SENSITIZATION BY KINETIC ENCOUNTERS 517 



but this mechanism is improbable in consideration of the large amount of energy re- 

 quired for such a dissociation (c/. page 484). 



(d) Oxidation-Reduction Reaction with the Reductant {Mechanism A^2) 



Weiss (1936) and Weiss and Fischgold (1936) suggested that the 

 primary process of sensitized oxidation is an oxidation-reduction reaction 

 between the dyestuff and the substrate of oxidation. The simplest 

 mechanism of this type is: 



(18.37a) D* -I- A > rD + oA 



(18.37b) rD -I- i O2 > D 



(18.37) A+IO2 )-oA 



This mechanism is highly probable in the case of dyestuffs forming 

 colorless leuco bases. Their fluorescence is strongly quenched by 

 reductants (I" ions, Fe++ ions, and autoxidizable organic compounds). 

 If these dyes are brought together with reductants whose oxidation- 

 reduction potentials are higher than their own, they are reduced (bleached) 

 even in the dark; in light, they can be reduced also by reductants with 

 an oxidation-reduction potential low^er than their own. Since this 

 bleaching is reversed in the dark, by the reoxidation of the leuco dye, 

 a stationary state is established during the illumination which differs 

 from the thermodynamic equilibrium. The dyestuff is bleached as long 

 as the system is illuminated. The best example of such reversible 

 bleaching is the reaction of thionine (Lauth's violet) with ferrous ions, 

 described in chapter 4 (page 77) and 7 (page 152). This system can be 

 bleached completely in a few seconds by sufl&ciently strong light, and 

 recovers its color almost instantaneously in the dark. Other similar 

 systems, e. g., thionine and potassium iodide, or eosine and ferrous ions, 

 are less sensitive, and their reversible bleaching can be discovered only 

 by means of photometric measurements (Rabinowitch and Weiss, 

 unpublished). In the presence of oxygen, some leuco thionine is re- 

 oxidized by oxygen, and ferric ions are accumulated, the net result being 

 a thionine-sensitized autoxidation of ferrous iron: 



(18.38a) Thionine* + 2 Fe++ + 2 H+ > leuco thionine + 2 Fe+++ 



(18.38b) Leuco thionine + 5 O2 > thionine + H2O 



(18.38) 2 Fe++ + § O2 + 2 H+ > 2 Fe+++ + H2O 



This is a particularly simple case of sensitized autoxidation. 



According to equation (18.38), autoxidation by reversibly reducible 

 dyes is due to the disturbance of the photostationary state by molecular 

 oxygen, which removes the leuco dye. A similar disturbance can also be 

 caused by the removal of the oxidized acceptor, e. g., of the Fe+++ ions. 

 Weiss (1936) found, for instance, that, if reaction (18.38a) is carried out 



