ACTIVATION OF FLUORESCENCE 771 



alcohols and amines to the formation of hydrogen bonds with the amino or 

 hydroxy group, shown in formula 23.1. 



I I I 



^c^\^ ^c^'^c^ -c<\^ 



H H I 



Chelated Enol Form Keto Form Stabilized Keto Form 



(noiifluorescent, stable (unstabilized) (fluorescent, stable in the 



in pure hydrocarbons) presence of amine or alcohol) 



Formula 23.1. Tautomerization and fluorescence of chlorophyll (after Livingston, 



Watson and McArdle, 1949). 



The fluorescence of ethereal chlorophyll solutions is not covered by this 

 hypothesis, and further experiments are needed to show Avhether this 

 fluorescence would persist upon more stringent purification. As an alterna- 

 tive explanation of the phenomenon of activation, Li\nngston considered 

 the possibility that chlorophyll forms nonfluorescent dimers in pure hydro- 

 carbons, and that these dimers are dissociated into fluorescent monomers 

 by association with polar molecules. If this were the case, however, one 

 would expect the extent of activation to depend on concentration of chloro- 

 phyll; while a few — admittedly preliminary — experiments showed no dif- 

 ference in the values of F/Fo between partially activated chlorophyll solu- 

 tions in benzene containing 4.8 X lO'S 1.5 X 10"* and 2.3 X 10"^ mole/1, 

 chlorophyll, respectively. 



Observations bearing obvious relation to those of Livingston and co- 

 workers, have l)een made also by Evstigneev, Gavrilova and Krasnovsky 

 (1949^. In studying the effect of oxygen on the absorption spectrum and 

 fluorescence of chlorophyll, they first found this effect to depend on the 

 solvent. In toluene, heptane and carbon tetrachloride, oxygen increased 

 the absorption (page 648) and activated the fluorescence, while in pyridine, 

 ethanol, ethyl acetate, acetone and (commercial) benzene, it had no effect 

 on absorption, and quenched fluorescence. Later (1949^) the same 

 investigators found that these effects were due not to oxygen, but to water 

 vapor. In moist toluene, oxygen quenched fluorescence in the same way 

 as in ethanol or other polar solvents. 



Evstigneev and co-workers also discussed two conceivable mechanisms 

 of the action of polar solvents. One was the same as Livingston's alterna- 

 tive hypothesis — solvent effect on dimerization. The other differed from 

 Livingston's preferred hypothesis: it assumed attachment of polar mole- 

 cules to the free co-ordination places at the central magnesium atom. This 



