SCHEME OF FLUORESCENCE AND SENSITIZATION 797 



dized to oA (two alternative direct reactions of Chi* and two alternative 

 reactions of the metastable tChl). It neglects all resonance effects. 

 The yield of fluorescence, ^, is according to scheme 23.11: 



(23.18) f = ^' 



(Av + A-. + kt) + A-JChI] + A-*[0.] + A-* [A] 



(This equation is to replace equation 19.5.) 



Some of the constants in (23.18) can be estimated. 



kf is 1.2 X 10^ (inverse of the natural life-time of Chi*, as calculated 

 from the intensity of the red absorption band on page 634) . 



ki may be small compared to A;, — in other words, tautomerization may 

 be a normal intermediate step of internal conversion. This is, however 

 not certain. The quantum yield 7 ^^ 1 was found (c/. Tables 18.11 and 

 18. Ill) at [A] ^ 5 X 10~^ — i. e., concentrations at which k% [A] may well 

 be high enough to make ki insignificant even if it is not small compared to 



ki + ki must be about 10^, if one assumes a fluorescence \aeld of ~ 10% 

 in the absence of quenching by O2 or A. 



fcj should be >10^2 according to Weiss and Weil-Malherbe (c/. page 

 773), but more probably is about 10^ (the value obtained by Weiss and 

 Weil-AIalherbe for dyes other than chlorophyll). Figure 23.7 indicates 

 half-quenching at [Chi] = 1.5 X 10~2 mole 1., and thus fc, = 67 and 

 kc^^7 X 10^ However, the sigmoid shape of the curve points to resonance 

 transfer rather than collisions as the main quenching mechanism. 



A;o is about 5 X 10^ calculated from a "half-quenching" concentration 

 of 2 X 10-2 mole/1, (cf. Table 23.IIIC). 



k\ could be calculated from Franck and Levi's curves for the quench- 

 ing of fluorescence by benzidine, if the concentration units used were known. 

 (The A'l values in Table 23.IIIC are bimolecular rate constants in sec. X 

 1. X mole"'^ the h, k* and A;a values in eciuation 25.18 are products of these 

 constants and the rate constant of monomolecular deactivation, kf -\- ki -\- 

 k,^ 10«sec.-i.) 



We refrain from an attempt to deduce from scheme 23.11 an equation 

 for the quantum >aeld of sensitized autoxidation (to replace equation 19.6'), 

 because the result is much too complicated to allow comparison with the 

 experimental data, e. g., with Gaffron's equation (18.32) for the sensitized 

 autoxidation of allylthioui-ea. The "self-quenching" reaction of the tau- 

 tomer tChl: 



(23.19) tChl + Chi » 2 Chi 



has been added to account for the decrease in the yield of chlorophyll- 

 sensitized photoxidation with increased pigment concentration [Chi], in 



