PHOTOniKMlSTRY H 



Therefore ks is about 10* sec~'. Approximately, 



The addition of 0.1 mole of allylthioiirea to a chlorophyll solution reduces 

 its fluorescence by less than 5 per cent. Accordingly, if a direct reaction 

 of this substance with the (singlet) excited chlorophyll is responsible for 

 the sensitized reaction, then k^lB] < 0.05 X 10^ or kf, < 5 X 10^ liters 

 raole~^ sec"'. However, the quantum yield for the reaction is 



k,[B] 



f 



10^ + k,[B] 



In the presence of 0.1 mole of allylthiourea the quantum yield is certainly 

 greater than 0.9, which reciuires that kf, be greater than 10^". Since the 

 difference between these two estimates is 200-fold or more, the postulate 

 that the reducing agent reacts directly with the singlet excited state must 

 be rejected. If it is assumed that a collision between an oxygen molecule 

 and an excited chlorophyll molecule is responsible for both the reaction 

 and the fluorescence quenching, the computed values of k^ are 10^" and 

 5 X 10" sec-i (moles/liter) -\ respectively. Not only is this fiftyfold 

 discrepancy too great to be explained in terms of experimental uncer- 

 tainties, but also the value (5 X 10") required by the photochemical data 

 is unreasonably large. The frequency factor for a bimolecular reaction 

 between molecules of ordinary dimensions and mass at room temperature 

 is given by the simple collision theory (Moelwyn-Hughes, 1947) as about 

 10" sec-i (liter/mole). Although the size of the chlorophyll molecule 

 might possibly increase this number fivefold, 5 X 10" is surely an upper 

 limit. In order for the rate constant to be as large as the frequency fac- 

 tor, the steric factor must equal unity and the energy of activation must 

 be zero. Moreover, the rate of such a reaction, occurring in solution, will 

 be determined by the frequency with which the reaction partners can 

 diffuse together (Fowler and Slater, 1938) and not by the total number of 

 collisions between them. In a condensed phase, collisions occur in 

 bursts of (probably) 100 or 1000, and the number of encounters is corre- 

 spondingly reduced. It must be concluded that this reaction cannot be 

 induced by a collision of the directly excited chlorophyll molecule with 

 either of the reaction partners. Therefore the sensitizer molecule must 

 be capable of existing in some long-lived energy-rich form. The same 

 conclusion may be reached from the results of analysis of kinetic data for 

 other sensitized reactions (Franck and Livingston, 1941). 



More direct evidence is offered by the experimental investigation 

 (Gaffron, 1937) of the photochemical autooxidation of the aromatic 

 hydrocarbon, rubrene. Since the quantum yield for this reaction remains 

 high even when the oxygen concentration is relatively low, it can be shown 



