Repulsion between charged radicals

This assures negligible repulsion between charged (surface active) oligomeric radicals and charged hydrophilic) particle surface with the same charge. 

Individual rate constants for homogeneous and heterophase DADMAC homopolymerizations have been calculated from the kinetic models above. These constants are summarized in Table 4. The overall activation energies, also given in Table 4, are relatively high. These values likely result from the electrostatic repulsion between the positively charged radicals and the monomer cations. 

Incorporation of an ionic component into a donor/acceptor molecule is a very effective way of suppressing electron back-transfer. One interesting example consists of the photo-oxidation of leuko crystal violet (LCV) to crystal violet (CV, the dye) by benzophe-none bearing a quaternary ammonium ion (Tazuke Kitamura 1984). In this case, the cation radical of LCV formed is repulsed by the ammonium positive charge. At the same time, the benzophenone anion radical remains stabilized by the attached cationic atmosphere (Scheme 5-16). As shown in the scheme, two favorable results are achieved the stabilization of an ion radical pair by counterion exchange and the charge separation by coulombic repulsion between the two positive charges. This leads to 100% efficiency of the photo-oxidation. With unsubstituted benzophenone itself, the efficiency does not exceed 20%. 

Recent studies by Cole-Hamilton and coworkers have shown that a family of neutral heteropentalenes will quench [Ru(bipy)3] at rates which are typically an order of magnitude greater than those observed for MV this is attributed to a lack of electrostatic repulsion between the chromophore and the quencher. Further, the radical anion of a related compound (2,1,3-benzothiadiazole-4,7-dicarbonitrile), generated by direct photolysis in the presence of EDTA, has been successfully stabilized in CTAB micelles revealing the possibility of very large charge separation yields for the sensitized reaction, especially when sensitized by a multi-negative chromophore. 

UV-Vis spectroscopy of the [2]catenane 214+ revealed - by virtue of the presence of a characteristic broad charge-transfer band with a maximum absorbance at 835 nm and the absence of a peak at 515 nm - that the sole co-conformer in a room temperature acetonitrile solution is the one in which the TTF unit resides inside the cavity of the tetracationic cyclophane. Upon either chemical or electrochemical oxidation of the TTF unit to its radical cationic (or dicationic) state, the Coulombic repulsion between the cyclophane and the positively charged TTF unit leads to its expulsion from the cavity of the cyclophane. As the 1,5-dioxynaphthalene site is left untouched by the TTF oxidation and has an intermediate affinity for residing within the cyclophane,... 

The gel dose in aqueous solution at a given concentration strongly increases with the degree of neutralization. Repulsion between the charged expanded coils would inhibit the radical recombination and be responsible of the observed effect. 

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