EXAMPLES OF SENSITIZATION BY CHLOROPHYLL 



513 



Table 18.11 

 Quantum Yield of Chlorophyll-Sensitized Autoxidation of Alltl Thiourea 



Equation (18.32) does not include the effect of variable oxygen 

 concentration. This effect was comparatively small (in the range investi- 

 gated by Gaffron). For example, at [A] = 1 X 10"^ mole per liter and 

 [Chi] = 0.153 X 10-2 mole per liter, t = 0.38 at [O2] = 3 X 10"^ mole 

 per liter and 0.48 at [O2] = 75 X 10"* mole per liter. Even smaller is 

 the influence of [O2] on the yield of autoxidation of rubrene sensitized 

 by chlorophyll (Gaffron 1933, 1937), particularly at the higher concentra- 

 tions of the acceptor. 



Ghosh and Sen-Gupta (1934) measured, in methanol and benzene, 

 the rate of the chlorophyll-sensitized reaction between methyl red and 

 phenylhydrazine discovered by Bohi. The quantum yields calculated 

 for the total absorption by both pigments were considerably below unity; 

 but when the yields were related to the absorption by chlorophyll alone, 

 7 values were found to approximate unity at the lower concentrations 

 of the sensitizer {cf. Table 18. III). The table shows that the yields 



Table 18.III 



Quantum Yield (7) of the Chlorophyll-Sensitized Oxidation-Reduction 



Reaction between Methyl Red and Phenylhydrazine 



(Calculated for Chlorophyll Absorption Alone) 



are higher in methanol then in benzene, and decline rapidly with in- 

 creasing concentration of chlorophyll (in agreement with Gaffron's 

 observations). They are independent of temperature (between 25° and 

 35° C.), and of the methyl red concentration (between 0.005 and 0.02 



