface, who studied the photooxidation of [Ru(bpy)3]2+ in the aqueous phase. The excited state was quenched by hexadecyl- 4,4 bipyridinium, which becomes hydrophobic on reduction and crosses to the toluene phase. There are other examples and mechanisms at the present time their main interest resides in their chemistry, and in the separation of products that can be achieved at the interface. 

The series of electron transfer reactions (14) for which we calculated rate constants involve quenching of the lowest ex-... 

H. Shizuka, Excited state proton-transfer reactions and proton-induced quenching of aromatic compounds, Acc. Chem. Res. 18, 141-147 (1985). 

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