Chapter 23 

 FLUORESCENCE OF PIGMENTS IN VITRO 



Fluorescence phenomena have two aspects. In the first place, the fluor- 

 escence spectrum offers a welcome addition to the absorption spectrum in 

 the study of the term system and molecular structure of a chemical com- 

 pound. In the second place, the yield and duration of fluorescence gives 

 significant information as to the fate of the excitation energy and thus 

 provide clues to the mechanism of photochemical reactions of the light- 

 absorbing compound. In the case of chlorophyll, we are particularly in- 

 terested in the second, photochemical aspect of the fluorescence phenomena. 



Because of the division of this treatise into a chemical and a physical 

 part, the photochemistry of chlorophyll already was dealt with in the first 

 volume (chapters 18 and 19), while fluorescence could first be discussed 

 in the present, second vohune. Certain conclusions derived from fluores- 

 cence studies had to be anticipated in Volume I ; some of them will have 

 to be repeated and amplified here. 



Some new facts and considerations have been added to this field since 

 the appearance of Volume I ; their fluorescence aspects are discussed in 

 the present chapter, and their photochemical aspects in chapter 35. 



A. Fluorescence of Chlorophyll in Vitro* 



1. Fluorescence Spectra of Chlorophyll and Its Derivatives in Solution 



The fluorescence of chlorophyll was discovered by Brewster more than 

 a hundred years ago (1834). It was first studied spectroscopically by 

 Stokes in 1852, and a by-product of this study was the discovery that the 

 leaf pigment consists of two green and two yellow components. Dher^ 

 (1914) and Wilschke (1914) contributed the first photographs of the 

 fluorescence spectrum. However, because the fluorescence bands of 

 chlorophyll are situated in the far red and infrared, and the sensitivity of 

 red- and infrared-sensitized plates varies strongly with wave length, the 

 photographic method is not very suitable for the quantitative study of 

 chlorophyll fluorescence. 

 * Bibliography, page 801. 



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