Electron transfer quenchers

Both energy and electron transfer quenchers have been employed to show that the quenching rates of the fullerene triplet state are decreased as a function of the size of the dendrimer shell [36]. These results further demonstrate that fullerene is an excellent functional group to probe the accessibility of a dendrimer core by external molecules. 

The deprotonation step, either by the sensitizer radical anion or by some adventitious base, is essential for the formation of any amine derived products. This step can be prevented if the a-hydrogens are arranged in a plane orthogonal to the singly occupied nitrogen n-orbital a requirement which is met for the radical cation of l,4-diazabicyclo[2.2.2]octane (DABCO). The low oxidation potential, due to the interaction of the pair of transannular nitrogens, makes this an excellent electron transfer quencher. Yet, no product formation is observed as a result of these interactions, with the possible exception of the zwitterionic adducts formed with highly electrophilic ketones [193]. 

The kinetic complexity seen in oriented micelles persists in inverse micelles. The distribution of electron transfer quenchers within the water pool follows Poisson statistics and enables the kinetic data to describe migration rates to and from the aqueous subphase [65]. These orientation effects also make possible topological control of non-electron transfer photoreactions occurring within AOT micelles [66]. 

Besides spectroscopic methods, quenching processes have been utilized to differentiate between various types of radical ion pairs, too. These chemical methods make use of the different reactivities of CIP and SSIP which are caused by the unequal solvation and distance of the charged species in the ion pairs. Depending on the ambivalent character of radical ions, these intermediates may be scavenged either by electron transfer quenchers (Q) or by nucleophilic and electrophilic scavengers (Scheme 7 and Eqs. (5—7)). 

Scheme 7. Quenching of PET processes between A and D by secondary electron transfer (only A is excited) A electron acceptor D electron donor P products Q electron transfer quencher.

Chemical evidence for the radical cation, 7, can be obtained from quenching experiments. A known electron transfer quencher,... 

Figure 8. Effect of electron-transfer quenchers on oxygen uptake.

Intramolecular electron transfer in cytochrome c has been investigated by attaching photoactive Ru complexes to the protein surface. Ru(bpy)2(C03) (bpy = 2,2 -bipyridine) has been shown to react with surface His residues to yield, after addition of excess imidazole (im), Ru(bpy)2(im)(His) +. The protein-bound Ru complexes are luminescent, but the excited states ( Ru ) are rather short lived (r 100 ns). When direct electron transfer from Ru to the heme cannot compete with excited-state decay, electron-transfer quenchers (e.g., Ru(NH3)6 + ) are added to the solution to intercept a small fraction (1-10%) of the excited molecules, yielding (with oxidative quenchers) Ru ". If, before laser excitation of the Ru site, the heme is reduced, then the Fe to Ru + reaction (ket) can be monitored by transient absorption spectroscopy. The ket values for five different modified cytochromes have been reported (Ru(His-33), 2.6(3) xlO Ru(His-39), 3.2(4) xlO Ru(His-62), 1.0(2) x 10 Ru(His-... 

Quenching rate constants for triplet energy transfer quenchers are often faster than for electron transfer quenchers. However the requirement for a triplet state... 

Stabilizers UVA 2,2 -methylenebis(6-(2H-benzotriazol-2-yl)-4-1,1,3,3-tet-ramethylbutyl)phenol 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hex-yloxy-phenol HAS 1,3,5-triazine-2,4,6-triamine, N,N [1,2-eth-ane-diyl-bis[[[4,6-bis[butyl(1,2,6,6-penlamethyl-4-piperidinyl) amino]-1,3,5-lriazine-2-yl]imino]-3,1-propanediyl]bis[N ,N -di-butyl-N ,N -bis(1,2,2,6,6-pentamethyl-4-piperidinyl)- Electron transfer quencher 1,2,4-trimethoxybenzene ... 

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