Back electron transfer organic radical ions

The problem of differentiating the rates of back electron transfer and of chemical reaction of radical ions can be addressed in one of two ways either the rate of reaction of the radical ions can be increased, or the rate of back electron transfer between the components of the radical ion pair can be suppressed. Although the reactivity of the component radical ions can be manipulated by standard physical organic techniques (which alter the electron density and steric access to sites of electron sufficiency or deficiency), it is very difficult to change the chemical reactivity of fixed members of a donor-acceptor pair. Yet the rate of chemical reaction, rearrangement, or trapping of the individual radical ions must be competitive with the rate of back electron transfer (the reverse of Eq. 1) if net chemistry is to be observed. Obviously, if the rate of reaction of these radical ions is extremely fast, the relative rate of back electron transfer may be slow enough to obviate this problem. 

Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

Polymerization of butane-1,4-diol dimethacrylate, sensitized by benzophenone in the presence of three different sulfides, has been described by Andrzejewska et al. [190]. The measurements show that in the absence and in the presence of propyl sulfide and 2,2 -thiobisethanol no polymer was formed. This can be explained by the effective back electron transfer process that occurs in the radical-ion pair in organic solvents. Effective polymerization was observed only in the presence of TMT. Laser flash photolysis studies performed for the benzophenone-TMT pair allow one to construct a scheme (Scheme 23) explaining characteristic features of the mechanism of polymerization initiated by the system. The results prompted the authors to study other symmetrically substituted 1,3,5-trithianes as electron donors for benzophenone-sensitized free-radical polymerization (Figure 38 Table 12) [191]. 

The role played by MWCNTs is to provide electrons into the Ti02 condnction band under visible light irradiation and to trigger the formation of very reactive radicals snch as the snperoxide radical ion 0 . Back electron transfer from adsorbed OH closes the electrocatalytic cycle and provides a sonrce for the hydroxyl radical HO, which is responsible for the degradation of the organic molecnles (Fignre 13.2). 

An entirely different consequence of electron (or hole) transfer to addends is provided by semiconductor-sensitized decomposition of electron acceptors A-X (or electron donors, i.e., hole acceptors, D-Y) [17]. If organic molecules whose redox states (anion radical or cation radical) are unstable with respect to scission into a free radical and an ion [Eqs. (5,6)] are adsorbed onto a wide band gap semiconductor such as Ti02, etc., then back electron (hole) transfer can be inhibited, if the scission process is rapid. 

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