796 



FLUORESCENCE OF PIGMENTS IN VITRO 



CHAP. 23 



given there to describe the most Ukely mechanism of fluorescence and sen- 

 sitized photoxidation in chlorophyll solutions.* This scheme, amplified 

 to include also internal conversion, self-quenching by collisions and photo- 

 chemical reaction with the oxidation substrate A— c. g., an amine— (but 

 simplified as far as the mechanism of sensitized photoxidation is concerned) , 

 is reproduced in scheme 23.11. To prevent the scheme from becoming too 



Chl" 



6 © 



tChl 



® <h 



(tChI) k, 



t + Oj) 

 ( + A) 



(♦Chi) 



© © 



(tO,) 



(+A) 



oChKtHO,) rChl(+oA) 



Chi 



Chi Chi 2Chl 2Chl Chi 



(toA-hHOj) ( + "»') 



(+A) 



Chi 2Chl 

 ( + oA) 



(+02) 



Chi 



Scheme 23.11. Fate of excitation energy in chlorophyll solutions containing o.xygen 



and an autoxidizable substrate A ^ 



complicated, it does not include reactions with the solvent ; it must, how- 

 ever, be kept in mmd that, as shown repeatedly in chapter 18, "pseudo- 

 monomolecular" reactions with the solvent may nearly always serve as 

 alternatives for truly monomolecular tautomerizations. The meaning of 

 the arrows in the scheme is: (1) internal conversion, (2) tautomerization 

 (followed by sensitized autoxidation of the substrate A, or by dissipative 

 return into the normal state Chi), (3) self-quenching, (4) fluorescence, (5) 

 sensitized autoxidation of A through primary reaction with O2, (6) sensi- 

 tized autoxidation of A through primary reaction with A. According to 

 the Lewis-Kasha theory, "tautomerization" may mean an electronic 

 (rather than nuclear) rearrangement, and "dissipation" may be achieved 

 by emission of phosphorescence. 



Scheme 23.11 shows eight different ways by which normal Chi can be 

 re-formed after excitation, and four ways by which substrate A can be oxi- 



*In the last two lines of page 546 in Volume I (first printing), a misprint and an 

 omission must be corrected: The lines should read: "If, at [O2] = 10-1000 mm., 

 A*[02] is not 'C k, the fluorescence yield, >p, nmst depend on oxygen pressure in this 

 rlnge; and if, at [Oil = 10 ■• to lO^" mole/1.. Aj !<),] i.s«; k, and Af, |(),] is»At', . . ." 



