SENSITIZATION BY KINETIC ENCOUNTERS 519 



tomeric chlorophyll either is converted monomolecularly back into 

 ordinary chlorophyll or oxidizes the acceptor by a bimolecular reaction, 

 (18.40b), leads directly to such a dependence of the yield on [A]. The 

 other mechanisms give more complicated kinetic equations. However, 

 since the quantum yield must be zero in the absence of A, and cannot 

 exceed unity at high values of A, all mechanisms must give "saturation 

 curves" for the function, 7 = /[A]; and the available experimental data 

 are not exact enough to allow one to assert that the quantum yield curve 

 follows exactly the simple Stern-Volmer formula. Thus, new, precise, 

 photochemical experiments appear desirable. 



None of our mechanisms explains the occurrence of a term propor- 

 tional to [Chi] in the denominator of (18.32). Additional hypotheses 

 are required to account for it (as well as for the similar decline of the 

 quantum yield of the chlorophyll-sensitized reaction between methyl red 

 and phenylhydrazine, observed by Ghosh and Sen-Gupta at the higher 

 concentrations of the sensitizer). 



This decline could be caused, for example, by a polymerization of 

 chlorophyll and consequent accelerated dissipation of energy. Another 

 possibility is energy dissipation by colhsions of excited and normal 

 chlorophyll molecules, Chi* + Chi -> 2 Chi. The occurrence of one or 

 both of these processes is indicated by the self-quenching of chlorophyll 

 fluorescence (Weiss and Weil-Malherbe 1944). Long-hved active mole- 

 cules also may be deactivated by such collisions, e. g., by dismutation, 

 which produces the alternative : 



(18.42a) tChl + A > rChl + oA or 



(18.42b) tChl + Chi > rChl + oChl 



and can thus lead to a decUne in the probability of the sensitized oxida- 

 tion of A with increasing concentration of chlorophyll. 



The mechanisms which envisage a bimolecular back reaction — e. g. 

 (18.34) and (18.41) — can be shown (by the reasoning employed on 

 page 489) to require a proportionality of the absolute yield with the 

 square root of light intensity at the low values of [A] (when 7 is small and 

 the stationary concentration of oChl is determined almost exclusively by 

 reaction (18.41a); while, at the higher concentrations of the acceptor, 

 when 7 approaches unity (i. e., when practically all oChl molecules 

 react according to 18.41b), the absolute yield should become proportional 

 to the first power of light intensity. Thus, the quantum yield should 

 decrease with increasing light intensity at low values of [A], and become 

 independent of this intensity at high values of [A]. On the other hand, 

 the mechanisms which imply a monomolecular deactivation, e. g., (18.33) 

 and (18.40), require that the quantum yield should be independent of 

 light intensity at all concentrations of the acceptor. New experimental 

 material would be required to apply these conclusions. 



