QUENCHING BY OXYGEN 779 



ethanol is reduced under one atmosphere oxygen by 30-35% (compared 

 with its intensity in the absence of oxygen). This indicates that 50% 

 quenching must require about two atmospheres oxygen, corresponding to a 

 concentration of about IQ-^ mole/1, (c/. table 23.IIIC). Life-time of the 

 fluorescent state of chlorophyll in ethanol solution was estimated on page 

 634 as 8 X 10-» sec. The average time that an excited chlorophyll mole- 

 cule has available before its first encounter with a molecule of a solute 

 whose concentration is of the order of 10 -^ mole/1, also is of the order of 

 10"^ sec. (No exact formulae for the calculation of encounter intervals in 

 solutions are available, but it appears likely that these intervals are some- 

 what — perhaps 10 or 100 times — longer than the collision intervals in gases 

 of the same concentration.) It thus seems that excited chlorophyll mole- 

 cules in the fluorescent state A undergo a quenching reaction by the very 

 first, or one of the first, encounters with an oxygen molecule. 



The nature of this interaction is not known, but it is most likely to be 

 the autoxidation of chlorophyll. A different hypothesis was suggested by 

 Kautsky and maintained by him despite much criticism. According to this 

 hypothesis, the interaction is a hulk transfer of electronic excitation energy 

 from Chi* to O2. This concept originated in certain obsei-vations made by 

 Kautsky and de Bruijn (1931) and by Kautsky, de Bmijn, Neuwirth and 

 Baumeister (1933) in the study of the autoxidation of leuco malachite 

 green, adsorbed on silica gel. They found that this reaction can be sensi- 

 tized, in an atmosphere of oxygen of very low pressure (10"^ mm.), by the 

 dyestuff trypaflavine, adsorhed on separate particles of the gel. Kautsky ex- 

 plained the "transmission" of the sensitizing action across the air gaps sepa- 

 rating the dyestuff from the acceptor by the assumption that excited dye- 

 stuff molecules transfer their energy to oxygen molecules : 



(23.14) D* + O2 > D + O2* 



This process he also made responsible for the quenching of fluorescence. 

 Energy transfer was supposed by him to convert ordinary oxygen into a 

 metastable active form, which Kautsky identified as the state ^S, known 

 from spectroscopic data to be situated 37.3 kcal/mole above the ground 

 state ^n. After Gaffron remarked that infrared excitation of bacterio- 

 chlorophyll provides <37 kcal, Kautsky (1937) suggested that the state 

 ^A (23 kcal/mole) could sei-ve the same purpose. Both states are meta- 

 stable because their multiplicity (singlet) is different from that of the 

 ground state (triplet). The same principle underlies Lewis and Kasha's 

 more recent theory of metastable triplet states in molecules with singlet 

 ground states (c/. p. 730). 



An alternative chemical explanation of the mechanism of quenching 



