778 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



bility (like the permanent dimerization discussed on page 761). Study 

 of the effect of the quencher on the absorption spectmm of the fluorescent 

 material, and of the dependence of quenching on the concentration of the 

 quencher, may help to distinguish between "quenching by complex forma- 

 tion" and "quenching by kinetic encounters"; but, at present, very few 

 such data are available for chlorophyll solutions. Finally, in analogy to 

 the above-discussed mechanisms of self-quenching (page 759), still 

 another possibility of quenching should exist when the quencher is "in 

 resonance" with the fluorescent molecule: quenching without complex 

 formation and without kinetic encounters, through transfer of excitation 

 energy over distances considerably larger than the collision diameters. 

 If the molecules of the quencher are nonfiuorescent, they will serve as 

 "traps" in the same way as was postulated by Forster for the dimers. 

 Vavilov and co-workers (Pekerman 1947, Vavilov et at. 1949^'^) found 

 convincing examples of this type in the quencing of dye fluorescence by 

 resonating nonfluorescent dyes. However, strong quenchers in Table 

 23.IIIC (p. 782) certainly do not owe their effectiveness to a resonance 

 transfer mechanism. They are all oxidants and this points to chemical 

 interaction rather than physical energy transfer. In the second place, 

 they have no absorption bands in the red, and are thus not in resonance 

 with excited chlorophyll molecules. Their lowest excited states must be 

 considerably higher than the fluorescent state A of chlorophyll. 



Among the substances whose quenching effect on the fluorescence of 

 chlorophyll has been investigated in some detail are the reaction partners 

 in chlorophyll-sensitized autoxidations — molecular oxygen and oxidation 

 substrates such as benzidine, allyl thiourea etc. Because of the importance 

 of these results for the analysis of the mechanism of sensitized autoxidation, 

 they have already been anticipated in part in Volume I (c/. chapter 18, 

 pages 483 and 518, and chapter 19, page 546). 



The quenching action of oxygen on the fluorescence of different dyestuffs 

 (including chlorophyll) was first investigated by Kautsky and co-workers. 

 Kautsky and Hirsch (1931) had discovered that the fluorescence of certain 

 dyestuffs adsorbed on silica gel is considerably weakened by oxygen at pres- 

 sures of several hundred millimeters, and that their afterglow (phosphores- 

 cence) is completely destroyed even by very much lower pressures of this 

 gas. Later, a similar ciuenching was observed in fluorescent dyestuff 

 solutions, including chlorophyll solutions in acetone. According to Kaut- 

 sky, Hirsch and Flesch (1935), the quenching of chlorophyll fluorescence 

 is proportional to the partial pressure of oxygen, and shows no "saturation 

 effect" even under a partial pressure of one atmosphere. 



Franck and Levi (1934) and Weil-Malherbe and Weiss (1942) found 

 that the fluorescence of chlorophyll or ethyl chlorophillide solutions in 



