CHLOROPHYLL-SENSITIZED AUTOXIDATIONS 1527 



It will be noted that 7 values show a decline with rising light inten- 

 sity; Warburg and Schocken recommended not to use light intensities 

 much in excess of 1 X 10 ~^ einstein absorbed per 15 min. in 5 cc. In 

 part, the decrease can be due to oxygen exhaustion in the illuminated bot- 

 tom layer, which may occur at higher hght intensities despite shaking. 



The nature and mechanism of the chemical change taking place in this 

 actinometer remains uncertain. Up to 2.5 moles oxygen were consumed 

 per mole thiourea after prolonged irradiation, while not more than 2 should 

 be used if the reaction were : 



(35.27) CS(NH2)2 + 2 O2 > CNNH2 + H.,S04 



About 75% of the amount of sulfuric acid to be expected according to this 

 equation was actually found; qualitative test for cyanamide formation 

 was positive. 17% of the thiourea consumed was recoverable as urea, 

 perhaps because of hydrolysis of the primarily produced cyanamide: 



(35.28) CNNH2 + H2O > CO(NH2)2 



While reacting with a quantum yield close to 1, the system continues to 

 fluoresce strongly, which should account for the loss of probably not less 

 than 10% of the absorbed quanta. That sensitization does not compete 

 with fluorescence indicates (c/. Vol. 1, page 483) that it proceeds by the 

 intermediary of a "long-lived active state" such as the "tautomer," tChl, 

 which is formed in about 95% of all cases of excitation. 



Warburg and Schocken, having observed that thiourea will react with 

 oxygen in ultraviolet light without sensitizer, concluded that the most 

 likely mechanism of sensitization is transfer of excitation energy from the 

 chlorophyllide to thiourea. However, the mechanism of sensitized photo- 

 chemical reactions is generally different from that of nonsensitized reac- 

 tions because the reaction substrate (thiourea) has no way of accepting a 

 "red quantum," either directly by light absorption or indirectly by transfer 

 from an excited chlorophyllide molecule. The more likely mechanisms of 

 sensitized autoxidation are, therefore, those involving intermediate revers- 

 ible chemical changes of the sensitizer, such as were described in Vol- 

 ume I, pages 514-521 and in section 5 above. Whether the long-lived acti- 

 vated chlorophyllide molecule, tChl, reacts first with thiourea or \vith oxy- 

 gen remains unknown. 



It also remains a matter of speculation whether the chlorophyllide 

 molecules enter into association with thiourea molecules (or oxygen mole- 

 cules, or both) in the dark. According to table 23. HID, thiourea does not 

 quench the fluorescence of chlorophjdl; and the same is probably true of 

 chlorophyllide fluorescence. Association, if any, must therefore be of the 

 type not affecting fluorescence ; in other words, the photochemical reaction 



