REVERSIBLE PHOTOCHEMICAL REACTIONS 491 



one must remember that they are based on experiments of an exploratory 

 character, and that only precise measurements with chemically well- 

 defined preparations, in different pure solvents, could provide a basis for 

 a more definitive theory. 



The effect of the solvent, in particular, must be studied more closely. 

 Phenomena ascribed by Franck and Livingston to a primary tautomer- 

 ization (and subsequent dismutation or oxidation), could equally well be 

 explained by a 'primary reversible reaction with the solvent. For example, 

 the bleaching of chlorophyll in oxygen-free methanol, could be attributed 

 (instead of reactions 18.11) to one of the following processes: 



light 



(18.14) Chi* + S . rChl + oS or 



dark 



light 



(18.15) Chi* + S V oChl + rS 



dark 



(where S is the solvent). Similarly to (18.11c), these equations indicate 

 a bimolecular back reaction, and thus lead to a proportionality of sta- 

 tionary bleaching with the square root of light intensity. 



If bleaching is a reduction, as assumed in (18.14), its inhibition by 

 oxygen can be attributed either to the reoxidation of reduced chlorophyll : 



(18.16) rChl + \ O2 > Chi + h H2O 



(leaving oxidized solvent as a net product), or to a catalysis of the back 

 reaction in (18.14) by the system O2-HO2, as suggested by Franck and 

 Livingston in (18.12). On the other hand, if bleaching is an oxidation, 

 as postulated in (18.15), the oxj-gen effect can be explained only by a 

 catalytic acceleration, according to (18.12), since an oxidation of the 

 reduced product, rS, by oxygen would leave oxidized chlorophyll as a net 

 reaction product. (This is a possible mechanism of irreversible photoxi- 

 dation of chlorophyll, which will be discussed on pages 494 et seq.; but 

 because of the low quantum yield of the latter process, it can account 

 only for a small fraction of the products of reversible bleaching.) 



In the interpretation of the reversible bleaching by ferric chloride, 

 too, a reaction with the solvent can be substituted for tautomerization. 

 For example, one may write, instead of (18.13): 

 (18.17a) Chi* + S . oChl + rS 



(18.17b) rS + Fe+++ > S + Fe++ 



(18.17c) oChl + Fe++ > Chi + Fe+++ 



Ferric ions remove the product, rS, according to (18.17b), and can thus 

 prevent the catalytic acceleration of the back reaction in (18.17a) by 

 oxygen. This makes the bleaching independent of oxygen (presumption 

 being that the Fe++ ions reduce oChl much more slowly than the radicals, 

 HO2). 



