514 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



mole per liter) . The dependence on wave length is not very pronounced 

 in methanol, but in benzene, 7 values obtained at 436 mju were as low as 

 0.05. Very low quantum yields were obtained when phenylhydrazine 

 hydrochloride was substituted for the free base. 



2. Different Mechanisms of Sensitization 



Several mechanisms by which autoxidations and oxidoreductions can 

 be sensitized have been mentioned in this chapter. They involve a triple 

 alternative : 



(1) the sensitizer may be either (A) free, or (B) associated with one 

 of the reaction partners; 



(S) the interaction of the sensitizer with the substrate may be either 

 (a) an energy transfer, or (jS) an oxidation-reduction (that is, electron 

 transfer or hydrogen transfer) ; 



(3) the component with which the light-activated sensitizer reacts 

 may be either (1) the oxidant, or (2) the reductant. 



This gives eight combinations — all theoretically possible; most, if not 

 all, have been discussed in the literature. 



3. Sensitization by Kinetic Encounters (Type-A Mechanisms) 



(a) Energy Transfer to the Oxidant (Mechanism Aal) 



Kautsky made the sweeping claim that all dyestuff-sensitized re- 

 actions — not only autoxidations, but even oxidation-reductions, including 

 photosynthesis — are initiated by the transfer of excitation energy from 

 the dyestuff to oxygen, bringing the latter into the metastable excited 

 state ^A (37.3 kcal above the ground level ^n). 



The origin of this hypothesis will be described in volume II, chapter 23. It will 

 be shown there that the quenching of chlorophyll fluorescence by oxygen indicates that 

 excited fluorescent chlorophyll molecules in fact react with oxygen, perhaps even by 

 the very first encounter, but that this interaction becomes fully effective only when the 

 partial pressure of oxygen reaches the order of one atmosphere, corresponding to more 

 than 0.01 mole per Uter. On the other hand, according to Gaffron (1933), the quantum 

 yield of chlorophyll-sensitized photoxidation of allyl thiourea is high, and almost 

 independent of oxygen concentration, between 10"^ and 10~^ mole per liter; and the 

 same is true of the chlorophyll-sensitized oxidation of rubrene. At these concentrations, 

 the probabihty of encounters of the short-hved fluorescent chlorophyll molecules with 

 oxygen molecules is too small to account for the high efficiency of sensitized oxidation. 



To explain this fact, Kautsky, Hirsch, and Flesch (1935) have postulated the ex- 

 istence of a long-lived excitation state of chlorophyll (c/. page 486). Kautsky suggested 

 that both the short-lived fluorescent, and the long-Uved metastable, chlorophyll mole- 

 cules can transfer their energy in bulk to oxygen molecules, and that this energy is 

 sufficient — even in the second case — to promote oxygen to the metastable state ^A. 

 (The excitation energy of this term corresponds to a wave length of 762 m^i.) An 

 objection was raised by Gaffron (1935), who found that autoxidation of allyl thiourea 

 can also be sensitized by bacteriochlorophyll in infrared fight (X > 760 m^). Kautsky 



