Living radical formation, efficiency

Efficient photoinduced electron transfer was observed in ternary complexes of xanthone and N,iV-diethylaniline associated to p- and y-CD. Time-resolved ESR in aqueous solutions and in dried powder samples revealed the formation of long-lived radical pairs [330],... 

Benzophenone is a common photosensitiser. Free radical formation by hydrogen abstraction from solvent leads to use in UV-curing applications such as inks, imaging, and clear coatings in the printing industry. By way of contrast, hydroxy benzophenones with the OH group adjacent to the carbonyl are useful photostable UV absorbers because reversible proton transfer is an efficient deactivation route leading to very short lived excited-states. 

A long life time of the triplet state tends to lead to a reduced overall quantum yield, because chances are higher that alternative deactivation routes may successfully compete with radical formation. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) is an example for an initiator with a rather short-lived first triplet state (t<0.1 ns)," causing a high quantum yield The quantum yield rm for the formation of macroradicals from the initiator fragment radicals is also known as the initiator efficiency, /, which is defined analogously to the efficiency of thermal initiators. 

CH4/O2 and CF4 as the reactant gases and observed the formation of [M — 4 H] ions in the Cl plasma (Scheme 13)166. Thus, repeated deprotonation and electron transfer processes appear to offer an efficient access to more highly unsaturated and/or ring condensed trimethylenemethane radical anions. The [M — 4 H]- ion is considered identical to the molecular radical anion (42) of acepentalene (43), which was generated as a short-lived species from the former by neutralization-reionization mass spectrometry167. Efforts to apply Squires methodology to triquinacene 41 and the tribenzotriquinacenes 44 have been made168. 

Laser flash photolysis is one of the most efficient methods for the direct spectroscopic observation of free radicals and for monitoring the kinetics of formation and decay in real-time. This method is an extension of conventional flash photolysis method [26] that was invented by Norrish and Porter in 1949, and who were awarded by the Nobel Prize in 1967. We have used this approach to investigate the generation and reactions of free radicals with DNA. In this technique, a laser light pulse is used to produce short-lived intermediates in solution contained in an optical cuvette, and the kinetics of their formation and decay are monitored by transient absorption spectroscopy. The apparatus we used is shown in Figure 4.1. 

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