1498 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



suggested that this product is the often postulated chlorophyll molecule in 

 a metastable electronic triplet state. (It could also be a tautomeric triplet 

 form; for the difference between the two, see pages 790-795.) 



The following set of reactions — most of which have been considered be- 

 fore — was stated to account satisfactorily for the flash measurements: 



(35.12A a) Chi > Chi* (excitation) 



-hi. 

 (35.12A b) Chi* > Chi (fluorescence) 



kt 



(35.12A c) Chi* > tChl (conversion to metastable 



state) 

 (35.12A d) tChl + Chi > 2 Chi ("self-quenching" of meta- 



stable molecules) 



(35.12A e) tChl + S > oChl + rS (oxidation of tautomer to 



a radical) 



(35.12A f) rS + oChl > Chi + S (back reaction) 



(35.12A g) O2 + tChl > tChlOo Cor Chl-Oa) (quenching of the meta- 



(35. 12 A h) tChlOz + Chi > 2 Chi + O2 stable state by oxygen) 



(35.12A i) Chi -|- tChlOa > Chi -|- OChl (irreversible oxidation) 



Immediately after the flash, the solution was estimated to contain 65- 

 70% of chlorophyll in the metastable form, tChl, and 15-20% in the oxi- 

 dized, free radical form, oChl. The relation between tChl and oChl then 

 shifted in favor of the radical, as indicated by figure 35.2B (the radical 

 alone is supposed to absorb at 524.5 mju). 



It will be noted, that in reaction sequence (35.12A), Franck's mechanism 

 of the oxygen effect (equations 35.2, 35.4) was adopted. The radical for- 

 mation was interpreted as reversible oxidation of chlorophyll (by the sol- 

 vent, or by an impurity, not as reversible reduction (as in 35.5 b, c). This 

 was done without special justification. Whether the stationary photo- 

 bleaching in air-free methanol is a reduction or an oxidation (or a combi- 

 nation of both?) remains open. 



The "concentration quenching" of tChl (reaction d) was included in 

 sequence (35.12A) because of Livingston and Knight's observations of the 

 effect of chlorophyll concentration on stationary bleaching (cf. above) ; by 

 themselves, the flash data could be accounted for also by a simple mono- 

 molecular deactivation, tChl — »■ Chi. 



Livingston, Porter and Windsor (1954) experimented with stronger 

 flashes (~ 125 joule) and a synchronized flashing absorption beam, and 

 found an aknost complete transformation of the chlorophylls and pheophy- 

 tins into the metastable form in (Os-free) methanol, and even benzene; it 

 decayed with a half-time of 2-5 X 10"^ sec. 



Reversible photobleaching of oxygen-free chlorophyll solutions was ob- 

 served also by Linschitz and Rennert (1952) in glassy ether-isopentane- 

 ethanol mixtures, at - 190° C. ; the change in spectrum (decrease of ab- 



