790 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



emit fluorescence, the addition of a sul^strate that is oxidized with a quan- 

 tum yield of 50% should reduce both self-quenching and fluorescence in the 

 same proportion— i. e., to 45 and 5%, respectively. 



Contrary to the suggestion of Weiss and Weil-Malherbe, self -quenching 

 could not explain also the dependence of the yield of sensitized autoxida- 

 tion on chlorophyll concentration (eq. 18.32). 



The statement that stimulation of fluorescence indicates chemical 

 change (p. 781) does not apply to sensitization. That energy absorbed 

 by other pigments can be utilized for chlorophyll fluorescence, was first 

 observed in vivo (chapter 24). According to Duysens (1951), in a 10"^ M 

 solution of chlorophyll a + 6 in acetone, one-half of the quanta absorbed 

 by h are available for the fluorescence of a (by the same token, a must 

 quench the fluorescence of 6). 



7. Long-Lived Active States and Afterglow of Chlorophyll 



As stated in chapter 18, "long-lived activations" can sometimes be due 

 to chemical changes (as weU as to the formation of metastable electronic 

 states— an explanation advocated by Kautsky and more recently by G. N. 

 Lewis). Several of the quenching processes discussed earlier in this 

 chapter may lead to transient formation of unstable products. Metasta- 

 ble active products may occur in the course of physical energy dissipation 

 as well as in that of chemical quenching. In the first case, strong vibrations, 

 excited during "internal conversion," can induce internal chemical changes— 

 e. g., one or two hydrogen atoms may be transferred to a different position 

 in the molecule, thus creating a metastable, tautomeric form. In the second 

 case— that of chemical quenching— metastable states may be produced 

 by reversible photochemical reaction with the solvent— e. g., an exchange of 

 electrons or hydrogen atoms. In this case, the long-lived, metastable 

 state of the pigment is an oxidized or reduced (rather than a tautomeric) 

 state. The active, oxidized or reduced product can be reconverted to the 

 original pigment either by reversal of the reaction by which it was formed — 

 thus leaving no net photochemical change at all— or by other reactions 

 (e. g., with the solvent, or with dissolved oxygen), thus leaving a sensitized 

 photochemical change. All these possibilities were discussed in some de- 

 tail in Volume I (chapter 18). 



The hypothesis that long-lived activated molecules are molecules in 

 metastable electronic states (Kautsky, G. N. Lewis) was dismissed as im- 

 plausible in Volume I, chapter 18 (page 486). Subsequent development of 

 this concept, supported by extensive experiments by Kasha and other 

 workers at Berkeley, makes it necessary to bring the subject up again here. 

 Both the ground state of a valence-saturated molecule, and the excited 

 states corresponding to intense absorption bands, usually are singlet 



