1308 THE PIGMENT FACTOR CHAP. 32 



phyll b -»- chlorophyll a in solution was mentioned on p. 1303.) Forster 

 proved, by quantitative measurements, that the observed effect could not 

 be attributed to re-absorption of fluorescent light of the "donor" by the 

 "acceptor" or to their association in a complex (the latter was shown by 

 the demonstration of a competitive relation between the quenching of the 

 trypoflavin fluorescence by i-hodamine B and by iodide). The observed 

 lack of an effect of glycerol addition indicated that the energy transfer 

 occurred by long-range resonance, rather than by kinetic encounters. A 

 theoretical analysis of the resonance energy transfer between two different 

 molecules — similar to that given earher for two identical molecules — was 

 first undertaken by Forster (1949-). A value of do(= 5.8 m/x) was calcu- 

 lated from experimental data for the "critical distance" (the distance 

 where the probability of transfer becomes equal to that of emission), for a 

 mixture of trypoflavin and riboflavin. An approximate theoretical calcu- 

 lation gave do = 6.3 m^. (This can be compared with do = 8.0 m^t given 

 on p. 1287 for the transfer between two chlorophyll a molecules.) 



The resonance transfer of energy between diff'erent dyestuffs in solution 

 has been investigated, theoretically and experimentally, also by Vavilov, 

 Galanin and Pekerman (1949; cf. also Vavilov 1950), and later by Galanin 

 and Franck (1951), and Galanin and Levshin (1951). They used mix- 

 tures containing a fluorescent and a nonfluorescent dye, and studied the 

 quenching of fluorescence by increasing concentrations of the nonfluores- 

 cent component. The excitation energy, transferred by resonance from 

 the fluorescent to the nonfluorescent molecule, is dissipated in the latter by 

 internal conversion, so that no sensitized fluorescence of the acceptor oc- 

 curs; the eflficiency of the transfer is measured by the quenching of the 

 fluorescence of the donor. The quenching eff"ect proved to be dependent 

 in the expected manner on the overlapping of the absorption band of the 

 energy acceptor with the emission band of the donor. Furthermore, the 

 half-life of the fluorescence decay was found to decrease proportionally with 

 the yield of fluorescence, and the yield to be independent of viscosity. Both 

 relationships are characteristic of resonance quenching; the second one 

 because resonance quenching depends on the average distance between the 

 fluorescer and the quencher, which is independent of viscosity. Quenching 

 by collisions, on the other hand, is slowed down as viscosity increases. 



Vavilov, Galanin and Pekerman (1949) and Vavilov and Galanin 

 (1949) (cf. Vavilov 1950) described experiments in which dyestuff fluores- 

 cence was excited in a thin layer between a plane and a convex lens. The 

 thickness of the layer could be determined by counting the Newton rings 

 in reflected light. The fluorescent light was analyzed by a monochroma- 

 tor, and the ratio of intensities determined for two wave length bands — one 

 subject to re-absorption in the layer (either by the fluorescent material 

 itself or by another added dyestuff with an overlapping absorption band) 



