1510 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP, 35 



the bands of the other two forms are shifted strongly toward the red in 

 methanol (from 406 to 447 m^u, and from 491 to 530 mju), with the result 

 that what was the "long wave band" in water, becomes the "central band" 

 in methanol. Livingston and Pariser tried to relate the quantum yield to 

 the total concentration of the dye, or to the concentration of any one of its 

 three constituents. It transpired that the simplest relationships are ob- 

 tained if form II alone is taken into consideration as a reactant. 



The phenylhydrazine concentration (free base + hydrochloride) was 

 varied between 0.02 and 1.0 mole/liter, the total methyl red concentration, 

 from 0.24 to 6.10 X 10 ~* mole/liter, and the (calculated) concentration of 

 "form II" from 0.085 to 6.10 X 10""* mole/liter. The quantum yields were 

 found to vary between 0.10 and 0.49 mole/einstein. A maximum yield 

 7 ~ 0.5 can be explained by assuming a reduction of the dye in two steps. 

 A single light quantum can form, at best, one molecule of the intermediate 

 semiquinone; two semiquinone molecules then dismute into one molecule 

 of the dye and one molecule of the leuco dye. 



From the effect of methyl red on the reversible bleaching of chlorophyll 

 (section 1), Livingston concluded that the first step of the sensitized reac- 

 tion possibly is the association of methyl red (in the form II) with the long- 

 lived (tautomeric?) active form of chlorophyll, tChl, 



35.15a) Chi + hv > Chi* (absorption) 



Ay 



(35.15b) Chi*— 



-^ Chi + hv (fluorescence, ^ 3%) 



-> tChl (tautomerization, ^97%) 



(35.15c) tChl > Chi (detautomerization) 



(35.15d) tChl + MR" '—^ tChlMR" (complex formation) 



(35.15e) tChlMR" — > Chi + MR" (complex decomposition + detau- 



tomerization ) 



The chlorophyll-MR" complex can be supposed to react with the reductant 

 (phenylhydrazine, symbolized by PH2), transferring one H atom: 



(35.16) PHo + tChlxMR" ^^ PH + Chi + MRH" 



and forming two radicals, PH and MRH". These can be stabilized, either 

 by dismutation, e. g.: 



(35.17) 2 MRH" > MRH2" + MR" (leuco dye + dye) 



or by dimerization : 



(35.18) 2PH > HP-PH 



