SENSITIZATION BT KINETIC ENCOUNTERS 515 



and Flesch (1936) attempted to explain this fact by the utiUzation of a certain amount 

 of thermal energy. Later, GafTron (1936) found that the efficiency of sensitization 

 remains practically the same even at 818 ran, and pointed out that, if the sensitization 

 in the infrared were an " anti-Stokes " process, in which thermal energy must help to 

 bring about the excitation of oxygen, the yield in the infrared should be much smaller 

 than in the visible. This objection was answered by Kautsky (1937) with a reference 

 to a second metastable state of the oxygen molecule, ^2, with an excitation energy of 

 only 22.5 kcal, which can be supplied by radiations up to 1261 mix. 



The general arguments against the transfer of electronic excitation 

 energy in bulk from a colored sensitizer to a colorless acceptor, to be dis- 

 cussed in chapter 23 (Vol. II), as well as the lack of positive evidence in 

 favor of the roundabout way of utilization of light energy assumed by 

 Kautsky, are suflScient for the rejection of his hypothesis. 



The energy transfer becomes less improbable if the oxidant is itself a 

 dyestuff — particularly one whose absorption bands overlap with those of 

 the sensitizer. On pages 503-505 (cf. Eq. 18.29), we have considered 

 the possibility that a mechanism of this type may account for the chloro- 

 phyll-sensitized reduction of azo dyes. An interesting demonstration of 

 such a transfer is the carotenoid-sensitized fluorescence of chlorophyll 

 in vivo (cf. Vol. II, Chapter 24). 



(6) Oxidation-Reduction Reaction with the Oxidant (Mechanism A^l) 



As stated on page 486, we do not believe that chlorophyll molecules, 

 which fail to emit fluorescence, pass into a metastable electronic state; 

 but we considered it possible that these molecules may pass into a 

 chemically changed active state of considerable duration. Similarly, 

 while we do not consider probable a purely physical transfer of excitation 

 energy from metastable chlorophyll molecules to oxygen, we admit the 

 possibility of a chemical reaction (e. g., an electron transfer) between 

 activated chlorophyll and oxygen. In presence of an oxidizable sub- 

 strate, A, this reaction may become a prelude to sensitized photoxidation, 

 in the following way, for example: 



(18.33a) Chi* v tChl 



(18.33b) tChl + O2 > HOj + oChl 



(18.33c) oChl + A > oA + Chi 



(18.33d) HO2 > i H2O + I O2 



(18.33) A + i O2 > oA 



The formation of the radicals, HO?, was first suggested by Weiss (1935) as a substi- 

 tute for that of metastable oxygen molecules, in the explanation of Kautsky's experiments 

 on the transfer of sensitization across air gaps; cf. volume II, chapter 23. 



Reaction (18.33c) is identical with (18.27b) — the reaction which was 

 assumed on page 502 to explain the protection of chlorophyll from 

 photoxidation by autoxidizable substances. 



