INFLUENCE OF SOLVENT ON YIELD 763 



photosynthesis. Since this summary was written, a number of pertinent 

 data have been collected by Livingston and co-workers. A discussion of 

 these will be found below. 



If we consider the few presently available data on the intensity of 

 chlorophyll fluorescence in different media, we acquire the impression that 

 physical energy dissipation and chemical quenching must both play a part 

 in these systems; but much remains to be done before their relative roles 

 will become clear. 



The possibility that the yield of chlorophyll fluorescence in solution 

 may be limited by photochemical dissociation of chlorophyll {e. g., into 

 "monodehydrochlorophyll" and a hydrogen atom) was suggested by 

 Franck and Wood (1936). This hypothesis was discussed in chapter 18 

 (Vol. I, page 484), and it was pomted out that a quantum of red light 

 (with an energy of about 40 kcal/emstein) is unlikely to disrupt a carbon- 

 hydrogen bond (whose standard energy is about 100 kcal/mole), even if 

 some energy might be gained by the solvation of the hydrogen atom. Thus, 

 if light absorption does cause a reversible photochemical change of chloro- 

 phyll, it is more likely to be either tautomerization, or reaction with the sol- 

 vent. Dissociation becomes more likely when excitation occurs in the blue- 

 violet band, with quanta of about 60 kcal./einstein; it was mentioned be- 

 fore that this is one possible explanation of the lower yield on fluorescence 

 in this region. 



It is not unlikely that energy dissipation by internal conversion is the 

 basic factor limiting the yield of fluorescence of chlorophyll in condensed 

 systems; it is also possible that— as suggested by Franck and Livingston 

 (1941)— tautomerization occurs as a more or less regular intermediary 

 stage in this dissipation (c/. Vol. I, p. 490). Chemical interactions with 

 solvent or admixtures, as well as self -quenching, are then to be considered 

 as contributmg factors, which further depress the yield of fluorescence 

 under certain conditions. (According to Lewis and Kasha — cf. pp. 790-2 

 —formation of a metastable triplet state could play the role ascribed above 

 to tautomerization.) 



4. Influence of Solvent on Yield of Chlorophyll Fluorescence 



Chlorophyll fluoresces in all (or most) organic solvents (as well as in 

 wax and paraffin), but with different intensity. No precise measurements 

 of the fluorescence intensity in different solvents are available, but Franck 

 and Levi (1934), Albers and Ivnorr (1935), Ilnorr and Albers (1935), 

 Knorr (1941) and Zscheile and Harris (1943) gave some preliminary results; 

 all these investigators agree that wide differences occur both in the initial 

 intensity of fluorescence in different solvents, and in its change with time 

 (cf. Table 23.III). 



