PHOTOREDUCTION OF CHLOROPHYLL 505 



above to methanol, only more efficiently; they accelerate the bleaching 

 of the azo dye, while preventing that of chlorophyll. 



Bohi carried out similar experiments with 25 different dyes, including 

 azofuchsin, ponceau 2R, Congo red, diamine green, etc. The bleaching 

 required from one-half hour to 15 hours (in direct sunlight) in binary 

 chlorophyll-dyestuff systems, but only five to ten minutes in ternary 

 systems containing phenylhydrazine. 



Bohi noticed that yellow dyestuffs, even the easily reducible ones, e. g., metanil 

 yellow and azoflavin, were not reduced by chlorophyll in light. In an earUer work, 

 Baur and Neuweiler (1927) formulated a rule for sensitization (reminiscent of Stokes' 

 rule), according to which the absorption band of the sensitizer must he on the violet 

 side of that of the substrate. Bohi used this rule to explain the lack of reaction of 

 chlorophyll with yellow dyes. If we assume that the primary interaction between 

 excited chlorophyll and the dyestuff is an oxidation-reduction, as in (18.28), the im- 

 portance of the relative position of the absorption bands appears unexplained. The 

 rule of Baur and Neuweiler, if confirmed, could be used as an argument in favor of an 

 alternative mechanism (similar to Gaffron's mechanism 18.22 of chlorophyll bleaching) 

 in which the first step is the transfer of excitation energy: 



(18.29) Chi* + A > Chi + A* 



(18.30) A* + Chi > rA + oChl or A* + S > rA + oS 



The energy transfer (18.29) is not improbable between two dyestuffs which absorb in 

 the same spectral region, (this case being different from that of energy transfer from a 

 dye to a colorless acceptor, which was considered in 18.22). 



In the experiments of Rabinowitch and Weiss, and Bohi, chlorophyll 

 reveals its capacity for reversible photochemical oxidation (analogous to 

 its capacity for reversible thermochemical oxidation, discussed in chapter 

 16, page 465). The irreversible photoxidation of chlorophyll probably is 

 merely a secondary and rather infrequent consequence of this primary 

 reversible oxidation. 



5. Photoreduction of Chlorophyll 



It was stated in chapter 16 that efforts to reduce chlorophyll reversibly 

 to a leuco compound, have not been successful. However, the difficulty 

 was not a general reluctance of chlorophyll to be reduced, but the occur- 

 rence of irreversible side reactions (c/. page 457). 



Similarly to all the other simple reactions of chlorophyll, its reduction 

 probably can be accelerated by light. However, chlorophyll has less 

 tendency for photochemical reduction than the typical reversibly re- 

 ducible dyes — e. g., indigo, thiazines, and oxazines. Thus, Windaus 

 and Borgeaud (1928) were unable to oxidize ergosterol by chlorophyll in 

 light (in the absence of oxygen), a reaction which could easily be accom- 

 plished by means of eosine; similarly Meyer (1935) found that diethyl- 

 amine can be dehydrogenated photochemically by eosine, but not by 

 chlorophyll. 



